bluuebunny/crossref_metadata_2025 huggingface dataset:
Half-collision studies of singlet-to-triplet energy transfer: Action spectroscopy and predissociation dynamics of electronically excited Cd⋅H2 and Cd⋅D2 complexes
Authors: Wallace Ingvar , Funk David J. , Kaup John G. , Breckenridge W. H.
Published: July 2002

The Cd⋅H2 and Cd⋅D2 van der Waals complexes were synthesized by expanding cadmium vapor and He/H2 mixtures into a supersonic free jet. The ‘‘half-collision’’ process, Cd(5s5p 1P1)⋅H2→Cd(5s5p 3PJ) +H2, was studied by fixing a probe dye laser pulse (delayed 10 ns) onto one of the Cd(5s6s 3S1←5s5p 3PJ) transitions while exciting the Cd⋅H2(Cd⋅D2) complex with a pump dye laser pulse tuned across frequencies near that of the free Cd(5s5p 1P1←5s5s 1S0) atomic transition. When the probe laser was tuned to detect Cd(5s5p 3P2), an action spectrum to the red of the atomic transition was obtained for Cd⋅H2 consisting of a broad continuum superimposed upon which was an anharmonic series of vibrational transitions with discernible, blue-shaded rotational structure.

A similar spectrum was recorded for Cd⋅D2, except that only very broadened blue-shaded rotational structure was observed. From the isotopic band-head splittings, computer simulations of the rotational band structure, and recent ab initio calculations of Cd⋅H2 potential surfaces by Jack Simons and co-workers, it is tentatively concluded that the vibrational resonances are due to the (1B1;K′a=0) ←(1A1;K■a=1) perpendicular transition of ortho Cd⋅H2 (para Cd⋅D2 ). The continuum is assigned to the other perpendicular transitions of para and ortho Cd⋅H2(Cd⋅D2) which excite 1B2(K′a=0,1,2) and 1B1(Ka=1,2) states. The attractive 1B2 state predissociates rapidly due to potential surface crossings with both the 3A1 and 3B1 repulsive states of CdH2, and we postulate that the 1B1 energy levels with K′a≥1 are strongly coupled with the 1B2 levels with Ka≥1. The 1B1(K′a=0) level is predissociated via a less efficient surface crossing with the more strongly repulsive 3A1 state.

The yield of Cd(5s5p 3P2) vs Cd(5s5p 3P1) for the ‘‘vibrational resonance’’ mechanism, 2.6±0.2, is quite different from that for the ‘‘continuum’’ mechanism, 1.17±0.05, consistent with this idea. The 3A1 state correlates only with Cd(5s5p 3P2), but the B2 spin–orbit component of the 3B1 state (which will be produced by predissociation of the 1B2 state) correlates with Cd(5s5p 3P1). We estimate that the CdH2(1B1) state has a value of De≊1700 cm−1 at R′e(Cd–H2)≊2.5 Å, qualitatively consistent with the ab initio calculations, which show that the CdH2(1B1) potential is quite flat, with no H–H bond stretching or appreciable repulsion until Cd–H2 distances as small as 2.0 Å are reached.
Direct observation of collision induced transitions between the a 1Πg (v=0) and a′ 1∑−u (v=0) levels of N2 via double resonance enhanced multiphoton ionization spectroscopy
Authors: Katayama Daniel H. , Dentamaro Anthony V.
Published: July 2002

We report on the first direct observation of collision induced electronic transitions in a diatomic molecule using double resonance enhanced multiphoton ionization (REMPI) spectroscopy. The collision partner to induce these transitions in N2 between the electronic states, a 1Πg (v=0) and a′ 1∑u− (v=0), is helium. A pump laser selectively excites a rotational level of the a 1Πg (v=0) state by tuning the laser to a suitable line of the (2+2) a 1Πg–X 1∑g+ (0,0) REMPI spectrum and a probe laser records the collision induced transitions to the a′ 1∑u− (v=0) manifold by scanning the (1+1) REMPI x 1∑g−–a′ 1∑u− (1,0) band. Both pulsed lasers are coincident in time to optimize single collision effects. The results show that the collision induced electronic transitions over energy gaps of approximately 1100 to 1200 cm−1 are as efficient as rotational energy transfer and this technique can be used to access high lying electronic states of the molecule.
Vibrational predissociation and intramolecular vibrational relaxation in dimethyl-s-tetrazine–argon complexes
Authors: Osborn David L. , Alfano Joseph C. , Dantzig Niels van , Levy Donald H.
Published: July 2002

The one- and two-argon van der Waals complexes of dimethyl tetrazine (DMT) have been investigated by means of laser-induced excitation and dispersed fluorescence spectroscopy. The band shifts D″0 − D′0 range from −23 to −31 cm−1 for DMT–Ar relative to DMT. The excited state dissociation energy of the DMT–Ar bond was estimated to be 270 ≤ D′0 ≤ 484 cm−1. In all levels with energy greater than the argon dissociation energy, predissociation is very fast. A dissociation rate constant kD = 3 × 109 s−1 has been determined for dissociation from the 6a1 level. This rate constant is an order of magnitude larger than that observed in tetrazine–argon. The dissociation rate constant of DMT–Ar2 from the 6a1 level is approximately 2 × 1010 s−1. Unlike aminotetrazine–Ar2, DMT–Ar2 shows no mode-selective variations in the branching ratios between the DMT and DMT–Ar photoproducts.
A high resolution infrared double resonance technique for molecular eigenstate spectroscopy in a free jet
Authors: Go Jungsug , Perry David S.
Published: July 2002

