Publications

ZEKE spectroscopy is based on delayed detection by pulsed field ionization. It is thereby possible to monitor the time evolution at a given excitation frequency. Moreover, by varying the depth of detection, one can harvest different Rydberg series. The qualitative features expected for such a spectrum are discussed. The quantitative theory required to compute spectra is outlined and applied to the realistic example of Na-2(+). The computed spectrum is found to very accurately exhibit two time scales, just as has been observed, with the shorter decay time being faster for lower Rydberg states. Extensive interseries coupling is noted.

The early quantum dynamics following the B((3) Pi(0u)+) <– X photoexcitation of I-2 in large rare gas clusters is studied and the resonance Raman spectrum of these systems is calculated by a novel time-dependent quantum mechanical simulation approach. The method used is the classically based separable potential (CSP) approximation, in which classical molecular dynamics simulations are used in a first step to determine an effective time-dependent separable potential for each mode, then followed by quantum wavepacket calculations using these potentials. In the simulations for I-2(Ar)(n) and I-2(Xe)(n), with n = 17, 47, all the modes are treated quantum mechanically. The Raman overtone intensities are computed from the multidimensional time-dependent wavepacket for each system, and the results are compared with experimental data on I-2 in Ar matrices and in liquid Xe. The main findings include: (i) Due to wavepacket dephasing effects the Raman spectra are determined well before the iodine atoms hit the rare gas `'wall'' at about 80 fs after photoexcitation. (ii) No recurrencies are found in the correlation functions for I-2(Ar)(n). A very weak recurrence event is found for I-2(Xe)(n). (iii) The simulations for I-2(Ar)(17) (first solvation layer) and for I-2(Ar)(47) (second solvation shell) show differences corresponding to moderate cluster size effects on the Raman spectra. (iv) It is estimated that coupling to the B `' ((1) Pi(1u)) state or to the a (1 g) state have a small effect on the Raman intensities. (v) For I-2(Ar)(47), the results are in very good quantitive agreement with I-2/Ar matrix experiments. The I-2(Xe)(n) results are in qualitative agreement with experiments on I-2 in liquid Xe. The reported calculations represent a first modeling of resonance Raman spectra by quantum dynamical simulations that include all degrees of freedom in large systems, and they demonstrate the power of the CSP method in this respect. (C) 1996 American Institute of Physics.

Reprinted in 'Israel' (2000).

Reprinted in 'Israel' (2000).

The vibrational ground state and the fundamental excited states of (Ar)(13) were studied by vibrational self-consistent field (VSCF) calculations. These calculations treat the interaction between different modes through a mean potential approximation, and incorporate anharmonicity in full. The good accuracy of VSCF for such systems was demonstrated by test calculations for (AT)(3) and other clusters. The study of (Ar)(13) focused on the properties of the wave functions and the excitation energies, on the role of the coupling between the modes and on the deviation from the harmonic approximation. It was found that SCF excitation energies for the fundamental transitions differ from the harmonic values by about 25% for the softest modes, and by about 10% for the stiffest modes. Coupling between the modes, treated by SCF; was found to be much more important than the intrinsic anharmonicity of the individual modes. For the ground state, the harmonic wave function compares well with VSCF, but for the fundamental excited states appreciable differences were found. The results for a potential field expanded to fourth-order polynomial in the normal mode displacements are found to be valid, almost indentical with those for a more elaborate sixth-order polynomial expansion. The fundamental excitation frequencies computed using the Aziz-Slaman Ar-Ar pair potential are very similar, with some quantitative deviations, to the values obtained with a Lennard-Jones potential. The differences are larger for certain specific modes, and very small for the others. These calculations demonstrate the computational power of VSCF as a tool for quantum-mechanical calculations for large clusters, at the level of specific wave functions. (C) 1996 American Institute of Physics.

