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15 Sep 2003

Volume 119, Issue 11, pp. 5311-5764

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Determination of absolute photoionization cross sections for vinyl and propargyl radicals

Jason C. Robinson, Niels E. Sveum, and Daniel M. Neumark

J. Chem. Phys. 119, 5311 (2003); http://dx.doi.org/10.1063/1.1606440 (4 pages) | Cited 49 times

Online Publication Date: 28 August 2003

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Photofragment translational spectroscopy experiments employing tunable vacuum ultraviolet photoionization yielded absolute photoionization cross sections for vinyl and propargyl radicals at 10 eV of 11.1±2.2 and 8.3±1.6 Mb, respectively. From these values, the photoionization efficiency curves from 7.8–10.8 eV for these radicals were placed on an absolute scale. © 2003 American Institute of Physics.
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33.80.Eh Autoionization, photoionization, and photodetachment

What caliber pore is like a pipe? Nanotubes as modulators of ionic gradients

Ariel Fernández

J. Chem. Phys. 119, 5315 (2003); http://dx.doi.org/10.1063/1.1606439 (5 pages) | Cited 3 times

Online Publication Date: 28 August 2003

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Carbon nanotubes are shown to be modulators of ionic gradients possibly impacting artificial nanophysiology. In contrast with wider nanotubes, the ion traffic inside narrow nanotubes (∼9.45 Å diameter) eventually reaches steady stoichiometries which may be dramatically different from bulk ratios. The narrow nanotube reduces the polarizability of confined water so that charge imbalances inside cannot be sustained. © 2003 American Institute of Physics.
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61.46.-w Structure of nanoscale materials
47.60.-i Flow phenomena in quasi-one-dimensional systems
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back to top Theoretical Methods and Algorithms

General-model-space state-universal coupled-cluster theory: Connectivity conditions and explicit equations

Xiangzhu Li and Josef Paldus

J. Chem. Phys. 119, 5320 (2003); http://dx.doi.org/10.1063/1.1599283 (14 pages) | Cited 73 times

Online Publication Date: 28 August 2003

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We present a new version of the state-universal (SU), multireference, coupled-cluster (CC) theory that is capable of handling completely general, incomplete model spaces. This is achieved by exploiting the concept of “locality” for the active molecular spin orbitals and by introducing the constraining conditions (C conditions) on cluster amplitudes that are associated with the internal excitations transforming one reference configuration into another one. These C conditions make it possible to represent the exact (i.e., full configuration interaction) wave function via the SU CC cluster ansatz based on an arbitrary model space. The C conditions are then taken into account together with the standard SU CC equations for the external amplitudes, thus enabling us to reach the exact result in the limit, while preserving the connectivity property and thus the size extensivity. We also present compact expressions for the matrix elements of the effective Hamiltonian as well as the explicit expressions for the most important coupling coefficients that are required at the single and double excitation level. All other expressions are the same as in the single reference CC formalism. © 2003 American Institute of Physics.
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31.15.bw Coupled-cluster theory

N-reference, M-state coupled-cluster method: Merging the state-universal and reduced multireference coupled-cluster theories

Xiangzhu Li and Josef Paldus

J. Chem. Phys. 119, 5334 (2003); http://dx.doi.org/10.1063/1.1599302 (12 pages) | Cited 43 times

Online Publication Date: 28 August 2003

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We propose a generalization of the reduced multireference coupled-cluster method with singles and doubles (CCSD) to the genuine MR, state-universal (SU) CC approaches. Two key ingredients of this generalization are (i) the algorithm for the cluster analysis of general MR configuration interaction (CI) wave functions that is based on the SU cluster ansatz of Jeziorski and Monkhorst, and (ii) the formulation of the SU CC method employing a general (incomplete) model space. These recent developments enable us to employ modest size MR CISD wave functions that are based on an N-dimensional reference space M1 as a source of higher-than-pair-cluster amplitudes in the externally corrected SU CCSD method that is based on an M-dimensional model space M0, forming a subspace of M1. An appropriate choice of M0 and M1 makes it then possible to avoid the most severe intruder-state problems. The method is illustrated on the often-investigated H4 and H8 model systems. © 2003 American Institute of Physics.
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31.15.bw Coupled-cluster theory
31.15.V- Electron correlation calculations for atoms, ions and molecules

