bluuebunny/crossref_metadata_2025 huggingface dataset:
Rearrangement dynamics of the hydrogen-bonded network of clathrate hydrates encaging polar guest
Authors: Koga Kenichiro , Tanaka Hideki
Published: July 2002

Mechanisms of rearrangement process in a defect-bearing solid hydrogen-bonded network system —a clathrate hydrate encaging polar guest—are examined in terms of the waiting time distribution functions for connectivity changes of water molecules. As previously found for the lifetime distributions of hydrogen bonds in supercooled water by Sciortino et al., a power-law-like behavior is observed for the waiting time distribution in short-time region in the clathrate hydrates; in long time region the distribution is close to an exponential form. It is revealed that the behavior is accounted for by assuming two kinds of processes of the connectivity changes: one is caused by thermal excitation in a perfect network region, which can be regarded as a Poisson process and another is induced by defects in the environmental network, which is responsible for the power-law-like behavior. Also the waiting time for the defect-induced connectivity changes of a molecule is related to how many other molecules are involved in local network rearrangement processes. By a simple model of single-axis rotator for reorientational jumps of a water molecule, we show that a power-law-like behavior appears in the dipole autocorrelation function when the waiting time for jump events is a strong power-law type distribution as observed in the defect-induced connectivity changes.
Pressure dependencies of the thermal diffusion factors, αT, of nine helium–fluoroethane systems at 300 K: Prediction of the variation with temperature of the limiting diffusion coefficients of these systems from αT values at zero pressure
Authors: Dunlop Peter J. , Bignell C. M.
Published: July 2002

Thermal diffusion factors, αT, are reported at 300 K for nine binary systems consisting of helium and a fluoroethane. The data are extrapolated to zero pressure to test the approximate theory of Oost et al. for the pressure dependence of αT. The zero pressure values of αT are used to predict the temperature dependence of the binary diffusion coefficients of these systems.
A density-functional study of cluster reactivity. III. NH3 on a free Ga5As+5 cationic cluster
Authors: Akpati H. C. , Mackey J. , Nordlander P. , Lou L.
Published: July 2002

Ammonia chemisorption on a small and positively charged gallium arsenide cluster in the gas phase has been investigated using density-functional methods. The reactivity at various cluster sites are characterized in terms of binding energy and charge transfer from the adsorbate to the cluster with a strong correlation shown between the latter two quantities. The calculations show a strong dependence of the reactivity of the cluster on its charge state. Ionizing the bare cluster to a cationic charge state enables multiple chemisorption of ammonia, which is much less favored by a neutral cluster and can be prohibited on a negatively charged cluster.
Trajectory calculations of relative center of mass velocities in collisions between Ar and toluene
Authors: Bernshtein V. , Oref I.
Published: July 2002

Average velocities of Ar relative to the center of mass of toluene in bimolecular collisions were determined using quasiclassical trajectory calculations. The collision durations were binned in 20 fs and 100 fs bins and for each bin the velocities of all trajectories were averaged. 10 000 trajectories were calculated. About 64% of all collisions were elastic and the rest were inelastic collisions. The remaining 36% inelastic collisions can be classified into four types. (a) Impulsive collisions of duration 0–300 fs (62%). (b) Chattering collisions of duration longer than 300 fs but shorter than intramolecular vibrational relaxation (IVR) times (≳30%). (c) Complex forming collisions which last longer than molecular IVR times but less than complex (molecular+transition modes) IVR times and complex forming collisions which last longer than complex IVR times. The latter may lead to statistical distribution of energy in the collision complex. These long lived trajectories have negligible contribution to the value of the average energy transferred. (d) Supercollisions (0.12%) which are collisions which transfer an inordinate amount of energy in one event. The details of the collisional process are discussed and sample distributions are presented. Analysis of the collision events indicate that out-of-plane vibrations and overall rotations play a major role in the energy transfer mechanism. A comparison with existing analytical energy transfer models is presented and it is shown that some of them do not agree with the present trajectory calculation results. It is suggested that supercollisions in the gas phase and in solution play a major role in chemical reactions.
Relaxation of the product state coherence generated through the photolysis of HgI2 in solution
Authors: Pugliano Nick , Szarka Arpad Z. , Hochstrasser Robin M.
Published: July 2002

Experimental results are presented for wave packet dynamics of the HgI molecules produced through the impulsive photolysis of HgI2 in ethanol solution. The results demonstrate that the HgI is formed in a vibrational superposition state. The phase of the beats fit to a value of 0.9π. This information is used to estimate the frequency of a transition state vibration of the HgI2 reactive state in solution. Both the oscillation frequency and decay rate exhibit a probe wavelength dependence. This is described as a result of each of the finite probe spectral bandwidths interrogating a specific set of vibrational eigenstates. The anharmonicity of HgI leads to a distinct oscillatory signal for each of the probes used. From this an approximate assignment of the level pairs being sampled by each probe is obtained. This information is used to construct solvated potential energy surfaces consistent with the data. The level dependent relaxation was not interpretable by a simple Bloch-type picture, in which each coherence is characterized by a complex frequency. A quantum mechanical relaxation matrix including coherence transfer terms adequately modeled the observed coherence retention. The data also show that an energy dependent pure dephasing rate is required for the anharmonic HgI level pairs. Other aspects of solvated wave packet dynamics are discussed.
Transient absorption spectra and bleaching of 4′-n-pentyl-4-cyanoterphenyl in cyclohexane — determination of cross sections and recovery times
Authors: Sander Rolf , Herrmann Volker , Menzel Ralf
Published: July 2002

