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1 Jun 1996

Volume 104, Issue 21, pp. 8183-8832

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The optical and optical/Stark spectrum of iridium monocarbide and mononitride

A. J. Marr, M. E. Flores, and T. C. Steimle

J. Chem. Phys. 104, 8183 (1996); http://dx.doi.org/10.1063/1.471573 (14 pages) | Cited 42 times

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Supersonic molecular beam samples of iridium monocarbide IrC and iridium mononitride IrN were generated using a laser ablation/reaction source and characterized using high resolution (Δν<30 MHz FWHM) laser induced fluorescence spectroscopy. This is the first identification of gaseous IrN. Numerous strong band systems in the 18 800 to 14 360 cm−1 spectral range were assigned as the (v′,0) progression of the A 1Π–X 1Σ+ band system of IrN. The (1,0) and (0,0) bands were analyzed to produce a set of fine and hyperfine parameters. The electric field induced effects on the R(0) line of these bands were analyzed to produce permanent electric dipole moments: A 1Π(v=0) 2.78(2) D, A 1Π(v=1) 2.64(2) D, X 1Σ+(v=0)=1.66(1) D. The (0,0) band of the D2 Φ7/2−X 2Δ5/2 system of IrC was recorded and analyzed to produce a set of fine and hyperfine parameters. The electric field induced effects on the R(2.5) branch feature were analyzed to produce permanent electric dipole moments: D 2Φ7/2(v=0) 2.61(6) D and X 2Δ5/2(v=0) 1.60(7) D. Plausible electronic configurations consistent with the experimental observations are given. © 1996 American Institute of Physics.
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33.57.+c Magneto-optical and electro-optical spectra and effects
33.50.Dq Fluorescence and phosphorescence spectra

Isotopic substitution of a hydrogen bond: A near infrared study of the intramolecular states in (DF)2

Scott Davis, David T. Anderson, John T. Farrell,, and David J. Nesbitt

J. Chem. Phys. 104, 8197 (1996); http://dx.doi.org/10.1063/1.471604 (13 pages) | Cited 14 times

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High resolution near infrared spectra of the two high frequency intramolecular modes in (DF)2 have been characterized using a slit‐jet infrared spectrometer. In total, four pairs of vibration–rotation–tunneling (VRT) bands are observed, corresponding to K=0 and K=1 excitation of both the ν2 (‘‘bound’’) and ν1 (‘‘free’’) intramolecular DF stretching modes. Analysis of the rotationally resolved spectra provides vibrational origins, rotational constants, tunneling splittings and upper state predissociation lifetimes for all four states. The rotational constants indicate that the deuterated hydrogen bond contracts and bends upon intramolecular excitation, analogous to what has been observed for (HF)2. The isotope and K dependence of tunneling splittings for (HF)2 and (DF)2 in both intramolecular modes is interpreted in terms of a semiclassical 1‐D tunneling model. High resolution line shape measurements reveal vibrational predissociation broadening in (DF)2: 56(2) and 3(2) MHz for the ν2 (bound) and ν1 (free) intramolecular stretching modes, respectively. This 20‐fold mode specific enhancement parallels the ≥30‐fold enhancement observed between analogous intramolecular modes of (HF)2, further elucidating the role of nonstatistical predissociation dynamics in such hydrogen bonded clusters. © 1996 American Institute of Physics.
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33.20.Ea Infrared spectra
33.20.Vq Vibration-rotation analysis
82.20.Tr Kinetic isotope effects including muonium

Zeeman studies of the first excited states of palladium and metal free phthalocyanines in Shpol’skii matrix

S.‐C. Huang and W.‐H. Chen

J. Chem. Phys. 104, 8210 (1996); http://dx.doi.org/10.1063/1.471574 (6 pages)

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Phthalocyanines (Pcs) are planar molecules of relatively high symmetry and exhibit magneto‐optic effects in many ways. We report the Zeeman effect of the first excited states of palladium (Pd–) and free‐base (H2–) Pcs in a 5 K Shpol’skii matrix by direct observation of their fluorescence position shifts. From the experimental data an apparent angular momentum integral, coupling the lowest two Jahn–Teller stabilized crystal field split first excited states, Λ′, was found to be 2.7±0.1 ℏ for PdPc. No conclusive Λ′ was deduced for H2Pc but its value can be estimated to be around the theoretical value of 3 ℏ. The experimental data of H2Pc revealed a complex structure in the fluorescence band examined, which complicated the analysis of Λ′ but helped unravel the biexponential decay problem of the structure. We infer one narrower band in the structure has the previously reported lifetime of τ  6.3 ns, and the other broader band being ∼1.8 cm−1 higher, has τ  3.7 ns. We believe this composite situation also exists for the other tautomeric structure since it also exhibited biexponential decay. © 1996 American Institute of Physics.
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33.57.+c Magneto-optical and electro-optical spectra and effects
33.50.Dq Fluorescence and phosphorescence spectra

