bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical studies on the femtosecond real-time measurement of ultrafast electronic decay in polyatomic molecules
Authors: Stock G. , Schneider R. , Domcke W.
Published: July 2002

We present a computer simulation of the real-time detection of ultrafast electronic decay dynamics in polyatomic molecules with femtosecond laser pulses. The intramolecular non-Born-Oppenheimer quantum dynamics is treated numerically exactly for a two-state three-mode vibronic coupling model representing the conically intersecting S1 and S2 excited states of pyrazine. The pump–probe signal is evaluated in lowest order perturbation theory with respect to the radiation–matter interaction by numerical integration over the pump and probe pulses. We discuss in some detail the dependence of the pump–probe signal on the properties of the laser pulses (frequencies and pulse durations). The calculations predict a dramatic (∼12 000 cm−1) and ultrafast (∼20 fs) red shift of the stimulated-emission signal as well as distinctive quantum beats in the pump–probe signal as a function of the delay time. Both effects are very pronounced and should therefore be relatively easily detectable experimentally. They are expected to be generic features of ultrafast internal-conversion processes in polyatomic molecules.
The 157 nm photodissociation of OCS
Authors: Strauss C. E. , McBane G. C. , Houston P. L. , Burak I. , Hepburn J. W.
Published: July 2002

The photodissociation of OCS at 157 nm has been investigated by using tunable vacuum ultraviolet radiation to probe the CO and S photoproducts. Sulfur is produced almost entirely in the 1S state, while CO is produced in its ground electronic state and in vibrational levels from v=0–3 in the approximate ratio (v=0):(v=1):(v=2):(v=3)=(1.0):(1.0):(0.5) :(0.3). The rotational distribution for each vibrational level is found to be near Boltzmann, with temperatures that decrease from 1350 K for v=0 to 780 K for v=3. Measurements of the CO Doppler profiles demonstrate that the dissociation takes place from a transition of predominantly parallel character (β=1.8±0.2) and that the CO velocity and angular momentum vectors are perpendicular to one another.
Intracluster reactions in phenylacetylene ammonia clusters initiated through resonant enhanced ionization
Authors: Breen J. J. , Tzeng W.-B. , Kilgore K. , Keesee R. G. , Castleman A. W.
Published: July 2002

Protonated ammonia clusters of specific size are observed to form following the resonant enhanced ionization of phenylacetylene–ammonia clusters PA⋅(NH3)n in the 000 region of the S1–S0 transition of the (unclustered) PA. The resonance enhanced absorption of two photons, ≊8.8 eV total energy, are found to cause intracluster reactions for n≥5. The findings imply that the adiabatic ionization potential of (NH3)4 is greater than 8.8 eV, while that of (NH3)5 is (slightly) less.
Branching ratios in the N+CH3 reaction: Formation of the methylene amidogen (H2CN) radical
Authors: Marston G. , Nesbitt F. L. , Stief L. J.
Published: July 2002

The branching ratios for the reaction N+CH3 →Products, have been determined in a discharge-flow system coupled with mass-spectrometric detection of both reactants and products. The major products are H2 CN+H, with about 10% of the reaction proceeding to give HCN+H2 . Experiments carried out on the reaction of N atoms with the deuterated methyl radical showed that the branching ratio for formation of D2 CN+D is about 0.9 and for DCN+D2 formation about 0.1 independent of T from 200 to 363 K. The results are consistent with the energetics and orbital symmetry properties of the reactant and product molecules. Implications for the atmosphere of Titan are discussed.
Basis set and polarization function effects on optimized geometries and harmonic frequencies at the second-order Mo/ller–Plesset perturbation level
Authors: Guo Hong , Karplus Martin
Published: July 2002

Basis set and polarization function effects on the ground state optimized geometries and harmonic frequencies at the second-order Mo/ller–Plesset perturbation (MP2) level have been studied for 11 small molecules (one or two heavy atoms) containing one or two carbon atoms, as well as for propene, propane, isobutene, acetaldehyde, methyl ether, cis- and trans-1,2-difuoroethylene. A series of basis sets ranging in quality from 4-21G to 6-311G** have been used for the small systems; for the larger systems 6-31G and 6-31G* basis sets were compared. In addition, three modified 6-31G basiss sets in which d basis functions are added to certain (but not all) heavy atoms were introduced to study the effect of polarization functions in systems containing heteroatoms. It was found that the inclusion of d functions in basis sets is important for calculating the equilibrium geometries, especially for CC and CX (X=N, O, and F) bonds. For vibrational frequencies, however, addition of d functions to basis sets often does not produce a significant improvement; for many alkenes and alkanes MP2/6-31G (MP2 with a 6-31G basis set) and MP2/4-21G calculations give good results for the frequencies that are comparable to those obtained with MP2/6-31G* and MP2/4-21G*, respectively. For molecules containing heteroatoms, the MP2/6-31G (MP2/4-21G) frequencies are generally rather close to the MP2/6-31G* (MP2/4-21G*) results except for the vibrations involving CX or XH stretching, for which the MP2/6-31G(MP2/4-21G) values are usually too low. Such deficiencies can be removed by addition of d basis functions to one of the atoms involved in CX or XH bonding. It is suggested that such basis sets with limited polarization functions can be usefully applied to larger molecules. Some experimental frequencies which are not consistent with the ab initio values are discussed and reassignments are proposed.
Photoabsorption and direct dissociation cross sections of C2H2 in the 1530–1930 Å region: A temperature dependent study
Authors: Wu C. Y. Robert , Chien T. S. , Liu G. S. , Judge D. L. , Caldwell J. J.
Published: July 2002

