bluuebunny/crossref_metadata_2025 huggingface dataset:
The photoionization spectra of effusive and supersonic molecular beams of monosilane
Authors: Ding A. , Cassidy R. A. , Cordis L. S. , Lampe F. W.
Published: July 2002

Effusive and supersonic molecular beams of monosilane have been ionized by photons from the Berlin synchrotron light source (BESSY). Photoionization efficiency curves have been taken as a function of wavelength (50 to 120 nm) for a number of fragment ions. The main products observed are SiH+3 and SiH+2 . No appreciable SiH+4 could be detected and less than 5% of the signal was due to lower fragments (SiH+, Si+). The threshold energies Ethr, the standard enthalpy of formation ΔH○f,g of the fragment ions SiH+2 and SiH+3 and the ionization potentials Iz of the corresponding radicals have been determined to be SiH+2:   Ethr=11.67±0.04 eV ΔH○f,g=276.3±0.9 kcal/mol, Iz(SiH2)=9.47±0.03 eV.

SiH+3: Ethr=12.23±0.02 eV, ΔHf,g=237.1±0.6 kcal/mol, Iz(SiH3)=8.32±0.07 eV. Discrete structure in the photoionization curves is observed in the range of Ethr=15.8–17.6 eV and interpreted as vibrational progressions of a highly excited autoionizing state of SiH4 which lies 15.8 eV above the ground state. Vibrational constants have been found to be 1790±80 and 680±50 cm−1 which correspond to the ground state constants 2187 and 711 cm−1 of the ν1 and the ν2 modes of monosilane. The broadening of some lines in the progression is interpreted as being caused by a transition into a dissociative excited neutral state with lifetimes as short as 3×10−14 s for v=3 of the ν2 mode. Additional ions (predominantly Si2H+4, Si2H+6, Si2H+7) have been observed when photoionizing a supersonic molecular beam of monosilane. These are shown to stem from the ionization and subsequent fragmentation of dimers and possibly higher multimeres. Threshold values have been determined to be Si2H+4: 11.58±0.02 eV; Si2H+6: 11.4 eV; and Si2H+7: 11.4 eV.
Chain organization and thermodynamics in micelles and bilayers. II. Model calculations
Authors: Szleifer I. , Ben-Shaul A. , Gelbart W. M.
Published: July 2002

Based on the theory presented in part I (preceding paper) we calculate molecular and thermodynamic properties of model chains packed in micellar aggregates of three typical geometries: spheres, cylinders, and planar bilayers. Each possible conformation of a model chain is equivalent to a sequence of walks on a regular cubic lattice. The internal energy of a given conformation is proportional to the number of ‘‘kinks’’ (π/2 bond angles). The kink (gauche) energy measures the inherent flexibility of the chain. We calculate bond order parameter profiles for chains packed in aggregates of various curvature and radius, and find that in all cases the degree of conformational freedom increases from the chain head towards its end. The same qualitative behavior is observed for entirely flexible (zero kink energy) chains. This implies that the internal energy of the chain plays only a secondary role, compared to that of the packing constraints in determining chain conformational statistics in micellar aggregates. In accordance with this conclusion we also find that the geometry dependence of the conformational free energy is dominated by the entropic contribution. The differences between the minimal free energies of chains in different geometries are generally small. Yet, they may be comparable in magnitude to the changes associated with the surface (‘‘opposing forces’’) contributions to the geometry dependence of the micelle’s free energy.
Sum rules for atomic form factors and total x-ray scattering intensities
Authors: Thakkar Ajit J. , Koga Toshikatsu
Published: July 2002

Sum rules relating the position moments 〈xlymzn〉 and 〈rn〉 to the atomic form factor are derived. Exactly analogous sum rules relate the intracular moments 〈x12ly12mz12n〉 and 〈r12n〉 to the total x-ray scattering intensity in Waller–Hartree theory.
A modified Poisson–Boltzmann equation in electric double layer theory for a primitive model electrolyte with size-asymmetric ions
Authors: Outhwaite C. W. , Bhuiyan L. B.
Published: July 2002

The modified Poisson–Boltzmann theory is extended to treat a primitive model electrolyte with unequal ionic radii in the neighborhood of a uniformly charged plane wall. The linear equation indicates that the transition from a damped exponential to a damped oscillatory asymptotic behavior in the mean electrostatic potential as the concentration is increased depends in a complicated manner on the ratio of the ionic radii, the ionic valences, and the ionic concentration. The nonlinear equation is solved numerically by a quasilinearization technique. Comparisons of the results are made with those from the Poisson–Boltzmann theory. A feature of the unequal radii model is its ability to predict a charge separation for an uncharged wall. The analysis is restricted to aj≤3ai where ai, aj are the radii of the smaller and larger ions, respectively.
Extensive theoretical studies of the hydrogen-bonded complexes (H2O)2, (H2O)2H+, (HF)2, (HF)2H+, F2H−, and (NH3)2
Authors: Frisch Michael J. , Del Bene Janet E. , Binkley J. Stephen , Schaefer Henry F.
Published: July 2002

