bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectroscopy and dynamics of the dipole-supported state of acetyl fluoride enolate anion
Authors: Marks Jeffrey , Brauman John I. , Mead Roy D. , Lykke Keith R. , Lineberger W. C.
Published: July 2002

High resolution photodetachment spectroscopy of acetyl fluoride enolate anion has revealed ≈200 narrow resonances near the photodetachment threshold, corresponding to excitation of the anion to a diffuse state in which the electron is weakly bound by the field of the molecular dipole. An analysis of the rotational transitions between the ground valence state and the excited dipole-supported state has been carried out, yielding spectroscopic constants for both states. The binding energy of the dipole-supported state is found to be less than 35 cm−1 . The dependence of autodetachment lifetimes upon rotational quantum numbers of the dipole-supported state has been measured. The selection rules and dynamics of autodetachment from the dipole-supported state are discussed. The results are compared with those obtained previously for acetaldehyde enolate anion.
Spectra and structure of nitrous oxide dimers and clusters
Authors: Gauthier Michel
Published: July 2002

Supersonic expansions of nitrous oxide seeded in argon were studied by single pass FTIR absorption at right angles with the jet axis. In very dilute mixtures, 0.2% N2O, dimer bands at 1282.4 and 2228.0 cm−1 were observed corresponding to the ν1 and ν3 stretches of the monomer. Cluster bands (due to clusters of undetermined size) were readily observed in 20%–100% N2O mixtures at 1165.1, 1295.2, 2245.6, 2467.4, 2578.3, 2811.7, and 3505.8 cm−1. All bands are shifted from the corresponding monomer bands more than in the crystal and in the same direction. Changing the dilution from 10% to 20% shifted all cluster bands 1–2 cm−1 towards low frequencies but still far away from the crystal frequencies. This is interpreted as being due to cluster melting. No increase in bandwidth is associated with the shifts, indicating that, if present at all, the coexistence region described by Berry and co-workers is rather limited in N2O clusters.
Photophysics of buckminsterfullerene and other carbon cluster ions
Authors: O’Brien S. C. , Heath J. R. , Curl R. F. , Smalley R. E.
Published: July 2002

The laser-induced fragmentation behavior of positive carbon cluster ions has been investigated by tandem time-of-flight techniques for the jet-cooled clusters up to 80 atoms in size. Two distinct photophysical regimes were found. The first applies to clusters with 34 atoms or more, all of which dissociate to produce even numbered fragments. Large even clusters fragment by the loss of the high energy species C2, odd ones lose a C atom. The second regime applies to clusters composed of 31 or less atoms, all of which fragment by the loss of C3. These two regimes are sharply separated by C+32 which fragments to produce small cluster ions in the 10–19 atom size range. Fragmentation of the large clusters occurs on a microsecond or faster time scale only at very high levels of excitation (>12.8 eV). These photophysical results are interpreted as consequences of the large even clusters having edgeless, spheroidal cage structures while the small ones have linear chain or ring structures.
Collision partner and level dependence of vibrational relaxation in S p-difluorobenzene. Stimulated emission pumping combined with single vibronic level fluorescence spectroscopy
Authors: Kable Scott H. , Thoman John W. , Knight Alan E. W.
Published: July 2002

Rate coefficients have been measured for vibrational relaxation in S0 p-difluorobenzene vapor under assault from eight collision partners. The dissipation of vibrational energy is monitored from each of four vibrational states with energies in the range ∼1500–3300 cm−1. Rate coefficients are found to vary between 1.5×10−10 cm3 molecule−1 s−1 for helium colliding with p-difluorobenzene in the lowest state explored, to 12×10−10 cm3 molecule−1 s−1 for p-difluorobenzene self-relaxation from the highest level studied. The dependence of collision partner on the vibrational relaxation efficiency for each initially prepared state is studied, revealing a dependence on the reduced mass of the collision pair, and/or the well depth of the system. In addition the dependence of the efficiency on the initially prepared state is investigated for any one collision partner using a modified version of the Parmenter–Tang propensity rule model. A comparison between these data for vibrational relaxation in the S0 state of p-difluorobenzene and previous data for vibrational relaxation in the S1 state of the same molecule is discussed.
Mobilities of cluster ions of Li+ in some molecular gases
Authors: Satoh Y. , Takebe M. , Iinuma K.
Published: July 2002

The mobilities of the singly clustered ions of Li+ with constituent gases have been measured with a conventional drift tube over the E/N range 8–89 Td in N2, 10–25 Td in O2, 10–139 Td in CO, 50–348 Td in CO2, and 40–80 Td in CH4. Two methods were employed to determine the mobilities; one was a difference method and the other was based on the pressure dependence of apparent mobilities. It is found that the mobilities of each cluster ion species remain constant over most of the E/N range used and the zero-field values are 1.87±0.07 for Li+(N2)–N2 at 303 K, 1.89±0.11 for Li+(O2)–O2 at 303 K, 1.70±0.06 for Li+(CO)–CO at 303 K, 0.92±0.04 for Li+(CO2)–CO2 at 303 K, and 2.22±0.08 cm2/V s for Li+(CH4)–CH4 at 302 K, respectively. The zero-field mobilities of these polyatomic ions considerably deviate from the values of the Langevin polarization limit. They are predicted with moderate accuracy by assuming a hard sphere-r−4 potential which takes a repulsive core size into account.
Intensity distribution in rotational line spectra. I. Experimental results for Doppler-free S1←S transitions in benzene
Authors: Sieber H. , Riedle E. , Neusser H. J.
Published: July 2002

