PROCEEDINGS
OF THE TOMSK ORDER OF THE RED BANNER OF LABOR POLYTECHNIC
INSTITUTE named after S. M. KIROV

Volume 199 1969

CURRENT CONCEPTS OF THE PHOTOLYSIS

AND RADIOLYSIS OF ALKALI METAL AZIDES

(REVIEW)

Yu. A. ZAKHAROV

(Presented by the scientific seminar of the Department of Radiation Chemistry, CTF)

A considerable number of works have been devoted to the study of processes developing in alkali metal azides when they are irradiated with light or ionizing radiation. In this respect azides should be regarded as the most fully studied—after silver halides and alkali metal halides. Nevertheless, the present level of knowledge in the radiation physics and chemistry of azides makes it possible at present to speak only with great caution about the fine mechanism of the processes of photolysis and radiolysis of azides, and only for sodium and potassium. The complexity of the processes occurring in this case is clearly evident from the most recent works (1—4).

Up to 1960–62, work on the photochemistry of alkali metal azides developed mainly along the line of studying the macrokinetics of the process of nitrogen evolution—the reaction product (for a detailed review of these works see [5]). As a rule, the initial stage of the process was studied (irradiation for 20–40 min at a light intensity of \(1 \div 6 \cdot 10^{15}\) quanta/cm\(^2\)·sec.).

The attempts contained in these works to construct a detailed scheme of photolysis took into account above all the kinetic regularities of nitrogen evolution (dependence on light intensity, temperature, irradiation time, etc.), and also, in part, the information obtained by that time on the electrophysical properties and coloration of crystals under irradiation.

The more detailed information obtained recently on the kinetics of photolysis, as well as the study of optical, IR, and radio-frequency spectra (EPR) in irradiated azides, and of some other properties of these compounds, provided material for the further development of ideas about the mechanism of photolysis (and, in part, radiolysis) of alkali metal azides, as presented in works [1—4, 6—7].

In the present work we devote primary attention precisely to these recently obtained results, as well as to data on microdefects in irradiated azides, which are described relatively little in [5].

EPR Spectra in Irradiated Azides

The first works on the observation of EPR spectra in irradiated azides date from 1958–60. At present, some of the ob—

...the observed signals have been correctly deciphered; at the same time, a number of signals have been assigned to certain radicals on less secure grounds, and there are signals that remain unstudied and undeciphered. In addition, for some studies carried out by different authors, it would seem under not very different conditions, differences and inconsistencies are characteristic in the nature of the observed EPR spectrum. Nevertheless, the EPR data already provide valuable information on the intermediate products of the photolysis and radiolysis of azides.

Information on the signals observed by various authors in irradiated KN$_3$ is summarized in Table 1.

Table 1

Nature of EPR signals at 77°K in irradiated KN$_3$

In work [8] it was shown that irradiation of KN$_3$ single crystals with ultraviolet light at 77°K or with Co$^{60}$ $\gamma$-rays gives an anisotropic signal consisting of 9 lines with equal splitting when the crystal is oriented $(001)//H$, and of twice the number of lines (two groups of 9 lines) if the magnetic field lies in the plane [011].

The ratio of the intensities of the lines in the group corresponds to the case of spin splitting on four equivalent nitrogen nuclei. The authors assign the signal to the linear ion-radical N$_4^-$, located in two magnetically nonequivalent positions in the lattice and situated in two neighboring anion vacancies with the radical axis along the [110] direction. The work also notes the presence of a group of weak lines. It is very interesting that the N$_4^-$ signal is observed only after irradiation at 77°K and is not observed after photolysis at room temperature.

In work [9], in addition to the signal from N$_4^-$, identical to that established in [8], the presence in KN$_3$, irradiated with light at 77°K, of one more five-component (for $H \parallel /110/$) signal was shown. Analysis of the spectrum enabled the authors to suppose that the paramagnetic centers are the radical N$_2^-$, lying at the site of an azide group and having an axis parallel to the [110] plane of the crystal. The radicals N$_2^-$ may be fragments of the ion N$_3^-$; in the crystal there are two nonequivalent N$_2^-$ radicals, corresponding to the number of nonequivalent sites of N$_3^-$ ions in the KN$_3$ lattice [10].

In contrast to the signals assigned to the ion-radicals N$_4^-$ and N$_2^-$ and having symmetry close to axial, work [9] also revealed the presence in irradiated KN$_3$ at low temperatures of a very anisotropic line, noticeable only at $H \parallel [010]$ and not studied in any detail by the authors.

Unlike [8, 9], in [11] irradiation of \(\mathrm{KN_3}\) was carried out at \(77^\circ \mathrm{K}\) with \(\gamma\)-radiation from \(\mathrm{Co}^{60}\). The character of the signals assigned to \(\mathrm{N_4^-}\) and \(\mathrm{N_2^-}\) differs comparatively little from the signals detected in [8] and [9] (a more clearly expressed axiality of \(\mathrm{N_2^-}\) is shown). However, in addition to the \(\mathrm{N_2^-}\) and \(\mathrm{N_4^-}\) radicals, [11] found the existence of an easily saturated signal consisting of a series of triplets with equal splitting of 5.5 gauss; the signal is isotropic and is observed only at low microwave-field powers in crystals irradiated with \(\gamma\)-radiation. Formation of this signal requires high-energy radiation, since the work showed that the triplet is absent in photolyzed crystals. The authors of [11], with sufficient justification, assigned this signal to atomic nitrogen trapped in the \(\mathrm{KN_3}\) lattice. At intermediate microwave-field powers the authors also observed a broad singlet of unknown origin.