The general-model-space state-universal coupled-cluster method exemplified by the LiH molecule

Xiangzhu Li and Josef Paldus

J. Chem. Phys. 119, 5346 (2003); http://dx.doi.org/10.1063/1.1599335 (12 pages) | Cited 52 times

Online Publication Date: 28 August 2003

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The salient features of the recently introduced general-model-space (GMS) state-universal (SU) coupled-cluster (CC) method are illustrated on the case of the LiH molecule. Describing the breaking of the Li–H bond by relying on an open-shell-type GMS reveals the importance of the connectivity conditions (C conditions), which represent a crucial new ingredient of the GMS SU CC theory. Only when we properly account for these C conditions can we uniquely represent the full configuration interaction (FCI) wave functions in terms of the multireference SU exponential cluster ansatz and recover the FCI energies via the GMS SU CC method, assuming that all the relevant clusters at a given level of the theory are considered. Drawing on various GMSs, we compute the potential energy curves for three 1Σ+, two 3Σ+, three 1Π, and three 3Π states, using the GMS SU CC method truncated at the singly- and doubly-excited level (GMS SU CCSD), as well as the externally corrected (N,M)-CCSD method that exploits the NR-CISD wave functions as the external source of higher-than-pair clusters in the MR SU CCSD method. In all cases we obtain excellent results: For Σ+ states, the maximum difference between the FCI and various SU CCSD energies is about 0.5 millihartree. These errors are further reduced when we employ the (N,M)-CCSD methods. For the Π states, the deviations of the SU CCSD energies relative to FCI amount to at most a few hundreds of a millihartree. We also report on the size-extensivity tests and the exactness of the formalism for two-electron systems. © 2003 American Institute of Physics.
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31.15.bw Coupled-cluster theory
31.15.vn Electron correlation calculations for diatomic molecules

Density fitting for the decomposition of three-electron integrals in explicitly correlated electronic structure theory

Seiichiro Ten-no and Frederick R. Manby

J. Chem. Phys. 119, 5358 (2003); http://dx.doi.org/10.1063/1.1600431 (6 pages) | Cited 52 times

Online Publication Date: 28 August 2003

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It is shown that the convergence of R12-type explicitly correlated electronic structure theories can be improved by rearranging the three-electron integrals before using a resolution of the identity to decompose them into expressions involving only two-electron integrals. The new scheme is illustrated for some test systems within the explicitly correlated second order many-body perturbation theory. © 2003 American Institute of Physics.
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31.15.xp Perturbation theory

Multiconfigurational system-bath dynamics using Gaussian wave packets: Energy relaxation and decoherence induced by a finite-dimensional bath

Irene Burghardt, Mathias Nest, and Graham A. Worth

J. Chem. Phys. 119, 5364 (2003); http://dx.doi.org/10.1063/1.1599275 (15 pages) | Cited 35 times

Online Publication Date: 28 August 2003

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The quantum dynamics of an anharmonic oscillator coupled to a bath of up to 60 harmonic oscillators is investigated by a new multiconfigurational hybrid method for wave packet propagation. The method, originally proposed in [Burghardt, Meyer, and Cederbaum, J. Chem. Phys. 111, 2927 (1999)], represents a variant of the multiconfiguration time-dependent Hartree method including a moving basis of Gaussian functions. Energy relaxation and quantum decoherence induced by the zero-temperature oscillator bath are shown to be accurately described by the new method. Decoherence rates for a bath with a discretized ohmic spectral density are found to be consistent with golden-rule predictions for T = 0. © 2003 American Institute of Physics.
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31.15.-p Calculations and mathematical techniques in atomic and molecular physics
03.65.Ge Solutions of wave equations: bound states