An infrared double resonance technique for the study of jet-cooled polyatomics is reported which offers state selection, access to one-photon-forbidden vibrations, sub-Doppler resolution, and high sensitivity. Molecular eigenstate spectra of the propyne 2ν1 band reveal a predicted doorway state which mediates the two-stage IVR coupling mechanism.
Theory of spin relaxation in bicontinuous cubic liquid crystals
Authors: Halle Bertil , Ljunggren Stig , Lidin Sven
Published: July 2002

A theoretical framework is presented for the interpretation of spin relaxation data from bicontinuous cubic lyotropic phases described by periodic minimal surfaces. Specifically, the two irreducible time correlation functions (TCFs) that determine the contribution from surface diffusion to the observable spin relaxation rates are considered. Simple analytical results are obtained that relate the initial TCFs to the fourth-rank orientational order parameter of the dividing interface and the initial decay of the TCFs to the average Gaussian curvature over the cubic unit cell. These exact results are used to construct single-exponential approximations for the TCFs. Explicit calculations are reported for the three cubic triply periodic minimal surfaces of simplest topology, i.e., Schwarz’s D and P surfaces and Schoen’s gyroid surface, as well as for the corresponding parallel surfaces that have been used to model the dividing interface (the locus of surfactant headgroups) in bicontinuous cubic phases. The theoretical results presented here demonstrate that spin relaxation data can provide quantitative information about microstructure in bicontinuous cubic phases in surfactant and lipid systems. In particular, the spin relaxation method can discriminate among different microstructures belonging to the same space group, such as the P and C(P) surfaces.
Theory of phase equilibria and closed-loop liquid–liquid immiscibility for model aqueous solutions of associating chain molecules: Water +alkanol mixtures
Authors: Green Daron G. , Jackson George
Published: July 2002

In this work we examine the phase equilibria exhibited by a number of model binary mixtures of water+alkanols. The water molecules are modeled as hard spheres with four off-center square-well hydrogen-bonding sites representing the two hydrogen atoms and the two electron lone pairs on the oxygen atom. Dispersion forces are included and are treated within the mean-field approximation. Each alkanol is modeled as a chain molecule formed from fused hard spheres with dispersion forces and two hydrogen-bonding sites. Thus water–water, water–alkanol, and alkanol–alkanol association is allowed. A simple thermodynamic perturbation theory is used to develop an augmented van der Waals equation of state which treats the asymmetry in the attractions which arise because of the directional hydrogen-bonding sites, and the asymmetry in the repulsions caused by the molecular shape of the chains. The effects of chain length and association on the global phase equilibria are studied. As well as the usual gas–liquid phase separation, liquid–liquid and gas–gas phase separation, positive, negative, and double azeotropes, and Bancroft points are found. Closed-loop liquid–liquid immiscibility corresponding to the type VI classification of Scott and van Konynenburg is seen for water+alkanol mixtures with intermediate chain lengths and bonding energies. We also discuss our serious reservations about the type VI behavior claimed to have been found in some recent work. Although the potential parameters used in our work were not chosen to model any specific systems, it is gratifying to see that the theory can reproduce the types of phase behavior exhibited by aqueous solutions of alkanols.
Photodissociation of hydrogen chloride at 157 and 193 nm: Angular distributions of hydrogen atoms and fine-structure branching ratios of chlorine atoms in the 2P
j levels
Authors: Tonokura Kenichi , Matsumi Yutaka , Kawasaki Masahiro , Tasaki Shintaro , Bersohn Richard
Published: July 2002

The H, D, and Cl atoms from the photodissociation of HCl and DCl at 157 and 193 nm were detected by laser-induced fluorescence (LIF) in the vacuum ultraviolet region. The Doppler profiles of the H and D resonance lines at 121.6 nm in the LIF spectra indicate that the absorption of HCl and DCl at 157 nm is a mixture of perpendicular and parallel transition (A 1Π, a 3Π0+←X 1Σ+), while that at 193 nm results almost solely from the A 1Π←X 1Σ+ transition. The fine-structure branching ratios, are measured for the chlorine atom by LIF. Combined with the previous results, the branching ratios [Cl*(2P1/2)]/[Cl(2P3/2)] are 0.81±0.09 at 157 nm, 0.50±0.05 at 193 nm for HCl, and 0.23±0.05 at 193 nm for DCl. These results suggest that the nonadiabatic couplings during the breakup of HCl (DCl) in the excited states play an important role in determination of the fine-structure branching ratios.
Photofragmentation of CF3I+ produced by resonant multiphoton ionization
Authors: Waits Leslie D. , Horwitz Ronald J. , Daniel Robert G. , Guest Joyce A. , Appling Jeffrey R.
Published: July 2002

Time-of-flight mass spectrometry has been used to examine the spatial anisotropy and translational energy of I+ and CF3+ fragments resulting from resonant multiphoton excitation of CF3I in the 300–306 nm range. These ion species are shown to be largely the photofragments from the single-photon direct dissociation of CF3I+ generated via a new resonant (2+1) multiphoton ionization (MPI) transition. Photoelectron spectroscopy shows that most of the parent CF3I+ is produced in its ground X̃ 2E3/2 state by the resonant MPI process. All the observed I+ is formed via this parent ion photodissociation, while the observed CF3+ is formed via statistical decomposition of the parent as well as parent ion photodissociation. The present results are discussed in the context of the current understanding of alkyl halide photodynamics.
Nonlinear aspects of band structure in liquids. II. Solute spectra
Authors: Chen Zhe , Stratt Richard M.
Published: July 2002

