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22 Jun 2000

Volume 112, Issue 24, pp. 10695-11109

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Detection and characterization of alkyl peroxy radicals using cavity ringdown spectroscopy

Michael B. Pushkarsky, Sergey J. Zalyubovsky, and Terry A. Miller

J. Chem. Phys. 112, 10695 (2000); http://dx.doi.org/10.1063/1.481705 (4 pages) | Cited 24 times

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Cavity ringdown spectra of the  2A′−math 2A″ electronic transition in the IR are reported for the methyl and ethyl peroxy radicals. Analysis of partially resolved rotational structure for the origin band of the transition provides information about both the and math states of CH3O2⋅. An estimate for the absorption cross section is determined from the CRDS absorption and the rate of radical–radical recombination. © 2000 American Institute of Physics.
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33.20.Ea Infrared spectra
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Sn Rotational analysis
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions

Ultrafast excited-state proton transfer and subsequent coherent skeletal motion of 2-(2′-hydroxyphenyl)benzothiazole

S. Lochbrunner, A. J. Wurzer, and E. Riedle

J. Chem. Phys. 112, 10699 (2000); http://dx.doi.org/10.1063/1.481711 (4 pages) | Cited 66 times

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The excited-state intramolecular proton transfer of 2-(2′-hydroxyphenyl)benzothiazole was investigated with 30 fs temporal resolution. The proton takes 60 fs to arrive at its S1 equilibrium position and the electron distribution changes on the same time scale. The molecule cannot adjust to the new structure of the H-chelate ring equally fast and starts to strongly vibrate in four low-frequency skeletal modes. This coherent wave packet motion is the cause of the experimentally observed strong signal oscillations that are only weakly damped despite the solvent environment. © 2000 American Institute of Physics.
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82.30.Qt Isomerization and rearrangement
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.53.-k Femtochemistry

Probing the nature of surface intersection by ab initio calculations of the nonadiabatic coupling matrix elements: A conical intersection due to bending motion in C2H

Alexander M. Mebel, Michael Baer, and Sheng H. Lin

J. Chem. Phys. 112, 10703 (2000); http://dx.doi.org/10.1063/1.481712 (4 pages) | Cited 28 times

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Nonadiabatic coupling matrix elements between the 1 2A′, 2 2A′, and 1 2A″ electronic states of the C2H radical are computed using ab initio full valence active space CASSCF method. The line-integral technique is then applied to study possible geometric phase effects. The results indicate the existence of a unique conical intersection due to CCH bending between the 1 2A′ and 2 2A′ states at the linear configuration in the vicinity of rCC = 1.35 Å and rCH = 1.60 Å. The line-integral calculations with ab initio nonadiabatic coupling terms confirm that when a path encircles the conical intersection, the line integral always produces the value π for the topological (Berry) phase and when a path encircles the two (symmetrical) conical interactions or none of them, the line integral produces the value of zero for the topological phase. © 2000 American Institute of Physics.
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31.15.A- Ab initio calculations
31.15.xr Self-consistent-field methods
02.10.Ud Linear algebra
02.10.Xm Multilinear algebra

Evidence of triplet ethylene produced from photodissociation of ethylene sulfide

Fei Qi, Osman Sorkhabi, and Arthur G. Suits

J. Chem. Phys. 112, 10707 (2000); http://dx.doi.org/10.1063/1.481713 (4 pages) | Cited 12 times

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Tunable synchrotron radiation has been used to probe the dissociation dynamics of ethylene sulfide, providing selective determination of the translational energy distributions of both excited (1D) and ground-state (3P) sulfur atoms, with momentum-matching to the ethylene cofragments. The results suggest the presence of a channel giving S(3P) in conjunction with triplet ethylene C2H4 (3B1u), and allow for the first experimental measure of the energy of the latter species near the equilibrium geometry, in which the two methylene groups occupy perpendicular planes. © 2000 American Institute of Physics.
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82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
33.80.Gj Diffuse spectra; predissociation, photodissociation
31.50.Df Potential energy surfaces for excited electronic states
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.80.Eh Autoionization, photoionization, and photodetachment
07.57.-c Infrared, submillimeter wave, microwave and radiowave instruments and equipment
07.60.-j Optical instruments and equipment

A computer simulation study of the static structure and dynamic properties of liquid C60 using Girifalco's potential

