bluuebunny/crossref_metadata_2025 huggingface dataset:
Ground state of random copolymers and the discrete random energy model
Authors: Gutin A. M. , Shakhnovich E. I.
Published: July 2002

We propose and investigate a modification of the random energy model where the energies of different states are still independent random values but may take only discrete values. This model appears naturally in studies of random heteropolymers with monomers of two types. We calculate the probability that the ground state of such a polymer is nondegenerate and test this result against a lattice model of a heteropolymer with exhaustively enumerated conformations. The theory is in excellent agreement with numerical experiment. Our results imply that the lower the energy of the ground state the less probable that it is degenerate. The probability of degeneracy decays exponentially as ground state energy decreases.
Theoretical study of linear and bent CrCO, NiCO, and CuCO
Authors: Fournier René
Published: July 2002

I performed density functional calculations on the electronic ground state of the monocarbonyls of chromium, nickel, and copper. CrCO and CuCO have bent equilibrium geometries with angles equal to 139.9° and 143.9° respectively, while NiCO is linear. The linear structures of CrCO and CuCO are less stable by 3 kcal/mol and are not minima, but have two imaginary frequencies. An analysis of the bonding suggests that the bent structures of CrCO and CuCO are more stable because the repulsion between the 5σ orbital of CO (essentially a lone pair on the carbon atom) and the half-filled orbital on the metal is smaller in that geometry. The calculated Ni–CO bond energy 53 kcal/mol is 13 kcal/mol larger than the most recent experimental determination [L. S. Sunderlin et al., J. Am. Chem. Soc. 114, 2788 (1992)]. The calculated bond energies of CrCO and CuCO are 14 and 20 kcal/mol, respectively.
Infrared-induced desorption of HD from lithium fluoride
Authors: Ferm Paul M. , McClelland Gary M.
Published: July 2002

Low intensity ir radiation at 2–15 μm desorbs HD from 1.5–4.2 K lithium fluoride (LiF) via four distinct channels. The channel most studied here involves coverages of one monolayer or less and 9–15 μm radiation, causing HD to desorb with a translational energy TB of 21 K. This process, which is attributed to a single phonon ejecting HD from its lowest bound state, is precisely linear in the radiation intensity, and its efficiency increases with increasing wavelength, reaching a maximum of ≊4×10−5 molecules per photon at 15 μm. At multilayer coverages and a surface temperature of 4.2 K, at which HD has a significant vapor pressure, we observe a photoinduced thermal desorption signal (TB=4.0–4.8 K), arising from a very small (≊0.005 K) perturbation of the crystal temperature. The evidence for thermal desorption of this channel strongly contrasts with the other three channels, which are nonthermal. A third channel involves radiation between 2.71 and 4.4 μm, which causes a very weak signal with TB≊20 K. While this region was chosen to include the HD vibrational transition at 2.75 μm, we present calculations which indicate that vibrational predesorption is not operative here. A fourth channel (TB≊34 K) is observed only for LiF containing bulk OH− impurities absorbing at 2.78 μm, and is attributed to phonons generated by the OH− excitation.
Resonance Raman excitation profiles of methyl iodide in hexane
Authors: Markel Frances , Myers Anne B.
Published: July 2002

Resonance Raman spectra, including absolute scattering cross sections, have been measured for methyl iodide in hexane at nine excitation wavelengths from 204.2 to 274.0 nm. Spectra excited in the 246–274 nm region, on resonance with the directly dissociative A state, exhibit overtone progressions in the C–I stretch and its combination bands with the methyl umbrella mode. The fundamental of the C–I stretch is weak relative to the overtones when excited near the peak of the A band, an effect shown to arise from interferences between the resonant amplitude and preresonant contributions from higher electronic states. Both the absorption spectrum and the Raman excitation profiles are quite similar in solution and vapor phases, suggesting that the dissociative A state is not strongly perturbed by solvation. This conclusion is consistent with a simple theoretical estimate of the solvation effect on the A state potential.
Spinodal and kinetic boundary of metastable region
Authors: Kiselev S. B. , Kostyukova I. G.
Published: July 2002

Equations for the thermodynamic spinodal and the kinetic boundary of the region of metastable states (physical spinodal) in vicinity of the critical point are given. These equations are based on the asymmetric scaled equation of state and the theory of relaxation of metastable states. The results of numerical calculations of binodal, thermodynamic, and physical spinodal, as well as the behavior of isochoric heat capacity and isothermal compressibility in the metastable region for water are presented.
Dissociative adsorption of H2 on Ni(111)
Authors: Yang Hong , Whitten Jerry L.
Published: July 2002

