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15 Jun 1991

Volume 94, Issue 12, pp. 7575-8640

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Time resolved stimulated light scattering from a solvated chromophore: A molecular dynamics study

Alan M. Walsh and Roger F. Loring

J. Chem. Phys. 94, 7575 (1991); http://dx.doi.org/10.1063/1.460144 (13 pages) | Cited 18 times

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We calculate the signal in a time‐resolved, coherent light scattering experiment on a dilute solution of a nonpolar solute dissolved in a nonpolar solvent. We model the solute and solvent molecules as electronic two level systems, and assume that only the solute interacts with radiation. The electronic degree of freedom is treated perturbatively within the Born–Oppenheimer approximation. The nuclear degrees of freedom are treated classically, and their effects are modeled by a molecular dynamics simulation. We assume the electronic absorption spectrum is dominated by inhomogeneous broadening. The validity of this assumption for our model is verified through the calculation of the absorption line shape with and without the inhomogeneous broadening assumption. A quantitative measure of the homogeneous dephasing time is obtained from a calculation of the photon echo signal for our model. The time dependence of the light scattering signal is shown to reflect both dynamics of the fluid in its electronic ground state and in its electronic excited state. We analyze the relative importance of contributions to the signal from these two types of dynamics as a function of experimental time scale and laser frequency.
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42.65.Es Stimulated Brillouin and Rayleigh scattering
42.65.Dr Stimulated Raman scattering; CARS
78.47.-p Spectroscopy of solid state dynamics

Influence of molecular reorientational processes on the circular intensity difference scattering by optically active media in dc electric field

S. Woźniak and S. Kielich

J. Chem. Phys. 94, 7588 (1991); http://dx.doi.org/10.1063/1.460145 (8 pages) | Cited 2 times

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The influence of reorientational processes on circular intensity difference scattering (CIDS) in a static electric field E0 is analyzed for systems of optically active dipolar molecules with the point group symmetries 3, 4, 6, and ∞ and of optically active nondipolar molecules with the symmetries 32, 422, and 622 for three experimental configurations: (i) at E0 perpendicular to the plane of scattering; (ii) E0 parallel to the incident light beam; and (iii) E0 parallel to the wave scattered in the direction of observation. The effect is described in terms of molecular reorientation functions and is dependent on the experimental field configuration, the electro‐optical properties of the molecules, and the field strength E0. Numerical calculations of molecular reorientation functions are performed for arbitrary degrees of electric saturation. The effects are accessible to observation in solutions of macromolecules by advanced laser and optoelectronic techniques.
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33.55.+b Optical activity and dichroism

Resonance‐enhanced multiphoton ionization‐photoelectron spectra of CO2. I. Photoabsorption above the ionization potential

Ming Wu, David P. Taylor, and Philip M. Johnson

J. Chem. Phys. 94, 7596 (1991); http://dx.doi.org/10.1063/1.460146 (6 pages) | Cited 10 times

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Photoabsorption above the first ionization potential of CO2 was observed at relatively low laser intensity, detected via resonant‐enhanced multiphoton ionization‐photoelectron spectra through several Rydberg states. This phenomenon can be explained by the presence of accidental resonances with long‐lived autoionizing states which make photon absorption within the ionization continuum possible. Laser powers are too low for this to be explained in terms of a ponderomotive potential and conventional above‐threshold ionization. This resonance‐enhanced above‐threshold absorption phenomenon is potentially useful in the study of excited and superexcited states. Photoelectron energies can be assigned to terminations on CO+2 ionic states at both the four‐ and five‐photon levels, allowing measurement of states up to 22 eV. Two unassigned bands may represent terminations on a new state of CO+2, with an ionization potential of 21.4 eV.
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33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.60.+q Photoelectron spectra

The dynamics of open‐shell Van der Waals complexes

Marie‐Lise Dubernet, David Flower, and Jeremy M. Hutson

J. Chem. Phys. 94, 7602 (1991); http://dx.doi.org/10.1063/1.460147 (17 pages) | Cited 117 times

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The theory of Van der Waals complexes formed from atoms and open‐shell (Σ and Π) diatomic molecules is developed, paying particular attention to the quantum numbers that are conserved in the complex and the angular momentum coupling cases that may be observed. Complexes formed from diatoms in multiplet Σ states may exhibit several different coupling schemes closely analogous to Hund’s coupling cases for diatomic molecules. Complexes formed from diatoms in Π states usually exhibit a coupling scheme in which the (signed) projection P of the diatom angular momentum j onto the intermolecular axis is nearly conserved. Correlation diagrams showing the bending energy levels as a function of potential anisotropy are given for complexes containing diatomic molecules in both Σ and Π states. The transition from free internal rotor quantum numbers to near‐rigid bender quantum numbers with increasing anisotropy is investigated. The cases of Ar–OH and Ne–OH are considered as examples.
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36.40.-c Atomic and molecular clusters
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

