bluuebunny/crossref_metadata_2025 huggingface dataset:
Picosecond studies of excitation transport in a finite volume: The clustered transport system octadecyl rhodamine B in triton X-100 micelles
Authors: Ediger M. D. , Domingue R. P. , Fayer M. D.
Published: February 2003

A detailed experimental and theoretical examination of electronic excited state transport in the finite volume system, octadecyl rhodamine B molecules in triton X-100 micelles, is presented. Picosecond fluorescence mixing and transient grating techniques were used to examine systems in which the average number of chromophores per micelle ranged from 0.1 to 11. Because of the clustering of chromophores in the small micelles, the energy transport observed is extremely efficient. A statistical mechanical theory, based on a density expansion with a Padé approximant, is developed for donor–donor transport on a spherical surface. This theory accurately accounts for the experimental data with only the micelle radius as an adjustable parameter. The radius obtained from this procedure is in good agreement with determinations by other methods. This demonstrates that quantitative information about the spatial extent of chromophore distributions in small volumes can be obtained when appropriate finite volume energy transport theories are employed. It is shown that theories developed for infinite volumes are not applicable to systems such as the ones considered here. Finally the partitioning of rhodamine B and octadecyl rhodamine B between aqueous and micellar phases is measured, and lifetimes and rotation times are reported.
The thermodynamics of dense fluid nitrogen by molecular dynamics
Authors: Johnson J. D. , Shaw M. S. , Holian B. L.
Published: February 2003

We present an extensive set of molecular dynamics results for the thermodynamics of dense fluid N2. The density and temperature regime is 1.3 g/cm3≲ρ≲2.3 g/cm3 and 500 K≲T≲12 000 K. These data are then analyzed to study the effects of internal degrees of freedom on the N2 equation of state. Most importantly, we demonstrate the existence of an effective spherical potential that models very accurately (to 1.5% or better in pressure and energy) the true equation of state for the anisotropic N2 potential. We discuss the relation of this effective potential to the median average over angles and other averaging methods, including the arithmetic mean.
Ab initio configuration interaction calculations of the hyperfine structure in small radicals
Authors: Feller David , Davidson Ernest R.
Published: February 2003

Extended basis set natural orbital CI wave functions have been used to compute the isotropic and anisotropic hyperfine coupling of ten small radicals for which experimental measurements exist. The configuration spaces employed in the CI’s were of two kinds: One involved all single and double excitations from the Hartree–Fock configuration plus limited triples and quadrupoles, while the other consisted of singles and doubles from a multireference set coupled with various configuration selection techniques. At the highest level of theory used in the present study, agreement with experiment was generally within 10%, but differences for the isoelectronic series H2CN, H2CO+, and H2BO as well as the related acetaldehyde radical cation H3C–CHO+ are larger than would be expected based on the data from the other six radicals.
An ESR investigation of the formaldehyde cation radicals (H2 12CO+ and H2 13CO+) in neon matrices at 4 K
Authors: Knight Lon B. , Steadman J.
Published: February 2003

The H2 12CO+ and H2 13CO+ radicals have been trapped in neon matrices at 4 K for detailed ESR (electron spin resonance) study. Both photoionization (17 eV) from an open tube neon discharge source and electron bombardment, recently developed techniques for generating matrix isolated cations, were employed in this investigation. The ESR spectra were sufficiently resolved to allow a complete characterization of the nuclear A and g tensors. The magnetic parameters are: gx=2.0069(2), gy=2.0015(2), gz=2.0025(2), Ax(H)=129.4(2), Ay=(H)=134.5(2), Az(H)=134.3(2) g, ‖Ax(13C)‖ =43.9(2), ‖Ay(13C)‖=37.3, and ‖Az(13C)‖ =35.1(2) g. Of particular interest is the conclusion that the direction of the largest g component (x) lies perpendicular to the radical plane rather than along the in-plane C2 symmetry axis. These H2CO+ results were compared with MINDO and ab initio CI type theoretical calculations and to previous experimental results for the isoelectronic radicals H2CN, H2BO, H2C2−, and the acetaldehyde cation CH3HCO+. Theoretical values for the isotropic nuclear hyperfine interaction for this series of H2XY radicals are consistently below the experimental results, although good agreement is obtained for the H and 13C dipolar interactions in H2 13CO+.
Photoionization of N2 X 1Σ+g, v″=0 and 1 near threshold. Preionization of the Worley–Jenkins Rydberg series
Authors: Dehmer P. M. , Miller P. J. , Chupka W. A.
Published: February 2003

The high resolution relative photoionization cross section for N2 is reported in the wavelength region from the ionization threshold to 650 Å, with particular attention given to the region between the N+2 X 2Σ+g, v′=0 and 1 ionization limits. Cross sections from both the X 1Σ+g, v″=0 and 1 vibrational levels were determined in this region, and preionized members of the npπu 1Πu Worley–Jenkins Rydberg series converging to N+2 X 2Σ+g, v′=1 are observed in both spectra. In the spectrum excited from v″=1, the Worley–Jenkins series appears prominently as a result of good Franck–Condon overlap between the N2 X 1Σ+g, v″=1 and the N+2 X 2Σ+g, v′=1 levels; the intensities of the series members decrease approximately as 1/n*3, in accord with simple theoretical predictions. However, in the spectrum excited from v″=0, the Worley–Jenkins series converging to N+2 X 2Σ+g, v′=1 is weak as a result of a poor Franck–Condon overlap with the ground vibrational level; the intensities of the series members show large deviations from the simple theory as a result of channel interactions with Rydberg states converging to N+2 A 2Πu. These perturbing Rydberg states have low photoabsorption oscillator strengths for excitation from v″=1 (in contrast to excitation from v″=0) and hence have only a small effect on the cross section from the excited vibrational level. Just as in the case of photoionization of H2, the results demonstrate that channel interaction can redistribute the oscillator strength of a perturbing Rydberg state of low principal quantum number over a number of members of an interacting Rydberg series and that the spectral range affected by the perturber can be much greater than the width of the perturbing level.
Fluorescence excitation spectra of indole, 3-methyl indole, and 3-indole acetic acid in supersonic jets
Authors: Bersohn Richard , Even Uzi , Jortner Joshua
Published: February 2003