Ab initio configuration interaction calculations are performed to study the dissociative adsorption of H2 on a Ni(111) surface. The lattice is modeled as an embedded three-layer 41-atom cluster. Ni 3d orbitals are explicitly included on seven Ni atoms on the surface. H is preferentially chemisorbed at a threefold site on Ni(111) and the calculated binding energy of 62 kcal/mol, H–Ni distance of 1.86 Å, and H vibrational frequency of 1176 cm−1 are in excellent agreement with experimental data. H adsorbed at bridge and on-top Ni sites is 2.5 and 8.1 kcal/mol less stable, respectively. The heat of reaction H2 (gas)→2 H (ads) is calculated to be 22.0 kcal/mol exothermic. When two H atoms are adsorbed as nearest neighbors to the same Ni atom, threefold sites are preferred with H atoms adsorbed at fcc–fcc, hcp–hcp, or across atom fcc–hcp sites. These structures are consistent with the observed (2×2)−2H low energy electron diffraction pattern. The average adsorption energy per H is calculated to be 62 kcal/mol for the across atom case. Adsorption of H at a threefold site on Ni(111) has a tendency to block adsorption at adjacent threefold sites; coadsorbed H atoms at adjacent sites are unbound by 13.5 kcal/mol relative to H2 at infinite separation. For three H atoms chemisorbed at separated threefold sites sharing a Ni atom with the equal H–H lateral distances of 2.48 Å, the calculated average adsorption energy is 60 kcal/mol per H. Dissociation of H2 at an on-top Ni site has the lowest activation barrier of 1.6 kcal/mol. For the transition state, the H–H bond length, H-surface height, and H–H vibrational frequency are 1.22 Å, 1.38 Å, and 2506 cm−1, respectively. Molecular H2 adsorbed over an on-top site is bound by 3.4 kcal/mol with a slightly stretched H–H bond length of 0.79 Å, H-surface height of 1.85 Å, and H–H stretching frequency of 3396 cm−1.
Comparison of local-density and Hartree–Fock calculations of molecular polarizabilities and hyperpolarizabilities
Authors: Guan Jingang , Duffy Patrick , Carter Jonathan T. , Chong Delano P. , Casida Kim C. , Casida Mark E. , Wrinn Michael
Published: July 2002

This paper presents a comparison between density functional theory local density approximation (LDA) and Hartree–Fock approximation (HFA) calculations of dipole moments, polarizabilities, and first hyperpolarizabilities, using ‘‘comparable’’ basis sets, in order to assess the relative quality of the LDA and the HFA for calculating these properties. Specifically, calculations were done using basis sets of roughly double or triple zeta plus polarization quality, with and without added field-induced polarization (FIP) functions, for the seven small molecules H2, N2, CO, CH4, NH3, H2O, and HF, using the HFA option in the program HONDO8 and the LDA options in the programs DMol and deMon. For the calculations without FIP functions, the results from HONDO8 HFA and deMon LDA, both of which use Gaussian basis sets, are very similar, while DMol, which uses a LDA numerical atomic orbital basis set, gives substantially better results. Adding FIP functions does much to alleviate these observed basis set artifacts and improves agreement with experiment. With FIP functions, the results from the two sets of LDA calculations (deMon and DMol) are very similar to each other, but differ markedly from the HFA results, and the LDA results are in significantly better agreement with experiment. This is particularly true for the hyperpolarizabilities. This appears to be the first detailed study of DFT calculations of molecular first hyperpolarizabilities. We note that closer attention to numerical details of the finite field calculation of β⇊ is necessary than would usually be the case with traditional ab initio methods. A proof that the Hellmann–Feynman theorem holds for Kohn–Sham calculations is included in the Appendix.
Hydrogen dissociation on copper: Importance of dimensionality in calculations of the sticking coefficient
Authors: Engdahl Christer , Nielsen Ulrik
Published: July 2002

The dissociative adsorption of H2 on copper is studied using classical trajectory and quantum wave-packet calculations. A multidimensional effective-medium potential is used which takes the molecular orientation as well as the surface structure into account. The emphasis in this work is on the effects of including all six molecular degrees of freedom in calculations of the sticking coefficient and the implications on comparisons with experimental data. We find that six-dimensional effects, among other things, lead to substantial shifts of the sticking onset energies. The change of onset energy due to vibrational excitation or isotope exchange is also sensitive to six-dimensional effects. This means that a two-dimensional analysis of the relation between observed onset energies and potential barriers sometimes may be misleading. The agreement between calculated and experimental sticking curves is improved by including six dimensions. The surface atom motion, however, is found to have a negligible effect on the sticking coefficient at surface temperatures used in recent molecular beam measurements. By comparisons in reduced dimensionality we find that a quasiclassical calculation gives a reasonable account of the true quantum sticking coefficient in the present case. There are significant quantum effects but these appear small compared to the multidimensional effects. A multidimensional combined quantum and classical approach seems to be appropriate for the dissociation dynamics problem.
Molecular dynamics studies of the melting of butane and hexane monolayers adsorbed on the basal-plane surface of graphite
Authors: Hansen Flemming Y. , Newton J. C. , Taub H.
Published: July 2002

The effect of molecular steric properties on the melting of quasi-two-dimensional solids is investigated by comparing results of molecular dynamics simulations of the melting of butane and hexane monolayers adsorbed on the basal-plane surface of graphite. These molecules differ only in their length, being members of the n-alkane series [CH3(CH2)n−2CH3] where n=4 for butane and n=6 for hexane. The simulations employ a skeletal model, which does not include the hydrogen atoms explicitly, to represent the intermolecular and molecule–substrate interactions. Nearest-neighbor intramolecular bonds are fixed in length, but the molecular flexibility is preserved by allowing the bend and dihedral torsion angles to vary. The simulations show a qualitatively different melting behavior for the butane and hexane monolayers consistent with neutron and x-ray scattering experiments. The melting of the low-temperature herringbone (HB) phase of the butane monolayer is abrupt and characterized by a simultaneous breakdown of translational order and the orientational order of the molecules about the surface normal. In contrast, the hexane monolayer exhibits polymorphism in that the solid HB phase transforms to a rectangular-centered structure with a short coherence length in coexistence with a fluid phase. A significant result of the simulations is that they demonstrate the importance of molecular flexibility on the nature of the melting transition. The formation of gauche molecules is essential for the melting process in the hexane monolayer but unimportant for butane. The effect of molecular length on the qualitative nature of the melting process is discussed for both monolayers.
Calculation of x-ray emission spectra of molecules and polymers by the Green’s function method
Authors: Liegener Christoph , Ågren Hans
Published: July 2002