If a set of solvent molecules are sufficiently weakly interacting, then their net effect on a solute is simply the sum of their individual effects. The shift and broadening of any solute quantum state would then be determined solely by the statistics of this sum (the local field at the solute). However, when the solvent–solvent interaction is comparable to that felt by the solute, the problem of ascertaining solvent effects on solute states becomes one of understanding how a band of collective solvent states couples to a solute. In this paper we show that a nonlinear liquid theory for band structure can be used to understand phenomena such as inhomogeneous broadening from precisely this solvent–band perspective. Inhomogeneous broadening in this language arises from configuration-to-configuration fluctuations in the solvent’s Green’s function, the size of which one can now evaluate.
Theoretical and neon matrix electron spin resonance studies of the methanol cation: CH3OH+, CH3OD+, CH2DOH+, and 13CH3OH+
Authors: Knight Lon B. , Kerr Kelly , Villanueva Martha , McKinley Allan J. , Feller David
Published: July 2002

Four isotopes of the methanol cation radical (CH3OH+) have been generated by three independent methods and isolated in neon matrices at 4 K for a detailed electron spin resonance (ESR) investigation. The ion generation methods employed were X irradiation, photoionization, and electron ionization. The nuclear hyperfine (A tensors) measurements were compared with those obtained from ab initio extended basis set multireference configuration interaction (CI) wave functions. The relationships between geometry and electronic structures were fully explored. The trend in the large isotropic methyl hydrogen A values for the isoelectronic series CH3F+, CH3OH+, and CH3NH2+ was found to follow the trend in dissociation energies for these radical cations. The neon magnetic parameters for CH3OH+ are gx=2.0036(4) and gz=2.010(1); Aiso (methyl hydrogens)=229(1) MHz, ‖Ax‖=54(2) and ‖Az‖=80(3) MHz for the hydroxy hydrogen; ‖Ax‖=40(2) and ‖Az‖=29(4) MHz for 13C. The observed magnetic parameters for CH2DOH+ indicate an unusually large deuterium effect Aiso (CH2)=329 MHz with ‖Aiso‖=4.1(3) MHz for the methyl deuterium. These results show that averaging of the methyl hydrogen environments is occurring on the ESR time scale.
The 3dNi8(3F)3dCu10σ2σ*1 manifold of excited electronic states of NiCu
Authors: Spain Eileen M. , Morse Michael D.
Published: July 2002

Diatomic NiCu has been supersonically cooled in a molecular beam and investigated by resonant two-photon ionization spectroscopy. A total of nine band systems connecting the ground 3d9Ni3d10Cuσ2, X 2Δ5/2 state to the 3d8Ni(3F)3d10Cuσ2σ*1 manifold of states have been found, and bands of eight of these systems have been rotationally resolved and analyzed. L- and S-uncoupling interactions have been found between two pairs of states, resulting in B[11.9]2.5∼C[11.9]1.5 and [10.4]2.5∼[10.4]1.5(?) heterogeneous perturbations. In the case of the B∼C interaction, the magnitude of the perturbation matrix element has been deduced. The detailed spectroscopic analysis of the 3d8Ni(3F)3d10Cuσ2σ*1 manifold of states presented in this paper allows the electronic structure of NiCu to be understood in depth and provides a database for comparison to the ligand-field plus spin–orbit calculation of the NiCu excited states presented in the following paper.
Molecular crystal surface dielectric properties: Microscopic theory for linear response
Authors: Munn R. W.
Published: July 2002

A microscopic molecular theory is developed for dielectric behavior at molecular crystal surfaces. Since dipolar interactions between layers of molecules decrease rapidly with separation, only layers within a short distance of the surface are affected by it. Algebraic results are obtained relating the local electric field and the macroscopic electric field in a surface layer to an applied uniform electric field, and hence to one another. These lead to expressions for the linear electric susceptibility of a layer and of an assembly of layers. The results provide a basis for calculations of various surface-induced and surface-modified properties.
A finite basis-discrete variable representation calculation of vibrational levels of planar acetylene
Authors: Bentley Joseph A. , Wyatt Robert E. , Menou Michel , Leforestier Claude
Published: July 2002

A methodology for the calculation of high energy vibrational eigenstates of S0 acetylene is described. Acetylene is modeled as a 5D planar molecule. The discrete variable representation (DVR) is employed for radial coordinates with a finite basis representation (FBR) for the angular coordinates. Symmetry adaptation of the primitive basis (dimension 2.7 × 106) coupled with a two level diagonalization/truncation scheme maintains relatively small basis sets (< 2500 functions) in all diagonalizations. Eigenvalues up to nearly 3700 cm−1 above the ground state are reported.
Molecular-dynamics investigation of surface-induced melting in sulfur hexafluoride
Authors: Boutin Anne , Fuchs Alain H.
Published: July 2002