The structures and binding energies of the complexes (H2O)2, (H2O)2H+, (HF)2, (HF)2H+, F2H−, and (NH3)2 have been examined using much higher levels of theory than has been previously applied to these systems. These methods including large basis sets and full optimization of structures with the effects of electron correlation included, are known to give single bond energies to an accuracy of about 2 kcal mol−1 and are found in this study to give excellent agreement with the extensive experimental data available for the hydrogen fluoride and water dimers. The Cs open

form of ammonia dimer remains a very shallow minimum energy structure at these levels, in agreement with previous theoretical results but seemingly in disagreement with experiment. The theoretical enthalpy of association of H5O+2 is found to be −35.0 kcal mol−1, in slight disagreement with the most recent experimental results, but in accord with earlier ones, which suggests that these experiments should be reexamined. The enthalpy of association of H2F+ is predicted to be −33.5 kcal mol−1, and that of F− with HF to be −46.4 kcal mol−1. A study of the effects of basis set expansion on the structure of the water dimer shows that the structure is much more sensitive to basis set at the Hartree–Fock level than when correlation is included. A valence triple-zeta basis plus two sets of first polarization functions and one set of diffuse functions appears to be necessary to approach the Hartree–Fock limiting structure. Counterpoise estimates of the effects of basis set deficiencies on the structure and binding energy of this complex are shown to be misleading. Examination of the complexes (HF)2, (H2O)2, (NH3)2, (H2O)2H+, (HF)2H+, and F2H− at the MP4/6-311++G(3df ,3pd)//MP2/6-311++G(2d,2p) level of theory indicates that previous studies using fourth order perturbation theory with some smaller basis sets and Hartree–Fock optimized structures are likely to be reliable, although part of the agreement reflects a cancellation of error. HF/6-31+G(d) estimates of zero-point vibrational energy contributions to association energies are found to be satisfactory for asymmetric complexes, but can both over and underestimate the contribution of this term for symmetrically bound complexes.
Gerade Rydberg states and n
s 3Σ+u(1u,0−u) photoionization spectra of the rare gas dimers (n=2–6)
Authors: Killeen K. P. , Eden J. G.
Published: July 2002

Transient absorption spectra of rare gas dimer Rydberg transitions in the ultraviolet, visible, and near infrared (220≤λ≤900 nm) spectral regions have been observed in electron beam excited rare gases. The most prominent molecular bands for He2, Ne2, Ar2, Kr2, and Xe2 are assigned to the mp 3Πg←ns 3Σ+u (He:n=2, 3≤m≤10; Ne:n=3, 4≤m≤10; Ar:n=4, 5≤m≤15; Kr:n=5, 6≤m≤16; Xe:n=6, 8≤m≤11) Rydberg series of the dimer. Adiabatic ionization potentials, relative to the ns 3Σ+u state, are determined by extrapolation of the series to their limits (m→∞) to be 34 361.4±30 cm−1 (4.260±0.004 eV) for He2, 34 396.3±25 cm−1 (4.265±0.003 eV) for Ne2, 29 351.9±15 cm−1 (3.639±0.002 eV) for Ar2, 28 428.5±10 cm−1 (3.525±0.001 eV) for Kr2, and 26 664.5±250 cm−1 (3.306±0.031 eV) for Xe2. Absorption bands which are ascribed to m′p 3Σ+g←ns 3Σ+u Rydberg series of Ne2(m′=4,6,8), Ar2(5≤m′≤10), Kr2(6≤m′≤10), and Xe2(m′=8,9) are also reported, and the ns 3Σ+u ionization potentials found to be identical to the respective values quoted above. All of the observed molecular Rydberg states have an A 2Σ+1/2u ion core. The lowest vibrational quanta (ΔG1/2) for the ns 3Σ+u states are determined from vibronic structure in the Rydberg transitions to be 1723 cm−1 for He2, 539 cm−1 for Ne2, 302 cm−1 for Ar2, 172 cm−1 for Kr2, and 135 cm−1 for Xe2, and are consistent with previously reported values. The instability of the ns 3Σ+u metastable state with respect to the dimer ion core potential [D0(A 2Σ+1/2u)−D0(ns 

3Σ+u)] is calculated to be 0.508±0.004 eV for He2, 0.681±0.003 eV for Ne2, 0.572±0.002 eV for Ar2, 0.560±0.001 eV for Kr2, and 0.52±0.03 eV for Xe2. Using these values and those reported in the literature for the dissociation energy of the dimer ion A 2Σ+1/2u state, D0(ns 3Σ+u) is estimated (Ne2: 0.67±0.07 eV; Ar2: 0.76±0.02 eV; Kr2: 0.62±0.02 eV; Xe2: 0.52±0.03 eV). By comparing energy level spacings, quantum defects, and absorption oscillator strengths for transitions between molecular or atomic states, the mp 3Πg Rydberg levels appear to correlate with the mp[3/2]2+1S0 atomic asymptotes. An important aspect of the experimental approach is that the diatomic excited states are studied in the vicinity of Re rather than at the van der Waals (ground state) minimum. Chemical forces (as opposed to ion-induced dipole interactions) dominate at small R and the resulting spectra are largely free of congestion arising from the perturbations and avoided curve crossings that are prevalent at large R. Therefore, insight can be obtained into the structure and behavior of molecular Rydberg states in the region near the first ionization limit A 2Σ+1/2u[1(1/2)u].
A vibrational study of the adsorption and decomposition of formic acid and surface formate on Al(111)
Authors: Crowell J. E. , Chen J. G. , Yates J. T.
Published: July 2002

