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15 Apr 2000

Volume 112, Issue 15, pp. 6507-6940

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A quantitative study of non-Condon effects in the S2O math→math emission spectrum

F. Iachello, F. Pérez-Bernal, T. Müller, and P. H. Vaccaro

J. Chem. Phys. 112, 6507 (2000); http://dx.doi.org/10.1063/1.481314 (4 pages) | Cited 2 times

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A novel technique has been developed for the quantitative study of vibronically-resolved transition intensities in polyatomic molecules beyond the Condon approximation. Matrix elements of coordinate-dependent transition moment operators are evaluated analytically with the pertinent vibrational wave functions obtained by means of Lie algebraic methods. Experimentally-observed S2O math 1A′–math 1A′(π∗−π) emission intensities, in conjunction with previous Franck–Condon calculations, reveal pronounced non-Condon effects for vibronic bands terminating on higher-lying vibrational levels of the ground electronic state. The transition dipole moment is examined as a function of both the S–O and S–S local stretching coordinates. © 2000 American Institute of Physics.
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33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
33.70.Fd Absolute and relative line and band intensities
02.10.Ud Linear algebra
02.10.Xm Multilinear algebra
02.20.Sv Lie algebras of Lie groups

Measurement of radiation damping rate constants in nuclear magnetic resonance by inversion recovery and automated compensation of selective pulses

Jin-Hong Chen, Brian Cutting, and Geoffrey Bodenhausen

J. Chem. Phys. 112, 6511 (2000); http://dx.doi.org/10.1063/1.481223 (4 pages) | Cited 11 times

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A method is proposed to measure the radiation damping rate constant based on the analysis of the nonexponential recovery of the magnetization after inversion. It is applicable when the recovery is dominated by radiation damping rather than by relaxation processes. The accurate measurement of the radiation damping rate constant can be used to simplify a recent method for the compensation of radiation damping effects during the application of selective pulses [Chen, Jerschow, and Bodenhausen, Chem. Phys. Lett. 308, 397 (1999)]. This procedure can now be automated, as illustrated by applications to selective inversion and excitation. © 2000 American Institute of Physics.
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07.57.Pt Submillimeter wave, microwave and radiowave spectrometers; magnetic resonance spectrometers, auxiliary equipment, and techniques
33.25.+k Nuclear resonance and relaxation
76.60.-k Nuclear magnetic resonance and relaxation
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

The singlet–triplet separation in dichlorocarbene: A surprising difference between theory and experiment

Christopher J. Barden and Henry F. Schaefer

J. Chem. Phys. 112, 6515 (2000); http://dx.doi.org/10.1063/1.481601 (2 pages) | Cited 15 times

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The recently reported experimental singlet–triplet energy separation in CCl2, namely 3±3 kcal/mol, lies far below extant theoretical predictions. This problem is investigated here at much higher levels of theory than previously applied. The present theoretical prediction is 19.5±2 kcal/mol. © 2000 American Institute of Physics.
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31.15.bw Coupled-cluster theory
33.15.Dj Interatomic distances and angles
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Efficient real-space approach to time-dependent density functional theory for the dielectric response of nonmetallic crystals

F. Kootstra, P. L. de Boeij, and J. G. Snijders

J. Chem. Phys. 112, 6517 (2000); http://dx.doi.org/10.1063/1.481315 (15 pages) | Cited 38 times

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Time-dependent density functional theory has been used to calculate the static and frequency-dependent dielectric function ϵ(ω) of nonmetallic crystals. We show that a real-space description becomes feasible for crystals by using a combination of a lattice-periodic (microscopic) scalar potential with a uniform (macroscopic) electric field as perturbation in a periodic structure calculation. The induced density and microscopic potential can be obtained self-consistently for fixed macroscopic field by using linear response theory in which Coulomb interactions and exchange-correlation effects are included. We use an iterative scheme, in which density and potential are updated in every cycle. The explicit evaluation of Kohn–Sham response kernels is avoided and their singular behavior as function of the frequency is treated analytically. Coulomb integrals are evaluated efficiently using auxiliary fitfunctions and we apply a screening technique for the lattice sums. The dielectric function can then be obtained from the induced current. We obtained ϵ(ω) for C, Si, and GaAs within the adiabatic local density approximation in good agreement with experiment. In particular in the low-frequency range no adjustment of the local density approximation (LDA) band gap seems to be necessary. © 2000 American Institute of Physics.
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77.22.Ch Permittivity (dielectric function)
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