Optimal reduced dimensional representation of classical molecular dynamics

Bijoy K. Dey, H. Rabitz, and Attila Askar

J. Chem. Phys. 119, 5379 (2003); http://dx.doi.org/10.1063/1.1599349 (9 pages) | Cited 6 times

Online Publication Date: 28 August 2003

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An optimal reduced space method for capturing the low-frequency motion in classical molecular dynamics calculations is presented. This technique provides a systematic means for carrying out reduced-dimensional calculations in an effective set of reduced coordinates. The method prescribes an optimal reduced subspace linear transformation for the low frequency motion. The method is illustrated with a dynamics calculation for a model potential, where the original six-dimensional space is reduced to two (three) dimensions, depending on the desired frequency cutoff value. © 2003 American Institute of Physics.
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71.15.Pd Molecular dynamics calculations (Car-Parrinello) and other numerical simulations
02.70.Ns Molecular dynamics and particle methods
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Kinetics of CH radicals with O2: Evidence for CO chemiluminescence in the gas phase reaction

Ghanshyam L. Vaghjiani

J. Chem. Phys. 119, 5388 (2003); http://dx.doi.org/10.1063/1.1599346 (9 pages)

Online Publication Date: 28 August 2003

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The production of CO vis-uv-chemiluminescence has been observed for the first time when a trace amount of CHBr3 vapor was photodecomposed in a pulsed-photolysis reactor using a 248-nm laser under multiphoton-dissociation conditions in an excess of O2 in diluent helium or argon carrier gas at 298 K and in the pressure range 2–95 Torr. The time-resolved chemiluminescent traces due to vibronic emissions at several band positions in the CO(A–X), CO(a–X), and CO(d–a) systems, and in the (1–0) and (0–0) bands of the OH(A–X) system were observed to decay nonexponentially. Their integrated intensities all showed quadratic dependence on the photolysis fluence employed. The OH and CO chemiluminescence data can be best interpreted by postulating the occurrence of O2 reactions with the methylidyne radicals in two different electronic states, CH(X 2Π) and CH(a 4Σ−). The production of CO(A) and CO(d) in these reactions require the methylidyne radicals to be internally (vibrationally) excited. The decay kinetics of the chemiluminescence was investigated in the presence of various added substrates, the buffer gas pressure, and in conditions of excess added CH4, which allowed us to isolate and study the CH(a 4Σ−)+O2 reaction, since the CH(X 2Π) could rapidly be removed from the system. The reactions of highly internally excited brominated radical species such as CBr, CHBr, and CBr2 with O2, in principle, can also produce CO chemiluminescence, but are argued to be of negligible importance in the present CHBr3/O2 photolysis system. © 2003 American Institute of Physics.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.80.Gj Diffuse spectra; predissociation, photodissociation

Dissociation of acetaldehyde in intense laser field: Coulomb explosion or field-assisted dissociation?

Mohamed E. Elshakre, Lirong Gao, Xiaoping Tang, Sufan Wang, Yafei Shu, and Fanao Kong

J. Chem. Phys. 119, 5397 (2003); http://dx.doi.org/10.1063/1.1598955 (9 pages) | Cited 6 times

Online Publication Date: 28 August 2003

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Dissociation of acetaldehyde in moderate strong laser field of 1013–1014 W/cm2 was investigated. Singly charged parent ion CH3CHO+ and fragmental ions CH3+, CHO+, C2H4+, O+, CH2CHO+, and H+ were produced by 800 nm laser of 100 fs pulse duration and recorded by time-of-flight mass spectrometer. The CH3+ fragment further dissociated to CH2+, CH+, and C+ ions at higher intensity. Ab initio calculated results show that the singly-, doubly-, and triply charged parent ions are stable. So, the dissociation mechanism was not due to Coulomb explosion of multicharged ion. A field-assisted dissociation (FAD) theory, which assumes that only one bond undergoes dissociation while the rest of the molecular geometry stays unchanged, was employed to treat the dissociation dynamics. Accordingly, the dressed potential energy surfaces of the ground state for the parent and the fragment ions were calculated. Corresponding quasiclassical trajectory calculations show that the bond ruptures take place in the order of C–C, C–O, and C–H, agreeing with the observation. The observed angular dependence and charge distribution of the product ions can also be interpreted by the FAD theory. © 2003 American Institute of Physics.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
82.50.Hp Processes caused by visible and UV light
82.37.Vb Single molecule photochemistry
33.15.Ta Mass spectra

Pulsed-field ionization zero electron kinetic energy spectroscopy and theoretical calculations of copper complexes: Cu–X(CH3)3 (X=N,P,As)

