Gas-Phase Studies of Collisions of Small Molecular Free Radical Species

The spectroscopy of several small molecular free radicals and the collision dynamics of reactions of such species has been investigated in a series of laser fluorescence experiments carried out with molecular beams and in a cell. The spectroscopic studies include observation of a number of new NCO ~ A^2Sigma ^+ - ~ X^2Pi (0,v _2,0) - (v _1,v_2,v_3 ) vibronic bands and the measurement of A ^3Pi - rm X^3Sigma^ -(v^', v^{'' } = v^' + 1) line positions for ND radicals from which the vibrational intervals in both the ND A^3Pi and X ^3Sigma^- electronic states were derived. Lifetimes for excited ND A^3 Pi rm v^' = 1-3 and NH v^' = 2 in specific rotational/fine structure levels were determined by excitation on isolated rotational lines in the Delta v = +1 sequence. The predissociation rates for specific levels were derived and were found to be consistent with a mechanism involving spin-orbit coupling to the repulsive 1^5Sigma^- state. Dynamical investigations include the determination of the OD/HNO products internal state distributions from the O(^3P) + ND_2 /NH_2 reactions for which the observed OD and HNO internal state product distributions were found to be extremely cold. These results are interpreted in terms of what is known about the H_2 NO potential energy surfaces. The nascent OH product internal state distribution for the NH(X^3 Sigma^-) + NO to OH + N_2 reaction was determined. Only a small percentage of the available reaction energy was observed as the OH product internal excitation energy. The results from this study, along with previous investigations of the H + N_2O reaction, sheds more light on the dynamics of these reactions. Relative cross sections were measured for the formation of various rovibrational/fine structure states in the NH(X ^3Sigma^-) product formed by collisional electronic quenching of NH(a^1 Delta) by Xe and CO. The vibrational distribution of the quenched product was found to be approximately statistical. The rotational distribution was found to be cold, and the population in the three ^3Sigma^- fine-structure manifolds was found to be nearly statistical. Collisional transfer between the A^2Pi and the X^2Sigma^+ states of CN with helium as the collisional partner was investigated, and the relative populations in the collisionally prepared levels were measured. These results were compared to results from previous experimental investigation of energy transfer in CN induced by Ar and with theoretically calculated CN-He cross sections.