A complete basis set model chemistry. VII. Use of the minimum population localization method

J. A. Montgomery, M. J. Frisch, J. W. Ochterski, and G. A. Petersson

J. Chem. Phys. 112, 6532 (2000); http://dx.doi.org/10.1063/1.481224 (11 pages) | Cited 282 times

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It is shown that localization is necessary to preserve size consistency in nonlinear extrapolations of molecular energies. We demonstrate that the unphysical behavior of Mulliken populations obtained from extended basis set wave functions can lead to incomplete localization of orbitals by the Pipek–Mezey population localization method, and introduce a modification to correct this problem. The new localization procedure, called minimum population localization, is incorporated into the CBS-QB3 and the new CBS-4M model chemistries, and their performance is assessed on the G2/97 test set. The errors found for CBS-QB3 are comparable with those for the G3 and G3(MP2) (mean absolute deviation of 1.10, 0.94, and 1.21 kcal/mol, respectively, on the G2/97 test set). The CBS-4M is less accurate than the other models (mean absolute deviation of 3.26 kcal/mol on the G2/97 test set), but can be applied to much larger systems. The modified localization method resolves several problem cases found with CBS-4 and improves the reliability of CBS-QB3. © 2000 American Institute of Physics.
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31.15.-p Calculations and mathematical techniques in atomic and molecular physics

Mixed quantum-classical surface hopping dynamics

Steve Nielsen, Raymond Kapral, and Giovanni Ciccotti

J. Chem. Phys. 112, 6543 (2000); http://dx.doi.org/10.1063/1.481225 (11 pages) | Cited 53 times

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An algorithm is presented for the exact solution of the evolution of the density matrix of a mixed quantum-classical system in terms of an ensemble of surface hopping trajectories. The system comprises a quantum subsystem coupled to a classical bath whose evolution is governed by a mixed quantum-classical Liouville equation. The integral solution of the evolution equation is formulated in terms of a concatenation of classical evolution segments for the bath phase space coordinates separated by operators that change the quantum state and bath momenta. A hybrid Molecular Dynamics–Monte Carlo scheme which follows a branching tree of trajectories arising from the action of momentum derivatives is constructed to solve the integral equation. We also consider a simpler scheme where changes in the bath momenta are approximated by momentum jumps. These schemes are illustrated by considering the computation of the evolution of the density matrix for a two-level system coupled to a low dimensional classical bath. © 2000 American Institute of Physics.
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82.20.Wt Computational modeling; simulation

Maximum-entropy calculation of energy distributions

Douglas Poland

J. Chem. Phys. 112, 6554 (2000); http://dx.doi.org/10.1063/1.481226 (9 pages) | Cited 19 times

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We use the maximum-entropy method to calculate molecular energy distributions from the moments of the distribution which in turn can be obtained from the temperature dependence of the heat capacity. If one knows the temperature expansion of the heat capacity through the nth power of the temperature, this then gives the exact first (n+2) energy moments. We illustrate the method for the ideal gas (the Maxwell–Boltzmann distribution of kinetic energy) and then use a model function to show that if one knows four or more moments of the energy distribution this allows one to resolve two or more distinct peaks in this function. We examine argon above the critical pressure, a one-dimensional model, and the protein barnase, all of which exhibit bimodal energy distributions. © 2000 American Institute of Physics.
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05.20.Dd Kinetic theory
51.10.+y Kinetic and transport theory of gases
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A dipole-bound dianion

Piotr Skurski and Jack Simons

J. Chem. Phys. 112, 6563 (2000); http://dx.doi.org/10.1063/1.481228 (8 pages) | Cited 15 times

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The possibility of binding two electrons by the dipole potential of a molecule was examined earlier by us using model potentials. That study suggested that large dipole moments μ = qR and large charge separation distances R (or equivalently large charges q) would be required to achieve binding two electrons. For example, even with a charge q = 1.5 a.u. which might be achieved using di- or tri-valent cations, a dipole moment exceeding 15.922 D is needed. The presence of inner-shell electrons even further increases the value of μ that is required because the dipole-bound electrons’ orbital must be orthogonal to and excluded from such inner shells. In the present work, we discuss our efforts to find a real molecule that can actually bind two electrons to a single dipole site. Numerical results are presented for the mono- and dianions of a double 5-member carbon ring system substituted with a Ca atom and three superhalogen −PF5 groups. The dianion of this molecule is found to be geometrically stable and to have a vertical electron detachment energy of ca. 0.8 eV. Its two excess electrons occupy the same fully symmetric a1 molecular orbital localized at the electropositive Ca end of the neutral system as is routinely observed in dipole-bound monoanions. Although our final candidate is chemically unusual, it is hoped that our predictions about it will encourage others to search for more synthetically tractable alternatives. © 2000 American Institute of Physics.
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31.15.-p Calculations and mathematical techniques in atomic and molecular physics
33.80.Eh Autoionization, photoionization, and photodetachment
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility

