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

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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, kv (or cross sections σv2), for electronic-to-vibrational transfer Hg1, 0*+CO→ lim kvHg+COv†were derived from observed sets of steady-state concentrations Nv under various experimental conditions. The best set of kv, expressed relative to kv=9=1.00, was; kv=2=80, kv=3=70, kv=4=60, kv=5=48, kv=6=43, kv=7=35, kv=8=15, kv=9=1.00, kv≥10≈0. These kv and the corresponding σv2 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 σvib2=(σvib2)1+(σvib2)0=Σv[(σv2)1+(σv2)0], was measured and found to be in the range 0.5–3.0 Å2. The most probable individual values for the total cross sections with Hg*1 and with Hg*0 as the donor atoms were, respectively, (σvib2)1≈1.3 Å2 and (σvib2)0≈0.09 Å2. Energy matching is of no importance in this transfer, since a ``resonant'' conversion of electronic-to-vibrational energy would require that kv=20=1 and kv2<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 rAB and the rBC coordinates, respectively.

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