bluuebunny/crossref_metadata_2025 huggingface dataset:
A log-derivative propagation scheme for the exact solution of two-state curve crossing problems
Authors: Alexander Millard H. , Parlant Gérard , Hemmer Terrence H.
Published: July 2002

A new method is presented for the exact quantum solution of certain two-state curve crossing problems, where electronic state ‖1〉 is energetically open at one end of the range of integration, while state ‖2〉, and, possibly, state ‖1〉, is energetically open at the other end of the range of integration. The method involves the use of log-derivative propagators, but differs from the usual log-derivative integration scheme in that one must propagate through the range of integration not only the log-derivative matrix but also a transformation matrix which permits one to reconstruct the initial wave function after the integration is completed. The method is numerically stable and, in a ‘‘solution following’’ approximation to the log-derivative propagators, converges as the fourth power of the step size. Application is made to several model problems. In one case the exact results are compared with the predictions of earlier semiclassical analyses [P. V. Coveney, M. S. Child, and A. Bárány, J. Phys. B 18, 457 (1985)]. The method is completely general, and can be applied to arbitrary potentials.
Proton transfer from 1-naphthol to water: Small clusters to the bulk
Authors: Knochenmuss Richard , Leutwyler Samuel
Published: July 2002

Excited-state proton transfer from 1-naphthol to water was studied as a function of solvent system size, from supersonically cooled neutral clusters, 1-naphthol⋅(H2O)n, n=1–50, to bulk ice and water. Occurrence or nonoccurrence of proton transfer was detected and studied using cluster-size-specific laser-spectroscopic techniques: resonant two-photon ionization (R2PI) and laser-induced fluorescence emission. Depending on cluster size or solution phase, three qualitatively different types of excited-state behavior were observed: (1) For small clusters, n≤7, both the R2PI and fluorescence spectra of the clusters were similar in nature to the spectra of bare 1-naphthol; (2) the medium-size clusters (n=8–20) show incremental spectral shifts which indicate successive stages of molecular solvation, and the spectra approach that of 1-naphthol in bulk ice at n≊20; (3) the fluorescence spectra for large clusters, n≥20, show increasing emission intensity below 25 000 cm−1, characteristic of the emission of the excited-state 1-naphtholate anion. Excited-state proton transfer appears to occur in the largest observed clusters (n≥30), yet the fluorescence spectra do not converge fully to that of 1-naphtholate anion in bulk water. These three behaviors are discussed in terms of a model based on three distinct excited states connected by two electronic and geometric rearrangement processes. The model accounts in a unified way for the complete range of aqueous solvation behavior observed here as well as in many other solvent systems studied previously. The extent of proton transfer reaction is largely solvent controlled, the major determinants being the proton affinity of the solvent or solvent cluster, and its ability to resolvate the nascent ion pair on a subnanosecond time scale. In bulk ice, the slow solvent relaxation results in complete absence of excited-state proton transfer from both 1- and 2-naphthol.
An accurate evaluation of the stationary points of the LiFH potential energy surface
Authors: Palmieri P. , Laganà A.
Published: July 2002

A new ab initio evaluation of the Li+HF potential energy values at the stationary points of the reaction channel is presented and compared with results of a previous calculation and estimates derived from empirical considerations.
State-resolved vibrational, rotational, and translational energy deposition in CO2(0001) excited by collisions with hot hydrogen atoms
Authors: Khan Farooq A. , Kreutz Thomas G. , Flynn George W. , Weston Ralph E.
Published: July 2002

Time-domain tunable diode laser absorption spectroscopy has been used to measure rotationally resolved transient absorption line shapes and nascent rotational populations for CO2 molecules excited into the (0001) vibrational state by collisions with translationally hot hydrogen atoms. High-accuracy measurements were made at both 292 and 223 K, providing information about the initial-state dependence of the collision dynamics and about the (temperature-independent) state–to–state scattering cross sections. The nascent rotational population distribution is found to be nearly independent of temperature, indicating broad rotational inelasticity in the 0000→0001 scattering process. The nascent populations and transient linewidths are interpreted in terms of two separate theoretical models which lend substantial insight into the H–CO2 collision dynamics.
Improved analysis of the infrared spectrum of D2H+
Authors: Polyansky O. L. , McKellar A. R. W.
Published: July 2002

The rotational analysis of the Coriolis-coupled ν2/ν3 bands in the infrared spectrum of the D2H+ molecular ion has been reexamined. By making four reassignments and adding one new transition, it has been possible to achieve a much better fit of the bands using fewer molecular parameters. The reassigned ν2/ν3 data were combined in a simultaneous least-squares analysis with existing ν1 band infrared data and with two newly measured pure rotational transitions. The resulting molecular parameters and calculated energy levels are the best currently available for this fundamental molecular ion.
Resonant two-photon ionization spectroscopy of jet-cooled NiPt
Authors: Taylor Scott , Spain Eileen M. , Morse Michael D.
Published: July 2002

