LOW-TEMPERATURE CANDOLUMINESCENCE

V. A. SOKOLOV

Introduction

The concept of candoluminescence, as is known, is associated with the unusual features of the glow of a number of substances (chiefly oxides), manifested under thermal excitation and, especially, under the action of high-temperature flames on these substances. Despite the fairly long practical use of this phenomenon in the nineteenth century for obtaining bright light sources, for a long time it received no theoretical explanation. The various assertions, hypotheses, and theories that subsequently appeared as a result of foreign work on candoluminescence proved to be confused and contradictory [1, 2].

In this light, the original investigations of candoluminescence carried out in 1945–48 under the direction of V. M. Kudryavtseva are of great importance. In a number of interesting works by V. M. Kudryavtseva and her colleagues it was convincingly shown that the features of the temperature glow of certain oxides that had been associated with candoluminescence are readily explained on the basis of the laws of thermal radiation and are determined solely by the nature of the absorption of these substances [3, 4, 5].

The works of V. M. Kudryavtseva concern the purely thermal glow of substances when they are heated by thermal conduction (outside a flame). In 1947 the author began investigations of the glow of oxides under the action of flames, since the Nichols school had assigned an exclusive role to this type of excitation of candoluminescence [1, 2]. By applying the Vavilov–Wiedemann criterion it was shown that, at high temperatures, the features of the glow of a number of oxides in flames are likewise determined solely by their thermal radiation and are not connected with any kinds of true luminescence [6, 7, 8, 9].

The success of the work of V. M. Kudryavtseva and of the aforementioned investigations of the glow of oxides in flames led to premature conclusions about the absence of candoluminescence in all cases without exception [8]. However, a more careful study of the action of flames on certain oxides and especially on practically used crystalline phosphors showed that, under certain conditions, the flame acts as an agent exciting true luminescence. By the present time

it was irrefutably proved that such luminescence can indeed arise under the action of flames, but only at comparatively low temperatures.

A review of works on high-temperature “candoluminescence,” which is merely a variety of thermal radiation, was published earlier by us in the journal Uspekhi Fizicheskikh Nauk [8]. The present article is devoted to a review of recent works on low-temperature candoluminescence, which is connected with true luminescence in the sense of the definition of Vavilov–Wiedemann.

I. Existence of candoluminescence under the action of flames

Observations of the action of flames on crystalline phosphors showed that contact of some of them with a flame causes a flash of light resembling, in color, the luminescence of these phosphors under the action of ordinary excitation agents (ultraviolet, cathode bombardment, etc.). However, such luminescence of a phosphor situated in a flame most often rapidly decreases in brightness and then goes out altogether. Further action of the flame leads to red heat, and subsequently to bright temperature radiation of the specimen.

It was natural to assume that the disappearance of the luminescence of the phosphor caused by the flame is connected with heating it to the quenching temperature of luminescence. Therefore, in order to observe and study the luminescence of a phosphor in a flame for a long time, it was necessary to create conditions that would make it possible, despite the action of the flame, to maintain the temperature of the phosphor below the quenching temperature of luminescence. The author succeeded in solving this problem by means of the following simple experiment [1].

Fig. 1.

A brass cylinder 1 (Fig. 1), filled with a cooling mixture, was set into slow rotation by means of a small electric motor 2. A phosphor, previously illuminated and unexcited, applied to the outer surface of the cylinder, while touching the colorless flame of a Bunsen burner, for a long time (as long as the cooling acted) exhibited bright luminescence, the spectrum of which could be photographed with a spectrograph 4. The described method always made it possible to maintain the temperature of the phosphor in the flame below the quenching temperature, which, depending on the phosphor, may be very different.

Comparison of the luminescence spectra of phosphors obtained in this way under the action of flames with the spectra of the ordinary luminescence of these phosphors, for example under the action of ultraviolet or cathode excitation, showed their complete identity. Figure 2 presents the luminescence spectrum of a ZnS, CdS—Cu phosphor under the action of fla-

...the luminous-gas flame (curve 1) and the luminescence spectra of the same phosphor under excitation by ultraviolet light (curve 2) and by cathode bombardment (curve 3) [11]. In order to show that the emission spectrum of the phosphor in the flame, obtained by the method described above, is not connected with thermal incandescence of the phosphor particles under the action of the flame, the same figure also gives a separately recorded spectrum of the temperature emission of the same phosphor (curve 4), obtained without the use of cooling, when the action of the flame leads to considerable heating of the phosphor, to quenching of the luminescence, and to visible temperature emission of its particles. It is easy to see that, in this wavelength region, this spectrum is the usual curve of thermal radiation, in no way connected with the luminescence spectra shown in the same figure.

