R. B. Gerber

Mo/ller–Plesset perturbation theory applied to vibrational problems

Mo/ller–Plesset perturbation theory is employed to improve the accuracy of static mean field computations in molecular vibration problems. This method is a simple and efficient way to get nearly exact frequencies for few‐mode model potentials. For more realistic potentials representing the dynamics of water and formaldehyde, the Mo/ller–Plesset treatment works equally as well. However, we find in general that MP2 level corrections give very accurate energies and additional corrections by higher level terms in the MP series are not substantial. Moreover, we find that for reference states on high energy manifolds degeneracies can result when higher level terms are included in the series. We discuss several ways to remove these degeneracies.

HKrF in solid krypton

A new krypton-containing compound, HKrF, has been prepared in a low-temperature Kr matrix via VUV photolysis of the HF precursor and posterior thermal mobilization of H and F atoms. All three fundamental vibrations have been observed in the FTIR spectra at ∼1950 cm−1 (H–Kr stretch), ∼650 cm−1 (bending), and ∼415 cm−1 (Kr–F stretch). Two distinct sites of HKrF have been identified. The energy difference between the H–Kr stretching vibrations for the two sites is remarkably large (26 cm−1), indicating a strong influence of the environment. In annealing after the photolysis of the precursor, HKrF is formed in two different stages: at 13–16 K from closely trapped H+F pairs and at T>24 K due to more extensive mobility of H and F atoms in the matrix. HKrF in a less stable site decreases at temperatures above 32 K, the other site being stable up to the sublimation temperature of the matrix. The photodecomposition cross section for HKrF has been measured between 193 and 350 nm and compared with the cross sections...

Validity of time-dependent self-consistent-field (TDSCF) approximations for unimolecular dynamics: A test for photodissociation of the Xe-HI cluster

The photodissociation dynamics of a collinear model of the van der Waals cluster Xe–HI is used as a testing ground for time‐dependent self‐consistent field (TDSCF) approximations. In this study, the quantum‐mechanical TDSCF and a combined classical/quantal TDSCF (in which the light atom is treated quantum mechanically, the heavy atoms are treated classically) are compared to numerically exact wave packet calculations. Very good agreement is found between the TDSCF approximations and the exact result over the entire subpicosecond time duration of the process. In particular, all the properties related to the quantal degree of freedom in the combined quantal/classical TDSCF method reproduce almost perfectly the exact results. However, the classical mode in the hybrid approximation is somewhat less well described due to insufficient representation of energy transfer between the modes. The conclusions are very promising as to the applicability of TDSCF methods, in particular the hybrid quantal/classical scheme...

Vibrational spectroscopy and matrix-site geometries of HArF, HKrF, HXeCl, and HXeI in rare-gas solids

The vibrational spectroscopy and the matrix-site geometries of several novel rare-gas compounds in the matrix environment were computed theoretically, and compared with experiment. Ab initio calculations are used in the fitting of analytical potential surfaces for the HRgY molecules and for the interactions between HRgY and the matrix atoms Rg. With these potentials, matrix-site geometries for the molecule in the solid are computed. Finally, the vibrational spectroscopy of HRgY in the Rg matrix is computed using the vibrational self-consistent field (VSCF) method. The VSCF includes anharmonic effects, that are essential in this case. The version of VSCF used here includes coupling between HRgY and the vibrations of the solid atoms. The vibrations of 72 matrix atoms are treated. The main results are: (1) The matrix shifts are considerably greater than typically found for neutral, strongly bond molecules, but are much smaller than discrepancies between theory and experiment. This can be attributed to the in...

Vibrational states and structure of Ar3: The role of three‐body forces

Vibrational energies and eigenfunctions of Ar3, including some pertaining to highly excited states, are computed, and insights into their dynamical and structural properties are obtained. The method used employs the vibrational self‐consistent‐field (SCF) theory in hyperspherical coordinates as a first approximation. Exact results are obtained by configuration interaction, using the SCF states as an efficient basis. A focal point of the study is the effect of three‐body potentials on the vibrational spectrum. Axilrod–Teller and other three‐body potentials are used to examine this. It is found that the effect of three‐body forces on the spectrum is substantial, and larger than effects due to uncertainties in the presently known two‐body Ar–Ar potentials. This suggests that experimental spectroscopy of Ar3 may be used to determine reliable three‐body forces among Ar atoms. It is also shown that the three‐body double‐dipole–quadrupole interaction, while less important than the Axilrod–Teller one, has a significant effect on the vibrational spectrum. Finally, a detailed analysis is made of the Ar–Ar distance distributions in the various states, of the structural distributions of Ar3, and of the properties of the wave functions. We find that the wave functions show well‐ordered nodal patterns even for the highly excited large‐amplitude states. Thus, these states do not correspond qualitatively to ‘‘liquid‐like’’ behavior of the cluster.