Completely resolved Doppler-free rotational line spectra of six vibronic two-photon bands in benzene C6 H6 and C6 D6 are presented. The excited final states possess different excess energies in S1 (1567 to 2727 cm−1 ) and are embedded in dense manifolds of background states with differing densities of states (1<ρ<60 1/cm−1 ). The bands are analyzed by a statistical procedure. The intensity distribution of several hundreds of lines of each band is investigated. It is found that all weakly perturbed bands display a similar, peaked intensity distribution while in strongly perturbed bands the number of lines decreases monotonically with increasing intensity. The origin of this difference is discussed in terms of coupling to the many background states.
Tricritical phenomena in quasi-binary mixtures. VI. The binary system ethane + n-eicosane and some revised scaling parameters
Authors: Goh M. Cynthia , Scott Robert L. , Knobler Charles M.
Published: July 2002

The three-phase coexistence curve for the binary system ethane + n-eicosane has been measured and its relation to the tricritical point in the quasi-binary system ethane +(n-heptadecane+n-octadecane) is discussed. In order to fit the curve into the scaling scheme developed from an extended classical theory of tricritical phenomena, it is necessary to include terms derived from a ninth-order free-energy polynomial.
H+ClCN→HCl+CN: Product excitations and reaction mechanism at Ec.m. ≂21.6 kcal mol−1
Authors: de Juan J. , Callister S. , Reisler H. , Segal G. A. , Wittig C.
Published: July 2002

Nascent CN(X 2Σ) rovibrational and kinetic energy distributions have been obtained for the title reaction using laser induced fluorescence, and regions of the HClCN potential energy surface appropriate to the observed HCl+CN channel, as well as the HCN+Cl and HNC+Cl channels, have been calculated ab initio at the MP4/3-21G* level. The CN spectator has low internal excitation; the average fractions of the available energy deposited in CN vibration and rotation are 〈 fV(CN)〉 ∼0 and 〈 fR(CN)〉 =0.06, respectively, with the rotational distribution corresponding to a temperature of ∼950 K. Sub-Doppler resolution spectroscopy on several CN B 2Σ←X 2Σ transitions provides an average value for the fraction of the available energy appearing as center-of-mass kinetic energy, 〈 fT〉 =0.33, as well as approximate kinetic energy distributions for specific CN V,R levels. Since the kinetic energy distributions are for specific CN levels, the corresponding HCl internal energy distributions are obtained by energy conservation, and the experimental results indicate a vibrational population inversion in HCl. Product energy disposal is similar to that of many ‘‘light+heavy–heavy’’ systems which exhibit repulsive energy release and which prefer end-on rather than broadside

entrance channels. The dominant reaction pathways are probably HCN+Cl and HNC+Cl, with HCl+CN being minor. Because of the competitiveness between the chemically distinct product channels, the entrance channel associated with the observed products is more stereospecifically constrained than for a similar reaction which has a single set of products. The ab initio calculations suggest that end-on encounters can be reactive, thus accounting for the observed channel. A local minimum exists for a three-center structure involving hydrogen, chlorine, and carbon, and the barrier from here to HCN+Cl is small. Broadside approaches give rise to HCN+Cl, and we therefore surmise that the observed products derive predominantly from encounters in which the H atom approaches the chlorine over a modest range of angles and impact parameters. Linear approaches at the nitrogen end of the molecule are unreactive at the energies of the present experiments. However, there is a cis transition state leading to HNC+Cl which is low enough to participate in the overall chemistry. Thus, the ab initio calculations are consistent with the experimental observations, and suggest that the title reaction is a minor channel, and that CN+HCl is produced by direct reaction via attack at the chlorine.
A nomenclature for Λ-doublet levels in rotating linear molecules
Authors: Alexander M. H. , Andresen P. , Bacis R. , Bersohn R. , Comes F. J. , Dagdigian P. J. , Dixon R. N. , Field R. W. , Flynn G. W. , Gericke K.-H. , Grant E. R. , Howard B. J. , Huber J. R. , King D. S. , Kinsey J. L. , Kleinermanns K. , Kuchitsu K. , Luntz A. C. , McCaffery A. J. , Pouilly B. , Reisler H. , Rosenwaks S. , Rothe E. W. , Shapiro M. , Simons J. P. , Vasudev R. , Wiesenfeld J. R. , Wittig C. , Zare R. N.
Published: July 2002

It is proposed that the two Λ-doublet levels of linear molecules with nonzero electronic orbital angular momentum be labeled Λ(A′) and Λ(A″), e.g., Π(A′) and Π(A″) for Π states, etc., according to the following prescription: All series of levels in which the electronic wave function at high  J is symmetric with reflection of the spatial coordinates of the electrons in the plane of rotation will be designated Λ(A′) for all values of J, and all those for which the electronic wave function is antisymmetric with respect to reflection will be denoted Λ(A″). It is emphasized that this notation is meant to supplement, and not replace, the accepted spectroscopic e/f labeling and the parity quantum number. The utility of the Λ(A′)/Λ(A″) notation is that it is of most relevance in the mechanistic interpretation of reactive or photodissociative processes involving open-shell molecules.
The B 1Σ+u, B′ 1Σ+u,C 1Πu, and D 1Πu states of hydrogen. A
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o calculation of rovibronic coupling in H2, HD, and D2
Authors: Senn P. , Quadrelli P. , Dressler K.
Published: July 2002

