Direct measurements of collision-induced state-to-state rotational energy transfer rates in C 2H 2 (ν″ 2 = 1)

Abstract

We report the direct measurement of state-to-state rotational relaxation rates in the IR-inactive (ν″ 2 = 1) vibrational mode of acetylene at room temperature. Stimulated Raman pumping in the Q-branch of the (ν″ 2 = 1) ← (ν″ 2 = 0) transition is used to prepare a significant population in single rotational states of the upper vibrational level. The population in this level is probed by laser-induced fluorescence via the à 1A u(ν′ 3 = 1) ← X̃ 1∑ + g(ν″ 2 = 1) electronic transition. Rotational relaxation rates are derived from the collisional redistribution of the population in the range between 0.02 and 0.4 hard-sphere collisions. This low collision number enables us directly to determine the collision matrix for single and multiple quanta transitions, which account for approximately 40–50% of the population loss in the initially prepared state. For J″ = 9, 11, and 13, total depopulation rate constants of about (8.8±1.0) × 10 −10 cm 3 s −1 ( J″ = 11, 13) are observed.