A study of the stability of the radicals showed that, when irradiated crystals are warmed from \(77^\circ\) to \(196^\circ \mathrm{K}\), the \(N\) centers disappear, while the concentration of the ion-radicals \(\mathrm{N_4^-}\) and \(\mathrm{N_2^-}\) increases by factors of 5 and 2, respectively; upon further annealing the number of \(\mathrm{N_4^-}\) increases in parallel with the decrease in the concentration of \(\mathrm{N_2^-}\).

Thus, in \(\mathrm{KN_3}\) crystals irradiated with hard electromagnetic radiation at \(77^\circ \mathrm{K}\), the radicals \(\mathrm{N_4^-}\) and \(\mathrm{N_2^-}\), \(N\), and also, apparently, other as yet unidentified paramagnetic particles are present; these include the broad singlet and the unresolved signal detected in [9] and [11]. The superposition of all the signals, as well as the presence in the \(\mathrm{KN_3}\) lattice of magnetically nonequivalent sites for each radical, considerably complicates the spectrum and makes the interpretation of the signals and quantitative kinetic measurements very difficult. In studies of nitrogen radicals, one must also take into account saturation of the signals from \(\mathrm{N_2^-}\) and the special features of atomic nitrogen.

In addition to the radicals described, [12] established the presence in \(\mathrm{KN_3}\), irradiated at room temperature with \(\gamma\)-rays, of a complex signal assigned by the author to defects of the \(F\)-center type, while in [13], for X-rayed \(\mathrm{KN_3}\), a signal was obtained consisting of 12–14 lines with different saturation. In more detail, in particular, the signal from \(F\)-centers at low temperatures, unfortunately, was not investigated. In view of the presumed important role of \(F\)-centers as primary centers in the photolysis of \(\mathrm{KN_3}\), reliable identification

Table 2

E.P.R. in \(\mathrm{NaN_3}\) after irradiation at \(77\text{–}90^\circ \mathrm{K}\)

$F$ centers in $\mathrm{KN}_3$, and the study of signals from it appear especially desirable.

Upon heating irradiated $\mathrm{KN}_3$ crystals to $300^\circ\mathrm{C}$, a singlet appears in the EPR spectrum with $g = 1.9982—2.0017$, associated with the appearance of metallic potassium nuclei (16—17) of size $10—100\,\text{\AA}$.

Among other data on EPR spectra in potassium azide that are not directly related to radiolysis of the salt, one should note work [14] on radicals in the irradiated $\mathrm{KN}_3—\mathrm{KNO}_3$ system, as well as work [15] on the EPR of $\mathrm{Mn}^{++}$ ions in an azide matrix.

Electron paramagnetic resonance in irradiated sodium azide has been studied in relatively greater detail. In works [19—20] it was shown that, in a $\mathrm{NaN}_3$ matrix after irradiation with X-rays, N atoms can be stabilized. The signal from atomic nitrogen in $\mathrm{NaN}_3$ is quite complex; it is composed of at least seven triplets [20], similar to the triplets of N in various matrices [21—22] (in work [19] only one triplet was obtained). The probable locations of the N atoms are interstitial sites in the $\mathrm{NaN}_3$ lattice. As in $\mathrm{KN}_3$, in sodium azide N atoms are not formed upon irradiation with light.

The spectrum of $F$ centers in sodium azide has been studied in relatively detail (spectroscopically, but not kinetically) (23, 18); the signal is readily saturated, isotropic, symmetric, and consists of 19 lines with a splitting of 9.1 gauss, thus agreeing with the de Boer model of $F$ centers (splitting of an electron captured by an anion vacancy on the six nitrogen atoms of the first coordination shell with nuclear spin $3/2$). $F$ centers are slowly destroyed even at $77^\circ\mathrm{K}$, converting in the process, and also upon irradiation with light from the $F$ band, into centers of type R [23]. To the latter is assigned a broad asymmetric singlet, less prone to saturation than the signal from $F$ centers. A small fraction of the $F$ centers (about 10%) is stable, which gave the authors [24] grounds to conclude that two types of $F$ centers are present in $\mathrm{NaN}_3$.

As in $\mathrm{KN}_3$, upon heating photolyzed $\mathrm{NaN}_3$ to $600^\circ\mathrm{K}$, a singlet is formed that is assigned to metallic Na nuclei; in this case the spectrum is similar for crystals heated both in vacuum and in air [24—25].

For rubidium and cesium azides, EPR spectra have not been described in the literature. When comparing the data obtained with $\mathrm{KN}_3$ and $\mathrm{NaN}_3$, however, what is noticeable first of all is the lower stability of paramagnetic centers in $\mathrm{NaN}_3$. Both azides are very convenient objects for studying $F$ centers by the EPR method. The latter is due to the fact that the electron density of $F$ centers on the surrounding nitrogen atoms is relatively large (in comparison with $F$ centers in alkali-metal halides), which makes it possible to obtain distinct signals without resorting to the method of double resonance.

For elucidating the mechanism of photolysis, another conclusion following from the cited works is also important: the absence in irradiated $\mathrm{NaN}_3$ of ion radicals $\mathrm{N}_4^{-}$ and $\mathrm{N}_2^{-}$.

Coloration of azides upon irradiation with light and ionizing radiation

Irradiation of alkali-metal azides at low temperatures leads to the appearance of a whole series of color centers having characteristic absorption bands. In this respect only sodium and potassium azides have been studied in relatively great detail. The absorption spectrum of irradiated potassium azide was first considered rather carefully

Tompkins and co-workers [26, 27]. The spectrum obtained by them at 77°K for KN₃ irradiated with light mainly at \(\lambda\)-2530 Å is shown in Fig. 1; the positions of the absorption bands are given in Table 3.