The nonadiabatic rovibronic structures of the B, B′, C, and D states of H2, HD, and D2 have been calculated up to the common dissociation limit of the first three of these electronic states using the ab initio adiabatic potential curves and nonadiabatic coupling functions published recently by Wolniewicz and one of us. The homogeneous (Σ+–Σ+ and Π–Π) and heterogeneous (Σ+–Π+) interactions are treated simultaneously by numerical solution of the coupled vibrational equations for each J value from J=0 to J=6. The calculated nonadiabatic energy shifts, which show strong v- and J-dependent variations within each electronic state and which spread over approximately ±50 cm−1 for J=6 in H2, yield rovibronic structures in very good agreement with spectroscopic term values. A number of published spectroscopic assignments, including states close to the dissociation limit, must be reevaluated in the light of this calculation. The remaining energy errors are nuclear-mass dependent and are attributed to the neglected nonadiabatic interactions with the higher members of the 1Σ+u and 1Πu Rydberg series. The convergence errors of the Born–Oppenheimer energies of all four electronic states treated here appear to be at most 2 cm−1 for the highest bound vibrational states and less than 1 cm−1 for the lowest ones.
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o self−consistent field (SCF) calculations on borane icosahedra with zero, one, or two substituted carbon atoms
Authors: Green T. A. , Switendick A. C. , Emin David
Published: July 2002

Restricted Hartree–Fock calculations at the double-zeta level are performed for the closed-shell isoelectronic molecules B12H−212, CB11H−12 , and 1,12-C2 B10 H12 . The geometries are optimized and the vibrational spectra are obtained. The behavior of the molecular orbitals upon carbon substitution is analyzed. The results are compared with the available experimental and theoretical results from the literature. The main features of the electronic structure are correctly described at this level of approximation. The new results can aid in the interpretation of photoemission, infrared absorption, and Raman scattering experiments on these molecules.
Comparison of the Ca+HF(DF) and Sr+HF(DF) reaction dynamics
Authors: Zhang Rong , Rakestraw David J. , McKendrick Kenneth G. , Zare Richard N.
Published: July 2002

A comparative study of the reaction family, Ca and Sr with rovibrationally selected HF or DF, has been carried out under single-collision conditions. A thermal beam of the alkaline earth atoms, Ca or Sr, is fired into a low-pressure gas of HF or DF in which the reagent molecules have been prepared in a selected vibration–rotation state by the use of a tunable infrared light source (optical parametric oscillator). The resulting alkaline earth monofluoride reaction products, CaF or SrF, are detected in a quantum-state-specific manner with a tunable visible light source (dye laser) using laser induced fluorescence. The HF molecule is aligned with its rotational angular momentum pointing preferentially either along or perpendicular to the metal atom beam. For both Ca+HF(v=1) and Sr+HF(v=1) the cross section and the product state distribution are found to be independent of the approach geometry, which is consistent with reaction through a bent transition state. The state-to-state reaction dynamics for Ca+HF(DF) and Sr+HF(DF) show marked differences. The Ca+HF(DF)→CaF+H(D) reaction has attributes of a statistical internal energy distribution, whereas Sr+HF(DF)→SrF+H(D) shows some deviation from this behavior. The findings for Ca+HF(DF) are consistent with a model in which the Ca atom inserts into the HF bond to form a long-lived H–Ca–F intermediate; the findings for Sr+HF(DF) are interpreted as competition between direct reaction and reaction which samples the deep H–Sr–F potential well.
Theory of time-resolved coherent anti-Stokes Raman scattering from molecules in liquids. Effects of coherence transfer
Authors: Hayashi M. , Nomura Y. , Fujimura Y.
Published: July 2002

Effects of the coherence transfer induced by the molecule–heat bath interactions on the ultrashort time-resolved coherent anti-Stokes Raman scattering (CARS) from molecules in liquids are theoretically studied. Based on the perturbative density matrix formalism an expression for the CARS intensity is derived taking into account the coherence transfer between the Raman active vibrational transitions of two molecules in liquids. The coherence transfer constants and dephasing constants are properly incorporated with the aid of Liouville space Feynman diagrams. The structure of the coherence transfer matrix element which expresses the time evolution of the coherence between the relevant transitions is clarified by solving the Master equation with the coherence transfer and dephasing constants in the Markoff approximation. Frequency shifts of the quantum beats appear in the time-resolved CARS as a result of the coherence transfer. A multispherical layer model is adopted in evaluating the coherence transfer effects in liquids in femtosecond time domains. Model calculations of time-resolved CARS spectra have been carried out to demonstrate the coherence transfer effects in both short and long range coherence transfer cases. It is predicted that the quantum beats are amplified in the time-resolved CARS spectra of molecules in liquids in a long range coherence transfer case when there exist differences in the coherence transfer constants between each spherical layer.
Electronic energy transfer in Na2
Authors: Astill A. G. , McCaffery A. J. , Taylor S. C. , Whitaker B. J. , Wynn M. J.
Published: July 2002

The emission spectrum of the A 1Σ+u–X 1Σ+g band of Na2 has been recorded following both multimode and single mode excitation of the B 1Πu state using an argon ion laser. The spectral profile has been investigated as a function of excitation frequency, cell temperature and buffer gas pressure. Simulation of the band profile shows that the A 1Σ+u state is vibrationally inverted. This is interpreted through a populating mechanism involving collisional transfer from the B 1Πu to the (2) 1Σ+g state followed by radiative transfer to the A 1Σ+u state. The collision partner is most likely to be 2S state Na atoms. The emission is also found to be significantly negatively polarized with respect to the laser polarization plane. The results are discussed in the light of various theoretical models.
Photoabsorption and photofragmentation studies of Ar+n cluster ions
Authors: Levinger Nancy E. , Ray Douglas , Alexander Michael L. , Lineberger W. C.
Published: July 2002