The adsorption and decomposition of formate and formic acid have been studied on the Al(111) surface using high resolution electron energy loss spectroscopy (EELS) and temperature programmed desorption (TPD). Formic acid reacts with clean Al(111) at 120 K to form a surface formate species. Molecular adsorption of formic acid occurs at 120 K only after saturation of the surface formate layer has been reached. Off-specular vibrational measurements have determined that the formate species is symmetrically bridge bonded through both oxygen atoms with C2v symmetry. A lower symmetry formate species of C1 symmetry is produced upon heating or electron bombardment of a condensed formic acid layer. Thermal or electron induced decomposition of a formate or a condensed formic acid layer is controlled by oxygen incorporation into the aluminum lattice, and results in complete decomposition and production of a carbon and oxygen covered surface. Only hydrogen is evolved from a formate-covered Al(111) surface.
A configuration interaction (CI) procedure for the evaluation of two-photon electronic transition probabilities. Program implementation with application to the A1g→B2u transition of benzene
Authors: Bendazzoli G. L. , Evangelisti S. , Palmieri P. , Rettrup S.
Published: July 2002

A CI program is described for the evaluation of two-photon electronic transitions, based on time dependent second order perturbation theory. The expression of the transition amplitudes is obtained by directly solving first order perturbation equations over the CI components, avoiding the second order expression with summation over the full set of the intermediate CI eigenstates. The procedure has been implemented as part of a general CI program for ab initio computations of these properties. To test the program, the relative intensities of the ‘‘hot’’ vibronic components of the A1g→B2u transition of the benzene molecule have been evaluated on a minicomputer and compared to the experiment.
Nonrelativistic Compton scattering in Furry’s picture. III. Kramers–Heisenberg transition amplitude by means of the complex-coordinate method
Authors: Froelich Piotr , Weyrich Wolf
Published: July 2002

The procedure for calculating the general Kramers–Heisenberg transition amplitude for scattering of light is presented. The procedure is based on L2 atomic structure calculations implemented within the framework of the complex-coordinate method, gives the transition amplitudes without explicit calculation of the final states, and includes the contribution from Ā 2 terms, p̄Ā terms to second order, and interference terms between these scattering contributions.
Accurate STF HF wave functions from B to Ca and accurate STF HF wave functions for N2 and P2
Authors: Sekiya Masahiro , Tatewaki Hiroshi
Published: July 2002

Accurate Slater type function (STF) Hartree–Fock (HF) wave functions are calculated from B to Ca. The STF’s have a form of rne−γr and power (n) of r is carefully chosen. The total atomic energies agree with those of numerical HF (NHF) within the errors of 3×10−6 and 1×10−5 a.u. for B to Ne and for Na to Ca, respectively. It will be also shown that for the N2 molecule, the present atomic STF HF set augmented with four 3d, three 4f, and one 5g STF polarization functions gives the total energy of −108.9937 a.u. at R=2.068 a.u. which is quite close to that of NHF, −108.9939 a.u. It is found that one 5g type polarization function is indispensable for getting the accurate HF value. The near HF wave function for the P2 molecule, which has the same quality as that of N2, will be also shown in the present work. The basis sets for N2 are applied to configuration interaction calculations in order to see the role of the polarization functions on the correlation effects.
Photodissociation as a quantum transition: Photofragment vibrational distributions of C2N2(C̃ 1Πu) predissociation
Authors: Dateo C. E. , Kresin V. Z. , Dupuis M. , Lester W. A.
Published: July 2002

Polyatomic indirect photodissociation is treated as a quantum transition between quasidiscrete and dissociative (photofragment) states. Our adiabatic method is followed to describe the nuclear dynamics of the dissociative state. Ab initio MCHF excited electronic potential energy surfaces are constructed and used to determine heavy-particle dynamics. The theory is applied to single-photon predissociation of C2N2(C̃ 1Πu) at 164, 158.7, and 153.6 nm to form CN(X 2Σ+)+CN(A 2Π). Theoretical predictions are found to be in good agreement with recent experimental product vibrational energy distributions.
Origin of the line shapes from intramolecular Penning ionization in benzene/argon clusters
Authors: Kamke B. , Kamke W. , Wang Z. , Rühl E. , Brutschy B.
Published: July 2002

A supersonic beam of benzene (Bz) seeded in argon was ionized by VUV radiation from the Berlin electron storage ring (BESSY). Resonance structures in the photoionization efficiency curves of Bz and Bz⋅Arn (n=1,...,4) are attributed to intramolecular Penning ionization after excitation of argon within a mixed cluster. The dependence of the shapes of the resonance lines on the seed ratio and on the cluster size is reported. It is concluded that formation of large mixed clusters causes broad resonances which are blue shifted relative to the monomer absorption line, whereas the excitation of a single argon atom bound to benzene causes narrow and red shifted lines.
Viscoelasticity of homogeneous polymer melts near a critical point
Authors: Fredrickson Glenn H. , Larson R. G.
Published: July 2002

The anomalous contribution to the shear stress caused by critical fluctuations in homogeneous polymer melts is investigated. Mean field expressions for the critical contributions to the stress tensor, dynamic shear modulus, and nonlinear steady shear viscosity are derived. These expressions are applied to the case of a homogeneous diblock copolymer near its microphase separation transition. The low frequency scaling behavior of the storage modulus is found to be G′(ω)∼ω2a−5/2, while the loss modulus is predicted to scale like G″(ω)∼ωa−3/2. The dimensionless parameter a is defined by a=2(χN)s−2χN, where χN is the product of the Flory interaction parameter and the number of Kuhn segments per chain, and (χN)s is the value of χN on the spinodal computed by Leibler. The linear steady shear viscosity is also found to be singular as a→0. A comparison is made between the present theoretical predictions and the recent experiments of Bates on 1,4/1,2 polybutadiene block copolymers.
Full CI benchmark calculations on N2, NO, and O2: A comparison of methods for describing multiple bonds
Authors: Bauschlicher Charles W. , Langhoff Stephen R.
Published: July 2002