M. M. G. Alemany, C. Rey, O. Diéguez, and L. J. Gallego

J. Chem. Phys. 112, 10711 (2000); http://dx.doi.org/10.1063/1.481714 (3 pages) | Cited 5 times

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Using Girifalco's potential to describe the interaction between two C60 molecules, we performed computer simulations to analyze the static structure and dynamic properties of liquid C60, and calculated its diffusion constant and shear viscosity. Our dynamical calculations predict that this yet-unobserved liquid does not support collective phenomena. © 2000 American Institute of Physics.
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61.25.Em Molecular liquids
61.20.Ja Computer simulation of liquid structure
66.10.C- Diffusion and thermal diffusion
66.20.-d Viscosity of liquids; diffusive momentum transport
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back to top Theoretical Methods and Algorithms

Thermodynamic consistency of reaction mechanisms and null cycles

Guy Schmitz

J. Chem. Phys. 112, 10714 (2000); http://dx.doi.org/10.1063/1.481715 (4 pages) | Cited 3 times

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At equilibrium, the relationship K = kf/kr between the equilibrium constant and the rate constants of an elementary reaction is a consequence of the principle of detailed balancing. Out of equilibrium, it remains valid for most elementary reactions. In general, it is not valid for complex reactions out of equilibrium but its validity for the elementary steps of the mechanism may have important consequences for the interpretation of the experimental results. A related criterion of thermodynamic consistency of reaction mechanisms is the absence of null cycles defined as sets of different reactions taking place under given conditions and leading to no net stoichiometric change. © 2000 American Institute of Physics.
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82.60.Hc Chemical equilibria and equilibrium constants
82.20.Pm Rate constants, reaction cross sections, and activation energies

A numerical study on the performance of the multiconfiguration time-dependent Hartree method for density operators

A. Raab and H.-D. Meyer

J. Chem. Phys. 112, 10718 (2000); http://dx.doi.org/10.1063/1.481716 (12 pages) | Cited 12 times

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A numerical study based on the multiconfiguration time-dependent Hartree (MCTDH) method for the propagation of density operators is presented. Within the MCTDH framework, there exist two types of expansions of the density operator which employ different kinds of so-called single-particle density operators. The latter may either represent Hermitian operators (type I), or else ket-bra products of so-called single-particle functions (type II). The performance of these two types of representations is tested on three models for closed and open system dynamics. The open dynamics is induced for each system by Lindblad-type dissipation operators. We find that the MCTDH representation of type I is most efficient if the coupling between the degrees of freedom is weak, but if the temperature of the initial state and/or the strength of the dissipation is moderate. On the other hand, for strong coupling between the degrees of freedom, but for lower temperatures and for weak dissipation type II is more efficient. Furthermore, considering the open dynamics of the systems both types of MCTDH density operators can be very efficiently used to calculate absorption spectra. The Lindblad-type dissipation operator is shown, however, to capture only partially the effects of a real environment. © 2000 American Institute of Physics.
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31.15.xr Self-consistent-field methods

Quantum wave packet dynamics with trajectories: Implementation with distributed approximating functionals

Robert E. Wyatt, Donald J. Kouri, and David K. Hoffman

J. Chem. Phys. 112, 10730 (2000); http://dx.doi.org/10.1063/1.481717 (8 pages) | Cited 30 times

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The quantum trajectory method (QTM) was recently developed to solve the hydrodynamic equations of motion in the Lagrangian, moving-with-the-fluid, picture. In this approach, trajectories are integrated for N fluid elements (particles) moving under the influence of both the force from the potential surface and from the quantum potential. In this study, distributed approximating functionals (DAFs) are used on a uniform grid to compute the necessary derivatives in the equations of motion. Transformations between the physical grid where the particle coordinates are defined and the uniform grid are handled through a Jacobian, which is also computed using DAFs. A difficult problem associated with computing derivatives on finite grids is the edge problem. This is handled effectively by using DAFs within a least squares approach to extrapolate from the known function region into the neighboring regions. The QTM–DAF is then applied to wave packet transmission through a one-dimensional Eckart potential. Emphasis is placed upon computation of the transmitted density and wave function. A problem that develops when part of the wave packet reflects back into the reactant region is avoided in this study by introducing a potential ramp to sweep the reflected particles away from the barrier region. © 2000 American Institute of Physics.
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03.65.-w Quantum mechanics
47.10.-g General theory in fluid dynamics

