R. O. Watts

The infrared predissociation spectra of water clusters

Infrared predissociation spectra of water clusters have been measured in the frequency range 3000–3800 cm−1 using a molecular beam–color center laser apparatus. The transition from a spectrum resembling that of liquid water to that of the dimer is clearly seen. Detailed theoretical analyses using normal mode theory, local mode theory, and a quantum simulation method are used to interpret the spectrum in terms of a potential surface that includes both intramolecular and intermolecular degrees of freedom.

Sub‐Doppler resolution infrared spectroscopy of the acetylene dimer: A direct measurement of the predissociation lifetime

Rotationally resolved, infrared predissociation spectra have been obtained for (C2H2)2 using a molecular beam apparatus in conjunction with a single mode F‐center laser operating near 3300 cm−1. Several well separated bands are seen in the low resolution spectra, possibly indicating that more than one stable dimer structure exists. The high resolution spectra indicate that the widths of the individual rovibrational transitions, and hence predissociation lifetimes, depend on the band excited. The lifetimes obtained from the widths vary from 80×10−9 to 1.6×10−9 s.

Study of the structure of molecular complexes. VII. Effect of correlation energy corrections to the Hartree‐Fock water‐water potential on Monte Carlo simulations of liquid water

The two‐body Hartree‐Fock potential for water‐water interactions previously used to describe small water clusters and used in Monte Carlo studies of liquid water has been used to assess the effect of neglecting the correlation energy correction. A number of semiempirical corrections based on the London, Kirkwood, and Wigner formulas were used. They have an energy spread such that they encompass the expected magnitude of the exact correlation energy correction. Short range correlation interactions were included using a modification of the functional of density proposed by Wigner and the corresponding long range interactions were given by a term proportional to r−6 with coefficients due to both London and Kirkwood. After adding the corrections to the Hartree‐Fock potential the various potentials were compared by generating a number of potential energy surfaces. Following this the water‐water potentials were used in a Monte Carlo simulation of liquid water to calculate thermodynamic and structural data. Afte...

Percus‐Yevick Equation Applied to a Lennard‐Jones Fluid

An efficient method of solving the Percus‐Yevick and related equations is described. The method is applied to a Lennard‐Jones fluid, and the solutions obtained are discussed. It is shown that the Percus‐Yevick equation predicts a phase change with critical density close to 0.27 and with a critical temperature which is dependent upon the range at which the Lennard‐Jones potential is truncated. At the phase change the compressibility becomes infinite although the virial equation of state does not show this behavior. Outside the critical region the PY equation is at least two‐valued for all densities in the range (0, 0.6).

Molecular beam infrared spectra of dimers formed from acetylene, methyl acetylene, and ethene as a function of source pressure and concentration

Infrared spectra have been obtained for dimers formed from acetylene, methyl acetylene, and ethylene using a wide variety of free jet source conditions. An F‐center laser was used to excite various C–H stretch fundamentals in the range 3000–3300 cm−1. Rotational structure was resolved in the spectra of both acetylene and methyl acetylene dimers when dilute mixtures of either gas in helium were expanded from high pressures. The spectrum of the ethylene dimer showed no such structure even when formed in very dilute mixtures. These results contradict previous suggestions that the vibrational relaxation lifetime is uniformly short for all polyatomic dimers.

A spherical potential for hydrogen from solid state and scattering data

Calculations of the ground state energy and pressure of solid hydrogen, and of the total differential scattering cross section for D2+H2, are used to assess existing isotropic potentials for the hydrogen pair interaction. Although several of the models give consistent results in the region of the potential minimum, they differ significantly at short distances. A comparison with solid state and scattering data indicates that ab initio calculations are in error and that there is a lack of consistency between this data and second virial coefficient measurements. The results are used to recommend a potential surface that is consistent with the experimental data.

Quantum simulation of systems with nodal surfaces

A modification of the diffusion Monte Carlo algorithm that enables the direct simulation of wave functions containing nodal surfaces is described. The use of orthogonality constraints during the simulation enables the properties of excited state systems to be calculated. For Fermi systems, spin variables and exchange operators are also included in the algorithm. The effectiveness of the methods is demonstrated using applications to several simple models.

Coordination number for the Li+–F− ion pair in water

A cluster of 200 molecules of water containing the Li+−F− ion pair has been studied using Monte Carlo techniques. Two temperatures (T = 298 and 500°K) and three different distances for the Li+−F− separation [R (Li−F) = 2.0, 6.0, and 10.0 A] have been considered. The water−water potential is obtained from quantum−mechanical Hartree−Fock−type computations corrected by inclusion of dispersion forces; the ion−water potential has been obtained from Hartree−Fock computations for the Li+−F−−H2O complex. The Monte Carlo simulation provides reproducible information on the cluster shape, on the cluster structure, and on the coordination numbers for the Li+ cation and the F− anion, when in presence of one another.

Vibrational predissociation of Van der Waals clusters of ethylene

Abstract Infrared laser spectroscopy and quadrupole mass spectrometry have been used to study clusters of ethylene molecules formed in a free jet expansion. Vibrational spectra in the range 2950—3200 cm −1 show structure corresponding to the predissociation of clusters as a result of exciting them near ν 1 -ν 11 and r u fundamentals, as well as the ν 2 + r 12 combination band. These results provide valuable information concerning the structures and dissociation mechanisms for ethylene clusters. In all cases the vibrational bands show lorentzian lineshapes of = 5 cm −1 fwhm, suggesting that the dominant broadening mechanism is homogeneous. Estimates of the excited-state lifetimes can therefore be obtained directly from these widths.

The argon hydrogen–fluoride differential scattering cross section

The total differential cross section for Ar–HF was measured at a collision energy of 1637 K (141 meV). Although diffraction oscillations were not resolved, a broad primary rainbow peak was observed. Scattering calculations were carried out for the Ar–HF interaction potentials developed by Douketis et al. and Hutson and Howard. The spherical potential and infinite order sudden approximations do not give a good description of the total differential scattering. The final‐l labeled coupled states approximation, on the other hand, is in good agreement with the more accurate close coupled approximation. Neither potential predicts cross sections which are in good agreement with the measured scattering intensities. Contributions to the total scattering from elastic and inelastic processes were investigated using the coupled states approximation. The contributions to the total scattering from different initial HF rotational states were also studied. The total scattering for j initial=0 differs significantly from t...