Full configuration interaction (CI) calculations on the ground states of N2, NO, and O2 using a DZP Gaussian basis are compared with single-reference SDCI and coupled pair approaches (CPF), as well as with CASSCF multireference CI approaches. The CASSCF/MRCI technique is found to describe multiple bonds as well as single bonds. Although the coupled pair functional approach gave chemical accuracy (1 kcal/mol) for bonds involving hydrogen, larger errors occur in the CPF approach for the multiple bonded systems considered here. CI studies on the 1Σ+g state of N2, including all single, double, triple, and quadruple excitations show that triple excitations are very important for the multiple bond case, and accounts for most of the deficiency in the coupled pair functional methods.
Thiocarbonyl spectroscopy: Methyl torsional vibrations and internal rotational barriers of thioacetaldehyde in its ã 3A″ and X̃ 1A′ states
Authors: Judge R. H. , Moule D. C. , Bruno A. E. , Steer R. P.
Published: July 2002

Thioacetaldehyde was prepared by the vacuum flash pyrolysis of 1, 3, 5-trimethyl-s-trithiane and was observed spectroscopically in a rapidly flowing system. The 614 nm absorption system of CH3CHS/CH3CDS/CD3CHS/CD3CDS was recorded photographically at path lengths of 72 m. Phosophorescence excitation and emission spectra of CH3CHS were observed with a cw dye laser. The assignment of the first absorption system was made to the spin forbidden transition ã 3A″←X̃ 1A′. The bands observed in the region of the origin proved to have a very complex structure, and were assigned to torsional transitions. The observation of several hot band progressions which could be fitted to levels derived from microwave data gave the parameters V″3 =534.3 cm−1, F″=6.9950 cm−1, and ν0=16 293.8 cm−1 for CH3CHS. The fit to the excited state torsional levels was less satisfactory and gave V3 =94.18 cm−1, F′=5.3608 cm−1. An analysis of the data for the other isotopic species showed that the torsional motion in the upper state was coupled to a large amplitude wagging of the aldehydic hydrogen. The observed phase shift in the torsional angle on electronic excitation was attributed to the effects of hyperconjugation between the methyl hydrogens and the π (CS) orbitals.
Doppler-free polarization spectroscopy of the B 1Πu –X 1Σ+g band system of K2
Authors: Heinze J. , Schühle U. , Engelke F. , Caldwell C. D.
Published: July 2002

The B 1Πu–X 1Σ+g band system of the K2 molecule is investigated by Doppler-free polarization spectroscopy in a heat pipe oven. The transitions are measured in the region of 615 to 660 nm including vibrational levels up to v″=17 in the X state and all vibrational levels of the B state up to the dissociation limit. A Fabry–Perot interferometer is calibrated using metastable neon and K2 secondary wavelength standards to provide absolute accuracy in the frequency determination of less than 20 MHz. The comprehensive study of the band system is completed by LIF measurements to determine vibrational levels of the ground state up to v″=62. A detailed analysis has been completed and the coefficients are given in a Dunham expansion that accurately represents the frequencies of lines with specified vibrational and rotational quantum numbers. The B and X potential curves and the Franck–Condon factors for the transitions between B and X states are determined. The dissociation energies have been found to be 4447±15 cm−1 for the X state and 2113±15 cm−1 for the B state. The height of the potential barrier of the B state above the 4 2P3/2+4 2S1/2 atomic levels is 295±17 cm−1 at an internuclear distance of 8.1±0.1 Å.
Structure and energetics of Xe−n
Authors: Martyna Glenn J. , Berne Bruce J.
Published: July 2002

Diffusion Monte Carlo simulations were performed to determine the absolute binding energies of an excess electron to small clusters of xenon atoms (n≤19). It was found that clusters as small as Xe6 could bind the electron. The ground state wave function of the excess electron and the decomposition of the binding energy of the electron into kinetic and potential parts were determined for a number of small clusters. Large (n>50) and small clusters anions were then studied at finite temperatures using path integral Monte Carlo. In all cases the excess electron in small clusters was found to exist in very diffuse state extending well beyond the radius of the cluster. However, in large clusters the electron was localized within the bulk of the cluster. Various properties are presented to characterize the electron in Xe−n as function of cluster size and the results compared to an electron solvated in fluid xenon.
High-pressure Raman study of liquid and crystalline C2H2
Authors: Aoki K. , Kakudate Y. , Usuba S. , Yoshida M. , Tanaka K. , Fujiwara S.
Published: July 2002

Raman spectra were measured for liquid (0–0.7 GPa), crystalline cubic (0.7–0.9 GPa), and orthorhombic (0.9–3.5 GPa) phases in C2H2 at room temperature. For the orthorhombic phase, the high-pressure behavior of the librational and internal vibrations was obtained in a wide pressure range. The frequencies of all the librational modes increased monotonically with pressure, while those of the internal modes showed a variety of frequency shifts depending on vibrational mode. These high-pressure data will be of great use for the construction of theoretical models for the intermolecular interactions in crystalline C2H2.
On the preponderance of near-prolate rotors among polyatomic molecules
Authors: Silbey Robert J. , Kinsey James L.
Published: July 2002