In this paper we report on the laser-induced fluorescence spectra of indole, 3-methyl indole, and 3-indole acetic acid in pulsed supersonic expansions of He. The electronic origin of the first excited (1Lb) state is 35 233 cm−1 for indole, 34 887 cm−1 for 3-methyl indole, and 34 044 cm−1 for 3-indole acetic acid. On the basis of the energetics and fluorescence lifetimes, the spectral features at excess vibrational energies EV=0–2200 cm−1 for indole and EV=0–1600 cm−1 for 3-methyl indole were assigned to the vibrational level structure within a single electronic manifold. The second electronically excited (1La) state is not located in the above energy ranges. The energetics of selective solvation of 3-methyl indole was inferred from the spectra of the van der Waals complexes with Ar, H2O, and D2O, which provide information on microscopic solvent shifts of the electronic origin and of some prominent vibrational excitations of the 1Lb states.
Reaction diffusion in a medium containing a random distribution of nonoverlapping traps
Authors: Kayser R. F. , Hubbard J. B.
Published: February 2003

The transient reaction diffusion kinetics in a system containing a random distribution of stationary spherical traps is analyzed. It is shown that recently obtained results concerning the long-time behavior of the trap-averaged density at the origin ρ(o,t) may be readily extended to the cases of partially absorbing and nonoverlapping traps, independently of the number density of traps. We also estimate the size of the relative fluctuations about ρ(o,t) and show that these fluctuations diverge at long times.
Close-coupling calculations of transport and relaxation cross sections for H2 in Ar
Authors: Hutson Jeremy M. , McCourt Frederick R.
Published: February 2003

We present benchmark close-coupling calculations of relaxation, transport, and Raman line shape cross sections for H2 colliding with Ar, using the BC3 (6,8) potential energy surface of Le Roy and Carley. The experimentally observable cross sections calculated include those for shear viscosity, binary diffusion, depolarized Rayleigh light scattering, and flow birefringence. The agreement with available experimental data is fair, except for the pressure shifting of the pure rotational Raman lines of H2 and the cross section for depolarized Rayleigh light scattering. It is shown that the Raman line shift cross sections are very sensitive to the dependence of the intermolecular potential on the H2 stretching coordinate, and the line shift data should be useful in determining this feature of the potential more accurately. Conversely, line shift data should not be used to determine intermolecular potentials when the vibrational dependence of the surface is neglected. The role of orbiting resonances in transport and relaxation cross sections is also investigated. Their effects are found to be small for transport cross sections, but they can contribute up to 30% to relaxation and linewidth cross sections at very low temperatures (<100 K). The resonance effects decrease quickly as the temperature is increased and are negligible at room temperature and above. Nevertheless, the existence of resonant effects must be remembered when performing thermal averaging of cross sections, since one energy point accidentally on resonance can greatly distort calculated results.
Ion kinetic energy spectroscopy of the doubly charged ion of carbon monoxide
Authors: Curtis J. M. , Boyd R. K.
Published: February 2003

Spontaneous and collision-induced dissociation processes of CO2+ ions, formed by electron impact, have been studied in a double-focusing mass spectrometer using techniques of ion kinetic energy spectroscopy. The predissociation process, responsible for unimolecular dissociation of CO2+ on the microsecond time scale, is almost certainly electronically adiabatic tunneling through a potential barrier, though predissociation via electronic curve crossing cannot be entirely ruled out. Semiempirical potential curves for states of CO2+ were revised in order to better accommodate all of the available data, including Auger spectra, appearance energies, and kinetic energy release. Collision induced dissociation processes with Ar, N2, and H2 proceed via charge exchange, and involve predissociation of the D 2Π state by the C 2Δ state of CO+. When He is used as collision gas, the dissociation processes involving charge exchange are different, and require an energetic contribution from the relative kinetic energy (kinetic energy loss). In addition, He is quite different in inducing dissociation of CO2+ without prior charge exchange, from states of CO2+ up to 13 eV above the dissociation limit.
Interaction energies between noble gas atoms from a trial density function in the Thomas–Fermi–Amaldi–Dirac formulation
Authors: Donnamarǐa Marǐa C. , Castro E. A. , Fernández F. M.
Published: February 2003

The Thomas–Fermi–Amaldi–Dirac density functional theory is applied to calculate interaction energies for noble gas atoms and atomic ionization energies. Our results are compared with other theoretical values as well as with available experimental data.
Energetic considerations of the vibrational potential function in the effective nuclear charge model. III
Authors: Ohwada Ken
Published: February 2003