Table 3

Positions of the maxima of absorption bands in irradiated azides (in mµ).

* — under irradiation with X-rays.
** — obtained during annealing; poorly resolved.

The interpretation of the main absorption bands in [26, 27] was carried out solely on the basis of the analogy of Fig. 1 with the picture obtained in the study of spectra in irradiated crystals of alkali-metal halides.

Fig. 1. Absorption spectrum of KN₃ irradiated with light (\(\lambda = 253\) mµ) for 5 min (1), 90 min (2), and 180 min (3). Irradiation and recording temperature — 196°C.

Accumulation of both \(F\)-centers and \(R\)-centers under irradiation of KN₃ is described by a first-order equation and occurs, for \(F\)-centers according to [26], as a result of electron capture from an excited azide anion \((\mathrm{N}_3^{-*})\) at anion vacancies \((\square a)\), the number of which is limited. The \(F\)-centers are thermally stable (they are destroyed only very slightly at 200°K) and optically stable; on this basis in [26] de-

...the conclusion is drawn that complex centers \((R)\), like \(F\)-centers, are primary products of photolysis.

Heating a crystal irradiated at 77 or 220°K to 273 or 300°K leads to radical changes in the spectrum. The \(F\)-band disappears with the simultaneous growth of a broad band with a maximum at 700–750 mµ and a weakly resolved structure in the region of 600 and 750 mµ (Fig. 2). These bands are apparently formed upon aggregation of \(F\)-centers and are assigned to complex centers of the \(R\) and \(M\) types. Heating to 323°K causes disruption of the band of complex centers and the formation of a colloidal band at 725–730 mµ and a peak at 440 mµ, associated with photoemission of electrons from metal particles.

Fig. 2. Absorption of a \(\mathrm{KN}_3\) single crystal irradiated with ultraviolet light at \(-78^\circ\mathrm{C}\) and then heated for 10 min at \(25^\circ\mathrm{C}\) (1), irradiated at \(25^\circ\mathrm{C}\) for 10 min (2).

In [27] some data are given concerning the kinetics of growth of the \(V\)- and \(F\)-bands upon irradiation of \(\mathrm{KN}_3\) with X-rays. The lag of the accumulation curves of \(V\)-centers relative to the analogous curves for \(F\)-centers under irradiation of potassium and sodium azides, established by the authors, is naturally connected with the irreversible destruction of \(V\)-centers and the formation of nitrogen—the product of radiolysis of azides. The accumulation of \(F\)-centers and \(V\)-centers is described by first-order equations; according to the authors, the rate of this process increases substantially when the irradiated azides are placed on an aluminum substrate, owing to emission of electrons from the substrate and their capture by anion vacancies in the azide.

The comparative study of color centers in \(\mathrm{NaN}_3\), \(\mathrm{KN}_3\), \(\mathrm{RbN}_3\), and \(\mathrm{CsN}_3\) carried out in [28, 31] made it possible to establish that, in the series of azides with the same type of structure (\(\mathrm{KN}_3\)—\(\mathrm{RbN}_3\)—\(\mathrm{CsN}_3\)), the position of the \(V\)- and \(F\)-bands shifts to the long-wavelength part of the spectrum, thus correlating with the interionic distance, the degree of ionicity of the bond, and the lattice energy of the azides. The regular relationships established in this respect among the indicated quantities are qualitatively similar to those found for a large group of ionic compounds [29]. In a similar typical manner, in a series of azides with a gradual decrease in lattice energy, the edge of the intrinsic absorption of the azides also shifts to the long-wavelength region (the deviation of \(\mathrm{RbN}_3\) in [2–8] may, according to [28], be connected with the increased defectiveness of the crystals).

Finally, despite the absence of such a conclusion in [28], on the basis of the results presented by the authors one may draw the definite conclusion that the tendency of azides to coloration upon irradiation with X-rays increases in the series from \(\mathrm{NaN}_3\) to \(\mathrm{CsN}_3\), in parallel with the decrease in lattice energy, which is also typical of ionic compounds [29]. For all azides, with the exception of \(\mathrm{NaN}_3\), the intensities of the \(V\)- and \(F\)-bands are approximately equal; the authors’ conclusion regarding the relatively weak intensity of the \(V\)-band in \(\mathrm{NaN}_3\) contradicts the results of [27]. As in [27], it is shown in [28] that the optical and ther-

thermal annealing of \(V\)- and \(F\)-centers in \(\mathrm{KN_3}\) proceeds independently, does not return the crystal to its initial state, but leads to profound chemical transformations—the liberation of metal and nitrogen.

In works [30—31], in parallel with optical spectra in irradiated azides, the accumulation and thermal stability of centers possessing absorption in the IR region—according to [31, 32], \(\mathrm{N_4^-}\) radicals—were studied. The conclusions of these works concerning the stability of \(\mathrm{N_4^-}\) in \(\mathrm{KN_3}\) generally agree with the data on this question obtained in the study of EPR spectra. Upon linear heating of crystals irradiated at low temperature, the concentration of \(F\)- and \(F'\)-centers in all azides first increases (by 10—20% of the initial concentration) and only upon further heating decreases.