Density functional study of carbonic acid clusters

P. Ballone, B. Montanari, and R. O. Jones

J. Chem. Phys. 112, 6571 (2000); http://dx.doi.org/10.1063/1.481229 (5 pages) | Cited 11 times

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Density functional calculations on carbonic acid H2CO3 are extended to clusters of up to five such units. The most stable forms are the linear, hydrogen-bonded analogs of the dimer with anti–anti orientation. We calculate structures and vibration frequencies, as well as the energy required to bend and stretch the linear isomers. Linear chains of up to ∼ 20 units should be favored over ring structures, and they have a tensile strength reminiscent of chains of water molecules. We also discuss planar, nonlinear structures as well as three-dimensional isomers. © 2000 American Institute of Physics.
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31.15.E- Density-functional theory
36.40.Mr Spectroscopy and geometrical structure of clusters
36.40.Cg Electronic and magnetic properties of clusters
33.15.Fm Bond strengths, dissociation energies
33.15.Bh General molecular conformation and symmetry; stereochemistry

Infrared spectra of the CS2−, CS2+, and C2S4+ molecular ions in solid neon and argon

Mingfei Zhou and Lester Andrews

J. Chem. Phys. 112, 6576 (2000); http://dx.doi.org/10.1063/1.481230 (7 pages) | Cited 6 times

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Laser ablation of transition metal targets with concurrent code position of CS2/Ne and CS2/Ar mixtures produces metal independent absorptions at 1206.8 and 1159.2 cm−1 in neon and 1200.5 and 1160.4 cm−1 in argon due to CS2+ and CS2−. Additional metal independent absorptions at 1385.2 cm−1 in neon and 1379.7 cm−1 in argon increase on annealing. Isotopic substitutions show that this vibration involves two equivalent CS2 subunits. Based on density functional theory calculations of structure and vibrational frequencies, the 1385.2 and 1379.7 cm−1 bands are assigned to the C2S4+ cation in solid neon and argon. Identical experiments using matrix samples doped with the electron trapping molecule CCl4 enhance the 1385.2 and 1379.7 cm−1 absorptions and further support the cation assignment. No evidence was found for the (CS2)2− anion in these experiments. © 2000 American Institute of Physics.
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33.20.Ea Infrared spectra
31.15.E- Density-functional theory
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.70.Jg Line and band widths, shapes, and shifts
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis

Photodissociation spectroscopy of Ca+(C2H4)

J. H. Holmes, P. D. Kleiber, D. A. Olsgaard, and K.-H. Yang

J. Chem. Phys. 112, 6583 (2000); http://dx.doi.org/10.1063/1.481231 (7 pages) | Cited 15 times

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We have studied Ca+(C2H4) by photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer over the spectral range 440–790 nm. Ca+ is the only photofragment observed. We find four absorption bands of the complex and assign them to metal-centered transitions correlating with excitation of Ca+(3d and 4p). Spectral assignment is supported by ab initio electronic structure calculations of the complex and isotope substitution experiments. Calculations find a weakly bound ground state equilibrium structure with C2V π-bonding geometry and a dissociation energy of De″ = 0.506 eV. Theoretical and experimental results show the 4pπ(2 2B2 & 2 2B1) excited states to be relatively weakly bound at long range. Spectral analysis gives vibrational constants for the Ca+--C2H4 intermolecular a1-stretch in the 1 2A1, 2 2B1, and 2 2B2 states, and for the CH2–CH2 a1-wag and the HCH a1-bend in 2 2B2. The results offer an interesting comparison with previous studies of similar weakly bound bimolecular complexes of light metal ions with alkene or alkane hydrocarbons. © 2000 American Institute of Physics.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
33.15.Ta Mass spectra
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Fm Bond strengths, dissociation energies
33.15.Mt Rotation, vibration, and vibration-rotation constants

OH vibrational activation and decay dynamics of CH4–OH entrance channel complexes

