bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure and energetics of Xe−n : Many-body polarization effects
Authors: Martyna Glenn J. , Berne Bruce J.
Published: July 2002

In a previous paper, Martyna and Berne, J. Chem. Phys. 88, 4516 (1988), diffusion Monte Carlo simulations were performed to determine the absolute binding energies of an excess electron to small clusters of xenon atoms (n≤19) using a pair additive pseudopotential. In this approximation, the electron–xenon polarization energy is treated as pair additive and therefore ignores the induced dipole–induced dipole interactions. Here we treat the many-body polarization problem in the dipole approximation. It is found that while the smallest stable cluster anion is Xe−6 for the pair polarization model this increases to Xe−7 for the many-body polarization model. In fact, the electron binding energy corresponding to the pair-polarization model was found to be a factor of 2.7 larger than for the many-body polarization model for all the clusters studied. In accord with this very large destabilization of electron binding energy (induced by many-body polarization), the spatial extent of the electronic ground state in the many-body polarization model increases compared to that of the pair polarization model. We also compare our results for both the many-body polarization and the two-body polarization models to corresponding dielectric continuum models developed by Stampfli and Bennemann, Phys. Rev. A 71, 1674 (1988). In the many-body polarization case, the continuum model agrees well with our results. However, the agreement in the pair polarization case is rather poor for all cluster sizes. If parameters of the continuum model are adjusted to obtain agreement for small clusters sizes, the model is found to break down for large clusters sizes where the spatial extent of the electron is small enough that the microscopic details of the cluster become extremely important. A new variant of the fast Fourier transform projector method suitable for use in problems involving electron attachment to clusters is also developed. The results obtained with this new method are shown to agree with those of diffusion Monte Carlo.
Spectroscopy and electronic structure of jet-cooled NiCu
Authors: Fu Zhenwen , Morse Michael D.
Published: July 2002

Diatomic nickel–copper, NiCu, has been investigated by resonant two-photon ionization spectroscopy in a jet-cooled molecular beam. Six band systems have been identified over the range 11 500–16 500 cm−1. The ground state of NiCu has been determined to be X 2Δ5/2, with ω″e =273.01±1.15 cm−1, ω″ex″e =1.00±0.38 cm−1, and r″e =2.233±0.006 Å. This state derives from a strongly bound (2.05±0.10 eV) 3d10Cu3d9Ni 4sσ2 configuration. Excited states observed in this work derive from the more weakly bound 3d10Cu3d8Ni 4sσ24sσ*1 configuration, and are characterized by smaller vibrational frequencies (191–208 cm−1) and a longer bond length (2.351±0.005 Å) than the ground X 2Δ5/2 state.
Photoionization spectra of cesium and cesium oxide clusters
Authors: Limberger H. G. , Martin T. P.
Published: July 2002

Cesium and cesium oxide clusters have been ionized in a one-photon process by means of a tunable cw dye laser and detected in a mass spectrometer. Mass spectra obtained using different photon energies and photoionization spectra of 7 metal and 86 oxidized metal clusters (in a mass range from 1 to 2024 amu and a maximum energy of 3.1 eV) are presented. Vertical ionization potentials were determined from the spectra. The interpretation of the ionization threshold is given in terms of the displaced harmonic oscillator model which leads to a relation between adiabatic and vertical ionization potentials (IP). The ionization energies of cesium clusters Csn decrease with cluster size. An unusually low IP was observed for the enneamer Cs9 and for the cesium monoxide Cs11O. With increasing oxidation of the cesium metal clusters the IP decreases (suboxides) reaches a minimum at Cs(Cs2O)n and then increases (superoxides).
Observation of electronic shell structure in large cesium oxide clusters
Authors: Bergmann T. , Martin T. P.
Published: July 2002

Photoionization spectra of CsnOx clusters have been recorded for 15≤n≤130 and 1≤x≤10. We present a method of interpreting these spectra and a computer algorithm for their evaluation. Several features of these photoionization spectra support the spherical-jellium-background model.
Photoelectron spectroscopy of argon clusters: Evidence for an Ar13 ionization chromophore
Authors: Carnovale Frank , Peel J. Barrie , Rothwell Richard G. , Valldorf Jürgen , Kuntz Philip J.
Published: July 2002

Ultraviolet photoelectron spectra have been measured for argon clusters prepared in pulsed supersonic expansions of both pure and seeded mixtures of argon. The broad band spectra show structural features which relate to the degree of condensation and hence the mean cluster size obtained at different stagnation pressures. These are interpreted in terms of an ion–core hypothesis with diatomics-in-molecules calculations being used to simulate spectral features. The analysis shows that Ar+3 cores are formed in the initial ionization process of very small clusters, with the Ar+13 core becoming dominant for all larger clusters and for the condensed state of argon.
Spectroscopy of the predissociated C state of Na3
Authors: Broyer M. , Delacrétaz G. , Ni G.-Q. , Whetten R. L. , Wolf J.-P. , Wöste L.
Published: July 2002