Auxiliary basis sets for grid-free density functional theory

Kurt R. Glaesemann and Mark S. Gordon

J. Chem. Phys. 112, 10738 (2000); http://dx.doi.org/10.1063/1.481763 (8 pages) | Cited 3 times

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Density functional theory (DFT) has gained popularity because it can frequently give accurate energies and geometries. The evaluation of DFT integrals in a fully analytical manner is generally impossible; thus, most implementations use numerical quadrature over grid points. The grid-free approaches were developed as a viable alternative based upon the resolution of the identity (RI). Of particular concern is the convergence of the RI with respect to basis set in the grid-free approach. Conventional atomic basis sets are inadequate for fitting the RI, particularly for gradient corrected functionals [J. Chem. Phys. 108, 9959 (1998)]. The focus of this work is on implementation of and selection of auxiliary basis sets. Auxiliary basis sets of varying sizes are studied and those with sufficient flexibility are found to adequately represent the RI. © 2000 American Institute of Physics.
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31.15.E- Density-functional theory

Basis set convergence of correlated calculations on He, H2, and He2

Jae Shin Lee and Sun Yong Park

J. Chem. Phys. 112, 10746 (2000); http://dx.doi.org/10.1063/1.481718 (8 pages) | Cited 49 times

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The convergence behavior of the total and correlation energies of He, H2, and He2 with the increase of basis quality in the correlation-consistent basis sets, cc-pVXZ and aug-cc-pVXZ(X=D,T,Q,5,6), was studied to search for a proper extrapolation scheme to predict the accurate complete basis set (CBS) limits at the MP2, CCSD, and CCSD(T) level. The functional form employed for extrapolation is a simple polynomial including inverse cubic power and higher-order terms of the cardinal number X in the correlation-consistent basis set as well as exponential function. It is found that a simple extrapolation of two successive correlation-consistent basis set energies (total or correlation energies) using (X+k)−3 [k = 0 for MP2 and k = −1 for CCSD and CCSD(T) level] gives in general the most reliable (and accurate in case of total energy) estimates to the CBS limit energies. It is also shown that the choice of proper basis set, which can represent the electronic motions in the fragment and complex equally well, appears necessary for reliable estimate of the relative energies such as the binding energy of the complex. From the extrapolation of aug-cc-pV5Z and aug-cc-pV6Z energies with (X+k)−3, we obtained 21.3(21.4), 28.4(29.0) and 33.2(33.8) microhartrees as the CBS limit binding energy of He2 at the internuclear separation of 5.6 a.u. at the MP2, CCSD, and CCSD(T) level, respectively, with the values in parentheses representing the exact CBS limit binding energies. © 2000 American Institute of Physics.
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31.15.xp Perturbation theory
31.15.bw Coupled-cluster theory
02.60.-x Numerical approximation and analysis
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Tunneling and decay dynamics of math 1Σg+ outer well states in hydrogen

E. Reinhold, W. Hogervorst, and W. Ubachs

J. Chem. Phys. 112, 10754 (2000); http://dx.doi.org/10.1063/1.481719 (7 pages) | Cited 9 times

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We present an experimental study of the decay dynamics of rovibrational states confined in the outer minimum of the Hmath 1Σg+ potential of the hydrogen molecule. Specific (v,J) quantum states are populated using a two-step excitation process involving a pulsed extreme ultraviolet laser source. Decay products are selectively probed with additional laser pulses, using the timing of these pulses to determine lifetimes. The competition between dissociation and ionization, occurring at short internuclear distance after tunneling through the potential barrier, is addressed. Observed lifetimes and decay channels of math levels are consistent with a semiclassical description of tunneling of the nuclear motion through the potential barrier towards small internuclear distance. For the HD isotopomer breaking of the g−u symmetry is found to strongly affect the decay dynamics. © 2000 American Institute of Physics.
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33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
31.30.Gs Hyperfine interactions and isotope effects
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Vq Vibration-rotation analysis

HCl photodissociation on argon clusters: Effects of sequential solvation and librational preexcitation