Using synchrotron radiation as a continuum light source, the absolute photoabsorption and direct dissociation cross sections of C2 H2 in the 1530–1930 Å region have been measured at 295 and 155 K. In comparison with cross sections obtained at room temperature, the cross sections at absorption peaks typically increase by 10%–40% while those at absorption valleys decrease by as much as 30% at 155 K. Further, the absorption band profiles become narrower as the gas temperature decreases. In the presently studied wavelength region, there are at least three electronic states, namely, the Ã 1 Au , the B̃ 1 Bu , and a continuum. Using the present low temperature data we have constructed the repulsive potential energy curve of the direct dissociation state according to the Franck–Condon principle. The potential energy curve thus constructed can be more accurate than that constructed by using room temperature cross section data because only the absorption cross section from the ν′=0 level contributes. The present cross section data have also been used in the analysis of oscillator strength distribution.
The vibrational frequencies of TiFnCl4−n, n=0,4
Authors: Bauschlicher Charles W. , Taylor Peter R. , Komornicki Andrew
Published: July 2002

Vibrational frequencies have been computed for TiFnCl4−n, for n=0,4 at the SCF level. The computed frequencies are in good agreement with the known experimental values for TiF4 and TiCl4. The calculations suggest that the band at 772 cm−1 observed by DeVore and Gallaher is due to the Ti–F stretching mode of TiFCl3.
Linear versus cyclic (HCN)3: An a
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t
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o study on structure, vibrational spectra, and infrared intensities
Authors: Kurnig Ingrid J. , Lischka Hans , Karpfen Alfred
Published: July 2002

Equilibrium structures and several other ground state properties of linear and cyclic (HCN)3 clusters were investigated with the aid of ab initio methods applying extended basis sets and including electron correlation. Computed equilibrium structures, stabilization energies, dipole moment, vibrational spectra and infrared intensities are compared with available experimental data. Corresponding results are also presented for the HCN monomer and the linear (HCN)2 dimer. Modifications of monomer properties induced by cluster formation are monitored and discussed. In agreement with our previous self-consistent field studies and with a recent vapor phase spectroscopic experiment, both the linear and the cyclic trimer are true minima on the energy surface and are almost equally stable.
Transition structures for the interchange of hydrogen atoms within the water dimer
Authors: Smith Brian J. , Swanton David J. , Pople John A. , Schaefer Henry F. , Radom Leo
Published: July 2002

High levels of ab initio molecular orbital theory were used to examine rearrangement processes in the water dimer corresponding to the interchange of various hydrogen atoms. Our most reliable calculations involve MP4/6-311+G(2df,2p) energy evaluations at MP2/6-311+G(d,p) optimized structures. The lowest energy rearrangement pathway corresponds to the interchange of hydrogen atoms of the acceptor molecule within the Cs water dimer structure (1). This proceeds via a transition structure of C1 symmetry (2) and requires an energy of 0.59 kcal mol−1. The interchange of donor and acceptor molecules can be achieved via a transition structure with Ci symmetry (4) and requires an energy of 0.87 kcal mol−1. Finally, the interchange of hydrogen atoms of the donor molecule, via a C2v transition structure (9), requires 1.88 kcal mol−1. The rearrangements via 2 and 4 lead to complete scrambling of hydrogen atoms within the individual H2O moieties at a cost of 0.87 kcal mol−1; the transition structure 9 is not necessary for this process. The implications of these results with regard to the interpretation of spectroscopic data on the water dimer will be of interest.
Laser vaporization generation of Al12C, Al13C, Al12C2, and Al13C2 for rare gas matrix electron spin resonance studies: Experimental–theoretical comparisons
Authors: Knight Lon B. , Cobranchi Scott T. , Herlong John O. , Arrington C. A.
Published: July 2002

The metal carbide radicals AlC and AlC2 have been generated by the laser vaporization of aluminum carbide and trapped in neon and argon matrices at 4 K for electron spin resonance (ESR) characterization. These results provide the first experimental evidence showing that AlC has a 4Σ ground electronic state and that AlC2 is X 2A1. Ab initio theoretical calculations were conducted for the geometries and various nuclear hyperfine parameters in both radicals which yielded A values in reasonable agreement with the observed. In AlC, the three unpaired electrons reside primarily on carbon with the following neon matrix magnetic parameters (MHz): g∥=2.000(1); g⊥=2.0010(5); ‖A⊥(Al)‖=33.2(5); ‖A∥(Al)‖=3(3); A⊥(13C)=52.1(5); A∥(13C)=52(2); and D(zero field splitting)=374(1). For AlC2, the spin density resides predominantly in an aluminum 3pz/3s hybrid directed away from C2. The neon magnetic parameters (MHz) are: g∥=2.0005(5); g⊥=1.9965(3); A⊥(Al)=941.5(5); A∥(Al)=1067(1); ‖A∥(13C)‖=59(1); and ‖A⊥(13C)‖=52(1).
Experimental and reduced dimensionality quantum rate coefficients for H2(D2)+CN→H(D)CN+H(D)
Authors: Sun Q. , Yang D. L. , Wang N. S. , Bowman J. M. , Lin M. C.
Published: July 2002

