The design and performance of a high resolution translational energy loss spectrometer

Abstract

An instrumental development is described that achieved high energy resolving power when studying collisions between fast ion beams, at keV energies, interacting with thermal molecular and atomic gaseous targets. This technique is referred to as translational energy spectroscopy (TES) and can reveal spectroscopic information on the states of the participating species, the dynamics of the collision, lifetimes of species formed, state-to-state collision cross-sections and populations of states. The instruments described are based on an unique symmetrical arrangement of cylindrical electrostatic analysers. These are placed in a double focusing ion-optical geometry leading to very small first and second order image aberrations, sufficient to obtain a primary energy resolving power of 105. Two such instruments were developed and their relative performances are compared experimentally and through computer modelling; the second, and larger, instrument has an improved sensitivity by an order of magnitude and a three times increase in resolving power. This has enabled collision spectroscopy to achieve performance similar to electron energy-loss spectroscopy and photoelectron spectroscopy, with a resolution on the unscattered ion beam measured to be 0.01 eV, at 3 keV collision energy, on energy loss processes of ≥ 0.03 eV. These developments allow TES to observe, (i) the vibrational/electronic spectroscopy of keV ion/molecule collisions; (ii) spin-selective spectra; (iii) the highly excited states close to and beyond the ionization limit of the target species; and (iv) involatile targets using a high temperature cell. The validity of optical quantum selection rules, relying on the electric-dipole interaction governing photon-induced transitions, has been tested for TES, which relies on the more general ion/neutral interaction potential responsible for collisional excitation. Spin conservation was found to be strongly adhered to, whilst orbital angular momentum and total angular momentum were not. The specific selection rules for homonuclear diatomics g<–>g and u<–>u were found to be allowed in TES, in contradiction to the case of optical spectroscopy, whilst there was some preliminary evidence that the optical Σ+–Σ− symmetry-forbidden transition was also forbidden for the TES reactions studied. For H2+ projectiles, excitation to triplet states is strongly favoured over excitation to singlet states. © 1997 John Wiley & Sons, Ltd.