We investigated the excited state absorption (ESA) of 5CT (4′-n-pentyl-4-cyanoterphenyl) dissolved in cyclohexane in the wavelength range from 400 to 870 nm. Time resolved measurements of these transient absorptions were used for the differentiation between singlet and triplet absorption. We detected a long living component, which could be related to a triplet band centered at 500 nm and 55 nm broad (FWHM) and a short living absorption indicating a S1→Sn-transition band centered at 576 nm with a width of 16 nm (FWHM). The life time of the triplet state was determined to be (340±5) ns for the not-degassed sample. Measurements of the ground state depopulation and the fluorescence as a function of the excitation intensity were modeled using rate equations. Thus we were able to determine the population densities of the relevant excited states and the cross section for the S1→Sn absorption. Finally, the relaxation time of the higher excited electronic singlet state Sn to the first excited singlet S1 was estimated to 0.5 ns from bleaching of this transient absorption.
State resolved probe of an energetic surface reaction: Phosgene on silver
Authors: Xin Q.-S. , Zhu X.-Y.
Published: July 2002

State resolved characterization of nascent gas phase products is used as probe for the dynamics of an energetic surface reaction. This is achieved in the photodissociation of monolayer phosgene adsorbed on Ag(110). Irradiation of adsorbed Cl2CO in a broad photon energy range (hν=1.9–6.4 eV) leads to dissociation, with Cl retained on the surface and CO desorbing into the gas phase. The translational energy of product CO (g), 〈Etrans〉=0.26 eV, is independent of hν, even at the threshold photon energy (1.9 eV). This result establishes a dissociative electron attachment mechanism involving a reactive intermediate, ClCO, whose prompt dissociation serves as a probe to surface dissociation dynamics. Consistent with translation, internal state distribution of product CO (g) also shows an energetic origin: The rotational distribution, with an overall flux-weighted mean rotational energy of 〈Erot〉=0.17 eV, can be approximated by a bimodal Boltzmann distribution with rotational temperatures of 700 K at low J(s) and 7000 K at high J(s); the relative vibrational population is Nν=1/Nν=0=0.30. Contrary to common expectation based on quenching rates, both translational and rotational energies of CO (g) from monolayer photodissociation are much higher than those from the direct photodissociation in multilayers. This is taken as evidence for concerted reaction dynamics on the surface: The high exothermicity in the Cl–Ag bond formation on the surface exerts part of the energy to the Cl–CO coordinate, leading to higher energies in CO (g).
Electronic spectral diffusion in glasses: The influence of coupling to the medium on experimental observables
Authors: Zimdars David , Fayer M. D.
Published: July 2002

The theory of electronic dephasing in low temperature glasses is extended to include the possibility that the strength of coupling of the chromophore to the solvent medium depends on the nature of the bath dynamical processes and the nature of the chromophore and, therefore, the chromophore-bath coupling can vary as a function of the rate of the dynamics of the medium. In the context of the sudden jump two-level system (TLS) model of low temperature glasses, this theory is used to reconcile the apparent contradiction implied by differences observed in spectral diffusion data for cresyl violet and metal-porphyrins in deuterated ethanol glass at 1.5 K. Previously, the coupling strength of a chromophore to the TLS has been assumed to be independent of rate of the transition between TLS states. Within the context of this approximation, spectral diffusion data yield, Pi(R), the intrinsic TLS fluctuation rate distribution. With the inclusion of the rate dependent coupling, C(R), it is shown that the spectral diffusion observables actually yield Pi(R)C(R). Therefore, the observed lack of spectral diffusion for a particular chromophore over some range of times can imply C(R) is zero rather than the current interpretation that Pi(R) is zero.

To illustrate the importance of C(R), a hueristic model is analyzed. A fluctuation rate distribution is introduced that consists of the sum of three log-normal functions each associated with a specific class of dynamics occurring over three overlapping ranges of rates. The uncharged and nonpolar metal porphyrins is taken to couple to TLS strain dipoles, while the charged and polar cresyl violet also couples to TLS electric dipoles. By taking one of the types of TLS dynamics to only give rise to electric dipole fluctuations, it is possible to fit all of the experimental data in deuterated ethanol with a single intrinsic distribution of TLS fluctuation rates. This analysis of previously reported data is supported by the presentation of new stimulated photon echo data on both cresyl violet and zinc meso-tetraphenyl porphine in deuterated ethanol.
A numerical test of activated rate theories for cusped and smooth potentials
Authors: Starobinets Alexander , Rips Ilya , Pollak Eli
Published: July 2002

A numerical study of the effect of dissipation on the radiationless transition rate in the adiabatic and solvent-controlled limits is presented. For light particle reactions, the nonlinearity of the potential surface in the vicinity of the barrier top is important, and the potential may be approximated as a cusped double well potential, provided that the nonadiabatic coupling is small compared to the thermal energy. Three different theoretical approaches for calculation of the thermally activated rate are analyzed and compared with exact numerical results. We find that Variational Transition State Theory (VTST) with a planar dividing surface, as well as the approach of Calef and Wolynes (CW), provide a good description of the rate of symmetric reactions. A rate expression suggested by Dekker is found to be the least accurate. The CW approach is most accurate in the strong damping regime, while VTST is better in the weak damping regime. The accuracy of both methods improves as the potential is smoothed. VTST and the CW expression are also found to give a reasonable description of asymmetric reactions, provided that the asymmetry is not too large.
Initial state laser control of curve-crossing reactions using the Rayleigh–Ritz variational procedure
Authors: Gross Peter , Gupta Ashish K. , Bairagi Deepa B. , Mishra Manoj K.
Published: July 2002