Shenggang Li, Bradford R. Sohnlein, Gretchen K. Rothschopf, Jason F. Fuller, and Dong-Sheng Yang

J. Chem. Phys. 119, 5406 (2003); http://dx.doi.org/10.1063/1.1598956 (8 pages) | Cited 9 times

Online Publication Date: 28 August 2003

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The copper complexes were produced in pulsed laser vaporization molecular beams and investigated by pulsed-field ionization zero electron kinetic energy (ZEKE) spectroscopy and second-order Møller–Plesset (MP2) perturbation and hybrid B3LYP density functional theory calculations. The ground electronic states of Cu–X(CH3)3 and Cu+–X(CH3)3 (X=N,P,As) are 2A1 and 1A1, respectively, both with C3v symmetry. From the ZEKE spectra, the adiabatic ionization potentials of the neutral molecules are determined to be 44 730, 41 508, and 42 324 cm−1, and the Cu+/Cu–X stretching frequencies are 268/199, 214/187, and 188/155 cm−1 for X=N, P, and As, respectively. The degenerate Cu+/Cu–P–C and Cu+/Cu–As–C bending frequencies are measured to be 146/83 and 118/52 cm−1, while the Cu+/Cu–N–C mode was not observed. In addition, the CH3 wag, X–C stretching, and XC3 umbrella modes are also measured for the phosphine and arsine complexes. From the MP2 theory, the dissociation energies of the Cu+ and Cu complexes are estimated to be 59/12, 70/15, and 65/11 kcal mol−1 down the X group. Both MP2 and B3LYP predictions of ionic vibrational frequencies compare well with the spectroscopic values, but the B3LYP calculations of neutral low frequency modes are less satisfactory. On the other hand, the B3LYP calculations yield better ionization potentials than the MP2 methods for these molecules. © 2003 American Institute of Physics.
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33.60.+q Photoelectron spectra
31.15.E- Density-functional theory
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
33.20.Tp Vibrational analysis

Twisted S1 excited state geometries in 4-dimethylaminobenzonitrile and dimethylaniline: New -d6 origin bands

Hiroyuki Saigusa, Naoki Miyakoshi, Chisato Mukai, Tomoyoshi Fukagawa, Shigeru Kohtani, Ryoichi Nakagaki, and Robert Gordon

J. Chem. Phys. 119, 5414 (2003); http://dx.doi.org/10.1063/1.1598959 (9 pages) | Cited 10 times

Online Publication Date: 28 August 2003

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The S1←S0 electronic spectra of 4-dimethylaminobenzonitrile-h6 and -d6 (DMABN) and dimethylaniline-h6 and -d6 have been reexamined, and new electronic origins have been observed for the -d6 species, approximately 65 cm−1 lower in energy than previously reported. The spectra of DMABN-h3d3 and several other isotopomers of DMABN are reported for the first time. A prominent low-frequency progression is assigned to dimethylamino torsion, and the S1 states are found to be twisted by about 26° with a small 190 cm−1 barrier to planarity. Other bands are tentatively assigned to inversion and methyl torsional motions. © 2003 American Institute of Physics.
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33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)

Success and failure of polarized-ion models: Bending and atomization energy of groups 2 and 12 dihalides

Kelling J. Donald, Willem H. Mulder, and László v. Szentpály

J. Chem. Phys. 119, 5423 (2003); http://dx.doi.org/10.1063/1.1599345 (14 pages) | Cited 4 times

Online Publication Date: 28 August 2003

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Polarized-ion models—with and without dipole reaction fields and charge-quadrupole interaction—are evaluated in calculating atomization energies, bond angles, and bending force constants of groups 2 and 12 dihalides. A study of the sensitivity to changes in the input parameters reveals a strong dependence on dipole polarizabilities. The charge–quadrupole bending terms of a multipole expansion are important for quasi-linear molecules, where the preceding terms in the expansion tend to cancel each other. They normally stabilize the linear geometry, but enhance bent structures, if a certain criterion for the ratio of quadrupole polarizabilities of metal and halide ions is fulfilled. The models explain the bent alkaline earth halides, but fail spectacularly for the group 12 halides—HgBr2 and HgI2 are even calculated as unstable relative to the constituent atoms. The unexpected weakness of ionic bonds is related to the large increase of the ionization energies by relativistic effects and the lanthanide contraction. © 2003 American Institute of Physics.
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31.15.-p Calculations and mathematical techniques in atomic and molecular physics
33.15.Fm Bond strengths, dissociation energies
33.15.Dj Interatomic distances and angles
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants

