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Hydrogen is efficiently released during water dissociation on zirconium (Zr), while even very rapid temperature programed heating of a hydrogen covered Zr surface predominantly leads to dissolution (∼99% dissolution). To help resolve these apparently contradictory observations, we have studied the dynamics of water (D2O) dissociation on a crystalline Zr surface by probing the rotational and vibrational energy distributions of the D2 produced using resonant enhanced multiphoton ionization spectroscopy. The internal-state energy distribution of the D2 product was found to be rotationally cold and vibrationally hot with respect to the temperature of the surface. The rotational distribution shows slight deviations from Boltzmann’s law, with a mean rotational temperature of 426 K while the surface is at 800 K. The population of the ν″=1 vibration is at least four times higher than a 800 K temperature would allow, this corresponding to a vibrational temperature of 1100 K. Information on the translational energy of the D2 product have also been obtained by time-of-flight spectroscopy and it is found to be nearly thermally equilibrated with the surface temperature. Similar results were obtained from studies of D2 scattered from a clean Zr surface, and of D2 released by a slow thermal desorption process which involves dissolved hydrogen as the source. The reconciliation of the present results with those for thermal desorption of preadsorbed hydrogen implies a role for both surface and subsurface adsorption sites on the Zr surface and clearly demonstrates that at high temperatures, the release of D2 arises from the recombinative desorption of adsorbed hydrogen formed by the complete dissociation of D2O. [ABSTRACT FROM AUTHOR]

Threshold ion-pair production spectroscopy (TIPPS) has been applied to two isotopomers, HF and DF. From the high resolution (∼0.3 cm-1) TIPP spectra, the ion-pair thresholds of HF/DF have been precisely measured. Combined with the ionization energy of H(D), the electron affinity of F, and the zero point energies of HF/DF, the difference between their classical bond dissociation energies was obtained as De(H-F)-De(D-F)=12.4±0.5 cm-1. Our result provides an experimental estimate of the Born-Oppenheimer breakdown in the ground electronic state. The present work also measured the total ion-pair yield spectra of HF and DF in the threshold region, and the ion-pair formation mechanisms of these two molecules were discussed in light of the high resolution results. [ABSTRACT FROM AUTHOR]

The spectroscopic technique of threshold ion-pair production spectroscopy (TIPPS) has been applied to the triatomic molecule HCN. We have recorded the total ion-pair yield and TIPP spectra for the HCN→H++CN- process using coherent vacuum ultraviolet excitation. From the simulation of our high-resolution TIPP spectrum we have precisely measured the HCN ion-pair threshold EIP0 to be 122 244±4 cm-1. This value could be used to determine the bond dissociation energy D0(H-CN) to unprecedented accuracy. Our fitting result also showed that rotationally excited instead of cold CN- fragment is favored as the ion-pair dissociation product in the threshold region. [ABSTRACT FROM AUTHOR]

The photofragmentation dynamics of Fe(CO)5 in a supersonic molecular beam have been studied for photolysis wavelengths of 193, 248, 266, and 351 nm. The CO photofragments formed under these collision-free conditions were detected by vacuum ultraviolet laser-induced fluorescence (VUV LIF). This allowed for the determination of the rotational and vibrational distributions for the CO products and, by using Doppler spectroscopy, the translational energy distributions were also determined. These data are presented along with the details of a statistical model calculation which reproduces the experimental data very precisely. The statistical model for the photodissociation dynamics, which works at all photolysis wavelengths studied, is based on sequential elimination of CO ligands, with complete energy randomization in the intermediate ground state Fe(CO)n fragments between elimination steps. The detailed mechanism for the ultraviolet photochemistry of Fe(CO)5 is discussed in light of these results and previous experimental and theoretical work. [ABSTRACT FROM AUTHOR]

The 193 nm dissociation of CS2 has been studied under supersonic molecular beam conditions, with the atomic S products detected by vacuum ultraviolet laser-induced fluorescence. These experiments measure the branching ratio between singlet and triplet channels to be S(3P)/S(1D) =2.8±0.3. Doppler spectroscopy on the S(3P2) and S(1D2) products was used to study the dynamics of each channel separately. The energy release in each channel was similar, with the CS fragments formed vibrationally inverted with about 50% of the available energy in CS rotation and vibration. The recoil velocity anisotropy in each channel was also measured, found to be the same for both, and was consistent with a 2 ps lifetime for the 1Σ+u(1B2) excited state of CS2. [ABSTRACT FROM AUTHOR]