We have measured the photoabsorption spectra of mass selected Ar+n clusters, n=3–40, from 355–1064 nm. The smaller clusters, n<15, display a visible photoabsorption spectrum similar to Ar+3, i.e., a broad, intense band peaking near 520 nm. From n=15–20 this photoabsorption band shifts smoothly to a longer wavelength, peaking near 600 nm for Ar+20. This band does not change appreciably as n increases from 20 to 40. These results clearly demonstrate that the Ar+n clusters have photophysical properties quite different from those of Ar+2. We have also studied the photoabsorption and subsequent photofragmentation of Ar+n cluster ions, n=3–60, at selected visible wavelengths. The ionic photofragment distributions both indicate that photofragmentation proceeds through the loss of individual Ar atoms and place an upper bound of 90 meV on the cluster ion binding energy in the large cluster limit.
Two-photon (vacuum ultraviolet + visible) spectroscopy of argon, krypton, xenon, and molecular hydrogen
Authors: McCann M. P. , Chen C. H. , Payne M. G.
Published: July 2002

Two-photon transitions have been examined in argon, krypton, xenon, and molecular hydrogen using a coherent vacuum ultraviolet (VUV) photon at a fixed wavelength of 118 nm and a tunable photon from a dye laser. The 118 nm VUV photon is produced by third harmonic generation in xenon using UV light at 355 nm from a frequency tripled Nd:YAG laser. Though the VUV intensity is very weak, (∼100 nJ per pulse) it was utilized very efficiently since most of VUV photons in the ionization region were absorbed. Spectra were obtained in the region of 12.5 to 13.5 eV and the corresponding two-photon rate constants were calculated. This is the first time that coherent VUV light has been employed with tunable visible light for the production of two-photon spectra and the measurement of two-photon rates. The two-photon ionization rate of xenon was measured using photons at fixed wavelengths of 118 and 355 nm. A new parameter is proposed for direct comparison of the data from various two-photon experiments.
Note on the vibrational spectrum of C4 and C5
Authors: Martin J. M. L. , François J. P. , Gijbels R.
Published: July 2002

The geometry and infrared spectrum of C5 have been computed ab initio using the MP2/6-31G* theoretical model. The inner and outer bond lengths are 1.291 and 1.300 Å, respectively. A very intense (1243 km mol−1) ∑+u band is predicted at 2193 cm−1, while another ∑+u mode at 1368 cm−1 is found to be much less intense (64 km mol−1). The results confirm the assignment of the experimental infrared (IR) frequencies of C4 and C5 suggested by Bernholdt et al. [J. Chem. Phys. 89, 3612 (1988)].
Electron–vibration coupling in semiconductor clusters studied by resonance Raman spectroscopy
Authors: Alivisatos A. P. , Harris T. D. , Carroll P. J. , Steigerwald M. L. , Brus L. E.
Published: July 2002

The resonance Raman spectrum of 45(+−3) Å diameter CdSe clusters was measured. The incident photons were resonant with the HOMO–LUMO transition in the clusters. At low temperature, one mode at 205 cm−1 is observed, as well as two overtones, with the integrated areas under these peaks in the ratio of 9:3:1. This mode is assigned as the longest wavelength longitudinal optical vibration of the cluster. The strength of the coupling between the lowest electronic excited state and the LO vibration is found to be 20 times weaker in these clusters than in the bulk solid. The CdSe cluster resonance Raman spectrum is shown to be consistent with the recently measured homogeneous cluster absorption spectrum.
Double-resonance studies of rotational autoionization of H2
Authors: O’Halloran M. A. , Dehmer P. M. , Pratt S. T. , Dehmer J. L. , Tomkins F. S.
Published: July 2002

Two-color resonantly enhanced multiphoton ionization combined with photoelectron spectroscopy detection was used to study rotational autoionization of np Rydberg states of H2 near the first ionization threshold. In this two-step experiment, one laser was used to excite a two-photon transition to the E,F 1Σ+g, v′=E0, J′=0–4 levels, and a second laser was used to probe single photon transitions to the rotationally autoionized np Rydberg series coverging to the X 2Σ+g, v+=0, N̄+=1–6 rotational levels of the ion. Assignments were made for a large number of Rydberg states converging to v+=0 and for several interlopers converging to v+=1 and 2. Of the five dipole allowed Rydberg series converging to v+=0 excited from each intermediate J′ level (J′>2), two are allowed to rotationally autoionize in a coupling scheme that assumes ejection of pure p waves in the ionization process and singlet coupling of the spins of the ion core and the outgoing electron. Members of these Rydberg series have large half-widths, and the v dependence of the half-widths is in good agreement with simple quantum defect theory predictions. Ionization via the change of four quanta of rotational energy was found to be significantly slower than ionization via the change of two quanta of rotational energy. Ionization was also observed for the series that are forbidden to rotationally autoionize according to this simple coupling scheme, and it is shown that both the inclusion of f waves in the autoionization process and singlet–triplet mixing may be required to explain these observations.
Speeds of sound in CF4 between 175 and 300 K measured with a spherical resonator
Authors: Ewing M. B. , Trusler J. P. M.
Published: July 2002

An apparatus is described that exploits the high precision of a spherical resonator in the temperature range 80 to 350 K at pressures up to 1.3 MPa. We have used the radial modes of the spherical resonator to measure the speed and absorption of sound in CF4 in the temperature range 175 to 300 K and the pressure range 0.08 to 1.0 MPa. The speeds of sound, which have a precision better than 0.001% have been used to obtain perfect-gas heat capacities and second and third acoustic virial coefficients of CF4. (p,V,T) virial coefficients calculated from the results are in good agreement with those in the literature. We also report vibrational relaxation times for CF4 determined from the linewidths of the radial modes.
Multireference coupled cluster and multireference configuration interaction studies of the potential surfaces for deprotonation of NH+4
Authors: Kaldor Uzi , Roszak Szczepan , Hariharan P. C. , Kaufman Joyce J.
Published: July 2002