The C state of Na3 has been explored in detail by the methods of resonant two-photon ionization spectroscopy and photofragmentation spectroscopy of a supercooled cluster beam. The photofragmentation spectrum, recorded by depletion of Na3 from the beam, reveals a long progression of bands that had been invisible to the two-photon ionization method, and it appears that ultrafast fragmentation occurs for all levels more than 400 cm−1 above the zero-point level. This is consistent with earlier observations of Na D-line emission following Na3 excitation in this region, assuming the responsible channel is Na2 2X and Na 2P production. The vibronic fine structure of the C band system is complex at lower energies, and yields to a detailed explanation only through consideration of the dynamical Jahn–Teller effect. This analysis demonstrates that the C state has electronic symmetry E″, and is subject to a symmetry-lowering deformation of 180 cm−1, or 1.4 times the frequency of the e′ vibrational mode of D3h Na3. The corresponding minimum energy structure is an obtuse isosceles triangle (65° apex angle), but the barrier to pseudorotation (estimated to be 40 cm−1) is small. Furthermore, the computed vibronic wave functions imply that only the lowest few states are well described within the adiabatic Born–Oppenheimer approximation; at intermediate energies the spectrum is correspondingly very irregular, while at higher energies a near harmonic regularity returns by virtue of the relatively small magnitude of the Jahn–Teller distortion. These quantitative conclusions are intermediate among those pertaining to the other known states of Na3, and, in combination with them, permit extensive comparison with the results of high quality electronic structure computations. The tentative assignment is to the 12E to 22E″ electronic transition (united atom 1S21Px,y to 1S21Dxz,yz).
A curve-crossing model for He*(23S)+Ne collisions consistent with differential and total scattering cross sections, and quenching rate constants
Authors: Fukuyama T. , Siska P. E.
Published: July 2002

He*(23S)+Ne differential elastic scattering measurements in crossed supersonic beams over the energy range 0.6–2.8 kcal/mol are combined with total cross section velocity dependence and quenching rate constant temperature dependence to determine a 3×3 potential matrix characterizing a double curve-crossing model for the collision dynamics and energy transfer. The coupled radial Schrödinger equations are integrated numerically to yield the appropriate cross sections. At low energy (E<4 kcal/mol) a mixed adiabatic/diabatic two-state model derived from the potential matrix accurately represents the data. Dynamics in the three-state model, which is topologically similar to non-adiabatic coupling involving charge transfer, is discussed, and the bearing of the model on collisional pumping of the HeNe 1.1 μ laser is assessed.
Multireference singles and doubles configuration interaction study of the photoelectron spectrum of HO−2
Authors: Vazquez Gabriel J. , Buenker Robert J. , Peyerimhoff Sigrid D.
Published: July 2002

Multireference singles and doubles configuration interaction (MRD-CI) electronic structure calculations are carried out on the hydroperoxyl radical and its negative ion. Potential energy curves of the ground state of the anion and of the ground and first excited states of the neutral molecule along the O–OH coordinate are employed to compute the Franck–Condon factors for the electron photodetachment processes HO2(X 2A″,A 2A′)+e←HO−2(X 1 A′). The theoretical spectrum agrees fairly well with the photoelectron spectrum reported by Oakes, Harding, and Ellison, and is consistent with their assignment of the peaks K and D as origins for the electron loss transitions to the HO2 A and X states, respectively. An alternative choice of peak E as X←X− origin yields better agreement between computed and experimental Franck–Condon factors than does the peak D assignment, but is less satisfactory when energetic considerations are taken into account. Two independent ab initio results lend support to the value of 1.078±0.017 eV for the measured adiabatic electron affinity of HO2, which is in good agreement with the best value inferred from the MRD-CI calculations of 1.069±0.05 eV. The matter is not settled, however, as some inconsistencies in the experimental data still cast some doubts on the assignment of the peaks and the accuracy of the HO2 EA.
A photoion–photoelectron coincidence study of (CO)2 and (CO)3
Authors: Norwood K. , Guo J.-H. , Luo G. , Ng C. Y.
Published: July 2002

The photoion–photoelectron coincidence (PIPECO) spectra for (CO)+2 in the wavelength region of 620–990 Å have been obtained at different nozzle stagnation pressures (P0). The ionization energy for (CO)2 to CO+(X̃)⋅CO is determined to be ≲12.73±0.05 eV (974±4 Å), indicating that CO+(X̃)⋅CO is bound by more than 1.29 eV. The PIPECO measurements also provide evidence that CO+(Ã,B̃)⋅CO are bound with dissociation energies >0.3 eV. At P0≤200 Torr and a nozzle temperature (T) of 120 K, nozzle expansion conditions which minimize the formation of (CO)n (n≥3), the intensities for the CO+(Ã,B̃)⋅CO PIPECO bands are found to be negligibly small compared to that for the CO+(X̃)⋅CO PIPECO band. This observation supports the conclusion that the excited CO+(Ã,B̃)⋅CO ions are dissociative, with dissociation lifetimes<42 μs. This conclusion is contrary to that of the preliminary study which is based on the PIPECO spectrum for (CO)+2 measured at P0=350 Torr and T=120 K. Assuming that the radiative lifetimes of CO+(Ã or B̃) and CO+(Ã or B̃)⋅CO are identical, we estimate that the dissociation lifetimes for CO+(Ã)⋅CO and CO+(B̃)⋅CO are ≲4 μs and ≲50 ns, respectively.
The infrared spectra of nitrous oxide–HF isomers
Authors: Lovejoy Christopher M. , Nesbitt David J.
Published: July 2002

Two spectroscopically distinct isomers of a hydrogen bonded complex between nitrous oxide and hydrogen fluoride are observed by direct infrared laser absorption detection in a slit supersonic expansion. The linear isomer FH–NNO contains a relatively rigid hydrogen bond to the nitrogen end of NNO. The bent isomer NNO–HF has a stronger hydrogen bond to the oxygen end of NNO, but this bond is characterized by a softer bending potential and thus the complex exhibits evidence of large amplitude bending motion. Rapid vibrational predissociation, as determined from the homogeneous broadening of the rovibrational absorption structure, is evidenced in both isomers. The linear isomer exhibits predissociation lifetimes which show structure as a function of the upper J′ rotational level, including narrow resonances which suggest excitation of NNO fragment vibrational modes.
Cu 3d covalency in chemisorption?
Authors: Wahlgren Ulf , Pettersson Lars G. M. , Siegbahn Per
Published: July 2002

