Ultraviolet Laser Photodissociation Research Articles

Cation−π Complexes of Silver Studied with Photodissociation and Velocity-Map Imaging

Ag+(aromatic) ion-molecule complexes of benzene, toluene, or furan are generated in the gas phase by laser vaporization in a supersonic expansion. These ions are mass selected in a time-of-flight spectrometer and studied with ultraviolet laser photodissociation and photofragment imaging. UV laser excitation results in dissociative charge transfer (DCT) for these ions, producing neutral silver atom and the respective aromatic cation as the photofragments. Velocity-map imaging and slice imaging techniques are employed to investigate the kinetic energy release in these photodissociation processes. In each case, DCT produces significant kinetic energy, and evidence is also found for excitation of the internal rovibrational degrees of freedom for the molecular cations. Analysis of the kinetic energy release together with the known ionization energies of silver and the molecular ligands provides new information on the cation-π bond energies.

Ion Imaging of MgI+ Photofragment in Ultraviolet Photodissociation of Mass-Selected Mg+ICH3 Complex.

We have observed images of MgI+ fragment ions produced in ultraviolet laser photodissociation of mass-selected Mg+ICH3 ions at 266 nm. Split distribution almost perpendicular to the polarization direction of the photolysis laser was observed in the photofragment image. Potential energy curves of Mg+ICH3 were obtained by theoretical calculations. Among these curves, the excited complex ion dissociated along almost repulsive potentials with several avoided crossings, which was connected to MgI+ + CH3. In the ground state of Mg+ICH3, the CH3I was bonded with Mg from the iodine side, and the Mg-I-C bond angle was calculated to be 101.1°. The theoretical results also indicated that the dissociation occurred after the 52A' ← 12A' photoexcitation, where the transition dipole moment was almost parallel to the Mg-I bond axis. The MgI+ and CH3 fragments dissociated each other parallel to the direction connecting those center-of-masses, which was 67° with respect to the transition dipole moment of 52A' ← 12A' photoexcitation. Therefore, the fragment recoil direction was assumed to approach perpendicular tendency against the polarization direction under the fast dissociation process. However, calculated potential energy curves showed a complicated reaction pathway for MgI+ production, including nonadiabatic processes, although the experimental results indicated the fast dissociation reaction.

Communication: Vacuum ultraviolet laser photodissociation studies of small molecules by the vacuum ultraviolet laser photoionization time-sliced velocity-mapped ion imaging method

We demonstrate that the vacuum ultraviolet (VUV) photodissociation dynamics of N(2) and CO(2) can be studied using VUV photoionization with time-sliced velocity-mapped ion imaging (VUV-PI-VMI) detection. The VUV laser light is produced by resonant sum frequency mixing in Kr. N(2) is used to show that when the photon energy of the VUV laser is above the ionization energy of an allowed transition of one of the product atoms it can be detected and characterized as the wavelength is varied. In this case a β parameter = 0.57 for the N((2)D°) was measured after exciting N(2)(o(1)Π(u), v(') = 2, J(') = 2) ← N(2)(X(1)Σ(g) (+), v(") = 0, J(") = 1). Studies with CO(2) show that when there is no allowed transition, an autoionization resonance can be used for the detection of a product atom. In this case it is shown for the first time that the O((1)D) atom is produced with CO((1)Σ(+)) at 92.21 nm. These results indicate that the VUV laser photodissociation combined with the VUV-PI-VMI detection is a viable method for studying the one-photon photodissociation from the ground state of simple molecules in the extreme ultraviolet and VUV spectral regions.

New Insights into the Vacuum UV Photodissociation of Peptides

Vacuum ultraviolet laser photodissociation (UVPD) of peptide ions leads to unusual dissociation channels involving backbone C-C bond breaking. However, the molecular basis for the observed behavior is not clearly understood. We now report theoretical investigations using ab initio/density functional theory (DFT) techniques on neutral and protonated dipeptides undergoing vacuum ultraviolet (VUV) induced fragmentation via a Rydberg excitation (and/or electron detachment) and subsequent C-C bond cleavage. New experimental results on VUV photodissociation of dipeptides (protonated Ala_Arg and Arg_Ala) provide strong support for our proposed model. Our mechanism also provides a natural explanation for the presence of immonium ions that are sometimes observed in such experiments.

Photodissociation Studies of Mass-Selected Complex Cations Mg+(N,N-dimethylformamide)1,2

Gas-phase complexes Mg+−[HCON(CH3)2]1,2 are mass-selected and studied by ultraviolet laser photodissociation in a time-of-flight mass spectrometer. Beside the dominant evaporation photofragments, some photoreaction products have also been detected from both complexes. Quantum mechanics calculations and deuterium-substitution experiments are employed to facilitate the structural, energetic, and mechanistic analysis of the photoreactions. For the most stable structures of the complexes, Mg+ is linked to the carbonyl oxygen atoms and situated trans to the −N(CH3)2 groups. Starting from these complex structures, photoproducts are generated through (1) H abstraction next to the carbonyl group by the photoexcited Mg+* (to form (CH3)2NCO+) and/or (2) the subsequent CO loss (to form CH3NH+CH2). For Mg+−[HCON(CH3)2]2, the further solvation tends to hold back the photoreactions because of the even more facile evaporation channels.

Rotational features in the fluorescence excitation spectrum of O(1D2) from vacuum ultraviolet laser photodissociation of O2

Seventh-order anti-Stokes Raman-shifted ultraviolet laser radiation is used to dissociate O2 in the 175–177 nm region of the Schumann–Runge band system, B 3Σ−u–X 3Σ−g. A cross section for the production of O(1D2) is deduced from the 762 nm fluorescence of O2(b 1Σ+g), a collisional de-excitation product of O(1D2) and O2(X 3Σ−g). Step structure observed in the spectrum is attributed to rotational thresholds for absorption from X 3Σ−g to energies above the B 3Σ−u dissociation limit. The threshold energies define a limiting rotationless dissociation energy of 57 136.4±0.9 cm−1. Shape resonances, quasibound by the rotational barrier of B 3Σ−u, are observed for the first time in O2. A theoretical calculation of the cross section is in agreement with the measured cross section when the near-dissociation outer limb of the B 3Σ−u potential has an R−5 long range form. The shape of the adopted potential is consistent with an avoided crossing with another 3Σ−u state near R=4.6 Å.

Low-temperature growth of polycrystalline Si and Ge films by ultraviolet laser photodissociation of silane and germane

Polycrystalline Si and Ge films have been grown on amorphous SiO2 substrates (average Tsubstrate <120 °C) by the photodissociation of SiH4/N2 or GeH4/He mixtures, respectively, using pulsed ArF (193 nm) and KrF (248 nm) excimer lasers. For both Si and Ge, the film growth rate exhibited a strong dependence on laser wavelength and beam intensity I, where 1≲I⩽20 MW cm−2. As-deposited films exhibited average grain sizes of up to 0.5 μm and the grains were equiaxed with a random orientation. Ge films doped with ∼1020 cm−3 Al were obtained by the simultaneous photodissociation of Al(CH3)3 and GeH4.