Theory of thermal effects in nuclear magnetic resonance spectra of metal hydrides undergoing quantum mechanical exchange

S. Szymański

J. Chem. Phys. 104, 8216 (1996); http://dx.doi.org/10.1063/1.471575 (14 pages) | Cited 10 times

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Thermal effects in nuclear magnetic resonance spectra of transition metal hydrides exhibiting resolved quantum mechanical exchange splittings are consistently explained. Interactions of the relevant spatial degrees of freedom of the hydride protons with a quantum mechanical thermal bath are described in terms of Wangsness–Bloch–Redfield (WBR) theory. Upon elimination of the vibrational modes which relax too quickly to be observed on the NMR time scale, the WBR equation for the remaining, slowly relaxing modes (exchange modes) is shown to be equivalent to the Alexander–Binsch equation for classically exchanging nuclei, where the standard spin–spin coupling term is replaced by (or augmented with) quantum exchange term. Numerical calculations were performed for a one‐dimensional model of the relevant spatial motions, where the vibrational relaxation effects were described in terms of two adjustable parameters only. The assumed motion includes correlated rotation of a pair of the hydride protons, where the interproton distance may vary with the rotation angle. These calculations confirm that the present approach affords a consistent theoretical reproduction of the effects observed experimentally, i.e., an increase of the effective splitting with increasing temperature, with a gradual emergence of stochastic exchange that ultimately leads to motionally narrowed NMR spectrum lacking any fine structure. © 1996 American Institute of Physics.
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76.60.-k Nuclear magnetic resonance and relaxation
71.70.Gm Exchange interactions

Dicarbocyanine dyes in methanol solution probed by depolarized Rayleigh and hyper‐Rayleigh light scattering

Ok‐Keun Song and C. H. Wang

J. Chem. Phys. 104, 8230 (1996); http://dx.doi.org/10.1063/1.471576 (7 pages)

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The hyper‐Rayleigh scattering (HRS) intensity of two symmetric carbocyanine dyes (1122 DEDC and 1144 DEDC, full names given in the text) in methanol is measured as a function of dye concentration. These dye molecules at equilibrium show a negligible permanent dipole moment. The low concentration data showing that the HRS intensity is proportional to the dye concentration are used to determine the first hyperpolarizability for each of these dyes. However, above a concentration ρb=0.1×10−3 M, the HRS intensity shows an anomalous concentration dependence. Above ρb, the HRS intensity shows a saturation behavior and it even decreases with increasing concentration at high dye concentration. The depolarization ratio of the HRS intensity is also measured as a function of dye concentration. At lowest concentration, the depolarization ratio is 0.18. As the dye concentration increases, the depolarization ratio also rapidly increases but the increase quickly saturates as the concentration exceeds ρb. The concentration dependence of the HRS intensity and depolarization ratio are interpreted as due to formation of molecular aggregates. The depolarized Rayleigh scattering (DRS) intensity is also measured as a function of dye concentration. The result of DRS corroborates well with that found in HRS. © 1996 American Institute of Physics.
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33.20.Fb Raman and Rayleigh spectra (including optical scattering)
33.70.Fd Absolute and relative line and band intensities

Theory of Raman scattering with pulses: Application to continuum Raman spectroscopy

J. Lu and Soo‐Y. Lee

J. Chem. Phys. 104, 8237 (1996); http://dx.doi.org/10.1063/1.471577 (8 pages) | Cited 11 times

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A theory of real‐time dependence of Raman scattering for a pulse‐mode laser is developed within second‐order perturbation theory and using the wavepacket terminology. The rate of spontaneous Raman emission with a pulse correctly reduces to the dynamical equivalent of the Kramers‐Heisenberg‐Dirac expression in the monochromatic limit. We apply the theory to continuum Raman scattering for short and long pulses and varying pulse carrier frequency. The rate of Raman emission as a function of time and pulse carrier frequency, from an initial ground vibrational state to various final vibrational states, is shown to be structureless for all pulses, and for pulses that are longer than the dissociation time the rate also rises and decays with the pulses. This is contrary to recent reports of recurring resonance fluorescence‐type structures at long times after the pulse has vanished. We explain why such structures are unphysical for continuum Raman scattering. Results are also presented for excitation from an initial first excited vibrational state. © 1996 American Institute of Physics.
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33.20.Fb Raman and Rayleigh spectra (including optical scattering)

