Schwingungsanregung von H2-Molekülen durch Zentralstoß mit Li+-Ionen

Abstract

An apparatus is described for measuring the inelastic differential cross section for vibrational excitation in collisions of diatomic molecules with monoenergetic ions at laboratory energies between 10 and 50 eV. The method consists of measuring the time of flight of single ions with a time amplitude converter and displaying the results on a 100 channel pulse height analyzer. From the shift in the time of flight relative to that expected for elastic scattering the final state of the molecule excited in a single collision is identified. By studying only central collisions with almost zero impact parameter rotational excitation is strongly suppressed. Measured times of flight after collisions of monoenergetic Li+ ions with H2 show that with increasing energy the most probable vibrational quantum jump increases from 0→1 to 0→2,0→3 etc. Contrary to the usual assumption of a small steric factor for vibrational excitation the results show that the inelastic cross section is larger than the elastic cross section. Using reported potential parameters the energy dependence of the most probable excited state is compared with the calculations of Secrest and Johnson for a one-dimensional collinear collision. The satisfactory agreement suggests that the steric factor is close to 1. From measurements at different scattering angles at 10 eV the integral inelastic cross section is found to be about 0.2 A2 corresponding to a differential cross section of 0.4 A2/sr. Measured values of integral and differential total cross sections for Li+-He andLi+-H2 are reported and compared with theory. Direct dissociation of D2 by Li+ in the energy range from 25 to 55 eV was not observed, yielding an upper limit for the cross section of 4 · 10−4 A2/sr.