The Parr–Gadre homogeneity condition for the molecular electronic energy with respect to the atomic number is examined in detail with the use of the model potential in the effective nuclear charge model previously proposed. The energy deviation from such a condition is closely related to that of the total molecular SCF energy from the eigenvalue sum, and it is shown that the R-dependent (R; internuclear distance) terms in the model potential contribute significantly to that deviation. The approximate analytic formulas for the chemical potentials of molecules are derived from both the model potential and the results of a low-order gradient expansion in the electron density. It is especially emphasized that the chemical potential change depends mainly upon the correction terms, in the model potential, describing the behavior of delocalized electrons in the molecule.
Interplay of vibrational and van der Waals type bonding
Authors: Manz J. , Meyer R. , Schor H. H. R.
Published: February 2003

The collinear IHI molecule and the I+HI reaction are studied using exact and approximate quantum mechanical techniques and employing two different potential energy surfaces: a minimum-free LEPS potential with a symmetrical IHI barrier and a semiempirical DIM-3C potential with a symmetrical barrier and two equivalent shallow wells for the I⋅⋅⋅HI and IH⋅⋅⋅I configurations. Comparison of the results for the two potentials yields a number of phenomena which are characteristic for vibrational bonding and its interplay with weak attractive forces: (i) The IHI→I+HI dissociation energy contains a contribution of approximately 9 kJ mol−1 due to vibrational bonding irrespective of the details of the potential. (ii) The symmetrical stretching frequency has a minimum value of ≳100 cm−1 due to vibrational bonding. (iii) Vibrational bonding reduces the average ground state interiodine distance from the large value implied by the van der Waals type minimum towards the saddle point configuration. (iv) Pure vibrational bonding allows only four collinear bound states with gerade parity, whereas the interplay of van der Waals type bonding enhances the number of bound states, including also levels with ungerade symmetry. The highest vibrationally excited levels describe very nonsymmetric I⋅⋅⋅HI or IH⋅⋅⋅I configurations. (v) Simultaneously, the interplay of both bonding mechanisms increases the number of scattering resonance energies.
Fractional gauge transformations of the first kind: Optimization of cross-section calculations
Authors: Brown C. W. , Robinson G. W.
Published: February 2003

A theory of matter-radiation interactions is presented that incorporates a fractional gauge transformation. The transformation is achieved with the Göppert-Mayer generator multiplied by a continuously variable gauge parameter. The derived Hamiltonian contains a mixture of A⋅p and E⋅x interaction terms: The field and matter variables within the full gauge-invariant, matter-radiation Hamiltonian are intermixed. Practical implications for calculating accurate transition cross sections using convenient, but inexact, matter eigenstates are developed by interpreting gauge-generalized perturbation theory as a gauge-variational theory. By minimizing the matter/field/interaction ground state energy with respect to gauge variations, the optimum matter-radiation basis for the chosen set of inexact matter states is obtained. The gauge optimization technique is demonstrated quantitatively for the 2pσu –1sσg electronic transition of H+2 using the simple LCAO-MO basis.
A quantum mechanical open system as a model of a heat engine
Authors: Kosloff Ronnie
Published: February 2003

A quantum model of a heat engine is analyzed. This engine is constructed from two coupled oscillators in interaction with a warm and cold reservoir. Power is extracted by an external periodic driving force. As a function of control parameters a maximum in power is obtained, and a decline of thermodynamic efficiency below the ideal Carnot value. This irreversibility is a consequence of the mechanism devised to extract power in its perturbing the energy level structure of the engine. In the limit of weak coupling to the driving force the efficiency at maximum power obtains the value of η=1−((Ta/Tb))1/2.
Study of vibrational energy transfer at a surface by a time-of-flight method
Authors: Yuan Wei , Rabinovitch B. S.
Published: February 2003

A single collision, time-of-flight extension of the VEM method for the study of molecule-surface vibrational energy transfer is introduced. The technique helps election between possible alternative trial analytic forms of the collisional transition probability function. A Gaussian form is preferred over a Boltzmann-exponential form for cyclobutene isomerization to 1.3-butadiene energized by collisions at a silica surface at 800 K.
State selection by resonant multiphoton ionization: N+2 A 2Πu, v+
Authors: Pratt S. T. , Dehmer P. M. , Dehmer J. L.
Published: February 2003

Three photon resonant, four photon ionization of N2 via the o3 1Πu, v′=1, 2 levels is shown to produce≥90% N+2 A 2Πu, v+=1, 2, respectively. These results suggest that resonant multiphoton ionization through Rydberg states with electronically excited ion cores may be a powerful technique for the production of electronically excited, vibrationally state-selected ions.
Dynamics of H+O2 → OH+O at high collision energies
Authors: Kleinermanns K. , Schinke R.
Published: February 2003

Classical trajectory calculations of total and state-to-state cross sections for the reaction H(2S)+O2(3Σ−g)  → OH(2Π)+O(3P) have been performed for collision energies E=28, 60, and 90 kcal/mol using an ab initio potential energy surface. The main conclusion is that with increasing collision energy the specifity of the initial conditions leading to reaction increases drastically. Thus, with increasing E, the range of impact parameters at which reactions occur becomes successively confined to a broad maximum at about half of the O2 equilibrium separation indicating a direct reaction mechanism. This results in a decrease of the total reaction cross section at high energies. The reaction specifity leads to increasingly nonstatistical rotational product distributions narrowly peaked at high rotational states. The vibrational product distributions are statistical at lower E and partly inverted at E=90 kcal/mol. At E=28 kcal/mol the angular product distribution is symmetric with respect to θ=90° as expected from reactive trajectories which proceed via a long-lived complex. At E=60 and 90 kcal/mol forward–backward scattering is observed with a slight preference for forward scattering. The bimodal angular distribution can be attributed to two essentially different types of trajectories. It is also observed at E=90 kcal/mol that trajectories leading to backward angles yield significantly enhanced product vibrational excitation.
Hot atom-laser induced fluorescence experiments on the reaction of H(2S) with O2
Authors: Kleinermanns K. , Wolfrum Jürgen
Published: February 2003