New Rydberg–Rydberg transitions of the ArH and ArD molecules. I. Emission from np states of ArD

I. Dabrowski, D. W. Tokaryk, M. Vervloet, and J. K. G. Watson

J. Chem. Phys. 104, 8245 (1996); http://dx.doi.org/10.1063/1.471578 (13 pages) | Cited 13 times

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The ground electronic state of argon hydride has a repulsive potential apart from a long‐range van der Waals minimum, but the Rydberg excited states have bound potentials similar to those of the ion ArH+. These states can be described approximately in terms of united‐atom quantum numbers nl. We report here rotational analyses of the bands 5p→5s, 5p→6s, and 6p→5s of ArD, which help to further characterize the np Rydberg series. In ArH the bands 5p→5s and 6p→5s have broad lines because of predissociation in the lower state, and 5p→6s is difficult to analyze without further information. The present data are fitted with a Hund’s case (d) effective Hamiltonian. In previous work the 4p state was found to have a very small σ‐π splitting, but this does not hold for the higher np states, and is probably due to an accidental cancellation between electrostatic and polarizability contributions. On the other hand, the spin–orbit coupling decreases monotonically with n. Features of the rotational levels are discussed in terms of the high‐J limiting quantum numbers lJ=N−R and sJ=J−N, where R≡N+, in particular the effect of spin–orbit coupling on the levels with (lJ,sJ)=(−1,1/2) and (0,−1/2), which produces a tendency to Hund’s case e behavior in 4p, and a sharp avoided crossing in 6p. The corresponding avoided crossing in 5p would occur beyond the present range of observed J values.
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33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.20.Kf Visible spectra
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.20.Sn Rotational analysis

Isotropic second‐order dipolar shifts in the rotating frame

Matthias Ernst, Andrew C. Kolbert, Klaus Schmidt‐Rohr, and Alexander Pines

J. Chem. Phys. 104, 8258 (1996); http://dx.doi.org/10.1063/1.471579 (11 pages) | Cited 8 times

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An experiment is described that utilizes the truncation of the Hamiltonian in the rotating frame by a radio‐frequency field designed to yield an isotropic shift for the dipolar coupling. This approach allows the measurement of a normally orientation‐dependent coupling constant by a single isotropic value. The dipolar isotropic shift is closely related to the field‐dependent chemical shift in solids due to the second‐order dipolar perturbation observed in magic‐angle spinning experiments. In the rotating frame, larger shifts of up to 1000 Hz can be observed for the case of a one‐bond C–H coupling compared to a shift of a few Hertz in the laboratory‐frame experiment. In addition to the isotropic shift, a line broadening due to the P4(cos β) terms is observed when the experiment is carried out under magic‐angle sample spinning (MAS) conditions, leading to the requirement of higher‐order averaging such as double rotation (DOR) for obtaining narrow lines. As an application of this new experiment the separation of CH, CH2, and CH3 groups in a 2D spectrum under MAS is demonstrated. Implemented under DOR it could be used as a technique to select carbon atoms according to the number of directly attached protons. © 1996 American Institute of Physics.
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33.70.Jg Line and band widths, shapes, and shifts

Evolution of excitonic energy levels in ArN clusters: Confinement of bulk, surface, and deep valence shell excitons

J. Wörmer, R. Karnbach, M. Joppien, and T. Möller

J. Chem. Phys. 104, 8269 (1996); http://dx.doi.org/10.1063/1.471572 (10 pages) | Cited 20 times

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The evolution of excitonic energy levels (Wannier and Frenkel type) is investigated for ArN clusters in the range N=200–106 using fluorescence excitation spectroscopy. In the case of Wannier excitons, a pronounced blue shift of the absorption bands relative to the position in the infinite solid is observed. As a consequence of the lower dimensionality, the shift of the transition energy of surface excitons is considerably smaller than that of the bulk states of clusters. The evolution with size is discussed within several theoretical models for exciton confinement. In addition, model calculations are performed for bulk excitons which give good quantitative agreement with the experimental results. In the case of n=1 Frenkel or intermediate type excitons, there are blue and red shifts observed. The spectral shift of (3p→4s) and deep valence (3s→4p) excitations differs considerably. From the shift of the transition energies the exciton mass of the (3p→4s) exciton is derived. © 1996 American Institute of Physics.
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71.35.Aa Frenkel excitons and self-trapped excitons
36.40.Mr Spectroscopy and geometrical structure of clusters
73.22.-f Electronic structure of nanoscale materials and related systems