The results of the cited works, which showed the presence of the \(\mathrm{N_4^-}\) radical in irradiated \(\mathrm{NaN_3}\), on the contrary, do not correspond to the conclusion on this question that can be drawn from analysis of the EPR data. The reasons for the absence of an EPR signal from \(\mathrm{N_4^-}\) remain unclear. The second discrepancy between the data of IR and EPR spectroscopy concerns the conclusion [30] that IR bands attributed to \(\mathrm{N_4^-}\)-centers appear in \(\mathrm{KN_3}\) only after the thermal destruction of \(V\)-centers, and the data of EPR studies on the presence of \(\mathrm{N_4^-}\) in \(\mathrm{NaN_3}\) at \(77^\circ\mathrm{K}\).

The conclusion of work [33] concerning the position of the band of \(F\)-centers (Table 3) differs somewhat from that established in the other cited works. Another feature of this article is the report of the presence, in \(\mathrm{KN_3}\) irradiated with \(\gamma\)-rays at room temperature, of a band not established in other works, with a maximum at \(830\,m\mu\), which is not comparable with other studies, and the authors’ [33] conclusions concerning the relatively high thermal stability of \(V\)-centers in \(\mathrm{KN_3}\) at room temperature.

The studies of spectra in irradiated \(\mathrm{NN_3}\) carried out before 1965 may naturally be divided into two groups: the early, already cited works [27, 28, 31, 34, 35], and the later investigations by Miller and coauthors [23, 36].

The results of the first group of works were compared and analyzed in detail in [5]. Both in the experimentally established regularities and in the interpretation of the absorption bands obtained in [27, 28, 31], there is much in common between potassium and sodium azides. In view of these circumstances, we shall not describe in detail the regularities established in these works for the coloration of \(\mathrm{NaN_3}\) crystals, but shall confine ourselves only to Table 3 and—below—to an analysis of the differences in the character of the spectra of \(\mathrm{KN_3}\) and \(\mathrm{NaN_3}\).

In contrast to the early investigations, in works [23, 36] a detailed joint study was carried out for the first time of EPR spectra and optical spectra in irradiated \(\mathrm{NaN_3}\). The data obtained demonstrate the exceptionally broad capabilities of the method; on their basis it proved possible to introduce substantial additions to the existing ideas about models of color centers in \(\mathrm{NaN_3}\) and about the mechanism of photolysis of this azide.

The authors [23] undertook a parallel study of the thermal and optical stability of the defects responsible in irradiated \(\mathrm{NaN_3}\) for the bands at 6100 and \(7300\,\text{\AA}\), using low-temperature absorption spectroscopy and EPR methods, as well as an investigation of the structure and symmetry of these centers by the method of anisotropy of EPR spectra upon rotation of \(\mathrm{NaN_3}\) single crystals.

They showed that the peak at \(730\,m\mu\) is destroyed optically or thermally (at a temperature of \(195^\circ\mathrm{K}\)), with an increase in both cases of the absorption at \(610\,m\mu\). Similar destruction is also characteristic of the 19-line signal of the \(F\)-center in the EPR spectrum. The increase in the intensity of

the 610 mμ band occurs in parallel with the growth of a broad asymmetric singlet belonging to the \(R_1\)-center. The 610 mμ band (and the singlet in the ESR spectrum) is photochemically stable, which is characteristic of coagulation centers. On the basis of these data the authors [23] concluded that the 790 mμ band belongs to \(F\)-centers, and the 610 mμ band to \(R_1\)-centers in irradiated NaN\(_3\).

It is also evident that in NaN\(_3\) \(R_1\)-centers can be obtained as a result of coagulation of \(F\)-centers during optical “bleaching” of crystals at 77°K by irradiating them in the \(F\)-band. This process does not occur at a temperature of about 4.2°K; irradiation in the \(F\)-band leads only to a decrease in the concentration of \(F\)-centers without growth of the 610 mμ band, evidently because of the immobility of anion vacancies at this temperature.

Annealing experiments on NaN\(_3\) irradiated at 77°K showed that two types of \(F\)- and \(R_1\)-centers exist in the azide, differing in thermal stability. In warmed NaN\(_3\) crystals two \(R\)-peaks are resolved: at 5470 and 6250 Å (Fig. 3), from centers composed of

Fig. 3. Absorption spectrum of a NaN\(_3\) single crystal at 77°K: \(a\)—spectrum of a crystal irradiated for 18 hours at 77°K; \(b\)—spectra after warming the crystal at 298°K for several minutes; \(c\)—spectrum \(b\) after irradiation at 77°K with light with \(\lambda \le 420\) mμ.

two possible combinations of neighboring paired anion vacancies in the lattice [10]. A theoretical estimate of the positions of the \(R_1\)-bands, carried out according to [37], gave satisfactory agreement with the observed absorption bands.

A more careful study of the spectra made it possible to further refine the processes in NaN\(_3\) [36]. It was shown that the rate of accumulation of \(R_1\)-centers is linearly related to the light intensity. The conversion \(F \longrightarrow R_1\)-centers is the most easily accomplished (thermally and optically); however, in contrast to the assertion of [23], the reverse optical transition \(R_1 \longrightarrow F\)-centers was carried out, and it was established that the ratio of \(F\)- and \(R_1\)-centers consequently depends on the irradiation spectrum. By the method of studying photoemission from colloidal Na centers in an annealed crystal it was shown that individual and paired anion vacancies are thermally relatively stable defects.