Martyn D. Wheeler, Maria Tsiouris, Marsha I. Lester, and György Lendvay

J. Chem. Phys. 112, 6590 (2000); http://dx.doi.org/10.1063/1.481232 (13 pages) | Cited 15 times

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Infrared spectroscopy has been utilized to examine the structure and vibrational decay dynamics of CH4–OH complexes that have been stabilized in the entrance channel to the CH4+OH hydrogen abstraction reaction. Rotationally resolved infrared spectra of the CH4–OH complexes have been obtained in the OH fundamental and overtone regions using an IR-UV (infrared-ultraviolet) double-resonance technique. Pure OH stretching bands have been identified at 3563.45(5) and 6961.98(4) cm−1 (origins), along with combination bands involving the simultaneous excitation of OH stretching and intermolecular bending motions. The infrared spectra exhibit extensive homogeneous broadening arising from the rapid decay of vibrationally activated CH4–OH complexes due to vibrational relaxation and/or reaction. Lifetimes of 38(5) and 25(3) ps for CH4–OH prepared with one and two quanta of OH excitation, respectively, have been extracted from the infrared spectra. The nascent distribution of the OH products from vibrational predissociation has been evaluated by ultraviolet probe laser-induced fluorescence measurements. The dominant inelastic decay channel involves the transfer of one quantum of OH stretch to the pentad of CH4 vibrational states with energies near 3000 cm−1. The experimental findings are compared with full collision studies of vibrationally excited OH with CH4. In addition, ab initio electronic structure calculations have been carried out to elucidate the minimum energy configuration of the CH4–OH complex. The calculations predict a C3v geometry with the hydrogen of OH pointing toward one of four equivalent faces of the CH4 tetrahedron, consistent with the analysis of the experimental infrared spectra. © 2000 American Institute of Physics.
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33.20.Ea Infrared spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Sn Rotational analysis
33.20.Tp Vibrational analysis
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.50.Dq Fluorescence and phosphorescence spectra
31.15.A- Ab initio calculations
33.70.Jg Line and band widths, shapes, and shifts
33.15.Bh General molecular conformation and symmetry; stereochemistry
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
33.40.+f Multiple resonances (including double and higher-order resonance processes, such as double nuclear magnetic resonance, electron double resonance, and microwave optical double resonance)
82.20.Rp State to state energy transfer

The third-order polarizability γ of C60: The role of low-lying two-electron excited singlet states Ag and Hg

Yasushi Nomura, Takashi Miyamoto, Toshiki Hara, Susumu Narita, and Tai-ichi Shibuya

J. Chem. Phys. 112, 6603 (2000); http://dx.doi.org/10.1063/1.481233 (5 pages) | Cited 4 times

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The third-order polarizability γ of the C60 molecule has been calculated for the third harmonic generation at several incident frequencies using two different schemes of the sum-over-state (SOS) method, and the contribution of the low-lying singlet excited states as the second intermediate states has been analyzed. The group-theoretical analysis of the SOS expression clarifies that the 1Ag and 1Hg states are the only states that contribute to the γ as the second intermediate states. For the numerical analysis, the electronic states previously obtained in the semiempirical CNDO/S approximation with the singly and doubly excited configuration interaction method are used. It is found that the inclusion of the doubly excited configurations is essential in evaluating the γ. Those excited 1Ag and 1Hg states which make significant positive contributions to the γ are practically the doubly excited states. In order to secure error-free SOS calculations, equivalent but independent calculations have been also carried out with the frequency-dependent moment method. © 2000 American Institute of Physics.
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36.40.Vz Optical properties of clusters
36.40.Cg Electronic and magnetic properties of clusters
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
31.50.Df Potential energy surfaces for excited electronic states
31.15.bu Semi-empirical and empirical calculations (differential overlap, Hückel, PPP methods, etc.)
31.15.vq Electron correlation calculations for polyatomic molecules

Theoretical investigation of the eight low-lying electronic states of the cis- and trans-nitric oxide dimers and its isomerization using multiconfigurational second-order perturbation theory (CASPT2)

