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.

Threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy of three unsaturated (C 2 F 4 , C 3 F 6 , 2-C 4 F 8 ) and one cyclic (c-C 4 F 8 ) perfluorocarbons has been performed using vacuum-ultraviolet radiation from a synchrotron source in the energy range 10-27 eV. Electrons and ions are detected by threshold electron analysis and time-of-flight mass spectrometry, respectively, allowing breakdown diagrams that show the formation probability of fragment/parent ions as a function of the internal energy of the parent ion to be obtained. Fixed-energy TPEPICO spectra with improved time resolution were performed on some of the fragment ions observed, from which the mean kinetic energy released in fragmentation was determined. Highresolution (ca. 0.01 eV at 10 eV) threshold photoelectron spectra of the three unsaturated species have been recorded, and assignments have been made of the type of electron removed by photoionization to the observed states of the parent ion. Unlike the saturated perfluorocarbons studied in the previous paper, the ground states of C 2 F 4 + , C 3 F 6 + , and 2-C 4 F 8 + are bound in the Franck-Condon region. The first photoelectron band of C 2 F 4 and C 3 F 6 shows vibrational structure associated with the V 2 CdC stretching mode. This suggests that upon ionization the CdC bond weakens. From the fragment ions observed by decay of excited states of these three parent ions, there is some correlation between the ions that are observed and the nature of the orbitals from which an electron has been removed. This observation indicates that decay from these states takes place impulsively, and many of these excited states are probably repulsive in the Franck-Condon region. From the c-C 4 F 8 study we have determined a new onset of ionization for c-C 4 F 8 (11.6 ( 0.2 eV) and a new upper limit for the heat of formation at 298 K of C 3 F 5 + (84 ( 20 kJ mol -1 ).

We have developed an efficient electron time-of-flight ͑TOF͒ selection scheme for high resolution pulsed field ionization ͑PFI͒ photoelectron ͑PFI-PE͒ measurements using monochromatized multibunch undulator synchrotron radiation at the Advanced Light Source. By employing a simple electron TOF spectrometer, we show that PFI-PEs produced by the PFI in the dark gap of a synchrotron ring period can be cleanly separated from prompt background photoelectrons. A near complete suppression of prompt electrons was achieved in PFI-PE measurements by gating the PFI-PE TOF peak, as indicated by monitoring background electron counts at the Ar(11sЈ) autoionizing Rydberg peak, which is adjacent to the Ar ϩ ( 2 P 3/2 ) PFI-PE band. The rotational-resolved PFI-PE band for H 2 ϩ (X 2 ⌺ g ϩ ,v ϩ ϭ0) measured using this electron TOF selection scheme is nearly free from residues of nearby autoionizing features, which were observed in the previous measurement by employing an electron spectrometer equipped with a hemispherical energy analyzer. This comparison indicates that the TOF PFI-PE scheme is significantly more effective in suppressing the hot-electron background. In addition to attaining a high PFI-PE transmission, a major advantage of the electron TOF scheme is that it allows the use of a smaller pulsed electric field and thus results in a higher instrumental PFI-PE resolution. We have demonstrated instrumental resolutions of 1.0 cm Ϫ1 full width at half maximum ͑FWHM͒ and 1.9 cm Ϫ1 FWHM in the PFI-PE bands for Xe ϩ ( 2 P 3/2 ) and Ar ϩ ( 2 P 3/2 ) at 12.123 and 15.760 eV, respectively. These resolutions are more than a factor 2 better than those achieved in previous synchrotron based PFI-PE studies.

Using vacuum-ultraviolet radiation from a synchrotron source, threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy has been used to study the decay dynamics of the valence electronic states of three saturated perfluorocarbon cations, C 2 F 6 + , C 3 F 8 + , and n-C 4 F 10 + , in the energy range 12-25 eV. Electrons and ions are detected by threshold electron analysis and time-of-flight mass spectrometry, respectively, allowing breakdown diagrams showing the formation probability of fragment ions as a function of the internal energy of the parent ion to be obtained. The threshold photoelectron spectra of C 3 F 8 and n-C 4 F 10 are reported for the first time in the literature. Higher resolution, fixed-energy TPEPICO spectra were also performed on some of the fragment ions, and the translational kinetic energy released in fragmentation was determined. By analysis of the breakdown diagrams of the three ions, nonstatistical effects were observed for states below 18 eV, indicating that decay takes place rapidly from these states before internal energy conversion can occur. This study indicates that impulsive decay can occur even for molecules with up to 14 atoms, implying that statistical decay cannot necessarily be expected even for large molecular species. Analysis of the mean kinetic energy releases also supports the suggestion that impulsive behavior is taking place for the fragmentation of C 2 F 6 + into C 2 F 5 + + F. For states above 18 eV, it is not obvious from this study whether decay is statistical or not. From the C 3 F 8 study, new upper limits for the adiabatic ionization energy of the CF 3 radical (8.8 ( 0.1 eV) and the heat of formation of C 3 F 7 + at 298 K (-360 ( 20 kJ mol -1 ) have been determined.

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.