Color effects in pressure‐tuned hole‐burned spectra

G. Gradl, J. Zollfrank, W. Breinl, and J. Friedrich

J. Chem. Phys. 94, 7619 (1991); http://dx.doi.org/10.1063/1.460148 (6 pages) | Cited 31 times

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We applied hydrostatic pressure to spectral holes burned into a resorufin doped ethanol/methanol glass. We found that the line shift is perfectly linear with pressure and showed a pronounced dependence on the burn frequency as predicted by theory [J. Chem. Phys. 90, 3274 (1989)]. We exploited the burn frequency dependence to determine the solvent shift of the dye probe and the compressibility of the alcohol glass used. On the other hand, the behavior of the hole width under pressure shows features not predicted by theory: The broadening is, like the line shift, dependent on the burn frequency within the inhomogeneous band, yet in a nonlinear fashion. We attribute the color effect in the pressure induced broadening of the hole to a breakdown of the Gaussian approximation.
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78.40.Ha Other nonmetallic inorganics

Simultaneous forward–backward Raman scattering studies of D2 broadened by D2, He, and Ar

G. J. Rosasco, W. J. Bowers, W. S. Hurst, J. P. Looney, K. C. Smyth, and A. D. May

J. Chem. Phys. 94, 7625 (1991); http://dx.doi.org/10.1063/1.460149 (9 pages) | Cited 3 times

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Unavoidable beam crossings within a spherical‐mirror, multipass stimulated Raman gain cell give rise simultaneously to forward‐ and backward‐scattering Raman signals. In the Doppler‐broadened/Dicke‐narrowed regime of density, the lineshape is a function of the momentum transfer in the scattering process and thus the observed spectra will have more complex lineshapes than those seen with simple forward or backward scattering geometries. The analyses necessary to quantitatively account for such forward–backward spectra are summarized. These spectra enable unique experimental tests of the lineshape functions used for the description of the Raman Q‐branch spectrum under conditions where Doppler contributions and Dicke narrowing are significant. Results for the D2@B:D2 and D2@B:He systems support the well‐known Galatry, or soft collision, lineshape function. However, in the case of D2@B:Ar, our results suggest the need to employ the more general, complex soft collision function. In addition, these studies have provided data on linear‐with‐density line broadening coefficients (previously published) and line shifting coefficients (reported here) for these molecular systems.
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33.20.Fb Raman and Rayleigh spectra (including optical scattering)
33.70.Jg Line and band widths, shapes, and shifts

(2+1) resonance‐enhanced multiphoton ionization–photoelectron spectroscopy of the OH radical

Esther de Beer, M. P. Koopmans, C. A. de Lange, Yumin Wang, and W. A. Chupka

J. Chem. Phys. 94, 7634 (1991); http://dx.doi.org/10.1063/1.460150 (6 pages) | Cited 23 times

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A (2+1) resonance‐enhanced multiphoton ionization–photoelectron spectroscopy (REMPI‐PES) study of the OH radical has been carried out in the two‐photon energy region between 81 300 and 88 900 cm−1. Translationally and rotationally hot OH radicals are generated via photodissociation of hydrogen peroxide or formic acid. The known D 2Σ− (v′=0–2) and hitherto unobserved 3 2Σ−(v′=0) intermediate states in this region (at 81 815.8 and 87 643.7 cm−1 above the ground state) are shown to possess predominant Rydberg character. From the rotational structure in the REMPI spectrum physical parameters have been derived for these states.
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33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.60.+q Photoelectron spectra