A very promising method, allowing detailed investigation of the stability of radiation-induced defects, is the combined study of thermoluminescence, thermal bleaching, and exoelectron emission during linear heating of specimens after low-temperature irradiation [40]. On azides, only the first studies in this direction have been carried out [41]. It has been shown that, upon warming irradiated azides, a number of luminescence peaks appear over a broad temperature interval (from −160°C to thermolysis) (with \(\lambda = 400\)—600 mμ), some of them

of them correspond to maxima of electron emission. Since no correlation was established in the work between the luminescence and thermobleaching curves, the authors concluded that the glow and electron emission are due to interconversion processes and the destruction of radical intermediate products of radiolysis. For \(KN_3\) these may be—the destruction of \(V\)-centers at low temperatures \((-196 \div -100^\circ C)\)

\[ \left|N_3TN_3\right|^- + E \begin{cases} \to N_2^- + 2N_2,\\ \to N_4^- + N_2, \end{cases} \]

and in the region up to \(0^\circ C\) the processes:

\[ \begin{aligned} N_2^- + T + N_2 &\to N_4^-,\\ N_2^- + T + N_2^- &\to N_4^- + e,\\ N_2^- + T &\to F\text{-center} + N_2^-, \end{aligned} \]

where the \(T\)-capture center is probably an anion vacancy. At higher temperatures destruction of \(N_4^-\) occurs. The interpretation of the phenomena in \(NaN_3\) is still more tentative, and for \(CsN_3\) and \(RbN_3\) is completely absent.

Table 4

Optical absorption of azides in the ultraviolet region of the spectrum

Data due to Debye on absorption in crystals in the deep ultraviolet region [38, 39] bear directly on the question of the processes occurring in azides under irradiation. The author [3—8] first obtained clearly resolved bands of exciton absorption in azides. In addition to these peaks, lying in the region \(140—190\ m\mu\) (Table 4), the existence in all azides of absorption in the region of \(225\ m\mu\) was established; it is characterized by a relatively small extinction coefficient and was assigned by the authors to transitions within the \(N_3^-\) anion with formation of an excited anion.

Contrary to the assumptions of [26—28], Debye showed that excitation of electrons in azides into the conduction band requires about

8.5 eV, and into exciton levels—about 8 eV, and was not carried out in the early work on photolysis. Absorption of high-energy light quanta leads to the appearance of photocurrents in azides [39], the spectral distribution of which has a complex form and in the general case does not correlate with the peaks of exciton absorption and excitation of the \( \mathrm{N}_3^- \) anion.

In NaN\(_3\) crystals irradiated with X-rays there is photoconductivity in the longer-wavelength region (220–250 \(m\mu\)), which also has a complex spectral distribution. The spectral distribution of external photoemission of electrons from azides, studied in the same work, has little in common with the optical spectra and the photoconductivity spectra.

Analysis of the material available in the literature shows that changes in the optical and photoelectric properties of azides as a result of their irradiation have been studied in relatively great detail only for two of them—sodium and potassium azides. The radiation-induced changes in the named properties in NaN\(_3\) and KN\(_3\) are in many respects similar in character; however, there are also a number of differences in their behavior under irradiation. The latter are of considerable interest in connection with discussion of the question of the specific features of the photolysis and radiolysis of azides.

These differences are as follows:

1. As was already noted above, the rate of formation of \(F\)-centers during the photolysis and radiolysis of KN\(_3\) is greater than in NaN\(_3\).

2. Like radical formations, color centers in KN\(_3\) are relatively more thermostable. In a very approximate estimate, analogous annealing phenomena proceed in the azides if the temperature of KN\(_3\) is 100–120° C higher than that of NaN\(_3\).

3. The form of the accumulation curves of \(R_1\)-centers in NaN\(_3\) and KN\(_3\) is different [3]. As for the accumulation of \(F\)-centers, in KN\(_3\) a monomolecular equation is valid; in NaN\(_3\) the kinetics is also close to monomolecular.

4. A special feature of the photolysis of NaN\(_3\) is the absence, in the early stages of photolysis, of the \(V\)-band, which apparently appears only after very prolonged irradiation.

5. In NaN\(_3\), in contrast to KN\(_3\), two types each of \(F\)- and \(R_1\)-centers have been found.

6. The question of the different spectral positions of the \(F\)- and \(R_1\)-bands in NaN\(_3\) and other alkali-metal azides deserves special discussion (taking into account Miller’s results—Table 3). This difference may be associated with the difference in the structure of NaN\(_3\) (rhombohedral) and other azides (tetragonal). It remains unclear, however, why, according to Miller, the \(R\)-band lies in the short-wavelength region of the spectrum (relative to the \(F\)-band), whereas for KN\(_3\) and alkali-metal halides the characteristic arrangement of coagulate bands is in the long-wavelength region. This discrepancy deserves close attention and requires both a check of Miller’s data and a more detailed study (in the spirit of works [23, 36]) of color centers in KN\(_3\), with the aim of confirming (or refuting) models of the centers.

7. In NaN\(_3\), optical and thermal conversion of \(F\)-centers into \(R\)-centers and the reverse optical conversion (more difficult) are possible. In KN\(_3\), conversion of \(R\)- into \(F\)-centers is not realized. On the basis of these, as well as other regularities (the ratio of band intensities, etc.), one may assume that in the photolysis of KN\(_3\) the formation of \(F\)- and \(R\)-centers occurs in parallel and independently, whereas in NaN\(_3\) the primary centers are \(R\)-centers, and \(F\)-centers are formed upon thermal decomposition of \(R\)-centers.

1. The appearance of the spectra in annealed \(KN_3\) and \(NaN_3\) is somewhat different. \(NaN_3\) is characterized by weaker photoemission of electrons from colloidal centers and by the absence of a colloidal band in the optical spectra (the latter despite the fact that a “colloidal” singlet is present in the spectra of E.P.R. of annealed \(NaN_3\)).