The thermal disordering of the three low-index faces of a molecular crystal: sulfur hexafluoride has been investigated by molecular dynamics simulation. The reliability of the model system used in this work has been tested several times in earlier works. Here, the triple point temperature Tm′ observed in the larger sample (1500 molecules) is equal to 225±5 K as compared to the calorimetric value of Tm=223 K. The following conclusions can be drawn from the temperature dependence of the structural and dynamics properties. A quasi-liquid-layer (QLL), having some liquidlike properties (for instance, translational mobility), but retaining some features of the solid (site-to-site molecular jumps and some long-range order), is observed in the temperature range of TQLL*<T*<TL*. Above TL* (very near the triple point), a truly liquid layer wets the solid surface. The onset of the liquidlike disordering (TQLL*) depends on the orientation of the free surface. More layers are disordered for a less close-packed face near Tm. However, the divergence of the thickness of the liquid layer is similar for three systems having the same size, in agreement with the predictions based on the isotropic nature of the local Van der Waals forces. Our results are basically consistent with the semi-infinite Landau type model of Lipowsky and Speth of surface disordering at a bulk first-order transition. We have shown that surface-induced melting occurs in a realistic polyatomic molecular model system which, on the other hand, does not show vacancy-induced premelting. This provides some evidence that interface-initiated melting not only exists, but might be the dominant mechanism for melting in this kind of material.
Rovibrational spectra of open-shell van der Waals complexes: H2–OH (X 2Π)
Authors: Miller Steven M. , Clary David C.
Published: July 2002

A variational basis function approach is described for the calculation of the rovibrational bound states and infrared spectrum of a van der Waals complex of a closed-shell diatom and a 2Π diatom using no angular momentum decoupling approximations. Using an ab initio potential surface, the method is then applied to calculate the bound states and spectra of the system H2–OH (X 2Π), and results are given for the complex containing both para and ortho H2. Interesting similarities and differences are discovered on comparing the results with those for the related Ar–OH and H2–HF complexes.
Diffusion of ammonia on Re(001): A monolayer grating optical second harmonic diffraction study
Authors: Rosenzweig Z. , Farbman I. , Asscher M.
Published: July 2002

Diffusion of ammonia was studied over the smooth Re(001) surface utilizing optical second harmonic diffraction from surface coverage grating. The decay of the first order diffraction peak as a function of surface temperature for different initial coverages was measured and then simulated by numerical solution of Fick’s second diffusion equation, employing a coverage dependent diffusion constant. For the first time the second order diffracted second harmonic signal was monitored during the diffusion process, supporting the calculated initial coverage profile and the diffusion model. The resulting diffusivity D(θ)=D0exp[−Em(θ)/RT], is defined by a barrier for diffusion Em(θ)=E0−ωZθ, with activation energy at zero coverage E0=3.4±0.6 kcal/mol, D0=2.8×10−3 cm2 sec−1 and the repulsion energy between the pair of nearest neighbor molecules ω=0.2±0.06 kcal/mol. The magnitude and effect of the repulsive interactions on diffusion are compared to their strong effect on the activation energy for desorption.
Photodissociation of water in rare gas matrices: Cage effect and local heating of the lattice
Authors: Tarasova E. I. , Ratner A. M. , Stepanenko V. M. , Fugol I. Ya. , Chergui M. , Schriever R. , Schwentner N.
Published: July 2002

The H fragment produced in the photodissociation of water (H2O→H+OH) is lighter than the lattice atoms and a local heating occurs over long time scales and large spatial regions. Therefore, use of a continuum model for the lattice is justified. The local heating is promoted by a decreasing heat conductivity with increasing temperature. Solutions of a nonlinear heat conductivity equation show that the heat release inside the cage leads to a temperature increase in its nearest surroundings up to the melting point in argon. Melting enables the light fragment to overcome the cage barrier. In the case of krypton, the thermal effect is less pronounced, while in Xe, there is none. The above results are in qualitative agreement with experimental data.
Dependence of the chemical dynamics of intercluster association reactions on the strength of the solute–solvent intermolecular potential
Authors: Hu Xiche , Hase William L.
Published: July 2002

Classical trajectory calculations are performed to investigate how microscopic solvation influences the H+CH3→CH4 reaction mechanism, rate constant, energetics, product energy, and angular momentum partitioning; and how these solvation effects depend on the solute–solvent interaction strength. Without solvation, the final energy and rotational angular momentum of CH4 strongly depend on the H+CH3 relative translational energy. However, for HAr2+CH3 with a normal H–Ar Lennard-Jones interaction strength εHAr0, a spectator-stripping mechanism dominates the reactive collisions so that both the final CH4 energy and rotational angular momentum do not significantly depend on the relative translational energy. The association cross section to form CH4 is slightly larger for HAr2+CH3 than for H+CH3. When the H–Ar interaction strength for HAr2 is increased from 1 to 100εHAr0, it is found that (1) the association cross section to form CH4 is insensitive to the H–Ar interaction strength, suggesting a long-range transition state; (2) the reaction mechanism changes from a spectator-stripping model to a complex one, which alters the character of the CH4+Ar2 product energy and angular momentum partitioning; and (3) the formation of the Ar2–CH4 complex leads to stabilized CH4 product, with substantial energy transfer from CH4 for the strongest H–Ar interaction strength of 100εHAr0.
On the quasiclassical calculation of fundamental and overtone intensities
Authors: Nikitin E. E. , Noda C. , Zare R. N.
Published: July 2002