We use the one-particle Green’s function for calculations of x-ray spectra of molecules and polymers. The x-ray rates are obtained as the residues of the one-particle Green’s function modulated by x-ray orbital transition moments. The latter are optionally decomposed into atomic contributions using local symmetry selection rules in line with the standard intensity model for molecular x-ray emission. Applications are performed for molecules in the sequence ethylene, butadiene, hexatriene, and polyacetylene. Site selectivity leads to surprisingly molecular-like structures in the theoretical polymer spectrum, although the bands are quite broad. The effect can be interpreted from the singularities of the spectral density of the one-particle Green’s function.
Li+FH reactive cross sections from J=0 accurate quantum reactivity
Authors: Laganà A. , Pack R. T , Parker G. A.
Published: July 2002

Accurate three dimensional quantum calculations have been carried out for the Li+HF reaction at zero total angular momentum J on a potential energy surface fitted to ab initio points. By adopting a J shifting approximation reactive cross sections are estimated and compared with the experiment. The agreement is very good.
Millimeter-wave spectrum of CH3CD3 in the three lowest torsional states
Authors: Moazzen-Ahmadi N. , Ozier I. , Mukhopadhyay I. , Amano T.
Published: July 2002

The pure rotational spectra of CH3CD3 in the three lowest torsional states has been observed using a mm-wave spectrometer. A total of 87 rotational frequencies were measured between 230–363 GHz for J=7←6 to 11←10 in v6=0, 1, and 2, where v6 is the torsional quantum number. For the lowest two torsional states, the spectra have the classic form expected for a symmetric top (with no internal rotation) in the ground vibrational state. For v6=2 and for a given (J+1)←J, a markedly different splitting pattern is observed as a result of the (K,σ) dependence of the effective rotational constant B̂, where σ labels the torsional sublevels. In order to identify the individual features in the (v6=2) spectrum, an assignment procedure was developed which is based on the fact that the ratio of moment of inertia of the top about the molecular symmetry axis to that of the whole molecule about the same axis is to a very good approximation 1/3. The torsion–rotation Hamiltonian discussed earlier in connection with CH3SiH3 [N. Moazzen-Ahmadi et al. J. Mol. Spectrosc. 119, 299 (1986)] was used to analyze the rotational frequencies along with the molecular beam anticrossing data and the origin of the torsional fundamental. Several constants which characterize the J-dependence of the energy levels were determined. Effective values for the barrier height Ṽ3 and the shape parameter Ṽ6 associated with the first-order correction in the Fourier expansion of the potential function were obtained. The effect of redundancies on the interpretation of the measurements is discussed.
Temperature programmed desorption of molecular hydrogen from a Si(100)-2×1 surface: Theory and experiment
Authors: Flowers Michael C. , Jonathan Neville B. H. , Liu Yong , Morris Alan
Published: July 2002

New experimental temperature programmed desorption (TPD) data have been obtained under carefully controlled conditions for atomic deuterium on the single crystal Si(100)-2×1 surface. A wide range of coverages from Θ=1.5 to 0.05 ML was used. A kinetic lattice-gas model has been developed which describes atomic hydrogen (or deuterium) adsorbed on the Si(100)-2×1 surface in terms of four basic units: dihydride (SiH2), doubly occupied dimers (H–Si–Si–H), singly occupied dimers (Si–SiH), and unoccupied dimers (Si=Si). The equilibria between these species have been determined by considering both the lattice partition functions and the vibrational partition functions associated with the Si–H bonds. By using a quasiequilibrium approximation and two competing desorption routes corresponding to formation of the β1 and β2 peaks, the TPD spectra for hydrogen (deuterium) molecules are determined and compared with the new experimental data. Fitting the experimental curves with the simulated data from the aforementioned model showed that the desorption process which leads to the β1 peak obeys first-order kinetics with an A factor of 2×1015 s−1 and activation energy of 57 kcal mole−1, whereas the process giving the β2 peak follows second-order kinetics with an activation energy of 47 kcal mole−1 and an A factor (expressed in 1st order units) of 3×1015 s−1.
Stark effect measurement in samarium monoxide: Dipole moments of the [16.6]1 and X− states
Authors: Linton C. , James A. M. , Simard B.
Published: July 2002