Energy levels and wave functions of weakly bound bosonic trimers using Pekeris coordinates and a symmetry-adapted Lanczos approach

Pierre-Nicholas Roy

J. Chem. Phys. 119, 5437 (2003); http://dx.doi.org/10.1063/1.1599348 (7 pages) | Cited 22 times

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An approach is presented for the accurate calculation of the energy levels of weakly bound bosonic rare gas trimers. A Pekeris coordinate system is used in order to avoid the interdependent range problem associated with internuclear distance coordinates. The present choice of coordinates also permits a straightforward treatment of boson symmetry and allows one to assess the importance of linear configurations through the calculation of one-body reduced distribution functions. The discrete variable representation is used and the eigenvalue problem is solved using a Lanczos recursion combined with projection operator techniques in order to extract totally symmetric states. Lennard-Jones systems corresponding to Argon and Neon are studied in order to assess the accuracy of the method by comparing them with those of recent benchmark calculations. The approach allows the accurate calculation of several bound states. Calculations are also performed for the case of a Morse interaction potential and the results are compared to those studies based on the commonly used internuclear distance coordinates. The present approach is found to be more accurate.© 2003 American Institute of Physics.
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05.30.Jp Boson systems
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

Vibrational analysis of small Hn+ hydrogen clusters

M. Barbatti and M. A. C. Nascimento

J. Chem. Phys. 119, 5444 (2003); http://dx.doi.org/10.1063/1.1599350 (5 pages) | Cited 10 times

Online Publication Date: 28 August 2003

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Anharmonic vibrational frequencies are calculated for the Hn+ clusters (n = 5,7,9) employing the correlation-corrected vibrational self-consistent field method. The present results confirm previous experimental assignments and also allow us to assign the 6690 cm−1 band of the H5+ cluster. The results explain the lack of features in the photodissociation spectra of the H7+ and H9+ clusters, in the region of 3000–3600 cm−1. The calculation also provides enough reliable data for the identification of these clusters in hydrogen atmospheres. The nature of the excitations near 4000 cm−1 in the vibrational spectra of the H7+ and H9+ clusters and their implication in the mechanism of photodissociation of these cluster are also discussed. © 2003 American Institute of Physics.
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33.20.Tp Vibrational analysis
36.40.Mr Spectroscopy and geometrical structure of clusters
33.80.Gj Diffuse spectra; predissociation, photodissociation

Exclusion surfaces for molecules in argon and helium

John Bentley

J. Chem. Phys. 119, 5449 (2003); http://dx.doi.org/10.1063/1.1600435 (8 pages)

Online Publication Date: 28 August 2003

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Molecular exclusion surfaces [J. Bentley, J. Phys. Chem. A 104, 9630 (2000)] have been determined for a number of molecules and molecular ions interacting with argon or helium atoms. These surfaces represent the boundaries between the molecules and their environment and have application, for instance, to continuum solvation models. To make these surfaces useful for such applications, we present some simple models for exclusion surfaces which depend only on properties of the electron densities of the isolated molecules. © 2003 American Institute of Physics.
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82.30.Nr Association, addition, insertion, cluster formation

Rotational spectroscopic investigation of carbonyl sulfide solvated with helium atoms