An electron–spin–echo study of the nonradiative triplet state of polyenals

Marianne Ros and Edgar J. J. Groenen

J. Chem. Phys. 94, 7640 (1991); http://dx.doi.org/10.1063/1.460151 (9 pages) | Cited 6 times

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The lowest triplet state T0 of the all‐trans isomers of dodecapentaenal, decatetraenal, octatrienal, and hexadienal dissolved in polyethylene films has been investigated by electron–spin–echo spectroscopy in combination with pulsed laser excitation. Microwave transitions between the spin sublevels have been observed both in magnetic field and in zero field. For these polyenals T0 is found to be of ππ∗ character. The direction of the principal axes of the fine‐structure tensors and the frequencies of the zero‐field transitions have been determined. The largest zero‐field splitting varies from 2496 MHz for dodecapentaenal to 5260 MHz for hexadienal. The zero‐field splitting turns out to be inversely proportional to the number of double bonds in the conjugated chain, which is found to be compatible with a description of the triplet excitation as the promotion of an electron from the highest occupied to the lowest unoccupied molecular orbital. The upper spin sublevel is preferentially populated in the intersystem crossing process and decays fastest with a lifetime of 14 μs for dodecapentaenal, of 28 μs for decatetraenal, and of 77 μs for octatrienal. This variation is adequately described by an energy‐gap law, which indicates that the Franck–Condon factors play a dominant role in the decay process.
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76.30.-v Electron paramagnetic resonance and relaxation
33.35.+r Electron resonance and relaxation

Theory of rotational coherence spectroscopy as implemented by picosecond fluorescence depletion schemes

Gregory V. Hartland, Leslie L. Connell, and Peter M. Felker

J. Chem. Phys. 94, 7649 (1991); http://dx.doi.org/10.1063/1.460152 (18 pages) | Cited 24 times

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We present a perturbation theory analysis of four time‐resolved fluorescence depletion schemes that are useful, or potentially useful, in rotational coherence spectroscopy. The analysis shows that ground‐state rotational constants determine the rotational coherence effects in fully resonant, time‐resolved stimulated Raman‐induced fluorescence depletion (TRSRFD), excited‐state rotational constants determine such effects in time‐resolved stimulated emission spectroscopy (TRSES), and both ground‐ and excited‐state constants do so in time‐resolved fluorescence depletion (TRFD). An analysis of a variant of the TRSRFD scheme in which the stimulated Raman process is not resonance‐enhanced shows that this method gives rise to qualitatively different rotational coherence effects than fully resonant TRSRFD. It is argued that the scheme may, nevertheless, be a viable means of ground‐state rotational coherence spectroscopy. Expressions for the calculation of rotational coherence effects in TRFD, TRSRFD, and TRSES traces are also presented. Such expressions are used to show that the magnitudes of rotational coherence transients are similar in all three schemes. Finally, experimental results on molecular iodine are presented to show that, indeed, both ground‐ and excited‐state rotational coherence effects are manifest in TRFD traces.
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07.60.Rd Visible and ultraviolet spectrometers
78.47.-p Spectroscopy of solid state dynamics
33.20.Sn Rotational analysis

Electronic transition moment variation and Einstein coefficients for the NO(B 2Π–X 2Π) system

Lawrence G. Piper, Thomas R. Tucker, and William P. Cummings

J. Chem. Phys. 94, 7667 (1991); http://dx.doi.org/10.1063/1.460722 (10 pages) | Cited 26 times

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This paper details an investigation of the variation in the electronic transition moment with internuclear separation for the NO(B2Π–X2Π) transition. Measurements of the relative intensities of a number of NO B–X vibronic transitions having a common upper level were used to construct a relative transition‐moment function between 1.27 and 1.60 Å. After normalizing this relative function by experimentally determined radiative lifetimes, the transition‐moment function was extended down to 1.23 Å by incorporating data from oscillator strength measurements. In contrast to empirical transition‐moment functions that have been proposed previously, the function in this paper decreases with increasing internuclear separation. Unlike these other functions, however, this one is consistent with theoretical predictions, with most available oscillator strength data, and with the observed trend in B‐state radiative lifetimes as a function of vibrational level.
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33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.70.Fd Absolute and relative line and band intensities
33.20.Lg Ultraviolet spectra

Picosecond–microsecond structural relaxation dynamics in polypropylene glycol: Impulsive stimulated light‐scattering experiments