The permanent electric dipole moments of the [16.6]1 (the Ω=1 state lying near 16 600 cm−1) and X0− (ground) electronic states of 152SmO and 154SmO have been determined in a pulsed molecular beam by measuring the Stark shifts of the R(0) and R(1) lines of the (0,0) band in the [16.6]1–X0− transition. Electric fields up to 7.9 kV/cm were used. The Stark measurements also gave a precise determination of the Ω doubling of the J=1 level of the [16.6]1 state. The magnitudes of the dipole moments for the X0− and [16.6]1 states were determined to be 3.517(20) and 4.022(24) D for 152SmO, and 3.451(28) and 3.967(40) D for 154SmO (2σ error bounds). The splitting due to the Ω doubling of the J=1 level of the [16.6]1 state was determined to be 0.0433(24) cm−1 for 152SmO and 0.0380(50) cm−1 for 154SmO. A field dependent perturbation affecting the J=1, MJ=0 level of the [16.6]1 state of the 154SmO isotopomer was observed and analyzed.
The 248 nm photodissociation of KI: Determination of the branching ratio of K(4 2P
J) doublets in the presence of Ar, H2, and N2
Authors: Ke Ching-Bin , Lin King-Chuen
Published: July 2002

By means of a three-level kinetic model developed previously, the nascent fine-structure branching ratios of K 4 2PJ doublets following photodissociation of KI at 248 nm can be determined accurately in the presence of foreign gases. With the forward and reverse Ar-induced collisional fine-structure mixing cross sections of 15 and 30 Å2 reported by Lijnse, and our measurement of Ar quenching cross section 0.81±0.08 Å2, the nascent branching ratio of the K 4 2P3/2 component is determined to be 0.611±0.002. Analogously, with the N2-induced collisional mixing cross sections of 100 and 190 Å2 reported by Lijnse and Hornman, and our measurement of N2 quenching cross section 18±2 Å2, the branching ratio of K 4 2P3/2 is determined to be 0.608±0.002. The agreement between these values confirms reliability of the kinetic model. However, a lack of confirmity is found in the presence of H2 quencher, using the collisional mixing cross sections of 53 and 75 Å2 reported by McGills and Krause, and our quenching cross section measurement of 2.8±0.3 Å2. The discrepancy is caused by the unsuitable adoption of mixing cross sections. Using the average branching ratio 0.610 determined from the cases of Ar and N2, the H2-induced collisional mixing cross sections can be evaluated to be 51±4 and 90±7 Å2. The obtained quenching cross sections by collisions with foreign gases are consistent with those reported elsewhere except for the case of Ar. The discrepancy of our Ar measurement of 0.81±0.08 Å2 from a reported value <0.07 Å2 does not cause a significant difference in the branching ratio determination; it is because the magnitude of the mixing cross sections are more than ten times larger than the relevant quenching cross sections, such that the dependence on the collisional quenching becomes insignificant.
The Ar–HF intermolecular potential: Overtone spectroscopy and ab initio calculations
Authors: Chang Huan-C. , Tao Fu-Ming , Klemperer William , Healey Catherine , Hutson Jeremy M.
Published: July 2002

The vibrational dependence of the intermolecular potential of Ar–HF is investigated through the spectra of levels correlating with HF(v=3). We have previously reported measurements of the (vbKn)=(3000), (3100), and (3110) levels of Ar–HF using intracavity laser-induced fluorescence in a slit supersonic jet [J. Chem. Phys. 98, 2497 (1993)]. These levels are found to be well reproduced (within 0.1 cm−1) by the Ar–HF H6(4,3,2) potential [J. Chem. Phys. 96, 6752 (1992)]. The second overtone experiments are extended to include the (3002) state which is coupled to (3110) through Coriolis interaction, and the (3210) state which is more sensitive to higher-order anisotropic terms in the potential. The observations establish that the level (3002) lies 0.229 cm−1 below (3110), with upper state rotational constant B=0.085 89 cm−1. This is in good accord with the predictions of the H6(4,3,2) potential. The (3210) state lies at 11 484.745 cm−1 with B=0.099 79 cm−1. The band origin is 1.7 cm−1 higher than predicted, and thus contains important new information on the vibrational dependence of the potential. Several detailed features of the spectra can be explained using the H6(4,3,2) potential. The Q-branch lines of the (3210)←(0000) band show evidence of a weak perturbation, which can be explained in terms of mixing with the (3112) state. The (3210) spectrum exhibits parity-dependent rotational predissociation and the widths of the P- and R-branch lines and the magnitude of the l-type doubling can be explained in terms of coupling to the (3200) state, which is estimated to lie 4 cm−1 below the (3210) state.

The Q-branch lines show a predissociation cutoff above Q(16); this is in reasonable agreement with the predictions of the H6(4,3,2) potential, but suggests that the binding energy calculated for the potential may be about 1 cm−1 too large. To examine the potential further, high-level ab initio calculations are performed, with an efficient basis set incorporating bond functions. The calculations give a well depth of 92%–95% of that of the H6(4,3,2) potential at θ=0° for v=0 and v=3, respectively; this is in line with earlier results on rare gas pairs. The calculations also reproduce the anisotropy of the H6(4,3,2) potential and its vibrational dependence. The dependence of the intermolecular potential on HF bond length is found explicitly.
Vibrational band shape analysis of the C–H vibration of CH2I2 molecules in liquid CCl4
Authors: Moser G. , Asenbaum A. , Döge G.
Published: July 2002