Yunjie Xu and Wolfgang Jäger

J. Chem. Phys. 119, 5457 (2003); http://dx.doi.org/10.1063/1.1598953 (10 pages) | Cited 47 times

Online Publication Date: 28 August 2003

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Rotational spectra of small- to medium-sized HeN–OCS clusters with N from 2 to 8 were investigated using a pulsed molecular beam Fourier transform microwave spectrometer and a microwave-microwave double resonance spectrometer. Spectra of five to eight isotopomers were measured for each HeN–OCS cluster. Nuclear quadrupole hyperfine structures were observed for 33S and 17O containing isotopomers. The isotopic data, together with the microwave-microwave double resonance experiments, were used to establish unambiguous assignments for these clusters. The assignment of N, the number of solvating helium atoms in a particular cluster, was supported by the observed intensity change under different backing pressure and nozzle cooling conditions and in particular by the systematic infrared vibrational band shifts from the concurrent infrared study. This sequence of clusters contains some of the largest clusters that have thus far been investigated with high-resolution spectroscopic techniques. This study provides detailed and important information for the understanding of the quantum solvation process of dopant molecules in helium nanodroplets. © 2003 American Institute of Physics.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.30.Gs Hyperfine interactions and isotope effects
82.30.Nr Association, addition, insertion, cluster formation
33.20.Bx Radio-frequency and microwave spectra
33.20.Sn Rotational analysis

High-resolution infrared spectra of carbonyl sulfide solvated with helium atoms

Jian Tang and A. R. W. McKellar

J. Chem. Phys. 119, 5467 (2003); http://dx.doi.org/10.1063/1.1598954 (11 pages) | Cited 45 times

Online Publication Date: 28 August 2003

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Infrared spectra of HeN–OCS clusters with N up to about 20 have been studied in the 2062 cm−1 region of the O–C stretching vibration using a tunable diode laser spectrometer to probe pulsed supersonic expansions from moderately high-pressure (<35 atm) cooled (>−60 °C) jet sources. Resolved rotation-vibration transitions have been assigned for the clusters with N = 2–8, and these assignments have been confirmed in detail by observations of the corresponding pure rotational transitions in the microwave region. The vibrational frequencies (band origins) were observed to move to higher frequencies (blueshift) for N = 2–5, continuing the shift previously observed for the binary complex, He–OCS. Then, for N = 6–8, the vibrational frequency moves back in the direction of lower frequencies (redshift), leading towards the limiting redshift previously observed in larger helium nanodroplets with N ≈ 103–104. These vibrational shifts are consistent with a model in which the first five helium atoms fill a “ring” around the “equator” of the OCS molecule, with the subsequent heliums then taking positions closer to the ends. The cluster rotational constants decrease monotonically (increasing rotational moments of inertia) for N = 1–8, falling below the value previously observed for the “free” rotation of OCS in helium nanodroplets. Strong, sharp spectral lines from clusters in the size range N ≈ 9–20 were also observed, but not assigned. This difficulty in extending the analysis for N>8 may be related to the onset of partially free internal rotation of the OCS within the helium cluster, which in turn has interesting implications for the nature of superfluid-type behavior in this finite scale system. © 2003 American Institute of Physics.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.20.Ea Infrared spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
33.20.Vq Vibration-rotation analysis
33.70.Jg Line and band widths, shapes, and shifts
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

Theoretical simulations on photoexcitation dynamics of the silver atom embedded in helium clusters

Akira Wada, Toshiyuki Takayanagi, and Motoyuki Shiga

J. Chem. Phys. 119, 5478 (2003); http://dx.doi.org/10.1063/1.1599351 (9 pages) | Cited 10 times

Online Publication Date: 28 August 2003

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Quantum molecular dynamics simulations have been performed to study the photoexcitation Ag(5p 2PJ)←Ag(5s 2S1/2) in size-selected helium clusters considering electronically nonadiabatic transitions. We employed the hybrid method in which the electronic degree of freedom of Ag(2P) was treated quantum mechanically while the motions of helium atoms were described by the semiclassical path integral centroid molecular dynamics method in order to take the quantum fluctuation effect into account. It has been found that the dynamics after photoexcitation is dominantly nonadiabatic in all cluster sizes studied, and that nonadiabatic transitions are enhanced by quantum fluctuation of helium motions. Most of the photoexcited AgHen clusters decompose into an isolated Ag atom and free helium atoms within several picoseconds. However, Ag∗Hen (n = 1–5) exciplex formation was also found to occur for all helium cluster sizes studied, although this process is found to be minor. © 2003 American Institute of Physics.
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33.80.-b Photon interactions with molecules
31.15.xv Molecular dynamics and other numerical methods
82.33.Fg Reactions in clusters
36.40.Mr Spectroscopy and geometrical structure of clusters
71.15.Pd Molecular dynamics calculations (Car-Parrinello) and other numerical simulations