New experimental rate coefficients are reported for the H2(D2)+CN→H(D)CN+H(D) reactions over the temperature range 209 to 740 K for H2 and 250 to 740 K for D2. Previous reduced dimensionality reaction probabilities for the reaction with H2, and new ones for the reaction with D2 are used to obtain analogous rate coefficients. In addition, reaction probabilities and rate coefficients for vibrationally excited reactants H2(v=1), D2(v=1), or CN(v=1) are presented. Comparisons of the calculated rate coefficients are made with the new and previous experiments, especially those of Sims and Smith [Chem. Phys. Lett. 149, 565 (1988)].
Density functional theory for freezing of a binary hard sphere liquid
Authors: Zeng X. C. , Oxtoby David W.
Published: July 2002

Our recently proposed effective liquid free energy model (ELFEM) density functional theory is extended to nonuniform binary systems. The theory is based on mapping the excess free energy of a nonuniform binary system onto that of an effective binary liquid. In the uniform density limit, the theory is required to reproduce the known properties of the liquid. The present theory is applied to the freezing of a binary hard sphere liquid into a substitutionally disordered fcc solid. Comparisons with previous density functional investigations and recent computer simulations are made.
Molecular orientation control for ‖J
K
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J〉 —selected CH3I via influence of electric field on nuclear hyperfine coupling
Authors: Gandhi Suketu R. , Bernstein Richard B.
Published: July 2002

Using the polarized laser photofragment photoionization technique, measurements have been made of the degree of orientation of CH3I in ‖JKMJ〉=‖111〉 and ‖222〉 parent rotational states under the influence of homogeneous electric fields (E) from 0 to 1.0 kV cm−1. From a series of experiments on hexapole-oriented molecules in weak fields, it has been found that the original degree of orientation of the symmetric-top molecules can be recovered after they pass through a homogeneous weak-field region provided that the field strength therein exceeds some minimum value (E≳0.3 V cm−1), sufficient to maintain an orientation axis. Even though the ‖JKMJ〉 parent states have ‘‘relaxed’’ via hyperfine coupling to an ensemble of ‖FJKIMF〉 states, when the molecules later pass into a strong E field, the ensemble is restored to its original degree of orientation. Another set of experiments in moderate to strong fields provided ‘‘saturation curves,’’ i.e., the dependence of the degree of orientation upon E. The results show that quite high field strengths (>0.5 kV cm−1) are required to ensure total decoupling of J from I and thus recovery of the full orientation. From these experiments it is now clear that one can control the degree of molecular orientation by placing the state-selected molecules in a know E field.
Hydrodynamic transport coefficients of random dispersions of hard spheres
Authors: Ladd Anthony J. C.
Published: July 2002

Accurate values for the hydrodynamic transport properties of random dispersions of hard spheres have been determined by numerical simulation. The many-body hydrodynamic interactions are calculated from a multipole-moment expansion of the force density on the surface of the solid particles; the singular lubrication forces are included exactly for pairs of particles near contact. It has been possible to calculate the transport properties of small periodic systems, at all packing fractions, with uncertainties of less than 1%; but for larger systems we are limited computationally to lower order, and therefore less accurate, moment approximations to the induced force density. Nevertheless, since the higher-order moment contributions are short range they are essentially independent of system size and we can use small system data to correct our results for larger systems. Numerical calculations show that this is a reliable and accurate procedure. The ensemble-averaged mobility tensors are strongly dependent on system size, with deviations from the thermodynamic limit varying as N−1/3. We show that these deviations can be accounted for by a straightforward calculation based on the length of the unit cell and the suspension viscosity and mobility. The remaining number dependencies are small. Problems involved in implementing this method for larger numbers of particles are considered, and alternative methods are discussed.
The dissociative adsorption of hydrogen: Two-, three-, and four-dimensional quantum simulations
Authors: Nielsen Ulrik , Halstead David , Holloway Stephen , No/rskov J. K.
Published: July 2002

A quantum wave packet calculation for the activated dissociative adsorption of H2 is presented. Restricting the motion of the molecule to lie within a plane normal to the surface we have treated all four molecular degrees of freedom exactly. We compare results obtained using two-, three-, and four-dimensional simulations on the same potential and show that by restricting the molecular orientation, important dynamical effects are lost. The potential employed in the calculations has been obtained using the effective medium approximation. In the simulations it has been possible to treat dissociation, rotations and diffraction on an equal footing. By including a rotational degree of freedom, it is seen that strong orientational effects occur near to the transition state and result in an anisotropic selectivity in the dissociation. By examining the state-to-state scattering probabilities, it is possible to use the nonreacting (scattered) fraction to provide information on the reactive potential energy surface.
Approximate constants of motion for vibrational spectra of many-oscillator systems with multiple anharmonic resonances
Authors: Kellman Michael E.
Published: July 2002

A theory of approximate dynamical constants of motion is presented for vibrational (and implicitly, rovibrational) spectra of polyatomics with multiple nonlinear resonances. The formalism is developed in terms of simple vector algebra. The theory is applied to Hamiltonians used in fits of experimental spectra of H2O, CHClF2, and acetylene, with attention to reduced dimension motion, assignability of spectra, and statistical analysis of chaotic spectra. The approximate constants may be of interest as bottlenecks to vibrational energy flow in polyatomics. They may also be useful in reducing the size of basis sets in quantum-mechanical calculations of rotation–vibration spectra.
When does a pair correlation function fix the state of an equilibrium system?
Authors: Zwicker Judith , Lovett Ronald
Published: July 2002