Protonation/deprotonation reactions are represented by H++B⇄HB+. The ionization potential of H (13.6 eV) is higher than that of B for organic and most inorganic molecules (it is 10.166 eV for NH3), and the separated pair H+B+ will be lower in energy than the closed-shell pair H++B. The reaction path involves, therefore, an avoided crossing, and its theoretical study requires multideterminant methods. The reaction with B=NH3 (or R1R2R3N) is of interest in several fields, and its study is described here. The multireference coupled-cluster method (MR-CCM) and multireference double-excitation configuration interaction (MRD-CI) were used. At each (H3N---H)+ separation, from 1 to 11 bohr, the ground state MRD-CI energy was optimized with respect to the angle θ between the NH bond in the NH3 group and the C3 axis; MR-CCM and MRD-CI calculations were performed for the two lowest 1A1 states and the lowest 3A1. Two different reference determinants had to be used for the MR-CCM calculations at different regions, but this created no difficulties and the transition was smooth. Close agreement (a few mhartree) was obtained between MRD-CI and MR-CCM results. The avoided crossing, near R(H3N---H)+=4 bohr, is manifested by the rapid change in the CI and coupled-cluster method (CCM) mixing coefficients and by the transition of the NH3 group from pyramidal at small R to planar at large R. The lowest 1A1 state dissociates adiabatically to NH+3(2A1)+H(2S), whereas the single determinant self-consistent field (SCF) function dissociates to NH3(1A1)+H+.
Geminate ion recombination in anisotropic media. Effects of initial distribution and external field
Authors: Pimblott Simon M. , Mozumder Asokendu , Green Nicholas J. B.
Published: July 2002

Recently we presented [Chem. Phys. Lett. 142, 385 (1987)] a method for calculating the diffusion-controlled geminate escape probability of an ion pair in an anisotropic medium. The escape probability depends on both the length and the direction of the initial interion vector, and is reducible to a function of two dimensionless variables describing the overall anisotropy of the medium. In this paper the treatment is extended to include an initial distribution of interion distances and an external field. Inclusion of a field increases the number of independently variable parameters to five: two to describe the anisotropy of the medium and three to describe the intensity and direction of the applied field. It is shown that the effects of including anisotropy are not negligible for the typical case of anthracene and that the slope-to-intercept ratio of the field-dependent escape probability for a spherically averaged distribution depends on the direction of the applied field.
Direct absorption spectra of jet-cooled Hg2
Authors: Schlauf M. , Dimopoulou-Rademann O. , Rademann K. , Even U. , Hensel F.
Published: July 2002

The electronic absorption spectrum of the van der Waals dimer Hg2 has been measured in the UV–spectral range by a new supersonic jet direct absorption technique. The data led to an improved analysis of the X 10+g–G 10+u transition, yielding the parameters ω′e =79 cm−1, ωex′e =0.22 cm−1, and Re =3.00 Å for the Morse potential of the G 10+u state.
Photodissociation kinetics of aluminum cluster ions: Determination of cluster dissociation energies
Authors: Ray Urmi , Jarrold Martin F. , Bower J. Eric , Kraus Joe S.
Published: July 2002

The photodissociation of aluminum clusters, Al+n (n=7–17), has been studied over a broad energy range (1.88–6.99 eV). Measurements of the lifetimes of the photoexcited clusters are described. Dissociation energies have been determined by comparing the measured lifetimes with the predictions of a simple RRKM model. The dissociation energies show an overall increase with cluster size, but there are substantial oscillations around n=7–8 and n=13–15. Cluster cohesive energies are derived from these results and from previous measurements of the dissociation energies of the smaller clusters. The cohesive energies of the larger clusters (n>6) are in good agreement with the predictions of a simple model based on the bulk cohesive energy and the cluster surface energy. However, the cohesive energies are substantially larger than the results of recent ab initio calculations. The photodissociation spectrum of Al+8 has been measured and shows a broad absorption feature with a maximum ∼470 nm.
Microwave and infrared electric-resonance optothermal spectroscopy of HF–HCl and HCl–HF
Authors: Fraser G. T. , Pine A. S.
Published: July 2002

Microwave and infrared spectra of HF–HCl and HCl–HF have been obtained using a molecular-beam electric-resonance optothermal spectrometer, which operates by quadrupole-field focusing of polar molecules onto a bolometer detector. The HF–HCl microwave measurements extend to Ka=1, the previous Ka=0 results of Janda, Steed, Novick, and Klemperer, allowing the determination of the Ka dependence and asymmetry of the Cl quadrupole coupling constant. For the metastable HCl–HF isomer no previous spectroscopic measurements have been reported. Here, microwave spectra are observed for the Ka=0 and 1 states and interpreted in terms of an L-shaped hydrogen-bonded structure for the complex, with a 3.235 Å center-of-mass separation between the HF and HCl subunits. The DJ distortion constant indicates that the harmonic stretching force constant for HCl–HF is ∼35% larger than that of HF–HCl. Infrared spectra of the Ka=0–0 and 1–0 subbands of the H–F stretching band for HF–HCl and of the Ka=0–0 subband of the H–F stretch for HCl–HF are also reported. The vibrational predissociation linewidths depend on vibration, Ka state, isotopic species, and isomer excited.
Strongly bound multiply excited states of B+2 and B2
Authors: Bruna Pablo J. , Wright James S.
Published: July 2002