Two problems are addressed in the present study: the degree of copper 3d covalency involved in the chemisorption of oxygen at the fourfold hollow site of Cu(100) and the separability of the correlation energy into contributions from the 3d shells on copper and from the valence sp band. The investigation was carried out at the all-electron level using a Cu5 cluster as a model of the Cu(100) surface. The analysis shows that the 3d covalency is of practically no importance in the system considered, contributing only 1–3 kcal/mol to the total chemisorption energy of 89 kcal/mol. The correlation energy was found to be separable to within 5 kcal/mol. A configuration-interaction calculation on the Cu5O system using the one-electron effective core potential developed previously yielded a correlation energy in close agreement with the all-electron results.
Comparison of the Ar(3P2) and Ar(3P) reactions with chlorine and fluorine containing molecules: Propensity for ion–core conservation
Authors: Sadeghi N. , Cheaib M. , Setser D. W.
Published: July 2002

Optical pumping has been used to select Ar atoms in the metastable 3P2 or 3P0 state in a flowing afterglow reactor. The relative concentrations of the two metastable states were assigned from observation of the N2(C,v′) emission spectra. The isolated reactions of the Ar(3P2) and Ar(3P0) atoms with F2, NF3, Cl2, CCl4, PCl3, and SOCl2 were examined at 300 K by observation of ArCl* and ArF* formation. The total quenching rate constants for Ar(3P0) are slightly larger than for Ar(3P2). The Ar(3P2) atom reactions give only the B and C states of ArF* and ArCl*; the Ar(3P0) atom reactions give a mixture of B, C, and D states with B and D being favored. Thus, a propensity for conservation of the Ar+ ion–core configuration was found. The branching fraction for ArX* formation from Ar(3P0) with Cl2, SOCl2, F2, NF3 are similar, but those for PCl3 and CCl4 are smaller, relative to Ar(3P2). Improved rate constants for formation of individual N2(C,v′) levels from Ar(3P0) and (3P2) reacting with N2 at 300 K are given in the Appendix.
An accurate calculation of the first-order interaction energy for the helium dimer
Authors: Rybak S. , Szalewicz K. , Jeziorski B.
Published: July 2002

The first-order interaction energy between two He atoms is calculated for a range of interatomic distances with a large explicitly correlated basis set reproducing over 99.998% of the correlation energy of the He atom. The interaction energy obtained in basis sets of increasing size converges to within 0.1 μhartree. This accuracy is comparable to accuracy of calculations which use orbital basis sets of spdfgh quality. Our results agree well with values extracted from experiments. We also show that our largest He wave functions behave properly in the outer region of the He atom.
Infrared diode laser spectroscopy of CaH and CaD(X 2Σ+). Determination of mass-independent parameters
Authors: Petitprez D. , Lemoine B. , Demuynck C. , Destombes J. L. , Macke B.
Published: July 2002

Rotation–vibration bands of CaH and CaD were observed in the 10 μm region using a diode laser spectrometer. The calcium hydride was produced in a dc discharge of H2/D2 over calcium or, in the case of CaH, by discharging CaH2 with Ar as buffer gas. Hot bands up to v=4←3 were observed. All the bands of both CaH and CaD were analyzed simultaneously, leading to the determination of a set of mass independent parameters including the mass scaling coefficients ΔH10,ΔH20,ΔH01 and ΔH11, and of the coefficients a1 to a6 of the Dunham potential. The molecular parameters are discussed and compared to other works. Moreover, the rotational line frequencies of CaH and CaD, which are of astrophysical interest, are predicted.
Oriented molecule beams: Focusing and orientation of t-butyl bromide with analysis by polarized laser photofragmentation
Authors: Xu Qi-Xun , Quesada Mark A. , Jung Kyung-Hoon , Mackay R. Scott , Bernstein Richard B.
Published: July 2002

Tert-butyl bromide is photodissociated at 277.3 nm (in the tail of the first absorption continuum). Both fragments, i.e., the t-butyl radical and the Br(2P1/2) atom, are detected by laser MPI-TOF mass spectrometry. Seeded, supersonic beams of this pseudosymmetric top molecule are readily focusable and orientable (via its first-order Stark effect) using the electrostatic hexapole lens. The linearly polarized laser-induced photofragmentation technique reveals unusually high degrees of orientation, significantly greater than that for t-C4H9I, due to the less serious hyperfine disorientation effect for the Br- (vs I-) containing molecule. Thus t-C4H9Br is an excellent candidate reagent for reactive asymmetry scattering studies of the steric effect via the oriented molecule beam method.
Calculation, display, and analysis of the nature of nonsymmetric nuclear magnetic resonance shielding tensors: Application to three-membered rings
Authors: Hansen Aage E. , Bouman Thomas D.
Published: July 2002