A new two-step procedure for laser control of photodissociation is presented. In the first step of the procedure, we show that control of photodissociation product yields can be exerted through preparation of the initial wave function prior to application of the photodissociation field in contrast to previous laser control studies where attention has focused on the design of the field which induces dissociation. Specifically, for a chosen channel from which maximum product yield is desired and a given photodissociation field, the optimal linear combination of vibrational eigenstates which comprise the initial wave function is found using a straightforward variational calculation. Any photodissociation pulse shape and amplitude can be assumed since the Schrödinger equation is solved directly. Application of this method to control of product yields in the photodissociation of hydrogen iodide is demonstrated. The second step of the control procedure involves the preparation of the coherent superposition of discrete levels obtained from the previous step; design of the preparatory field can be done analytically for two or three level systems as demonstrated here or with other well-studied iterative field design methods.
Comment on ‘‘Short time dynamics of glass-forming liquids’’ [J. Chem. Phys. 103, 1152 (1995)]
Authors: Wuttke Joachim
Published: July 2002

Roland and Ngai [J. Chem. Phys. 103, 1152 (1995)] challenged the interpretation of short-time dynamics of glass-forming liquids in terms of mode coupling theory by suggesting that the usual analysis shows critical singularities even when applied to smooth synthetic data. However, their analysis relies on a nonreproducible fit and overlooks several inconsistencies that in any experimental work would have been interpreted as clearly contradicting mode coupling theory.
Termolecular proton transfer reactions assisted by ionic hydrogen bond formation: Reactions of aromatic cations with polar molecules
Authors: Daly G. M. , Meot-Ner M. , Pithawalla Y. B. , El-Shall M. S.
Published: July 2002

We present a new method that applies resonant-two-photon ionization to generate reactant ions selectively in the source of a high-pressure mass spectrometer (R2PI-HPMS) for kinetic and equilibrium studies. Applications to reactions that would be obscured otherwise in a complex system are illustrated in mixtures of benzene with polar solvent molecules (S). We observe a novel type of proton transfer reactions from C6H6+• to two S molecules where S=CH3CN, CH3OH, C2H5OH and CH3COOC2H5, and from C6H5CH3+• to two S molecules where S=CH3OH and C2H5OH to form protonated solvent S2H+ dimers. The reactions are driven by the strong hydrogen bonds in the S2H+ dimers and therefore require the formation of the hydrogen bond concertedly with proton transfer, to make the process energetically feasible. The adducts (C6H6+•)S are observed with blocked solvent molecules where the subsequent switching reaction to yield S2H+ is slow, but not with alcohol reactants that can form hydrogen-bonded chains that facilitate fast subsequent proton extraction. Correspondingly, kinetic simulations suggest that the mechanism proceeds through (C6H6+•)S+S→S2H++C6H5• and C6H6+•+2S→S2H++C6H5• reactions, respectively. The rate coefficients of these reactions are in the range 10−13−10−12 cm3 s−1 for the reaction through a bimolecular switching channel and in the range 10−26−10−28 cm6 s−1 for reaction through a direct termolecular proton extraction mechanism. The relation to energetics and reactant structure is examined.
Effect of carrier-gas pressure on nucleation
Authors: Kashchiev Dimo
Published: July 2002

A theory is proposed for the rate of isothermal stationary nucleation of liquid or solid phases from vapors in the presence of inert carrier gas. Formulas are derived for the nucleation rate and the critical supersaturation as functions of the carrier-gas pressure. It is found that, depending on the interactions between the molecules of the carrier gas and the condensing vapors, the carrier-gas pressure can either stimulate or inhibit the nucleation process. The analysis shows that, typically, this effect is relatively small for carrier-gas pressures not much greater than the atmospheric pressure and is more pronounced for less supersaturated condensing vapors.
Zeeman studies of the first excited states of palladium and metal free phthalocyanines in Shpol’skii matrix
Authors: Huang S.-C. , Chen W.-H.
Published: July 2002

Phthalocyanines (Pcs) are planar molecules of relatively high symmetry and exhibit magneto-optic effects in many ways. We report the Zeeman effect of the first excited states of palladium (Pd–) and free-base (H2–) Pcs in a 5 K Shpol’skii matrix by direct observation of their fluorescence position shifts. From the experimental data an apparent angular momentum integral, coupling the lowest two Jahn–Teller stabilized crystal field split first excited states, Λ′, was found to be 2.7±0.1 ℏ for PdPc. No conclusive Λ′ was deduced for H2Pc but its value can be estimated to be around the theoretical value of 3 ℏ. The experimental data of H2Pc revealed a complex structure in the fluorescence band examined, which complicated the analysis of Λ′ but helped unravel the biexponential decay problem of the structure. We infer one narrower band in the structure has the previously reported lifetime of τ  6.3 ns, and the other broader band being ∼1.8 cm−1 higher, has τ  3.7 ns. We believe this composite situation also exists for the other tautomeric structure since it also exhibited biexponential decay.
Acid–base chemistry in the gas phase: The trans-1-naphthol⋅NH3 complex in its S and S1 electronic states
Authors: Humphrey Susan J. , Pratt David W.
Published: July 2002