The distribution with respect to Ray’s asymmetry parameter κ of polyatomic molecules listed in the Landolt–Börstein tables is skewed towards κ=−1 (prolate rotor limit). A plausible definition of ‘‘randomly shaped’’ objects leads to a prediction with this qualitative behavior but less strongly biased toward κ=−1 than the experimental distribution.
A three-mode large-amplitude model for the ground electronic state of CH3I
Authors: Johnson Bruce R. , Kinsey James L. , Shapiro M.
Published: July 2002

A large-amplitude model is developed to describe the three A1 vibrational modes of CH3 I. The use of a relatively simple choice of curvilinear coordinates is shown to be suitable for the regions of the ground electronic state probed by resonance Raman experiments. The essentials of the earlier two-mode linear triatomic model are regained by an adiabatic separation of the symmetric C–H stretch coordinate, but with a slightly different kinetic energy operator. A potential energy surface is constructed for the CH3 umbrella and C–I stretching coordinates. The resulting two-mode levels agree with almost all of the observed transitions to within experimental uncertainty up to 9300 cm−1 . Use of all three modes allows us to approximately apply the same potential surface to CD3 I. It is also hoped that the present model will be useful in excited state calculations, and in other CX3 Y molecules as well.
Resonance states using the optical potential model. Study of Feshbach resonances and broad shape resonances
Authors: Jolicard Georges , Leforestier Claude , Austin Elisabeth J.
Published: July 2002

A previous paper presented the study of resonance states by adding a trial imaginary potential to the Hermitian Hamiltonian and reported results for the Hazi Taylor potential. This study shows that this model can be applied successfully to Feshbach resonances and to broad shape resonances for a spherical van der Waals potential. In the case of shape resonances a nonstationary test takes advantage of the strong dependence of the resonance eigenvalue on the shape of the wall through which the wave function tunnels.
Reactions of carbon cluster ions with small hydrocarbons
Authors: McElvany Stephen W.
Published: July 2002

The ion/molecule reactions of laser-generated carbon cluster ions, C+n (n=3–20), with the neutral hydrocarbons CH4, C2H2, and C2H4 are studied using Fourier transform mass spectrometry. Branching ratios and rate constants are reported for the observed reactions. The reactions and proposed mechanisms are consistent with previous results and suggest a structural change of the cluster ions from linear to cyclic between n=9 and 10. The proposed reaction mechanisms include carbene insertion into both the C–H bonds and the unsaturated C–C bonds of the reactant neutrals. The reactions of the cluster ions with the unsaturated hydrocarbons C2H2 and C2H4 are the first systems in which the cyclic (n≥10) cluster ions are reactive. The results of the reactivity of the carbon cluster ions with the carbon-containing neutrals are discussed in terms of the stability of the clusters toward carbon atom addition and cluster growth. Further evidence is reported for the presence of structural isomers (linear and cyclic) of C+7 including a novel reaction sequence used to remove the linear isomer so that the reactivity of the cyclic isomer with C2H2 may be studied independently.
Photochemistry of adsorbed molecules. IV. Photodissociation, photoreaction, photoejection, and photodesorption of H2S on LiF(001)
Authors: Harrison I. , Polanyi J. C. , Young P. A.
Published: July 2002

The UV surface-aligned photochemistry (SAP) of submonolayers of H2S physisorbed on LiF(001) has been examined. Translational energy and angular distributions for photodissociation products and for H2S molecules leaving the surface after pulsed laser irradiation at 193 and 222 nm were measured by angularly resolved TOF to a quadrupole mass spectrometer. Single photon surface-aligned photodissociation (PDIS) of adsorbed H2S produced H with structured translational energy distributions P(ET) indicative of vibrational excitation within the complementary SH fragments. The SH vibrational distribution was bimodal and varied markedly with H2S coverage. Photoreaction (PRXN) within the adsorbate layer occurred as the H2S coverage increased beyond ∼0.1 monolayer. Molecular hydrogen was produced by PRXN of H with adjacent H2S(ad) molecules. The product H2 translational energy distribution showed evidence of both direct and indirect PRXN dynamics. At coverages greater than one monolayer, photoejection (PEJ) of H2S molecules with translational energies up to several eV was observed; PEJ was thought to be due to interadsorbate quenching of electronically excited H2S. At all the coverages examined, absorption of UV by the LiF substrate was found to photodesorb (PDES) H2S molecules with low translational energies (0–0.5 eV). The PDES was ascribed to an acoustic wave produced by laser excitation of color centers in the LiF, which were seen to fluoresce.
Lowest energy excited singlet state of 2,2′:5′,2′′-terthiophene, an oligomer of polythiophene
Authors: Birnbaum Duane , Kohler Bryan E.
Published: July 2002

Fluorescence excitation and emission spectra have been measured for solid solutions of 2,2′:5′,2″-terthiophene in n-decane at 77, 10, and 4.2 K. At 4.2 K the spectra exhibit full vibrational resolution (origin inhomogeneous FWHM approximately 5 cm−1). At 4.2 K narrow band excitation and detection establish that there are four independent but nearly identical excitation/emission pairs with origins at 24 806, 24 827, 24 835, and 24 879 cm−1. Whether this multiplicity is caused by the presence of different isomers or comes from a single isomer that can occupy in the n-decane lattice in four different ways is not known with certainty. However, the similarity of the vibronic development and energetic considerations suggest that the spectra arise from a single isomer present in four rather similar n-decane sites or from four slightly different conformations of a single isomer. The overlap of excitation and emission origins and the vibrational development of the spectra establish that the S0 to S1 transition is symmetry allowed (probably reasonably described as the 1 1A1 to 1 1B2 transition where the excited state is derived from the ground state by the promotion of one electron from the HOMO to LUMO).
Observation of l-dependent rate constants for ion production in Rydberg electron transfer reactions
Authors: Carman Howard S. , Klots Cornelius E. , Compton Robert N.
Published: July 2002