The components of the full nuclear shielding tensor are analyzed with particular regard to the origins of the antisymmetric component of the shielding of nuclei at low-symmetry molecular sites. As an aid we propose the study and display of the shielding response vector, i.e., the nuclear shielding field per unit applied magnetic field. The analysis is based on ab initio calculations in the localized orbital-local origin method for cyclopropane, cyclopropene, ethylene oxide, ethylene imine, and diazirine, and also includes a discussion of the mechanism for the large antisymmetric component predicted for the unsaturated nuclei in cyclopropene and diazirine, both in terms of bond and lone pair contributions and in terms of a decomposition of the dominant paramagnetic contributions into molecular point group species. Display of the component of the shielding vector parallel to the applied field is shown to be a valuable alternative to the common ellipsoid representations. Display of the components perpendicular to the applied field provides a picture of the relation of the antisymmetry to molecular structure and suggests its direct observation through monitoring the perpendicular response. Finally, we show that, contrary to a recent claim, the eigensolutions of the full tensor in the presence of antisymmetry play no role in the observed anisotropies.
A simple model for correcting the zero point energy problem in classical trajectory simulations of polyatomic molecules
Authors: Miller William H. , Hase William L. , Darling Cynthia L.
Published: July 2002

A simple model is proposed for correcting problems with zero point energy in classical trajectory simulations of dynamical processes in polyatomic molecules. The ‘‘problems’’ referred to are that classical mechanics allows the vibrational energy in a mode to decrease below its quantum zero point value, and since the total energy is conserved classically this can allow too much energy to pool in other modes. The proposed model introduces hard sphere-like terms in action–angle variables that prevent the vibrational energy in any mode from falling below its zero point value. The algorithm which results is quite simple in terms of the cartesian normal modes of the system: if the energy in a mode k, say, decreases below its zero point value at time t, then at this time the momentum Pk for that mode has its sign changed, and the trajectory continues. This is essentially a time reversal for mode k (only!), and it conserves the total energy of the system. One can think of the model as supplying impulsive ‘‘quantum kicks’’ to a mode whose energy attempts to fall below its zero point value, a kind of ‘‘Planck demon’’ analogous to a Brownian-like random force. The model is illustrated by application to a model of CH overtone relaxation.
Theoretical study of the structures and electron affinities of the dimers and trimers of the group IB metals (Cu, Ag, and Au)
Authors: Bauschlicher Charles W. , Langhoff Stephen R. , Partridge Harry
Published: July 2002

The molecular structure of both the neutral and negatively charged diatomic and triatomic systems containing the Cu, Ag, and Au metals are determined from ab initio calculations. For the neutral triatomic systems, the lowest energy structure is found to be triangular. The relative stability of the 2A1 and 2B2 structures can be predicted simply by knowing the constituent diatomic bond distances and atomic electron affinities (EAs). The lowest energy structure is linear for all of the negative ions. For anionic clusters containing Au, the Au atom(s) preferentially occupy the terminal position(s). The EAs of the heteronuclear systems can be predicted relatively accurately from a weighted average of the corresponding homonuclear systems. Although the theoretical EAs are systematically too small, accurate predictions for the EAs of the triatomics are obtained by uniformly scaling the ab initio results using the accurate experimental EA values available for the atoms and homonuclear diatomics.
Diode laser spectroscopy of gas phase C5: The ν3 fundamental and associated hot bands
Authors: Moazzen-Ahmadi N. , McKellar A. R. W. , Amano T.
Published: July 2002

The spectrum of the linear carbon chain molecule C5 in the gas phase has been studied around 2170 cm−1, the region of the highest asymmetric stretching vibration ν3. The results were obtained using a tunable diode laser spectrometer and a cooled hollow cathode discharge in a flowing mixture of acetylene and helium. Four vibration–rotation bands were assigned and analyzed: the fundamental, a hot band arising from the v7=1, l=1 vibrational level, a second hot band arising from v7=2, l=0, and a third hot band tentatively ascribed to v5=1, l5=1. Small local perturbations were found to affect the upper vibrational states of two of the bands. Analysis of the data yielded accurate values for a number of molecular parameters for C5, e.g., the band origin ν3= 2169.4410(2) cm−1, the rotational constant, B0 =2557.63(9) MHz, and the l-type doubling parameters, q7=3.99(6) MHz, and q5=2.36(9) MHz. The value of q7 may be used to estimate a value of 118 cm−1 for the lowest bending frequency of the molecule. There is no evidence in C5 for quasilinear behavior such as that shown by C3 and C3O2.
Lifetime and deactivation of NH and ND (A 3Π, v=0 and 1)
Authors: Kenner R. D. , Kaes A. , Browarzik R. K. , Stuhl F.
Published: July 2002

A reinvestigation of the lifetime of NH(A,v=0) using ArF laser photolysis of NH3 to produce NH(A) is reported. In addition, the lifetime of NH(A,v=1) and ND(A,v=0 and 1) have been investigated. The lifetime of NH(A,v=0,J=8–12) is 424±8 ns and that for NH(A,v=1,J=4–6) is 461±10 ns. The lifetimes of protonated and deuterated A 3Π imidogen are the same within 5% suggesting no predissociation of the low rotational levels. Rate constants for quenching of NH/ND(A,v=0) and for deactivation of NH/ND(A,v=1) for a few selected collision partners have been measured and no isotope effect is found. The results of this work supersede those reported previously from this laboratory as the latter contain a small uncorrected systematic error.
Energetic and thermodynamic size effects in molecular clusters
Authors: Bixon M. , Jortner Joshua
Published: July 2002