2. The more complex form of the thermoluminescence curves of irradiated \(KN_3\) gives grounds for considering that the annealing processes here are more complex and multistage. As for rubidium and cesium azides, information on radiation-induced changes in their properties is extremely scanty; the available data nevertheless suggest that in many respects they are similar to potassium azide.

Kinetic regularities of the photolysis and radiolysis of azides

In early works devoted to the photochemistry of potassium and sodium azides, the initial stage of photolysis (corresponding to 20–30 min of irradiation of the preparation with a mercury lamp) was studied [42–44]. The principal regularities of photolysis established in these works are as follows:

1. The rate of isothermal processes in \(KN_3\) and \(NaN_3\) is constant (over the short portion of the process studied).

2. The activation energy for \(KN_3\) is \(4\ \text{kcal/mol}\).

3. The rate of photolysis is proportional to the square of the light intensity \([J]\) for \(KN_3\) and depends linearly on the intensity for \(NaN_3\) [45].

4. Nitrogen evolution continues even after irradiation of the azides is stopped (“dark reaction”).

5. The quantum yield of photolysis is 0.001 for \(KN_3\). Subsequent investigations, carried out first with \(BaN_6\) [46–47] and then also with alkali-metal azides [45, 48–49], showed, however, that the kinetic regularities of azide photolysis are much more complex; the typical curves obtained by the authors [49] for the change in the photolysis rate during the process itself are shown in Figs. 4–7.

The form of the photolysis curves realized experimentally is given in Fig. 8. For \(NaN_3\), a curve with a gradually decreasing rate is characteristic under irradiation with any light source.

In the case of \(KN_3\) and \(RbN_3\), an increase in the rate after a minimum occurs if short-wavelength radiation is present in the spectrum of the source \((\lambda < 200\,m\mu)\). In preparations that have been photolyzed beforehand, by contrast, the descending portion of the curve is absent and the rate after the onset of irradiation increases, tending toward saturation. In the photolysis curve of \(CsN_3\), two regions of rate saturation are expressed; the process rate increases linearly with increasing light intensity.

For all azides except \(NaN_3\), nitrogen evolution after cessation of illumination is characteristic. The rate of such a “dark reaction” and the amount of nitrogen evolved in it depend on many factors: the experimental temperature, the extent of transformation, the illumination intensity, etc.

For rubidium, cesium, and potassium azides, the dark reaction is bimolecular with respect to the nitrogen evolved.

For \(NaN_3\), appreciable dark evolution of \(N_2\) is absent even when photolysis is carried out in the low-temperature region. Radiolysis and chemical changes in irradiated alkali-metal azides have been investigated only in general outline.

Fig. 4–7. Dependence of the photolysis rate of azides on irradiation time at 0°C—4—NaN\(_3\); 5—KN\(_3\); 6—RbN\(_3\) (at 80°C). 7—CsN\(_3\) (a—at 0°C; b—at 80°C)

In this direction (the kinetics and chemistry of radiolysis), the most detailed studies were carried out by Hill [50–51].

The results of his work, as well as of certain other studies in this area, are discussed in sufficient detail in [5]; therefore, in the present review, without considering the details, we shall indicate only the principal kinetic features of the radiolysis processes. The accumulation of basic products in an aqueous solution of irradiated sodium azide (OH\(^{-}\) and NH\(_3\)) increases linearly with increasing dose (up to \(\approx 3 \cdot 10^{23}\) eV/g); the rate of accumulation depends linearly on the dose rate. The radiation yield of products is almost independent of temperature in the low-temperature region (approximately up to 0°C) and increases from two to nine in the temperature range 0°C–200°C; the activation energy of the process is

Fig. 8. Types of curves realized experimentally for the dependence of the photolysis rate on irradiation time, together with their components corresponding to individual stages of the process. a—reaction involving “special” lattice sites (T); b—linear growth of metal nuclei; c—process on metal nuclei.

in this temperature interval is 500 cal/mole. The ratio of the main radiolysis products—$G_{\mathrm{N_2}} : G_{\mathrm{OH}} : G_{\mathrm{NH_3}} = 4 : 3 : 1$—changes little over a wide temperature interval; moreover, all the nitrogen formed is retained by the NaN$_3$ matrix. The radiolysis of sodium azide proceeds according to the equation $\mathrm{NaN_3} \rightarrow \mathrm{Na} + {}^{3}/_{2}\mathrm{N_2}$; no nitride is formed.

The kinetics and special features of the radiolysis of other alkali-metal azides have not been studied in detail. Works devoted to certain particular questions of radiation transformations in azides are considered in [5].

Among the most recent investigations we note the work [52], in which it was shown that, in the series of azides from Na to Cs, radiation stability decreases (the initial radiation yield of products increases) in parallel with the increase in free volume in the azide lattice (or with the decrease in the energy of the crystal lattice).

Mechanism of the processes developing in azides under irradiation

The experimental material reported in the most recent works on the kinetics of photolysis of alkali-metal azides, and on the nature and regularities of the accumulation and disappearance of radicals and color centers under irradiation, has made it possible to broaden substantially and to revise earlier ideas about the mechanism of azide photolysis.

