High-resolution pulsed field ionization photoelectron-photoion coincidence spectroscopy using synchrotron radiation

We have developed a sensitive and generally applicable scheme for performing pulsed field ionization (PFI) photoelectron (PFI-PE)-photoion coincidence (PFI-PEPICO) spectroscopy using two-bunch and multibunch synchrotron radiation at the Advanced Light Source. We show that this technique provides an ion internal state (or energy) selection limited only by the PFI-PE measurement. Employing a shaped pulse for PFI and ion extraction, a resolution of 0.6 meV [full width at half maximum (FWHM)] is observed in the PFI-PEPICO bands for Ar+(P-2(3/2,1/2)). As demonstrated in the PFI-PEPICO study of the process, O-2+h nu --> O-2(+)(b (4)Sigma(g)(-), v(+)=4, N+)+e(-)--> O+(S-4)+O(P-3)+e(-), the dissociation of O-2(+)(b (4)Sigma(g)(-), v(+)=4) in specific rotational N+ levels can be examined. The simulation of the experimental breakdown diagram for this reaction supports the conclusion that the threshold for the formation of O+(S-4)+O(P-3) from O-2(+)(b (4)Sigma(g)(-), v(+)=4) lies at N+=9. We have also recorded the PFI-PEPICO time-of-flight (TOF) spectra of O+ formed in the dissociation of O-2(+)(b (4)Sigma(g)(-), v(+)=4-7). The simulation of these O+ TOF spectra indicates that the PFI-PEPICO method is applicable for the determination of kinetic energy releases. Previous PFI-PE studies on O-2 suggest that a high-n O-2 Rydberg state [O-2*(n)] with a dissociative ion core undergoes prompt dissociation to yield a high-n' O-atom Rydberg state [O*(n')] [Evans , J. Chem. Phys. 110, 315 (1999)]. The subsequent PFI of O-*(n(')) accounts for the formation of a PFI-PE and O+. Since the PFI-PE intensities for O+ and O-2(+) depend on the lifetimes of O-*(n(')) and O-2(*)(n), respectively, the PFI-PE intensity enhancement observed for rotational transitions to O-2(+)(b (4)Sigma(g)(-), v(+)=4, N(+)greater than or equal to 9) can be attributed to the longer lifetimes for O-*(n(')) than those for O-2(*)(n). The PFI-PEPICO study of the dissociation of CH3+ from CH4 also reveals the lifetime effects and dc field effects on the observed intensities for CH3+ and CH4+. The high resolution for PFI-PEPICO measurements, along with the ability to distinguish the CH3+ fragments due to the supersonically cooled CH4 beam from those formed by the thermal CH4 sample, has allowed the determination of a highly accurate dissociation threshold for CH3+ from CH4. (C) 1999 American Institute of Physics. [S0034-6748(99)02210-8].