The vibrational band shapes and their parameters of the C–H vibration of CH2I2 diluted with liquid CCl4 were measured as a function of composition. The linewidth increases from 8.7 cm−1 with decreasing mole fraction X of CH2I2 reaches a maximum value of 15.92 cm−1 near X=0.4 and decreases again in the limit of the diluted case. The frequency of the band center increases with decreasing X from 2965.37 to 2984.75 cm−1. These results are compared with the model of Knapp and Fischer for the concentration dependence of the vibrational linewidths and shifts. For the line shift, good agreement is found between theory and experiment by introducing a microscopic mole fraction. The model can be used to get an estimation on the difference between macroscopic and microscopic concentration. The predictions for the band shape are rather satisfactory in the whole concentration range. Deviations are due to the assumption that the band shapes in the model are Lorentzian for both neat liquids, whereas the experimental line shape in neat CH2I2 is not.
Molecular rotation and the observation of dipole-bound states of anions
Authors: Brinkman Elizabeth A. , Berger Susan , Marks Jeffrey , Brauman John I.
Published: July 2002

Electron photodetachment spectra have been measured in an ICR spectrometer for the enolate ions of acetone, cyclobutanone, cyclopentanone, cyclohexanone, cycloheptanone, methyl vinyl ketone, pinacolone, propionaldehyde, and 1,1,1-trifluoroacetone enolates. Electron affinities have been determined for acetone enolate radical 1.758±0.019, cyclobutanone enolate radical 1.801±0.008, cyclopentanone enolate radical 1.598±0.007, cyclohexanone enolate radical 1.526±0.010, cycloheptanone enolate radical 1.444+0.02/−0.002, tert-butyl methyl ketone (pinacolone) enolate radical 1.755+0.05/0.005, propionaldehyde enolate radical 1.621±0.006, and 1,1,1-trifluoroacetone enolate radical 2.625±0.010 eV. Autodetaching dipole-bound states are observed in some but not all of these spectra. The mechanism for autodetachment of these states is discussed and it is seen that the binding of an electron by a dipole is very sensitive to the motions of the dipole. The motions of the dipole can be predicted from the rotational motions of the molecule, allowing us to correlate the observation of dipole-bound states with the rotational motions of the dipole moment.
The Fock-space coupled-cluster method: Electron affinities of the five halogen elements with consideration of triple excitations
Authors: Hughes S. R. , Kaldor Uzi
Published: July 2002

The Fock-space coupled-cluster method with single and double excitations (CCSD or SUB2) is applied to the calculation of the electron affinities of all five halogen elements, F–At. Excellent agreement with experiment is obtained, the average error being 0.04 eV. The inclusion of triple excitations for F and Cl spoils the agreement. Comparison with other methods is made.
Reactions of hot deuterium atoms with OCS in the gas phase and in OCS–DI complexes
Authors: Böhmer E. , Mikhaylichenko K. , Wittig C.
Published: July 2002

Reactions of photolytically prepared hot deuterium atoms with OCS have been investigated: (i) under gas phase, single collision, arrested relaxation (i.e., bulk) conditions; and (ii) by photoinitiating reactions within weakly bound OCS–DI complexes. Nascent SD(X 2Π, v=0) rotational, spin–orbit, and Λ-doublet populations were obtained for the photolysis wavelengths 250, 225, and 223 nm by using A 2Σ←X 2Π laser induced fluorescence (LIF). The reason for using deuterium is strictly experimental: A 2Σ predissociation rates are considerably smaller for SD than for SH. The SD (v=0) rotational distribution was found to be very cold and essentially the same for both bulk and complexed conditions; the most probable rotational energy is ∼180 cm−1. No bias in Λ-doublet populations was detected. Spin–orbit excitation for bulk conditions was estimated to be [2Π1/2]/[2Π3/2]∼0.25, where 2Π1/2 is the upper spin–orbit component. This ratio could not be obtained with complexes because of limited S/N. The complete set of present and past experimental findings, combined with recent theoretical results of Rice, Cartland, and Chabalowski suggest a mechanism in which SD derives from a very short lived HSCO precursor. This can result from direct hydrogen attack at the sulfur and/or the transfer of hydrogen from carbon to sulfur via the HCOS intermediate.
Theoretical simulation of molecular Auger spectra: The carbon and oxygen KLL Auger spectra of formaldehyde
Authors: Minelli D. , Tarantelli F. , Sgamellotti A. , Cederbaum L. S.
Published: July 2002

A theoretical ab initio simulation of the carbon and oxygen KLL Auger spectra of formaldehyde is presented and discussed. The effects of nuclear vibrational motion on the energy position and broadening of the Auger peaks in the two very different spectra are explicitly accounted for using a method derived from the time-dependent theory of the nuclear dynamics of decaying states. The underlying vertical double ionization spectrum, comprising hundreds of relevant electronic states, is computed using Green’s function methods, while charge distribution effects and Auger intensities are estimated via a two-hole population analysis of the eigenstates. The resulting theoretical spectra reproduce accurately the experimental band shapes and positions, showing that the observed spectra are complex convolutions of a very large number of transitions, with strong correlation and intensity redistribution effects. The nuclear motion analysis is found to be particularly important for the correct reproduction of the spectra, accounting very well for the substantial differences in energy shifts and band widths in the two spectra.
Excitation profile of coherent anti-Stokes Raman scattering from the MnO−4 ion doped in a KClO4 crystal at low temperature
Authors: Leuchs M. , Kiefer W.
Published: July 2002