The potential curves, transition energies (Te), and spectroscopic constants (Re, ωe) of several low-lying electronic states of B+2 and selected doubly excited states of B2 are given. The data have been obtained by using a multireference single- and double-excitation (MRD) configuration interaction (CI) approach and a triple-zeta plus polarization AO basis set. The B+2 ground state, which is found here to be X 2Σ+g, shows a rather shallow potential curve (Re =4.015 bohr, ωe =423 cm−1) when compared with that of X 3Σ−g of B2(Re =3.00 bohr, ωe =1051 cm−1, exptl.). The first excited state of B+2, namely 1 2Πu, lies at Te =0.30 eV. Moreover, double excitations relative to X 2Σ+g are essential for the description of a large number of excited states, such as σuσg→π2u(1 4Σ−u, 1 2Σ−u, 1 2Δu, 1 2Σ+u) and σ2u→π2u(1 4Σ−g,

1 2Σ−g, 1 2Δg, 1 2Σ+g). Similarly, 2 2Πu arises from the triple excitation σ2uσg→π3u. In the same order as given above, such multiple excitations lead to a significant gain in bond strength (i.e., shorter Re, larger ωe) as a result of the simultaneous depopulation and population of strongly antibonding and bonding MOs, respectively. For neutral B2, similar features are observed for the excitation σ2u→π2u relative to X3Σ−g (leading to 3 1Σ+g) and 1 3Πu (leading to 2 3Πu). As a result of the loss of the two πu electrons, the ground state dissociation energy

(De) of B+2(X 2Σ+g), with a calculated value of 1.94 eV, is about 1.12 eV smaller than the experimental De of B2 (X 3Σ−g).
An investigation of the properties of large krypton cluster ions (development of the P=3 and P=4 Mackay icosahedral shells)
Authors: Lethbridge P. G. , Stace A. J.
Published: July 2002

A double-focusing mass spectrometer in conjunction with a cluster beam source has been used to study the unimolecular fragmentation patterns of krypton cluster ions, Kr+n, for n in the range 10–340. All cluster ions are observed to undergo the reaction Kr+n →Kr+n−1 + Kr; however, some clusters lose up to five atoms on the time scale of the experiment. The six naturally occurring isotopes of krypton complicates the cluster-ion mass spectra, with the number of ion peaks increasing rapidly with n for a given Kr+n. A computer model which successfully accounts for the influence of isotopes reveals that isotope enrichment of the cluster ions occurs. The relative intensities of the fragment ions exhibits pronounced intensity fluctuations, many of which coincide with similar observations on large argon cluster ions. It is proposed that particularly intense fragments, i.e., Kr+111 and Kr+120, arise from reactions which lead to the formation of kinetically favorable products. In contrast, intense ion peaks in the mass spectrum, i.e., Kr+147 and Kr+309, are attributed to the formation of thermodynamically favorable products. The latter coincide with shell closure in Mackay icosahedra.
The photofragmentation of Ar+3
Authors: Woodward C. A. , Upham J. E. , Stace A. J. , Murrell J. N.
Published: July 2002

The photofragment kinetic energy spectrum of Ar+3 has been recorded in a crossed-beam apparatus at 532 nm; the only photofragment product observed is Ar+. For the purposes of analysis, a spectrum for the photofragmentation of Ar+2 was recorded under similar experimental conditions. In each case, the ions were prepared by the electron impact ionization of a neutral argon cluster beam. The Ar+3 spectrum consists of two quite distinct features, a high-energy component which closely resembles the result observed for Ar+2, and a second, low-energy feature, which is peculiar to Ar+3 alone. The two high-energy wings appear to arise from a very rapid dissociation process where approximately 70% of the excess energy appears as Ar+ kinetic energy. A computer simulation of this region of the spectrum gives an anisotropy parameter, β, of 1.1±0.2. The low-energy, component to the spectrum arises from a two-step dissociation process, in which a weakly bound atom carries away a relatively large fraction of the available excess energy to leave a quasibound dimer ion. The energetics associated with this latter process can be accounted for using a partitioning scheme proposed by Baer et al. [J. Chem. Phys. 76, 5917 (1982)]. A computer simulation of the low-energy component gives β≂0.0. A detailed discussion of the results in terms of electronic structure, photofragmentation dynamics, and allowed electronic transitions, concludes that Ar+3 takes the form of a stable dimer ion and a weakly bound atom, and that it is most probably nonlinear.
High-resolution near-infrared spectroscopy of water dimer
Authors: Huang Z. S. , Miller R. E.
Published: July 2002

High-resolution near-infrared spectra are reported for all of the O–H stretch vibrational bands of the water dimer. The four O–H vibrations are characterized as essentially independent proton-donor or proton-acceptor motions. In addition to the rotational and vibrational information contained in these spectra, details are obtained concerning the internal tunneling dynamics in both the ground and excited vibrational states. These results show that for tunneling motions which involve the interchange of the proton donor and acceptor molecules, the associated frequencies decrease substantially due to vibrational excitation. The predissociation lifetimes for the various states of the dimer are determined from linewidth measurements. These results clearly show that the predissociation dynamics is strongly dependent on the tunneling states, as well as the Ka quantum number, indicating that the internal tunneling dynamics plays an important role in determining the dissociation rate in this complex.
Nuclear magnetic relaxation in small nematic droplets induced by molecular self-diffusion
Authors: Z̆umer S. , Kralj S. , Vilfan M.
Published: July 2002