It was previously assumed that the principal elementary stages of photolysis are processes involving excitation of an electron from the azide anion into the conduction band or to an exciton level:

\[ \begin{aligned} 1.\quad & \mathrm{N_3^-} \rightleftarrows \mathrm{N_3^0} + e.\\ 2.\quad & \mathrm{N_3^-} \rightleftarrows \mathrm{N_3^{*-}}. \end{aligned} \]

with subsequent formation of simple color centers by the reactions:

\[ \begin{aligned} 3.\quad & \mathrm{N_3} + \square k \rightleftarrows V\text{-center}.\\ 4.\quad & e + \square a \rightleftarrows F\text{-center}. \end{aligned} \]

\[ 5.\quad \mathrm{N_3^{-*}} \begin{array}{c} \xrightarrow{+O} V + e\\ \xrightarrow{+O_2} F + \mathrm{N_3^0}. \end{array} \]

or complex centers:

\[ \begin{aligned} 6.\quad & \mathrm{N_3^{-*}} + \square a \rightarrow R + \mathrm{N_2} + \mathrm{N}.\\ 7.\quad & \mathrm{N_3^{-*}} + \square a \cdot \mathrm{N_3^{-*}} \rightarrow R + F + 3\mathrm{N_2}. \end{aligned} \]

Analysis of EPR data in irradiated azides showed, however, that none of the signals detected can be assigned to the linear molecule $\mathrm{N_3^0}$—this compels reactions 1 and 3 to be regarded as improbable.

It was further shown that, under ordinary conditions of irradiation with light, one should not expect the formation of excitons and conduction electrons, since the width of the forbidden band in azides is about 8.5 eV, while excitation to exciton bands, as already mentioned, requires vacuum ultraviolet ($\lambda < 1500^\circ$ Å).

Finally, a careful study of the kinetics of photolysis showed that it differs at different stages of the process and for different azides.

According to [48–49], the complex photolysis curves (Fig. 8) can be represented as the summation of three or two simple monotonic curves, each of these curves corresponding to a process with its own specific mechanism and features of occurrence.

For the initial stage of photolysis of all alkali-metal azides, a decrease in the rate with time is characteristic. A probable cause of this may be the participation in the process of “special” sites of the crystal lattice, the number of which is limited and which are irreversibly consumed (destroyed) as a result of the reaction. Such special sites may be dislocation thresholds, impurity ions, or vacancies located at special sites of the lattice. It is unlikely that normal anion vacancies are such special sites, since the latter accumulate in the lattice during photolysis, and this should have ensured an acceleration of the process, not the retardation observed experimentally.

Already at this initial stage of photolysis, the reaction mechanisms in \( \mathrm{KN_3} \) and \( \mathrm{NaN_3} \) must differ substantially. For sodium azide the probable elementary stages of photolysis in this period may be the following:

\[ \text{8.}\quad \mathrm{N_3^-}\ \underset{}{\stackrel{-h\nu}{\rightleftarrows}}\ (\mathrm{N_3^-})^* . \]

\[ \text{9.}\quad (\mathrm{N_3^-})^* + \mathrm{T} \rightleftarrows (\mathrm{N_3^-})^* \cdot \mathrm{T}. \]

\[ \text{9a.}\quad (\mathrm{N_3^-})^* + \Box_a \to \mathrm{R} + \mathrm{N_2} + \mathrm{N}. \]

\[ \text{10.}\quad \mathrm{N} + \mathrm{N} \to \mathrm{N_2}. \]

\[ \text{11.}\quad (\mathrm{N_3^-})^* \cdot \mathrm{T} \to \mathrm{N_2} + \mathrm{Te} + \mathrm{N} \]

\[ \text{12.}\quad (\mathrm{N_3^-})^* + (\mathrm{T}-\mathrm{N_3^-}) \begin{cases} \to \mathrm{Te} + (\mathrm{N_3}-\mathrm{N_3})^- \to \mathrm{N_4^-} + \mathrm{N_2},\\ \to \mathrm{R} + \mathrm{N_4^-} + \mathrm{N_2}. \end{cases} \]

Absorption of a light quantum from the sources ordinarily used (low- or high-pressure mercury lamps, containing no light with \( \lambda < 1800 \ \text{\AA} \)) leads to the formation of the excited molecule \( (\mathrm{N_3^-})^* \) (but not the exciton \( \mathrm{N_3}^{-*} \)—according to [38–39]).

The valence electron of the excited molecule \( (\mathrm{N_3^-})^* \), by means of a tunneling transition, is captured by a “special” lattice site—the capture center \( \mathrm{T} \); if the role of the latter is played by an anion vacancy, then an \( \mathrm{R} \)-center and atomic nitrogen are formed.

During photolysis the nitrogen atoms \( \mathrm{N} \), mobile on the surface, recombine, forming molecular nitrogen; in the case of radiolysis they may be captured in the bulk by the matrix.

The question of the formation of the ion-radical \( \mathrm{N_4^-} \) upon irradiation of \( \mathrm{NaN_3} \) is, as already noted, controversial. \( \mathrm{N_4^-} \) may be formed by reaction (12), in parallel with the formation of the molecule \( (\mathrm{N_3}-\mathrm{N_3})^- \), which, according to the assumption of [27], is identified as a \( V \)-center. The low intensity of the \( V \)-band in \( \mathrm{NaN_3} \) indicates the low probability of process (12); the uncertainty in establishing the presence of \( \mathrm{N_4^-} \) in irradiated \( \mathrm{NaN_3} \) may also be connected with this. The scheme presented requires the formation of \( \mathrm{R} \)-centers as primary products of photolysis, a linear dependence of their accumulation rate on the intensity, and the same dependence for the rate of gas evolution, i.e., it corresponds to the experimental results.

For photolysis of \(KN_3\), in contrast to the process in \(NaN_3\), there is a characteristic independent formation, by first-order reactions, of both \(F\)- and \(R\)-centers as primary intermediate products, the presence of distinctly identifiable radicals \(N_4^-\) and \(N_2^-\), and a dependence of the photolysis rate on the square of the light intensity.