Relative rate constants for negative ion production have been measured for the Rydberg electron transfer reactions Cs**(ns,np,nd) +SF6→Cs++SF−6 and Cs**(ns,np,nd) +CCl4→Cs++Cl−+CCl3. We find that rate constant for production of Cl− from CCl4 are dependent upon the angular momentum of the Rydberg electron, with knd>kns for values of n as large as 40. Preliminary measurements for potassium (ns,nd) Rydberg atoms show that the l dependence for K is much smaller than that for Cs. These results are inconsistent with the ‘‘free electron model’’ used to describe these reactions. It is suggested that the observed l dependence is related to postattachment interactions with the Rydberg atom core which affect the survival of the Cl− ions.
Relaxation processes following excitation and ionization of SiF4 in the vicinity of the silicon 2p threshold. I. Electronic relaxation processes
Authors: de Souza G. G. B. , Morin P. , Nenner I.
Published: July 2002

The photoelectron and Auger spectra of SiF4 have been measured, using synchrotron radiation in the 100 to 125 eV photon energy range in the vicinity of the Si2p ionization threshold. Partial photoionization cross sections have been obtained for outer, inner valence states and satellite states in the same energy range, together with the threshold photoelectron spectrum. At the energies of the discrete resonances observed below 112 eV, the core excited molecule is found to decay mostly by resonant Auger and to a small extent by autoionization. Among the resonant Auger pathways, those in which two electrons (rather than one) are ejected are found to play a dominant role. Above threshold, especially around the energy of the first continuum resonance, normal Auger processes are observed. Also, we offer a new interpretation of the Auger spectrum based on large configuration interaction in the final state and on the screening of the Si2p hole by the lone-pair electrons of the fluorine atoms. We also find some evidence of cascade Auger processes which explain the formation of triply ionized molecules.
Sequential reactions of bare silicon clusters with SiD4: Constrained heterogeneous nucleation of deuterated silicon particles
Authors: Mandich M. L. , Reents W. D.
Published: July 2002

Bare silicon cluster ions are observed to undergo exothermic sequential clustering reactions with SiD4 at room temperature. Si+1–7 and Si−1–7 are created by laser evaporation and trapped in the ion cell of a Fourier transform mass spectrometer in the presence of SiD4. Clustering reactions are observed only for Si+1–3 and Si+5. Si+4,6,7 and the negatively charged silicon clusters do not react exothermically with SiD4. All of the reactive silicon clusters encounter chemical constraints to rapid growth of increasingly larger SixD+y species. Ab initio electronic structure calculations are used in concert with phase space theory calculations to deduce the structures of the products of the clustering reactions. These structures are found to be closely related to the lowest energy structures of the bare clusters if the degree of deuterium saturation is low. The inertness of unreactive clusters with 2–5 silicon atoms is correlated to unusually stable structures. Larger unreactive clusters with six or more silicon atoms appear to lack the divalent silicon center required to activate the Si–D bonds of SiD4. These findings are related to the phenomenon of hydrogenated silicon particle formation in silane plasmas.
Theoretical study of metal noble-gas positive ions
Authors: Bauschlicher Charles W. , Partridge Harry , Langhoff Stephen R.
Published: July 2002

Theoretical calculations have been performed to determine the spectroscopic constants for the ground and selected low-lying electronic states of the transition-metal noble-gas ions VAr+, FeAr+, CoAr+, CuHe+, CuAr+, and CuKr+. Analogous calculations have been performed for the ground states of the alkali noble-gas ions LiAr+, LiKr+, NaAr+, and KAr+ and the alkaline-earth noble-gas ion MgAr+ to contrast the difference in binding energies between the simple and transition-metal noble-gas ions. The binding energies increase with increasing polarizability of the noble-gas ions, as expected for a charge-induced dipole bonding mechanism. We find that the spectroscopic constants of the X 1Σ+ states of the alkali noble-gas ions are well described at the self-consistent field level. In contrast, the binding energies of the transition-metal noble-gas ions are substantially increased by electron correlation. The difference arises from the contribution of metal-neutral noble-gas-ion character in the wave functions. This correlation effect increases as the ionization potential of the noble gas decreases from He to Kr and as the ionization potential of the metal atom increases.
Calculation of hyperfine splitting constants with Slater-type cusp basis by the symmetry adapted cluster-configuration interaction theory
Authors: Nakatsuji H. , Izawa M.
Published: July 2002

