Time of flight measurements of the vibrational excitation of CO2 and N2O by collision with 3–8 eV Li+ ions

Abstract

The relative differential cross sections for the excitation of the first few vibrationally excited states of CO2 and N2O by collision with Li+ ions have been measured by the time of flight technique in a crossed beam scattering experiment. The individual vibrational transitions between the ground state and the first excited bending mode and stretching mode have been resolved for various center of mass energies between 3.0 and 6.9 eV and center of mass angles in the range of 9° to 35°. These angles are larger than the rainbow angle, indicating that the repulsive part of the intermolecular potential was sampled. Both CO2 and N2O exhibit little rotational excitation. At nearly all angles and energies, CO2 shows more vibrational excitation than the equally massive N2O. The bending mode excitation is particularly strong in CO2. Evidence is presented to show that these differences and the lack of rotational excitation cannot be explained purely by differences in the masses of the constituent atoms but must be due to significant differences in the intermolecular potentials. The CO2 observations can be qualitatively explained by a simple potential model involving coulomb forces between the Li+ ion and charges on the atoms of the molecule which are chosen to give the known quadrupole moment. From the rainbow structure of the measured total differential cross sections, the effective depth of the intermolecular potential well was estimated to be 0.52 eV for both systems.