A method has been developed to study the dynamics of reactions with substantial threshold energies by combining hot atom production by laser photolysis and fast product detection by laser induced fluorescence. The nascent OH rotational, vibrational, and fine structure state distributions produced in the endothermic reaction H+O2→OH(K,v, f )+O have been measured with hot hydrogen atoms from the photodissociation of HBr at 193 nm. OH rotational excitation is high and corresponds to the phase space statistical distribution over the measured range of rotational states. Also OH vibrational excitation is substantial. The OH spin doublets are produced statistically, but the partitioning in the λ doublets is very nonstatistical with a strong preference for the π+ component demonstrating the reaction to occur essentially in a plane at high collision energies. By measuring reactant and product densities at short times, an absolute total cross section of 0.42±0.40.2 (Å2) has been obtained.
Charge distribution from infrared intensities: Charges on hydrogen atoms and hydrogen bond
Authors: Gussoni M. , Castiglioni C. , Zerbi G.
Published: February 2003

It is known that from infrared intensities it is possible to derive atomic charges and charge fluxes. The intensity parameters of hydrogen atoms which are able to form hydrogen bonds are calculated in this work and compared with those of other hydrogen atoms not showing the same property; they are found to be different. From gas phase infrared intensity measurements one can then determine whether a given hydrogen atom is capable of forming hydrogen bond.
An experimental verification of incoherent light scattering
Authors: Härtl W. , Versmold H.
Published: February 2003

Dynamic light scattering from two liquidlike ordered latex suspensions with particle diameters 85 and 109 nm, as well as the scattering from a 4:1 mixture of these suspensions are investigated. Both the static structure factor SM(K) and the intermediate scattering function FM(K,τ) of the mixture strongly deviate from those of the neat suspensions at low scattering angles. This discrepancy is discussed in terms of an incoherent light scattering contribution due to scattering-power polydispersity of the mixture. Our experiments confirm theoretical expressions recently derived by Pusey et al.
Spectroscopic study on the iodine molecule by a sequential three-photon excitation
Authors: Ishiwata Takashi , Ohtoshi Hirokazu , Sakaki Mamoru , Tanaka Ikuzo
Published: February 2003

A three-photon absorption technique which utilizes a visible B 3Π0+u−X 1Σ+g transition followed by a simultaneous two-photon absorption was applied to study an ion-pair state of molecular iodine. The derived molecular parameters were Te=51 707 cm−1, ωe=131 cm−1, and Be=0.021 90 cm−1 for the F′(0+u) ion-pair state, which dissociates to I−(1S)+I+(1D). The excitation of I2 to a single rovibronic level of the F′ state was achieved and its fluorescence spectrum showed two discrete band systems corresponding to the transitions to: (1) the ground state at higher vibrational levels; and (2) the weakly bound state (Te=19 286 cm−1, ωe=64 cm−1, and re=3.65 Å) converging to the I(2P3/2)+I(2P1/2) products.
Electron diffraction studies of supersonic jets. VII. Liquid and plastic crystalline carbon tetrachloride
Authors: Valente Edward J. , Bartell Lawrence S.
Published: February 2003

Carbon tetrachloride at 3–11 mol % in neon carrier produces clusters of CCl4 molecules in flow through a Laval nozzle. Electron diffraction patterns of the clusters formed at a total pressure below 2.5 bar are intermediate between x-ray diffraction patterns of bulk liquid CCl4 and neutron patterns of the amorphous material formed by condensation at 10 K. Strong, sharp interference features at low angle imply appreciable long range order, perhaps beyond 50 Å, in the clusters, whose temperatures are about 210 K. When carrier partial pressure is increased beyond 2.6 bar, clusters begin to exhibit crystallinity. Crystallites belong to the rhombohedral plastic-crystalline phase Ib, a=14.27 Å and α=90.0°.
Conformational kinetics of methyl nitrite. I. NMR spectral evidence for statistical intramolecular vibrational redistribution
Authors: Chauvel J. Paul , Conboy Claire B. , Chew Wilbert M. , Matson Gerald B. , Spring Cheryl A. , Ross Brian D. , True Nancy S.
Published: February 2003

Pressure dependent rate constants for syn⇄anti conformational exchange in gaseous methyl nitrite and in gaseous methyl nitrite–CO2 mixtures have been obtained from line shape analyses of 1H NMR spectra. The pressure dependence of the exchange rates is consistent with a specific reaction rate constant of ∼1×109/s which agrees with RRKM calculations demonstrating that intramolecular vibrational redistribution is occurring at the statistical limit in methyl nitrite molecules with ∼12 kcal/mol of internal vibrational energy. At 12 kcal/mol methyl nitrite has a state density of ∼160/cm−1. These results indicate strong anharmonic and/or Coriolis coupling between vibrational levels. Bimolecular rate data for methyl nitrite and methyl nitrite–CO2 mixtures are consistent with a collisional efficiency βp for CO2 of 0.95(8) for activation of syn–anti conformational exchange at 258.8 K.
A study of the unimolecular decomposition of the (C2H4)+2 complex
Authors: Ono Y. , Linn S. H. , Tzeng W.-B. , Ng C. Y.
Published: February 2003