Evidence for a cage effect in the UV photolysis of HBr in ArHBr. Theoretical and experimental results

Abstract Evidence for a cage effect in the 193 nm photodissociation of HBr in the ArHBr cluster is found. This effect manifests itself as a tail extending toward lower energies in the hydrogen photofragment kinetic energy distribution (KED). This is a consequence of energy transfer in collisions between the light and the heavy atoms. There is good agreement between the experimental and theoretical KEDs.

Molecular dynamics simulations of reactions in solids: Photodissociation of Cl2 in crystalline Xe

The photodissociation of isolated Cl2 impurities in a Xe crystal was investigated by molecular dynamics simulations. The calculations were carried out for a photodissociation energy of 3.775 eV, and for several temperatures in the range from 10 to 150 K (the melting point is 162 K). The focus was on the physical mechanisms whereby the product atoms exit from the cage, on the properties of the final sites occupied by the Cl atom, and on the temperature dependence of the processs. The main findings were: (1) exit of a Cl atom from the original reagent cage, when it occurs, is always direct upon photodissociation, and does not involve multiple collisions with the surrounding cage walls. This is in qualitative contrast with the dynamics of cage exit in the case of HI photodissociation in Xe at very low temperatures, found in a previous study. (2) The occurrence of product exit from the cage depends entirely on whether the reagent molecule has been oriented at the direction of a transition state for the exit at the instant of photodissociation. (3) The temperature threshold of Cl exit from the cage is 95 K, and essentially coincides with onset of free rotation for the reagent molecules in the host crystal. (4) The temperature dependence of the probability for cage exit is strongly nonmonotonic: The probability increases as T increases from 95 to 110 K, falls off to 0 around 125 K, then increases again as T approaches melting. (5) At the photodissociation energy used, the only site that the Cl atoms occupy in the new cage is the octahedral interstitial site. Various aspects of reaction dynamics in crystalline solids are discussed in the light of the above results and by their comparison with findings of a previous study on photodissociation of HI in Xe.

Ultraviolet spectroscopy of water clusters: Excited electronic states and absorption line shapes of (H2O)n, n=2–6

A semiempirical model is developed, based on ab initio calculations, to provide an analytic representation of excited-state potential energy surfaces for (H2O)n, n=2–6. Using quantum calculations of the ground vibrational states, the UV absorption spectra are computed by a semiclassical approximation, showing a strong blue-shift with extended blue tails relative to the monomer, but with an additional red tail in the case of the dimer absorption band. The nature of the excitonic states is discussed.

Mid-IR spectra of different conformers of phenylalanine in the gas phase

The experimental mid- and far-IR spectra of six conformers of phenylalanine in the gas phase are presented. The experimental spectra are compared to spectra calculated at the B3LYP and at the MP2 level. The differences between B3LYP and MP2 IR spectra are found to be small. The agreement between experiment and theory is generally found to be very good, however strong discrepancies exist when -NH2 out-of-plane vibrations are involved. The relative energies of the minima as well as of some transition states connecting the minima are explored at the CCSD(T) level. Most transition states are found to be less than 2000 cm(-1) above the lowest energy structure. A simple model to describe the observed conformer abundances based on quasi-equilibria near the barriers is presented and it appears to describe the experimental observation reasonably well. In addition, the vibrations of one of the conformers are investigated using the correlation-corrected vibrational self-consistent field method.

Dynamics of molecular photodissociation in clusters: a study of Ar…HCl

Abstract The photodissociation of HCl in the air—HCl cluster has been studied theoretically using a classical trajectory method to explore the effect of a “solvent” atom on the process. The calculated kinetic energy and angular distributions of the photofragments show a “cage” effect, due to collisions between H and the heavy atoms before H leaves the cluster. Also, significant differences are seen depending on whether a classical distribution is used to describe the initial Ar—HCl bending distribution.