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Notes: A study is made of the vibrational energy levels and the corresponding oscillation dynamics of the clusters Xe4He2, Xe3He2, and I24He. XeHe2 is a representative of the "three ball'' clusters, while I2He is a prototype of the "stick and ball'' systems. The treatment is based on the vibrational self-consistent field (SCF) method, which introduces an approximate separation of the modes involved. Success of the method depends on an adequate choice of the coordinates that are being mutually separated. We use physical arguments, based on mass ratios and potential function considerations, as well as comparative SCF calculations in different coordinate systems, to determine the appropriate modes for each system. Numerically exact results are also obtained by configuration interaction (CI) calculations using a basis of SCF states. The SCF and CI calculations include all modes and employ realistic potentials. Several states that are both rotationally and vibrationally excited are also calculated. The main conclusions are: (1) Hyperspherical coordinates are the best modes for XeHe2; ellipsoidal coordinates are best for I2He. In each case, the "good modes'' SCF gives energies in remarkable agreement with the exact (CI) ones. (2) XeHe2 resembles a quantum liquid drop: Even in the ground state, it is delocalized over and between the (two) classical equilibrium structures. (3) Structural distributions, rather than rigid geometry, are essential for the description of such floppy clusters. The single-mode SCF wave functions offer a highly accurate description of the structural distributions. (4) There is a sequence of bound, excited rotational states of I2He in which the He precesses around the I2 axis. The amplitude of the I2He bending vibrations are very large (θA〈20°), but none of the bound states involves a full rotational motion around the I2 stick (with angular momentum normal to the axis). The SCF method with the "good coordinates'' proposed here is expected to yield results of similar high accuracy for any cluster of the "three balls'' or "stick and ball'' types.

Notes: Total cross sections for He scattering from isolated imperfections on surfaces are calculated using the Sudden approximation, and in some cases also by a numerically exact, time-dependent quantum-mechanical wave packet method. Systems studied include: CO adsorbates on Pt(111); mono-, di-, and trivacancies on Pt(111). The main results are: (1) the incidence angle and energy dependence of the cross section for He/[Pt(111)+CO] are very sensitive to the CO distance from the Pt plane. Interactions with the adsorbate image have little effect on the cross section. (2) The cross sections for clusters of vacancies are given within 10% or better, by the geometric sum of the monovacancy cross sections, the latter being treated as circles centered at each monovacancy. (3) The dependence of the cross section on the energy is sufficiently sensitive to distinguish between the "electron density hole'' and "electron density hump'' models for vacancies and vacancy clusters. (4) The Sudden approximation compares well with the exact quantum-mechanical results at typical experimental energies, when the incidence angle is not too far from the normal. These results indicate that experimental measurements of He scattering cross sections as a function of energy and incidence angle, combined with Sudden or wave packet scattering calculations, can provide detailed information on surface defects and their interactions with gas-phase atoms.

Notes: The vibrational predissociation dynamics of a collinear model of the I2(v)He cluster is studied by numerically exact time-dependent quantum mechanics, and by the time-dependent self-consistent field (TDSCF) approximation. The time evolution for the initial excitation levels v=5, 11, 22 is explored. Excellent agreement is found between the TDSCF and the exact evolution of the wave packet; in particular the approximation reproduces well the dephasing events in the dynamics, and the measurable predissociation lifetimes. The results are very encouraging as to the applicability of quantum TDSCF as a quantitative tool in the study of van der Waals predissociation dynamics.

Notes: We used a randomly corrugated hard wall model and the sudden approximation to analyze two experiments on atom scattering from disordered surfaces. In one, the structural surface disorder was caused by ion bombardment. In the other, the disorder was due to an incomplete overlayer of adsorbed atoms. We also present a study of the scattering of a rigid rotor by a randomly corrugated hard wall using the sudden approximation.

Notes: 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.