The energetics of the unimolecular reactions C2H+4⋅C2H4→C3H+5+CH3 and C4H+7+H have been reinvestigated by the molecular beam photoionization method. At nozzle expansion conditions where the concentrations of (C2H4)3 and heavier clusters produced in the beam are high, the appearance energies (AE) for C3H+5 and C4H+7 were found to shift to energies lower than the expected thermochemical thresholds for the formation of C3H+5 and C4H+7 from ethylene dimers. This indicates that ethylene trimers and heavier clusters can give rise to the same product ions as ethylene dimers. The AE’s for C3H+5 and C4H+7 from (C2H4)2 measured in an ethylene beam, which mainly consists of C2H4 and (C2H4)2, are 10.21±0.04 eV (1214±5 Å) and 10.05±0.04 eV (1234±5 Å), respectively. Taking into account the error estimates of the thermochemical thresholds for the formation of C3H+5 and C4H+7 from (C2H4)2, the latter values are in accordance with the conclusion of previous studies that the barriers for the reverse reactions of the ion–molecule reactions C2H+4+C2H4 are negligible. From the observed ionization energies of (C2H4)2 (9.84±0.04 eV) and C2H4 (10.505±0.004 eV) and the estimated binding energy of (C2H4)2 (0.02 eV), the bond dissociation energy for C2H+4⋅C2H4 is calculated to be 15.8±1 kcal/mol.
Quasiclassical trajectory studies of rigid rotor–rigid surface scattering. II. Corrugated surface
Authors: Park Seung C. , Bowman Joel M.
Published: February 2003

The quasiclassical trajectory method, previously applied to rigid rotor–rigid flat surface scattering [J. M. Bowman and S. C. Park, J. Chem. Phys. 77, 5441 (1982)] is applied to a rigid rotor–rigid corrugated surface, i.e., a N2–LiF(001), system. The mechanisms for rotational excitation at low and high collision energies are studied as well as their dependence on initial beam orientation and corrugation strength. A significant correlation between long-lived trajectories and high rotational excitation is found for low energy collisions and rotational rainbows are clearly observed in the high energy regime, although these features are broadened relative to the flat surface reported previously.
Anomalous increase of the time lag in nucleation near the critical point
Authors: Gitterman M. , Rabin Y.
Published: February 2003

Application of critical dynamics results in a Fokker–Planck equation for the distribution function of nuclei. The time lag associated with the establishment of a steady state following a fast quench is exponentially larger than that for a slow quench. Numerical estimates are presented.
A Monte Carlo method for quantum Boltzmann statistical mechanics
Authors: Doll Jimmie D. , Freeman David L.
Published: February 2003

An alternate approach to quantum Boltzmann statistical mechanics in calculations on interacting many-body systems is presented in the form of the Monte-Carlo method. (AIP)
Direct observation of nonlinear Jahn–Teller effects in the 1 1A1g→3s 1E
g two-photon spectrum of cyclohexane
Authors: Whetten Robert L. , Grant Edward R.
Published: February 2003

The ultraviolet two-photon absorption spectrum of the lowest excited singlet state of jet-cooled cyclohexane reveals a highly perturbed system of very sharp bands in the 55 000–58 000 cm−1 region, which we assign as transitions to vibronic elements of the 3s 1Eg Rydberg state. On the basis of polarization, isotope, and variable-temperature data and by use of vibronic coupling calculations, we construct a detailed assignment of the vibronic level structure which demonstrates that the proliferation of low-energy states is a direct result of the introduction of an entirely new type of low-frequency motion (150 cm−1) in the excited state, that of the Jahn–Teller vibronic pseudorotation. We observe moderate to high activity in five of the eight possible Jahn–Teller modes, and estimate a total Jahn–Teller stabilization energy of 2550 cm−1. Importantly, we find large splittings (15% of the zeroth-order frequency) of states which would remain degenerate given only linear Jahn–Teller activity (free pseudorotation), and interpret these in terms of nonlinear vibronic coupling as the first experimental evidence for significant quenching of vibronic angular momentum in an isolated molecule.
Raman study of solid N2O4: Temperature induced autoionization
Authors: Bolduan F. , Jodl H. J. , Loewenschuss A.
Published: February 2003

Raman spectra of solid N2O4 condensed from gaseous NO2/N2O4 were investigated. When deposited at 80 K the O2NNO2 dimer is formed. Deposition at 15 K produces additionally bands of the NO2 monomer and the O2NONO isomers. Warming to 180 K induces autoionization to NO+NO−3 which remains stable over the 15–180 K range. The transition is probably an intermolecular process invoked through the low energy lattice modes or ν4 twist vibration of the dimer.
Direct studies of proton tunneling in hydrogen bonded mixed molecular crystals by optical excitation
Authors: Clemens John M. , Hochstrasser R. M. , Trommsdorff H. P.
Published: February 2003

