Ultraviolet Mass-analyzed Threshold Ionization Spectroscopy Research Articles

Conformational structure of cationic tetrahydropyran by one-photon vacuum ultraviolet mass-analyzed threshold ionization spectroscopy.

Isolating and identifying the conformational forms of molecules are imperative processes to investigate the chemical reaction pathways of individual conformers. Herein, we explored the conformational structures of tetrahydropyran in the neutral (S0) and cationic (D0) states by varying the supersonic expansion conditions using one-photon vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy. The constructed 2D potential energy surfaces associated with conformational interconversion between the chair and boat forms in the S0 and D0 states revealed that the ionic transitions observed in the MATI spectra correspond to the most stable chair conformer. Accordingly, based on the 0-0 band in the VUV-MATI spectrum supported by the VUV photoionization efficiency curve, the adiabatic ionization energy for the conversion of the chair conformer to a cationic state was determined to be 74 687 ± 4 cm-1 (9.2600 ± 0.0005 eV). Definitive vibrational assignment of the measured MATI spectra using Franck-Condon fitting revealed the cationic structure of the twisted chair conformer. The geometrical change upon ionization promoted the vibrational modes associated with ring inversion and deformation motions in the cationic state. This behavior, which was attributed to the effect of electron removal from the highest occupied molecular orbital (HOMO) consisting of the nonbonding orbital of the oxygen atom, reveals the role of electrons in the HOMO.

Determination of precise pyrimidine cationic structure by vacuum ultraviolet mass-analyzed threshold ionization spectroscopy

The vibrational spectrum of a pyrimidine cation in the ground electronic state was obtained using vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy. Accurate ionization energy of pyrimidine was determined from the 0-0 band position in the VUV-MATI spectrum and was measured by varying the PFI field to the zero field limit, which is 75,258 ± 7 cm(-1) (9.3308 eV). The spectrum displayed a large number of vibrational peaks, which could be nearly completely assigned through Franck-Condon analysis performed with variations of geometrical parameters at the B3LYP/cc-pVTZ level. Based on the excellent agreement between experimental and calculated results, the definite geometry of the pyrimidine cation in the ground electronic state was determined to be a planar structure with C2v symmetry with a decreased N-N distance in the ring.

Vacuum ultraviolet mass-analyzed threshold ionization spectroscopy of methylcyclohexane in the supersonic jet

Vacuum ultraviolet (VUV) mass-analyzed threshold ionization (MATI) spectrum of supersonically cooled methylcyclohexane has been obtained to give the precise adiabatic ionization energy of 9.6958±0.0025eV for the chair equatorial conformer. Vibrationally resolved MATI spectrum has been analyzed with the aid of density functional theory and Franck–Condon calculations. The MATI spectrum reflects the structural change upon ionization and its origin is discussed by inspecting the shapes of the valence orbitals involved in the ionization process. The spectroscopic implication of the structural interconversion above the certain energy level is discussed with theoretical calculations of molecular structures and energetics.

Vacuum ultraviolet mass-analyzed threshold ionization spectroscopy of vinyl bromide: Franck–Condon analysis and vibrational assignment

Vibrational spectrum of vinyl bromide cation in the ground electronic state was obtained by one-photon mass-analyzed threshold ionization (MATI) spectroscopy using coherent vacuum ultraviolet radiation generated by four-wave difference frequency mixing in Kr. From MATI spectrum, ionization energy to the ground state of the cation was determined to be 9.8171±0.0006 eV (79 180±5 cm−1). Almost complete vibrational assignments for the peaks in the MATI spectrum were possible by utilizing vibrational frequencies and Franck–Condon factors calculated at the Becke three parameter Lee–Yang–Parr (B3LYP)/6-311++G(df,pd) level. Franck–Condon analysis for one-photon MATI spectra is especially useful because calculations of only the ground electronic states are involved while that for two-photon MATI spectra requires excited state calculations.

Vacuum ultraviolet mass-analyzed threshold ionization spectroscopy of benzene: Vibrational analysis of C6H6+ and C6D6+ in the X̃ 2E1g state

Vibrational spectra of C6H6+ and C6D6+ in the ground electronic state have been measured by one-photon mass-analyzed threshold ionization (MATI) spectroscopy using coherent vacuum ultraviolet radiation generated by four wave mixing in Kr gas. The ionization energies of C6H6 and C6D6 determined by one-photon MATI, 74551±5 and 74579±5 cm−1, respectively, are similar to those reported previously. Vibrational spectra are much simpler than the previous zero kinetic energy photoelectron and MATI spectra obtained by two-photon excitation. Almost complete vibrational assignments for the cations have been possible, which will be useful for future theoretical studies of the Jahn-Teller effect in these cations. Implication from the present one-photon spectra agrees with the previous suggestion that the geometry of benzene cation in the ground electronic state belongs to the D6h symmetry.