The wavelength 205.14 nm is absorbed by many hydrogen-containing molecules, which then dissociate to form hydrogen atoms. These in turn can absorb two more 205.14 nm photons and reach the 3s or the 3d state. They can absorb a third photon and form hydrogen ions or decay to the ground state by stepwise fluorescence, first emitting the Balmer α line at 656.2 nm (3s or 3d→2p)and then the Lyman α line (2p→1s)at 121.6 nm. Thus the hydrogen atom kinetic energy can be probed in three different ways. This method broadens greatly the possibilities of investigating photodissociations leading to hydrogen atom products. It has the advantage of simplicity and the disadvantages of a single-color experiment. The method is tested with a molecule that has been extensively investigated, H2S, and then applied to three other molecules, formic acid (HCO2H), vinyl radical (C2H3), and allyl radical (C3H5). H2S has a perpendicular transition with a large release of kinetic energy. Studies of the latter molecules lead to the conclusion that formyl, carboxyl, vinyl, and allyl radicals absorb 205.1 nm light and release hydrogen atoms with a large fraction of the available energy. The dissociation pathways of formic acid are clarified.

Photodissociation of iodine monochloride has been investigated at 235–248 nm by studying Doppler profiles of (2+1) resonance enhanced multiphoton ionization spectra of fragment chlorine atoms and two-photon laser induced fluorescence spectra of iodine atoms. At 235.3–237.8 nm, a measured branching ratio of [Cl*(2P1/2)]/[Cl(2P3/2)] is 0.68±0.10. At 248 nm, the [I*(2P1/2)]/[I(2P3/2)] ratio is 0.71±0.27. From the measured Doppler profiles and the ab initio calculation, the photoprepared states leading to Cl and Cl* production are assigned to the 3Π0+ (and possibly 3∑−0+) and the 1Π1 states, respectively. Nonadiabatic couplings between the potential curves have been discussed.

Vibrational spectra of p-, m-, and o-difluorobenzene cations in the ground electronic states have been measured by vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy. From the MATI spectra, ionization energies of p-, m-, and o-difluorobenzenes to the ionic ground states have been determined to be 73 861±5, 75 332±5, and 75 003±5 cm−1, respectively. A number of vibrational modes, including fundamentals, their combinations, and overtones were distinctly observed in the MATI spectra and could be assigned successfully by comparing with the results from density functional theory calculation at the B3LYP/6-311++G (2df,2pd) level. Appearance of the overtone progression of some totally symmetric vibrations could be explained also in terms of the overlap between the geometrical changes upon ionization and normal mode eigenvectors. Excellent correlation between the experimental and calculated results suggests that p-, m-, and o-difluorobenzene cations in the ground states retain D2h, C2v, and C2v symmetries, respectively, of the corresponding neutrals.

The phenomenon of interference of the amplitude for absorption of one photon of frequency 3ω and the amplitude for absorption of three photons of frequency ω was theoretically predicted by Shapiro, Hepburn, and Brumer. The interference was demonstrated experimentally by varying the relative phase between the tripled frequency photon and three photons with the fundamental frequency by the groups of Elliott and Gordon in atoms and small molecules. In order to see how general this phenomenon is, five compounds were studied, ammonia, trimethylamine, triethylamine, cyclooctatetraene, and 1,1-dimethylhydrazine. CH3I was used as the tripling gas for light in the range 604–600 nm. Interference was observed in all cases. The last four compounds have low ionization potentials and interference was observed between a 3+1 and a 1+1 ionization process with a maximum modulation of 22%. NH3 with a higher ionization potential requires absorption of 3+2 or 1+2 photons and exhibits a maximum modulation of 33%. We conclude that molecular size is no obstacle and that as long as a molecule has sufficiently strong absorption at the tripled frequency, and sufficient vapor pressure, and the laser fundamental beam is very strong, phase control of interference is observable.

Vibrational spectra in the ground electronic states, X̃ 2B1, of fluoro-, chloro-, bromo-, and iodobenzene cations and in the excited electronic states, B̃ 2B2, of chloro- and bromobenzene cations have been measured by mass-analyzed threshold ionization (MATI) via one-photon excitation with vacuum ultraviolet light. The spectra consist mostly of strong 0–0 bands and fundamentals, overtones, and combinations of totally symmetric a1 modes. In particular, the observed vibrational bands in the B̃ states have been assigned by referring to the assignments for the ionic ground states and those for the neutral molecules. From the MATI spectra, the ionization energies of fluoro-, chloro-, bromo-, and iodobenzene to the ionic ground states have been determined to be 74 229±5, 73 177±5, 72 570±5, and 70 638±5 cm−1, respectively. The ionization energies to the B̃ states have also been determined to be 91 404±5 and 85 822±5 cm−1 for chloro- and bromobenzene, respectively.