INTERNAL ENERGY-DISTRIBUTIONS OF TUNGSTEN HEXACARBONYL IONS AFTER NEUTRALIZATION-REIONIZATION

The internal energy distributions P(epsilon) transferred to W(CO)(6)(.) during the kiloelectronvolt collisions that occur upon neutralizati on-reionization (NR) have been estimated based on the relative abundan ces of the W(CO)(0-6)(+.) products present in NR spectra (thermochemic al method). The average internal energy of the incipient {W(CO)(6)(+.) } ions arising after near thermoneutral neutralization with trimethyl amine followed by reionization with O-2 is similar to 9 eV for 8-keV p recursor ions and is mainly deposited during reionization. For compari son, the mean internal energy of {W(CO)(6)(+.)} after electron ioniza tion (EI) or collisionally activated dissociation (CAD) is similar to 6 eV. Making the neutralization step endothermic slightly increases th e overall excitation gained; however, a large increase in endothermici ty (> 16 eV) causes only a modest rise of the average internal energy (< 2 eV). The P(epsilon) curve for NR increases exponentially up to si milar to 6 eV, and levels off at higher energies, showing that the pro bability of imparting large internal energies (6-17 eV) is hight. In s harp contrast, the most probable excitation on CAD is less than or equ al to 6 eV, and the probability of deposition of larger energies decli nes exponentially. The mean internal energies after CAD and NR decreas e steadily when the kinetic energy is lowered. The structure (minima-m axima) observed in the P(epsilon) distribution for EI, which most like ly originates from Franck-Condon factors, is not reproduced in the dis tributions for NR or high energy CAD, despite the fact that all three methods involve electronic excitation. Because of the large internal e nergies transferred upon NR, NR mass spectrometry could be particularl y useful in the differentiation of ionic isomers with high dissociatio n but low isomerization thresholds.