State Selected Charge Transfer and Chemical Reactions by the Tesico Technique

The study of state-selected ion-molecule reactions using the vacuum ultraviolet pulsed field ionization-photoion technique

This paper presents the methodology to generate beams of ions in single quantum states for bimolecular ion-molecule reaction dynamics studies using pulsed field ionization (PFI) of atoms or molecules in high-n Rydberg states produced by vacuum ultraviolet (VUV) synchrotron or laser photoexcitation. Employing the pseudocontinuum high-resolution VUV synchrotron radiation at the Advanced Light Source as the photoionization source, PFI photoions (PFI-PIs) in selected rovibrational states have been generated for ion-molecule reaction studies using a fast-ion gate to pass the PFI-PIs at a fixed delay with respect to the detection of the PFI photoelectrons (PFI-PEs). The fast ion gate provided by a novel interleaved comb wire gate lens is the key for achieving the optimal signal-to-noise ratio in state-selected ion-molecule collision studies using the VUV synchrotron based PFI-PE secondary ion coincidence (PFI-PESICO) method. The most recent development of the VUV laser PFI-PI scheme for state-selected ion-molecule collision studies is also described. Absolute integral cross sections for state-selected H2+ ions ranging from v+ = 0 to 17 in collisions with Ar, Ne, and He at controlled translational energies have been obtained by employing the VUV synchrotron based PFI-PESICO scheme. The comparison between PFI-PESICO cross sections for the H2+(HD+)+Ne and H2+(HD+)+He proton-transfer reactions and theoretical cross sections based on quasiclassical trajectory (QCT) calculations and three-dimensional quantum scattering calculations performed on the most recently available ab initio potential energy surfaces is highlighted. In both reaction systems, quantum scattering resonances enhance the integral cross sections significantly above QCT predictions at low translational and vibrational energies. At higher energies, the agreement between experiment and quasiclassical theory is very good. The profile and magnitude of the kinetic energy dependence of the absolute integral cross sections for the H2+(v+ = 0-2,N+ = 1)+He proton-transfer reaction unambiguously show that the inclusion of Coriolis coupling is important in quantum dynamics scattering calculations of ion-molecule collisions.

Vacuum ultraviolet spectroscopy and chemistry by photoionization and photoelectron methods

The recent developments of vacuum ultraviolet (VUV) laser and third generation synchrotron radiation sources, together with the introduction of pulsed field ionization (PFI) schemes for photoion-photoelectron detection, have had a profound impact on the field of VUV spectroscopy and chemistry. Owing to the mediation of near-resonant autoionizing states, rovibronic states of ions with negligible Franck-Condon factors for direct photoionization can be examined by VUV-PFI measurements with rotational resolutions. The VUV-PFI spectra thus obtained have provided definitive ionization energies (IEs) for many small molecules. The recent synchrotron-based PFI-photoelectron-photoion coincidence experiments have demonstrated that dissociative photoionization thresholds for a range of molecules can be determined to the same precision as in PFI-photoelectron measurements. Combining appropriate dissociation thresholds and IEs measured in PFI studies, thermochemical data for many neutrals and cations can be determined with unprecedented precision. The further development of two-color excitation-ionization schemes promises to expand the scope of spectroscopic and chemical applications using the photoionization-photoelectron method.