Negative ion formation mechanism and velocity distribution in laser desorption/ionization of C60

Abstract

In UV laser desorption/ionization (LDI) of C60, both molecular cations and anions are formed. While the C 60 + formation has been discussed in a number of studies and is fairly well understood, the C 60 − formation has received less attention. Here, we present measurements of C 60 − and C 60 + formation and its fluence dependence in UV-LDI of C60 from both metallic and non-metallic substrates. The data show clearly that the C 60 − formation is coupled to the C 60 + formation, indicating that C 60 − is predominantly formed through electron transfer early in the expanding laser desorption plume. The velocity distribution of desorbed C 60 − ions was also determined and was found to be best described by two Maxwell-Boltzmann distributions with superimposed stream velocities.