Fig. 2.

Figure 3 shows the luminescence spectrum of a ZnS—Cu phosphor under the action of a luminous-gas flame (curve 1), its luminescence spectrum under excitation by ultraviolet light (curve 2), and the spectrum of temperature emission of the same phosphor (curve 3), obtained by the same method as in the preceding case [12]. Figure 4 presents the luminescence spectrum in the flame of a ZnS, CdS—Ag phosphor (curve 1) in comparison with the spectrum of its photoluminescence (curve 2) [12].

Fig. 3.

Fig. 4.

Both the above-described method of observing candoluminescence in flames and the spectra obtained make it possible to conclude that, in the present case, we are dealing with a variety of true luminescence in the sense of the definition of Vavilov—Wiedemann.

The next series of similar experiments concerns the study of the emission in flames of white metallic oxides, with which the concept of candoluminescence was once directly associated...

cence. In this connection, it was natural to suggest that, if certain oxides do produce true luminescence in a flame, they do so only when they also exhibit it under other methods of excitation, i.e., in other words, when they possess the properties of crystallo-phosphors. Such assumptions have been fully confirmed by experiment.

The most characteristic example is provided by the results of experiments carried out by the author and A. N. Gorban’ [13] on the study of lime (CaO), whose bright glow upon excitation by hot flames was widely used in the last century (under the name “Drummond light”) for obtaining intense and bright light. The features of this glow were explained by Nichols’s school as due to the capacity of CaO for candoluminescence [1, 2].

Specially designed experiments established that at high temperatures (>650–700°C) CaO gives in a flame only purely thermal radiation, whose characteristics are associated with the selective absorption properties of this oxide [9]. At the same time, the study of natural samples of lime showed that at lower temperatures (<650–700°C) they are capable, when excited by a flame, of producing true luminescence, which appears especially distinctly at temperatures of 400–450°C and finally disappears at 650–700°C [13]. These same samples also luminesced under the influence of ultraviolet radiation and cathode bombardment, and the temperature limits of the photo- and cathodoluminescence of lime are approximately the same as those of luminescence under the influence of a flame. The spectrum of the luminescence of lime upon excitation by a flame is similar to the spectra of its luminescence obtained by other methods of excitation. This is clearly seen, for example, from Fig. 5, where the luminescence spectra of one and the same lime sample are shown together, obtained under the influence of a flame (curve 1) and as a result of cathode bombardment (curve 2). In the same figure the spectrum of the thermal radiation of lime (curve 3), obtained as a result of excitation of it by a flame at a temperature >700°C, is also presented.

Fig. 5.

Since lime has a comparatively high temperature of complete quenching of luminescence, the latter, upon excitation by a flame, may under appropriate conditions be superposed on the visible thermal radiation of the samples. Figure 6 gives the emission spectrum of lime in a flame, obtained at the temperature of red heat (~600°C). As is seen from the figure, in the long-wavelength region there is a superposition of the orange luminescence band on the spectrum of the temperature glow of the sample (curve 1).

When the background of thermal radiation is subtracted (curve 2), the maximum of the candoluminescence observed against this background is shifted toward shorter wavelengths (curve 3) and occupies exactly the position of the orange luminescence band shown in Fig. 5 (curves 1 and 2).

Spectral analysis of the investigated samples of natural lime showed the presence in them of small amounts of the impurities Ag, Cu, and Mn, which, apparently, are the cause of the appearance of several bands in the luminescence spectra of lime shown in Figs. 5 and 6.

Fig. 6.

All this indicates that oxides give true candoluminescence under the action of a flame only in the case when they are, in essence, crystallophosphors (the presence in the oxide crystals of small amounts of impurities), capable of luminescing also under ordinary methods of excitation (under the action of ultraviolet light, cathode bombardment, etc.). Moreover, as with other methods of excitation, luminescence under the action of flames can occur only at temperatures below the temperature of luminescence quenching.

All this indicates that oxides give true candoluminescence under the action of a flame only in the case when they are, in essence, crystallophosphors (the presence in the oxide crystals of small amounts of impurities), capable of luminescing also under ordinary methods of excitation (under the action of ultraviolet light, cathode bombardment, etc.). Moreover, as with other methods of excitation, luminescence under the action of flames can occur only at temperatures below the temperature of luminescence quenching.