Petra Žďánská, Petr Slavíček, and Pavel Jungwirth

J. Chem. Phys. 112, 10761 (2000); http://dx.doi.org/10.1063/1.481720 (6 pages) | Cited 16 times

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Photolysis of the HCl molecule surface solvated on clusters with 2 to 12 argon atoms is investigated by means of quantum molecular dynamics simulations. Two basic questions are addressed: (i) How does the cage effect change upon increasing the size of the cluster, and (ii) how can caging be influenced by an infrared (IR) excitation of HCl hindered rotation (libration) prior to ultraviolet (UV) photolysis. The efficiency of caging is discussed in terms of measurable quantities. In the time domain, temporary populations of the trapped hydrogen atom are monitored, while in the energy domain short-lived vibrational resonances are observed as a fine structure in the hydrogen kinetic energy distribution. While caging is negligible for the smallest clusters, it becomes more efficient upon increasing the cluster size, and for 12 solvent atoms the cage effect is already very strong. Finally, it is shown that while in the ground state the hydrogen atom points essentially toward the rare gas cluster, in excited librational states hydrogen is directed mostly away from argon atoms. As a consequence, caging of the photodissociating hydrogen atom in the case of a surface solvated HCl molecule can be efficiently “turned off” by librational preexcitation. © 2000 American Institute of Physics.
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82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
33.80.Gj Diffuse spectra; predissociation, photodissociation
82.30.Nr Association, addition, insertion, cluster formation
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.80.-b Photon interactions with molecules

High resolution pulsed field ionization–photoelectron study of CO2+(X 2Πg) in the energy range of 13.6–14.7 eV

Jianbo Liu, Wenwu Chen, C.-W. Hsu, M. Hochlaf, M. Evans, S. Stimson, and C. Y. Ng

J. Chem. Phys. 112, 10767 (2000); http://dx.doi.org/10.1063/1.481721 (11 pages) | Cited 22 times

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The vacuum ultraviolet pulsed field ionization–photoelectron (PFI–PE) spectra for CO2 have been measured in the energy range of 13.6–14.7 eV, revealing complex vibronic structures for the ground CO2+(X 2Πg) state. Many vibronic bands for CO2+(X 2Πg), which were not resolved in previous photoelectron studies, are identified in the present measurement based on comparison with available optical data and theoretical predictions. As observed in the HeI photoelectron spectrum of CO2, the PFI–PE spectrum is dominated by the symmetry allowed ν1+ (symmetric stretch) vibrational progression for CO2+(X 2Πg). However, PFI–PE vibronic bands due to excitation of the symmetry disallowed ν2+ (bending) and ν3+ (asymmetric stretch) modes with both odd quanta, together with the symmetry allowed even quanta excitations, are clearly discernible. The simulation of rotational contours resolved in PFI–PE vibronic bands associated with excitation to the (ν1+ = 0–1, ν2+ = 0–2, ν3+ = 0) vibrational levels has yielded accurate ionization energies for the formation of these vibronic states from CO2(X 1Σg+). © 2000 American Institute of Physics.
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34.90.+q Other topics in atomic and molecular collision processes and interactions (restricted to new topics in section 34)
33.60.+q Photoelectron spectra
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Sn Rotational analysis
33.80.-b Photon interactions with molecules

Time-dependent Hartree approaches for the study of intramolecular dynamics in dimer systems

Pierre-Nicholas Roy and John C. Light

J. Chem. Phys. 112, 10778 (2000); http://dx.doi.org/10.1063/1.481722 (9 pages) | Cited 2 times

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We apply and the time-dependent Hartree (TDH) method to the study of intramolecular dynamics in dimer systems. The HCl dimer is chosen as test case. Model calculations are performed on reduced dimensional representation of this system namely two-, three-, and four-dimensional ones. We assess the validity of different implementations of the TDH method including the account of direct correlations between coordinate pairs, and mixed quantum-classical and quantum-Gaussian wave packets treatments. The latter yields very good results compared to the fully quantal treatment. © 2000 American Institute of Physics.
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33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
31.15.xr Self-consistent-field methods

Vibrationally excited states of CH3CFCl2: Intramolecular vibrational redistribution and photodissociation dynamics