In this paper we explore the interrelationship between the energetics and the thermodynamic properties of molecular clusters. We advance simple models for the energy spectrum, which are used to derive analytical results for the thermodynamic properties of these clusters. The energy spectrum is characterized by the distribution of the energies of the local minima of the nuclear potential energy hypersurface, i.e., the inherent structures. On each of these energy levels the vibrational density of states of the particular inherent structure is superimposed. The energy spectra were specified in terms of the energy gap, Δ, between the (single) ground state and the excited-state inherent structures, the number, R, of the inherent structures and their energetic spread W. Four classes of energy spectra were considered. (1) A large energy gap with nearly degenerate excited-state manifold, i.e., Δ≫W. (2) A large energy gap with a considerable spread of the excited-state manifold, i.e., Δ≪W and W/R≪Δ. (3) A gapless spectrum with W/R≳Δ. (4) A multiple bunched spectrum with several energy gaps. Explicit analytical relations for the temperature dependence of the internal energy were derived for energy spectra of types (1), (2), and (3) both for the canonical and for the microcanonical ensembles. For energy spectra of types (1) and (2) the caloric curve exhibits a single inflection, which marks the ‘‘transition’’. A unified description of multistate isomerization with large R, which corresponds to rare-gas clusters, and of molecular isomerization with R≂1, which prevails for alkali-halide clusters, was provided. For energy spectra of type (3) the transition disappears, while for energy spectra of type (4) hierarchical isomerization is exhibited. Our analytical models have established the ensemble dependence of the transition for types (1) and (2), which is manifested by a considerable broadening of the transition range for the canonical ensemble, reflecting the role of energy fluctuations in the finite system.
Stretch–bend coupling between van der Waals modes in the S1 state of substituted benzene–Ar1 complexes
Authors: Bieske Evan J. , Rainbird Mark W. , Atkinson Ian M. , Knight Alan E. W.
Published: July 2002

The van der Waals vibrations of aniline–, phenol–, fluorobenzene–, and chlorobenzene–Ar1 complexes have been measured using one-color resonance enhanced multiphoton ionization spectroscopy, together with time-of-flight mass spectrometry, in a skimmed supersonic molecular beam. A delayed ionization extraction technique is used to suppress contributions to the spectra from dissociating complexes. The S1–S0 electronic origins for the van der Waals complexes are found to be shifted towards lower energy (red shift) relative to the parent molecule electronic origin for all the Ar1 complexes. The red shifts increase in magnitude in the order: fluorobenzene, chlorobenzene, phenol, aniline. Progressions, overtones and combination transitions involving the low frequency van der Waals vibrations, i.e., the symmetric bend (bx), the asymmetric bend (by) and the stretch (sz) are observed clearly in the S1←S0 excitation spectra. Intensity profiles are found to deviate substantially from those expected on the basis of harmonic Franck–Condon factors. A model involving stretch–bend anharmonic coupling via cubic terms in the vibrational potential is found to account for the observed spectral features and intensity anomalies.
Symmetry breaking and symmetry restoring in ions of loosely bound systems
Authors: Tarantelli F. , Cederbaum L. S. , Campos P.
Published: July 2002

We present a theoretical analysis of the effects that lead to distortion or symmetry restoration of the nuclear configuration in ions of weakly bound molecular systems and clusters. The vibronic coupling mechanisms involved are reviewed and discussed in terms of simple and general models. Guided by a qualitative characterization of the wave function of a loosely bound system, we study the symmetry determining effects for such ions in terms of first order (Koopmans’), relaxation and correlation distortion energies associated with electron removal from the parent system. Our discussion suggests that the Koopmans’ approximation can reasonably be expected to provide a qualitatively correct dependence of the ion energy on nuclear motion. Exceeding these expectations, it is found in numerical applications to the He+3 and He+4 ions that the relaxation and correlation energy changes accompanying symmetry breaking tend to cancel exactly. This implies, and we show in practice, that while insufficiently accurate correlation energy estimates may easily lead to incorrect geometries, the inexpensive Koopmans’ approximation can often suffice to obtain the correct energy curvature along symmetry breaking coordinates. Further preliminary results for N3 and CO+2 suggest that these findings have a much wider applicability than anticipated.
Excited state lifetimes and appearance potentials of benzene dimer and trimer
Authors: Shinohara Hisanori , Nishi Nobuyuki
Published: July 2002

Van der Waals clusters of benzene are formed in a supersonic jet, subjected to two-color resonance-enhanced two-photon ionization (2C-RE2PI) via the S2 state, and analyzed in a time-of-flight mass spectrometer. The lifetimes of the excited states of benzene dimer and trimer are obtained by the 2C-RE2PI technique. The observed lifetime data of the dimer and trimer show a biexponential decay, which corresponds to the lifetimes of the lowest singlet 1B1g state of a parallel spaced benzene excimer and an excited S1-van der Waals dimer state. The two excited states are likely to be separated by a shallow potential barrier. The observed lifetimes of the parallel stacked lowest singlet excimer and the vibrationally excited S1-van der Waals states of the benzene dimer are 122 and 24 ns, respectively. These excited states are produced as a result of the internal conversion from an optically excited S2-van der Waals dimer state. The appearance potentials of the benzene dimer and trimer are also measured. The photoionization curves of the dimer and trimer in the threshold region show very slow rise at 8.53±0.03 and 8.54±0.03 eV, respectively. The observed photoionization thresholds are much lower than those reported previously.
Time-resolved measurements of the photodissociation and recombination dynamics of I−2 in mass selected cluster ions
Authors: Ray Douglas , Levinger Nancy E. , Papanikolas John M. , Lineberger W. C.
Published: July 2002