Various approximations to the transition dipole moment matrix element <n′‖M‖n≳ are compared with each other and to exact (numerical) values of this overlap integral for different n→n′ transitions in a Morse potential with a linear dipole moment function. By partitioning the numerical integral into different contributions that involve the classically allowed and forbidden regions of each wave function, we have learned what conditions must be satisfied for validity of the different approximations. In particular, we consider the Landau approximation to the quasiclassical matrix element in which the exact wave function for the upper state is replaced by the Wentzel–Kramers–Brillouin (WKB) wave function in the classically allowed region of that state. We find that the Landau approximation is more accurate than might have been expected because of the compensation of the neglected tunneling contribution by the singular behavior of the WKB wave function in the classically allowed neighborhood of the turning point. Based on this study, we suggest an improved semiclassical approximation for transition dipole matrix elements that involve an arbitrary dipole moment function. This method is applied to the n′−0 transition of a Morse oscillator using a linear dipole moment function; it can reproduce the exact values of the transition dipole moment matrix element to better than 5% for n′=1 to n′=15. Under the condition that the dipole moment function is slowly varying or decreases monotonically with increasing internuclear separation, a simple expression is presented for estimating relative strengths of neighboring high overtone transitions.
Photodissociation of molecules physisorbed on inert crystalline surfaces
Authors: Hose Gabriel , Freed Karl F.
Published: July 2002

This paper presents a theoretical treatment within the Franck–Condon and impulse approximations of direct photodissociation of polyatomic molecules sparsely physisorbed on static and UV transparent crystals. A separable local mode model is adopted for the ground state adsorbate and for the initially excited molecule in order to evaluate the multidimensional Franck–Condon excitation and impulse fragmentation amplitudes. Since physisorption forces are weak, the adsorbate excitation and decomposition processes remain intramolecular in nature. The photodissociation of surface physisorbed molecules thus differs from the corresponding gas phase process mainly because of substrate confinement on adsorbate rotation and translation degrees of freedom, which are all described by our theory. One result is that the photodissociation cross section depends on the laboratory frame observation angles. Another is the availability of fragment–substrate rearrangement channels which involve combined dissociation, fragment adsorption, and surface migration excitations. In particular, the excitation may lead to adsorbate desorption and/or migration in addition to adsorbate photofragmentation. Accordingly, the cross section contains six contributions depending on the excitation process and the fragment–substrate rearrangements.
A lattice dynamics study of a Langmuir monolayer of monounsaturated fatty acids
Authors: Levine Yehudi K. , Kolinski Andrzej , Skolnick Jeffrey
Published: July 2002

A Monte Carlo dynamics (MCD) scheme has been applied in a study of the effects of unsaturated double bonds on the internal conformational dynamics and orientational order of hydrocarbon chains arranged in a monolayer on the surface of an impenetrable interface. The MCD algorithm makes use of the high coordination {2 1 0} lattice for the representation of both sp3 and sp2 valence states of the carbon atoms. The chain dynamics are considered to arise from a superposition of local conformational rearrangements. The simulations reproduced the principal features of the experimentally observed order parameter profiles of the C–H bonds on taking into account the intramolecular conformational energy of the molecules and excluded volume effects. The results show that the introduction of a rigid, planar, unsaturated segment enhances the orientational order in the monolayer. The extent of the enhancement is larger for the trans unsaturated chains than for the cis unsaturated ones. The increase in orientational order is accompanied by a marked increase in the effective rotational correlation times, indicating that the unsaturated segments undergo slow and restricted motion. In addition, the C–H bonds of the saturated chain segment between the cis double bond and the headgroup of the chain undergo slower motions than the corresponding vectors in the saturated and trans unsaturated chains. This arises from the anchoring of the headgroup at the impenetrable monolayer interface.
A defect theory for the glass transition and residual entropy of hyperquenched water
Authors: Johari G. P.
Published: July 2002

The ground state structure of thermodynamically equilibrated metastable amorphous water at 0 K is considered as a continuous random network of H-bonded water molecules. At higher temperatures, this structure contains a fixed number of broken H bonds, or frozen-in fluctuations of local density, termed defects, where molecules have a considerable degree of configurational freedom to allow viscous flow. Their concentration has been calculated from the known change in the heat capacity at its Tg of 136 K, and statistical thermodynamics. The configurational enthalpy and entropy of a thermodynamically equilibrated glassy water, according to this theory, become zero at 0 K, and not at 20%–30% below its Tg, and lead to a residual entropy of 0.20 J/K/mol for the 0.23% defects containing structure. This is in addition to the entropy of R ln(3/2) due to the orientational disorder, as in ice. Any attempt to include an extra configurational enthalpy leads to a further lower value for residual entropy. The metastable equilibrium at different temperatures and the relaxation towards this equilibrium structure have been expressed in terms of free energy-defect concentration diagrams for glassy water.
Vibrational relaxation of HCl as a function of xenon density: The role of HCl–Xe complexes
Authors: Moustakas Antonis , Weitz Eric
Published: July 2002

A treatment of the effect of vibrational predissociation of van der Waals complexes on vibrational relaxation rates [Chem. Phys. 29, 253 (1978)] has been extended by including V–V energy transfer between complexed and uncomplexed species and the possibility of different V–T/R relaxation rates for complexed and uncomplexed species. These modifications allow us to reproduce experimentally observed relaxation data for HCl(v=1) over two orders of magnitude in xenon density. The dependence of the vibrational relaxation rate on density is discussed as are the implications of the observed behavior on isolated binary collision formulations of vibrational energy relaxation processes.
Proton-donor properties of water and ammonia in van der Waals complexes. Be–H2O and Be–NH3
Authors: Chal/asiński G. , Szczȩśniak M. M. , Scheiner S.
Published: July 2002