A. Melchior, X. Chen, I. Bar, and S. Rosenwaks

J. Chem. Phys. 112, 10787 (2000); http://dx.doi.org/10.1063/1.481752 (9 pages) | Cited 12 times

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Utilizing vibrationally mediated photodissociation of jet-cooled CH3CFCl2 enabled monitoring of the second (3νCH) and third (4νCH) overtones of the methyl in the ground electronic state. The excited molecules are photodissociated by ∼235 or 243.135 nm photons that further tag Cl(2P3/2)[Cl] and Cl(2P1/2)[Cl∗] isotopes or H photofragments via mass selective (2+1) resonantly enhanced multiphoton ionization. The vibrational spectra are characterized by a multiple peak structure related to C–H stretches and to Fermi resonating levels involving the CH3 deformation. The cooling in the expansion reduces the rotational and vibrational congestion and affords a determination of the splittings and the upper limits for homogeneous broadening of the transitions. The highest-frequency peak of 4νCH exhibits an additional splitting, related to coupling of the mixed stretch–deformation states with other modes of the molecule. The yield of all three photofragments increases as a result of preexcitation, demonstrating that the energy is not preserved in the excited bond but rather flows to the C–Cl bond. The initial vibrational state preparation not only enhances C–Cl and C–H bond cleavage but also affects the Cl∗/Cl branching ratio, as compared to the nearly isoenergetic one-photon 193 nm photolysis of vibrationless ground state CH3CFCl2, implying that it alters the photodissociation dynamics. © 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.Mt Rotation, vibration, and vibration-rotation constants
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)

Ab initio study of the electron-spin magnetic moments (g-factors) of C2−, CSi−, Si2−, LiC2, NaC2, and LiSi2

Pablo J. Bruna and Friedrich Grein

J. Chem. Phys. 112, 10796 (2000); http://dx.doi.org/10.1063/1.481723 (10 pages) | Cited 3 times

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The electron-spin g-shifts of the title radicals are calculated using second-order perturbation theory, a Breit–Pauli Hamiltonian, and multireference configuration interaction (MRCI) wave functions. The isotropic 〈Δg〉 values (Δg = g−ge) are −830 for C2−(X2Σg+),−6515 for CSi−(X2Σ+), and −7595 for Si2−(12Σg+), in ppm, with ppm = 10−6. For the X1A1 ground states of LiC2, NaC2, and LiSi2, they are −315, −290, and −7770 ppm, respectively. Experimental 〈Δg〉 values in Ar matrices, 3900 ppm for LiC2 and 3535 ppm for NaC2, do not agree with calculated results. However, those predicted for the π-type 12B1 excited state of LiC2 and NaC2 (3415 and 3635 ppm, with large parallel components of 9185 ppm for LiC2, and 10 350 ppm for NaC2) are in good agreement with the measured data. Thus, it appears that LiC2 and NaC2 were experimentally prepared, and their ESR spectra measured, in their lowest excited state 12B1. For the 12B1 state of LiSi2, 〈Δg〉 is about 9820 ppm. The latter value is mainly determined by a large g∥-shift of about 28 000 ppm. © 2000 American Institute of Physics.
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31.15.A- Ab initio calculations
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
31.15.vn Electron correlation calculations for diatomic molecules
31.15.vq Electron correlation calculations for polyatomic molecules

Fourier transform emission spectrum of the HCSi radical,  2Σ+–math 2Πi transition

R. Cireasa, D. Cossart, M. Vervloet, and J. M. Robbe

J. Chem. Phys. 112, 10806 (2000); http://dx.doi.org/10.1063/1.481724 (5 pages) | Cited 6 times

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A Fourier transform emission spectrum of the HCSi,  2Σ+–math 2Πi transition was observed by means of a Schüler-type discharge tube in the 9000–14 000 cm−1 spectral region. Three bands whose origins appeared at 12 934.406, 11 766.721, and 10 752.430 cm−1 were rotationally analyzed and assigned, respectively, to the (100)–(000), (000)–(000), and (000)–(100) bands. The new ab initio calculated A(math 2Π) spin–orbit constant agrees well with the experimental value. Weak rotational perturbations in the (100) level of the upper electronic state are likely due to highly excited vibrational levels of the ground state, whereas the “anomalous” increase of the spin–rotation constant in the (100) level of the ground state relative to the (000) level was shown to result of a Fermi interaction with the (020) 2Π Renner–Teller components. A comparison was made between the rotational constants of the (000) ground state level with those obtained in concomitant works. © 2000 American Institute of Physics.
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33.50.Dq Fluorescence and phosphorescence spectra
33.20.Kf Visible spectra
33.20.Ea Infrared spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants

Resonance enhanced multiphoton ionization of the hydrogen halides: Rotational structure and anomalies in Rydberg and ion-pair states of HCl and HBr

