Competitive two-channel photodissociation of n-butylbenzene ions in the Fourier-transform ion cyclotron resonance mass spectrometer

Abstract

The photochemistry of n-butylbenzene ions was studied in the ion cyclotron resonance ion trap using Fourier-transform and double-resonance techniques. The branching ratio for formation of the m/e 92 and 91 product ions was determined at several wavelengths. The effect of thermal excitation of the ions was analyzed, and values were derived for the branching ratio as a function of ion internal energy. The results are in agreement with dissociative charge-transfer ionization experiments, after similar correction for thermal excitation, but previous ion-beam photodissociation results are badly divergent as a result of large excess internal energy of the parent ions. RRKM calculations of the branching ratio suggest that the dissociation behavior is consistent with a quasiequilibrium-theory description, and that the activation energy for m/e 91 formation is 0.6 eV higher than that for m/e 92 formation. The actual values of these activation energies are less well determined, but the results are consistent with a value of 1.1 eV for m/e 92 formation, in agreement with existing thermochemical information, assuming a methylenecyclohexadiene product ion structure.