The classical demonstrations that a unique single particle external field is associated with each equilibrium single particle density field (Hohenberg and Kohn, Mermin) are reinterpreted in the language of functional Legendre transformations. This picture is readily extended to the pair distribution function problem and the extension offers a context for understanding how the singlet and pair number densities fix the state of a system. It is shown that one can be sure that there are closure relations to integral equations in general and that in principle the correct closure relation fixes not only the distribution functions but also the complete thermodynamic state of a system. It also follows that a correctly closed integral equation possesses a unique solution. Integral equations for the radial distribution function alone, however, are typically produced by projecting out the singlet density field and for this reason they provide an incomplete characterization of the system. The failure to specify a unique state leads to the existence of multiple solutions.
An a
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o calculation of the intramolecular stretching spectra for the HF dimer and its D-substituted isotopic species
Authors: Jensen Per , Bunker P. R. , Karpfen Alfred , Kofranek Manfred , Lischka Hans
Published: July 2002

We have carried out an ab initio calculation of the intramolecular stretching spectra (wave numbers and transition moments) of (HF)2, (DF)2, and HFDF involving v1+v2≤3, where v1 and v2 are the local mode quantum numbers for the two intramolecular (HF or DF) stretches. The ab initio surface used as a basis for these calculations has already been published [M. Kofranek, H. Lischka, and A. Karpfen, Chem. Phys. 121, 137 (1988); P. R. Bunker, P. Jensen, A. Karpfen, M. Kofranek, and H. Lischka, J. Chem. Phys. 92, 7432 (1990)], but in the present work we have extended the 1520 nuclear geometry points previously available with 198 points in order to explore further the variation of the intramolecular stretching energies and the dipole moment along the minimum energy (trans tunneling) path. We compute the intramolecular stretching energies and transition moments by making an adiabatic separation of the intramolecular stretching motion and the other vibrational motions of the molecules, and we use the semirigid bender Hamiltonian to average over the trans-tunneling motion. For HFHF, we obtain the fundamental level corresponding to the ‘‘free-H’’ stretch ν1 at 3925 cm−1 and that corresponding to the ‘‘bound-H’’ stretch ν2 at 3874 cm−1, in very good agreement with the experimental results of 3930.9 and 3868.1 cm−1, respectively [A. S. Pine, W. J. Lafferty, and B. J. Howard, J. Chem. Phys. 81, 2939 (1984)]. For the higher excited states, we obtain the 2ν1 energy level at 7674 cm−1 (7700±20 cm−1), 2ν2 at 7570 cm−1 (7555±15 cm−1), 3ν1 at 11 259 cm−1 (11 260 cm−1), and 3ν2 at 11 085 cm−1 (11 060  cm−1), where the experimental values [K. von Puttkamer and M. Quack, Chem. Phys. 139, 31 (1989)] are given in parentheses.
State resolved photofragmentation of Ni(CO)4 at 193, 248, and 308 nm: A detailed study of the photodissociation dynamics
Authors: Schlenker F. J. , Bouchard F. , Waller I. M. , Hepburn J. W.
Published: July 2002

The vibrational, rotational, and translational energy distributions for the CO photofragments from the 193, 248, and 308 nm photolysis of Ni(CO)4 in a supersonic molecular beam have been determined by vacuum ultraviolet laser-induced fluorescence. The measured product energy distributions appeared to be statistical, with equilibrium between the degrees of freedom investigated. The distributions were significantly colder than those calculated with a microcanonical statistical model using published bond energies. To model the measured distributions, it was necessary to postulate that the unsaturated nickel carbonyl products are formed in a stable electronically excited state. By using an excited state energy consistent with published fluorescence experiments, excellent agreement was obtained between the measured distributions and those calculated using a microcanonical statistical model. These results indicate that for 193 nm photolysis, essentially all of the Ni(CO)n products are electronically excited, with about 2.8 eV of electronic excitation. The Ni(CO)n products from 248 nm photolysis are formed in both the ground and excited states, with a 3:1 branching ratio. The data taken at 308 nm also indicate the presence of both channels, with the excited state channel still important. These results are consistent with predictions of earlier ab initio work, although the detailed mechanism is somewhat different. This earlier work is discussed in light of the present results.
Solvation effects on molecular pure radiative lifetime and absorption oscillator strength in clusters
Authors: Penner Abraham , Amirav Aviv , Jortner Joshua , Nitzan Abraham , Gersten Joel
Published: July 2002

The solvation effects on the molecular pure radiative lifetime, its absorption line shape, oscillator strength, and spectral red shift are studied for 9, 10-dichloranthracene embedded in clusters of argon, krypton, and xenon. The clusters are synthesized in a supersonic free-jet coexpansion of the organic molecule with the rare gas. Cluster size is controlled by the nozzle backing pressure and its pure radiative lifetime is obtained by measuring both the fluorescence lifetime and the absolute emission quantum yield. For small (jet atom) clusters the pure radiative lifetime is increased by subsequently adding rare gas atoms. For up to two rare gas atoms this increase correlates with the atomic polarizabilities and with the spectral red shift. For Ar and Kr this trend of increasing radiative lifetime continues up to clusters of six rare gas atoms. These results are in good agreement with calculations based on classical electromagnetic theory using point polarizable dipoles for the atom and a simplified charge distribution for the molecule. For large clusters of Ar (up to ∼1000 atoms) both the spectral red shift and the lifetime become cluster size independent. This is also in agreement with classical electromagnetic theory using a model of a point (molecular) dipole embedded in a dielectric (rare gas) sphere. Both experiment and theory indicate that the radiative lifetime in this limit is still larger than that of the free molecule, which in itself is longer than that expected in the bulk solvent. This implies that a further cluster size evolution of this quantity is expected upon increasing the cluster size above the radiation wavelength. We also report on a sudden lineshape broadening and a slight spectral blue shift that accompanies the growth of large molecule–argon clusters and which we interpret as originating from a possible transition towards the surface of the molecule in order to minimize growth strain.
Mixed cluster ions as a structure probe: Experimental evidence for clathrate structure of (H2O)2H+ and (H2O)21H+
Authors: Wei S. , Shi Z. , Castleman A. W.
Published: July 2002