We present picosecond time-resolved pump-probe measurements of the photodissociation and recombination dynamics of I−2 surrounded by a specific number of CO2 molecules in mass selected I−2 (CO2)n clusters. The transient bleaching data can be fit to an exponential absorption recovery of 30±10 ps in I−2 (CO2)9 clusters and 10±5 ps in I−2 (CO2)16 clusters. The data demonstrate the feasibility of measurements of real-time reaction dynamics in microsolvent environments.
Wave packet studies of predissociation in H3 Rydberg states
Authors: Orel A. E. , Kulander K. C.
Published: July 2002

The predissociation of H3 Rydberg states by the two lowest electronic states of H3 is studied using time-dependent wave packet techniques. The final vibrational state distribution function and branching ratios between two- and three-body channels are calculated within a two-dimensional approximation. The effect of initial vibrational excitation on these distributions is studied, and comparison is made to recent experiments.
High resolution vibrational overtone spectroscopy of hydrogen peroxide in the Δv=4 region
Authors: Douketis Constantine , Reilly James P.
Published: July 2002

The vibrational overtone spectrum of H2 O2 vapor between 7400 and 7600 Å is recorded under both bulk gas and supersonic beam conditions. An absorption band corresponding to a Δv=4 O–H stretch is observed in this spectral region. Rotational analysis indicates that it is a hybrid band with mainly parallel character. In addition to the 59 rovibrational transitions that could be assigned to this band, another 83 lines are observed that did not fit an interpretable asymmetric rotor pattern. Spectral linewidths are found to be Doppler limited in all cases (7 MHz FWHM with the supersonic beam).
Picosecond mass-selective measurements of phenol-(NH3)n acid–base chemistry in clusters
Authors: Steadman Jhobe , Syage Jack A.
Published: July 2002

The rate of proton transfer from the acidic S1 state of phenol to the basis solvent (NH3)n was measured as a function of solvent cluster size n. A distinct reaction threshold was observed for solvent size n=5 for 266 nm picosecond excitation. The proton transfer rate was measured to be ka=(60±10 ps)−1 for n=5–7. A competitive recombination rate of k−a =(350±100 ps)−1 occurs for n=5. Additional solvation stabilizes the product side causing the reaction enthalpy and consequently k−a to decrease. No evidence of proton transfer was observed when phenol was seeded in the less basic solvent clusters (CH3OH)n and (H2O)n.
Exploring potential energy surfaces with transition state calculations
Authors: Davis Heidi L. , Wales David J. , Berry R. Stephen
Published: July 2002

Means are presented for using stationary points in two ways. One, for well-understood potentials, elucidates relations between the form of the surface and the dynamics that it supports, including the determination of the effective molecular symmetry group. The other, for potentials of uncertain quality, provides a test for unphysical characteristics and suggests how the surface might be improved if it is found to be unsatisfactory in some respect. Our approach involves comparison of transition state calculations using the slowest slide and Cerjan–Miller algorithms for two example systems: the Lennard-Jones Ar7 cluster and the Handy–Carter many-body-expansion potential for the ground state of formaldehyde.
Additivity of the one-third power of the electron density in the hydrogen molecule–ion and hydrogen molecule
Authors: Lee Chengteh , Zdravkovic Slobodan , Parr Robert G.
Published: July 2002

It is established that in the hydrogen molecule−ion and hydrogen molec the one−third power of the ground−state electron density is very well represented as a superposition of spherically symmetric densities on the two nuclei.(AIP)
Photoelectron spectra of the excited states of an intermediate case molecule: Pyrazine
Authors: Hillenbrand Susan , Zhu Langchi , Johnson Philip
Published: July 2002

The photoelectron spectrum for the mixed singlet–triplet levels populated through excitation of the origin of the S11B3u state in the intermediate case molecule pyrazine contains structure identifiable with the different spin components of the molecular eigenstates. The triplet part of the spectrum has a broad background and a few resolved peaks. It is suggested that the structured component is comprised of the transitions of a few high frequency vibrational combinations in the triplet, while the broad distribution of electron energies results from ionization of a dense manifold of low frequency combinations which are likely to undergo IVR. We also report the photoelectron spectrum of ‘‘relaxed’’ triplet states produced by collisional relaxation from levels populated after ISC from the origin of the S1 state.
A comparative kinetic study of xenon adsorption on a flat Pt(111) and stepped Pt(557) and Pt(112) surfaces
Authors: Siddiqui H. R. , Chen P. J. , Guo X. , Yates J. T.
Published: July 2002

The adsorption and desorption kinetics for Xe on three Pt single crystalline surfaces have been measured using threshold temperature programmed desorption measurements. It has been found that the first-order desorption kinetic parameters at zero coverage for Xe on Pt(111) (E0d =5.73±0.50 kcal/mol; ν0d=1012.5±0.9 s−1) are in excellent agreement with measurements made by a modulated molecular beam method [E0d =5.65±0.35 kcal/mol; ν0d=1012.1 s−1]. The presence of step defect sites of varying density on Pt(557) (1/6 defects) and on Pt(112) (1/3 defects) is shown to have a marked influence on Xe desorption kinetics, producing larger values of E0d≂8.5–8.7 kcal/mol and larger values of ν0d ≂1014 s−1 compared to Pt(111). In addition, for Pt(111), slight attractive Xe–Xe interactions are observed, whereas for Xe adsorption on step sites, repulsive Xe–Xe interactions are observed as the steps fill. Xe adsorption kinetics on Pt(111) at 87 K are consistent with an island growth mechanism. In contrast, on Pt(557) and Pt(112), the adsorption process does not seem to produce island growth on the terraces. These results, taken together, show that using Xe, defect sites on metals may be probed effectively with adsorption and desorption kinetic methods.
Isotopic fractionation in low temperature ion–molecule exchange reactions: Enrichment of 22Ne in Ne+n clusters formed by association in an ionized free jet
Authors: DeLuca Michael J. , Cyr Donna M. , Chupka William A. , Johnson Mark A.
Published: July 2002