A method of calculating hyperfine splitting constants (hfsc) of open-shell radicals is reported. Since the hfsc reflects a very local property of the wave function at the position of the nuclei, we use Slater-type orbitals (STOs) which have cusps at the centers. Spin correlation (spin polarization) and electron correlation are taken into account with the use of the SAC-CI (symmetry adapted cluster-configuration interaction) theory. Configuration selection is not done since energy and spin density are very different properties. The dimensions of the matrices involved in the SAC-CI method are small enough to permit such a procedure. The integrals necessary for the SCF and SAC-CI methods are calculated by the STO-6G method with the use of the available Gaussian program, and the hfsc’s and the cusp values are calculated from the original STOs. Several STOs are examined for H+2 and H2O+. The resultant standard method is applied to the organic π and σ radicals CH3, CH3CH2, CH3NH, CH3O, CH3OCH+3, H2CO+, CH2CH, and HCO. The present results of hfsc’s show generally better agreement with experiment than the previous calculations based on the Gaussian-type orbitals (GTOs). In particular, the present results are free from the theoretical dilemma seen for the GTO calculations that a variationally better wave function does not necessarily give a better hfsc. This is important in developing a reliable theory for calculating hfsc’s.
Metal cluster cation reactions: Carbon monoxide association to Cu+n ions
Authors: Leuchtner R. E. , Harms A. C. , Castleman A. W.
Published: July 2002

Copper cluster cations (Cu+n,n=1–14) were produced in a laser vaporization/flow tube apparatus and equilibrated to room temperature. The association rate constants of carbon monoxide onto these ions were measured; low-pressure, termolecular behavior was observed for the smaller species while for clusters greater than Cu+7, the longer lifetimes due to the increased number of degrees of freedom leads to pressure independence (>0.3 Torr) of the effective bimolecular rates. Unimolecular decay theory (RRKM) is used to explain the overall trend and when intrinsic surface site reactivity is taken into account, excellent agreement with measured reactivity is obtained.
Theoretical analysis of heteropolymeric transitions in superhelical DNA molecules of specified sequence
Authors: Benham Craig J.
Published: July 2002

This paper develops accurate and computationally tractable, theoretical methods to analyze equilibrium heteropolymeric transitions in superhelical DNAs of specified sequence and kilo-base length. Although these methods are generally applicable, their development here is focused primarily on superhelical strand separation, a heteropolymeric transition of great biological importance to which every base pair in a duplex DNA molecule is susceptible. Because the total number of states of this transition grows exponentially with molecular length, exact analytic methods that consider all possible states are not feasible for DNAs of practical interest. First, an approximate statistical mechanical analysis is developed in which all states are considered whose free energies do not exceed that of the minimum energy state by more than a specified threshold amount. An approximate partition function is constructed using these states, from which estimates of the equilibrium values of important transition parameters are calculated. Next, a density of states analysis is performed to estimate the influence of the neglected, high energy states. Its results are used to refine the computed equilibrium parameter values, correcting for the effects of the neglected states. Only the transition profile (the probability of transition of each base pair in the sequence) cannot be refined in this manner, although its overall accuracy can be assessed. Sample calculations analyzing the strand separation transition in superhelical pBR322 DNA show that the analytic methods developed here yield accurate results in a computationally feasible manner. Moreover, the predictions of this analysis agree closely with experimental results.
Nuclear hyperfine structure in the electronic spectrum of propynal
Authors: Bitto H. , Docker M. P. , Schmidt P. , Huber J. Robert
Published: July 2002

The laser-induced fluorescence spectrum of propynal (S1←S0) (Ã 1A″←X̃ 1A′) has been investigated and a series of perturbations due to singlet–triplet coupling resolved. The singlet–triplet interaction matrix elements are of the order of 280 MHz. Fluorescence decays for both the mixed singlet–triplet eigenstates have been measured and are shown to contain quantum beats due to coherent excitation of nuclear hyperfine levels. This Doppler-free measurement in the time domain leads to an accurate determination of the nuclear hyperfine splittings produced by the two nonequivalent hydrogen atoms. The hyperfine structure has been analyzed for two rotational levels in the 91 vibrational state and gives the following values for combinations of the Fermi contact constants and dipole–dipole constants of the two protons in the pure triplet state: T1(ã 3A″): A(1)FC+ 1/2 (T(1)xx−T(1)yy) ∼46 MHz; A(2)FC+ 1/2 (T(2)xx−T(2)yy) −4 MHz; T(1)zz∼T(2)zz ∼0 MHz. A simple model for the spin distribution in propynal predicts that the Fermi contact constant should be positive for the aldehydic proton and smaller and negative for the acetylenic proton and suggests that the Fermi contact constants make the dominant contribution to the effective hyperfine constants.
Experiments on the effects of external periodic variation of constraints on the thermodynamics of an oscillatory system
Authors: Lazar James G. , Ross John
Published: July 2002

The first experiments are presented which show that an external periodic perturbation imposed on a nonlinear isothermal biochemical reaction far from equilibrium may change the dissipation and hence the efficiency of that reaction; the results confirm earlier theoretical and numerical predictions. The reaction is driven and maintained far from equilibrium, a process thermodynamically equivalent to the establishment and maintenance of a concentration gradient across a membrane, i.e., a biochemical pump. We study the highly nonlinear oxidation of NADH catalyzed by the enzyme horseradish peroxidase (HRP) under conditions of continuous oxygen influx. NAD+ is recycled to NADH by a second enzymatic reaction. Experimental conditions are set so that the (HRP) reaction is in a stable limit cycle (autonomous oscillations). We then apply a periodic perturbation to the oxygen influx such that the average influx during the perturbation equals the previous steady influx. We present data obtained for a variety of frequencies and amplitudes of sinusoidal variations in the oxygen influx. Upon imposition of the periodic perturbation, we find changes in the average steady-state concentration of NADH, the average ΔG of the (HRP) reaction, the average rate of the (HRP) reaction, the phase difference between ΔG and the rate, the response of the system to the perturbation, and the dissipation of the reaction. Changes in dissipation which occur upon the imposition of an external periodic perturbation are a result of a combination of changes which occur in the thermodynamic quantities listed. These results confirm the possibility of variable efficiency of biological pumps since the establishment and maintenance of a chemical potential difference across a membrane is thermodynamically equivalent to the establishment and maintenance of a nonequilibrium stationary state in a single phase, open, homogeneous system. Moreover, the results confirm the possibility of an alternating current chemistry of control and optimization of thermodynamic efficiency, dissipation, and yield by means of an external variation of constraints in nonlinear (bio)chemical reactions and biological pumps.
Some model calculations of carbon cluster growth kinetics
Authors: Creasy William R.
Published: July 2002

