The formation of methyl cation (CH 3 ϩ ) from methane (CH 4 ) has been investigated in high resolution using the newly perfected pulsed field ionization photoelectron-photoion coincidence ͑PFI-PEPICO͒ scheme. The PFI-PEPICO data reveal that fragmentation of CH 4 in high-n Rydberg states occurs at energies above the dissociation threshold prior to pulsed field ionization. The crossover point of the breakdown curves is found to depend strongly on the Stark field in the ion source and thus traditional simulation procedures based on such a feature for ion dissociation energy determination are not appropriate in PFI-PEPICO studies. We show that for a prompt dissociation process, the disappearance energy of the parent molecule provides an accurate measure of the 0 K ion dissociation threshold, as that for CH 3 ϩ from CH 4 is 14.323Ϯ0.001 eV.

The pulsed-field ionization zero-electron kinetic-energy ͑PFI-ZEKE͒ threshold photoionization spectrum of NO 2 from 9.58 to 20 eV is obtained using vacuum ultraviolet synchrotron radiation by means of the Chemical Dynamics Beamline at the Lawrence Berkeley National Laboratory Advanced Light Source. The high resolution afforded by PFI threshold discrimination yields new or refined spectroscopic constants for a number of known excited states of the cation, including the first estimate of the A rotational constant in the a 3 B 2 state, as well as new fundamental frequencies for the A 1 A 2 and B 1 B 2 states, a precise determination of the singlet-triplet splitting in the c 3 B 1 -C 1 B 1 complex and the first observations of the states, d 3 A 1 and D 1 B 2 . Most significantly, ZEKE photoelectron detection resolves vibrational structure in the linear X 1 ⌺ g ϩ ground state of NO 2 ϩ . Vibrational positions in the first electron volt of the spectrum are found to conform with the predictions of a Hamiltonian that includes Fermi resonance and other anharmonic terms derived from earlier multiresonant laser spectroscopic experiments on the lower bending excited states.

: Using tunable vacuum-UV radiation from a synchrotron in the range 12-26 eV, we have measured the threshold photoelectron and threshold photoelectron -photoion coincidence spectrum of

The bimolecular reactions of the high recombination energy cations Ar + , F + and Ne + with four fully saturated (CF 4 , C 2 F 6 , C 3 F 8 and n-C 4 F 10 ) and three unsaturated (C 2 F 4 , C 3 F 6 and 2-C 4 F 8 ) Phys. Chem. 100 (1996) 17166), are compared with those determined from the threshold photoelectron-photoion coincidence spectra of the PFCs at the recombination energies of the reagent cations. This comparison provides information that helps to interpret the dynamics of charge-transfer, and whether it occurs via a long-range or a short-range mechanism. Energy resonance and goodFranck-Condon factors connecting the ground electronic state of a reactant neutral molecule to one of its ionic states, at the recombination of the reagent cation, are generally considered to be sufficient for long-range charge-transfer to occur. However, the results from this study imply that good Franck-Condon factors are not critical in determining the efficiency of a long-range chargetransfer. Instead, the results suggest that, in addition to the requirement for energy resonance, the electron taking part in the charge-transfer process must be removed from a molecular orbital which is unshielded from the approaching reagent cation. This enables the cation to exert an influence on the electron at large impact parameters.