R. Sayós, R. Valero, J. M. Anglada, and Miguel González

J. Chem. Phys. 112, 6608 (2000); http://dx.doi.org/10.1063/1.481234 (17 pages) | Cited 36 times

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In this work we have carried out ab initio electronic structure calculations, CASSCF/CASPT2 and CASSCF/MRCI-SD+Q with several Pople’s and correlation-consistent Dunning’s basis sets, of the planar cis- and trans-NO dimers for the lowest eight electronic (singlet and triplet) states. The geometry, frequencies, dipole moment, binding energy, and vertical excitation energies are predicted with an accuracy close to or even better than the best reported ab initio previous results for some of these properties, and in very good agreement with the available experimental data. CASPT2 optimized geometries show the existence of at least four shallow NO-dimers (i.e., two cis-(NO)2 (1A1 and 3B2) and two trans-(NO)2 (1Ag and 3Au)), although CASSCF optimization with CASPT2 pointwise calculations indicate the existence of other less stable dimers, on the excited states. Vertical excitation energies were calculated for these four dimers. For the cis-NO dimer, the ordering and the energy spacings between the excited states (i.e., 1A1, 3B2, 1B2, 2nd 1A1, 1A2, 3A2, 3B1, 2nd 3B1) are very similar to those found in a recent MRCI-SD study. The singlet cis-NO dimer (1A1) is the most stable one in almost quantitative accord with the experimental data, and in disagreement with previous density functional theory studies. A nonplanar transition state for the singlet trans ↔ cis isomerization has also been fully characterized. This leads to an almost negligible energy barrier which would originate a rapid isomerization to the most stable cis-NO dimer at low temperatures, in accord with the experimental difficulties to measure the properties of the trans-NO dimer. Not only are basis set superposition error corrections necessary to evaluate accurately the binding energies, but also to determine the NN distance of these symmetrical dimers. Some problems regarding the symmetry of the wave function were found for the symmetrical NO dimers and for the NO+NO asymptote, and several approximate solutions were proposed. © 2000 American Institute of Physics.
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31.15.E- Density-functional theory
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
82.30.Qt Isomerization and rearrangement
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
31.50.Df Potential energy surfaces for excited electronic states
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.15.xr Self-consistent-field methods

Proton-transport catalysis and proton-abstraction reactions: An ab initio dynamical study of X+HOC+ and XH++CO (X=Ne, Ar, and Kr)

Michael A. Collins, Simon Petrie, Andrew J. Chalk, and Leo Radom

J. Chem. Phys. 112, 6625 (2000); http://dx.doi.org/10.1063/1.481235 (10 pages) | Cited 9 times

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Ab initio potential energy surfaces have been constructed and used to carry out classical simulations of the reactions of X with HOC+ and of XH+ with CO (X=Ne, Ar, and Kr). The competition between rearrangement, X+HOC+→OCH++X, and abstraction, X+HOC+→XH++CO, has been examined, and found to favor abstraction in the cases where both processes are energetically allowed. The reaction of XH+ with CO is found to produce highly vibrationally excited [CHO]+ products. © 2000 American Institute of Physics.
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82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
31.15.A- Ab initio calculations
82.20.Kh Potential energy surfaces for chemical reactions
33.15.Mt Rotation, vibration, and vibration-rotation constants
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions

Ab initio calculation of energies and lifetimes of metastable dianions: The C22− resonance

T. Sommerfeld, F. Tarantelli, H.-D. Meyer, and L. S. Cederbaum

J. Chem. Phys. 112, 6635 (2000); http://dx.doi.org/10.1063/1.481236 (8 pages) | Cited 15 times

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Most small dianions known in the solid state and solutions cannot exist as isolated entities and decay in the gas phase by electron autodetachment. These dianions show rare-gas-like closed-shell electronic ground states and represent a new type of metastable system. Here we study the prototype closed-shell resonance C22− in the framework of the complex absorbing potential method. We investigate in detail a number of unsettled methodological issues. In particular, there is no “natural” choice of orbital set for closed-shell metastable states and we study several orbital sets as well as other basis set and correlation effects on resonance energy and width. Closed-shell resonances typically show several open decay channels and we compute partial widths for the three open channels of C22−. Finally, we study the complex potential energy curve and compare our bond lengths and vibrational frequencies with geometrical parameters which have been obtained ignoring the metastable character of C22−. © 2000 American Institute of Physics.
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33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
31.15.A- Ab initio calculations
33.15.Dj Interatomic distances and angles
33.20.Tp Vibrational analysis

Solvent-induced stabilization of the naphthalene anion by water molecules: A negative cluster ion photoelectron spectroscopic study

Svetlana A. Lyapustina, Shoujun Xu, J. Michael Nilles, and Kit H. Bowen

J. Chem. Phys. 112, 6643 (2000); http://dx.doi.org/10.1063/1.481237 (6 pages) | Cited 20 times

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We show that (a) only a single water molecule is needed to stabilize the naphthalene anion, (b) the EAa of naphthalene is −0.20 eV, in agreement with determinations by electron transmission spectroscopy, (c) the energetics are consistent with the number of waters required to stabilize the naphthalene anion, and (d) the excess electron is located on the naphthalene moiety of Nph1−(H2O)n. © 2000 American Institute of Physics.
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33.60.+q Photoelectron spectra
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy

