Metastable noble gas apparatus for laser photoionisation studies

Abstract

Metastable noble gas atoms are produced by crossing a low-energy electron sheet beam with a jet of gas injected by an automotive fuel injector. A laser beam is directed at right angles through the wide dimension of the electron beam, and the gas jet is directed at a small (19 degrees ) angle to the laser beam. An electric field is used to extract photoionised atoms from the interaction region which are then detected in a time-of-flight (TOF) mass spectrometer. The apparatus has two novel features. First the metastable atoms are created directly in the interaction region where they are photoionised instead of being created in a discharge cell and transported as a molecular beam to the interaction region. This simplification eliminates at least one level of differential pumping and creates a relatively high density of metastable atoms. Because the low-energy electron gun which creates the metastable atoms can also be operated at higher energies, it can be used to ionise any atom or molecule present. In this mode the apparatus is self-diagnostic for system alignment and can readily detect the presence of impurities in the target gas.