We have performed polarized resonance coherent anti-Stokes Raman experiments on the permanganate ion doped in potassium perchlorate single crystals at temperature T=15 K. At this temperature the m (Cs) site splitting of the excited degenerate 1T2-electronic level of the permanganate ion and the vibronic structure are well resolved. We report on the A′-ν1 coherent anti-Stokes Raman excitation profile which shows a strong dependence on the frequency of the pump laser. The simulation of the experimental results is performed by using the transform theory, which enables one to calculate the resonance Raman excitation profile solely from the measured absorption spectrum. According to the well known relation between the third-order nonlinear susceptibility and the Raman polarizability the coherent anti-Stokes excitation profile is given by a simple product of two Raman excitation profiles which are shifted relative to each other. The linear and the quadratic electron-phonon coupling as well as the influence of non-Condon terms were taken into account. Since the transform theory is mode selective in case there are no mode mixing effects the results derived from the description of the Raman excitation profiles and the coherent anti-Stokes excitation profiles are more definite. For this purpose one only needs the model parameters of the mode of interest. The simulation of the A′-ν1 coherent anti-Stokes profile shows that a small amount of non-Condon coupling yields to a better agreement with the experiment.
Hyperfine and spin–orbit structure of the 4Δi ground state of CoO
Authors: Clouthier D. J. , Huang G. , Merer A. J. , Friedman-Hill E. J.
Published: July 2002

A detailed examination of the structure of the 4Δi ground state of gaseous CoO has been carried out from analysis of the electronic spectrum near 6300 Å, using intracavity dye laser and wavelength-resolved fluorescence methods. The intracavity experiments have given the details of the hyperfine structure of the two lowest spin–orbit components at sub-Doppler resolution, permitting the hyperfine parameters, a=0.02295 cm−1 and (b+c)=−0.01117 cm−1, to be derived; from the negative value of the contact interaction the electron configuration is established as (4sσ)2(3dδ)3(3dπ)2. The wavelength-resolved fluorescence experiments have given the relative positions of the four spin–orbit components of the X 4Δi state to ±0.3 cm−1. Based on these measurements, a pair of subbands sharing a common electronic upper level, but with the Ω=5/2 and 7/2 components of the ground state as lower levels, has been recognized. Rotational analysis of Doppler-limited intracavity spectra of these two subbands has given an accurate value for the Ω=5/2–7/2 separation as 304.321±0.007 cm−1.
Molecular tunneling dynamics of NH3D+ in ammonium sulfate and ammonium thiocyanate
Authors: Burrows Wes , Strauss Herbert L.
Published: July 2002

The kinetics of NH3D+ rotation in ammonium sulfate and ammonium thiocyanate are measured at low temperature (8–60 K). Infrared hole burning of the N–D stretching bands causes reorientation of the ions among distinct positions relative to the crystal lattice, producing nonequilibrium population distributions. The slow relaxation (time scale of minutes to hours) is monitored with a Fourier-transform infrared spectrometer as the ions tunnel back to their equilibrium distributions. Kinetic models are used to extract rate constants for rotation between orientations. The rate constants range from 0.01 h−1 at 8 K, to about 10 h−1 above 50 K. The higher temperature data is described by barrier transmission calculations with barrier heights of 1000 to 1600 cm−1. The low temperature data agrees with rates calculated from a phonon-assisted tunneling model for transitions between the ground states of the orientations, with energy differences between ground states on the order of 10 cm−1. Calculations of the tunneling rates between quantum states in a potential for the rotation of the NH3D+ ion in ammonium sulfate generated by a point-charge model are also performed and compared to the experimental results.
Transition state structures and angular momentum effects in the dissociation dynamics of energy-selected C4H+8 ions
Authors: Booze Jon A. , Schweinsberg Matthias , Baer Tomas
Published: July 2002

The photoionization and dissociation dynamics of energy-selected 1-butene ions have been investigated by the technique of threshold photoelectron photoion coincidence (TPEPICO) time of flight mass spectrometry. The absolute dissociation rates for the reactions leading to the loss of H, CH3, and CH4 have been measured for two samples prepared with very different internal energy and angular momentum distributions. First rotationally cold ions were prepared by photoionizing 1-butene molecules cooled in a seeded molecular beam. These rates were analyzed within the framework of RRKM theory with vibrator transition state structure for all three channels. Excellent agreement between theory and experiment was obtained when ab initio calculated transition state frequencies were used for the H loss and the CH3 loss transition states. A variational transition state theory (VTST) analysis shows that the CH3 loss transition state lies about 11 kJ/mol below the dissociation limit. Second, dissociation rates using an effusive source which contained a 298 K distribution of vibrational and rotational energy were measured. The vibrator-type transition state model, with inclusion only of the vibrational energy distribution, gives a good account of the total rates but significantly overestimates the H loss branching ratio. Excellent agreement is obtained, however, when the energies of the molecular ions and vibrator transition states are corrected for the rotational energy of each structure. K-rotor mixing with the vibrations does not change the calculated rates significantly. Finally, the analysis confirms a previous proposal [Faraday Discuss. Chem. Soc. 75, 57 (1983)] that an orbiting transition state (a la phase space theory) is not the rate limiting bottleneck at the energies used in this experiment.
Photoelectron angular distributions and vibrational branching ratios of CO (2+1)-photon ionization via the B 1Σ+ state
Authors: Sha G. , Proch D. , Rose Ch. , Kompa K. L.
Published: July 2002