Resonant two-photon ionization spectroscopy of jet-cooled NiPt has been used to investigate the possibility of d-electron contributions to the bonding in this species. Based on an abrupt onset of predissociation, the bond strength of NiPt is assigned as D0(NiPt)=2.798±0.003 eV. Comparisons of scans using ArF (6.42 eV) or F2 (7.87 eV) radiation as the ionization laser yield IP(NiPt)=8.02±0.15 eV, from which we derive D0(Ni+–Pt)=2.41±0.15 eV and D0(Ni–Pt+) =3.58±0.35 eV. High resolution studies of the 6–0 and 8–0 bands of one of the three identifiable progressions demonstrate an Ω′=0←Ω″=0 transition with r′e =2.3396±0.0039Å and r″0 =2.2078±0.0023Å. The short bond length and large bond strength of NiPt, as compared to the corresponding values (re=2.330±0.003Å and D0=2.34±0.10 eV) for the coinage metal analog, CuAu, demonstrate significant d-orbital contributions to the bonding in NiPt.
Rotational autoionization of n
p Rydberg series of D2
Authors: Pratt S. T. , Dehmer P. M. , Dehmer J. L.
Published: July 2002

Two-color resonantly enhanced multiphoton ionization combined with photoelectron spectroscopy was used to study rotational autoionization of np Rydberg states of D2 near the first ionization threshold. This work represents an extension of the study by O’Halloran et al. [J. Chem. Phys. 90, 930 (1989)] on the analogous states of H2. As in that work, one laser was used to excite a two-photon transition to the E,F 1Σ+g, v′=E0, J′=0–4 levels, and a second laser was used to probe single-photon transitions to the rotationally autoionized np Rydberg states converging to the X 2Σ+g, v+=0, N̄+=1–6 rotational levels of D+2. Of the five dipole-allowed Rydberg series converging to v+=0 excited from each intermediate J′ level (J′≥2), two are allowed to rotationally autoionize in a coupling scheme that assumes ejection of pure p waves in the ionization process and singlet coupling of the spins of the ion core and the outgoing electron; members of these Rydberg series have large half-widths. As in H2, autoionization requiring the change of four quanta of rotational energy was found to be significantly slower than that requiring the minimum change of two quanta. Ionization was also observed for the series that are forbidden to rotationally autoionize according to this simple coupling scheme, and it is shown that both the inclusion of f waves in the autoionization process and singlet–triplet mixing may be required to explain these observations. By extending the wavelength region of the J′=4 spectrum beyond that of the earlier study on H2, rotational autoionization requiring even higher partial waves was observed.
Time dependent thermal lensing measurements of V–T energy transfer from highly excited NO2
Authors: Toselli Beatriz M. , Walunas Theresa L. , Barker John R.
Published: July 2002

The time dependent thermal lensing technique has been used to measure the vibrational relaxation of NO2 (initially excited at 21 631 cm−1) by Ar, Kr, and Xe. The energy transfer analysis was carried out in terms of 〈〈ΔE〉〉, the bulk average energy transferred per collision. This quantity was found to have a very strong dependence on vibrational energy, with a marked increase at energies greater than about 10 000 cm−1, where several electronic excited states (2B2, 2B1, and 2A2) mix with the ground state (2A1). This effect may be due to large amplitude vibrational motions associated with the coupled electronic states. Even at low energies, deactivation is faster than in other triatomic systems, probably because NO2 is an open shell molecule and electronic curve crossings provide efficient pathways for vibrational deactivation. The V–T rate constant for deactivation of NO2(010) by argon is estimated to be (5.1±1.0)×10−14 cm3 s−1. Results obtained for NO*2–NO2 collisions gave 〈〈ΔE〉〉 values in good agreement with literature results from fluorescence quenching experiments, indicating that V–T may be more important than V–V energy transfer in the quenching process.
Theoretical investigation of collision induced rotational alignment in N+2–He
Authors: Follmeg Bernd , Rosmus Pavel , Werner Hans-Joachim
Published: July 2002

Collision induced rotational alignment of N+2 ions drifting in a helium buffer gas is studied by quantum closed coupled calculations using an ab initio interaction potential obtained from multireference configuration interaction wave functions. New formulas are derived for the tensor cross sections. For a given velocity distribution of the collisional partners a set of kinetic equations is solved under steady-state conditions. The resulting alignment parameters are found to be smaller than the experimental values for the velocity distribution assumed so far in drift tube experiments. However, by modification of the anisotropy of this distribution, good agreement between the theoretical quadrupole moments of the rotational angular momentum distributions and the corresponding experimental data can be obtained. It is shown that the attractive part of the potential has a significant influence on the collision dynamics of the N+2–He system. The closed coupled m-resolved cross sections indicate that collision induced transitions between magnetic sublevels of a single rotational state contribute more to the alignment effect than transitions between different rotational states.
Calculation of the total scattering cross section for the collision of hard spheres: The atom–diatom case
Authors: Gislason Eric A. , Sizun Muriel
Published: July 2002

A simple expression for the total scattering cross section Q for the collision of A+BC, where A, B, and C are hard spheres, is derived. It is assumed that B and C are initially touching. The result represents an average over all spatial orientations of the diatomic BC. The cross section depends only on the radii of the three spheres. If RB=RC, the total cross section is given, to a good approximation, by Q=π(RA+RB)(RA+2RB). A number of combining rules for total cross sections involving hard spheres are also derived. In addition, the results for hard spheres are used to develop a very simple procedure for computing classical atom–diatom total cross sections on realistic potential energy surfaces. The method is applied to collisions of H+H2, Li++N2, and Li++CO, and is seen to work very well.
Rate constant and branching fraction for the reaction of O+(2D,2P) with CO2
Authors: Viggiano A. A. , Morris Robert A. , Paulson John F.
Published: July 2002