Ground state potential energy curves for He–Kr, Ne–Kr, Ar–Kr, and Kr2: Coupled-cluster calculations and comparison with experiment

Terence P. Haley and Sławomir M. Cybulski

J. Chem. Phys. 119, 5487 (2003); http://dx.doi.org/10.1063/1.1600434 (10 pages) | Cited 16 times

Online Publication Date: 28 August 2003

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Ab initio calculations were performed with the coupled-cluster single and double excitations with perturbative triples correction method using the augmented correlation consistent polarized triple, quadruple, and quintuple zeta basis sets [aug-cc-pVnZ (n = T,Q,5)] for four complexes: He–Kr, Ne–Kr, Ar–Kr, Kr2. For each complex and each basis set a fitted potential energy curve is given. The most accurate results were obtained with the aug-cc-pV5Z basis set supplemented with an additional (3s3p2d2 f1g) set of bond functions. Ab initio potentials were compared to experimental data and the best empirical potentials available. The agreement was good although calculated potentials were found to be too shallow. To better reproduce spectroscopic data, ab initio potentials were modified using a nonlinear least-squares procedure. The modified potentials were developed for He–Kr, Ne–Kr, Ar–Kr, and Kr2, as well as Ne–Ar and Ar2. They all compare favorably with the best available empirical potentials. © 2003 American Institute of Physics.
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31.15.A- Ab initio calculations
31.50.-x Potential energy surfaces
31.15.bw Coupled-cluster theory

Conformational analysis of the jet-cooled peptide mimetic ethylacetamidoacetate from torsion-rotation spectra

R. J. Lavrich, A. R. Hight Walker, D. F. Plusquellic, I. Kleiner, R. D. Suenram, J. T. Hougen, and G. T. Fraser

J. Chem. Phys. 119, 5497 (2003); http://dx.doi.org/10.1063/1.1599353 (8 pages) | Cited 7 times

Online Publication Date: 28 August 2003

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Rotational spectra of two conformers of the peptide mimetic, ethyl-acetamidoacetate, were measured in a molecular beam using a Fourier-transform microwave spectrometer. In each conformer, internal rotation of the acetyl methyl group gives rise to observable splittings in the spectrum. From analysis of the torsion-rotation interactions, the methyl group’s orientation has been determined in the principal axis frame of each conformer and is shown to unambiguously identify its conformational form. One conformer exists in the all-trans configuration and belongs to CS point group and the second, higher-energy conformer has C1 symmetry. Two separate theoretical fitting procedures are applied to assess the reliability of the structural information and are shown to be essentially equivalent. For example, methyl torsional barriers are 63.7(1) cm−1 versus 67.1(1) cm−1 and 64.8(1) cm−1 versus 67.5(1) cm−1 for the CS and C1 conformers, respectively, and principal axis orientations of the methyl groups agree to ±0.1°. The small differences in the torsional barriers and rotor axis angles for the two conformers are a result of a change in the orientation of the ethyl group on the other end of the molecule. The predicted energy ordering of these two conformers at the MP2/6-311G(d,p) level of theory is in disagreement with experimental observations. © 2003 American Institute of Physics.
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87.15.B- Structure of biomolecules
87.15.M- Spectra of biomolecules
33.20.Bx Radio-frequency and microwave spectra
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

Rho-axis-method Hamiltonian for molecules having one methyl rotor and C1 point-group symmetry at equilibrium

Isabelle Kleiner and Jon T. Hougen

J. Chem. Phys. 119, 5505 (2003); http://dx.doi.org/10.1063/1.1599354 (5 pages) | Cited 6 times

Online Publication Date: 28 August 2003

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Two modifications of the rho-axis-method torsion-rotation Hamiltonian for fitting spectra of methyl top molecules belonging to the Cs point group at equilibrium which are required to treat molecules with C1 equilibrium configurations are described: (i) The permutation-inversion group must be changed from G6 to G3, which causes the A1 and A2 species to coalesce into a single A species, and which also introduces the slight complication of separably degenerate E species. (ii) One term must be added to the second-order contact-transformation-reduced Hamiltonian and seven terms must be added to the fourth-order reduced Hamiltonian. Programming strategies for this Hamiltonian are also discussed. In particular, portions of the C1 computer code requiring complex arithmetic are indicated, and three schemes for checking the internal consistency of the C1 code are suggested. Application of this C1 Hamiltonian to ethylacetamidoacetate is given in the preceding paper. © 2003 American Institute of Physics.  
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31.15.xh Group-theoretical methods
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
33.15.Mt Rotation, vibration, and vibration-rotation constants

