Isotope effects in the quenching of electronically excited atoms. Quenching of I(5²P_½) by methane and deuteromethanes studied by time-resolved resonance fluorescence

Abstract

The quenching of electronically excited iodine atoms, I(52P&), by CH4, CDJH and CD4 has been s tudjed using time-resolved resonance fluorescence : k(CH4) = (9.4+ 0.4) x em3 molecule-' s-'These isotope effects are shown to be consistent with a mechanism involving resonant electronic to vibrational + rotational (E-VR) energy transfer. A calculation, based on long-range coupling between transition multipole moments, is presented for the quenching of I(52P+) by CH4 and is in good agreement with the experimental result, supporting this mechanism for resonant E-VR transfer.The observation of lmge isotope effects in the quenching of electronically excited atomic states led us to suggest that resonant electronic to vibrational + rotational (E-VR) energy transfer can occur with high efficiency in some systems, e.g., Te(53P,)+H,(v'f = 0, J" = 1) + Te(53P2)fH,(v' = 1, J' = 3)A& = -6 cm-l. This suggestion has since been substantiated both experimentally 2-13 and theoretically. Wodarczyk and Sackett showed that quenching of Br(42P+) and I(52Pt) by HF occurs with high efficiency and leads to resonant vibrational excitation of the HF molecule. More recently Grimley and Houston l 3 have shown that quenching of 1(52P,) by H 2 0 and HDO leads to efficient ( w 100 %) resonant excitation of two 0-H stretching quanta in these molecules.Such efficient E-VR transfer, where there is typically a large (> 1 order of magnitude) decrease in quenching rate on fully deuterating the quencher, appears in many cases to involve coupling at long-range between transition multipole moments in the atom and quenching molecule. Such a mechanism implies particular selection rules for AJ, depending on the coupling term involved, and gives a strong dependence of the quenching rate for any piu-ticular channel on Av and on the energy mismatch to be accommodated as a change in relative kinetic energy of the collision partners. This mechanism has been applied successfully to the calculation of quenching crosssections for a number of E-VR processes. Thus French and Lawley l4 have presented full scattering calculations for quenching of various excited atoms by H2, HD and D2, which show that resonant coupling between transition multipole moments gives rise

Deactivation of I(52P1/2) by CF3I, CH3I, C2H5I, and CH4

“…As shown in Table I, however, our value is slightly lower than those reported by other groups [9,20,26,271. In particular, recent observations using flash photolysis followed by either resonance absorption spectroscopy [271 or resonance fluorescence detection [91 have settled on a value of roughly 9.5 x cm3 molecule-' s-'.…”

“…Thus, we feel quite confident of the accuracy of our measurement. In any case, the difference between our measurements and those made previously [9,27], while not readily explainable, amounts to only 20%.…”

“…Previous results on the I* deactivation rate constant by these precursors have been in serious disagreement. Since Donovan et al [9] have recently used time-resolved resonance fluorescence to measure the deactivation of I* by CHI, this quencher was included in our study in order to extend the range of comparison between results produced by different monitoring techniques.…”

Deactivation of I(2P1/2) by CH3Cl, CH2Cl2, CHCl3, CCl3F, and CCl4

“…From the slopes of these plots, the second-order quenching rate constants are derived. Table I collects the values of obtained, together k Q with spin-orbit relaxation constants pertaining to other 10 cm molecule s pounds [8,12]. Furthermore, they follow a logical trend of having lower values as the number of hydrogen atoms in the molecule decreases.…”