PUMP-PROBE MEASUREMENTS OF STATE-TO-STATE ROTATIONAL ENERGY-TRANSFER RATES IN N-2 (UPSILON = 1)

We report direct measurements of the state-to-state rotational energy transfer rates for N2 (v=1) at 298 K. Stimulated Raman pumping of Q-branch (v=1 ←0) transitions is used to prepare a selected rotational state of N2 in the v=1 state. After allowing an appropriate time interval for collisions to occur, 2 + 2 resonance-enhanced multiphoton ionization is used (through the a 1Π8←X 1Σ8+ transition) to detect the relative population of the pumped level and other levels to which rotational energy transfer has occurred. We have performed a series of measurements in which a single even rotational level (Ji = 0-14) is excited and the time-dependent level populations are recorded at three or more delay times. This data set is then globally fit to determine the best set of state-to-state rate constants. The fitting procedure does not place any constraints (such as an exponential gap law) on the J or energy dependence of the rates. We compare our measurements and best-fit rates with results predicted from phenomenological rate models and from a semiclassical scattering calculation of Koszykowski et al. [J. Phys. Chem. 91, 41 ( 1987) ]. Excellent agreement is obtained with two of the models and with the scattering calculation. We also test the validity of the energy-corrected sudden (ECS) scaling theory for N2 by using our experimental transfer rates as basis rates (J = L→0), finding that the ECS scaling expressions accurately predict the remaining rates. © 1990 American Institute of Physics.