Cationic clusters of neon atoms (Ne+n ) formed by association of neutrals onto seed ions in an ionized supersonic expansion are found to favor incorporation of the heavier isotope (22Ne) by as much as a factor of 15 (in the dimer and trimer ions) when compared to a simple statistical distribution based on natural abundances. This enrichment is attributed to the small difference in zero-point energies among species formed with the two major isotopes of neon (20Ne and 22Ne), which is of the same order as the collisional energy of particles in the expanding jet. This enrichment is anticipated by current models of isotope exchange which are invoked to explain the anomalous isotope abundance patterns in interstellar clouds.
Water hydrogen bonding: The structure of the water–carbon monoxide complex
Authors: Yaron D. , Peterson K. I. , Zolandz D. , Klemperer W. , Lovas F. J. , Suenram R. D.
Published: July 2002

Rotational transitions between J≤3 levels within the K=0 manifold have been observed for H2O–CO, HDO–CO, D2O–CO, H2O–13CO, HDO–13CO, and H217O–CO using the molecular beam electric resonance and Fourier transform microwave absorption techniques. ΔMJ=0→1 transitions within the J=1 level were also measured at high electric fields. A tunneling motion which exchanges the equivalent hydrogens gives rise to two states in the H2O and D2O complexes. The spectroscopic parameters for H2O–CO in the spatially symmetric tunneling state are [∼(B0) =2749.130(2)MHz, D0=20.9(2)kHz, and μa=1.055 32(2)D] and in the spatially antisymmetric state are [∼(B0) =2750.508(1)MHz, D0=20.5(1)kHz, and μa=1.033 07(1)D]. Hyperfine structure is resolved for all isotopes. The equilibrium structure of the complex has the heavy atoms approximately collinear. The water is hydrogen bonded to the carbon of CO; however the bond is nonlinear. At equilibrium, the O–H bond of water makes an angle of 11.5° with the a axis of the complex; the C2v axis of water is 64° from the a axis of the complex. The hydrogen bond length is about 2.41 Å. The barrier to exchange of the bound and free hydrogens is determined as 210(20) cm−1 (600 cal/mol) from the dipole moment differences between the symmetric and antisymmetric states. The tunneling proceeds through a saddle point, with C2v structure, with the hydrogen directed towards the CO subunit. The equilibrium tilt away from a linear hydrogen bond is in the direction opposite to the tunneling path.
Monte Carlo simulation of aqueous solutions of Li+ and Na+ using many-body potentials.  Coordination numbers, ion solvation enthalpies, and the relative free energy of solvation
Authors: Cieplak Piotr , Kollman Peter
Published: July 2002

We present the results of the N-P-T ensemble Monte Carlo simulations for the systems containing 125 H2O and Li+ or Na+ ions at 25 °C using a many-body potential. Ion solvation enthalpies are calculated in excellent absolute agreement with experiment, once corrections for nonbonded cutoffs are made. The relative free energy of hydration of Li+ and Na+ is calculated in only qualitative agreement with experiment, which is surprising given the quantitative free energies found in such calculations on ion–water clusters. For Li+, we determine a coordination number equal to 4, whereas for Na+ that value is between 5 and 6; these values are in the range of various experiments.
A coupled cluster study of As2
Authors: Scuseria Gustavo E.
Published: July 2002

Theoretical calculations at the self-consistent field (SCF) and coupled-cluster method including all single and double excitations (CCSD) have been carried out for As2. A variety of basis sets employing both segmented contractions and atomic natural orbitals have been developed and investigated. The predicted equilibrium bond length (re=2.107 Å), harmonic vibrational frequency (ωe=447 cm−1), and ionization potentials (9.75, 10.08, and 14.78 eV) are in excellent agreement with experimental values 2.103 Å, 430 cm−1, and 9.89 eV, 10.22 eV, and 15.32 eV; respectively. The best theoretical estimates for the dissociation energy (71.6 kcal/mol) and for the electron affinity (0.56 eV) were obtained at the CCSD level of theory employing large basis sets. However, given the importance of correlation effects on these two properties (the corresponding SCF values are 12.4 kcal/mol and 0.42 eV), the theoretical predictions could substantially be affected by the inclusion of connected triple excitations not considered in this work. The open-shell atomic As(4S) and diatomic calculations for As+2 and As−2 were carried out using a recently developed CCSD program that is based on restricted Hartree–Fock references and does not include any spin contamination for the correlation energies. The importance of correlating 3d electrons and the basis-set superposition error for De are analyzed. The effect of including d- and f-type polarization functions in the basis set is also discussed.
Infrared photodissociation of size-selected methanol clusters
Authors: Buck U. , Gu X. J. , Lauenstein Ch. , Rudolph A.
Published: July 2002

