Photodissociation dynamics and the dissociation energy of vanadium monoxide,VO, investigated using velocity map imaging

Velocity map imaging has been employed to study multiandndash;photon fragmentation of vanadium monoxide (VO) via the Candnbsp;4andSigma;andndash;andnbsp;state. The fragmentation dynamics are interpreted in terms of a dissociation at the threeandndash;photon level, with the first photon weakly resonant with transitions to vibrational energy levels of the Candnbsp;4andSigma;andndash;andnbsp;state. The dissociation channels accessed are shown to depend strongly on the vibration level via which excitation takes place. Analysis of the evolution of the kinetic energy release spectrum with photon energy leads to a refined value for the dissociation energy of ground state VO of D0(VO) = 53126 andplusmn; 263 cm-1.</p

A velocity map imaging study of multiphoton photodissociation and photoionisation dynamics in niobium oxide, NbO

Velocity map imaging has transformed our capabilities in terms of determining photofragmentation and photoionisation thresholds. Here, we present an investigation into the multi-photon dissociation and photoionisation dynamics of niobium monoxide (NbO) probed using velocity-map imaging via the&nbsp;C&nbsp;4&Sigma;&ndash;&nbsp;&larr;&nbsp;X&nbsp;4&Sigma;&ndash;&nbsp;and&nbsp;B&nbsp;4&Pi; &larr;&nbsp;X&nbsp;4&Sigma;-&nbsp;transitions. Analysis of the kinetic energy release spectra permits determination of refined values of the dissociation energies for NbO and NbO+&nbsp;of 60,650&thinsp;&plusmn;&thinsp;124 and 57,414&thinsp;&plusmn;&thinsp;124&thinsp;cm&minus;1, respectively, in good agreement with previous values using other methods. With the same instrument, we have further recorded photoelectron images which exhibit a long progression assigned to the production of the ground state,&nbsp;X&nbsp;3&Sigma;&minus;&nbsp;NbO+&nbsp;(v&nbsp;+), for which a vibrational constant of 1002&thinsp;&plusmn;&thinsp;11&thinsp;cm&minus;1&nbsp;is determined.</p

Photodissociation dynamics and the dissociation energy of vanadium monoxide,VO, investigated using velocity map imaging

Velocity map imaging has been employed to study multi–photon fragmentation of vanadium monoxide (VO) via the C 4Σ– state. The fragmentation dynamics are interpreted in terms of a dissociation at the three–photon level, with the first photon weakly resonant with transitions to vibrational energy levels of the C 4Σ– state. The dissociation channels accessed are shown to depend strongly on the vibration level via which excitation takes place. Analysis of the evolution of the kinetic energy release spectrum with photon energy leads to a refined value for the dissociation energy of ground state VO of D0(VO) = 53126 ± 263 cm-1.</p

C-I and C-F bond-breaking dynamics in the dissociative electron ionization of CF3I

We present a comprehensive experimental study into the dissociative electron ionization dynamics of CF3I at energies ranging from 20 to 100 eV. A beam-gas instrument has been used to measure the absolute total ionization cross-section for the molecule over the energy range from 0 to 300 eV. Coupled with data from an electron-molecule crossed beam velocity-map imaging instrument, this allows absolute partial ionization cross-sections to be determined for formation of each ionic fragment. These reveal a number of fragmentation channels involving both C–I and C–F bond cleavage, in some cases followed by further fragmentation of the resulting molecular ion. Velocity-map images have been recorded for the I+ and CF3+ products of C–I bond cleavage and the CF2I+ products of C–F bond cleavage. Analysis of fragment kinetic energy distributions extracted from the images reveals that CF3+ product of C–I bond cleavage appears to be formed via a statistical mechanism occurring over long timescales, while the CF2I+ products of C–F cleavage are formed via a much faster, more direct dissociation mechanism involving one or more repulsive states of the parent molecular ion. The I+ fragments arising from C–I bond cleavage display behaviour intermediate between the two extremes. For all fragments, the images show little or no dependence on the energy of the incident electron, implying that the initially excited ion state or states undergo rapid relaxation to the dissociative state(s) in all cases. Only a very small fraction of the incident electron's kinetic energy is released into kinetic energy of the recoiling atomic and molecular fragments, implying that most of the available energy remains with the two departing electrons. The kinetic energy distributions obtained for the various fragments of dissociative electron ionization are compared with the corresponding distributions reported from photoionization studies in order to gain insight into the electronic states involved

C-I and C-F bond-breaking dynamics in the dissociative electron ionization of CF3I

We present a comprehensive experimental study into the dissociative electron ionization dynamics of CF3I at energies ranging from 20 to 100 eV. A beam-gas instrument has been used to measure the absolute total ionization cross-section for the molecule over the energy range from 0 to 300 eV. Coupled with data from an electron-molecule crossed beam velocity-map imaging instrument, this allows absolute partial ionization cross-sections to be determined for formation of each ionic fragment. These reveal a number of fragmentation channels involving both C–I and C–F bond cleavage, in some cases followed by further fragmentation of the resulting molecular ion. Velocity-map images have been recorded for the I+ and CF3+ products of C–I bond cleavage and the CF2I+ products of C–F bond cleavage. Analysis of fragment kinetic energy distributions extracted from the images reveals that CF3+ product of C–I bond cleavage appears to be formed via a statistical mechanism occurring over long timescales, while the CF2I+ products of C–F cleavage are formed via a much faster, more direct dissociation mechanism involving one or more repulsive states of the parent molecular ion. The I+ fragments arising from C–I bond cleavage display behaviour intermediate between the two extremes. For all fragments, the images show little or no dependence on the energy of the incident electron, implying that the initially excited ion state or states undergo rapid relaxation to the dissociative state(s) in all cases. Only a very small fraction of the incident electron's kinetic energy is released into kinetic energy of the recoiling atomic and molecular fragments, implying that most of the available energy remains with the two departing electrons. The kinetic energy distributions obtained for the various fragments of dissociative electron ionization are compared with the corresponding distributions reported from photoionization studies in order to gain insight into the electronic states involved

Observation of direct vibrational excitation in gas-surface collisions of CO with Au(111): a new model system for surface dynamics.

We report vibrational excitation of CO from its ground (v = 0) to first excited (v = 1) vibrational state in collision with Au(111) at an incidence energy of translation of EI = 0.45 eV. Unlike past work, we can exclude an excitation mechanism involving temporary adsorption on the surface followed by thermalization and desorption. The angular distributions of the scattered CO molecules are narrow, consistent with direct scattering occurring on a sub-ps time scale. The absolute excitation probabilities are about 3% of those expected from thermal accommodation. The surface temperature dependence of excitation, which was measured between 373 and 973 K, is Arrhenius-like with an activation energy equal to the energy required for vibrational excitation. Our measurements are consistent with a vibrational excitation mechanism involving coupling of thermally excited electron–hole pairs of the solid to CO vibration