2. On the Physicochemical Nature of True Candoluminescence

The principal cause of candoluminescence should be considered to be physicochemical processes associated with the recombination, on the surface of the phosphor, of free atoms and radicals of the flame into molecules. In this connection, hydrogen atoms may be especially effective; when they recombine into a molecule, an energy of 4.47 eV is released. This reaction can proceed only through a triple collision and usually takes place readily on a solid surface, which plays the role of the third particle and takes up the energy released during recombination.

In studies of hydrogen recombination on solid surfaces, luminescence of the surfaces (glass, quartz, etc.) was noted in a number of cases [14, 15, 16]. Bonhoeffer observed the glow of phosphors specially introduced into a stream of atomic hydrogen obtained as a result of an electrical discharge [21].

Luminescence of phosphors may occur not only under the influence of atomic hydrogen, but also as a result of recombination on their surface of other atoms or radicals, if energy sufficient to excite luminescence is released in the process. Thus, in labo-

in the laboratory of N. A. Prilezhaeva, P. N. Kokhanenko observed the luminescence of zinc sulfide during recombination on its surface of acetone radicals into molecules [17].

Thus, an explanation of the luminescence of phosphors in flames on the basis of the concept of surface recombination of atoms and radicals is entirely natural, all the more so because hot flames contain, in rather large amounts, both radicals and free atoms.

Let us consider in more detail hydrogen-containing flames, which are the most effective for excitation of candoluminescence.

N. N. Semenov established the chain mechanism of the hydrogen combustion reaction [18]. According to this mechanism, atomic hydrogen must inevitably be present in a hydrogen or hydrogen-containing flame, and this was confirmed by the corresponding experiments. According to determinations by V. N. Kondrat’ev, the partial pressure of hydrogen atoms in a flame is of the order of tens of percent of the total pressure [19]. According to determinations by other authors, this value is 30–35% [20]. There are data according to which, in individual cases, an even higher concentration of atomic hydrogen in the flame is experimentally found [21].

Consequently, the presence of atomic hydrogen in hydrogen-containing flames is obligatory and is confirmed by experiment; moreover, the concentration of hydrogen atoms is quite appreciable and, in any case, cannot be less than 20% of the total number of \(H_2\) molecules entering the flame.

The recombination of atomic hydrogen on solid surfaces has in most cases been studied not under flame conditions (in atomic hydrogen obtained as a result of an electric discharge, etc.). It is therefore of particular interest that, for example, V. N. Kondrat’ev convincingly proved that such recombination of hydrogen atoms on solid surfaces also takes place under flame conditions [19]. All this directly leads to the idea that a phosphor introduced into a hydrogen-containing flame inevitably plays the role of a solid body, on whose surface recombination of hydrogen atoms into molecules will occur. The energy released during recombination and absorbed by the phosphor is, as will be shown below, the principal cause of the phosphor’s luminescence under flame conditions.

3. Experimental evidence for a surface-recombination mechanism

Recombination of flame atoms and radicals on the surface of phosphors. A vivid proof of the participation, in candoluminescence, of physicochemical processes associated with recombination on the surface of phosphors of atoms and radicals of the flame is provided by the following, very simple experiment proposed by A. N. Gorban’ [22].

It is known that recombination of atoms and radicals proceeds readily on metallic surfaces, and also on the surfaces of oxides of certain metals of Groups II and III of the periodic system; among metals, Pt, V, and Cu possess especially high activity in this respect [23]. If the candoluminescence of a phosphor is due to a significant extent to recombination on its surface of atoms and radicals of the flame, then introducing a metal grid into the flame together with the phosphor should

reduce its luminescence, since most of the recombinations will occur on the grid. To carry out the experiment, the simple apparatus described in Section 1 was used for observing the candoluminescence of phosphors by means of a slowly rotating cylinder filled with a cooling mixture. If, now, as shown in Fig. 7, a copper grid is introduced into the flame (after the grid has been heated, the flame passes through it), then the luminescence of the phosphor in the region of the flame located directly above the grid is sharply weakened. When the grid is removed, the bright luminescence of the phosphor is immediately restored to its former intensity.

Fig. 7.

This experiment serves as direct proof that the recombination processes mentioned above participate in candoluminescence.

In this connection it was of interest to determine experimentally the quantity of atomic hydrogen and radicals in the flame of illuminating gas used to excite the phosphors, and also to determine directly, under the conditions of this flame, the recombination coefficients of atoms and radicals on several phosphors investigated for candoluminescence; this was done by means of the thermoelectric-probe method [22].