Size-selective infrared photodissociation spectra of (CH3OH)n clusters from n=2 to n=9 were measured near the absorption band of the C–O stretching mode of the monomer at 1033.5 cm−1 . The experiments were carried out in a molecular-beam apparatus in which the clusters are generated in a supersonic expansion and afterwards size selected in a scattering experiment with helium atoms. The internally excited clusters are dissociated by the radiation of a cw CO2 laser in a new antiparallel arrangement of the scattered cluster beam and laser beam. The observed spectra vary from the dimer, for which a double-peak structure appears, to the single-peak spectra of the trimer, tetramer, and pentamer which are continuously shifted to higher frequencies. A special transition is seen from the pentamer to the hexamer, for which again a double-peak structure is observed which continues to larger clusters. Applying an intermolecular model potential, a correlation between the observed spectra and the cluster configuration of minimum energy is derived. The line shifts of the dimer to the red and blue are caused by the nonequivalent position of the donor and acceptor in the hydrogen bond. The next three larger clusters are nearly planar rings, while from the hexamer onwards, only distorted rings and similar isomeric structures appear. These calculations together with the evaluation of the integrated dissociation cross section show that only internally excited dimers and trimers can be dissociated with one or two CO2 laser photons, respectively.
The vinyl radical investigated by infrared diode laser kinetic spectroscopy
Authors: Kanamori Hideto , Endo Yasuki , Hirota Eizi
Published: July 2002

A c-type band was observed at around 895 cm−1 by infrared diode laser kinetic spectroscopy combined with the excimer laser photolysis of vinyl halides at 193 nm and was assigned to the CH2 wagging mode of the vinyl radical. The band was found to consist of two component bands separated by 0.0541(11) cm−1. Both component bands showed clearly the statistical weight in an alternative way, that is, if one shows the weight 1:3 for even:odd Ka levels, the other exhibits 3:1, indicating that the radical is of C2v effective symmetry, executing a double-minimum motion probably associated with the C–H in-plane rocking vibration. The upper states of the two bands were found to be perturbed weakly, possibly by a Coriolis interaction with the first overtone state of the C–H rocking mode.
High-resolution jet absorption study of nitrogen near 800 Å
Authors: Huber K. P. , Jungen Ch.
Published: July 2002

The absorption spectrum of a supersonic free jet expansion of 14N2 in He has been photographically recorded in the region of the He continuum from ∼900 to 650 Å at reciprocal dispersions of 0.26 and 0.19 Å/mm. The low rotational temperature of close to 20 K results in a drastic simplification of the band structures compared to room or even liquid nitrogen temperature spectra. This report concentrates on the region from 835 to 790 Å and presents a survey of vibronic structures up to the first ionization potential. The Rydberg series of np complexes converging to v=0 in the ground state X 2Σ+g of the ion can be followed to n=39, the low-J rotational structure being clearly discernible in complexes as high as n=19. The application of multichannel quantum defect theory to a least-squares analysis of four series of rovibronic levels associated with the lowest rotational levels N+=0, 1, and 2 of the ion leads to very substantially reduced error limits for the ionization potential of 14N2 and reveals a number of irregularities that point to perturbations by Rydberg levels derived from A 2Πu of N+2.
Multiphoton ionization and dissociation of mixed van der Waals clusters in a linear reflectron time-of-flight mass spectrometer
Authors: Ernstberger B. , Krause H. , Kiermeier A. , Neusser H. J.
Published: July 2002

Slow metastable fragmentation of benzene/toluene and benzene/para-difluorobenzene clusters is observed in a newly developed linear reflectron time-of-flight mass spectrometer after two-photon ionization. The breakdown of the metastable intensity with decreasing two-photon energy is measured and yields the appearance potential for the main dissociation channels of the homo- and the heterodimers. Based on these values, the dissociation energies of the neutral dimers are obtained and shown to be consistent with the changes of the polarizability and dipole moment of the components. In addition, from the appearance potentials and the measured ionization potentials, the dissociation energies of the charged dimer clusters are found. The binding energies of the heterocluster ions and the para-difluorobenzene homodimer ion are smaller than the respective binding energies in the benzene and toluene homodimer ions. This is explained by a larger contribution of charge transfer resonance interaction to the binding energy of the latter homodimer ions. On the basis of these results we present an energetic scheme for prediction of the dissociation pathways of the trimer ions in agreement with the measured results.
Kinetic energy dependence of dissociative charge–transfer reactions of He+, Ne+, Ar+, Kr+, and Xe+ with silane
Authors: Fisher Ellen R. , Armentrout P. B.
Published: July 2002

Guided ion-beam techniques are used to measure the cross sections as a function of kinetic energy for reaction of SiH4 with He+, Ne+, Ar+, Kr+, and Xe+. State-specific data for the 2P3/2 ground spin–orbit states of Kr+ and Xe+ are also obtained. The products observed in the He, Ar, and Kr systems are SiH+x for x=0–3. For the Ne system, formation of SiH+x x = 0–2, is seen, while in the Xe system only SiH+3 and SiH+2 are observed. Reactions of He+, Ne+, Kr+, and Xe+ show little dependence on kinetic energy, but for the case of Ar+, the reaction probability and the product distribution are highly sensitive to the kinetic energy of the system. Thermal reaction rates for all of the reactions are derived and compared with previous measurements. The results for these reactions are explained in terms of vertical ionization from the 1t2 and 3a1 bands of SiH4. The relationships of these reactions to plasma deposition and etching are also discussed.
The dissociation energy of N3
Authors: Martin J. M. L. , François J. P. , Gijbels R.
Published: July 2002

The dissociation energy of N3 has been computed ab initio using coupled cluster techniques and large basis sets. Our best theoretical estimates (accuracy ±2 kcal/mol) are ∑De=234.0, ∑D0=228.4, ΔH0 f =109.3, and ΔH0 f,298=108.4 kcal/mol. These values are in excellent agreement with an ICR determination of ΔH0 f=112±5 kcal/mol. Very large basis set effects are observed.