The rate constant and products of the reaction of O+(2D,2P) with CO2 have been measured. The measurements were carried out in a selected ion flow tube by the monitor ion method. The rate constant was found to be 1.06×10−9 cm3 s−1. The reaction was found to proceed by charge transfer (≥95%). This is in contrast to the reaction of the ground O+(4S) state for which the product is O+2. The rate constant for the excited state reaction is the same as that for the ground state reaction and is slightly less than the collisional limiting value.
The atmospheric water continuum in the infrared: Extension of the statistical theory of Rosenkranz
Authors: Ma Q. , Tipping R. H.
Published: July 2002

The statistical theory proposed by Rosenkranz to calculate the continuous absorption by water molecules in the high-frequency (infrared) wing of the pure rotational band is reviewed and extended. In the review there is a discussion, in particular, of the approximations that are made, including those that are necessary and which limit the applicability of the theory to other spectral regions, and those that are made for calculational convenience. Then, several extensions to the theory are discussed, including increasing the number of rotational states used to calculate the band-average relaxation parameter, modifying the definition of this parameter to account for near-wing effects, and eliminating the boxcar approximation. This last modification, effected by using asymmetric-top functions instead of symmetric-top functions to calculate matrix elements of the density operator and to diagonalize the dipole–dipole interaction, results in significant enhancement of the relaxation parameter. This improvement, in turn, allows one to eliminate an inconsistency in the original formulation of Rosenkranz while obtaining substantially the same final results. The implications of the present results for the calculation of the absorption in the high-frequency wing of the ν2 fundamental vibration-rotational band of H2O are discussed briefly.
Excitation, deexcitation, and fragmentation in the core region of condensed and adsorbed water
Authors: Coulman D. , Puschmann A. , Höfer U. , Steinrück H.-P. , Wurth W. , Feulner P. , Menzel D.
Published: July 2002

Using synchrotron radiation, Auger electron, and H+/D+-ion yields have been studied at and above the O 1s excitation energies for condensed H2O/D2O layers of varying thickness, and for two reproducible adsorbate layers (so-called bilayers and monolayers) on Ru(001). Decay electron spectra as well as polarization dependences, angular distributions, and energy distributions of desorbing ions have been investigated. For polarizations with sufficient E component perpendicular to the surface, a sharp peak in the H+ NEXAFS spectrum is seen for all layers which has no direct counterpart in the Auger NEXAFS spectra, and whose intensity maximizes for E oriented in the detection direction. This observation is interpreted as due to the 1a1→4a1 core-to-bound transition of the surface molecules whose final state decays electronically and dissociates on comparable time scales. This appears to have the consequence that the symmetry of the coupled excitation is different from that expected for the primary photoabsorption process. There appears also to be an influence of hydrogen bonding on these effects. Similarities and differences between the various layers investigated are also analyzed.
Temperature-dependent molecular dynamics of liquid carbon disulphide: Polarization-selected impulsive stimulated light-scattering data and Kubo line shape analysis
Authors: Ruhman S. , Nelson Keith A.
Published: July 2002

Femtosecond time-resolved impulsive stimulated scattering (ISS) data from carbon disulphide at three temperatures are presented and analyzed in terms of molecular orientational motions which show weakly oscillatory, librational dynamics at short times (<1 ps) and rotational diffusion at longer times. The data are collected with a light polarization combination that eliminates signal due to electronic polarizabilities. Compared to ISS data reported earlier [S. Ruhman et al., Chem. Phys. Lett. 141, 16 (1987)], this permits improved examination of molecular motion at times shorter than 150 fs. The data also extend to longer times (several picoseconds) with good signal/noise ratios, permitting a more complete analysis of the molecular response. Finally, an improved analysis of the data is presented. Angular position correlation functions, which are determined from the data, are related to angular velocity correlation functions (AVCFs) through a Kubo line shape analysis. Approximate analytical models of liquid-state molecular dynamics which account for inhomogeneity and stochastic forces are then used to fit the AVCFs, rather than to fit the data itself as was done earlier. In the context of the models, we extract information about intermolecular torques and the extent of inhomogeneity in the torques.
Hot muonium and muon spur processes in nitrogen and ethane
Authors: Kempton James R. , Senba Masayoshi , Arseneau Donald J. , Gonzalez Alicia C. , Garner David M. , Pan James J. , Fleming Donald G. , Percival Paul W. , Brodovitch Jean-Claude , Leung Siu-Keung
Published: July 2002