We deduce information about the dynamics of a proton transfer reaction between an acid and a base. Our probe is the fully resolved S1←S0 fluorescence excitation spectrum of the 1:1 complex of 1-naphthol and ammonia in the gas phase. Analysis of this spectrum shows that the complex is planar in both electronic states, with the NH3 forming a nearly linear hydrogen bond to the hydroxy hydrogen atom of 1-naphthol. The O–H...N heavy atom separation is R=2.86 Å and the barrier to rotation of the NH3 group about its C3 axis is V3=39.9 cm−1 in the S0 state. Excitation of the complex to its S1 state increases the acidity of 1-naphthol, decreases the heavy atom separation to R=2.72 Å, and increases the torsional barrier to V3=46.5 cm−1. Modeling these changes using the Lippincott–Schroeder potential for the hydrogen bond shows that the photoinitiated heavy atom motion produces a significant decrease in the barrier to proton transfer in the S1 state.
The microwave spectrum of the NCl radical in the electronically excited (a 1Δ) state
Authors: Kobayashi Kaori , Goto Masahiro , Yamamoto Satoshi , Saito Shuji
Published: July 2002

The pure rotational spectrum of the 14N35Cl radical in the first electronically excited a 1Δ state was detected by microwave spectroscopy. The NCl radical was produced by a dc-glow discharge of an N2 and Cl2 mixture between 175–210 K. Seven rotational transitions for υ=0 and five for υ=1, showing hyperfine structures due to the nitrogen and chlorine nuclei, were observed in the 162–404 GHz region. The rotational, centrifugal distortion, and hyperfine coupling constants including nuclear spin–rotation coupling constant of the chlorine nucleus were determined accurately by a least-squares analysis of the measured frequencies. The equilibrium structural parameters were derived and discussed.
Kinetic study of the reaction of Mn(a6S5/2) with N2O from 448 to 620 K
Authors: Campbell Mark L.
Published: July 2002

The gas phase reactivity of Mn(a6S5/2) with N2O in the temperature range 448–620 K is reported. Manganese atoms were produced by the photodissociation of 2-methylcyclopentadienyl manganese tricarbonyl and detected by laser-induced fluorescence. The reaction rate of the a6S5/2 state is very slow and temperature dependent. The rate constants are independent of total pressure indicating a bimolecular reaction. The rate constants are described in Arrhenius form by (2.05±0.45)×10−10 exp(−44.7±1.0 kJ/mol/RT) cm3 s−1.
Potential energy surface for and pure rotational spectra of isotopomeric Cl2–Ar van der Waals complexes
Authors: Wang Feng , McCourt Frederick R. W.
Published: July 2002

Pure rotational spectra have been calculated for the three isotopomers of the Cl2–Ar van der Waals complex formed by Cl2 in its ground electronic state. The potential energy surface involved is a sum of pairwise Morse atom–atom interactions, which are merged asymptotically into an anisotropic van der Waals form. The Morse atom–atom interactions have been refined by adjusting their parameters to obtain agreement with both excitation spectra and recent microwave transitions observed for the 35Cl2–Ar van der Waals complex, whereas the anisotropic long-range part of the potential surface has been taken from ab initio results [Mol. Phys. 80, 533 (1993)]. The present model potential surface predicts a T-shaped structure for the complex, in agreement with experiment. Excellent agreement has been found between the calculated and experimental microwave transition frequencies for the 35Cl37Cl–Ar complex. Good agreement has also been found between the experimentally determined bond energy, bond length, and average ‘‘bond angle’’ governing the overall geometry of the complex and the corresponding quantities determined from dynamical calculations based upon the present potential surface. The new potential surface has also been utilized to predict the microwave spectrum of the 37Cl2–Ar isotopomer.
One-electron second-order optical activity of a helix
Authors: Maki Jeffery J. , Persoons André
Published: July 2002

The second-order nonlinear-optical response of a chiral molecule is calculated. We model the optical response classically using a single electron bound to a helical path. The helical motion of the electron causes optical activity in the second-order response. The hyperpolarizability tensor of a single helix and the susceptibility tensor for a thin film of helices are given. We examine the process of second-harmonic generation from a chiral surface using the calculated susceptibility tensor. The efficiency of the harmonic generation is different for left- and right-hand circularly polarized fundamental light, which is ascribed to be a form of nonlinear optical activity. The roles of pitch and radius of the helix are readily seen in the microscopic and macroscopic second-order optical responses and in the surface second-harmonic generation, which may provide some insight for synthesizing new chiral compounds. Our results also allow us to draw conclusions about the relative strength and importance to second-order optical activity of electric- and magnetic-dipole transitions. For instance, we confirm that optical activity can occur in surface second-harmonic generation from electric-only response, but we find that magnetic response can make a similar contribution and thus should not be ignored.
Orientational correlation functions and polarization selectivity for nonlinear spectroscopy of isotropic media. II. Fifth order
Authors: Tokmakoff A.
Published: July 2002