Absorption, excitation, and fluorescence spectra have been observed for thioindigo in benzoic acid host crystals at 1.4–4.2 K. The spectra reveal sites which arise from different tautometic forms of neighboring host dimer molecules. These sites exist in thermal equilibrium and interconvert during the excited state lifetime of thioindigo. The site interconversion process arises from a change in the tautomer configuration of a neighboring benzoic acid dimer due to proton tunneling. Fluorescence lifetime measurements for these sites give a direct measure of the proton tunneling rates in the excited state system at liquid helium temperatures k(H)tunneling =(4.45±0.10)×108 s−1. The deuteron tunneling rate is considerably slower in the acid deuterated host crystal k(D)tunneling =(4.9±1.5)×106 s−1. The acid protonated host crystal exhibits reversible hole burning phenomena for all thioindigo sites with recovery of the original line profile on the time scale of 1 min. The homogeneous linewidths obtained from hole burning measurements are not determined by the fluorescence lifetime of thioindigo. The additional contribution to the linewidth can be attributed to dephasing arising from site interconversion processes in both the ground and excited state system. A comparison of hole burning and fluorescence lifetime measurements provides a value for the proton tunneling rate in the ground state interconversion (equilibrium) process, k(H)tunneling =(3.5±1.3)×108 s−1. These results show that the hole burning phenomenon does not arise directly from these tunneling processes occurring in neighboring host dimers.
Polarization line shape of Balmer-β from electron impact dissociation of H2
Authors: Freund Robert S. , Donohue David E. , Fisanick Georgia J.
Published: February 2003

This work reports for the first time an example of varying polarization within the Doppler linewidth. The Doppler broadened Balmer-β line from electron impact dissociation of H2 consists of two major components, a central peak due to one set of dissociative states and broad wings due to another. Polarization of the total line peaks at 37 eV with a value of 16%, passes through 0% near 500 eV, and reaches ∼−1.5% at 1000 eV. Polarization of the wings is more positive than the central peak, and reaches values as high as 20%. These data may be helpful in characterizing the dissociative states of H2.
Photoionization of sodium clusters
Authors: Peterson K. I. , Dao P. D. , Farley R. W. , Castleman A. W.
Published: February 2003

Sodium clusters, Nax (2≤x≤8), were produced in a supersonic jet by coexpansion with argon and subjected to analysis via photoionization coupled with mass spectrometry. The present measurements, made at somewhat higher resolution than earlier literature studies, yield ionization potentials in excellent agreement for x=1 to 4 and x=7, and reveal heretofore unreported structure in the post-threshold spectra. The present measurements enable a more definitive assignment of the ionization potentials, giving a lower value for x=6 and pointing to lower ones for x=5 and 8. The frequently discussed odd–even alternation in ionization potentials is no longer evident beyond the pentamer. Values for the binding energies of Na to Na+x (3≤x≤8) are deduced.
Excited-state dynamics of hydrogen-bonded dimers of benzoic acid
Authors: Poeltl David E. , McVey Jeffrey K.
Published: February 2003

The fluorescence emission spectra of jet-cooled hydrogen-bonded dimers of benzoic acid and isotopic analogs are presented and the vibrational assignments of the ground state complexes are made. Intramolecular vibrational relaxation is observed in S1 at excess vibrational energies as low as 373 cm−1. The role of the hydrogen bonding network as a heat bath is considered and the results are consistent with an argument based on an estimate of the density of states. Electronic energy transfer is examined in the weak coupling limit and transfer is observed in mixed dimers of perhydro (d0) and ring deuterated (d5) benzoic acid. Transfer from d5 to d0 is observed when excess vibrational energy is present. The jet-cooled, isolated dimer provides an excellent case for the examination of energy transfer in hydrogen-bonded complexes.
An exponentiated DWBA formula for H-atom transfers. Extensions to lower barrier potentials and to higher energies
Authors: Babamov Vasil K. , Lopez Vicente , Marcus R. A.
Published: February 2003

Analytic formulas obtained earlier for the probability of H-atom transfer between two heavy particles in the threshold region are extended to higher energies and to systems with lower barrier potentials. The calculated reaction probability vs energy curve is in good agreement with numerical results for the model system investigated involving transfer between two nearly degenerate states.
Spectroscopy and dynamics of the chemiluminescent reactions N+(1D)+H2→NH+(B 2Δ)+H and N+(1D)+D2→ND+(B 2Δ)+D
Authors: Kusunoki I. , Ottinger Ch.
Published: February 2003

Optical emission in the 3500–5000 Å range has been observed resulting from N++H2(D2) collisions at 1–9 eVCM. It is due to the B 2Δ→X 2Π transition of NH+ (ND+) formed in an exchange reaction. Numerous previously unobserved bands were resolved and the vibrational molecular constants determined. Vibrational and rotational product level populations were obtained through spectrum simulation. A comparison with other chemiluminescent ion–molecule reactions indicates a very high degree of internal excitation. From the measured reaction threshold, metastable N+ (1D) ions were identified as the reactants, and the product B 2Δ state formation is discussed in terms of electronic structure. Surprisingly no product is observed in the closely related A 2Σ− state, and reasons for this are suggested.
Deductions from the master equation for chemical activation
Authors: Snider Neil
Published: February 2003

