[Time resolved distribution of excitation energy in collisions of vibrationally excited KH with CO2].

ABSTRACT

The vibrational levels of KH(X1 sigma+ v" = 0-3) were generated in the reaction of K(5P) with H2. The vibrationally excited KH(v" = 17) was populated by an overtone pump-probe configuration Different characteristics of collisional energy transfer in highly and lowly excited vibrational levels of KH and CO2 were investigated through measuring the time-resolved distribution of vibrational energy in KH(v" = 17.3) + CO2 collisions. For KH(v" = 17), there existed three principal regions of vibration temperature (T(v)) in this equilibration process. The initial phase consists of very rapid fall in T(v) within - 5 micros, and the vibrational energy of KH(v" = 17) is mainly transferred to the vibrational levels of CO2 (00 degrees 1) or high rotational levels of CO2 (00 degrees 0). The second phase (5-20 micros) has a slight decline in T(v), and the process of energy transfer to vibrational levels or high rotational levels of CO2 has already finished. The vibration temperature of the third phase has a slightly more rapid decline compared with the last phase. This phase shows that the process of transfer to lowly rotational levels and translation energy of CO2 is accelerated. The equilibration of vibrationally excited KH (v" = 3) in CO2 was also investigated. There are similarities to the behavior of KH (v" = 17) in CO2 plot, but also are significant differences. Once the initial resonant V-R exchange has equalized vibrational temperatures, there is a very slow linear decline in T(v) with equilibrium attained within -80 micros. This same point is reached within 15 micros for KH (v" = 17). The data demonstrate that single rate coefficient measurements are unlikely to capture the complex nature of processes that generally are multistaged with different relaxation rates characterizing each different stage. Examination of the quantum state distributions reveals that these distinct stages reflect the dominance of specific energy transfer mechanisms, some of which are inherently fast and others are much slower. The energy gain into CO2 resulting from collisions with excited KH was probed using transient absorption techniques. Distributions of nascent CO2 rotational populations in both ground (00 degrees 0) state and the vibrationally excited (00 degrees 1) state were determined. A kinetic model was developed to describe rate coefficients for appearance of CO2 states resulting from collisions with excited KH. These experiments show that collisions resulting in CO2 (00 degrees 0) are accompanied by substantial excitation in rotation while the vibrationally excited CO2 (00 degrees 1) state has rotational energy distributions near the initial distributions.