Anil R. Duggal and Keith A. Nelson

J. Chem. Phys. 94, 7677 (1991); http://dx.doi.org/10.1063/1.460153 (12 pages) | Cited 32 times

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Picosecond time‐resolved impulsive stimulated light‐scattering experiments are conducted on polypropylene glycol at temperatures above the glass transition temperature Tg. Through the use of a wide range of scattering angles, longitudinal acoustic waves are characterized in the 20 MHz–5 GHz frequency range. In addition, time‐dependent thermal expansion is observed on nanosecond–microsecond time scales. The results are consistent with an empirical description of structural relaxation dynamics in terms of a stretched exponential relaxation function with exponent β=0.4 and with the average relaxation time given by the Vogel–Tamman–Fulcher form. Comparisons to Brillouin scattering and photon correlation spectroscopy results indicate that this description holds for a dynamic range of over 10 orders of magnitude. Comparison to dielectric relaxation measurements which probe mainly orientational motions of polymer segments indicates that density fluctuations involve different segmental motions with significantly faster dynamics for T>Tg.
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78.35.+c Brillouin and Rayleigh scattering; other light scattering
42.65.Es Stimulated Brillouin and Rayleigh scattering

Structure of the benzene–Ar2 cluster from rotationally resolved ultraviolet spectroscopy

Th. Weber and H. J. Neusser

J. Chem. Phys. 94, 7689 (1991); http://dx.doi.org/10.1063/1.460154 (11 pages) | Cited 57 times

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Rotationally resolved spectra of the two vibronic bands 610 and 1620 and a vibronic van der Waals band of the benzene–Ar2 cluster are presented, whose vibronic assignments are based on the analysis of their rotational structures. A fit to the rotational line positions in the symmetric top spectra yields an accurate set of rotational constants in the ground and the excited electronic state and the exact values for the band origins of the bands. From these values the spectral shift between corresponding cluster and monomer bands as well as the frequency of the van der Waals symmetric stretching vibration in the excited electronic state are precisely determined. The structure of the cluster is identified to be symmetric with one Ar atom located on the C6 axis on each side of the benzene ring at a distance of 3.58 Å in the S0 state and 3.52 Å in the S1 state. These bond lengths exactly agree with our recent values for benzene–Ar. From the result that the bond lengths are equal for the dimer and the trimer we conclude that there is no Ar–Ar interaction through the intermediate benzene ring plane.
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36.40.-c Atomic and molecular clusters
33.20.Lg Ultraviolet spectra
33.15.Bh General molecular conformation and symmetry; stereochemistry

Direct absorption spectra of jet‐cooled benzene in 130–260 nm

Atsunari Hiraya and Kosuke Shobatake

J. Chem. Phys. 94, 7700 (1991); http://dx.doi.org/10.1063/1.460155 (7 pages) | Cited 58 times

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The direct absorption spectrum of benzene in a free jet has been measured in the 130–260 nm region (S1, S2, and S3 states, Rydberg series, and the first ionization limit) using synchrotron radiation as a light source. The absolute molar extinction coefficients (ϵ) of benzene in jets have been determined by scaling measured free‐jet values to the known value in the vapor phase for a broadband at 200.1 nm in the S2 state. The vibrational temperature for ν16 mode was estimated to be 185 K. The maximum value of ϵ of the S1 absorption system was found to be 1400 l mol−1 cm−1 (spectral bandwidth=0.065 nm). A shoulder observed at 205.45 nm in the S2 absorption system is assigned to the S2 origin, induced by pseudo‐Jahn–Teller distortion.
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33.20.Ni Vacuum ultraviolet spectra

Microwave spectrum of alkali metal tetrahydroborate. I. Rotational transitions and molecular structure of NaBH4 in the ground vibrational state

Yoshiyuki Kawashima, Chikashi Yamada, and Eizi Hirota

J. Chem. Phys. 94, 7707 (1991); http://dx.doi.org/10.1063/1.460156 (6 pages) | Cited 8 times

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The rotational spectrum of NaBH4 was observed in the millimeter‐wave region using a high temperature absorption cell. The observed spectrum of NaBH4 showed the pattern of a symmetric top molecule: Strong and weak for K=3n and 3n±1, respectively, because of the nuclear spin statistical weight for C3v symmetry. The rotational and centrifugal distortion constants for the 11B and 10B species were determined. The observed rotational constants of Na11BH4 and Na10BH4, combined with the assumption that r(B–Hb)−r(B−Ht)=0.04 Å and θ (Hb–B–Ht)=111°, gave estimates for r(Na–B) and r(B–Hb) to be 2.308±0.006 Å and 1.28±0.10 Å, respectively, where the uncertainties are mainly due to those of the assumed values. This bond length obtained for Na–B is much shorter than the reported value in crystal: 3.08 Å. The bond lengths derived indicate that NaBH4 has a tridentate molecular structure with three bridging hydrogens. This result agrees with those of ab initio calculations.
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33.20.Bx Radio-frequency and microwave spectra
33.15.Dj Interatomic distances and angles
33.15.Mt Rotation, vibration, and vibration-rotation constants