Nuclear magnetic relaxation caused by self-diffusion induced modulation of the intramolecular nuclear interactions in nematic liquid crystal droplets embedded in a solid polymer matrix is analyzed in order to show when it allows the determination of the structure of such confined systems. Both the frequency and temperature dependences of the relaxation rates T−11 and T−11ρ are calculated for radial and bipolar nematic droplet structures. Special attention is given to the effect of the boundary layer nematic phase. Translationally induced modulation is in most cases an effective relaxation mechanism only in the low frequency range (<MHz for micronsize droplets). The available experimental rotating-frame relaxation data are in excellent agreement with theoretical predictions.
Theoretical studies of the energetics and structures of atomic clusters
Authors: Xie J. , Northby J. A. , Freeman David L. , Doll J. D.
Published: July 2002

Comparative calculations of the binding energy and structure of relaxed closed-shell clusters of icosahedral and cuboctahedral point group symmetry are reported. The atoms are presumed to interact via either the Lennard-Jones or the Aziz–Chen (HFD–C) pair potential. The IC structure is found to be lower in total energy for less than 14 shells (10 179 atoms) in the Lennard-Jones case and for less than 13 shells (8217 atoms) in the HFD–C case. Detailed energetics are analyzed in order to elucidate the mechanism for the transition from icosahedral to cuboctahedral symmetry.
Monte Carlo simulations of partially ionized polyelectrolytes: Shape and distribution functions
Authors: Christos George A. , Carnie Steven L.
Published: July 2002

A novel modification to the ‘‘slithering snake’’ algorithm is used to enable Monte Carlo simulations of partially ionized polyelectrolyte chains where the charged beads interact via screened Coulomb interactions. The use of ‘‘stroboscopic’’ averaging enables detailed shape characteristics of the chains to be calculated. The moment of inertia tensor, the distribution function of the end-to-end distance and the segment density distribution functions from the center of mass and both ends of the polyion are combined to give an overall characterization of the shape changes of a polyion as added salt concentration, chain length (up to 240 beads) and degree of ionization are varied. Some abrupt behavior seen for high degree of ionization may be smoothed once ionic degrees of freedom are included. The present work—by characterizing the shape of short chains—may also help to formulate models for very long polyions consisting of many segments, each of a few hundred beads.
Time-resolved photodissociation study of relaxation processes in gas-phase styrene ion
Authors: Dunbar Robert C.
Published: July 2002

Time-resolved photodissociation of styrene ions at 308 nm was observed in the ion cyclotron resonance ion trap. The rate of m/z 78 product ion formation was shown to be a sensitive function of the internal energy of the dissociating parent ions. The dissociation rates, ranging up to 5×105 s−1, were near the upper limit of the technique, and an improved signal equation was derived for quantitative interpretation of the data. By observing the m/z 78 photoappearance rate as a function of pressure and delay time between electron impact ionization and the laser pulse, the relaxation of initially excited styrene ions was characterized, giving a collisional relaxation rate constant of 4.5×10−10 cc molecule−1 s−1, and an IR radiative relaxation rate constant of 0.65 s−1. At 11.5 eV electron impact ionizing energy the parent ions were found to be formed with an average initial excess internal energy of 0.45 eV.
Selective spectroscopy of rigid and fluxional carbazole–argon clusters
Authors: Knochenmuss R. , Leutwyler S.
Published: July 2002

Two size-selective spectroscopic techniques were used to experimentally differentiate between nearly rigid (solid-like) and highly fluxional (liquid-like) carbazole⋅Arn (n=4–6) clusters produced and cooled in supersonic molecular beams: (1) ionization potential selective resonant two-photon ionization (IP selective R2PI) spectroscopy; and (2) spectral hole-burning with R2PI detection. For each cluster size, separate and qualitatively very different electronic spectra were obtained by IP selective R2PI, depending on total ionization energy. At low ionization energies, broad bands of halfwidth ≊50 cm−1 (FWHM) were obtained, which are interpreted as due to fluxional clusters of high internal energy. When ionizing slightly above an abrupt step in the ionization efficiency curve, additional narrow (Δν≊5 cm−1) features appear superimposed on the semicontinuous spectra; these are interpreted as due to (near) rigid clusters with low internal energy. The spectral hole-burning experiments support this interpretation in that deep holes of ≊5 cm−1 width could be burned in the rigid cluster spectra, but no spectral holes could be observed in the broad bands. The latter fact is interpreted as due to spectral diffusion of the fluxional subpopulation on a 10−8 s or faster time scale. These results are in good agreement with conclusions drawn from recent numerical simulations.
Dynamics of molecular reactions in solids: Photodissociation of F2 in crystalline Ar
Authors: Alimi R. , Gerber R. B. , Apkarian V. A.
Published: July 2002