For this purpose two thin-walled quartz capillaries (with an external diameter less than 1 mm) were introduced into the flame, inside which the junctions of a differential thermocouple were placed. One of the capillaries was wetted in an aqueous suspension of the mixed oxide $\mathrm{ZnO}\cdot\mathrm{Cr}_2\mathrm{O}_3$, in order to ensure practically complete recombination of all atoms and radicals of the flame interacting with the surface (the recombination coefficient on $\mathrm{ZnO}\cdot\mathrm{Cr}_2\mathrm{O}_3$ is equal to 1 [22]). The second capillary, on the contrary, was washed in nitric acid in order to exclude the possibility of recombination of atoms and radicals on its surface (the recombination coefficient on passivated quartz is close to 0 [31]).

When both capillaries are introduced simultaneously into the flame, the capillary treated with $\mathrm{ZnO}\cdot\mathrm{Cr}_2\mathrm{O}_3$ undergoes relative heating, due to the energy released upon recombination on its surface of atomic hydrogen and flame radicals.

E. N. Kondrat’eva and V. N. Kondrat’ev [24] obtained an equation relating the magnitude of the heating \(\Delta T\) to the relative concentration of hydrogen atoms

\[ \Delta T = 1000\,\frac{P_{\mathrm{H}}}{P}, \]

where \(P_{\mathrm{H}}\) is the partial pressure of atomic hydrogen, and \(P\) is the total gas pressure.

The recombination energy of radicals is close to the value of the recombination energy of atomic hydrogen. Therefore the formula given above is applicable both in the case of recombination of hydrogen atoms and in the case of recombination of radicals.

As a result of carrying out the experiment described above under conditions of an illuminating-gas flame, heating of a capillary with \(\mathrm{ZnO}\cdot\mathrm{Cr}_2\mathrm{O}_3\), relative to pure quartz, was obtained in the amount \(\Delta T = 75^\circ\). Hence

\[ \frac{P_{\mathrm{H}}}{P}=\frac{\Delta T}{1000}=\frac{75}{1000}=0.075 \]

or \(7.5\%\) of the amount of the combustible mixture, which, relative to the amount of gas in the stoichiometric mixture (the theoretically required amount of air \(\simeq 2.0\,m^3/m^3\) of gas), is

\[ \frac{P_{\mathrm{H}}}{P_{\mathrm{gas}}}=22.5\%. \]

Thus, in the illuminating-gas flame used to excite candoluminescence in the experiments described above [11, 12, 13], there was a sufficient quantity of hydrogen atoms and radicals to cause, as a result of recombination, the glow of the phosphor introduced into the flame.

The recombination coefficients were determined [22] for the phosphors \(\mathrm{ZnS}\), \(\mathrm{CdS—Cu}\), on which true candoluminescence was observed in flames [11, 12], and also on the natural phosphor—\(\mathrm{CaO}\) (lime), which, as was established, is likewise capable of exhibiting true luminescence when excited by flames [13].

To determine the recombination coefficients of the phosphors, the rectilinear dependence between the relative heating \(\Delta T\) and the value of the recombination coefficient \(\varepsilon\) was used. The calibration graph was constructed on the basis of data on the recombination coefficients for \(\mathrm{ZnO}\cdot\mathrm{Cr}_2\mathrm{O}_3\) (\(\varepsilon \simeq 1\)) and passivated quartz (\(\varepsilon \simeq 0\)) [24, 25].

In the case of the \(\mathrm{ZnS}\), \(\mathrm{CdS—Cu}\) phosphor, heating \(\Delta T\) of the capillary treated with this substance was obtained at \(25\text{–}30^\circ\), which gives, for the recombination coefficient, a value equal to \(0.33\text{–}0.4\). For lime the recombination coefficient proved to be high and approximately equal to 1, since the heating of the capillary treated with lime was the same as in the case of \(\mathrm{ZnO}\cdot\mathrm{Cr}_2\mathrm{O}_3\) (\(\Delta T \simeq 75^\circ\)).

The investigated samples of the \(\mathrm{ZnS}\), \(\mathrm{CdS—Cu}\) phosphor and of lime gave bright luminescence under the action of a flame [11, 12, 13]. Thus, phosphors that candoluminesce well possess high recombination coefficients.

It may be considered that the facts set forth above indicate the decisive importance, in candoluminescence in flames, of physicochemical processes associated with the recombination on the surface of the phosphor of atoms and radicals of the flame into molecules.

