Pump–probe measurements of state-to-state rotational energy transfer rates in N2 (v=1)

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Abstract
We report direct measurements of the state‐to‐state rotational energy transfer rates for N2 (υ=1) at 298 K. Stimulated Raman pumping of Q‐branch (υ=1←0) transitions is used to prepare a selected rotational state of N2 in the υ=1 state. After allowing an appropriate time interval for collisions to occur, 2+2 resonance‐enhanced multiphoton ionization is used (through the a 1Πg←X 1Σ+g 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. 9 1, 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.