Classical molecular dynamics simulations of F2 photodissociation in a host Ar crystal are presented. At temperature T=12 K, the photodissociation yield shows a sharp threshold for an excess energy of ∼0.6 eV, and it reaches nearly unity for excess energies above 2 eV. For a given excess energy, the quantum yield at 4 K is higher than a 12 K, and is predicted to remain finite even at 0 K. The transition state for photofragment exit from the reagent cage is found to be located in well-defined windows in the unit cell of the surrounding solid. The quantum yields (or photodissociation probabilities) are extremely high, especially at low T, in comparison with the values found in previous studies, e.g., for Cl2 in Xe and in Ar. Indeed, for high excess energy the near-unit quantum yields indicate the virtual absence of an inhibiting cage effect on the reaction. The anomalous behavior of F2 in Ar is attributed to the short effective range of the repulsive F/Ar interaction, which enables the F atom to exit the cage and migrate in the crystal. It is also due in part to the F/Ar attractive potential, which is found strong enough to focus and stabilize the migration of the F product in ‘‘channels’’ within the lattice, following photolysis. Classical trajectories show long-range migration of the product atoms, of the scale of 30 Å, following the initial impulse provided by the photodissociation. This is the first system for which such long-range impulse-induced migration was found. The results of the simulations are analyzed focusing on the role of the initial state of F2 in the crystal, on the final sites occupied by the product atoms, and on the migration dynamics. Implications of the results for mechanisms of reactions in solids are discussed.
Vibronic perturbations in the Ã 2Πu–X̃ 2Πg transition of BO2: K-resonance crossings and the onset of chaotic behavior
Authors: Adam A. G. , Merer A. J. , Steunenberg D. M.
Published: July 2002

A detailed re-investigation of the (0,0) and 211 bands of the Ã 2Πu–X̃ 2Πg transition of 11BO2, near 545 nm, has been carried out from spectra taken at sub-Doppler resolution. Avoided crossings are found to occur between the 2Σ and 2Δ vibronic components of the v2=1 level of the Ã 2Πu state. These perturbations are the first known examples of K-resonance crossings, which result from an interplay of the electronic Λ-type doubling and the vibrational l-type doubling in a state where the electrostatic parameter ε is small. It turns out that perturbations of this type must always occur in a 2Π electronic state if the ratio A/εω2 is large enough, but have not been seen until now for lack of suitable data. Their existence allows an unusually complete description of the orbital angular momentum effects in both the Ã 2Πu and X̃ 2Πg states. It has been necessary to fit the avoided crossings by a full matrix treatment of the K resonance. In addition the Ã 2Πu state, although it appears to be unperturbed at lower resolution, shows an astonishing number of very small random rotational perturbations. These arise from vibronic coupling between the Ã 2Πu and X̃ 2Πg states of BO2 through the ν3 vibration, which has species σ+u; their presence is consistent with the explanation given by Kawaguchi et al. [Mol. Phys. 44, 509 (1981)] for the sizable negative anharmonicity of the ground state ν3 vibration. About 20% of the available ground state energy levels at 18 300 cm−1 appear to be active in causing level shifts and splittings in the Ã 2Πu state. The matrix elements are quite small, ranging up to about 600 MHz, but the great number of perturbations indicates the onset of chaotic behavior.
Vibrational energy and bimolecular reactions: Enhancement of the electron transfer derived product channels for quenching of Xe(3P2) and Kr(3P2) atoms by CFnCl4−n, C2FnCl6−n, and CF3CFClCF2Cl
Authors: Sobczynski R. , Setser D. W. , Slagle A. R.
Published: July 2002

The yields of XeCl(B,C) and KrCl(B,C) from the reactions of Xe(3P2) and Kr(3P2) metastable atoms with chlorofluoromethanes and chlorofluoroethanes are enhanced by the addition of vibrational energy to the molecule. The reactions were studied in a fast flow reactor with He or Ar carrier gas; vibrational energy was added to the molecules by multiphoton absorption from a CO2 laser pulse at fluences of 0.2–1.0 J cm−2. The enhancement of the XeCl(B,C) or KrCl(B,C) product was observed by monitoring the B–X fluorescence intensity in real time following excitations by the CO2 laser. Enhancement factors are reported for reactions with CF2Cl2, CFCl3, CF3CFCl2, CF2ClCF2Cl, CF2ClCFCl2, and CF3CFClCF2Cl; survey experiments are presented to demonstrate enhancement of XeCl* and XeBr* from vibrationally excited CF2BrCl and CF3CH2Br. The decay times of the XeCl(B) and KrCl(B) signals provide information about the relaxation rates of the vibrationally excited CFnCl4−n and C2FnCl6−n molecules. In order to have a reliable reference, the total quenching rate constants and the branching fractions for XeCl(B,C) and KrCl(B,C) formation for 300 K conditions also were measured for most of the compounds; the branching fraction for XeCl* formation form CF3CCl3 is large for a polyatomic molecule.
Features of the electronic potential energy surfaces of nitric acid below 7 eV
Authors: Bai Y.Y. , Segal G. A.
Published: July 2002

Large scale configuration interaction calculations have been carried out for the potential energy surfaces of the four lowest singlet states of nitric acid along the dissociation paths leading to OH (X 2Π) and NO2 in its ground X 2A1 and B 2B1 states. In order to gain a detailed picture of the dissociation dynamics, correlations are drawn between the forces experienced by these species upon photodissociation and observed oscillator strengths, vector coupling parameters, relative Λ doublet intensities and available energy distribution where this data is available. The calculations are in good agreement with experiment and support the conclusion that the transition to the lowest such excited state, 1 1A″, is a vibronic transition whose electric dipole transition polarization is in plane, i.e., that the overall vibronic state observed is of A′ symmetry. Dissociation from the ground state is shown to be associated with a tumbling motion of the OH due to the predominance of quadrupolar forces at intermediate distances.
On the modeling of the self-polarizing field for a coating of point polarizable species on an isotropic dielectric sphere
Authors: Xu Muliang , Afara Suzane Mamiche , Dignam Michael J.
Published: July 2002

Our previous model dealing with the optical properties of small uniformly coated spheres is used iteratively to give the effective polarizability for homogeneous isotropic dielectric spheres and compared with results predicted by the Lorentz expression. Better agreement with the Lorentz expression is obtained using a modification of the model used to evaluate the self-polarizing field for the coating.