Translational energy distributions for dissociation of the van der Waals cation species (C6H6 center dot center dot center dot Ar-n)(+) (n=1,2) measured by velocity map imaging

The kinetic energy distributions associated with ejection of Ar from the ca tion van der Waals species (C6H6. . . Ar)(+) and (C6H6. . . Ar-2)(+) have b een measured by ion imaging with a velocity mapping configuration, The (C6H 6. . . Ar-2)+ dissociation was observed for the isomer with an Ar atom on e ach side of the benzene ring. The cations were created by (1+1) resonance-e nhanced multiphoton ionization via their 6(0)(1) transitions. The initial c ation vibrational state population distributions were deduced from photoele ctron spectra of benzene measured with the velocity map imaging spectromete r, The cations are produced with an average vibrational energy similar to 1 800-1900 cm(-1). For dissociation of (C6H6. . . Ar-2)(+) on average enough energy remains in the (C6H6. . . Ar)(+) fragment to eject the remaining Ar. Tile experiment views fragmentation of the subset of the (C6H6. . . Ar-2)( +) cations that lose a single Ar and fur this reason the initial internal i on energy is significantly lower than 1800 cm(-1) for the (C6H6. . . Ar-2)( +) dissociations monitored. The average initial energies above dissociation are estimated to be similar to 1310 and similar to 350 cm(-1) for (C6C6. . . Ar)(+) and (C6H6. . . Ar-2)(+), respesctively. The kinetic energy distri butions are well fitted by the function P(E) = E-1/2{C-1 exp(-k(1)E) + c(2) exp(-k(2)E)}. The average kinetic energies released were 92 +/- 4 and 78 /- 5 cm(-1) for (C6H6. . . Ar)(+) and (C6H6. . . Ar-2)(+), respectively. Th e lower average kinetic energy released for (C6W6. . . Ar-2)(+) is attribut ed to its lower initial internal energy. For (C6H6. . . Ar)(+) the kinetic energy released represents only a small fraction of the total energy that r equires redistributing. A large proportion of the total energy is therefore taken up as rotational and vibrational energy of the benzene cation fragme nt.