The influence of orientational relaxation on the fifth-order nonlinear polarization is evaluated for off-resonant Raman and dipole resonant experiments in the perturbative limit. The symmetry properties of the χ(5) tensor are discussed, and correlation functions for orientational diffusion are calculated for all fifth-order tensor elements relevant to isotropic media. The results allow generalization of the effect of orientational motion on the nonlinear experiment of arbitrary order. The general polarization dependence of the fifth-order experiments are obtained and used to address the selectivity of certain polarization geometries to orientational dynamics. It is shown that for χ(5) Raman spectroscopies, no combination of polarizations will give complete selectivity to the isotropic, anisotropic, or nonresonant contributions to the Raman polarizability tensor. Similar restriction of selectivity exists for resonant experiments. Polarization conditions are suggested to facilitate the comparison of third- and fifth-order spectroscopies.
Collisional deactivation of highly vibrationally excited pyrazine
Authors: Miller Laurie A. , Barker John R.
Published: July 2002

The collisional deactivation of vibrationally excited pyrazine (C4N2H4) in the electronic ground state by 19 collider gases was studied using the time-resolved infrared fluorescence (IRF) technique. The pyrazine was photoexcited with a 308 nm laser and its vibrational deactivation was monitored following rapid radiationless transitions to produce vibrationally excited molecules in the electronic ground state. The IRF data were analyzed by a simple approximate inversion method, as well as with full collisional master equation simulations. The average energies transferred in deactivating collisions (〈ΔE〉d) exhibit a near-linear dependence on vibrational energy at lower energies and less dependence at higher energies. The deactivation of ground state pyrazine was found to be similar to that of ground state benzene [J. R. Barker and B. M. Toselli, Int. Rev. Phys. Chem. 12, 305 (1990)], but it is strikingly different from the deactivation of triplet state pyrazine [T. J. Bevilacqua and R. B. Weisman, J. Chem. Phys. 98, 6316 (1993)].
Collisional deactivation of N2O(001) studied by time-resolved infrared fluorescence
Authors: Poel Kathleen L. , Alwahabi Zeyad T. , King Keith D.
Published: July 2002

The time-resolved infrared fluorescence (IRF) technique has been used to study the vibrational deactivation of excited N2O by large polyatomic colliders at ambient temperature (295±2 K). N2O(0001) molecules were prepared by direct pumping with the P(18) line of a pulsed CO2 laser at 9.536 μm. The bimolecular rate constant for self-deactivation was determined to be (0.763±0.006)×103 Torr−1 s−1, in very good agreement with previous work. The rate constants for deactivation by Ar and H2 were found to be (0.103±0.003) and (4.89±0.52)×103 Torr−1 s−1, respectively. The deactivation rate constants for the large polyatomic molecules, c-C6H10, c-C6H12, C6H6, C6D6, C7H8, C7D8, C6H5F, p-C6H4F2, C6HF5 and C6F6, were found to be (176±10), (153±22), (115±4), (201±2), (127±11), (407±52), (144±14), (173±13), (129±8), and (48±9)×103 Torr−1 s−1, respectively. Experimental deactivation probabilities and average energies removed per collision are calculated and compared. There is little difference in deactivation probabilities between the acyclic ring compounds and their aromatic analogues and the partially-fluorinated benzenes but the perfluorinated compound, C6F6 is much less efficient than the other species. The perdeuterated species, C6D6 and C7D8, especially the latter, show enhanced deactivation relative to the other species, probably as a result of near-resonant intermolecular V–V energy transfer. The results are compared with our recent work on the deactivation of CO2(0001) by the same group of large polyatomic colliders [K. L. Poel, Z. T. Alwahabi, and K. D. King, Chem. Phys. 201, 263 (1995)].
Electron-impact excitation of low-lying preionization-edge n→σ* and Rydberg transitions of CHF2Cl and CHFCl2: Absolute generalized oscillator strength measurement
Authors: Ying J. F. , Leung K. T.
Published: July 2002

Angle-resolved electron energy loss spectroscopy has been used to determine the absolute generalized oscillator strengths (GOSs) of valence-shell electronic transitions of difluorochloromethane (CHF2Cl) and dichlorofluoromethane (CHFCl2) as functions of energy loss and momentum transfer at an impact energy of 2.5 keV. Absolute GOS profiles of the prominent low-lying preionization-edge energy loss features of CHF2Cl and CHFCl2 were determined and found to be consistent with the previous assignments of the underlying transitions made by VUV photoabsorption spectroscopy. In particular, the lowest-lying features at 8.0 eV in CHF2Cl and at 7.5 eV in CHFCl2 have been attributed predominantly to electronic excitations from the Cl 3p nonbonding (n) orbitals to the C–Cl σ* antibonding orbital, in good accord with single-excitation configuration interaction (CI) excited-state calculations. The corresponding GOS profiles of these n(Cl 3p)→σ*(C–Cl) (HOMO→LUMO) transitions revealed an interesting trend of increased dipole character with increasing Cl content, i.e., from an essentially quadrupole-dominated profile, characterized by a maximum at K2∼0.9 a.u., in CHF2Cl to a mixed dipole-quadrupole profile in CHFCl2 and CHCl3. The CI calculations further showed that some of the underlying n(Cl 3p)→σ*(C–Cl) transitions in CHF2Cl, CHFCl2, and CHCl3, like the other chlorofluorocarbons: CF3Cl, CF2Cl2, CFCl3, and CCl4, could lead to dissociation of the C–Cl bond. In addition, the GOS profiles of the remaining low-lying preionization-edge features at 9.8 and 11.2 eV in CHF2Cl and at 9.4, 10.7, and 11.6 eV in CHFCl2 were also determined. These features have been previously assigned as Rydberg transitions originated from the nonbonding HOMOs. In particular, these experimental GOS profiles were found to be dominated by a strong maximum at K=0, which is indicative of strong dipole interactions. The weak secondary maxima observed at K2∼2.8–3.5 a.u. could be interpreted qualitatively in terms of the spatial overlaps between the initial-state and final-state orbital wave functions. Together with our earlier work on CHF3 and CHCl3, the present work on the remaining members of the CHFmCl3−m (m=0–3) series, CHF2Cl and CHFCl2, provides further evidence for the empirical trends on the preionization-edge structures observed in the CFnCl4−n (n=0–4) series.
Semiclassical calculations on the energy dependence of the steric effect for the reaction Ca(1D)+CH3F(jkm=111)→CaF+CH3
Authors: Meijer Anthony J. H. M. , Groenenboom Gerrit C. , van der Avoird Ad
Published: July 2002

