Full Text
MASS-SPECTROGRAPHIC STUDY OF ION REACTIONS IN HYDROGEN
In 1931 it was established that, when hydrogen is ionized, the ions \(H_2^+\) and \(H_3^+\) are formed in considerably larger amounts than \(H^+\) ions, and that the relative quantity of \(H_3^+\) ions increases proportionally to the pressure in the ion source.
Fig. 1.
The purpose of the work reported here was to determine the relative content of diatomic and triatomic ions formed during the ionization of hydrogen, deuterium, and a mixture of hydrogen and deuterium, and to study the processes of their formation. To obtain ions, the phenomenon of ignition of an arc discharge at low pressures in the presence of a homogeneous magnetic field was used.
The experiments were carried out with the apparatus shown in Fig. 1. Gas entered the discharge chamber 1 through gas line 2 and was ionized in an arc discharge with a heated cathode 3. The ions formed were accelerated by an electric field, passed through slit 4 into a transverse homogeneous magnetic field, and were focused through \(180^\circ\). Currents of the order of \(0.1\)—\(10\) microamperes flowed to collector electrode 5. With the aid of a special motor, the ion collector was moved, which made it possible to record mass spectrograms. At a small discharge current (about \(10\ \mu a\)) and a pressure of \(3 \cdot 10^{-3}\) mm Hg in hydrogen, a considerably smaller quantity of \(H^+\) ions was found than
ions \(H_2^+\) and \(H_3^+\). At the same time it was observed that an increase in the magnetic-field intensity causes a regular growth in the ratio \(H_3^+/H_2^+\).
To explain the observed phenomena, the following assumptions were made.
The predominant process of primary ionization is represented by the equation
\[ H_2 + e \to H_2^+ + 2e . \tag{1} \]
The resulting \(H_2^+\) ions undergo collisions in the discharge with neutral molecules, becoming \(H_3^+\) ions:
\[ H_2^+ + H_2 \to H_3^+ + H . \tag{2} \]
As the magnetic-field intensity increases, the rate of radial displacement of the ions decreases, the number of collisions increases, and, correspondingly, the number of transitions of \(H_2\) into \(H_3\) increases.
In the graphs: vertical axis—percentage content in the beam; horizontal axis—magnetic field (kilooersteds). The left graph shows \(H_3^+\) and \(H_2^+\); the right graph shows \(D_3^+\) and \(D_2^+\).
Fig. 2.
The experiments were repeated with deuterium. As was to be expected, ions of mass 4 and 6, i.e. \(D_2^+\) and \(D_3^+\), were predominantly detected.
The change in the relative content of various ions in hydrogen and deuterium is shown in Fig. 2. It is evident from this figure that \(D_3^+\) ions in deuterium begin to play the predominant role at stronger magnetic fields than \(H_3^+\) ions in hydrogen. Assuming that the discharge characteristics in hydrogen and deuterium—the current density, pressure, and ionization rate—coincide, this phenomenon can be explained by the greater mobility of the light ions.
Further, the relative content of ions in isotopic mixtures was studied; these mixtures were obtained by electrolysis of solutions containing the corresponding amounts of heavy and light water.
Mixtures of the following composition were investigated: 20% H\(_2\), 80% D\(_2\), and 59% H\(_2\), 41% D\(_2\).
The curves constructed from the results of measurements carried out in these mixtures are presented in Fig. 3. As in hydrogen, ions of masses 1, 2, and 3 were observed here, and, as in deuterium, masses 2, 4, and 6. In addition, ions of mass 5 were found—apparently HD\(_2^+\) ions. The formation
Fig. 3.
of these ions may occur as a result of any of the collisions listed below:
\[ \mathrm{H_2^+ - D_2,\quad D_2^+ - HD,\quad HD^+ - D_2,\quad and\quad HD^+ - HD.} \]
The relative content of ions of different masses and the change in this content with variation of the magnetic field (Fig. 3) can in a number of cases be explained. But there are also certain anomalies, such as, for example, the difference in the course of the curve for ions of mass 4 and the decrease in the relative content of ions of mass 3.
The authors carried out a theoretical investigation of the probability of formation of ions of various types; as a result, curves were constructed (Fig. 4) in fairly good agreement with the experimental results (Fig. 3). This agreement made it possible to conclude that the formation of ions of various kinds, registered in the arc, is governed by the rules of sta-
No preference is observed in the ionization of any one of the isotopic molecules \(\mathrm{H_2}\), \(\mathrm{HD}\), or \(\mathrm{D_2}\).
The collision reaction (2), given above for hydrogen, may in the most general form be expressed as
\[ (\mathrm{AB})^{+}+(\mathrm{CD}) \to (\mathrm{ACD})^{+}+\mathrm{B} \quad \text{or} \quad (\mathrm{BCD})^{+}+\mathrm{A}. \]
Reactions with the formation of \((\mathrm{ABC})^{+}+\mathrm{D}\) or \((\mathrm{ABD})^{+}+\mathrm{C}\) are improbable, since the dissociation of ions is considerably more probable than the dissociation of neutral molecules.
Fig. 4.
The energy of formation of \(\mathrm{H_2^{+}}\) ions, for example, is \(60.95\ \text{kcal/mol}\), while that of \(\mathrm{H_3}\) molecules is \(102.62\ \text{kcal/mol}\).
It was found that the probabilities of formation of \((\mathrm{ACD})^{+}+\mathrm{B}\) and \((\mathrm{BCD})^{+}+\mathrm{A}\) are the same.
L. L.
Cited Literature
- R. L. Murray, J. Appl. Phys. 23, 6 (1952).
- H. D. Smyth, Rev. Modern Phys. 3, 147 (1931).