Muon polarizations are reported for nitrogen and ethane over a wide pressure range from below 1 to 200 atm for N2 and up to 245 atm for C2H6. The N2 measurements were made at ambient temperature, while those for C2H6 were made at temperatures both above and below the critical temperature (305.3 K). This is the first μSR study of muonium and diamagnetic muon formation to cover the entire range from a low pressure gas to densities typical of liquids. The data are discussed in terms of hot atom and spur models. In the lowest pressure range, below 1.5 atm for N2 and about 10 atm for C2H6, the muonium polarization increases with pressure. This is well understood in terms of epithermal charge exchange. In N2 there is a small diamagnetic fraction, which is ascribed to the N2Mu+ molecular ion. This fraction approaches zero as the pressure is increased to 200 atm, with a corresponding increase in the muonium fraction, consistent with charge neutralization of the molecular ion by electrons from the radiolysis track. In C2H6, there is a decrease in the muonium fraction and a concomitant increase of the diamagnetic fraction with density, the changes occurring in two stages. The initial change is explained by stabilization of the vibrationally excited substitution products of hot muonium reactions. The second one is explained by proton transfer from the molecular ion adduct, C2H6Mu++C2H6→C2H5Mu+C2H+7, trapping the muon in a diamagnetic product. Both N2 and C2H6 have a missing fraction of polarization above 10 atm, most likely due to spin exchange of Mu with paramagnetic species created in the muon track. In N2, the missing fraction is recovered at pressures beyond about 150 atm, which is explained by scavenging of electrons by positive ions. In C2H6 the missing fraction is roughly constant for densities beyond 5 mol ℓ−1 (≊50 atm), and about twice the maximum found for N2. Both facts are consistent with the existence of ethyl radicals and hydrogen atoms in C2H6, which are longer lived than the spur electrons.
Ab initio study of intermolecular potential for ArHCl
Authors: Chal/asiński G. , Szczȩśniak M. M. , Kukawska-Tarnawska B.
Published: July 2002

The combination of supermolecular Mo/ller–Plesset treatment with the perturbation theory of intermolecular forces is applied in the analysis of the potential energy surface of ArHCl. Two minima have been found, a primary for collinear Ar–HCl and a secondary for collinear Ar–ClH. The depths of these minima are about 12% below the empirical estimates, due to basis set unsaturation of the dispersion contribution. The Ar–HCl structure is favored by the induction and dispersion terms whereas Ar–ClH by the exchange–repulsion term. The total ab initio potential, as well as its components, are compared with related terms of recent Hutson’s H6(3) potential [J. Chem. Phys. 89, 4550 (1988)] and the anisotropy of interaction is analyzed. It is concluded that the one-center multipole expansions of induction and dispersion contributions do not reproduce the correct anisotropy of induction and dispersion terms. Ab initio estimates of three-body effects in the Ar2HCl complex are also discussed.
Derivation of femtosecond pump–probe dissociation transients from frequency resolved data
Authors: Krause Jeffrey L. , Shapiro Moshe , Bersohn Richard
Published: July 2002

We present a theoretical synthesis of Zewail’s femtosecond transition state (FTS) experiments. The FTS transients are obtained by convoluting the continuous wave, bound–free, and free–free frequency-resolved spectra with the pump and probe laser pulses. We accomplish this by deriving a simple semiclassical expression for the free–free (as well as the bound–free) matrix elements and regularizing the divergent integrals occurring in the free–free integrals, by subtracting a δ-function singularity. This method allows us to write the FTS transient signal as a coherent superposition of a ‘‘molecular’’ term and an ‘‘atomic’’ term, which shows clearly the effects of the interaction between the on-resonance and off-resonance transients observed in Zewail’s experiments. Our method, which achieves essentially the same accuracy as the purely numerical wave-packet propagation techniques, is much more efficient. It enables an easy exploration of all possible pump–probe configurations and clearly confirms the fact that quantum mechanics does not allow a unique definition of a ‘‘bond-breaking’’ time. In fact, we show that the observed transients are a strong function of the laser preparation and probing. In addition, we demonstrate that the assumption that the initial wave packet is localized after absorption of a 50–100 fs pulse is wrong, and that the classical approximation that there exists a one-to-one correspondence between the probe frequency and the internuclear distance of the photofragments is inaccurate. We show that the parameters obtained by the use of these classical approximations for the free–free spectrum lead to the extraction of the wrong potential parameters which, in turn, leads to a discrepancy between the calculated and measured bound–free absorption spectrum. This discrepancy does not exist if the potential parameters are extracted quantum mechanically.
A new Raman cross section measurement technique monitors the tyrosine environmental dependence of the electromagnetic field strength
Authors: Larkin Peter J. , Gustafson William G. , Asher Sanford A.
Published: July 2002

We have developed a new Raman spectroscopic technique which can be used to determine relative Raman intensities as a function of the liquid medium refractive index. We utilize teflon as an external standard. The refractive index dependence of the solid angle of light collection disappears in our sampling geometry and the self-absorption bias is minimized. We observe a refractive index dependence for the Raman intensity for N-acetyl-L-tyrosinamide (NATYR) close to that calculated using previously derived relations which determine the effect of the refractive index on the electromagnetic field strength and upon the Raman scattered intensities. This dependence gives a ca. 25% increased resonance Raman intensity of NATYR in ethylene glycol compared to water for ca. 243 nm excitation. The strong dependence of the Raman intensity upon the environment suggests that UV resonance Raman intensity measurements can be used to monitor the average environmental refractive index of aromatic amino acids in proteins.
Far-infrared spectra of hydrogen dimers: Comparisons of experiment and theory for (H2)2 and (D2)2 at 20 K
Authors: McKellar A. R. W. , Schaefer Joachim
Published: July 2002