Time-resolved k(E∗) measurements for dissociation of allyl iodide vibrationally excited via C–H overtones (v = 6)

Alexey V. Baklanov, Mattias Aldener, Bosse Lindgren, and Ulf Sassenberg

J. Chem. Phys. 112, 6649 (2000); http://dx.doi.org/10.1063/1.481239 (7 pages)

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The direct time-resolved measurements of the energy dependent rate constant k(E∗) have been carried out for dissociation of allyl iodide (AI) vibrationally excited via C–H overtones (v=6). Resonant two-photon ionization (R2PI) technique has been used for the detection of atomic iodine I(2P3/2) arising from the dissociation of photoexcited AI molecules. For R2PI detection a method with narrow-band vacuum ultraviolet radiation (VUV) was used. VUV radiation was generated by means of nonresonant frequency tripling of visible dye-laser radiation in gaseous xenon. Measured k(E∗) values were found to be in excellent agreement with those calculated within the microcanonical version of the statistical Rice–Ramsperger–Kassel–Marcus theory in its “phase space” or “loose” transition state limit. The canonical version of the same model is also in good agreement with experimental data from the literature on the dissociation of allyl iodide under thermal heating conditions where collisional excitation takes place. © 2000 American Institute of Physics.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
33.80.Gj Diffuse spectra; predissociation, photodissociation
31.50.Df Potential energy surfaces for excited electronic states
82.20.Pm Rate constants, reaction cross sections, and activation energies
34.50.Ez Rotational and vibrational energy transfer
33.80.Eh Autoionization, photoionization, and photodetachment

Photodissociation dynamics of propyne at 157 nm

S. Harich, J. J. Lin, Y. T. Lee, and X. Yang

J. Chem. Phys. 112, 6656 (2000); http://dx.doi.org/10.1063/1.481316 (10 pages) | Cited 10 times

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Photodissociation of propyne at 157 nm has been investigated using photofragment translational spectroscopy. Detailed investigation of various photofragments from the deuterated compounds CD3CCH and CH3CCD, as well as the unlabeled propyne provides a uniquely clear picture of an inherently complex process. Hydrogen atom elimination processes from both the CH3 group and the C☒C–H group have been clearly observed. H atom elimination from the methyl group appears to be a single dynamical process, while ethynyl H elimination shows two distinctive dynamical pathways with a ratio of 0.30 (fast): 0.43 (slow). The relative contribution of the atomic hydrogen elimination from the two different sites was determined to be 0.73 (ethynyl): 0.27 (methyl). Molecular hydrogen elimination processes have also been observed, but with a much smaller yield compared to the atomic hydrogen elimination (1:9.6). Comparison of the H2 HD and D2 photoproducts from various deuterated propyne molecules shows that the molecular hydrogen elimination process is not sensitive to the origin of the two hydrogen atoms. This implies that scrambling (or isomerization) of H atoms is important prior to dissociation at 157 nm excitation of propyne. Two different C–C bond breaking processes have also been observed; one process breaks the C–C single bond to form methyl and C2H radicals, while the other process forms CH2 and C2H2. The existence of the CH2 channel also indicates that isomerization of propyne is significant prior to dissociation. The relative branching ratio of these two channels is estimated to be 2.2:1 for CH3 and CH2 formation, respectively. © 2000 American Institute of Physics.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
82.30.Qt Isomerization and rearrangement

The resonant Auger electron spectrum of C 1s−1π∗ excited ethene: A combined theoretical and experimental investigation

Reinhold F. Fink, Stacey L. Sorensen, Arnaldo Naves de Brito, Andrus Ausmees, and Svante Svensson