The potential energy surfaces (PES) of Be–H2O and Be–NH3 are studied with particular attention to characterization of proton-donor properties of water and ammonia. Calculations were performed by means of both supermolecular and intermolecular Mo/ller Plesset perturbation theory. The Be–H2O PES reveals two van der Waals minima: the C2v minimum (De=176 cm−1, Re=6.5 bohr), and the H-bonded minimum (De=161 cm−1, Re=7.5 bohr), separated by a barrier of 43 cm−1 at the T-shaped configuration. The Be–NH3 PES reveals only one van der Waals minimum, at the C3v configuration (De=260 cm−1, Re=6.5 bohr) and a saddle point at the H-bonded geometry. The locations of the minima as well as the anisotropy of the interaction are determined by the anisotropy of electric polarization contribution, embodied by the self-consistent-field (SCF)-deformation and perturbation induction energies.
Anharmonic corrections to variational transition state theory calculations of rate constants for a model activated reaction in solution
Authors: McRae Robin P. , Garrett Bruce C.
Published: July 2002

Previous errors in harmonic variational transition state theory calculations of rate constants for a model of an activated reaction in solution are corrected by including anharmonicities in the calculations of bound state energy levels. Harmonic variational transition state theory including multidimensional quantum mechanical corrections for reaction coordinate motion has previously been used to calculate the rate constant for a reaction model corresponding to a single solute coordinate coupled to a harmonic bath. Comparison of these rate constants with accurate quantum mechanical rate constants showed good agreement, in general, with the worst discrepancy to be a factor of 3.6. By including anharmonicities in the bound vibrational potentials to infinite order and calculating the energy levels by a uniform semiclassical prescription, the maximum error is reduced to 42%.
Effect of hyperfine depolarization upon creation and detection of alignment in free-jet expansions via selective photodissociation
Authors: Yan Chun , Kummel Andrew C.
Published: July 2002

The creation of alignment by photodissociation is a well-accepted process. When an isotropic sample of gas is subjected to a strong linearly polarized laser pulse at a frequency at which the photodissociation cross section is large, the surviving molecules are usually aligned. If the transition is parallel, μ lies along the internuclear axis (ΔΛ=0) and the surviving molecules will be peaked around M=0, while for a perpendicular transition (ΔΛ=1) the surviving molecules will be peak around M=J. Although this effect has been seen in laser cavities and in the focus of laser beams, it has not been used to create aligned pulses of gas from free-jet expansions. We present the theoretical calculations for the practical creation of alignment in short free-jet gas pulses via saturation photodissociation.

Our methodology allows the propagation of the laser light along any direction and with any polarization, the quantification of the effect of hyperfine and electron spin depolarization upon the creation of alignment, direct comparison of the degree of alignment created in parallel versus perpendicular transitions when the polarizations are set to cause M=J versus M=0 peaked distributions, experimental determination of the degree of alignment after depolarization using a second fixed frequency laser, and experimental determination of the degree of alignment prior to depolarization without the use of an additional laser. Our calculations show that hyperfine and electron spin depolarization are the limiting forces in the creation of aligned pulses of gas. These effects are most pernicious in free-jet expansions where only the lowest rotational states are populated and therefore even modest values of nuclear spin and electron spin can effect large depolarizations.

The calculations show that these depolarization effects can be effectively mitigated by three methods: (1) using molecules with small Be values, (2) limiting the free-jet expansion so the rotational temperature is above about 50 K, and (3) employing parallel photodissociation transitions (ΔΛ=0). For very cold expansions, parallel transitions are less susceptible to depolarization than perpendicular transitions (ΔΛ=1) because in parallel transitions, the low J states are selectively photodissociated.
Rate and mechanism of intramolecular vibrational redistribution in the ν16 asymmetric methyl stretch band of 1-butyne
Authors: Bethardy G. A. , Perry David S.
Published: July 2002

The spectrum of the ν16 asymmetric methyl stretch vibration of 1-butyne near 2991 cm−1 has been studied via direct absorption infrared spectroscopy at a resolution of 35 MHz. Analysis by ground state combination difference indicates that the ν16 band is extensively perturbed by dark vibrational bath states. All of the transitions appear as multiplets of about five eigenstates in a window of about 0.017 cm−1. A detailed analysis is presented for the upper state levels K′a = 0–2 and J′=0–6. A lack of J′ dependence implies anharmonic coupling is dominant and that b- and c-type Coriolis interactions are not important at these low J′ values. However, the average dilution factor goes from 0.72 at the K′a = 0 to 0.46 at the K′a = 2 suggesting weak a-type Coriolis interactions. For the K′a = 0 levels, the measured average level density of 17 states/cm−1/symmetry species is comparable to the value of 14 vibrational states/cm−1/symmetry species obtained from a symmetry specific direct count. This is an indication that the dynamics explore all of the energetically available vibrational phase space. The nearly Gaussian distribution of matrix elements suggests that there is significant coupling among the bath states. At the K′a = 0 level, the rms anharmonic coupling matrix element is <vsj2≳1/2=0.0125 cm−1. From the frequency-resolved data, a coherently prepared asymmetric methyl stretch in 1-butyne is deduced to decay with a 276 ps time constant to the asymptotic probability of 0.6.
Time dependent nucleation. II. A semiclassical approach
Authors: Demeio Lucio , Shizgal Bernie
Published: July 2002