Interactions between the B’ 2Δ non-Rydberg state and nearby 2Δ Rydberg states of NO have been studied by vacuum ultraviolet laser-induced fluorescence spectroscopy. By using tunable, coherent radiation in the 60 000–70 000 cm-1 range, NO molecules in a supersonic jet were excited and their radiative decay was monitored by time-resolved fluorescence detection. In this way accurate, collision-free radiative lifetimes were measured for several levels of the B’ 2Δ, F 2Δ, N 2Δ, and D 2Σ+ excited states of NO. Using these lifetimes the oscillator strengths for the B’–X and F–X transitions were calculated. The radiative lifetimes for the perturbed states were quite different from the unperturbed lifetimes, and these changes are discussed in terms of the known interactions between states in this spectral region. For the unperturbed vibrational levels of the B’ 2Δ state, radiative lifetimes between 129–139 ns were measured, while for the F 2Δ state, a radiative lifetime of 30 ns was measured. For the perturbed bands of the F2Δ and B’ 2Δ states, the radiative lifetimes were accurately described by a two-state homogeneous perturbation model, but for the B’(v=7)∼N(v=0) mixed states, the lifetimes were dramatically different from those expected on the basis of two interacting states. This is caused by a third-interacting state whose identity is not known. The B’(v=8,9) states, while having unperturbed line positions, have shorter radiative lifetimes, indicating the onset of predissociation in those levels. [ABSTRACT FROM AUTHOR]

The photodissociation of glyoxal has been investigated by monitoring the CO internal energy distribution using tunable vacuum ultraviolet laser-induced fluorescence on the A←X system. Appearance times for the CO are in excellent agreement with the glyoxal fluorescence decay times, indicating that there is no long-lived intermediate in the dissociation. The quantum yield for CO production is independent of the K quantum number describing the glyoxal rotation. The CO is formed almost entirely in v=0 but is spread over a broadly excited rotational distribution peaking at J[bar_over_tilde:_approx._equal_to]42. Analysis of the CO Doppler profiles shows that the velocity of the CO increases with increasing rotational level and that the CO recoil velocity vector is oriented predominantly perpendicular to its angular momentum vector. These observations, which are in agreement with both previous time-of-flight data and molecular orbital calculations, are consistent with a model for the dissociation involving planar intermediates for the two channels leading to CO+H2CO or to 2 CO+H2. It appears that the highest rotational levels of CO are produced in coincidence with the H2CO channel. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of the 1B2 state of CS2 have been studied for excitation energies between 46 600 and 50 500 cm-1 (214–198 nm). These experiments used a tunable pulsed uv laser to photodissociate jet-cooled CS2 and the resulting atomic S products were detected by single photon VUV laser-induced fluorescence. These data allow for the measurement of the S(1D2):S(3PJ) product branching ratios as a function of vibrational level in the 1B2 state. Because of the low rotational temperature of the jet cooled CS2, we were able to resolve vibrational bands, and simulate the rotational contours for K=0 and K=1 vibrational bands up to 50 100 cm-1 excitation energy, obtaining homogeneous linewidths for these levels. For vibrational levels close to the zero point, the dissociation dynamics, both lifetimes and product branching ratios, were very different for K=0 and K=1 levels at the same energy. In particular, K=1 levels have shorter lifetimes, and a substantially higher yield of S(1D2) product. We propose a model for the dissociation dynamics of the 1B2 state of CS2, based on our results and preliminary ab initio calculations of the potential energy curves for excited states of CS2. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report pulsed field ionization spectra of an ionic electronically excited state, specifically, the NO+(a 3Σ+ v+=0,1,2)←NO(X 2Π1/2) transition. A coherent vacuum ultraviolet light source provides single photon excitation, which enables a direct measurement of the ionization potentials for the three vibrational levels — I.P.(v+=0)=126 392±1.5 cm-1, I.P.(v+=1)=127 653±1.5 cm-1, and I.P.(v+=2)=128 884±1.5 cm-1. The rotational structure of these spectra has been simulated using a standard model for rotational line strengths in a photoelectron spectrum, and this simulation shows the strong atomic p orbital character of the initial NO orbital from which the electron is photoionized. However, our data show that the rotational line strengths are strongly dependent on the final vibrational state of NO+, in disagreement with the theoretical model. Also, the spectrum for the v+=1 state of NO+ is strongly affected by the presence of a strong autoionizing Rydberg resonance at the same photon energy, which leads to a complex resonance. [ABSTRACT FROM AUTHOR]