Radiative and nonradiative decay of electronically excited NCO

Scott A. Wright and Paul J. Dagdigian

J. Chem. Phys. 104, 8279 (1996); http://dx.doi.org/10.1063/1.471580 (13 pages) | Cited 11 times

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A study to observe higher vibrational levels in NCO(math 2Σ+) and the onset of predissociation in this molecule has been carried out. Laser fluorescence spectra have been recorded over the wave number range 27 300–32 900 cm−1, from the math(0,0,2)–math(0,0,0) band up through the math(1,0,0)–math(0,0,0) band. Vibrational assignments have been made for a number of newly observed math–math bands, and band origin wave numbers and upper level rotational constants have been derived from comparison of experimental spectra with simulations. Decay lifetimes for excitation of a large number of both assigned and unassigned excited vibronic levels have been determined. The onset of predissociation appears to occur at energies slightly below that of the math(0,0,0) level. © 1996 American Institute of Physics.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.20.Tp Vibrational analysis
33.50.Dq Fluorescence and phosphorescence spectra

Infrared laser jet spectroscopy of transition metal hexacarbonyl‐rare gas dimers

G. M. Hansford and P. B. Davies

J. Chem. Phys. 104, 8292 (1996); http://dx.doi.org/10.1063/1.471581 (9 pages) | Cited 4 times

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High resolution infrared absorption spectra of nine van der Waals complexes M(CO)6⋅Rg (M=Cr, Mo, W; Rg=Ar, Kr, Xe) formed in a supersonic jet expansion have been recorded near the 5 μm carbonyl stretching fundamental bands of the hexacarbonyl monomers. In each case a single red‐shifted perpendicular band was observed. It is shown that the spectral results are only consistent with a C3v symmetric top structure for each dimer; no effects due to internal motions are seen in the spectra. The M–Rg separations deduced from analysing the spectra are slightly larger than the separations calculated from the van der Waals radii. Red‐shifts of the band origin are partly explained by a simple vibrational dipole‐induced dipole model. © 1996 American Institute of Physics.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.20.Ea Infrared spectra

Orientation and energy dependence of NO scattering from Pt(111)

R. J. W. E. Lahaye, S. Stolte, S. Holloway, and A. W. Kleyn

J. Chem. Phys. 104, 8301 (1996); http://dx.doi.org/10.1063/1.471582 (11 pages) | Cited 27 times

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A classical molecular dynamics study is applied to simulate the scattering of NO from Pt(111) in the energy range of 0.3–1 eV. The solid consists of a large number of crystal atoms that interact via an anharmonic nearest‐neighbor potential. The NO–Pt(111) interaction potential is constructed as a pairwise additive potential with a well depth of 1 eV for the N end of the molecule towards the surface and purely repulsive for the O end. The in‐plane scattering results obtained with this model potential are compared with recent experiments for NO–Pt(111). The angular intensity distributions, the final translational energy, as well as the rotational energy distributions with the corresponding alignment are in qualitative agreement with those experimental results. A detailed examination of the collision dynamics shows that multiple collisions with the surface results predominantly in superspecular scattering. The rotational angular momentum of the scattered molecules exhibits a preference for cartwheeling alignment and the rotational energy distributions for specular and normal exit angles can be described with a Boltzmann distribution, whereas for grazing exit angles they are distinctly non‐Boltzmann. The latter structure results from a cutoff in the rotational excitation by the attraction of the well. The high rotational excitation clearly originates from molecules that initially are oriented with the O end towards the surface, whereas for the low rotational excitation this orientation preference disappears. © 1996 American Institute of Physics.
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34.35.+a Interactions of atoms and molecules with surfaces
34.50.Lf Chemical reactions

Two‐color multiphoton transitions in molecular beam electric resonance studies: Rotating wave versus Floquet, and on‐ versus off‐resonance, calculations