Far-infrared spectra of weakly bound complexes of molecular hydrogen have been studied using an infrared Fourier transform spectrometer and a long absorption path (98 m) through equilibrium gas at low temperature. The dimer transitions accompany pure rotational transitions of H2 or D2 monomers. Para-H2 was studied in the S0(0) region (350 cm−1 ), normal H2 in the S0(1) region (590 cm−1 ), and ortho-D2 in the S0(0) region (180 cm−1 ). The extensive and well resolved (13 sharp lines) spectrum observed for (D2)2 was of special interest. A new empirical (rigid rotor) fit potential of the H2–H2 system has been used for calculating eigenvalues and numerical eigenfunctions of the dimers in the close coupled formalism. Dipole moment transition probabilities were calculated by using the previously tested induced dipole moment surface of Meyer. In order to compare with experiment, bound–bound transition frequencies have been calculated for the three cases, along with the full collision-induced spectrum for the para-H2 S0(0) case. It was found that the dimer binding energies give quite good agreement with experiment, and that the line and continuum intensities were also in satisfying agreement with the measurements. However, there is evidence that the rigid rotor potential function is not completely adequate, especially for dimers containing rotationally excited monomers. The next step will be to utilize a nonrigid rotor potential including the effects of the individual hydrogen monomer bond lengths.
Electronic states and dynamical behavior of LiXen and CsXen clusters
Authors: Martyna Glenn , Cheng Ching , Klein Michael L.
Published: July 2002

A variety of theoretical techniques have been used to explore the electronic states, approximate interaction potentials and dynamical behavior of LiXen and CsXen clusters at finite temperature. Although alkali atoms prefer to reside on the outer perimeter of the cluster, nevertheless, the electronic states (s–p transition) suffer a perturbation which should be seen in appropriate experiments. The pair approximation for the guest host interaction is shown to yield an excellent description of the structural and dynamical properties of the system. Comparisons are made with recent related studies on the lowest energy geometries of NaArn clusters and the behavior of Li and Na in bulk xenon.
Accurate three-dimensional quantum cumulative reaction probabilities for Ne+H+2→NeH++H
Authors: Kress Joel D.
Published: July 2002

Accurate three-dimensional quantum cumulative reaction probabilities and their energy derivatives are presented for the reaction Ne+H→NeH++H.
Resonant photodissociation of Mo(CO)6 adsorbed on graphite and Ag(111)
Authors: So S. K. , Ho W.
Published: July 2002

The adsorption and photodissociation of Mo(CO)6 on the basal plane of graphite and Ag(111) are studied by photoinduced desorption, high-resolution electron-energy-loss spectroscopy, electronic electron-energy-loss spectroscopy (EELS), and thermal desorption spectroscopy. Mo(CO)6 is found to absorb in pure molecular form, without dissociation, on each surface at 85–90 K. Electronic EEL spectra confirm that the electronic structure of the molecule remains relatively unperturbed on the surface. Similar to the gas phase, electronic transitions of the molecule, including the ligand-field transition and the metal-to-ligand charge transfer, were observed for absorbed Mo(CO)6. Upon low-power UV irradiation (λ<360 nm), the adsorbed molecules readily photodissociate and release CO. The mechanism of photodissociation on each surface is separately identified to be direct photoelectronic excitation of the adsorbed Mo(CO)6 by careful photon power and wavelength-dependence studies. Linear power dependence was found in each case, indicating the initial excitation is due to the absorption of a single photon. Resonances in the photodissociation spectra of the molecules were observed at ∼290 nm and 325 nm, due to the metal-to-ligand charge transfer and the ligand-field transition, respectively. For graphite, the relative photoyield was observed to closely match the Mo(CO)6 absorption spectrum. In contrast, a relative enhancement in the photoyield was observed for Ag(111) at ∼325 nm due to the increase of the surface electric field associated with the onset of the d band to the Fermi-level transition in this wavelength region. In addition, small quantities of the fragments from Mo(CO)6 were observed as a result of bombardment of the surface by low-energy (20 eV) electrons. The photodissociation yields are very sensitive to the adsorbate coverage. For a coverage of about 0.25 monolayer or less, there is no observable photodissociation for Mo(CO)6 on Ag. On both Ag and graphite, the photodissociation yield increases as the coverage approaches and exceeds one monolayer; photodissociation competes efficiently with relaxation into the substrate.
Coverage dependent desorption kinetics of CO from Rh(111) using time-resolved specular helium scattering
Authors: Peterlinz K. A. , Curtiss T. J. , Sibener S. J.
Published: July 2002

Linearized measurements of the coverage dependent desorption rates of CO from Rh(111) were made with a novel three molecular beam apparatus. To measure these isothermal and essentialy isosteric rates we introduce a new kinetic response amplifier, namely time-resolved specular helium scattering, which makes use of the large attenuation cross section that CO has for specular helium scattering. The measurements are made by using one intense and continuous CO beam to establish a specific adsorbate coverage while another low intensity and chopped CO beam is used to weakly modulate the adsorbate density around the selected steady state. The transient helium reflectivity waveforms we measure during the modulated CO scattering contain the desired kinetic information, and are typically one to almost two orders of magnitude more sensitive to the desorption kinetics than are the signals arising from direct detection of desorbing CO. Desorption rates are reported for 0≤θCO≤0.22 and 440 K ≤TS≤555 K.

