RATE COEFFICIENTS FOR STATE-TO-STATE ROVIBRONIC RELAXATION IN COLLISIONS BETWEEN NO(X(2)II, NU=2,OMEGA,J) AND NO, HE, AND AR AT 295, 200, AND 80 K

The state-to-state rates of collisional energy transfer within and bet ween the rotational level manifolds associated with the Omega=1/2 and Omega=3/2 spin-orbit states of NO(X(2) Pi, nu=2) have been measured us ing an infrared-ultraviolet double resonance (IRUVDR) technique. NO mo lecules were initially prepared in a specific rovibronic level, for ex ample, nu=2, Omega=1/2, J=6.5, by tuning the output from an optical pa rametric oscillator (OPO) to a suitable line in the (2,0) overtone ban d. Laser-induced fluorescence (LIF) spectra of the A (2) Sigma(+)-X (2 ) Pi (2,2) band were then recorded at delay times corresponding to a s mall fraction of the average time between collisions in the gas sample . From such spectra, the relative concentrations of molecules in level s populated by single collisions from the initially prepared state cou ld be estimated, as could the values of the rate coefficients for the state-to-state processes of collisional energy transfer. Measurements have been made with NO, He, and Ar as the collision partner, and at th ree temperatures: 295, 200, and 80 K. For all collision partners, the state-to-state rate coefficients decrease with increasing Delta J (i.e ., change in the rotational quantum number and rotational angular mome ntum) and increasing Delta E(rot) (i.e., change in the rotational ener gy). In NO-NO collisions, there is little propensity for retention of the spin-orbit state of the excited molecule. On the other hand, with He or Ar as the collision partner, transfers within the same spin-orbi t state are quite strongly preferred. For transfers between spin-orbit states induced by all collision partners, a propensity to retain the same rotational state was observed, despite the large change in intern al energy due to the spin-orbit splitting of 121 cm(-1). The results a re compared with previous experimental data on rotational energy trans fer, for both NO and other molecules, acid with the results of theoret ical studies. Our results are also discussed in the light of the conti nuing debate about whether retention of angular momentum or of interna l energy is the dominant influence in determining the rates of state-t o-state rotational energy transfer. (C) 1995 American Institute of Phy sics.