OPTIMIZATION OF A COMMERCIAL QUADRUPOLE MASS-SPECTROMETER FOR TIME-OF-FLIGHT MEASUREMENTS OF LASER-DESORPTION

Quadrupole mass analyzers (QMAs) are receiving increasing attention for applications in time-of-flight (TOF) studies of laser-induced desorption, ablation and surface reactions in order to extend the information attainable from these experiments. With such a complicated detection device, however, it is easily possible to inadvertantly manipulate the real time-of-flight distribution, for example by missing fast or slow particles. In this work a systematic investigation of the influence of the different QMA parameters on the signal and their interplay was undertaken in laser-induced desorption experiments. The particle pulses were generated by resonant vibrational excitation of condensed benzene films with a TEA CO2 laser. Our investigations give the optimized configuration and setting of potentials for the instrument studied, in terms of sensitivity and energy resolution, and an accurate procedure to determine the drift time of ions in the quadrupole. This makes it possible to detect undistorted time-of-flight distributions with a time resolution of about 5 mus for a range of incident kinetic energies of at least 0-3 eV.