Interpolated potential energy surface and classical dynamics for H3++HD and H3++D2

Gloria E. Moyano and Michael A. Collins

J. Chem. Phys. 119, 5510 (2003); http://dx.doi.org/10.1063/1.1599339 (8 pages) | Cited 21 times

Online Publication Date: 28 August 2003

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A potential energy surface for H5+ has been constructed by a modified Shepard interpolation on a sparse set of data points, using second order Möller–Plesset perturbation theory. An improved version of the surface was also obtained by substituting the energy values at the data points with values evaluated using a coupled cluster treatment (with single and double excitations, and perturbative treatment of triple excitations). Classical simulations for the collisions between H3++HD and H3++D2 were carried out in order to calculate the total integral cross sections and rate coefficients for these systems. There is good agreement with earlier experimental data for rate coefficients at temperatures between 80 and 300 K, but the predicted rate coefficient for the reaction of H3++HD at 10 K deviates from the most recent experimental measurement, suggesting that quantum rather than classical reaction dynamics are necessary. © 2003 American Institute of Physics.
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82.20.Kh Potential energy surfaces for chemical reactions
31.15.xp Perturbation theory
31.15.bw Coupled-cluster theory
82.20.Db Transition state theory and statistical theories of rate constants

Current density maps, magnetizability, and nuclear magnetic shielding tensors of bis-heteropentalenes. I. Di-hydro-pyrrolo–pyrrole isomers

I. García Cuesta, R. Soriano Jartín, A. Sánchez de Merás, and P. Lazzeretti

J. Chem. Phys. 119, 5518 (2003); http://dx.doi.org/10.1063/1.1599337 (9 pages) | Cited 10 times

Online Publication Date: 28 August 2003

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Magnetic susceptibility and nuclear magnetic shielding at the nuclei of bis-heteropentalenes formed by two pyrrole units ([2,3−b], [3,2−b], [3,4−b], and [3,4−c] isomers) have been evaluated by a series of different approximations and a large Gaussian basis set. An ab initio model of magnetic field induced current density was obtained for four isomeric systems, showing that strong diamagnetic flow takes place within the π electrons. The π currents are responsible for exalted magnetic anisotropy and proton deshielding. The theoretical findings are used to assess a diatropicity scale for these molecules. © 2003 American Institute of Physics.
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33.25.+k Nuclear resonance and relaxation

Photodissociation of methyl iodide clusters in the A-band excitation: Photofragmentation excitation spectra of (CH3I)n by ultraviolet pump-CRD probe measurement

Fumiyuki Ito and Taisuke Nakanaga

J. Chem. Phys. 119, 5527 (2003); http://dx.doi.org/10.1063/1.1599832 (7 pages) | Cited 11 times

Online Publication Date: 28 August 2003

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Methyl iodide clusters (CH3I)n in a supersonic jet are photolyzed in an excitation of the valence A band with a tunable ultraviolet light source, and the iodine molecule (I2) thus produced is detected by cavity ring-down (CRD) spectroscopy. A production yield of I2 is measured in the region of 245–275 nm to obtain photofragment excitation (PHOFEX) spectra of (CH3I)n. The PHOFEX spectra show maximum at 250 nm ( ∼ 1000 cm−1 blueshifted from monomer) at low concentration, and a new peak at 260–265 nm ( ∼ 450 cm−1 redshifted) at high concentration. These two peaks are assigned to dimer and higher cluster (hexamer to decamer), respectively, based on our infrared-CRD study [Chem. Phys. 286, 337 (2003)]. The redshifted peak of the higher clusters indicates that perturbation of the excited state due to intermolecular interaction is stronger than in the dimer. © 2003 American Institute of Physics.
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36.40.Jn Reactivity of clusters
82.33.Fg Reactions in clusters
82.50.Hp Processes caused by visible and UV light
33.80.-b Photon interactions with molecules
33.20.Lg Ultraviolet spectra
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