Translational Energy Distributions for Dissociation of the van der Waals Cation Species (C6H6···Arn)+ (n = 1,2) Measured by Velocity Map Imaging

Abstract

The kinetic energy distributions associated with ejection of Ar from the cation van der Waals species (C6H6···Ar)+ and (C6H6···Ar2)+ have been measured by ion imaging with a velocity mapping configuration. The (C6H6···Ar2)+ dissociation was observed for the isomer with an Ar atom on each side of the benzene ring. The cations were created by (1+1) resonance-enhanced multiphoton ionization via their 610 transitions. The initial cation vibrational state population distributions were deduced from photoelectron spectra of benzene measured with the velocity map imaging spectrometer. The cations are produced with an average vibrational energy ∼1800−1900 cm-1. For dissociation of (C6H6···Ar2)+, on average enough energy remains in the (C6H6···Ar)+ fragment to eject the remaining Ar. The experiment views fragmentation of the subset of the (C6H6···Ar2)+ cations that lose a single Ar and for this reason the initial internal ion energy is significantly lower than 1800 cm-1 for the (C6H6···Ar2)+ dissociations monitored. The average initial energies above dissociation are estimated to be ∼1310 and ∼350 cm-1 for (C6H6···Ar)+ and (C6H6···Ar2)+, respectively. The kinetic energy distributions are well fitted by the function P(E) = E1/2{c1 exp(−k1E) + c2 exp(−k2E)}. The average kinetic energies released were 92 ± 4 and 78 ± 5 cm-1 for (C6H6···Ar)+ and (C6H6···Ar2)+, respectively. The lower average kinetic energy released for (C6H6···Ar2)+ is attributed to its lower initial internal energy. For (C6H6···Ar)+ the kinetic energy released represents only a small fraction of the total energy that requires redistributing. A large proportion of the total energy is therefore taken up as rotational and vibrational energy of the benzene cation fragment.