First direct experimental evidence for clathrate structures of (H2O)nH+ (n=20,21) is reported based on a technique allowing the number of nonhydrogen-bonded surface hydrogens to be counted. Neutral clusters (H2O)n⋅((CH3)3N)m, prepared in a pulsed nozzle supersonic expansion, are ionized by multiphoton ionization and investigated with a reflectron time-of-flight mass spectrometry technique. The magic numbers (n,m) in the ion intensity distributions of (H2O)n⋅((CH3)3N)m⋅H+ studied under various experimental conditions are well correlated to the stable hydrogen-bonding structures. For the mixed cluster ion (H2O)20⋅((CH3)3N)m⋅H+, the intensity distribution displays an abrupt intensity drop after the magic number at (20,11), while for (H2O)21⋅((CH3)3N)m⋅H+ the magic number appears at (21,10). The findings provide experimental evidence for a stable clathrate structure of (H2O)20H+, with the proton residing on the surface, while for (H2O)21H+, the H3O+ ion is encaged inside the clathrate structure of (H2O)20; the latter structure provides a total of 10 hydrogen-bonding sites for (CH3)3N.
Ground state calculations of di-π-cyclooctatetraene cerium
Authors: Dolg Michael , Fulde Peter , Küchle Wolfgang , Neumann Carl-Stefan , Stoll Hermann
Published: July 2002

Quantum chemical calculations are presented which predict that in the ground state of di-π-cyclooctatetraene cerium (cerocene) the Ce ion is almost entirely in a 4f1 configuration corresponding to Ce3⊕(C8H1.5■8)2. The 4f electron forms with an electron of the ligand e2u highest-occupied molecular orbital a 4f1e32u singlet in close analogy to a Kondo ion in a metal. Due to coupling of the 4f1e32u with the 4f0e42u configuration, the latter corresponding to Ce4⊕(C8H2≤8)2, the splitting between the ground state singlet and the first excited triplet is of the order 0.5 eV. The self-consistent-field and multiconfiguration self-consistent-field parts of the calculations are done by employing recently developed pseudopotentials for cerium using basis sets of up to 626 basis functions. The correlation energy is accounted for by means of various correlation-energy density functionals and also by limited coupled electron-pair approximation calculations. Similar results are found in both cases.
State-resolved collision energy dependence of Penning ionization cross sections for N2 and CO2 by He*23S
Authors: Ohno Koichi , Takami Tomohide , Mitsuke Koichiro , Ishida Toshimasa
Published: July 2002

The state-resolved collision-energy dependences of Penning ionization cross sections σ(E) were measured in an energy range (60<E<400 meV) for N2 and CO2 with He*23S by using a very high-intensity He* beam (1.8×1015 s−1 sr−1) and detecting energy-analyzed electrons as functions of time-of-flight of He*. The partial ionization cross sections for Π states (N+2B:A2Πu, CO+2B:X2Πg, A2Πu) were observed to increase more rapidly with the increase of the collision energy than those for Σ states (N+2B:X2Σ+g, B2Σ+u, CO+2B:B2Σ+u,C2Σ+g). In the studied energy range, the repulsive walls for end-on collisions were indicated to be harder than those for side-on collisions. The directional peculiarity of the potential surfaces was related to the anisotropy in the hybridization of He* orbitals interacting with the target molecules.
Desorption and molecular interactions on surfaces: CO/Cu(001) and Cu(011)
Authors: Peterson L. D. , Kevan S. D.
Published: July 2002

We report time-resolved electron energy loss spectroscopy results for the desorption of CO from Cu(001) and Cu(011). The measurements were performed isothermally and effectively at constant coverage. We present an analysis based on a quasiequilibrium model that expresses the pseudo-first-order desorption rate constant in terms of a virial expansion. The results of this analysis demonstrate the existence of small, long-range attractive interactions as well as the expected short-range repulsive interactions between adsorbed molecules. The interplay between these two types of interactions leads to a kinetic compensation effect that can be easily understood in terms of vanishing virial coefficients. Thus, in these simple systems, the origin of the kinetic compensation effect lies in the statistical thermodynamics of the adsorbed phase, while the dynamical events associated with the desorption event apparently play a lesser role. The relationship of this analysis technique to others is investigated, and its generality is emphasized.
Crossed beam studies of state-to-state vibrational energy transfer from the v=5 excited state of I2(X 1Σ+g) prepared by simulated emission pumping
Authors: Ma Zhiliu , Jons Steven D. , Giese Clayton F. , Gentry W. Ronald
Published: July 2002