The master equation for unimolecular decomposition and collisional deactivation of an energetic species A formed by chemical reaction was investigated. Particular attention was paid to the validity of the approximation of neglecting upward transitions, i.e., collisional transitions in which energy is added to A. Calculations of κa , the collision frequency ω times the ratio of the rate of decomposition of A to that of deactivation, were performed for the stepladder model for very large and very small ω and for the separable model for small ω. Neglect of upward transitions reduces κa. For the stepladder model the amount by which κa is reduced is greater for large ω than it is for small ω. It was also found that κa is more sensitive to changes in 〈Δε〉−, the average internal energy loss from A in deactivating collisions, when ω is small than it is when ω is large. Thus, when fitting computational results to experimental data, one derives a 〈Δε〉− which is too small if one neglects upward transitions, and the amount by which 〈Δε〉− is too small is considerably greater for large ω than it is for small ω. This result explains both qualitatively and semiquantitatively the discrepancies in 〈Δε〉−’s derived from stepladder model calculations of low pressure and high pressure κa’s for sec-butyl radicals in the presence of rare gas diluents.
Theoretical investigation of the X 2Σ+ and C 2Σ+ states of BeH
Authors: Cooper David L.
Published: February 2003

Recent experimental evidence that the rotationless X 2Σ+ state potential of BeH possesses a small maximum is examined and is shown to be somewhat equivocal. Complete configuration interaction calculations involving the three valence electrons are carried out for the two lowest states of 2Σ+ symmetry. The X 2Σ+ state potential shows no potential maximum and spectroscopic constants for the C 2Σ+ agree very well with experiment.
Proton transfers between first- and second-row atoms: (H2OHSH2)+ and (H3NHSH2)+
Authors: Scheiner Steve
Published: February 2003

A b initio molecular orbital methods are used to study the transfer of the central proton along the hydrogen bonds in (H2OHSH2)+ and (H3NHSH2)+. Proton transfer potentials are generated using the 4-31G* basis set at the Hartree–Fock level for various values for the hydrogen bond length R(XS). Full geometry optimizations are carried out at each stage of proton transfer. The barrier to proton transfer increases as the hydrogen bond is lengthened. For a given bond length, the highest barriers are observed for transfer from N to S and the smallest for the reverse process. Intermediate between these two extremes are transfers between O and S for which the forward and reverse transfers lead to nearly identical barrier heights. Adiabatic transfers, in which the intermolecular separation is allowed to change as the transfer progresses, are studied as well. The barrier to adiabatic transfer from OH2 to SH2 is 2.6 kcal/mol; 1.9 for the reverse process. Similar relaxation of R(NS) leads to no stable (NH3)(SH3)+ structure and hence transfer from N to S is not expected. Application of larger basis sets and inclusion of correlation effects through second and third-order M≂ller–Plesset corrections support the reliability of the HF/4-31G* results.
A correlated variational wave function for the orientational ground state of solid methane
Authors: Sprik Michiel , Klein Michael L.
Published: February 2003

We introduce a correlated variational wave function of the Jastrow type that is able to represent the ground state of solid CH4 through the orientational order–disorder transition. To describe the local orientation in a weakly ordered phase a new kind of pocket state is introduced, that is derived from the imaginary time propagator of a free top. The effect of short range correlations between the orientational zero point fluctuations is evaluated for pure meta (total spin=2) methane in the disordered and the ordered phases of the James and Keenan (JK) model. The T-A tunnel energy in fully converted methane is calculated in the same approximation and corrections to the mean field values are found to be large. While the difference in energy between the partially ordered phase II and the fully ordered JK phase III is found to be small, the latter remains marginally the more stable, in contrast to experiment.
Polarization propagator calculations with an AGP reference state
Authors: Weiner B. , Jensen Hans-Jo/rgen Aa. , Öhrn Y.
Published: February 2003

A consistent approximation to the polarization propagator is developed and compared to others viz Tamm–Dancoff and random phase approximations (TDA and RPA, respectively). The role of a properly defined vacuum as the reference state for the propagator is examined. It is shown that the closest one can come to a ‘‘consistent RPA’’ is a generalized TDA which corresponds to a polarization propagator based on an energy optimized generalized antisymmetrized geminal power (GAGP) state. The theory is applied to the nitrogen molecule in a slightly simplified form and calculated excited electronic states are compared to experimentally observed ones.
Bonding and potential energy curves for NiH and NiH2
Authors: Ruette F. , Blyholder G. , Head J.
Published: February 2003

The MINDO/SR procedure does well in reproducing experimental observations and theoretical ab initio calculated properties of NiH and NiH2. The occupied Ni d orbitals are nonbonding and remain lower in energy than the bonding Ni s and p orbitals in both NiH and NiH2. In agreement with experiment the lowest state of NiH2 is calculated to be a triplet with σ and δ holes in the d orbitals. A number of bent metastable states of NiH2 have been found, with the geometry being influenced indirectly by the d-orbital occupancy. One triplet state with a triangular geometry can be readily formed from H2 plus Ni and is stable by 9 kcal/mol with respect to dissociation into Ni plus H2. The H–H bond is stretched and reduced in energy but is not broken. This is a highly activated H2 molecule but there is still a large activation evergy, 42 kcal/mol, to completely break the H–H bond.
Microscopic motions and the local environments of atoms in simple liquids
Authors: Endo Yoshikazu , Endo Homare
Published: February 2003