J. Chem. Phys. 112, 6666 (2000); http://dx.doi.org/10.1063/1.481241 (12 pages) | Cited 15 times

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The resonant Auger electron spectrum for ethene has been calculated with an ab initio approach using configuration-interaction energies and wave functions for the intermediate core-excited and final states. The transition rates were determined by the “one-center approximation.” The role of vibrational relaxation on the line shapes was described by a moment method which considers the case of symmetric core holes and their localization due to the vibrational relaxation of the core-excited state. The core hole localization is investigated in some detail and is found to be extremely efficient in the C 1s−1π∗ excited state of ethene. Another property of the core-excited state is found to be the polarization of the valence electron density toward the core hole. We demonstrate this by using three different symmetric configuration interaction representations and one nonsymmetric Hartree–Fock representation for this state. A modified improved virtual orbitals method is described and employed to obtain virtual orbitals which give a compact description of this effect. The theoretical spectra obtained in this way are compared with a measured spectrum and assignment of the structures in the spectrum to electronic configurations is made. We find strong configuration mixing in the higher excited final states which is evidence for the breakdown of the one-particle picture. © 2000 American Institute of Physics.
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33.80.Eh Autoionization, photoionization, and photodetachment
31.50.Df Potential energy surfaces for excited electronic states
31.15.A- Ab initio calculations
31.15.vj Electron correlation calculations for atoms and ions: excited states
33.70.Jg Line and band widths, shapes, and shifts
34.50.Ez Rotational and vibrational energy transfer

The electronic origin and vibrational levels of the first excited singlet state of isocyanic acid (HNCO)

H. Laine Berghout, F. Fleming Crim, Mikhail Zyrianov, and Hanna Reisler

J. Chem. Phys. 112, 6678 (2000); http://dx.doi.org/10.1063/1.481242 (11 pages) | Cited 5 times

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The combination of vibrationally mediated photofragment yield spectroscopy, which excites molecules prepared in single vibrational states, and multiphoton fluorescence spectroscopy, which excites molecules cooled in a supersonic expansion, provides detailed information on the energetics and vibrational structure of the first excited singlet state (S1) of isocyanic acid (HNCO). Dissociation of molecules prepared in individual vibrational states by stimulated Raman excitation probes vibrational levels near the origin of the electronically excited state. Detection of fluorescence from dissociation products formed by multiphoton excitation through S1 of molecules cooled in a supersonic expansion reveals the vibrational structure at higher energies. Both types of spectra show long, prominent progressions in the N–C–O bending vibration built on states with different amounts of N–C stretching excitation and H–N–C bending excitation. Analyzing the spectra locates the origin of the S1 state at 32 449±20 cm−1 and determines the harmonic vibrational frequencies of the N–C stretch (ω3 = 1034±20 cm−1), the H–N–C bend (ω4 = 1192±19 cm−1), and the N–C–O bend (ω5 = 599±7 cm−1), values that are consistent with several ab initio calculations. The assigned spectra strongly suggest that the N–C stretching vibration is a promoting mode for internal conversion from S1 to S0. © 2000 American Institute of Physics.
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31.50.Df Potential energy surfaces for excited electronic states
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.50.Dq Fluorescence and phosphorescence spectra
33.20.Fb Raman and Rayleigh spectra (including optical scattering)
31.15.A- Ab initio calculations
33.80.Wz Other multiphoton processes
33.50.Hv Radiationless transitions, quenching

Rotational and translational energy distributions of CN(v = 0,J) from the hot atom reactions: H+XCN→HX+CN(v = 0,J), where X=Br, Cl, and CN

G. He, I. Tokue, and R. Glen Macdonald

J. Chem. Phys. 112, 6689 (2000); http://dx.doi.org/10.1063/1.481243 (11 pages) | Cited 2 times

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The dynamics of the reactions of translationally energetic H atoms with BrCN, ClCN, and (CN)2 was studied by determining both the rotational state distribution and the translational energy disposition of the CN product ground vibrational level. The reaction was carried out using H atoms with a most probable translational energy of 92 kJ mol−1. The CN radical was monitored by time- and frequency-resolved absorption spectroscopy using the CN red system (A 2Π←X 2Σ) (2,0) band near 790 nm. Sub-Doppler resolution spectroscopy was used to determine the initial translational temperature of the CN(0,J) product. The fraction of the available reaction exothermicity that appeared as CN(0) rotational energy, fR, for H+XCN→HX+CN was 0.034±0.006, 0.061±0.02, and 0.13±0.007, for X=Br, Cl, and CN, respectively. Likewise, the fraction of the available reaction exothermicity that appeared as relative product translational energy, fT, was 0.52±0.25, 0.52±0.20, and 0.59±0.05, for X=Br, Cl, and CN, respectively. The absolute reaction cross sections for the H+XCN→HX+CN reactions were also measured to be 0.03, 0.02, and 0.3×10−16 cm2 for X=Br, Cl, and CN, respectively. © 2000 American Institute of Physics.
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82.20.Rp State to state energy transfer
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.30.-b Specific chemical reactions; reaction mechanisms