Ágúst Kvaran, Huasheng Wang, and Áshildur Logadóttir

J. Chem. Phys. 112, 10811 (2000); http://dx.doi.org/10.1063/1.481725 (10 pages) | Cited 15 times

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(2+1) resonance enhanced multiphoton ionization spectra have been recorded, simulated, and used to derive energies of rovibrational levels in the F(1Δ), E(1Σ+), and V(1Σ+) states for HCl (H35Cl and H37Cl) and HBr (H79Br and H81Br). Spectroscopic parameters derived for the F states compare nicely with those derived by others using conventional analysis methods. Clear evidence for near resonance interactions between the F and the V states is seen for the first time, both in HCl and HBr. Shape of curves for rotational level energy spacings versus rotational quantum numbers are found to depend characteristically on the nature of off-resonance interactions observed between the E and the V states. Model calculations for state interactions, based on perturbation theory, are performed for HCl. These prove to be useful to interpret observed perturbations, both qualitatively and quantitatively. Interaction strengths are evaluated for F to V and E to V state interactions. Variations observed in the intensity ratios of O and S line series to Q line series in vibrational bands of the V state for HCl are discussed and mechanisms of two-photon excitation processes are proposed. © 2000 American Institute of Physics.
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33.80.Wz Other multiphoton processes
33.80.Eh Autoionization, photoionization, and photodetachment
33.20.Sn Rotational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.50.Df Potential energy surfaces for excited electronic states
33.20.Vq Vibration-rotation analysis

Electron-impact ionization of the chlorine molecule

Pietro Calandra, Caroline S. S. O’Connor, and Stephen D. Price

J. Chem. Phys. 112, 10821 (2000); http://dx.doi.org/10.1063/1.481753 (10 pages) | Cited 17 times

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Relative partial ionization cross sections for the formation of Cl2+, Cl+ and Cl2+ from molecular chlorine have been recorded as a function of the ionizing electron energy. In these measurements particular attention has been paid to the efficient collection of fragment ions with high translational energies and the minimization of any mass-dependent discrimination effects. The cross sections show that at electron energies above the double ionization threshold the yield of fragment ions can be comparable with the ion yield of nondissociative ionization. Further analysis shows that at electron energies above 50 eV the yield of fragment ions from multiple ionization is comparable with the yield of fragment ions from single ionization: dissociative multiple ionization contributes 14% of the ion yield at 50 eV electron energy and 26% at 100 eV. The decay of Cl22+ by heterolytic cleavage to form Cl2+ is a result of approximately 5% of the dissociative double ionization events. This heterolytic process has a threshold of 41.8±1.5 eV. Electron-impact induced triple ionization to form long-lived Cl23+ ions has been detected for the first time. This nondissociative triple ionization process makes up approximately 2% of the triple ionization events and triple ionization is responsible for approximately 2% of the ion yield above 100 eV. The threshold for dissociative triple ionization is determined to be 65.3±1.5 eV, a value in good agreement with a trication precursor state energy derived from the kinetic energy release for the fragmentation of Cl23+ to Cl2+ and Cl+, which provides the first experimental estimate of the triple ionization energy of molecular chlorine. © 2000 American Institute of Physics.
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34.80.Gs Molecular excitation and ionization

Vibrational mode and collision energy effects on proton transfer in phenol cation–methylamine collisions

Ho-Tae Kim, Richard J. Green, and Scott L. Anderson

J. Chem. Phys. 112, 10831 (2000); http://dx.doi.org/10.1063/1.481726 (7 pages) | Cited 5 times

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Mass-analyzed threshold ionization has been used to prepare vibrationally state-selected phenol cations, that were then reacted with methylamine at collision energies ranging from 0.1 to 2 eV. Integral cross sections and product recoil velocity distributions are reported. Ab initio calculations of stationary points on the surface and RRKM (Rice–Ramsperger–Kassel–Marcus) analysis of complex lifetimes are also presented for comparison. The only reaction observed over the entire energy range is exoergic proton transfer (PT). For ground-state reactants, the PT cross section is reduced by increasing collision energy, such that the reaction efficiency declines from ∼71% at low Ecollision to ∼50% at 2 eV. Excitation of either v6a or v12 vibrations inhibits reaction over the entire collision energy range, with the effect being somewhat mode-specific and increasing with increasing Ecollision. At low Ecollision, both vibrational and collision energy inhibit reaction with similar efficiency. Collision energy effects diminish at high Ecollision, while vibration continues to have a strong effect. Product ion velocity distributions are approximately forward–backward symmetric at Ecollision ⩽ 1 eV, but are backward peaked at high energies. Mechanistic implications of these results are discussed. © 2000 American Institute of Physics.
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82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.20.Pm Rate constants, reaction cross sections, and activation energies