Vibrational energy levels, wave functions, and ir absorption intensities are computed for (H2O)(n) clusters with n = 2, 3, 4, and 5. The calculations were carried out by the vibrational self-consistent field (VSCF) approximation, with corrections for correlations between the modes by perturbation theory. This correlation corrected VSCF (CC-VSCF) is analogous to the familiar Moller-Plesset method in electronic structure theory. Test calculations indicate that this method is of very good accuracy also for very anharmonic systems. While the method is of highest relative accuracy for the stiffest modes, it works very well also for the soft ones. Some of the main results are (1) the frequencies calculated are in good but incomplete agreement with experimental data available for some of the intramolecular mode excitations. The deviations are attributed to the inaccuracy of the coupling between intramolecular and intermolecular modes for the potential function used. (2) Insight is gained into the pattern of blue- or redshifts from the corresponding harmonic excitation energies for the various modes. (3) Anharmonic coupling between the modes dominates in general over the intrinsic anharmonicity of individual modes in determining the spectrum. (4) The anharmonic corrections to the frequencies of some intermolecular modes (shearing, torsional) are extremely large, and exceed 100% or more in many cases. (5) An approximation of quartic potential field in the normal mode displacement is tested for the clusters. It works well for the high and intermediate frequency modes, but is in error for very soft shearing and torsional modes. (6) The relative errors of the VSCF approximation are found to decrease with the cluster size. This is extremely encouraging for calculations of large clusters, since the VSCF level is computationally simple. (C) 1996 American Institute of Physics.

The time-dependent self-consistent-field (TDSCF) approximation is used to study the photodissociation of the Ar-HCl cluster in a three-dimensional framework. The results are compared with numerically exact quantum calculations, and the properties and accuracy of the TDSCF approach are evaluated on this basis. The TDSCF approximation is used in Jacobi coordinates, and the total wave function is factorized into a wave packet for two coordinates associated with the H atom, and a wave packet for a single coordinate that describes the relative motion of the heavy particles. Quantitative agreement between the TDSCF and the exact results is found for most quantities calculated. The calculations show that photodissociation, and in particular the departure of the H atom is predominantly a direct process, but an appreciable amount of wave packet amplitude moving in excited state resonances is also found. This amplitude seems significantly larger than obtained in recent calculations by Schroder et al. [J. Chem. Phys. 100, 7239 (1994); Chem. Phys. Lett. 235, 316 (1995)]. The validity and computational efficiency of the TDSCF approach for realistic systems of this type is discussed. (C) 1995 American Institute of Physics.

Molecular dynamics simulations are presented showing that four-center reactions with high activation barriers occur readily, and in a concerted manner, under conditions of cluster impact. At the high velocities which prevail inside an impact heated cluster, we show that one can conveniently examine the reactive event as a sequence of elementary, suddenlike steps. The importance of the timing of these steps is emphasized. The bond-switching step is described by a kinematic model, which is shown to account for the energetic and steric requirements and for energy disposal of the concerted four-center reaction. Three different cases are examined in detail: the bimolecular N-2 + O-2 and H-2 + I-2 reactions and the unimolecular norbornadiene –> quadricyclane isomerization. The cluster is shown to have a significant role not only in activating the reactants but, equally important, in stabilization of the energy-rich nascent products. The event is over when the cluster beings to fragment, which occurs after a small number of collisions of its constituents. Comparison is made between clusters of different rare gases. In terms of the overall reactivity, all rare gases are similar and the role of the mass can be compensated by a suitable scaling of the initial velocity of impact. There are, however, differences related to the mass ratio of the reactants to that of the rare-gas atom. The high yield of the four center concerted mechanism in impact heated clusters and the very essential role of the cluster in overcoming the constraints on such a mechanism suggest that the designation `'cluster catalyzed reaction'' is appropriate.

The osteopenia associated with advanced age appears to be a universal phenomenon in humans and animals, but the mechanisms by which it occurs are understood incompletely. However, the explanation must lie in an absolute or relative diminution in the level of osteoblastic bone-forming activity when compared with osteoclastic bone-resorbing activity. The authors postulated that with old age there would be a reduction in the number or function or both of osteoblastic stem cells that could account for part of the diminution in bone formation. They further postulated that there would be either no change or an increase in osteoclastic potential and bone resorption. To test these concepts, bone marrow cells were isolated from 4- to 6-month-old or 24-month-old mice and cultured in vitro under a variety of circumstances that permitted an assessment of the stromal osteogenic cells and marrow hemopoietic progenitor cells belonging to the monocyte and osteoclast series. These data show a marked reduction in the number and in vitro activity of stromal osteogenic cells from old animals. There is an increase in old mice in the number of marrow cells capable of forming osteoclasts in coculture and responsive to the growth factors believed operational in the monocyte and osteoclast series. The authors now are exploring the hypothesis that an age-related diminution in transforming growth factor-beta levels is responsible for these changes in progenitor cell levels in marrow and their functional status as expressed in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)