State specific relaxation of neutral molecules at low temperatures

Abstract

This dissertation presents the results of several investigations into vibrational and rotational relaxation of neutral molecules at low temperatures. The use of the free jet flow reactor technique for production of very low local temperatures and the determination of relaxation data and derivation of rate coefficients are discussed. Four different molecules, HBr, CO, OH and OD, are examined in the free jet with various buffer gases to determine their vibrational and rotational relaxation properties. For HBr and CO, terminal rotational temperatures are measured in various buffer gases with the resonance enhanced multiphoton ionization (REMPI) technique, and results are analyzed and compared with translational temperatures. For OH and OD, discussion includes experimental production of radicals and relaxation within the X²Π and A²Σ states is made. With the use of laser induced fluorescence (LIF) and a novel method for production of radicals in the free jet, radical relaxation rates are determined for the first time at extremely low collision energies. We have measured the low temperature (T(trans) near 1 K) rate coefficients for: OH(A²Σ,vᵢ,Nᵢ)+Ar →(k) OH(A²Σv(f)N(f)) +Ar where v and N refer to the quantum numbers for vibration and pure rotational angular momentum, while the subscripts i and f refer to the initial and final states, respectively. The experiments reported in this dissertation help lead to a better understanding of collisions on a molecular level at very low collision energies.