Molecular dynamics simulations are carried out in order to understand the microscopic mechanism of atomic motions in simple liquids. The radial distribution function is classified into subgroups by taking a partial average over the atoms having the same coordination number in the first shell. By considering this classified distribution of atoms as an initial one, the time evolution of the radial distribution function, and the velocity auto- and cross-correlation functions are calculated in each subgroup. Investigation through these ‘‘microscopic’’ correlation functions reveals the details of atomic motions which would be obscured if the totally averaged correlation functions were used. It is found that an atom in a region of low local density oscillates weakly for a long period, because the low-density region is surrounded, on the average, by the high density region of atoms. On the other hand, an atom in a region of high local density receives a strong rebound from the neighboring atoms, but behaves less oscillatory at subsequent times, because the neighboring atoms move toward the outer region of low density.
Electronic energy level and intensity correlations in the spectra of the trivalent actinide aquo ions. III. Bk3+
Authors: Carnall W. T. , Beitz J. V. , Crosswhite H.
Published: February 2003

The solution absorption spectrum of Bk3+(aquo) was measured and the observed band structure interpreted in terms of a free-ion energy level model. The band intensities were successfully analyzed using the Judd–Ofelt theory for transitions within the f N configuration. Parameters of the theory were then used to compute fluorescence branching ratios from most probable fluorescing states, and an experimental search was successful in yielding evidence for a transition from one excited state to the ground state in D2O solvent. Absorption bands attributed to f → d transitions were observed and an interpretation of the electronic structure is presented. Band intensities were compared to those observed for Tb3+(aquo).
Spectral analysis of light scattered by liquid films. II. Light scattering in the long wavelength limit
Authors: Joosten J. G. H.
Published: February 2003

This paper describes what information can be obtained when the spectral distribution of light scattered by thin liquid films, like free soap films, is analyzed. To this end we have worked out the so-called long wavelength limit (i.e., when the wavelengths of the surface waves are much greater than the film thickness) of the thermal fluctuation power spectrum describing the dynamics of the interfacial displacements occurring on a film. The analysis reveals that the squeezing mode spectrum as well as the bending mode spectrum exhibit a three peak structure: two peaks shifted symmetrically around zero frequency (Brillouin doublet) and one peak centered around zero frequency (pure diffusive component). For the squeezing mode the diffusive component, which is typical for a film, is strong, whereas for the bending mode the energy contained in the Brillouin doublet is large when compared with the central peak. These observations and also the large difference in characteristic time scales, in which the two modes occur, enables one to study their features separately by means of dynamic light scattering. The width of the central peak of the squeezing mode depends on: the film thickness, the viscosity of the film liquid, the interfacial tension, and the second derivative with respect to film thickness of the free energy of film formation. The free energy used in this paper consists of a sum of electrical double layer repulsion and London–van der Waals attraction. The effect of the so-called surface elasticity on the central peak width of the squeezing mode is calculated. We also derived the thermal fluctuation power spectrum for interfacial waves when the film is suspended in a second medium. It turns out that for the bending mode the effect of the film bounding medium on the frequency shift of the Brillouin doublet is important, even if this is a low density vapor phase. The shift frequency for the bending depends on the film tension and the total mass that participates in this kind of fluctuation. The width of these shifted peaks depends mainly on the properties of the film bounding medium.
Resolution of the Ã/B̃ photoionization branching ratio paradox for the 12CO+2 B̃(000) state
Authors: Johnson M. A. , Zare R. N. , Rostas J. , Leach S.
Published: February 2003

The interaction between the 12CO+2 B̃(000) level and vibronic levels of the Ã state has been investigated. The detailed nature of this mixing proves to be the key to the quantitative understanding of the Ã/B̃ electronic state branching ratio discrepancy in which photoelectron spectroscopy gives about one-third the value of the Ã/B̃ ratio as that derived from fluorescence measurements. Upon excitation of rotational lines of the 12CO+2 B̃(000)–X̃(000) band near 2900 Å, part of the resulting dispersed fluorescence is found to be shifted to the red (λ>3000 Å). Detailed high resolution spectroscopic studies using jet-cooled ions and population labeling optical–optical double resonance show that this red shift is caused by perturbations of the B̃(000) state by two states belonging to the Ã2Πu manifold. When the red-shifted (to λ>3300 Å) fluorescence quantum yield for the perturbed B̃(000) state is summed over all populated rotational levels of the excited state, this gives 0.37±0.05, in excellent agreement with that previously obtained from photoelectron–photon coincidence measurements following photoionization of CO2 by the He 584 Å resonance line. After corrections for emission occurring in the region 3000 <λ< 3300 Å, the total quantum yield for emission at λ>3000 Å from the B̃(000) level is found to be 0.42±0.07. These results show that the principal mechanism responsible for the Ã/B̃ branching ratio paradox is interaction of specific rotational states of the B̃(000) level and perturbing levels of the Ã state of 12CO+2.
Semiclassical collision theory within the Feynman path-integral formalism: The perturbed stationary state formulation
Authors: Jolicard Georges
Published: February 2003

Classical equations are derived for the relative motion of two molecules as the internal degrees of freedom make a given quantum transition by extending the semiclassical theory of Peckhuks into the adiabatic representation. Using a stationary phase approximation to Feynman’s path integral representation of quantum mechanics as a starting point, classical trajectories are obtained which fulfill angular momentum and energy conservation. The features of the adiabatic treatment are analyzed through the numerical integration of the collinear harmonic oscillator-atom collision. The calculations show that the use of a perturbed basis appreciably reduces the computational time required by the iterative procedure of integration. However no solution can be obtained as in the diabatic approach for some classically forbidden transitions.