The continuum approximation of the Becker–Döring birth and death equations [B. Shizgal and J. C. Barrett, J. Chem. Phys. 91, 6505 (1989)] leads to a Fokker–Planck equation for the continuous cluster distribution. This linear Fokker–Planck equation is solved with the expansion of the cluster distribution function in the eigenfunctions of the Fokker–Planck operator. The Fokker–Planck eigenvalue problem can be transformed into an equivalent Schrödinger equation. In this paper, the semiclassical Wentzel–Kramers–Brillouin (WKB) method and the corresponding supersymmetric WKB method are employed in the determination of the eigenvalues and eigenfunctions of the equivalent Schrödinger equation. We compare the approximate results with those obtained with a standard discretization scheme. We obtain eigenvalues and eigenfunctions of the Fokker–Planck operator which are in good agreement with the exact ones. The nucleation fluxes and associated time lags are also considered.
Photochemistry of alkyl halide dimers
Authors: Fan Y. B. , Randall K. L. , Donaldson D. J.
Published: July 2002

Dimers and other small clusters of CH3I, C2H5I, i- and n-C3H7I, HI, CF3I, CH3Br, and C2H5Br formed in a supersonic expansion are irradiated at 248 and 193 nm and the halogen molecule product probed via laser induced fluorescence spectroscopy. Both dimers and larger clusters of RI (R=H, alkyl) excited at each wavelength yield I2 in its ground electronic state with very little internal energy. Clusters of CF3I and those containing alkyl bromides do not give halogen molecule products after excitation at either wavelength. A model for the dynamics in the dimer excited state which explains these results is presented.
The bending frequency δNS of dinitrogen sulfide (N2S): A theoretical analysis demonstrating the importance of Coriolis coupling terms
Authors: Collins Charlene L. , Yamaguchi Yukio , Schaefer Henry F.
Published: July 2002

There exists a discrepancy between theory and experiment for the N2S bending vibrational frequency. Ab initio molecular electronic structure theory has been used in order to investigate the electronic ground state of the dinitrogen sulfide (N2S) molecule. Self-consistent field (SCF), configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD) wave functions were employed using the three basis sets, the double zeta plus polarization (DZP), triple zeta plus double polarization (TZ2P), and TZ2P with one set of higher angular momentum polarization functions (TZ2Pf). For CCSD wave functions the connected triple excitations were included through perturbation theory [CCSD(T)]. Using the CCSD(T) method with the larger basis sets, experimental physical properties including bond lengths, rotational constant, centrifugal distortion constant, l-type doubling constant, vibrational frequencies, and isotopic shifts for the N–N stretching frequency have been reproduced with quantitative accuracy. This analysis proves the essential correctness of an earlier theoretical prediction of the N2S harmonic bending vibrational frequency.
Integral rate constant measurements of the reaction H +D2O → HD(v′, j′)+OD
Authors: Adelman David E. , Filseth Stephen V. , Zare Richard N.
Published: July 2002

The reaction H+D2O was studied by intersecting a pulsed beam of HI with an effusive spray of D2O in a high vacuum chamber. Translationally hot H atoms were generated by UV photolysis of HI in the intersection volume, and the HD product of the reaction H+D2O was detected in a quantum-state-specific manner by (2+1) resonance-enhanced multiphoton ionization. Because the same UV laser beam was used to initiate the reaction and detect the product, the relative collision energy varied as a function of product state detected—∼2.8 eV for v′=0, ∼2.6 eV for v′=1, and ∼2.5 eV for v′=2. Under these conditions, approximately 35% of the available energy is partitioned into the internal modes of the HD product. For the products, the HD ‘‘new bond’’ receives 15 times more energy than the OD ‘‘old bond.’’ A significant amount of energy appears as HD vibration with v′=0 and 1 having comparable populations. The fraction of available energy partitioned into HD rotation, gR(v′), is found to be essentially independent of HD vibration. This invariance may be rationalized in terms of a counterbalancing of two mechanisms for rotational excitation of the HD product. We find qualitative agreement between recent quasiclassical trajectory calculations by Kudla and Schatz for the HD product internal-state distributions and the present experimental results.
Stimulated emission pumping spectroscopy of CH3O (X̃ 2E, ν6): New observations on the Jahn–Teller effect
Authors: Geers A. , Kappert J. , Temps F. , Sears T. J.
Published: July 2002

Experiments have been carried out using hot band laser induced fluorescence (LIF) excitation and stimulated emission pumping (SEP) spectroscopy to characterize the excited doubly degenerate ν6 vibrational mode of CH3O (X̃ 2E) which is of interest with respect to the Jahn–Teller effect in the molecule. Rotationally resolved spectra were recorded for the Ã 2A1↔X̃ 2E, 610 and 310610 bands. The observed line structures indicate that these bands must be assigned to Δj=3/2 parallel (ΔK=0) vibronic bands. No evidence could yet be obtained for the corresponding Δj=1/2 vibronic transitions assumed previously. The results imply that the linear Jahn–Teller effect for ν6 is very small. Instead, coupling between the Ã 2A1 and the X̃ 2E states by a pseudo-Jahn–Teller interaction operator is suggested as a plausible mechanism for the observation of the forbidden transitions.
Constrained-search method to determine electronic wave functions from electronic densities
Authors: Zhao Qingsheng , Parr Robert G.
Published: July 2002

