J. W. Hepburn

A simple but reliable method for the prediction of intermolecular potentials

Abstract A simple but reliable method for the prediction of intermolecular potentials is presented and tested for the case of the lighter noble gases and some of their mixtures.

Simplified laser control of unimolecular reactions: Simultaneous (ω1, ω3) excitation

Abstract We show that product yields in photodissociation may be controlled by varying the relative phase and amplitude of two simultaneously applied lasers of frequency ω 1 and ω 3 =3ω 1 . The result is a substantially simplified experimental proposal for demonstrating coherent radiative control over yields in photoinduced unimolecular decay.

State‐resolved photodissociation of OCS monomers and clusters

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...

Determination of bond dissociation energies by threshold ion-pair production spectroscopy: An improved D0(HCl)

A recently developed form of threshold ionization spectroscopy has been used to determine the bond energy for HCl to spectroscopic accuracy (±0.8u200acm−1). This method is based on excitation to highly vibrationally excited ion-pair states using single-photon transitions from the ground state of HCl. These metastable Rydberg-like states were selectively detected using electricfield induced dissociation. By systematically varying the electric fields involved, and scanning the exciting photon energy, it was possible to determine the field-free energetic threshold for H35Cl+hν→H++35Cl−. Using this energy, together with the known values of the ionization potential of H and electron affinity of Cl, a new estimate for the dissociation energy of HCl was obtained: D0(H35Cl)=35u200a748.2±0.8u200acm−1.

Photofragment spectroscopy of CS2 at 193 nm: Direct resolution of singlet and triplet channels

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.

State‐resolved photofragmentation dynamics of Fe(CO)5 at 193, 248, 266, and 351 nm

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...

State‐to‐state photodissociation dynamics of trans‐glyoxal

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≂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 chann...

Pulsed field ionization threshold photoelectron spectroscopy with coherent vacuum ultraviolet : NO+(a 3Σ+ v=0,1,2)←NO(X 2Π1/2)

We report pulsed field ionization spectra of an ionic electronically excited state, specifically, the NO+(au20093Σ+ v+=0,1,2)←NO(Xu20092Π1/2) 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)=126u2009392±1.5 cm−1, I.P.(v+=1)=127u2009653±1.5 cm−1, and I.P.(v+=2)=128u2009884±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...

Pulsed‐field ionization threshold photoelectron spectroscopy with coherent extreme ultraviolet radiation: A comparison of CO and N2

Single‐photon zero‐kinetic‐energy pulsed‐field‐ionization spectra have been measured for the v^+=0 and 1 levels of CO^+ (Xu2009 ^2Σ^+) and the v^+=0 level of N_2^+ (Xu2009 ^2Σ_g^+) by coherent XUV radiation. In spite of similarities in the electronic structure of CO and N_2, 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 (113u2009025.6 and 115u2009211.2±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.

Dynamics of HI photodissociation in the A band absorption via H-atom Doppler spectroscopy

The dynamics of HI photodissociation following absorption in the A band (200 to 300 nm) was investigated by Doppler spectroscopy at the Lyman α transition of the H-atom photofragments. Measurements of both the branching ratios for the formation of spin-orbit excited I(2P1/2) and ground state I(2P3/2) atoms and the angular distributions of the recoil velocity for these two photofragment channels were obtained at nine photolysis wavelengths between 212.5 and 266 nm. These results show that ground state products result from a transition of perpendicular symmetry (ΔΩ=1), while the excited state atoms are produced from a parallel transition (ΔΩ=0). These experimental results, in combination with total absorption cross section data obtained prior to the present study, have enabled a calculation of the potential curves for the dissociative excited states. The outcome of the calculation was found to be in good qualitative agreement with the model of HI electronic structure originally suggested by Mulliken.