In a previous article [A. J. H. M. Meijer, G. C. Groenenboom, and A. van der Avoird, J. Chem. Phys. 101, 7603 (1994)] we investigated the energy dependence of the steric effect of the reaction Ca (1D)+CH3F (jkm=111)→CaF (A 2Π)+CH3 using a quasiclassical trajectory method. It was found that we could not reproduce the experimental results for this reaction [M. H. M. Janssen, D. H. Parker, and S. Stolte, J. Phys. Chem. 95, 8142 (1991)]. In this article, we reinvestigate this reaction using a semiclassical method, in which the rotation of the molecule and the electronic states of the interacting atom and molecule are treated quantum mechanically. For the chemical reaction we use a model which correlates the projection of the electronic orbital angular momentum of the Ca atom on the intermolecular axis with the projection of the electronic orbital angular momentum of the CaF product on the diatomic axis [M. Menzinger, Polon. Phys. Acta A 73, 85 (1988)]. This model is applied to examine the CaF (A 2Π, B 2Σ+, A′ 2Δ) exit channels separately. We conclude that we can reproduce the experimental results for the steric effect using this model. The improvement with respect to the classical trajectory results is shown to be due primarily to the extended reaction model rather than to the semiclassical description of the dynamics. We find trapping and reorientation in the semiclassical calculations, as in the previous classical trajectory results, but also non-adiabatic effects are present. The latter do not affect the reactive cross sections very much.
The vibrational spectra of molecular ions isolated in solid neon. XIII. Ions derived from HBr and HI
Authors: Lugez Catherine L. , Jacox Marilyn E. , Thompson Warren E.
Published: July 2002

When a Ne:HBr or a Ne:HI sample is codeposited at approximately 5 K with discharge-excited neon atoms, the infrared spectrum of the resulting solid includes not only the absorptions of the HX molecule and its multimers but also a prominent absorption of HBr+ or HI+. The absorption of each of the two cations lies within a few cm−1 of its gas-phase band center. The vibrational fundamentals of DBr+ and DI+ are also identified in experiments on deuterium-enriched samples. Other infrared absorptions are assigned to normal and deuterium-substituted (HBr)+2 and (HI)+2, as well as to BrHBr−, BrDBr−, and IHI−. Studies of changes in the absorption spectrum following exposure of the deposit to filtered visible and near ultraviolet radiation support these assignments and provide further information on photodissociation and photodetachment processes occurring in these systems.
Electron localization and metal–nonmetal transition in fluid KxKCl1−x: An electron spin resonance study of the magnetic properties with in situ variation of x (10−4⩽x⩽10−1)
Authors: Schindelbeck T. , Freyland W.
Published: July 2002

We have measured the electron spin resonance (ESR) spectra of saltrich KxKCl1−x melts at temperatures near 800 °C and over a broad composition range, 10−4≤x≤10−1, approaching the metal–nonmetal (M–NM) transition in these solutions. Emphasis has been given to a precise in situ variation of composition which has been achieved for the first time in a high temperature ESR experiment by Coulometric titration. The spectra are characterized by a motional linewidth narrowing with a Lorentzian line shape. The ESR characteristics as determined from the Lorentz fits of the spectra exhibit the following features as a function of x: In the nonmetallic regime the g factor (g=1.9938±0.0002) and the peak-to-peak halfwidth (ΔBPP=0.2 mT) stay constant up to x∼0.05. Above this composition a clear increase of ΔBPP is observed indicating the NM–M transition. The spin susceptibility ϰs has been determined from the imaginary part of the Lorentz fits and has been calibrated against a sapphire signal measured simultaneously with the KxKCl1−x liquid samples. The spin susceptibility strongly deviates from simple Curie behavior even at low x which gives evidence for spin paired states in the nonmetallic solutions. Above x∼0.05 ϰs rises steeply with x. The overall variation of ϰs with x is interpreted by two limiting models: a thermodynamic defect model focusing on the particle character of localized paramagnetic and diamagnetic states for x≤0.05, and a simple band model describing the strong increase of ϰs approaching the NM–M transition. Both approaches yield a satisfactory description of the observed variation of ϰs. Further support of this interpretation is found in the spin dynamics which are qualitatively discussed.
Transition state and dynamics of unimolecular no-barrier fragmentation: Thermal dissociation of N2O4
Authors: Katō Toshiko
Published: July 2002