Determination of electronic wave functions from electronic densities is discussed. A systematic new method based on the constrained-search formulation of density functional theory is developed, and the method is tested for Be, Ne, and Ar. The results show that the Kohn–Sham kinetic energy and the Kohn–Sham orbitals can be obtained readily and accurately by this method. Since Kohn–Sham orbitals are very close to Hartree–Fock orbitals, the problem of finding a physical meaningful wave function from an electron density is thereby solved.
Dipole moment analysis of excited van der Waals vibrational states of ArH35Cl
Authors: Cooksy A. L. , Elrod M. J. , Saykally R. J. , Klemperer W.
Published: July 2002

The far-infrared laser electric resonance spectra of the prototypical atom–diatom complex ArH35Cl are analyzed using improved zero-field molecular constants, yielding accurate permanent and transition dipole moments for the three lowest excited van der Waals vibrational states. The constants are obtained from a multistate fit to previous microwave, far-infrared laser electric resonance, and far-infrared tunable laser spectra, as well as new far-infrared measurements of the Σ-stretch state, which are reported here. The signs of the dipole moments and Coriolis coefficients establish the relative orientations of the HCl subunit in these states. The fit is found to converge only if these signs correspond to the HCl pointing in opposite directions along the a inertial axis in the Σ-stretch and Σ-bend states. A weak preference, near the experimental error limit, is found for the Ar–Cl–H average angle in the Π-bend state to be greater than 90°, contrary to expectation. For the best fit, we obtain the a-axis dipole moment components −0.5413(11) D (Σ bend), −0.263 45(29) D (Π bend), and 0.6754(36) D (Σ-stretch); and the b-axis components 0.365(12) D (Π and Σ-bend) and −0.0465(43) D (Π and Σ stretches), where the signs of the Coriolis coefficients and μa for the Σ stretch have been arbitrarily fixed positive. For the expected Π-bend configuration, with the Ar–Cl–H angle less than 90°, the magnitudes along the a axis change only slightly, but the b-axis components become 0.149(12) and −0.1403(64) D for the Π–Σ-bend and Π–Σ-stretch interactions, respectively.
Direct measurements of state specific unimolecular dissociation rate constants of highly excited single rotation vibration quantum states of CH3O (X̃ 2E)
Authors: Geers A. , Kappert J. , Temps F. , Wiebrecht J. W.
Published: July 2002

Vibration rotation quantum state resolved unimolecular dissociation lifetimes of highly excited CH3O (X̃ 2E) molecules have been measured using the method of stimulated emission pumping (SEP) in connection with transient laser induced fluorescence (TLIF) excitation spectroscopy for preparing selected single target states and for their time resolved detection. The measured decay rate constants for six closely spaced levels with precisely known total excitation energies around E(X̃)≊7450 cm−1 and J=0.5 (or 1.5) were found to vary erratically between 9⋅105 s−1≤k≤3⋅107 s−1. The results are compared to predictions by unimolecular rate theory and discussed with respect to the question of mode specific vs statistical reactivity.
Dielectric relaxation of dipolar liquids
Authors: Chandra A. , Wei Dongqing , Patey G. N.
Published: July 2002

An approximate expression is derived for the dielectric function ε(k,ω). The theory includes inertial and non-Markovian effects and is free of adjustable parameters. For the k=0 case, detailed comparisons are made with computer simulation results for dipolar soft-sphere and Stockmayer fluids, and the theory is shown to be qualitatively sound at both low and high frequencies. The present approximation should be very useful in developing a theory of solvation dynamics which properly includes important inertial effects.
Centroid-density quantum rate theory: Dynamical treatment of classical recrossing
Authors: Schenter Gregory K. , Messina Michael , Garrett Bruce C.
Published: July 2002

A new method is presented for the calculation of quantum mechanical rate constants for activated processes. This method is a hybrid approach involving Feynman path integrals and classical dynamics that is an extension of previous work of Messina, Schenter, and Garrett [J. Chem. Phys. 98, 8525 (1993)]. We make an ansatz for the quantum mechanical analog to the classical flux correlation function expression for the rate constant. This expression involves an imaginary-time, phase-space Feynman path integral, with the dividing surface and characteristic function expressed as a function of the phase-space centroid variables. The reactive flux correlation function is obtained from a classical-like expression in which the characteristic function is evaluated by evolving the phase-space centroid variables as if they were classical dynamical variables. We show that the theory gives exact analytic results in the high temperature and harmonic limits. The theory is further tested on a model anharmonic two-dimensional system of an Eckart barrier coupled to a harmonic oscillator. The results of the theory compare favorably to accurate numerical calculations.
Electronic decay of vibrationally selected core excited states in molecular N2
Authors: Rubensson J-E. , Neeb M. , Biermann M. , Xu Z. , Eberhardt W.
Published: July 2002

We have measured and analyzed the vibrational fine structure of the electronic participator decay of the 1s−11πg state in gas-phase N2. The monochromator resolution allowed us to limit the vibrational excitations in the excited state so that unequivocally assignable vibrational fine structure in the decay spectra could be resolved. The excitation-emission process is shown to be well-described within the two-step Franck–Condon model, and the data represents individual decay of vibrationally selected core excited states.