Fluorescence and electronic absorption spectra of phthalan: Two-dimensional vibrational potential energy surface for the ring-puckering and flapping in the S1(π,π∗) state

Eugene Bondoc, Sachie Sakurai, Kevin Morris, Whe-Yi Chiang, and Jaan Laane

J. Chem. Phys. 112, 6700 (2000); http://dx.doi.org/10.1063/1.481244 (7 pages) | Cited 3 times

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The ring-puckering and ring-flapping vibrations of phthalan in its S1(π,π∗) electronic excited state have been studied using fluorescence excitation spectroscopy of jet-cooled molecules, dispersed fluorescence spectroscopy, and ultraviolet absorption spectroscopy. This electronic state has A1 symmetry resulting from a B2→B2 orbital transition. Thus type A absorption bands result from A1→A1 and B2→B2 transitions to the S1 vibronic levels. The ring-puckering levels for the S1(π,π∗) electronic state were determined for both the flapping ground (vF = 0) and excited states (vF = 1) and these were used to calculate both one- and two-dimensional potential energy surfaces which fit the observed spectra. In the S1(π,π∗) state phthalan was found to be planar and more rigid than in the ground state in terms of the puckering coordinate. However, the molecule is less rigid along the flapping coordinate. This study shows how several types of spectroscopy and computations must be used in conjunction with each other to attain a comprehensive analysis of the electronic excited state. © 2000 American Institute of Physics.
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33.50.Dq Fluorescence and phosphorescence spectra
33.20.Lg Ultraviolet spectra
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.50.Df Potential energy surfaces for excited electronic states
31.90.+s Other topics in the theory of the electronic structure of atoms and molecules (restricted to new topics in section 31)
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
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Low-energy electron-energy-loss spectroscopy of condensed acetone: Electronic transitions and resonance-enhanced vibrational excitations

M. Lepage, M. Michaud, and L. Sanche

J. Chem. Phys. 112, 6707 (2000); http://dx.doi.org/10.1063/1.481245 (9 pages) | Cited 4 times

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We report electron-energy-loss spectroscopy, within the incident electron energy range 1 to 19 eV, of solid films of acetone condensed at 18 K. The strong Rydberg progressions, which usually dominate the spectra in the gas phase, are found to completely disappear in the solid phase. In the absence of these transitions, the remaining broad bands centered at 4.3, 4.5, 6.2, 8.7, and 9.8 eV energy loss can be assigned to the 1 3A2(n→π∗), 1 1A2(n→π∗), 1 3A1(π→π∗), 1 3B1(σ→π∗), and 2 3A2(σ→π∗) valence electronic transition of acetone, respectively. A broad feature ranging from 11 to 16 eV and having a maximum around 13.8 eV is ascribed to several overlapping autoionizing excited states. From a comparison with infrared and Raman spectra, the energy-loss peaks observed below 1 eV are found to be due to excitation of the fundamental, overtone, and combination vibrational modes of the molecule. Their incident energy dependence is showing broad vibrational enhancement maxima at 4, 7, and 9 eV, which are attributed to the formation of single-particle or shape resonances of 2B1, 2A1, and 2A2 (or 2B2) symmetries, respectively. © 2000 American Institute of Physics.
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34.80.-i Electron and positron scattering
31.50.Df Potential energy surfaces for excited electronic states
33.15.Mt Rotation, vibration, and vibration-rotation constants

A phenomenological description of the anomalous behavior of the electrical double layer at low temperatures

Douglas Henderson

J. Chem. Phys. 112, 6716 (2000); http://dx.doi.org/10.1063/1.481246 (3 pages) | Cited 2 times

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A simple description that accounts for some recent simulations of the electric double layer of a charged hard sphere fluid that show anomalously large negative adsorption of the ions at a surface and an unusual positive temperature derivative for the capacitance at low temperatures is discussed. The treatment is phenomenological since it does not result from any controlled approximation but it is reasonably well founded. The mean spherical approximation expressions for the density profiles of this system, which do not exhibit anomalous behavior, are modified in light of an exact contact value condition, the Ornstein–Zernike theory of the critical point, and some plausible reasoning. The agreement with the simulation results is fairly good. At the moment, there is no theory that accounts for these recent simulation results. © 2000 American Institute of Physics.
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61.20.-p Structure of liquids
68.08.-p Liquid-solid interfaces
68.43.-h Chemisorption/physisorption: adsorbates on surfaces
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