A theoretical study on the ionization of CO2 and CS2 with analysis of the vibrational structure of the photoelectron spectra

Kouichi Takeshita, Norihiro Shida, and Eisaku Miyoshi

J. Chem. Phys. 112, 10838 (2000); http://dx.doi.org/10.1063/1.481754 (7 pages) | Cited 6 times

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Ab initio calculations have been performed to study the molecular structures and the vibrational levels of the low-lying ionic states (2Πg, 2Πu, 2Σu+, and 2Σg+) of CO2 and CS2. The global regions of the potential energy surfaces have been obtained by multireference single and double excitation configuration interaction calculations. The vibrational calculations using the explicit vibrational Hamiltonians have been used for the vibrational analysis. The equilibrium molecular structures and the vibrational analysis of the ionic states are presented. The theoretical ionization intensity curves including the vibrational structures of the ionic states are also presented and are compared with the photoelectron spectra. © 2000 American Institute of Physics.
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34.50.Gb Electronic excitation and ionization of molecules
31.15.A- Ab initio calculations
33.60.+q Photoelectron spectra
31.15.vq Electron correlation calculations for polyatomic molecules
33.15.Mt Rotation, vibration, and vibration-rotation constants

Spectroscopy and relaxation kinetics of the perturbed CO(b 3Σ+,v′ = 0,1,2) and CO(a′ 3Σ+, v′ = 31–36, 40, and 41) levels and reinterpretation of CO(a′ 3Σ+, v′ = 34 and 35) formation in the Kr(5s′[1/2]0)+CO reaction

G. Zikratov, D. W. Setser, and N. Sadeghi

J. Chem. Phys. 112, 10845 (2000); http://dx.doi.org/10.1063/1.481756 (13 pages) | Cited 2 times

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The spectroscopic and kinetic consequences associated with the strong homogeneous interactions between (i) CO(b 3Σ+,v′ = 0) and CO(a′ 3Σ+, v′ = 30, 31, and 32), (ii) CO(b 3Σ+,v′ = 1) and CO(a′ 3Σ+, v′ = 34, 35, and 36), (iii) CO(b 3Σ+,v′ = 2) and CO(a′ 3Σ+, v′ = 40 and 41) levels are evaluated. Mixing of b 3Σ+ character into the a′ 3Σ+ levels reduces the radiative lifetimes of the latter, because the lifetimes of pure b 3Σ+ and a′ 3Σ+ states are approximately 50 ns and 3 μs, respectively. The strength of the interaction changes with rotational level, and the rotational line intensities and the radiative branching to different v″ levels of the CO(a′ 3Σ+–a 3Π) transition are strongly affected. Comparison of high resolution CO(a′, v′ = 31, 34, and 35) experimental spectra with calculated spectra shows a marked underestimation of the rotational temperature of these CO(a′,v′) levels unless the mixing is explicitly recognized. With benefit of this knowledge, some results from the Kr(5s′[1/2]0)+CO excitation-transfer reaction need to be reinterpreted. Emission spectra for 300 K rotational distributions from CO(b,v′ = 0, 1, and 2) and CO(a′,v′ = 30, 31, 34, and 35) were used to obtain vibrational band intensities for comparison with model calculations. Analysis of the pressure and time dependence of the laser-induced fluorescence data permit the electronic relaxation mechanism of the CO(b,v′ = 0, 1, 2) and CO(a′,v′ = 31 and 35) levels in He buffer gas to be discussed. The experimental radiative lifetimes of CO(b,v′ = 0,1,2) were measured as 60±6, 63±4, and 58±4 ns, respectively. The role of the homogeneously perturbed levels in the collisional relaxation mechanism is discussed. © 2000 American Institute of Physics.
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33.50.Dq Fluorescence and phosphorescence spectra
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.70.Fd Absolute and relative line and band intensities
34.50.Ez Rotational and vibrational energy transfer

The dynamic disorder of azulene: A single crystal deuterium nuclear magnetic resonance study

Thomas Bräuniger, Raphy Poupko, Zeev Luz, Peter Gutsche, Christoph Meinel, Herbert Zimmermann, and Ulrich Haeberlen