Thermal dissociation reaction into polyatomic molecules on the ground state no-barrier potential energy surface is studied by classical molecular dynamics simulations of N2O4⇌2NO2. A phase space surface ET=Veff(Rl)≳0 is identified as the transition state (TS), where ET is the sum of the potential and kinetic energies of interfragment motion and Veff(Rl) is the orbital angular momentum-dependent effective barrier. By dividing the motion of the system into fragments’ vibrational (V), rotational (R), and interfragment (T) modes, where the T mode is composed of translational (TT) and orbital (TL) modes, a scheme of reactive energy transfer for fragmentation is presented. The present energy condition for the TS is in accord with the one of phase space theory (PST). The observed photofragmentation rates of NCNO and CH2CO which increase with energy slower than predicted by PST suggest that intrareactant energy redistribution may influence the rate. Dissociation is found to occur by energy redistribution among T−R−V modes followed by the one among TT−TL−R modes, which determine the product vibrational and rotational distributions, respectively. This scheme supports separate statistical ensemble method in reproducing the nascent distributions from unimolecular photofragmentation at excess energies above the vibrational threshold.
Theoretical calculation of photodetachment intensities for H3O−
Authors: Thompson Ward H. , Karlsson Hans O. , Miller William H.
Published: July 2002

We have calculated total and arrangement-selected photodetachment intensities for the H3O− anion (and its deuterated form, D3O−) using a Green’s function in a discrete variable representation with absorbing boundary conditions. A multiply-shifted quasiminimal residual method is used to obtain the Green’s function for many energies at once. We present spectra obtained by explicitly treating two and four degrees of freedom. Comparison with experiment indicates that the bending angles in the anion and neutral are more similar than in the current potential energy surfaces. The calculated spectra are also consistent with the suggestion that the barrier should be ‘‘earlier.’’
A relativistic Kohn–Sham density functional procedure by means of direct perturbation theory. II. Application to the molecular structure and bond dissociation energies of transition metal carbonyls and related complexes
Authors: van Wüllen Christoph
Published: July 2002

The implementation of analytical geometry gradients within the framework of the relativistic density functional procedure described earlier allows the calculation of the geometrical structure and bond dissociation energies of polyatomic molecules. This has been done for the nine transition metal carbonyls M(CO)n (n=6: M=Cr, Mo, W; n=5: M=Fe, Ru, Os; n=4: M=Ni, Pd, Pt). To determine the first metal–carbonyl bond dissociation energy, a complete geometry optimization of the fragments M(CO)n−1 has been performed, and the energy differences have been corrected for the basis set superposition error (BSSE). The same procedure has been applied to the molecular structure of the nine complexes M(CO)5L (M=Cr, Mo, W; L=N2, CS, NO+) and their M–L bond dissociation energies. The results are in good agreement with quasirelativistic density functional and high-level ab initio calculations. In most cases, the agreement with experimental values, where available, is good as well.
Vibrational predissociation of ArCl2: Toward the determination of the potential energy surface of the B state
Authors: Janda Kenneth C. , Roncero Octavio , Halberstadt Nadine
Published: July 2002

Accurate quantum mechanical calculations are carried out to test the sensitivity of the spectroscopy and dynamics of the B state of ArCl2 to the steepness of the Morse term, α, of an atom–atom potential. It is discovered that the predissociation dynamics for this molecule are very complicated even in the Δv=−1 regime due to resonances in the continuum manifold of states. In both the Δv=−1 regime and the Δv=−2 regime the rate of vibrational predissociation and the product rotational distribution are extremely sensitive to the value chosen for α, but not in a regular way. For the Δv=−2 regime the variations can be attributed to spacings between resonances and the overlaps of the bright state wave functions with nearby dark states as expected from the intramolecular vibrational relaxation model. In the Δv=−1 regime, the variations are shown to originate from resonances in the v−1 continuum set of states. Although this makes it difficult to determine the value for α, a value of 1.8 Å−1 is probably close to the true value. The most useful new data to determine the potential would be measurements of the lifetimes for as many vibrational levels as possible and rotational distributions for excitation to low vibrational levels of the B state.
Adsorbate–adsorbate repulsions—the coverage dependence of the adsorption structure of CO on Cu(110) as studied by electron-stimulated desorption ion angular distribution
Authors: Ahner Joachim , Mocuta Dan , Ramsier R. D. , Yates John T.
Published: July 2002

The coverage dependent orientation of CO adsorbed on a Cu(110) surface was studied by the electron-stimulated desorption ion angular distribution (ESDIAD) technique. A neutral excited (CO*) species is imaged and in addition positive ions are measured. The adsorption temperature was varied between 32 K and 150 K. By applying the ESDIAD technique at a temperature below 80 K it was possible to decrease the beamwidths drastically, to determine the angular distributions better than ±0.5°, and to study the adsorption of CO chemisorbed and physisorbed on the surface. With increasing CO coverage we observe three distinct ESDIAD patterns. Starting from a normal beam pattern with an elliptical cross section with the major axis oriented in the 〈11̄0〉 direction for coverages up to 0.2 monolayer (ML), a transformation of the ESDIAD pattern into a pattern of two separated beams is observed for a coverage of about 0.5 ML, indicating a tilting of the molecules in the 〈11̄0〉 directions by ∼9°. With further increasing CO coverage an additional central peak develops with an elliptical broadening now in the 〈001〉 direction. The changes of the pattern are reversible as shown by decreasing the coverage by thermal desorption. Based on these ESDIAD and digital low energy electron diffraction results, a linear-chain model for CO adsorption is proposed. Temperature programmed desorption measurements also indicate the presence of repulsive CO–CO interactions in the adlayer.
Molecular orientation of submonolayer rhodamine-6G on quartz substrates: A comparative study using reflection and transmission UV-Vis spectroscopy
Authors: Elking Mark D. , He Gregory , Xu Zhi
Published: July 2002

