Abstract：The authors have investigated collision vibrational energy transfer rate constants in NaK[1 3Π(ν)] and He system. Pump laser excitation of the spin-forbidden band was used to produce very highly vibrationally excited metastable state NaK[1 3Π(ν＝22, 21, 20)]. The probe laser was used to excite the 1 3Π(ν＝22, 21, 20) to 5 3Π(ν′). Laser induced fluorescence (LIF) from 5 3Π→1 3Σ+ transition was used to follow the collision dynamics. The semilog plots of time-resolved LIF was obtained. The slopes yielded the effective lifetimes. From such data several Stern-Volmer plots could be constructed and the relaxation rate constants could be extracted for the sum of all processes that give rise to the decay of the prepared vibrational state. The rate constants (in units of 10-11 cm3·s-1) for ν being 22, 21 and 20 are 1.4±0.1, 1.2±0.1 and 1.0±0.1, respectively. The vibrational relaxation rate is increasing with vibrational quantum number. In order to determine the importance of multiquantum relaxation, it is necessary to measure the relative population of both the prepared state and collisionally populated states. By the kinetic equations governing up to Δν=2 transitions, the time dependence of populations of the vibrational states were obtained. With the help of the integrating the population equations over all time, the importance of the two-quantum relaxation could be studied experimentally. By varying the delay between the pump and the probe laser, the He pressure dependent vibrational state specific decay could be measured. The time evolutions and relative intensities of the three states ν＝22, 21 and 20 by preparing ν＝22 were obtained. Using experimental data the rate constants (in units of 10-11 cm3·s-1) for ν＝22→21 and ν＝22→20 are 0.67±0.15 and 0.49±0.12, respectively. The single quantum relaxation accounts for only about 48% of the total relaxation out of ν＝22. Multi-quantum relaxation (Δν>1) was found to be important at high vibrational states.
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