Notes: The photodissociation of HI impurities in a crystalline Xe host is studied by molecular dynamics simulations. From the calculated trajectories, analyses are given for: The behavior of the HI molecule before dissociation, the motions of the fragments following photon absorption, and the sites and vibration dynamics of the H fragments long after dissociation. The main findings include: (i) The photodissociation yield as a function of temperature is not monotonic (0 at 0 K, 0.2 at 17 K, 0.1 at 35 K). (ii) The nascent H atoms, at early time (∼0.5 ps), exhibit well defined, high frequency (∼900 cm−1) vibrations in the cage. The H trajectories acquire increasingly a random walk character with the progress of time. (iii) H exit from the cage is virtually never direct. (iv) After relaxation to equilibrium, the product H atoms occupy dilated interstitial sites (nearest-neighbor xenons displace by 0.3 and 0.6 A(ring) at the octahedral and tetrahedral interstitial sites, respectively). (v) The H atom dynamically distorts the octahedral site and exhibits three well-separated local vibrational frequencies, corresponding to motions along well-defined axes of the site. (vi) The reagent HI molecular rotations are strongly hindered at low temperatures, and are more aptly described as large amplitude bendings associated with the complex Xe⋅⋅⋅HI. The experimental implications of the above findings and the possible consequences of quantum effects are discussed.

Notes: A study has been made of the vibrational energy flow mechanisms and time scales pertaining to the overtone stretch excitations of methyl and acetylenic CH stretches in propyne. Classical trajectories are used to interpret the experimental data for the overtone linewidths, as well as to analyze the role that individual modes play in determining energy flow. The full anharmonic potential surface for these calculations, including all modes, has been developed from spectroscopic and structural information, including the linewidth data. The principal results are: (1) The trajectory calculations show a localization transition, corresponding to a switch over from normal-mode behavior for CH3 excitations up to v≅3 to a local-mode CH excitation within the CH3 moiety for excitations of v(approximately-greater-than)6, with transition behavior for v=4,5. (2) The acetylenic CH shows local-mode behavior from v=1. Extremely long lifetimes are found for the excitations of this mode, and the trajectories indicate that the experimental width is predominantly rotational. (3) The rocking and deformation modes are dominant receiving modes in the relaxation of the methyl stretch. (4) A shorter lifetime is calculated for the v=6 vs the v=5 or v=7 overtones of the methyl C–H stretch. Experimental results are qualitatively consistent with this prediction. The origin of this shorter lifetime is a band of resonances between the stretch excitation and combinations of rocking, deformation, and pseudorotation modes. (5) CH3 internal rotation figures importantly in the relaxation of some levels (v=5, 8 of CH3) where it "closes the energy gap'' for achieving resonant energy transfer. (6) For v=8 of the methyl CH, some direct energy transfer to both C–C(Triple Bond)C stretching modes is seen. The switching on of the stretches as receiving modes is a consequence of sufficiently strong interactions between the excited H and the C–C(Triple Bond)C chain, which take place at these high vibrational energies. (7) Evidence is found for long distance "through-space'' energy transfer due to long-range dipole–dipole forces. This transfer occurs from the acetylenic to the methyl CH stretches. This result is illustrated for the v=2 excitation of the acetylenichydrogen, and constitutes a direct demonstration of intramolecular long-distance, through-space v–v energy transfer. These results demonstrate the potential importance of large amplitude modes such as rocking and deformation as initial receiving modes for vibrational energy from excited CH overtones. On the time scale probed here (∼1 ps), despite the availability of many degrees of freedom, the transfer process is dominated by specific energy transfer channels and by the specific behavior of individual modes, rather than by statistical considerations, which will certainly prevail on longer time scales.

Notes: Exact quantum mechanical and quasiclassical trajectory results for rotationally inelastic N2 -corrugated surface collisions are compared over the energy range 0.01–0.04 eV. It is found that the degeneracy averaged, diffraction summed, rotationally inelastic transition probabilities display quantum oscillations.