A. E. Kondo and William J. Meath

J. Chem. Phys. 104, 8312 (1996); http://dx.doi.org/10.1063/1.471583 (9 pages) | Cited 8 times

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A nonzero difference, d, between the diagonal dipole moment matrix elements, μjj, of two molecular states involved in either one‐ or two‐color multiphoton transitions, can have substantial impact on the temporal evolution and spectral behavior of the states. The effects of d≠0 are investigated in this paper for two‐color transitions in a two‐level system previously studied in one‐color molecular beam electric resonance (MBER) experiments on symmetric top molecules. The calculations suggest a two‐color analog to the one‐field experiments, where the flexibility furnished by the field parameters of the two continuous wave electric fields, including relative phase, can be used to advantage. Both exact Floquet calculations and the rotating wave approximation (RWA) are used in this study. Analytic RWA expressions for the one‐ and two‐color molecule‐laser(s) couplings are particularly useful in helping to interpret and/or predict the effects of d≠0. The novel aspects of two‐color laser‐molecule interactions, relative to the one‐field case, are emphasized. In addition to investigations related to MBER studies, this work contributes to the more formal aspects of two‐color laser‐molecule interactions. It is shown that very useful analytical two‐level RWA solutions for the on‐resonance temporal behavior of the molecular states are available, even in the presence of competing resonances, whereas off‐resonance numerically useful analytical results are available only when one multiphoton resonance dominates a transition. © 1996 American Institute of Physics.
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34.50.Lf Chemical reactions
31.50.Df Potential energy surfaces for excited electronic states

Theoretical study of bath‐induced coherence transfer effects on a time‐ and frequency‐resolved resonant light scattering spectrum. II. Energy mismatch effects

Y. Ohtsuki and Y. Fujimura

J. Chem. Phys. 104, 8321 (1996); http://dx.doi.org/10.1063/1.471603 (11 pages) | Cited 5 times

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Bath‐induced coherence transfer effects on a time‐ and frequency‐resolved resonant light scattering spectrum is theoretically investigated using the Markoff master equation. According to Eberly and Wódkiewicz, a general expression for an experimentally observable spectrum in terms of a molecular response function is derived within the density matrix formalism. To generalize our previous results of the bath‐induced coherence transfer which were derived based on a displaced harmonic oscillator model [Y. Ohtsuki and Y. Fujimura, J. Chem. Phys. 91, 3903 (1989)], an eigenstate basis is used to represent a relevant system for investigating characteristics of the transfer. By the present model, we clarify the dependence of the bath‐induced coherence transfer on the energy‐level structure of the intermediate states associated with the transfer, i.e., energy mismatch effects. It is shown that if the energy mismatch of these states is smaller than dephasing rates, the bath‐induced coherence transfer occurs resonantly. In the other cases, the energy mismatch brings about a modulation in the time evolution of the superposition state created by the bath‐induced coherence transfer, which strongly diminishes the efficiency of the transfer. The resonance condition is derived analytically and is confirmed by numerical calculations of quantum beats induced by the bath‐induced coherence transfer. The possibility of very rapid dephasing of a quantum beat signal which cannot be explained in terms of dephasing rates is also shown, when the transition moments have such values that give π‐phase‐shifted quantum beats in bath‐induced fluorescence. © 1996 American Institute of Physics.
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42.25.Fx Diffraction and scattering
42.30.Lr Modulation and optical transfer functions

Acid–base chemistry in the gas phase: The trans‐1‐naphthol⋅NH3 complex in its S0 and S1 electronic states

Susan J. Humphrey and David W. Pratt

J. Chem. Phys. 104, 8332 (1996); http://dx.doi.org/10.1063/1.471584 (9 pages) | Cited 21 times

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We deduce information about the dynamics of a proton transfer reaction between an acid and a base. Our probe is the fully resolved S1←S0 fluorescence excitation spectrum of the 1:1 complex of 1‐naphthol and ammonia in the gas phase. Analysis of this spectrum shows that the complex is planar in both electronic states, with the NH3 forming a nearly linear hydrogen bond to the hydroxy hydrogen atom of 1‐naphthol. The O–H...N heavy atom separation is R=2.86 Å and the barrier to rotation of the NH3 group about its C3 axis is V3=39.9 cm−1 in the S0 state. Excitation of the complex to its S1 state increases the acidity of 1‐naphthol, decreases the heavy atom separation to R=2.72 Å, and increases the torsional barrier to V3=46.5 cm−1. Modeling these changes using the Lippincott–Schroeder potential for the hydrogen bond shows that the photoinitiated heavy atom motion produces a significant decrease in the barrier to proton transfer in the S1 state. © 1996 American Institute of Physics.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
33.50.Dq Fluorescence and phosphorescence spectra
33.80.-b Photon interactions with molecules
31.15.A- Ab initio calculations