Candoluminescence and surface-recombination luminescence of phosphors in an atmosphere of active gas. In connection with the facts just presented, it was of interest

it is of interest to obtain the emission spectrum of the phosphor in an atmosphere of atoms and radicals of the gas in whose flame candoluminescence is observed, and to compare the emission spectrum of the luminophore obtained in this way with the spectrum of its candoluminescence under flame conditions; this was done by A. N. Gorban’ and the author for the case of illuminating gas [26].

For activation of the gas (production of atoms and radicals), a specially constructed gas-discharge tube was used, provided in the middle with a branch tube connected to a fore-vacuum pump. The branch tube had an enlargement, on the inner walls of which the phosphor under study was deposited (Fig. 8), which excluded the possibility of excitation of the phosphor by the ultraviolet radiation of the discharge. In addition, in order to absorb the light radiation arising in the discharge, the entire outer surface of the discharge tube (with the exception of the enlargement in the branch tube) was densely coated with black lacquer.

Fig. 8.

The tube was evacuated of air and filled with illuminating gas, after which an electric discharge was excited in it (at a pressure of \(0.1\) mm Hg).

A considerable portion of the hydrogen atoms (illuminating gas contains up to 40% hydrogen) and radicals arising in the discharge, owing to their comparatively long lifetime \(\tau\) (for hydrogen atoms \(\tau \approx 1\) sec), following in the direction of the pump, enters the branch tube and, recombining on the surface of the phosphor, causes its luminescence,* which can readily be photographed with a spectrograph.

Fig. 9.

Figure 9 shows the luminescence spectrum of a ZnS, CdS—Cu phosphor obtained in this way (curve 2). In the same figure is shown the luminescence spectrum of the same phosphor under the action of a flame (curve 1). It is easy to see that these spectra are identical.

* It should be noted that the effect is observed only for comparatively large dimensions of the discharge tube and electrodes, when the number of free atoms and radicals formed during gas dissociation proves to be sufficiently large. Figure 8 gives the dimensions (in mm) of the discharge tube used in the work.

The fact that free atoms and radicals exist under the conditions of the flame that causes the candoluminescence of phosphors, and the fact that phosphor luminescence (of identical spectral composition) is present under the action of atoms and radicals of the same gas outside the flame, constitute an additional argument in favor of the surface-recombination mechanism of candoluminescence.

4. Semiconductor model of candoluminescence

In connection with the experimental proof of the surface-recombination character of candoluminescence, the question arose of a semiconductor model for the excitation of luminescence in crystalline phosphors in this case.

Such a model was proposed by A. N. Gorban’ and the author [27], on the basis of a development of the ideas of F. F. Vol’kenshtein, V. V. Voevodskii, and N. N. Semenov [28—30] concerning the relation between the processes of recombination of atoms and radicals on a solid surface and its semiconductor properties.

Using the band scheme of luminescence of crystalline phosphors, one can depict the individual stages of the process of recombination of atoms and radicals on the surface of a phosphor, which lead to luminescence. As an example, the recombination of atomic hydrogen is considered.

In scheme 1 (Fig. 10) the energy band diagram of a crystalline phosphor and the corresponding model of the crystal lattice are shown. Upon adsorption of a hydrogen atom with formation of a “weak” (one-electron) homeopolar bond 5, the bonding electron can play the role of a local center represented in the energy scheme of the crystal by acceptor level 6 (scheme 2). The appearance of an electron at this level is possible in two ways: either an electron is promoted to the level from the valence band (scheme 3), or a free electron from the conduction band falls onto it (scheme 4). This corresponds to the appearance of a “strong” (two-electron) homeopolar bond 7.

When an electron is promoted to level 6 from the valence band, a hole 8 arises in the latter; its appearance creates the possibility of exothermic ionization of the luminescence center 9. Such a process may lead to an electron from the conduction band being able to recombine with the ionized luminescence center 10 (scheme 6).

An electron may appear in the conduction band as a result of the recombination of hydrogen atoms into a molecule after another atom from the gas phase reaches the adsorbed hydrogen atom:

\[ (\mathrm{X})_{\mathrm{ads}} + \mathrm{X} \to (\mathrm{X}_2)_{\mathrm{ads}}. \]

In this case energy is released, and the molecule is desorbed. At the expense of the recombination energy, the localized electron may be ejected into the conduction band (scheme 7)*). In the case of similar processes taking place directly on the activator atom, direct ionization of the luminescence center is possible.

