Rotational energy transfer in OH (A 2Σ+, v′=0): A method for the direct determination of state-to-state transfer coefficients

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Abstract
We have determined state‐to‐state rate coefficients for rotational and fine structure transitions of OH (A 2Σ+, v’=0) in thermal collisions with He and Ar at 300 K. The temporal evolution of single fluorescence lines within the A–X, 0–0 band of OH were measured, exciting either the F2(4) or F2(5) state by a nanosecond laser pulse. The OH radical was produced in a discharge flow cell, containing predominantly He or Ar, at various pressures between 1 and 6 mbar. The time resolution in the experimental setup was sufficient to evaluate the rotational energy transfer coefficients directly from the time dependence of two fluorescence lines. The observed average rate coefficients for collisions of OH (A, v’=0) with Ar are approximately 3 times larger than those with He. The two rare gases show different qualitative behavior. Whereas the almost isoenergetic transitions with ΔJ=1 and ΔN=0 are favored in collisions with Ar, those with ΔJ=ΔN=−2 are favored in collisions with He. In addition, a strong preference for transitions conserving the parity of the OH, a propensity rule, previously reported for rotational relaxation in the A state of OH, was found for collisions with He but not for collisions with Ar. Our experimental results for He and for Ar are in good agreement with recent quantum mechanical calculations of the energy transfer coefficients.