Cross sections were measured as a function of kinetic energy E, for vibrationally inelastic collisions of He with I2(X 1∑+g) prepared in the v=5 excited state by stimulated emission pumping. The results for the transitions v=5→4 and v=5→6 are compared with those obtained previously for v=0→1 excitation of the ground state. Near threshold, the v=5→6 cross section displays a linear energy dependence, with a slope approximately 6 times that for v=0→1. The v=5→4 cross section has a finite intercept at E=0, and a slope approximately 5 times that for v=0→1. The scaling of the vibrationally inelastic cross sections with kinetic energy and quantum number can be understood semiquantitatively in terms of relationships derived from the forced harmonic oscillator model and microscopic reversibility.
Adiabatic potentials for alkali–inert gas systems in the ground state
Authors: Patil S. H.
Published: July 2002

We have developed perturbative expressions for the adiabatic potential for alkali–inert gas systems. The predictions of the model for the equilibrium separation Rm and the minimum potential εm are in good agreement with the experimental values. The model provides a unified approach to the interaction potential which indicates a conformal structure for some groups, implying a useful relation between Rm, εm, and van der Waals constant C6.
Parametrization of the trajectory calculations on ion-quadrupolar molecule collision rate constants
Authors: Bhowmik Pradip K. , Su Timothy
Published: July 2002

A trajectory treatment is performed to calculate collision rate constants for ion-quadrupolar molecule interactions. The results are parameterized into two equations for the odd and even cases. Rate constants for the odd case are slightly larger than those for the even case.
Photodissociation of CO−3⋅H2O: Observation of the O−⋅H2O+CO2 product channel
Authors: Roehl Coleen M. , Snodgrass Joseph T. , Deakyne Carol A. , Bowers Michael T.
Published: July 2002

A new product channel, O−⋅H2O+CO2 was discovered in the photodissociation of CO−3⋅H2O. Experiments were conducted by crossing a mass selected 8 kV ion beam with a linearly polarized laser beam and measuring the kinetic energy distributions of the charged photodissociation products. Distributions were collected at photon energies of 2.41, 2.54, and 2.71 eV. By varying the angle between the ion and the laser polarization vector, product angular distributions were obtained. Relative photodissociation cross sections of this process were determined by comparisons with known processes: a) CO−3⋅H2O+hν→CO−3+H2O and b) CO−3+hν→O−+CO2. Ab initio calculations focusing on geometric structure and vibrational frequencies of the ground state of O−⋅H2O were performed using GAUSSIAN 82 and GAUSSIAN 88 programs. With both MP2/6-31G** and MP2/6-31+G* basis sets, the lowest energy structure obtained was quasilinear with an O−⋅⋅⋅H–O angle of 3°. Vibrational frequencies and rotational constants obtained from these calculations were employed in statistical phase space modeling. Phase space theory indicated a photoinduced ‘‘intracluster’’ reaction mechanism was responsible for this novel product channel.
Inelastic scattering of NO from Ag(111): Internal state, angle, and velocity resolved measurements
Authors: Rettner C. T. , Kimman J. , Auerbach D. J.
Published: July 2002

We have determined the velocity distributions of individual quantum states of NO scattering from Ag(111) at specific scattering angles θf using molecular beam techniques to control the incidence energy Ei and angle θi. We find that the mean energies of scattered species Ef depend weakly on θf at low collision energies, but become increasingly independent of this parameter as Ei approaches 1.0 eV. This is true for all final rotation states J. The previously reported insensitivity of the final kinetic energy to J is found to apply at all scattering angles, so that Ef vs θf curves for high J fall only slightly below those for low J. This system is highly translationally inelastic at high incidence energies, with up to 55% of Ei being lost to phonons at Ei=1.0 eV. Angular distributions are relatively insensitive to J at low Ei, but for high Ei the peak flux is found to shift away from the surface normal as Ei increases. The effect of the surface temperature only becomes apparent at low incidence energies. A search for supernumerary rotational rainbows reveals no discernible oscillations even for the lowest surface temperatures. We believe that these supernumerary oscillations may be damped by ‘‘surface corrugation’’ effects for this system. Discussion focuses on the observed anticorrelation between kinetic energy transfer to phonons and to rotation, the extent to which parallel momentum is conserved in this system, and energy-angle scaling laws for energy transfer. In this latter case we show that energy transfer in this system scales approximately with the quantity Ei cos θi, over the full range of conditions covered.
Inelastic scattering of rotationally excited glyoxal by H2 at E=80 meV
Authors: Kroes G.-J. , Rettschnick R. P. H.
Published: July 2002

Results of quantum calculations on rotationally and vibrationally inelastic scattering of rotationally excited 1Au(S1) trans-glyoxal (in collisions with H2, at a collision energy of 80 meV) are reported. These results are used to assess the influence of sequential collisions on a recent experiment on glyoxal +H2. For ‖ΔK‖>3, the cross sections for rotationally inelastic scattering σ(00,K=k→00,K′=K+ΔK) depend exponentially on (ΔK)2 (and not on the energy difference between the initial and final states), which shows that the rotationally inelastic scattering is dominated by angular momentum transfer rather than by energy transfer. Taking into account sequential collisions leads to a substantial improvement in the agreement between the calculated and experimental values of the rotationally resolved cross sections for excitation of the lowest frequency ν7 torsional mode.
A solvable model of polydisperse charged particles with sticky interactions
Authors: Herrera J. N. , Blum L.
Published: July 2002

