Die Bestimmung des effektiven lonisierungsquerschnitts in kondensierten Molekularstrahlen

Abstract

Clusters of condensed CO2-molecules that form a “condensed molecular beam” are being ionized by electron impact in the energy range 40 eV < Eel < 580 eV. The average size of clusters is controlled by the pressure p0 of CO2-gas in the nozzle of the beam source. By calibrating the apparatus with uncondensed N2-beams the ion currents can be converted into an effective cross section σeff for ionization per molecule contained within the clusters relative to the corresponding cross section σ0 of uncondensed CO2. In the range covered by the experimental parameters it is found that always σeff σ0. With increasing size of clusters σeff tends to decrease and its maximum as function of electron energy moves to higher energies. At the highest value of p0 σeff=0.31 σ0 for Eel=150 eV, and the maximum of σeff is situated at 290 eV compared to 95 eV for uncondensed CO2.