Ion pair production from CH3Cl and CH3Br with 118 NM laser light

Characterization of a vacuum ultraviolet light source at 118 nm

Vacuum ultraviolet (VUV) light at 118 nm has been shown to be a powerful tool to ionize molecules for various gas-phase chemical studies. A convenient table top source of 118 nm light can be produced by frequency tripling 355 nm light from a Nd:YAG laser in xenon gas. This process has a low efficiency, typically producing only nJ/pulse of VUV light. Simple models of the tripling process predict that the power of 118 nm light produced should increase quadratically with increasing xenon pressure. However, experimental 118 nm production has been observed to reach a maximum and then decrease to zero with increasing xenon pressure. Here, we describe the basic theory and experimental setup for producing 118 nm light and a new proposed model for the mechanism limiting the production based on pressure broadened absorption.

Long term puzzles of the CH and CD energetics and related phenomena revisited; solutions sought through REMPI-photofragmentations of bromomethanes

Ever since the pioneering work by Herzberg and Johns in 1969 (The Astrophysical Journal, 1969, 158, 399) the spectral assignment and the energetics of the fundamental molecular fragment CH, in the region of 63 000-65 000 cm(-1) (7.81-8.06 eV), have remained a puzzle to a large extent. The dissociation of bromoform and deuterated bromoform following two-photon resonance excitations to molecular Rydberg states forms the fragment species CH* and CD* in the excited state A(2)Δ(v' =0) as well as carbon and bromine atoms in the ground and first excited states, C/C* and Br/Br*. Further (1r + 1i)REMPI of CH* and CD* resonance excites the fragments to the energy region of concern, whereas the atom fragments were identified by further (2r + 1i)REMPI. Analysis based on spectral simulations, isotope shifts and comparison with other data allowed spectral identifications, assignments and partial characterization of four highly excited bound states for each of the molecular fragments (CH**/CD**); including the (3)(2)Π valence state and the (4)(2)Π Rydberg state, for the first time. Perturbations, shown as line-shifts, line-intensity and/or line-width alterations, due to the level-to-level state interactions between the bound states and predissociations by a repulsive state are recognized. Recording of C(+) signals in REMPI of several bromomethanes for a one-photon energy of about 40 333 cm(-1) allows the clarification of a mystery concerning a broad C(+) band frequently observed. This work, presented, demonstrates the usefulness of molecular REMPI for fragment analysis.

Ab initio study of valence and Rydberg states of CH3Br

We performed configuration interaction ab initio calculations on the valence and 5s, 5p(a(1)), and 5p(e) Rydberg bands of the CH(3)Br molecule as a function of the methyl-bromide distance for frozen C(3v) geometries. The valence state potential energy curves are repulsive, the Rydberg state ones are similar to the one of the CH(3)Br(+) ion with a minimum at short distance. One state emerging from the 5p(e) band has valence and ion-pair characters as distance increases and the corresponding potential curve has a second minimum at large distance. This state has a very strong parallel electric dipole transition moment with the ground state and plays a central role in UV photon absorption spectra. It is also responsible for the parallel character of the anisotropy parameters measured in ion-pair production experiments. In each band, there is a single state, which has a non-negligible transition moment with the ground state, corresponding to a transition perpendicular to the molecular axis of symmetry, except for the 5p(e) band where it is parallel. The perpendicular transition moments between ground and valence states increase sharply as methyl-bromide distance decreases due to a mixing between valence and 5s Rydberg band at short distance. In each band, spin orbit interaction produces a pair of states, which have significant transition moments with the ground one. In the valence band, the mixing between singlet and triplet states is weak and the perpendicular transition to the (1)Q(1) state is dominant. In each Rydberg band, however, spin-orbit interaction is larger than the exchange interaction and the two significant transition moments with the ground state have comparable strengths. The valence band has an additional state ((1)Q(0)) with significant parallel transition moment induced by spin-orbit interaction with the ground state at large distance.

ION PAIR DISSOCIATION: Spectroscopy and Dynamics

Ion pair dissociation processes may be studied using coherent vacuum ultraviolet laser sources in a manner entirely analogous to photoelectron spectroscopy, albeit with the anion playing the role of a heavy electron. If the excitation energy is above the dissociation energy and the kinetic energy of the fragment is measured using ion imaging, this approach is termed ion pair imaging spectroscopy (IPIS) and is related to conventional photoelectron spectroscopy. If the excitation energy is just below the dissociation energy and pulsed-field dissociation is employed, this approach is analogous to mass analyzed threshold ionization (MATI) spectroscopy and is termed threshold ion pair production spectroscopy (TIPPS). These approaches provide a novel means of investigating ion thermochemistry and spectroscopy and superexcited state decay dynamics at high resolution.

Photodissociation of the Ethyl Bromide Cation at 355 nm by Means of TOF-MS and Ion Velocity Imaging Techniques

Photodissociation of ethyl bromide cation, C