He diffraction measurements revealed that the CO overlayer was disordered for all conditions for which kinetics were measured, and that the sticking coefficient varied with coverage as S0(1−3θ). At least a second order expansion of the chemical potential in terms of CO coverage was needed to explain these rates. The experimentally determined desorption rates can be fit equally well by placing the coverage dependence in either the pre-exponential factor or in the activation energy: k(θ,TS) =1.33×1014 exp[0.344θ+48.8θ2 −(32.3 kcal/mol)/(kbTS)]s−1, k(θ,TS) =1.33×1014 exp[−(32.3−1.62θ −38.3θ2 kcal/mol)/(kbTS)] s−1. The measured He diffraction data, sticking coefficient, specular He scattering attenuation vs CO coverage, and increase in CO desorption rate with increasing coverage imply nearest-neighbor repulsive interactions. The use of time-resolved specular He scattering for studying coverage dependent reactions is also discussed.
Monte Carlo precise determination of the end-to-end distribution function of self-avoiding walks on the simple-cubic lattice
Authors: Dayantis Jean , Palierne Jean-François
Published: July 2002

Chains have been generated on the simple-cubic lattice to determine, by Monte Carlo simulation, the end-to-end distribution function of self-avoiding walks. The modulus r of the end-to-end distribution vector r, the square of this modulus, and the interactions of all orders were recorded for each chain. The Alexandrowicz dimerization procedure has been used to circumvent attrition and thus obtain statistically significant samples of large chains. This made it possible to obtain samples involving 12 000–16 000 chains, within ‘‘windows’’ of width Δρ=0.2, where ρ=r/Nν, N being the number of steps in the walk and ν  the scaling exponent. It was found that the mean value 〈r〉=aNν, with ν=0.5919 and the prefactor a very close (perhaps strictly equal) to unity. The above value of ν is slightly larger than that calculated by Le Guillou and Zinn-Justin, but in accord with the Wilson ε=4−d expansion, where d is the space dimensionality. Also, 〈r 2〉=bN2ν, with b=1.136±0.05. An attempt was made to fit the data to the Fisher–McKenzie–des Cloizeaux distribution, P(r=Nνρ)=N−νdA ρθ exp(−βρδ), where θ, β, and δ are adjustable parameters. Taking δ=(1−ν)−1=2.45, in accordance with the Fisher law, it was found that plots for various N values of ln L(ρ)+βρδ vs ln ρ, where L(ρ) is the number of chains falling within subwindows of width dρ=0.02, were linear (except for the lowest values of ρ), if β was taken equal to 1.07. From the slope of the linear plot it was found that θ=0.27, in accord with theory. Finally, in three dimensions the end-to-end distribution function P(r) can be expressed by the relation P(r=Nνρ)=N−νd 0.2393ρ0.27 exp(−1.07ρ2.45). This relation is in good agreement with that proposed by des Cloizeaux and Jannink. A brief discussion of the two-dimensional case as well as fluctuations of the end-to-end distance is also presented.
Excited state structures and vibronic spectra of H2CO+, HDCO+, and D2CO+ using molecular gradient and Hessian techniques
Authors: Cesar Amary , Ågren Hans , Helgaker Trygve , Jo/rgensen Poul , Jensen Hans Jo/rgen Aa.
Published: July 2002

We choose H2CO+ and its deuterated species to demonstrate the potential for using second-order multiconfigurational self-consistent field theory to optimize structures and calculate properties of ionized and excited states. We focus on the calculation of multidimensional vibronic spectra using only the local information of the potential hypersurface, viz. the molecular energy, gradient, and Hessian. Second-order multiconfigurational self-consistent field optimization on lowest excited states using the trust radius algorithm is found to give the same stable convergence as for neutral ground states, while for higher lying states, the problem of multidimensional potential crossings renders the calculations more difficult.
Hyperfine splitting of the 1 3Δg Rydberg state of 7Li2
Authors: Li Li , An T. , Whang T.-J. , Lyyra A. M. , Stwalley W. C. , Field R. W. , Bernheim R. A.
Published: July 2002

The hyperfine structure of the 1 3Δg Rydberg state of Na2 has been studied by sub-Doppler CW perturbation facilitated optical–optical double resonance (PFOODR) spectroscopy via A 1Σu+∼b 3Πu mixed intermediate levels.
Potential energy surfaces for Pd2+H and Ni2+H interactions
Authors: Balasubramanian K. , Feng P. Y.
Published: July 2002

Complete active space multiconfiguration self-consistent field (CAS-MCSCF) followed by multireference singles+doubles configuration interaction (MRSDCI) calculations are carried out for Pd2+H and Ni2+H interactions. Spin–orbit effects are also included through the relativistic configuration interaction (RCI) method for Pd2H. We find that both Ni2H and Pd2H form M MH (M=Pd,Ni) bridge equilibrium structures. Three nearly-degenerate electronic states are found for both Pd2H and Ni2H. The nature of the metal hydrogen bridge bonds is discussed. For Pd2H the spin–orbit contamination of nearly-degenerate low-lying states was found to be significant.
Experimental and theoretical study of H+HI→H2+I reaction dynamics at 1.3 eV collision energy
Authors: Aker Pamela M. , Germann Geoffrey J. , Valentini James J.
Published: July 2002

The H2 (v′, J′) partial reaction cross sections arising from collisions of 1.3 eV H atoms with HI have been measured using coherent anti-Stokes Raman scattering (CARS) spectroscopy. Quasiclassical trajectory (QCT) calculations on a representative potential energy surface have been used to simulate the dynamics of this reaction. Comparison of the QCT and experimental results shows good agreement in average energy disposal, total cross section, and rotational state distributions. The only significant disagreement is in the detailed vibrational state distribution, for which the calculations give monotonically decreasing population with increasing v′, while the experimental results show a significant population inversion between v′=0 and v′=1. Based on the QCT-experiment comparison we make suggestions for improving the H+HI potential energy surface.
The stepwise dissociation of NH3 on the Si(111)–(7×7) surface: Low-temperature dissociative adsorption and thermal effects
Authors: Colaianni M. L. , Chen P. J. , Yates J. T.
Published: July 2002

