Y. T. Lee

Molecular beam studies of the F+H2 reaction

The dynamics of the F+H2 reaction have been investigated in a high resolution crossed molecular beam study. Differential cross sections and kinetic energy distributions were obtained for each HF vibrational state. The v=1 and v=2 states were predominantly backward scattered, but substantial forward scattering was observed for HF (v=3) over the range of collision energies accessible in our apparatus, from 0.7 to 3.4 kcal/mol. The results strongly suggest that dynamical resonances play a significant role in the reaction dynamics of F+H2 and that resonance effects are most prominent in the v=3 product channel. Quantal reactive scattering calculations on F+H2 predict that the v=2 channel should be most strongly affected by resonances. This discrepancy is attributed to inadequacies in the potential energy surface used in the calculations, and several modifications to the surface are proposed based on the experimental results. Other features of the reaction are also discussed, including the integrated partial c...

Photofragmentation of CH3I: Vibrational distribution of the CH3 fragment

The photodissociation of CH3I at 266 nm is investigated by means of high resolution photofragment spectroscopy. The resolution is sufficient to determine the vibrational population of the CH3 umbrella motion for the I*(2P1/2)+CH3 product channel. An approximate vibrational distribution for the I(2P3/2)+CH3 product channel is also determined. The rotational energy distribution for the CH3+I*(2P1/2) channel is estimated to be on the order of or less than 600 cm−1 wide for each of the CH3 vibrational states. A refined value for the C–I bond dissociation energy of 53.3±0.7 kcal/mole is determined from the energy threshold for the I*+CH3 channel. The vibrational energy distribution for the I*+CH3 channel is discussed in relation to a recent model calculation by Shapiro and Bersohn and possible explanations for the discrepency between the calculated and the measured distributions are considered.

Infrared vibrational predissociation spectroscopy of water clusters by the crossed laser‐molecular beam technique

Water clusters formed in a molecular beam are predissociated by tunable, pulsed, infrared radiation in the frequency range 2900–3750 cm−1. Absorption spectra of the clusters are obtained by detecting the recoiling fragments off‐axis from the molecular beam as a function of laser frequency using a rotatable mass spectrometer. By carefully adjusting the expansion conditions of the molecular beam and monitoring the largest cluster observable, excessive contamination by clusters larger than the specific one of interest is avoided. It is found that the spectra of clusters containing three or more water molecules absorb over the same frequency range as the liquid. Dynamical information on the predissociation process is obtained from the measured angular and velocity distributions of the fragments. An upper limit to the excited vibrational state lifetime of ∼1 μs is observed for the results reported here. The most probable dissociation process concentrates the available excess energy into the internal motions of...

Infrared spectra of the cluster ions H7O+3⋅H2 and H9O+4⋅H2

Infrared spectra of hydrated hydronium ions weakly bound to an H2 molecule, specifically H7O + 3 ·H2 and H9O + 4 ·H2, have been observed. Mass-selected parent ions, trapped in a radio frequency ion trap, are excited by a tunable infrared laser; following absorption, the complex predissociates with loss of the H2, and the resulting fragment ions are detected. Spectra have been taken from 3000 to 4000 cm^−1, with a resolution of 1.2 cm^−1. They are compared to recent theoretical and experimental spectra of the hydronium ion hydrates alone. Binding an H2 molecule to these clusters should only weakly perturb their vibrations; if so, our spectra should be similar to spectra of the hydrated hydronium ions H7O + 3 and H9O + 4 .

Molecular beam studies of the F+D2 and F+HD reactions

The F+D2 and F+HD reactions were investigated in a high resolution crossed molecular beams experiment at several collision energies. The DF product from both reactions was predominantly backward scattered although some forward scattered DF(v=4) was observed at the highest energy studied. The HF angular distributions from F+HD were quite different, showing considerable forward scattered (v=3) and no other identifiable structure. These results disagree with classical trajectory studies, which predict only small variations in the product angular distributions among F+H2 and its isotopic variants. They agree, however, with the predicted dependence of dynamical resonance effects on isotopic substitution. The results therefore support the conclusions drawn in the previous paper regarding the role of dynamical resonances in the F+H2 reaction.