Single and multiple photon ionization of triethylamine

J. E. Mathis and R. N. Compton

J. Chem. Phys. 104, 8341 (1996); http://dx.doi.org/10.1063/1.471585 (7 pages) | Cited 3 times

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Single and multiple photon ionization photoelectron spectroscopy of triethylamine (TEA) was studied using a newly developed high‐resolution electron spectrometer which utilizes position sensitive detection. The adiabatic ionization potential of TEA was accurately determined using both single (7.47±0.04 eV) and multiphoton (7.53±0.10 eV) ionization photoelectron spectroscopy. Although excitation to both the S1 and S2 states can occur, multiphoton ionization always occurs out of the S1 state. When the cation dissociates, the distribution of photoelectron energies similarly reflects this partitioning between S1 and S2. © 1996 American Institute of Physics.
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33.60.+q Photoelectron spectra
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)

Quasiclassical trajectory calculations of photodissociation of Ar–H2O(math–math) and H2O(math–math)

Kurt M. Christoffel and Joel M. Bowman

J. Chem. Phys. 104, 8348 (1996); http://dx.doi.org/10.1063/1.471586 (9 pages) | Cited 18 times

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We present results of full‐dimensional quasiclassical trajectory calculations of the photodissociation of H2O(3νOH,math–math) and Ar–H2O(3νOH,math–math) at 243 and 218 nm, and compare the resulting OH rotational distributions, and also relate them to recent experiments of Nesbitt and co‐workers [D. F. Plusquellic, O. Votava, and D. J. Nesbitt, J. Chem. Phys. 101, 6356 (1994)]. The dynamics calculations make use of a new six degree‐of‐freedom potential for Ar–H2O(math), which is reported here. The potential is based on a previously reported ab initio H2O math‐state potential, a semiempirical Ar–OH(2Π) potential, and a semiempirical Ar–H potential, together with an appropriate switching function to ensure permutation symmetry with respect to the two H atoms. Initial conditions for the trajectories are obtained from a product of a Husimi phase‐space density for the Ar–H2O(math) intermolecular modes and a Wigner/classical phase‐space density for the H2O(math) intramolecular modes. The Husimi phase‐space density is derived from the ground‐state wave function for Ar–H2O(math), using a previous spectroscopically empirical potential. To assess the accuracy of the trajectory approach, trajectory calculations are also reported for math–math photodissociation of H2O in the ground vibrational state at 166 nm and compared with the corresponding full‐dimensional quantum wave packet calculations of von Dirke and Schinke. To further assess the accuracy of the  math‐state potential surface for H2O, calculations for H2O(4νOH,math–math) are also reported at 218 nm and compared with experiment. Rotation/vibration distributions of the OH fragment are also calculated for photodissociation of Ar–H2O(4νOH,math–math) at 218 nm. © 1996 American Institute of Physics.
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33.80.Gj Diffuse spectra; predissociation, photodissociation

Theoretical study of the unimolecular dissociation HO2→H+O2. II. Calculation of resonant states, dissociation rates, and O2 product state distributions

Abigail J. Dobbyn, Michael Stumpf, Hans‐Martin Keller, and Reinhard Schinke

J. Chem. Phys. 104, 8357 (1996); http://dx.doi.org/10.1063/1.471587 (25 pages) | Cited 49 times

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Three‐dimensional quantum mechanical calculations have been carried out, using a modification of the log‐derivative version of Kohn’s variational principle, to study the dissociation of HO2 into H and O2. In a previous paper, over 360 bound states were found for each parity, and these are shown to extend into the continuum, forming many resonant states. Analysis of the bound states close to the dissociation threshold have revealed that HO2 is a mainly irregular system and in this paper it is demonstrated how this irregularity persists in the continuum. At low energies above the threshold, these resonances are isolated and have widths that fluctuate strongly over more than two orders of magnitude. At higher energies, the resonances begin to overlap, while the fluctuations in the widths decrease. The fluctuations in the lifetimes and the intensities in an absorption‐type spectrum are compared to the predictions of random matrix theory, and are found to be in fair agreement. The Rampsberger–Rice–Kassel–Marcus (RRKM) rates, calculated using variational transition state theory, compare well to the average of the quantum mechanical rates. The vibrational/rotational state distributions of O2 show strong fluctuations in the same way as the dissociation rates. However, their averages do not agree well with the predictions of statistical models, neither phase space theory (PST) nor the statistical adiabatic channel model (SACM), as these are dependent on the dynamical features of the exit channel. The results of classical trajectory calculations agree well on average with those of the quantum calculations. © 1996 American Institute of Physics.
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33.15.Fm Bond strengths, dissociation energies
33.15.Mt Rotation, vibration, and vibration-rotation constants