The statistical mechanics of a polydisperse system of charged particles with sticky interactions is studied in the mean spherical/Percus–Yevick approximation. The excess thermodynamic properties are calculated, taking as a reference system the sticky hard spheres mixture and are given in terms of a single scaling parameter ΓT. When the sticking probability is of the form λij=λiλj, then all the equations are explicitly solvable for arbitrary mixtures of charged systems. We obtain explicit expressions for the thermodynamic properties, structure function, and the Laplace transform of the pair distribution function.
Huge steric effect in the reaction Li+HF(v=1, j=1)→LiF+H
Authors: Loesch H. J. , Stenzel E. , Wüstenbecker B.
Published: July 2002

In a crossed beam experiment we have investigated the influence of the collision geometry on the (in plane) flux of products for the reaction Li+HF(v=1, j=1, m=0)→LiF+H. The spatial alignment of the molecular axes was achieved by applying the infrared radiation pumping technique in a strong electric field. Two geometries have been selected; they feature alignments where the axes stand either preferentially perpendicular to the relative velocity of the approaching reagents (V) and the scattering plane, or lie parallel to V. We find at a scattering angle in the center-of-mass frame of roughly 115° that for the parallel geometry (end-on attacks) a factor of 2.2 more LiF molecules are detected than for the perpendicular one (side-on attacks). This huge steric effect is well recovered by a trajectory calculation based on the ab initio potential energy surface (PES) of Chen and Schaefer. The main reason for the magnitude of the effect is that the products separate along the molecular axis and thus vanish to a large extent from the scattering plane when the molecules stand perpendicular to it.
Angle-energy distributions of Penning ions in crossed molecular beams. II. Effect of Penning electron recoil in Ne*(3s 3P2)+H,D→Ne+H+,D++e−
Authors: Khan A. , Siddiqui H. R. , Siska P. E.
Published: July 2002

Relative doubly differential cross sections for Penning ionization in the title systems are reported at a mean collision energy of 10.4 kcal/mol in a crossed supersonic beams experiment. As in paper I of this series on He*(21S)+H,D, the H+,D+ products are scattered sharply forward with respect to the incident H,D in the center-of-mass frame, but with the release of an additional 30 kcal/mol into translational energy at the peak of the distribution. Unlike the results of paper I, we find that a single optical potential, corresponding to an autoionizing state of NeH of assumed 2Σ+ symmetry, consistently represents the main features of both previously reported low-energy Penning ionization electron spectroscopy (PIES) data and the present results. Agreement between theory and experiment in the angular distributions is improved by including coupling between the angular momenta of the Penning electron and the atoms after ionization. The resonance width for PI is found to be nonexponential, showing a ‘‘saturation’’ effect at small distance. This is interpreted in terms of the ionic (charge transfer) character of the bonding in NeH*.
The spectrum of polarization fluctuations in an atomic liquid
Authors: Chen Zhe , Stratt Richard M.
Published: July 2002

Recent theoretical developments have shown how such examples of excitation properties as the electronic band structure and the set of vibrational normal modes of a liquid can be studied by traditional classical-liquid-theory methods. In this paper, we add another example to this collection: the set of polarization modes of a liquid. The basic notion is that in any polarizable but nonpolar fluid, the dynamics of the instantaneous dipoles can be represented as a linear combination of harmonic contributions from independent, microscopically defined, polarization modes. We note first how many of the properties one would like to know about the liquid—its full dielectric behavior, its optical absorption spectrum, its effect on the absorption spectrum of a solute, and even how the net polarization of the liquid fluctuates with time—are available from these polarization modes. We then point out how the requisite information about the modes can be ascertained by the same liquid theory methods used to treat p-orbital-based electronic problems. These considerations allow us to show how the mean spherical approximation can be used to obtain more accurate versions of optical spectra than was possible heretofore. It also suggests how one might begin to look at the dynamics of polarization in polar liquids.
Dissociation and predissociation on coupled electronic potential energy surfaces: A three-dimensional wave packet dynamical study
Authors: Manthe U. , Köppel H. , Cederbaum L. S.
Published: July 2002

The dissociation process of a triatomic molecule with strongly coupled electronic states is investigated by wave packet dynamics. Rotational and vibronic motion are separated for vanishing total angular momentum (J=0) in a linear molecule with conically intersecting Σ and Π electronic potential energy surfaces. All three remaining nuclear degrees of freedom are treated exactly. The time evolution of a wave packet prepared initially by photoionization of the neutral molecule is studied for different values of the nonadiabatic couplings and for different values of other relevant parameters. The potential surfaces studied include those modeling important aspects of the HCN+ system. Time dependent populations of diabatic and adiabatic electronic states are presented. The results can be interpreted by adapting ideas developed for vibronic coupling in bound systems to the dissociative situation.
Optical potentials for Ne*(3P2,0)–Ar, N2 interactions
Authors: Baudon J. , Feron P. , Miniatura C. , Perales F. , Reinhardt J. , Robert J. , Haberland H. , Brunetti B. , Vecchiocattivi F.
Published: July 2002

The differential elastic cross sections for Ne*(3P2,0)–Ar and Ne*(3P2,0)–N2 collisions have been measured in crossed beam experiments at 0.064 and 0.318 and at 0.071 and 0.295 eV, collision energies, respectively. These results have been analyzed simultaneously with integral cross sections and total ionization cross sections already available and optical spherical potentials for these two systems have been obtained. These potentials appear to be rather accurate in the distance range from 2.5 to ∼9 and from 3.0 to ∼9 Å for Ne*–Ar and Ne*–N2, respectively. The well depths and equilibrium distances are 5.12 meV and 4.9 Å for Ne*–Ar and 3.56 meV and 5.40 Å for Ne*–N2. The short-range repulsion in both cases exhibits a change in slope which can be correlated with the influence, for the interaction at shorter distances, of the Ne+ core of the metastable atom, which becomes less effective when the intermolecular distance increases. The optical potentials proposed here are given in analytical form suitable for the calculation of dynamical observables of these systems.
Spectroscopy and dynamics of He2Cl2: A quantum liquid cluster?
Authors: Sands William D. , Bieler Craig R. , Janda Kenneth C.
Published: July 2002

