ADIABATIC IONIZATION-POTENTIAL OF BENZYL RADICAL BY 2-COLOR RESONANT 2-PHOTON IONIZATION

The two-color resonant two-photon ionization (R2PI) technique is applied to the benzyl radical produced by photolysis of toluene in a pulsed Ar expansion and internally cooled by subsequent collisions. Benzyl cation yield curves vs omega-2 for fixed omega-1 show sharp thresholds whose total energy depends on the ion extraction field from 15 to 174 V/cm. Extrapolation to zero field yields the adiabatic ionization potential IP(benzyl) = 7.2477 +/- 0.0017 eV. We observe a slight but significant dependence of the extrapolated zero-field threshold on the R2PI scheme employed, i.e., on the electronic character of the benzyl excited state through which ionization proceeds. Combination of the new IP with a recent measurement of the enthalpy of formation of neutral benzyl radical yields the cation enthalpy of formation at 0 K: DELTA-H(f)-degrees-0(benzyl+) = 928 +/- 4 kJ/mol. The closed-shell benzyl cation binds a hydrogen atom much more weakly than the open-shell, neutral benzyl radical: D0(0)(C7H7+-H) = 220 +/- 4 kJ/mol vs D0(0)(C7H7-H) = 372 +/- 4 kJ/mol.