State-to-state vibrational energy transfer in OH A2Sigma+ with N2.

Vibrational energy transfer from v' = 0 to v' = 1 in the excited OH A2Sigma+ electronic state is investigated in the collisional region of a free-jet expansion. Laser excitation is used to prepare high rotational levels in OH A2Sigma+ (v' = 0, N' = 14-22), which lie above the energetic threshold for OH A2Sigma+ (v' = 1). Subsequent collisions with N2 result in population of a distribution of OH A2Sigma+ (v' = 1) product rotational levels that is characterized through dispersed fluorescence spectra. The majority of products are found in the most near-resonant rotational level of v' = 1, with population falling off exponentially in lower rotational levels. Additionally, the efficiency of vibrational energy transfer is determined by comparing the emission from v' = 1 products with that from the initially prepared v' = 0 level. The fractional transfer decreases by an order of magnitude from the highest to lowest initial rotational levels investigated. This decrease is correlated with an increasingly large change in rotational angular momentum between the initial and final states. The results show that angular momentum constraints are the dominant factor in the efficiency of OH A2Sigma+ v' = 0 to v' = 1 vibrational energy transfer at low collision energies.