The two color pump–probe technique has been used to study the B←X excitation spectrum and vibrational predissociation product state distribution of He2Cl2. Although the excitation spectrum of the complex shows distinct rotational structure and the expected rotational temperature of the complex is below 1 K, we are unable to even approximately reproduce the band shape with a rigid rotor analysis. The main dissociation channel for the complex involves the transfer of two quanta of Cl2 stretching motion, almost certainly in a sequential process. The product Cl2 rotational distribution is remarkably similar to that of HeCl2 undergoing Δv=−2 dissociation. One reason for the similarity between the HeCl2 and He2Cl2 dissociation product state distributions is that vibrational excitation of the HeCl2 van der Waals modes has little effect on the product state distribution. He2Cl2 provides an example of an extremely floppy, liquidlike cluster whose spectroscopy and dynamics can be studied with detailed state resolution.
Local modes of HOOH probed by optical-infrared double resonance
Authors: Fleming Patrick R. , Li Mengyang , Rizzo Thomas R.
Published: July 2002

We have used an optical-infrared double resonance technique to probe the nature of the eigenstates prepared by 4νOH vibrational overtone excitation in hydrogen peroxide. A visible dye laser excites the 4←0 OH stretch transition and an optical parametric oscillator promotes the molecules above the dissociation threshold by a ΔvOH=2 transition from the 4νOH level. Fixing the overtone excitation laser wavelength and scanning the wavelength of the infrared photon while monitoring the dissociation fragments by laser-induced fluorescence generates an infrared predissociation spectrum of the vibrationally excited molecule that contains information about vibrational state mixing at the 4νOH level. This spectrum indicates that the zeroth-order state that gives oscillator strength to the 4←0 OH stretch transition (i.e., the 4νOH bright state) is almost entirely comprised of a single vibrational eigenstate. Since the bright state is predominantly an OH stretch, the vibrational eigenstate prepared by 4νOH vibrational overtone excitation is well localized on the OH bond. This localization allows us to perform sequential local mode–local mode excitation of the two equivalent OH oscillators in HOOH.
Potential energy surface of H⋅⋅⋅H2O
Authors: Zhang Q. , Sabelli N. , Buch V.
Published: July 2002

We investigated the potential energy surface for the H2O⋅⋅⋅H interaction in the van der Waals well region. Calculations were carried out using the Mo/ller–Plesset second- and fourth-order perturbation theory in a [12s,7p,2d]→(6s,5p,2d) basis set for the O atom, and [6s,2p,1d]→(5s,2p,1d) for the H atoms. Basis set and superposition error effects were analyzed to gauge the reliability of the calculated potential. The potential was investigated in five physically distinct directions. The deepest potential well was found in the H2O molecular plane 3.30–3.45 Å from the H2O center of mass, near the H end of the OH bond. The following parameters are suggested for the spherically averaged potential: well depth 53±6 cm−1; minimum distance from the center of mass 3.25–3.40 Å.
A simple predictive model of chemical potentials: H2(1Σg) and Li2(1Σg)
Authors: Tang K. T. , Toennies J. P. , Meyer W.
Published: July 2002

A simple model for van der Waals potentials presented earlier [J. Chem. Phys. 80, 3726 (1984)] has been extended to describe chemical bonds by including the exchange-dispersion term of Herring and Flicker. For H2, the 1Σ ground state potential is predicted in excellent agreement with the accurately known ab initio potential, the well depth being reproduced to within 0.6%. New two configuration self-consistent-field (SCF) calculations for the 1Σ and the 3Σ states of Li2 have made it possible to test the model for this system as well. Here the discrepancy is only 3% in the well depth for the 1Σ Li2 potential.
Adsorption, desorption, and surface diffusion kinetics of NH3 on MgO(100)
Authors: Arthur D. A. , Meixner D. L. , Boudart M. , George S. M.
Published: July 2002

Laser-induced thermal desorption (LITD) techniques were used to study the adsorption, desorption, and surface diffusion kinetics of NH3 on MgO(100) single-crystal surfaces. Isothermal LITD adsorption measurements revealed that the sticking coefficient of NH3 on MgO(100) was constant vs coverage and decreased with increasing surface temperature. The adsorption kinetics were consistent with a mobile precursor intermediate. In addition, the saturation NH3 coverage on MgO(100) was strongly dependent on temperature, and decreased by a factor of 5 as temperature increased from 130 to 165 K. Isothermal and linear temperature ramp LITD experiments indicated that the desorption kinetics of NH3 from MgO(100) could be modeled using a first-order rate law with a coverage-dependent desorption activation energy. The coverage dependence of the desorption activation energy was consistent with repulsive lateral interactions between NH3 adsorbates on the MgO(100) surface. The surface diffusion of NH3 on MgO(100) was also examined using LITD techniques. In contrast to recent predictions of NH3 surface diffusion on MgO(100), no evidence of NH3 surface mobility (D≤10−9 cm2/s) was observed for temperatures as high as 165 K.