Upon adsorption of a gas atom on an activator atom, it is evident that a shift of the level (scheme 8) may occur, analogous to the formation of

*) The possibility of such delocalization of an electron was quantum-mechanically demonstrated by F. F. Vol’kenshtein in work [31].

the localization level (scheme 2). This will lead to the fact that, for the ejection of an electron into the conduction band (scheme 9), less energy will be required than the energy of the luminescence quantum, which will remain of the same magnitude as in the other cases, since after recombination of the atoms and desorption of the molecule the activator level will be in its usual position. Thus, when atoms are adsorbed on the activator, luminescence may be the result of recombination of atoms (or radicals) whose binding energy is less than the luminescence energy (anti-Stokes luminescence).

Fig. 10.

Consequently, in the case of adsorption of atoms or radicals on lattice ions, the energy released upon recombination migrates to the luminescence centers by means of the electron–hole mechanism described above; adsorption of gas atoms on the activator, however, leads to direct ionization of the emitting centers and promotes anti-Stokes chemiluminescence.

Scheme 10 shows transitions of electrons from the free band to sticking levels and back.

Thus, all stages of the recombination process leading to the emission of crystallophosphors find a satisfactory explanation in the band scheme of luminescence of crystallophosphors. Moreover, the model considered is suitable both for the interpretation of candoluminescen-

tion, as well as for explaining the radical-recombination luminescence of phosphors in an active-gas atmosphere.

Recently this model of radical-recombination luminescence has been developed on the basis of the electronic theory of catalysis on semiconductors, which made it possible not only to interpret the mechanism of such luminescence, but also to calculate its intensity. Such a calculation, carried out by F. F. Wolkenstein in [31], showed that the intensity of radical luminescence depends on the position of the Fermi level at the surface of the crystal and is determined by the following formula:

\[ I = A \left[ 1 + B \exp \left( \frac{\varepsilon - \vartheta}{\kappa T} \right) \right]^{-1}, \]

where \(\varepsilon\) and \(\vartheta\) are, respectively, the distances of the Fermi level \(F\) and of the localization level of the chemisorbed particle from the conduction band.

\[ A = aN^{*}P, \]

\[ B = 1 + \frac{b}{P}. \]

Here \(N^{*}\) is the maximum number of chemisorbed particles per unit surface area at pressure \(P = \infty\); \(a\) is a proportionality coefficient depending on the temperature \(T\); \(b\) is an adsorption coefficient (also depending on \(T\)).

Thus, if radical luminescence proceeds by the mechanism described above, its intensity should be sensitive to factors that shift the Fermi level at the surface of the phosphor.

One such factor is an external electric field applied to the phosphor, for example perpendicular to the adsorbing surface. Moreover, changing the sign of the field should lead to a change in the direction of the Fermi-level shift and, consequently, cause an increase or decrease in the intensity of the glow, depending on what type of bond (with respect to the given phosphor) the atoms or radicals of the gas in whose atmosphere the luminescence is observed correspond to.

Experiments on the influence of the field on cando- and radical luminescence were carried out [31] first in the low-temperature flame of illuminating gas, and then in an atmosphere of radicals produced by an electric discharge in hydrogen and air. In this case the scheme shown in Fig. 8 was supplemented by a capacitor, in whose field the branch tube with the phosphor was placed. A high voltage of \(\pm 5\) kV was applied to the capacitor (from an “Orekh”-type unit). A noticeable change in the intensity of the phosphor glow (ZnS, CdS—Cu) was observed, which was recorded with the aid of a photomultiplier. The results of the experiment are given in Table 1.

As can be seen from the table, the transition from hydrogen to the products of dissociation of air (mainly nitrogen and oxygen) leads to the opposite-

…character of the dependence of \(J\) on \(V\). If one assumes that in one case the chemisorption of particles corresponds to a donor type of bond, and in the other to an acceptor type (with respect to the given phosphor), then the data obtained, as shown in [32], are fully consistent with the theoretically predicted result in accordance with the above formula for the dependence of \(J\) on the position of the Fermi level at the crystal surface. This is additional evidence for the correctness of the proposed mechanism of radical-recombination luminescence. In the future, similar experiments on the field effect may serve to determine the character of the bonds of chemisorbed particles on the surface of a semiconductor.

Thus, low-temperature candoluminescence in the form of radical-recombination emission of crystalline phosphors, having gained the right to exist, already indicates the first (as yet untrodden) paths toward its scientific and practical applications.

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