Infrared-Emission Studies of Electronic-to-Vibrational Energy Transfer. II. Hg*+CO

Abstract

In an earlier work (Part I of this series) electronically excited mercury Hg^* was formed in a low‐pressure flow system by the 2537‐Å irradiation of Hg+CO; electronic‐to‐vibrational energy transfer was studied by recording the infrared emission spectrum of vibrationally excited carbon monoxide (symbolized CO^†) in its ground electronic state. In the present work the earlier experiments have been placed on a more quantitative footing. Absolute concentrations of Hg^*₁(6³P₁), Hg^*₀(6³P₀) and CO^†, v=2–9, have been measured under a variety of experimental conditions. The direct excitation of CO to v≤9 by electronic‐to‐vibrational transfer has been confirmed. Both Hg^*₁ and Hg^*₀ were found to be effective in bringing this about; the efficiency of Hg^*₁ being, however, about an order‐of‐magnitude greater than that of Hg^*₀. Vibrational relaxation was examined by observing the decay of the CO^† infrared afterglow down the nonilluminated portion of the reaction tube. A set of rate constants, k_v (or cross sections σ_v²), for electronic‐to‐vibrational transfer $${\mathrm{Hg}}_{\text{1, 0}}^{*}+\mathrm{CO}\to {\displaystyle {lim}^{k_v}}\mathrm{Hg}+{\mathrm{CO}}_v^{†}$$ were derived from observed sets of steady‐state concentrations N_v under various experimental conditions. The best set of k_v, expressed relative to k_v=9=1.00, was; k_v=2=80, k_v=3=70, k_v=4=60, k_v=5=48, k_v=6=43, k_v=7=35, k_v=8=15, k_v=9=1.00, k_v≥10≈0. These k_v and the corresponding σ_v² can be converted to approximate absolute units, since the over‐all cross section for electronic‐to‐vibrational transfer in collisions Hg^*_{1 0}+CO, symbolized ${\sigma }_{\mathrm{vib}}^2{\mathrm{=(\sigma }}_{\mathrm{vib}}^2{)}_1{\mathrm{+(\sigma }}_{\mathrm{vib}}^2{)}_0{\mathrm{=\Sigma }}_v{\mathrm{[(\sigma }}_v^2{)}_1{\mathrm{+(\sigma }}_v^2{)}_0]$ , was measured and found to be in the range 0.5–3.0 Ų. The most probable individual values for the total cross sections with Hg^*₁ and with Hg^*₀ as the donor atoms were, respectively, ${\mathrm{(\sigma }}_{\mathrm{vib}}^2{)}_1{\text{≈1.3\hspace{0.17em}Å}}^2\hspace{0.17em}\mathrm{and}{\mathrm{\hspace{0.17em}(\sigma }}_{\mathrm{vib}}^2{)}_0{\text{≈0.09\hspace{0.17em}Å}}^2$ . Energy matching is of no importance in this transfer, since a ``resonant'' conversion of electronic‐to‐vibrational energy would require that k_v=20=1 and k_v2<0=0, in marked contrast to the findings. The results are discussed in terms of a complex HgCO^* which undergoes intersystem crossing onto a potential‐energy surface which correlates with electronic ground‐state Hg(6¹S₀); the electronic potential energy being released into nuclear motion as the system ABC gives A+BC across this lower surface. Two possible types of energy release are distinguished, corresponding to relaxation predominantly along the r_AB and the r_BC coordinates, respectively.