Novel technique for real‐time monitoring of electron attachment to laser‐excited molecules

Lal A. Pinnaduwage and Panos G. Datskos

J. Chem. Phys. 104, 8382 (1996); http://dx.doi.org/10.1063/1.471588 (11 pages) | Cited 10 times

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We report a new experimental technique that is capable of monitoring electron attachment to laser‐excited molecules in real time; the time resolution is limited only by the time constant of the detection circuit and was ∼100 ps for the experiments reported here. This technique provides information on the lifetime of the excited states responsible for electron attachment, and also allows determination of electron attachment cross sections involved. Results on dissociative electron attachment to ArF‐excimer‐laser‐irradiated NO are reported: Electron attachment occurred to the A 2Σ+(ν=3) state populated via the absorption of a single photon, and to highly excited states populated via two‐photon absorption; the cross section for low‐energy electron attachment to the A 2Σ+(ν=3) state was ∼3 orders of magnitude larger compared to that for the A 2Σ+(ν=0). Decay of the electrons over the ∼200 ns lifetime of the A 2Σ+(ν=3) state was directly monitored. Negative‐ion formation that occurred via the A 2Σ+(ν=3) state was suppressed in the presence of CO2 due to collisional quenching of that state by CO2, and the reduction in the A 2Σ+(ν=3) state lifetime with increasing CO2 pressure was also observed. © 1996 American Institute of Physics.
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34.80.Ht Dissociation and dissociative attachment
34.80.Qb Laser-modified scattering

The sulfur reaction in small ionized carbonyl sulfide clusters

J. M. A. Frazão, J. M. C. Lourenço, M. Áurea Cunha, M. F. Laranjeira, J. Los, and A. M. C. Moutinho

J. Chem. Phys. 104, 8393 (1996); http://dx.doi.org/10.1063/1.471589 (12 pages)

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Ionic species obtained by electron impact on (OCS)n clusters produced by expansion of OCS mixed with argon at two different (OCS/argon) concentrations are studied as a function of stagnation pressure and electron energy. Ionization efficiency curves of homogeneous cluster ions (OCS)+n, homogeneous sulfur cluster ions S+m and inhomogeneous ions (OCS)nS+m are analyzed. Several reactions paths, ending in S+m, have been identified. The strong correlation between reaction path and cluster size is discussed. Finally, by minimization of the potential function created by dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions between OCS molecules, we propose the geometrical configurations of neutral (OCS)n (n=2–5) clusters. © 1996 American Institute of Physics.
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34.80.Gs Molecular excitation and ionization
36.40.Wa Charged clusters

Quasiclassical dynamics of the I2–Ne2 vibrational predissociation: A comparison with experiment

A. García‐Vela, J. Rubayo‐Soneira, G. Delgado‐Barrio, and P. Villarreal

J. Chem. Phys. 104, 8405 (1996); http://dx.doi.org/10.1063/1.471590 (8 pages) | Cited 31 times

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The vibrational predissociation dynamics of the I2(B,v)–Ne2 complex is investigated for several vibrational levels of I2, using a quasiclassical trajectory approach. The time evolution of the population of nascent I2 fragments is calculated. A model is proposed which reproduces the results of the classical trajectories, and allows to obtain the lifetimes associated with the dissociation of the two van der Waals (vdW) bonds. The classical lifetimes are higher in general than the experimental ones of Zewail and co‐workers [J. Chem. Phys. 97, 8048 (1992)]. The classical method appears to overestimate mechanisms of energy redistribution between the modes, which slow down the dissociation of the cluster. However, the behavior of the lifetimes with the initial iodine vibrational excitation is in very good agreement with experiment. A sequential path of fragmentation of the two weak bonds via direct predissociation is found to dominate, producing I2(B,v–2)+2Ne fragments. Although with smaller probability, alternative dissociation paths are observed involving statistical mechanisms of internal energy redistribution. In these paths, the energy initially transferred by the iodine heats the vdW modes without breaking the complex. Further energy transfer produces either simultaneous or sequential dissociation of the two weak bonds in a rather evaporative way, populating the v–2 and v–3 exit channels. © 1996 American Institute of Physics.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
36.40.Qv Stability and fragmentation of clusters
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