The molecular and dissociative adsorption of NH3 on a clean Si(111)–(7×7) surface has been studied using high-resolution electron-energy-loss spectroscopy (HREELS), Auger electron spectroscopy, and temperature-programmed desorption (TPD). All NHx (3≥x≥1) species have been observed vibrationally under varying experimental conditions. Adsorbed molecular ammonia is observed by both HREELS and TPD below 200 K. The absence of a strong δs(NH3) mode in our vibrational spectra suggests that NH3 is bound as a tilted species to the Si(111)–(7×7) surface. Ammonia exposures above 1.7×1014 NH3/cm2 at 110 K populate a weakly bound NH3(a) state which desorbs upon heating with a peak desorption temperature of 115 K. The NH2(a) species is observed to form at 80 K at all coverages and exhibits an enhanced thermal stability at higher ammonia exposures. The NH(a) species is detected above 200 K by an energy-loss feature at 1100 cm−1, which we assign to the δ(NH) mode. NH(a) is stable to 750 K on high coverage layers. Finally, we observe an increase in the Si–H stretching frequency as the concentration of NH(a) and N(a) species is increased by heating.
Natural orbitals from single and double excitation configuration interaction wave functions: their use in second-order configuration interaction and wave functions incorporating limited triple and quadruple excitations
Authors: Grev Roger S. , Schaefer Henry F.
Published: July 2002

As an alternative to orbitals obtained from a molecular complete-active-space self-consistent-field (CASSCF) wave function, we have investigated the use of natural orbitals (NOs) obtained from configuration interaction (CI) wave functions including all single and double excitations (CISD) for use in multireference CI (MRCI) studies. The specific MRCI methods investigated are (1) second-order CI (SOCI), which includes all single and double excitations with respect to a full CI in the valence space and (2) a wave function that includes all single and double excitations out of a valence space CISD reference function. The latter wave function can also be described as a single–double–triple–quadruple excitation CI in which only two electrons are allowed to simultaneously reside outside of the valence space, ‘‘which we call CISD[TQ].’’ Comparison is made with CASSCF-SOCI and full CI results for NH2 (2B1), CH3 (2A″2), and SiH2 (1B1) at equilibrium bond distances (Re) 1.5 and 2.0Re, and with full CI results for the dissociation energy of N2. The dissociation energies of N2 and C2 are also obtained using large atomic natural orbital basis sets and the results compared to CASSCF-SOCI and internally contracted MRCI results. In all, the MRCI results with CISD NOs are very similar to the CASSCF-MRCI results, and at geometries where the reference wave function is dominant, the relatively compact CISD[TQ] method yields results that are very close to SOCI. In addition to their ease of generation, the CISD NOs offer the added advantage of allowing for truncation of the CI configuration list on an orbital basis by simply deleting high-lying virtual orbitals. The errors introduced by this truncation are almost quantitatively obtained at the CISD level of theory.
Photoionization of As2 and As4: Implications for group V clusters
Authors: Yoo R. K. , Ruscic B. , Berkowitz J.
Published: July 2002

The vacuum ultraviolet photoionization mass spectrum of As4 is presented, from the ionization threshold to 600 Å. The apparent adiabatic ionization potential is ≤8.49 eV, but the true value may be significantly lower. Three broad autoionization features are observed, probably comprising members of a Rydberg series converging to the B̃ 2A1 state of As+4. The first fragment, As+3, has an appearance potential (0 K) of 11.23±0.05 eV, from which we extract ΔH0f0(As+3)≤228.7±1.3 kcal/mol. The photoion yield curve of As+2(As2) is obtained under conditions where As2 is dominant in the vapor. The adiabatic ionization potential is 9.69±0.02 eV. Two prominent autoionizing Rydberg series are observed, converging to the A 2Σ+g state of As+2, with an ionization potential of 10.238±0.002 eV. At higher energy, three members of a window resonance series can be seen, converging to the B 2Σ+u state of As+2, with an ionization potential of 15.37 eV. From an upper limit to the partial pressure of As3, equilibrium conditions, and assuming a triangular As3, we deduce ΔH0f0(As3)≥60.0 kcal/mol; other criteria suggest ΔH0f0(As3)≂63 kcal/mol. Consequently, the adiabatic ionization potential of As3 is <7.32 eV, and probably ≤7.19 eV. Several implications are drawn, relevant to recent studies of antimony and bismuth clusters.
Basis set superposition error effects on electronic and νFX, νF⋅⋅⋅N stretching modes of hydrogen bonded systems FX⋅⋅⋅NCX (X=H,D)
Authors: Bouteiller Y. , Behrouz H.
Published: July 2002

The FH⋅⋅⋅NCH hydrogen-bonded complex, which is a reference structure, has been reviewed from an electronic and vibrational point of view. Ab initio calculations have been made, using a large basis set and thus providing first structural and electrical properties on the subunits when the electronic correlation effects are taken into account in very close agreement with the experimental data. The structure and dipole moment of the complex are well represented. The νFH and νF⋅⋅⋅N stretching modes are calculated by means of the variational method, taking into account the mechanical anharmonicity and anharmonic coupling between both vibrations. It is shown that the νF⋅⋅⋅N stretching frequency, which is larger than the experimental value at the self-consistent-field level and especially when electronic correlation effects are taken into account, is strongly decreased, and so approaches the experimental value, when corrected for basis set superposition error.
Pressure induced shifts of CO2 lines: Measurements in the 003–000 band and theoretical analysis
Authors: Thibault F. , Boissoles J. , Le Doucen R. , Bouanich J. P. , Arcas Ph. , Boulet C.
Published: July 2002