Single-photon zero-kinetic-energy pulsed-field-ionization spectra have been measured for the v+=0 and 1 levels of CO+ (X 2Σ+) and the v+=0 level of N2+ (X 2Σg+) by coherent XUV radiation. In spite of similarities in the electronic structure of CO and N2, the measured ion spectra show dramatically different intensities for the Q branches. These threshold spectra are interpreted on the basis of ab initio calculations of the ion rotational distributions. Agreement between the calculated and measured spectra is very encouraging. Improved values for the ionization potentials of CO (113 025.6 and 115 211.2±1.5 cm-1 for v+=0 and 1, respectively) are reported and the unusual dynamics favoring ΔN<0 transitions are discussed. The CO spectra show quite different behavior for the ΔN<0 transitions for v+=0 and v+=1 bands, which is interpreted in terms of the relative importance of rotational autoionization in the two bands. [ABSTRACT FROM AUTHOR]

The vibrational, rotational, and translational energy distributions for the CO photofragments from the 193, 248, and 308 nm photolysis of Ni(CO)4 in a supersonic molecular beam have been determined by vacuum ultraviolet laser-induced fluorescence. The measured product energy distributions appeared to be statistical, with equilibrium between the degrees of freedom investigated. The distributions were significantly colder than those calculated with a microcanonical statistical model using published bond energies. To model the measured distributions, it was necessary to postulate that the unsaturated nickel carbonyl products are formed in a stable electronically excited state. By using an excited state energy consistent with published fluorescence experiments, excellent agreement was obtained between the measured distributions and those calculated using a microcanonical statistical model. These results indicate that for 193 nm photolysis, essentially all of the Ni(CO)n products are electronically excited, with about 2.8 eV of electronic excitation. The Ni(CO)n products from 248 nm photolysis are formed in both the ground and excited states, with a 3:1 branching ratio. The data taken at 308 nm also indicate the presence of both channels, with the excited state channel still important. These results are consistent with predictions of earlier ab initio work, although the detailed mechanism is somewhat different. This earlier work is discussed in light of the present results. [ABSTRACT FROM AUTHOR]

The photodissociation of OCS at 157 nm has been investigated by using tunable vacuum ultraviolet radiation to probe the CO and S photoproducts. Sulfur is produced almost entirely in the 1S state, while CO is produced in its ground electronic state and in vibrational levels from v=0–3 in the approximate ratio (v=0):(v=1):(v=2):(v=3)=(1.0):(1.0):(0.5) :(0.3). The rotational distribution for each vibrational level is found to be near Boltzmann, with temperatures that decrease from 1350 K for v=0 to 780 K for v=3. Measurements of the CO Doppler profiles demonstrate that the dissociation takes place from a transition of predominantly parallel character (β=1.8±0.2) and that the CO velocity and angular momentum vectors are perpendicular to one another. [ABSTRACT FROM AUTHOR]

Photodissociation of OCS in the region from 222–248 nm has been investigated by monitoring the CO and S(1D2) primary photoproducts; as well as the secondary production of S(3P2), S(3P1), and S(3P0) using fluorescence induced by a tunable vacuum ultraviolet laser source based on four-wave mixing in magnesium vapor. The quantum yield of S(3P) was found to be 0.00±0.02 at 222 nm. Thus, in contrast to our preliminary report, the present more detailed investigation shows that the sole sulfur product appears to be S(1D). The CO photofragment is produced almost exclusively in v=0 [CO(v=1)/ CO(v=0)≤0.02], but the rotational distribution is inverted and peaked at very high rotational levels. The peak shifts from J=56 for dissociation at 222 nm to J=31 at 248 nm. Doppler profiles of the CO rotational transitions reveal (1) that all observed levels are produced in coincidence with S(1D), (2) that for 222 nm photolysis the fragment recoil anisotropy shifts from a distribution characterized by β=1.9 at J=67 toward one characterized by β=0 near J=54, (3) that the CO velocity vector is aligned nearly perpendicular to its angular momentum vector, and (4) that the CO angular momentum vector is also aligned parallel to that component of the transition dipole which lies perpendicular to the recoil velocity. These results are interpreted in terms of a model for the dissociation in which excitation takes place to two surfaces of A’ and A‘ symmetry derived from a bent 1Δ configuration. Dissociation of OCS clusters was also investigated and was found to produce a photochemistry completely different from that of the monomers. Rotationally cold CO as well as S2 in both the X3Σ-g and a1Δg states was observed. [ABSTRACT FROM AUTHOR]