Four‐center reactions: A quantal model for H4

Marta I. Hernández and David C. Clary

J. Chem. Phys. 104, 8413 (1996); http://dx.doi.org/10.1063/1.471591 (11 pages) | Cited 23 times

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We develop a quantal model for studying four‐center reactions, A2+B2→2AB, and collision induced dissociation A2+B2→A+B2+A. The method involves using hyperspherical coordinates to describe vibrations of the A2 and B2 bonds and a global vibration and rotation of the exchange products. Application to the H4 system is presented, using a realistic potential energy surface. The reaction goes through a four‐center linear transition state located just above the dissociation threshold. In the energy range studied (5–5.5 eV), collision induced dissociation competes with the four‐center reaction and is the dominant process. It is found that vibrational energy, originally deposited in one of the diatomic partners, is much more efficient than translational energy in promoting reaction. Vibrational and rotational final distributions show that the products are internally hot. This simple quantal model, yet very demanding computationally, illustrates in detail many features of the H4 dynamics above the dissociation threshold, and could serve to study other four center reactions with trapezoidal or linear transition states. © 1996 American Institute of Physics.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.20.Kh Potential energy surfaces for chemical reactions

Tunneling currents in electron transfer reactions in proteins

Alexei A. Stuchebrukhov

J. Chem. Phys. 104, 8424 (1996); http://dx.doi.org/10.1063/1.471592 (9 pages) | Cited 38 times

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A new theoretical method for the analysis of the superexchange coupling and localization of electron tunneling pathways in long distance electron transfer reactions is introduced. The new method allows one to examine spatial distribution of microscopic quantum mechanical tunneling currents flowing through individual atoms, or to evaluate the relative probability that the tunneling electron will pass through an individual atom, in the intervening medium between donor and acceptor in the course of an electron transfer reaction. It is shown how the interatomic tunneling currents introduced in this paper can be calculated using methods of quantum chemistry. The method provides a rigorous theoretical framework for the description of the tunneling process in long‐range electron transfer reactions in proteins. The relation of the present theory of tunneling currents to the theory of pathways of Beratan and Onuchic is discussed. © 1996 American Institute of Physics.
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87.10.-e General theory and mathematical aspects
87.15.B- Structure of biomolecules

Calculation of dissociative attachment of electrons to diatomic molecules by the Schwinger–Lanczos approach

J. Horáček, F. Gemperle, and H.‐D. Meyer

J. Chem. Phys. 104, 8433 (1996); http://dx.doi.org/10.1063/1.471593 (9 pages) | Cited 7 times

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Numerical studies of resonant scattering of electrons by diatomic molecules with full account of the nonlocal level shift and resonance width operators are carried out with emphasis on the various approximations of the nonlocal potentials. The Schwinger–Lanczos approach proposed recently by Meyer, Horáček and Cederbaum [Phys. Rev. A 43, 3587 (1991)] is applied and its performance is investigated. The efficiency of the method is further improved by introducing a new local complex potential. Very accurate values of the dissociative attachment cross sections for a d‐wave resonance model are reported. © 1996 American Institute of Physics.
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34.80.Ht Dissociation and dissociative attachment

Velocity correlation functions, Fickian and higher order diffusion coefficients for ions in electrostatic fields via molecular dynamics simulation

Andreas D. Koutselos

J. Chem. Phys. 104, 8442 (1996); http://dx.doi.org/10.1063/1.471543 (7 pages) | Cited 9 times

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The dynamic and transport properties of swarms of ions in a uniform electrostatic field are studied by using a molecular dynamics method. For a representative system, K+ in Ar, using a universal interaction model potential, second and third order ion‐velocity correlation functions are determined at various field strengths. From them, Fickian diffusion coefficients parallel and perpendicular to the field, as well as higher order diffusion coefficients, Qzzz, are obtained within estimated overall accuracy 5% and 7%, respectively. Comparisons of the Fickian diffusion coefficients against results of the moment solution of Boltzmann kinetic equation and a Monte Carlo simulation method using the same interaction potential as well as against experimental data, reveal consistency among all calculation procedures and in addition agreement with drift tube measurements. These comparisons provide new tests for the accuracy of the employed interaction potential. The method has been applied for up to third order velocity correlations and diffusion coefficients but it is extendible to higher order dynamic and transport properties. © 1996 American Institute of Physics.
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34.50.Gb Electronic excitation and ionization of molecules
31.15.xv Molecular dynamics and other numerical methods
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