Reflection and transmission UV-Vis spectroscopy have been applied to study the molecular orientation and surface density of rhodamine-6G molecules physisorbed on optically flat quartz (SiO2) substrates. Our results have shown that for the s-polarized excitation, the submonolayer of physisorbed rhodamine-6G dye molecules causes enhanced reflection in the wavelength region from 400 nm to 600 nm where the electronic transition takes place. For the p-polarized excitation, the reflection is enhanced when the angle of incidence is smaller than Brewster’s angle of quartz at 55.6°, and the reflection is reduced when the angle of incidence is larger than Brewster’s angle of quartz. An independent method has been established in this paper by which the molecular orientation can be determined accurately by carrying out optical measurements in both the reflection and transmission directions.
Topology of electron–electron interactions in atoms and molecules. I. The Hartree–Fock approximation
Authors: Cioslowski Jerzy , Liu Guanghua
Published: July 2002

Topologies of the electron intracule and extracule densities, I(R) and E(R), are analyzed. These topologies are found to be inherently more complex than those of the one-electron density. The main topological features of I(R) and E(R) are already present in the densities calculated within the Hartree–Fock (HF) approximation. Results of test calculations on several planar systems show that the positions and properties of attractors in I(R) and E(R) are predicted with a surprising fidelity by a naive independent-atom model, making it possible to index distinct types of electron pairs present in atoms and molecules. In general, each pair of atoms in a given molecule has the potential of producing a pair of attractors in I(R). At the HF level of theory, all the atoms collectively furnish a single attractor in I(R) at R=0, but this topological pattern is bound to change upon the inclusion of electron correlation. The attractors in E(R) stem from both individual atoms and atomic pairs. In addition, attractors that are not associated with either of these entities are observed. The plethora of attractors present in I(R) and E(R) give rise to complicated patterns of other critical points. Unusual topological features, such as attractors linked by multiple interaction lines and bifurcations at bond points, are also encountered.
The intermolecular potential energy surface for CO2–Ar: Fitting to high-resolution spectroscopy of Van der Waals complexes and second virial coefficients
Authors: Hutson Jeremy M. , Ernesti Andreas , Law Mark M. , Roche Christine F. , Wheatley Richard J.
Published: July 2002

Two potential energy surfaces for CO2–Ar are obtained by least-squares fitting to the high-resolution spectra of Van der Waals complexes and the second virial coefficients of Ar+CO2 gas mixtures. The potentials incorporate a repulsive wall based on monomer ab initio calculations and the assumption that the repulsion potential is proportional to the overlap of the monomer charge densities. The dispersion energy is represented in a two-site model, with dispersion centers located along the C–O bonds of CO2. The resulting potentials give a good representation of all the experimental data with only three or four adjustable parameters. They are quite different from previous empirical CO2–Ar potentials, which all have either a poor representation of the attractive well or a poor representation of the repulsive wall.
Low temperature optical dephasing in polymethylmethacrylate: A photon echo study below 1 K
Authors: Zilker S. J. , Haarer D.
Published: July 2002

The first photon echo experiments on polymethylmethacrylate (PMMA) down to temperatures below 1 K are reported. A linear temperature dependence of the linewidth of the chromophore zinc-meso-tetraphenylporphine (ZnTPP) is obtained below 1 K. Based on this result a flat distribution of two-level system energies E on the time scale of our experiment is deduced. Above 1 K an activated process with an energy of 5 cm−1 is found, which explains the power law dependence usually measured by photon echo experiments in the temperature region 1.2–2.1 K. The fit parameters of the proposed process are shown to be in agreement with hole burning measurements and with specific heat data on PMMA.
Study of polar dumbbell fluids from the gaseous to the liquid densities by the reference interaction site model-1 and -2 integral equations
Authors: Takebayashi Y. , Kimura Y. , Hirota N. , Ohba M.
Published: July 2002

The structures and the thermodynamic properties of a fluid composed of a nonpolar or a polar dumbbell molecule have been studied from the gaslike to the liquidlike density regions based on two types of the reference interaction site model (RISM) integral equation theories (RISM-1 and RISM-2). This is the first application of the RISM-2 theory to a polar dumbbell fluid. We have proven that the RISM-2 theory with the hypernetted-chain (HNC) approximation has an inconsistency with respect to the zeroth-order relation between site–site total correlation functions and site–site direct correlation functions in the Fourier space. An hypothetical bridge function is introduced to remedy this inconsistency, which works well to give good information on the structure and the dielectric constant in the lower-density region. On the other hand, the RISM-1 theory works well in the higher-density region, but not well in the lower-density region. Complemental application of these theories, that is, the RISM-1 theory for the high-density region and the RISM-2 theory for the low-density region, allows us to understand the properties of fluid over wide density regions.