High resolution laser spectroscopy up to the dissociation limit of the NaK B 1Π state, and predissociation near the dissociation limit

Shunji Kasahara, Masaaki Baba, and Hajime Katô

J. Chem. Phys. 94, 7713 (1991); http://dx.doi.org/10.1063/1.460157 (8 pages) | Cited 15 times

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Doppler‐free high resolution spectrum of the B 1Π(v′,J′)–X 1Σ+(v″,J″) transitions of 23Na39K was measured by the optical–optical double resonance (OODR) polarization spectroscopy. The transition lines up to the v′=43 level, which was estimated to be 1.8 cm−1 below the dissociation limit, were observed. The potential energy curve for internuclear distance from 3.25 to 15.6 Å was calculated by the RKR method, and the inverse‐power coefficient was determined by analyzing the long‐range RKR turning points. The B 1Π state dissociates to Na(3s2S1/2)+K(4p2P3/2) atoms without a potential hill near the dissociation limit. The dissociation energy was determined to be 1324.3±0.3 cm−1 from the LeRoy–Bernstein plots. Remarkable line broadenings were observed for transitions higher than the dissociation energy to Na(3s2S1/2)+K(4p2P1/2) atoms. This is identified as originating from the predissociation to Na(3s2S1/2)+K(4p2P1/2) atoms. The predissociation is shown to be caused by a spin–orbit interaction between the B 1Π and (2)3Σ+ states, and the potential curves are expected to cross around the inner turning point of the B 1Π (v′=34) level.
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36.40.-c Atomic and molecular clusters
33.20.Kf Visible spectra

Methyl group torsional dynamics from rotationally resolved electronic spectra. 1‐ and 2‐methylnaphthalene

X.‐Q. Tan, W. A. Majewski, D. F. Plusquellic, and D. W. Pratt

J. Chem. Phys. 94, 7721 (1991); http://dx.doi.org/10.1063/1.460158 (13 pages) | Cited 55 times

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Rotationally resolved fluorescence excitation spectra of three vibronic bands in the S1←S0 transitions of 1‐ and 2‐methylnaphthalene (1 and 2MN) have been obtained. Each band exhibits perturbations that are produced by an interaction between the restricted torsional motion of the attached methyl group and the overall rotational motion of the entire molecule. A complete analysis of these effects yields values of the torsional barrier heights, the rotational constants, and the torsion–rotation perturbation coefficients of all vibronic levels that participate in the transitions. These values depend significantly on the position of the methyl group attachment, on the electronic state of the naphthalene chromophore, and on its vibrational state, as well. For example, V3 (the threefold torsional barrier) decreases from 809 cm−1 in 00 1MN to 128 cm−1 in 00 2MN. D (the largest first‐order torsion–rotation perturbation term) increases from 0.03 MHz in 00 1MN to 406 MHz in 00 2MN, a change of more than 4 orders of magnitude. The V3 values of 00 and 81 1MN are 563 and ≤ 373 cm−1, respectively. A full discussion of these dynamically relevant effects and their dependence upon both electronic and geometric factors is given.
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33.20.Lg Ultraviolet spectra
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
33.15.Mt Rotation, vibration, and vibration-rotation constants

The jet‐cooled fluorescence excitation spectrum and ring‐bending potential‐energy function and conformation of 2‐cyclopenten‐1‐one in the S1(n,π∗) electronic excited state