The electronic state‐selective photodissociation of CH2BrI at 248, 210, and 193 nm

The primary photodissociation channels of CH2BrI following excitation at 193.3, 210, and 248.5 nm have been studied with the crossed laser‐molecular beam technique. Product translational energy distributions and polarization dependences were derived for the primary dissociation processes observed. The data demonstrate bond selective photochemistry as well as some selective formation of electronically excited photofragments in bond fission and concerted dissociation. Excitation at 248.5 nm, which is assigned to excitation of primarily a n(I)→σ*(C–I) transition with some contribution from an overlapping n(Br)→σ*(C–Br) transition, results in both C–I and C–Br bond fission. C–I bond fission is the dominant channel, producing I atoms in both the 2P3/2 and spin‐orbit excited 2P1/2 states in a ratio of 1.0:0.75. Excitation at 193.3 nm, assigned to a transition to primarily predissociated Rydberg levels on the I atom, leads to C–Br bond fission, some C–I bond fission, and significant concerted elimination of IBr....

ENERGETICS OF GAS PHASE PROTON SOLVATION BY NH3

The absolute proton affinity of NH3 (203.6±1.3 kcal/mole at 298 K) and the proton solvation energies by more than one NH3 have been determined by the molecular beam–photoionization method. In addition, the NH3+–NH3 interaction energy (0.79±0.05 eV) has been measured by photoionization of the neutral van der Waals dimer. These experiments have shown that photoionization of van der Waals clusters is a very powerful method for determining the energetics of gas phase proton solvation.

Infrared spectroscopy of the cluster ions H+3⋅(H2)n

The vibrational spectra of the clusters H + 3 (H2)n were observed near 4000 cm−1 by vibrational predissociation spectroscopy. Spectra of mass-selected clusters were obtained by trapping the ions in a radio frequency ion trap, exciting vibrational transitions of the cluster ions to predissociating levels, and detecting the fragment ions with a mass spectrometer. Low resolution bands of the solvent H2 stretches were observed for the clusters of one to six H2 coordinated to an H + 3 ion. The red shift of these vibrations relative to the monomer H2 frequency supported the model of H + 9 as an H + 3 with a complete inner solvation shell of three H2, one bound to each corner of the ion. Two additional bands of H + 5 were observed, one assigned as the H + 3 symmetric stretch, and the other as a combination or overtone band. High-resolution scans (0.5 and 0.08 cm−1) of H + n , n=5, 7, and 9 yielded no observable rotational structure, a result of either spectral congestion or rapid cluster dissociation. The band contour of the H + 5 band changed upon cooling the internal degrees of freedom, but the peaks remained featureless. The observed frequencies of H + 7 and H + 9 agreed well with ab initio predictions, but those of H + 5 did not. This deviation is discussed in terms of the large expected anharmonicity of the proton bound dimer H + 5 .

Photoionization of dimeric polyatomic molecules: Proton affinities of H2O and HF

Photoionization studies of (H2O)2 and (HF)2 producing H3O+ and H2F+ yield 7.18±0.08 eV (165.8±1.8 kcal/mole) and 4.09±0.06 eV (94.3±1.4 kcal/mole) as the proton affinities of H2O and HF, respectively. The measured ionization potential of (H2O)2, 11.21±0.09 eV, along with the known ionization potential of H2O, 12.615±0.001 eV, allow the deduction of a lower bound for the dissociation energy of (H2O)2:1.58±0.13 eV (36±3 kcal/mole). The experiments have demonstrated that photoionization of dimers is one of the most useful general methods for the determination of proton affinities.

Molecular beam photoelectron spectroscopy and femtosecond intramolecular dynamics of H2O+ and D2O+

The 584 A photoelectron spectra of supersonic molecular beams of H2O and D2O have been obtained with improved resolution. The spectroscopic constants of the Xu20092B1 and Au20092A1 state ions, including ω01, x011, ω02, x022, and x012, are reported. For the first two electronic states of the ion, precise line splittings were evaluated with a least squares fitting procedure, employing sums of empirical instrument response functions and a linear background. A simulation of the vibrational manifolds of the Bu20092B2 state ions with combination progressions in the symmetry‐allowed modes ν1 and ν2 failed to reproduce the diffuse photoelectron bands observed for both H2O and D2O. Autocorrelation functions were calculated from the photoelectron bands of all three electronic states. The Bu20092B2 state correlation functions exhibit ultrafast decay, occurring on a 10−14 s time scale. The ν2 motion appears to define the decay in the correlation function. This behavior supports a previously proposed Bu20092B2–Au20092A1 curve‐crossing ...