Influence of laser parameters on the detection efficiency of sputtered neutrals mass spectrometry based on non-resonant multiphoton ionization

Abstract

Non-resonant multiphoton ionization experiments on sputtered neutral particles have been performed with nanosecond and picosecond laser pulses. The ionization yields for particles removed from copper and silicon samples have been measured with respect to the laser intensity, the focused laser beam profile and the laser pulse duration. An experimental set-up is described for measuring the focused beam profile of the high power laser and to correlate it directly with the ionization yield. The results suggest that there are two competing channels of ionization: namely, non-resonant ionization of neutral atoms and dissociative ionization of sputtered molecules. This is further evidenced by experiments with copper atoms evaporated from a heated wire. If the laser beam profile and the ion extraction optics of the mass spectrometer are not optimized, post-ionization originates dominantly from the dissociative ionization process which makes calibration difficult. Calculations show that the nature of the detection process depends on laser intensity. At low laser intensities the system works like a particle density detector whereas at intensities beyond the saturation intensity the system can become a particle flux detector. Within this approximation we give the optimum pulse durations and decay times for the ion and laser pulses to make the post-ionization yield quasi-independent of the sputtered particle velocities.