We present vibrational branching ratios and photoelectron angular distributions for resonantly enhanced (2+1)-photon ionization of CO. The excitation ladder involves the B 1Σ+(vi=0 or 1) Rydberg state. Contrary to expectations fostered by the Franck–Condon principle, ionization via vi=0 branches into vibrational states v+=0–4. Such phenomena are also observed in the case of vi=1, but only to a minor extent. The angular emission patterns of the photoelectrons ejected during the ionizing step are of distinct character in that they are highly anisotropic for Δv=vi−v+=0 processes, but show isotropy when due to Δv≠0 transitions. The photoelectron angular distribution which accompanies the Δv=0 ionization of B(vi=1) shows p-wave character, and hence we may postulate a spherical potential of the Rydberg ion core. The same approximation should hold for the vi=0 state. The dissimilar appearance of the angular distributions when ionizing from this level invites the hypothesis of two individually different ionization channels.
Microwave spectrum, quadrupole coupling constants, structure and ab initio calculations of N-bromocyanofluoromethanimine
Authors: Durig J. R. , Nanaie H. , DesMarteau D. D.
Published: July 2002

The microwave spectrum of N-bromocyanofluoromethanimine CF(CN)NBr in the ground vibrational state in the region from 18.0 to 38.0 GHz has been investigated. The assigned spectrum is only consistent with a planar molecule with the cyano group trans to the bromine atom. The hyperfine structure due to the nuclear quadrupole coupling of the bromine nucleus has been analyzed for the a-type R transitions of both the 81Br and 79Br isotopic species. Quadrupole splittings due to the 14N nuclei were not resolved. The rotational constants, the centrifugal distortion constant ΔJ, and the nonzero values of the diagonal and off-diagonal elements of the bromine quadrupole coupling tensor were determined by a least-squares fit for both isotopic species of bromine. The r0 structural parameters r(C–C), r(C–F), and ∢(N=C–C) were determined to have values of 1.422(9) and 1.349(15) Å, and 121.6(9)°, respectively, whereas other structural parameters were fixed at the values obtained for the corresponding parameters of similar molecules. The equilibrium geometry of the two possible isomeric forms of CF(CN)NBr were determined by an ab initio calculation which employed the various basis sets with and without electron correlations. These results are compared with the corresponding values of some similar molecules.
Evaluation of thermal rates for reactions with intermediate wells: Removal of bound state contributions to quantum flux correlation functions
Authors: Thachuk Mark , Mayne Howard R. , Schatz George C.
Published: July 2002

We consider the calculation of rates of chemical reactions that have bound intermediate states (i.e., wells along the reaction path) using flux correlation function methods. When time-dependent wave packets are used to evaluate the propagator matrix elements, and the dividing surface is located at a point where bound states have nonzero probability density, the standard expression for the flux correlation function shows infinitely long lived oscillations due to these bound states, making the evaluation of rate constants numerically ill-behaved. However, if the bound part of the initial wave packet is projected out, the resulting continuum-only propagators produce rapidly decaying correlation functions, and numerically well-behaved rate constants. We illustrate this projection operator approach by considering a one-dimensional reaction path model in which the potential is taken to be an Eckart well, and the dividing surface is located at the minimum. In another application, we consider a two degree of freedom model of H2 dissociative chemisorption on a rigid metal surface. This application is sufficiently complex that it is impractical to calculate the chemisorption rate using conventional flux correlation function methods, but with the projected wavepacket approach, the problem is made relatively easy. We also consider a second approach to the treatment of bound states in which the flux correlation function is altered to remove implicitly the bound state contributions to the propagator at long times. This second expression can be used with the full propagator, eliminating the need to construct and project explicitly the bound states. This should be advantageous when many bound states are present.
Solvation by nonpolar solvents: Shifts of solute electronic spectra
Authors: Stratt Richard M. , Adams John E.
Published: July 2002

It is only relatively recently that it has become possible to use spectroscopy to track the solvation of a molecule as one proceeds from small solute-plus-solvent clusters, through bulk liquids, and into cryogenic matrices. One of the more surprising findings of such studies is that, in a number of noteworthy instances—such as with benzene dissolved in Ar—the solvent shifts of spectral lines in even apparently sizable clusters seem not to go smoothly into the bulk results. In this and the following paper we consider just what level of theoretical treatment is necessary in order to be able to account for the solvent shift of electronic spectra consistently in environments ranging from clusters to the bulk. As we discuss in some detail, neither continuum dielectric approaches nor sums of pair potentials can adequately describe the solvation. What we propose here, instead, is that the effects of nonpolar solvents can be treated fully microscopically by a model incorporating both local repulsive effects and longer-ranged dielectric effects. The latter contribution, resulting from the solvent’s polarizability, is formulated in terms of the so-called polarization modes of the solvent, which change with the detailed arrangement of the solute’s environment. We illustrate the ideas by showing that one can understand the optical spectroscopy of benzene in liquid Ar more or less quantitatively by using this model, and we point out some connections with analogous time-dependent solvation studies. The application of this same approach to clusters is described in the succeeding paper.
Monte Carlo simulations of a hydrophobic weak polyelectrolyte. Charge distribution as a function of conformation
Authors: Abascal J. L. F. , Sassi Alexander P. , Blanch Harvey W. , Prausnitz John M.
Published: July 2002