Self-broadened and N2, Ar, He-broadened halfwidth and pressure shift coefficients of the rotational transitions in the 0003–0000 band of 12C16O2 have been measured from laboratory absorption recorded at room temperature with a Fourier transform spectrometer. Comparison is made with the results of a theoretical calculation based on a semiclassical model. Good agreement is obtained for all the IR vibrational bands for which data are available. It has been shown that the shifts mainly originate from vibrational dephasing. Infrared line shifts have been compared with results obtained from stimulated Raman spectroscopy and we show that they are not consistent. A number of possible explanations have been discussed to account for this discrepancy.
Intra–inter polyad mixing and breaking of symmetric–antisymmetric selection rule in the vibrational spectra of CS2 molecule
Authors: Pique J. P. , Manners J. , Sitja G. , Joyeux M.
Published: July 2002

A laser system composed of tunable lasers pumped by a copper vapor laser (Oxford Lasers Cu60) is described in this paper. The high resolution obtained with this system has allowed excitation of selective rotational levels of the 15 V and 10 V vibrational bands of the V 1B2 excited electronic state of the CS2 molecule in its vapor phase (∼100 mTorr). The rotational assignment of the excitation spectra was accomplished by observing the dispersed fluorescence. We show that it is not necessary to use a supersonic jet in order to assign the emission spectra of CS2. The goal of this work is to study the highly excited vibrational states of the ground electronic state of CS2 up to the first dissociation limit. For our purpose, there are two important consequences of the particular geometry of the 15 V excitation, which is just below and close to the bending potential barrier of the V 1B2 state. First, a very good Franck–Condon overlap in the excitation and a large Franck–Condon access to high vibrational states, as high as 20 000 cm−1, allow observation of the dispersed fluorescence through a high resolution monochromator with an OMA detector. This avoids the need for more complicated techniques, like SEP spectroscopy. Moreover, we show for the first time, that, in the V 1B2 excited state, a strong Coriolis interaction Q32 Q3 Jc couples the bending (0,3,0) and antisymmetric stretching levels (0,0,1). This breaking of the symmetric–antisymmetric selection rule gives access to the antisymetric stretching levels of the ground electronic state X̃ Σ+g from the 15 V excitation. We also show that, below 12 000 cm−1, the vibrational couplings of CS2 can be described by a model of 2 degrees of freedom, which includes a strong 1:2 Fermi resonance and accidental resonant perturbations between adjacent polyads which is probably a first step in the transition to a chaotic regime in CS2.
Determination of absolute thermal rate constants for the charge-transfer reaction DBr+(2∏I,v+)+HBr→HBr+(2∏I′,v′+)+DBr
Authors: Xie Jinchun , Zare Richard N.
Published: July 2002

The charge transfer reaction DBr+(2∏I,v+,J+)+HBr→HBr+(2∏I′,v′+,J′+)+DBr is studied in a state-to-state manner under thermal conditions in a slowly flowing gas mixture of HBr and DBr. The DBr+ reagent is prepared in a selected vibronic level by using (2+1) resonance-enhanced multiphoton ionization. The HBr+ product is detected in a quantum-state-specific manner using laser-induced fluorescence. From the measurements of the molecular density and the populations of both HBr+ product and DBr+ reagent, the absolute thermal rate constants k(i,v+→i′,v′+) are determined for this charge-transfer process. The rate constants for near-resonant charge transfer in which Δv+=0 and Δi=0 are much large than charge-transfer channels in which either Δi≠0; the smallest rate constants are for those channels in which both Δi≠0 and Δv+≠0. The rotational distribution of the HBr+(i′,v′+) products fits a temperature well in each case. For near-resonant charge transfer, the rotational temperature is slightly warmer than thermal, whereas for nonresonant charge transfer, the rotational temperature is much hotter than thermal. A model in which the excess energy of a charge-transfer process is statistically partitioned among all the degrees of freedom of the complex is able to predict closely the observed rotational temperature.
Polymer dynamics in binary blends
Authors: Wilson Jeffrey D. , Loring Roger F.
Published: July 2002

We develop a theory of the dynamics of flexible linear polymers in a melt composed of macromolecules of two molecular weights and of the same chemical species. A polymer is represented by a freely jointed chain that moves by two dynamical processes. The first is a local jump motion that may be blocked by obstacles, and the second is a slithering mode that mimics reptation. The dynamics of the obstacles are determined self-consistently by an ansatz that associates their relaxation with the dynamics of the slowest mode of conformational relaxation of a chain. The calculations of the autocorrelation function of the end-to-end vector and of the mean squared displacement of the center of mass are related exactly to the solution of a random walk problem with dynamical disorder. We calculate the necessary random walk propagator by applying the dynamical effective medium approximation. Calculations of the dependence of the self-diffusion coefficient of both components on blend composition and on molecular weights are presented. The theory is shown to provide a unified description of diffusion in the unentangled and entangled regimes.