Simple kinetic mechanisms are used to model the features of ionic carbon clusters that have been observed by laser vaporization mass spectrometry. The basis of the mechanism involves only stepwise addition of small species (C to C3) followed by collisional quenching. The model is used to examine experimental observations including the variation of cluster size distributions with plasma density, the formation of ‘‘magic’’ numbers, the formation of predominantly even clusters, the loss of hydrogen from the clusters in the presence of a hydrogen-containing plasma, and the effect of isomerization of n>31 on the high mass distribution. Substantial agreement between experimental observations and the kinetic model are obtained, and the effect of more complicated processes are discussed.
Dynamic friction on rigid and flexible bonds
Authors: Berne B. J. , Tuckerman M. E. , Straub John E. , Bug A. L. R.
Published: July 2002

A wide variety of problems involving molecular motion in liquids can be formulated in terms of the generalized Langevin equation (GLE). The friction coefficient on a molecular bond or on some more complicated reaction coordinate is then required. An often used approximation is to set the dynamic friction constant equal to the autocorrelation function of the fluctuating force exerted on the frozen bond by the remaining unfrozen coordinates. The true friction involves projection operators and should differ from this approximation. In this paper we derive various identities and show that the rigid bond approximation is the high frequency limit of the true dynamic friction coefficient. We compute the ‘‘true’’ dynamic friction and the friction approximated on the basis of the rigid or frozen bond and show that the asymptotic limit is very accurate even for frequencies not much larger than the peak frequency of the solvent spectral density. Two different dynamical systems are studied using MD simulations with our newly devised NAPA integrator for systems with disparate time scales. In one the molecule is not allowed to rotate and in the other it is allowed to rotate. Interestingly, even for very long rotational reorientation times, small but significant differences in the long time decay of the bond dynamic friction are observed for rotational and nonrotational molecules—differences, however, that do not produce large differences in the static friction constants.
Picosecond measurements of vibrational dynamics using pump–probe laser photoelectron spectroscopy
Authors: Smith J. M. , Lakshminarayan C. , Knee J. L.
Published: July 2002

A new technique is presented for measuring vibrational dynamics in excited electronic states. A vibronic state is prepared by a picosecond laser pump pulse. This state is then probed by monitoring transitions to vibrational states of the ion by a delayable picosecond probe pulse. The cross section for transitions to specific ion vibrational states gives information about the time evolution of the initially prepared intermediate state. This technique is demonstrated for several vibronic states in the S1 state of fluorene.
Cluster optimization simplified by interaction modification
Authors: Stillinger Frank H. , Stillinger Dorothea K.
Published: July 2002

Even when interactions are known exactly, it is generally very difficult to determine the lowest-energy configuration (a global potential energy minimum) for clusters with more than very few atoms or molecules. In mathematical parlance this is an NP-complete problem. A nonlinear optimization strategy, the ‘‘ant–lion method,’’ has been proposed to accelerate the search for global minima, and works by adroitly deforming the potential surface to produce overwhelming dominance by global minimum potential energy basins. This strategy is illustrated by application to clusters of 13 noble gas atoms. Monte Carlo results demonstrate that reduction of p in the pair potential 4(r−2p−r−p) below the ‘‘physical’’ value 6 produces a dramatic rise to essentially unity in probability of random encounter with the global minimum basins (icosahedral clusters).
High resolution time-of-flight mass spectrometry of carbon and carbonaceous clusters
Authors: Rohlfing Eric A.
Published: July 2002

We produce carbon (Cn) and carbonaceous (CnHm) clusters by the addition of hydrogen to the helium carrier gas in a laser-vaporization cluster source, and probe the neutral cluster distributions for n≥20 by single-photon ionization in conjunction with high resolution time-of-flight mass spectrometry using an angular reflectron. By adjusting the growth conditions in the cluster source, we produce Cn distributions with wide variations in their degree of local maxima, i.e., magic numbers. We find low-growth conditions under which unstable Cn (n=30–40 and odd n for n>40) are produced in nearly the same abundances as the more stable clusters, particularly the even n for n>40. Increasing the carrier gas density and residence time in the source leads to distributions in which the even clusters dominate their odd-numbered neighbors at n>40. The carbonaceous cluster distributions produced by hydrogen addition always show a dramatic preference for the hydrogenation of clusters with odd n; the even clusters are observed predominantly as pure Cn. We perform a quantitative hydrogenation analysis of the CnHm that yields the hydrogenation fraction, the distribution of hydrogens, and the C/H ratio for each n. The results of this analysis provide evidence for the formation of long-chain polyacetylenes, CnH2, for even n up to n=44 and show the dramatic even/odd alternation in hydrogenation behavior beginning near n=40. We discuss our results for the large clusters in terms of the proposed spherical structures for even clusters and the corresponding model for particle growth via nuclei comprised of incomplete carbonaceous shells.