Monte Carlo simulations were performed to obtain the distribution of charged segments around the center of mass in a lattice-model, isolated weak polyelectrolyte with attractive segment–segment (hydrophobic) interactions. The charge distribution (relative to the mean ionization for a given state) is shown to depend essentially on chain conformation (stiffness).
Collisions of protic and aprotic gases with a perfluorinated liquid
Authors: Saecker Mary E. , Nathanson Gilbert M.
Published: July 2002

Collisions of Ne, CH4, NH3, and D2O with a perfluoropolyether (PFPE) liquid are explored by directing molecular beams at the surface of PFPE in vacuum and monitoring the velocity of the scattered products. The protic and aprotic gases undergo similar impulsive energy transfer and thermal accommodation when striking the surface of PFPE. When compared with glycerol and squalane, PFPE absorbs less energy and exhibits smaller trapping-desorption fractions than the alcohol or hydrocarbon. We correlate these results with the energy lost in single and multiple hard spherelike collisions between the incoming particle and the protruding functional groups of the liquid phase molecules. Lacking the lighter CH and OH groups of glycerol and squalane and the capacity to hydrogen bond, PFPE exposes heavy CFx groups which provide a hard and weakly attractive surface that resists energy transfer and impedes thermal accommodation.
SiC2: A molecular pinwheel
Authors: Ross Stephen C. , Butenhoff Thomas J. , Rohlfing Eric A. , Rohlfing Celeste McMichael
Published: July 2002

We present the results of a combined experimental and theoretical study of the large-amplitude motion in SiC2 in which the C2 fragment undergoes hindered internal rotation. Stimulated emission pumping (SEP) is used to obtain rovibrational term energies for levels with up to 14 quanta of excitation in the large-amplitude vibration. We analyze the SEP data, as well as other available experimental data, using a semirigid bender model that allows for complete internal rotation within a triatomic molecule. From the least-squares fitting of this model to the data, we determine the potential energy along the minimum energy path of the large-amplitude vibration, the harmonic energies of the small-amplitude vibrations, and the variations of these energies and of the molecular geometry with the large-amplitude coordinate. The fitting is aided by results obtained from ab initio calculations we perform on the triangular and linear configurations of the molecule. The current data set is consistent with a large-amplitude potential energy function in which the energy difference between the triangular and linear configurations is 1883 cm−1. The statistical error on this energy difference is 22 cm−1, but we estimate the physical uncertainty to be about 200 cm−1. This result is in excellent agreement with the energy difference of 1819 cm−1 we obtain in our best ab initio calculations. The semirigid bender fitting and our best ab initio calculations are also both consistent with a potential energy function having no local minimum at linearity.
A water–water potential derived using a quantum Monte Carlo vibrational analysis
Authors: Franken Kenneth A. , Dykstra Clifford E.
Published: July 2002

A difficulty in the generation and testing of intermolecular interaction potentials has to do with the effects of vibrational motion. For weak modes, the vibrational excursions can be and often are sizable. So, not only do model potentials have to be suitable over a considerable range, they must also be accurate with respect to experimental data that reflects the vibrational averaging (i.e., on-average structures). In this report, we present an explicit interaction potential for the water dimer that has been devised with a systematic treatment of vibrational effects by means of quantum Monte Carlo treatment of the vibrational ground state. The model potential uses intrinsic electrical properties of isolated water and four empirical parameters. An iterative procedure based on matching measured on-average rotational constants to calculated values was used to select the atom–atom Lennard-Jones (empirical) parameters. We present the results of these calculations, including zero point vibrational energies and on-average separations.
Generation and detection of coherent magnetron motion in Fourier transform ion cyclotron resonance mass spectrometry
Authors: Chen Ruidan , Guan Shenheng , Marshall Alan G.
Published: July 2002

One of the most useful recent ion cyclotron resonance (ICR) developments is the conversion of magnetron motion to cyclotron motion by azimuthal quadrupolar excitation in the presence of ion-neutral collisions. The technique offers a mass-selective means for ‘‘shrink-wrapping’’ an ion cloud into a tight packet along the central axis of an ICR ion trap for enhanced signal to noise ratio, mass resolving power, and other advantages. However, the process itself is not directly observable. In this paper, we show that the conversion may be rendered observable by converting coherent magnetron motion (produced by off-axis ionization during a period short compared to the magnetron frequency) to coherent cyclotron motion, followed by subsequent dipole detection at ω+ (reduced cyclotron frequency) or quadrupolar detection at ωc (unperturbed cyclotron frequency) and 2ω+. Detection at ωc eliminates the ICR frequency shift due to the electrostatic trapping potential, providing for increased mass accuracy; detection at 2ω+ may offer increased mass resolving power. The observed signal behavior as a function of excitation amplitude–duration product is predicted theoretically and confirmed experimentally for both types of detection. Unlike the otherwise analogous 180° pulse in nuclear magnetic resonance (NMR), the magnetron-to-cyclotron interconversion may be phase-coherent with respect to both initial and final states. Finally, we show how coherent magnetron motion of two ion packets of different magnetron phase can be converted to cyclotron motion of two ion packets of different cyclotron phase, and we discuss the implications of that process.