Initial velocity of secondary ions from XY-TOF technique, simultaneous calibration by residual gas ionization

Abstract

A crucial parameter to distinguish the prompt secondary ion emission from surfaces after particle impact from the delayed one is T 0( m/ q), the time-of-flight (TOF) of ions with a given mass m and charge q “emitted” with zero velocity. This quantity is also an important reference for the measurement of prompt ion emission velocity distribution. Presented is a novel and accurate method to determine T 0( m/ q), based on position sensitive XY-TOF analysis of residual gas ionization along the projectile trajectory, which is a low-pressure version of the traditional cloud chamber technique. Measurements using a mixture of He, Ne and Ar gases at low pressure (10 −5 mbar) were performed to illustrate this new T 0-gas target calibration method. Secondary ion emission of H n +, C n H n + and Li + ions from C, Al and LiF targets, bombarded by MeV Ar 0 and N 0 projectiles, is analyzed. It is found that, in contrast to Li +, hydrogen and hydrocarbon ions are always promptly emitted. The initial velocity distribution of H 2 + is determined and discussed.