J. Chem. Phys. 112, 10858 (2000); http://dx.doi.org/10.1063/1.481727 (13 pages) | Cited 5 times

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Deuterium nuclear magnetic resonance measurements on single crystals of azulene, specifically deuterated in the 1 and 3 positions, are reported. The quadrupole coupling tensor of these deuterons was determined by rotation experiments, yielding Qzz = 182.2 kHz and η = 0.056, with Qxx, the intermediate component (magnitude wise), oriented perpendicular to the molecular plane. The deuterium signals are inhomogeneously broadened and their widths are strongly anisotropic. This is quantitatively interpreted in terms of alignment disorder, induced by polar (up–down) disorder already known to be present in azulene from earlier x-ray measurements. It is shown that the alignment disorder is due to a planar distribution in the orientation of the molecules about the short axis, with a root mean square deviation of ±1°. The linewidths are strongly temperature dependent and reduce from about 8.9 kHz (maximum width) at room temperature to 1.6 kHz at 67 °C. This effect is interpreted in terms of molecular “up–down” flips, which average out both the polar and the alignment disorder. The rate of this process is found to be 104 s−1 at 40 °C, with an activation energy of 65 kJ/mol. Magnetization transfer experiments were performed by selectively inverting the magnetization of one of the deuterium doublets, followed by monitoring the subsequent approach to equilibrium of the whole spectrum. The results show the presence of additional dynamic processes in the ultraslow motion regime. These include molecular π flips about their long axes, as well as jumps between different sites in the lattice. The rate of both processes is about 0.084 s−1 at 57 °C. The intersite jumps are predominantly of the flip type, which interchange crystallographic symmetry related deuterons. From the rate of this process, a self-diffusion constant of 0.35×10−22 m2 s−1, at 57 °C, is estimated. © 2000 American Institute of Physics.
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76.60.-k Nuclear magnetic resonance and relaxation
61.66.Hq Organic compounds

Time-dependent density functional calculations of molecular photoionization cross sections: N2 and PH3

M. Stener and P. Decleva

J. Chem. Phys. 112, 10871 (2000); http://dx.doi.org/10.1063/1.481755 (9 pages) | Cited 28 times

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A method based on the time-dependent density functional theory (TD-DFT) is proposed to calculate the photoionization cross section employing the explicit continuum wave-function, within a One Center Expansion and B-Splines radial basis set. The LB94 exchange-correlation potential with correct asymptotic behavior is employed. The results obtained for N2 and PH3 are in excellent agreement with the experimental data and are of comparable accuracy of ab initio methods. A deterioration is still present in the inner valence. For PH3 the effect of TD-DFT is dramatic and recovers completely the Kohn–Sham deficiency. The method has proven efficient for both valence and core ionization. © 2000 American Institute of Physics.
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31.15.E- Density-functional theory
33.80.Eh Autoionization, photoionization, and photodetachment

Collision-induced dissociation dynamics of Ar2+ at high levels of vibrational excitation

Yu-hui Chiu, Steve Pullins, Dale J. Levandier, and Rainer A. Dressler

J. Chem. Phys. 112, 10880 (2000); http://dx.doi.org/10.1063/1.481728 (10 pages) | Cited 10 times

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The collision-induced dissociation (CID) dynamics of the Ar2++Ar collision system are investigated at different Ar2+ internal energy distributions in a guided-ion beam (GIB) apparatus. The internal energy of reactant ions, assumed vibrational in a first approximation, is controlled by varying the position of ionization in a supersonic jet, electron impact ion source. Three conditions are investigated: cold, in which the ions are produced as vibrationally relaxed as possible; intermediate, in which a substantial shift in the CID onset is observed; hot, in which the apparent CID threshold is at near thermal collision energies. The vibrational distribution of the Ar2+ ions is probed at the same conditions by measuring the kinetic energy release of photofragment Ar+ following 2Σg+←2Σu+ photodissociation. The derived internal energy distributions are then used to model the observed CID cross sections with a modified line-of-centers approach to assess vibrational effects in the single-collision cross sections. The intermediate CID cross sections are consistent with a negligible vibrational enhancement beyond the statistical predictions. A substantial increase in cross section is observed when going from intermediate to hot conditions, despite a weak increase in internal energy, as apparent from the photodissociation measurements. Contributions from metastable states, not registered in the photodissociation experiment, can explain this disparity. © 2000 American Institute of Physics.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
34.50.Lf Chemical reactions
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.50.Df Potential energy surfaces for excited electronic states
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