Taking this into account, the probable sequence of stages is as follows:

\[ 8.\quad N_3^- \rightleftarrows (N_3^-)^* \]

\[ 14.\quad (N_3^-)^* + T\cdot (N_3^-)^* \to 2F + 3N_2. \]

\[ 15.\quad (N_3^-)^* + T\cdot (N_3^-)^* \to R + N_4^- + N_2. \]

\[ 16.\quad (N_3^-)^* + T\cdot (N_3^-)^* \to R + N_2^- + 2N_2. \]

Of the stages given, reaction 14 is evidently relatively more probable for \(KN_3\) (from the ratio of the intensities of the \(F\)- and \(R\)-bands in the irradiated azide). The photolysis of rubidium and cesium azides has been studied clearly insufficiently for constructing systems of probable elementary stages of the process. The fact reported in [48] of a linear dependence of the photolysis rate of \(CsN_3\) on the light intensity, however, gives grounds to suppose that photolysis of \(CsN_3\)[B] in the initial stage proceeds more according to the \(NaN_3\) type than according to the \(KN_3\) type. The products obtained in reactions 8–16 are final products in the photolysis of azides in the low-temperature region (\(\approx 77^\circ K\)).

If irradiation is carried out at higher temperatures, then the radicals and color centers—the products of the primary reactions—undergo further thermal transformations, already briefly described above. In this case the electron centers coagulate into more complex formations (the depth of coagulation depends on temperature), ultimately giving metallic centers. \(V\)-centers perish with a parallel growth of the \(F\)-band. Radical formations undergo thermal transformations, according to [19], in the sequence:

\[ 17.\quad N_2 + T\cdot N_2^- \to N_4^- + F\text{-center}. \]

\[ 18.\quad N_4^- \to N_3^- + N;\qquad N + N \to N_2. \]

The increase in the photolysis rate at later stages of the process, characteristic of \(KN_3\) and \(RbN_3\), occurs only when the azides are irradiated at this stage with light containing the short-wavelength part of the spectrum \((\lambda < 2000^\circ A)\), and is connected by the authors [48–49] with reactions of excitons and metallic nuclei that photoemit electrons:

\[ 2.\quad N_3^- \xrightleftharpoons[h\nu = 1849\,\mathring{A}]{} N_3^{-*} \]

\[ 19.\quad \text{metal nucleus } M_n \xrightarrow{h\nu} M_n^+ + e \]

\[ 20.\quad N_3^- \xrightarrow{h\nu = 2573\,\mathring{A}} (N_3^-)^*. \]

\[ 21.\quad (N_3)^* + M_n^+ \to N_3M_n. \]

\[ 22.\quad N_3^{-*} + N_3M_n \to M_n + N_4^- + N_2 + F\text{-center}. \]

\[ 23.\quad M_n + M^+ + e \to M_{n+1}. \]

(where \(M_n\) is a metal nucleus containing \(n\) atoms).

It should be noted that the regularities of this stage of photolysis have not been studied to a sufficient extent, and therefore the mechanism presented for it should be regarded as purely preliminary and subject to refinement.

The process represented by reactions 19–23 is absent in sodium azide. This is undoubtedly connected with differences in the fine mechanism of azide photolysis. It is not possible to name with any great certainty the specific causes of the kinetic differences between the processes in $\mathrm{NaN_3}$ and $\mathrm{KN_3}$. Tentatively, however, they may be associated with the relative weakness and with the location in the short-wavelength part of the spectrum of the absorption maximum in $\mathrm{NaN_3}$ that is connected with photoemission from colloidal sodium centers (Table 3).

The kinetic regularities of azide radiolysis have been studied to a much lesser extent than the photolysis of these salts. Study of the optical spectra and E.P.R. in azides irradiated with X-rays has shown that, in radiolysis, processes 9a, 10, and 11 are relatively more probable. Along with reaction 8, direct ionization of anions must play a major role here:

$$ 24.\ \mathrm{N_3^- \rightleftarrows N_3^0 + e.} $$

All these, as well as other possible differences in the course of photolysis and radiolysis of azides, should not, however, lead to fundamental, sharp differences in the mechanisms of photolysis and radiolysis, since the principal regularities of coloration and the form of the E.P.R. spectra in azides at low temperatures, as well as their change upon heating, are qualitatively similar both in photolyzed azides and in those irradiated with X-rays. The substantial differences between them and the spectra of azides irradiated with corpuscular radiation may be the consequence of sharper differences also in the mechanisms of decomposition in that case.

The materials presented in the present review show that investigations in recent years have considerably deepened our ideas about the kinetics and mechanism of the photochemical decomposition of alkali-metal azides. This has been promoted above all by the study of E.P.R. spectra and optical spectra in irradiated azides, by efforts to identify the E.P.R. spectra, and by the careful study of the kinetics of azide photolysis at different stages of the process.

All this makes it possible, for the first stage of photolysis, to describe with sufficient rigor the elementary steps through whose occurrence the photolysis process develops.

Radiolysis of azides has been investigated definitely less fully. This field is characterized by an almost complete absence of investigations performed in recent years and at the modern scientific level.

It seems especially desirable to obtain data on the kinetic regularities of the processes (the order of the accumulation reactions of individual centers, the dependence of the rate on dose rate, etc.). Only with such results available and by comparing them with spectroscopic data can one count on constructing sufficiently correct ideas about the sequence of elementary stages of the radiolysis process.

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ERRATA NOTED