Investigation of Early Ionization Processes in Shocked Xenon

Measurements of ionization rates in noble gases have been of considerable interest, with much of the recent experimental work performed at very high temperatures with shock‐tube techniques. As a result, there have been several conflicting theories formulated to trace the history of ionization phenomena. A major difficulty has been the problem of precisely identifying the earliest stage of the ionization process. Since the probability of atom—atom collisions leading to ionization is small, another source of energy sufficient to promote the necessary early stage of the process must be provided. It appears that under conditions of relatively low temperatures, as in weak shock waves, the assumption of a photoexcitation mechanism of the gas is plausible. Biberman and Veklenko have suggested such a mechanism for the production of excited states of rare gases in shock waves. This mechanism has been used to explain the initial shock‐front ionization. The nature of the processes responsible for visible emission at the shock front, which has been designated as impurity radiation, appears to be a result of the transfer of energy from the excited rare‐gas atoms to the impurity molecule. Spectral observations of the shock zone in xenon have been made in a carefully outgassed shock tube (10^—6 mm Hg). The absorption spectrum is typical of molecular fragments of carbon (C₂) which has been shown earlier to be localized in the region of 4500 to 4960 Å. Calcium or sodium, usual alkali impurities, were not observed in any measurement. The emission appeared to be predominantly from the xenon and possible carbon impurities. Microwave absorption measurements were made normal to the shock front. By use of a plasma model which assumes a linear varying electron density, microwave attenuation in the region of cutoff was measured to determine the rate of ionization and the plasma collision frequency. The observed ionization rate was of second order with respect to xenon pressure for the temperature range of 4000° to 9000°K. The observed activation energy was 1.60 eV±0.2. This was the approximate activation energy for the following reactions in xenon: $$\begin{array}{c}\hspace{0.17em}{\mathrm{Xe}}^{*}+\mathrm{Xe}\Rightarrow {\mathrm{Xe}}_2^{+}{\mathrm{+e}}^{-}\\ \mathrm{or}\\ \hspace{0.17em}{\mathrm{Xe}}^{*}+{\mathrm{C}}_2\Rightarrow {\mathrm{XeC}}_2^{+}{\mathrm{+e}}^{-}\end{array}\}\mathrm{collision\; of\; the\; second\; kind}.$$ The experimentally determined rate expression for ionization is given as $$k_i{\text{=3.58×10}}^{14}{T}^{\frac{1}{2}}\exp \text{[−1.6±0.2/kT]}{\mathrm{cm}}^3{\mathrm{moles}}^{\text{−1}}{\sec }^{\text{−1}}.$$ Radiation at the shock front appears to be best described by the excitation of impurity or by the dissociation of Xe₂ as suggested by Tanaka: $$\begin{array}{cc}{\mathrm{Xe}}^{*}+{\mathrm{C}}_2\mathrm{\hspace{0.17em}\Rightarrow }\mathrm{Xe}+{\mathrm{C}}_2^{*},& \mathrm{collision\; of\; second\; kind},\\ {\mathrm{C}}_2^{*}{\mathrm{(A}}^3\mathrm{\pi )\hspace{0.17em}\Rightarrow }{\mathrm{C}}_2{\mathrm{(X}}^3\mathrm{\pi )+}\mathrm{h}\mathrm{\nu ,}& \mathrm{radiative\; transition},\\ \begin{array}{c}{\mathrm{XeC}}_2^{+}{\mathrm{+e}}^{-}\Rightarrow {\mathrm{XeC}}_2^{*}\Rightarrow {\mathrm{XeC}}_2+\mathrm{h}\nu \\ {\mathrm{Xe}}_2^{+}{\mathrm{+e}}^{-}\mathrm{\hspace{0.17em}\Rightarrow }{\mathrm{Xe}}_2^{*}\mathrm{\hspace{0.17em}\Rightarrow }{\mathrm{Xe}}_2+\mathrm{h}\nu \end{array}\}& \mathrm{dissociated\; recombination}\mathrm{\hspace{0.17em}(}\mathrm{visible\; emission}\mathrm{).}\end{array}$$