C. M. Cheatham and Jaan Laane

J. Chem. Phys. 94, 7734 (1991); http://dx.doi.org/10.1063/1.460159 (10 pages) | Cited 9 times

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The jet‐cooled fluorescence excitation spectra of 2‐cyclopenten‐1‐one and its 5,5‐d2 isotopomer have been recorded in the 370–340 nm region. The electronic origin for the undeuterated species occurs at 27 210 cm−1 for the S1(n,π∗) electronic excited state. The vibrational frequencies for the three carbonyl motions and the nine ring modes were observed for the excited state. Bands at 67, 158, and 256 cm−1 for the d0 species, at 63, 147, and 240 cm−1 for the 5‐d1 isotopomer, and at 59, 138, and 227 cm−1 for the d2 species were assigned to the ring‐puckering motion in the S1 state. A single one‐dimensional potential‐energy function accurately fits the data for all three isotopomers. This function is nearly purely quartic in character and shows the ring to be planar in the electronic excited state. However, it has become less rigid, and this is ascribed to a decrease in initial angle strain within the ring. The C=O and C=C stretching frequencies occur at 1418 and 1357 cm−1 for the d0 molecule. The ring‐twisting frequency for the S1 state occurs at 274 cm−1. Previous electronic absorption measurements had resulted in a misassignment for this motion.
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33.20.Lg Ultraviolet spectra
33.50.Dq Fluorescence and phosphorescence spectra
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

Cluster ion dip spectroscopy of hydrogen bonded phenol(H2O)n clusters, n=0–4

R. J. Stanley and A. W. Castleman

J. Chem. Phys. 94, 7744 (1991); http://dx.doi.org/10.1063/1.460160 (13 pages) | Cited 72 times

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Employing a laser based time‐of‐flight mass spectrometer system, ion dip spectra for phenol (Ph), Ph(H2O)1, Ph(H2O)3, and Ph(H2O)4 were obtained in the range of 500–1300 cm−1 from a variety of initially pumped states. Dramatic enhancement of the signal‐to‐noise ratio of the cluster ion dip spectra relative to that of the bare phenol is attributable to the increase in the excited state singlet lifetime of the hydrated phenol chromophore. Several dips in the Ph(H2O)1 spectrum exceed the ‘‘saturation’’ limit of 50%, indicating that significant relaxation of the downpumped ground state is occurring via low frequency vibrational modes of the H2O solvent ‘‘bath.’’ Excitation of the hydrogen bond stretch (σ 10=156 cm−1 ) in the S1 state of the Ph(H2O)1 cluster reveals that the ground state (S0) hydrogen bond stretch, σ 01, is 151(±1) cm−1, a mode which appears to be built off of phenol fundamental and combination bands. A second intermolecular band is also evident at 141(±2) cm−1. There is no evidence of an analogous wag mode when pumping the S1 bend (β10), suggesting that the intermolecular modes in S1 are highly coupled. Attempts to obtain ion dip spectra for Ph(H2O)2 went unrewarded, presumably due to the anomalously short S1 lifetime of the Ph(H2O)2 cluster. Spectra for Ph(H2O)3 and Ph(H2O)4 were obtained which show prominent phenol bands, with low frequency (∼10 cm−1 ) progressions built off of these bands. The intermolecular hydrogen bond stretch for Ph(H2O)3 and Ph(H2O)4 in the ground state are 189(±1) cm−1 and 185(±1) cm−1, respectively.
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36.40.-c Atomic and molecular clusters
82.30.Nr Association, addition, insertion, cluster formation
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)

Radiative lifetimes of van der Waals heteroclusters

Eli Shalev, Narda Ben‐Horin, and Joshua Jortner

J. Chem. Phys. 94, 7757 (1991); http://dx.doi.org/10.1063/1.460161 (12 pages) | Cited 15 times

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In this paper we advance and apply a molecular theory of pure radiative lifetimes τr of (aromatic molecule)⋅(rare gas)n heteroclusters. The modification of τr in the heterocluster, relative to the bare molecule value, originates from intermolecular interactions between molecular multicenter transition monopoles, which are described by π electron approximation, and the rare‐gas atom transition dipoles, which are specified in terms of the static atomic polarizability α. The calculated changes of τr of 9,10 dichloroanthracene⋅A1 (A=Ar, Kr, Xe) heterodimers are in good agreement with experiment; the theory accounting for the nearly linear dependence of the change of τr on α.
Calculations of the radiative lifetimes of 9,10 dichloroanthracene⋅Arn (n=1–34) heteroclusters were performed using structural information from potential optimization for small (n=1–3) heteroclusters, static structural data for small and medium‐sized (n=1–8) heteroclusters, and (constant energy) finite‐temperature molecular dynamics simulations for medium‐sized and large (n=5–34) heteroclusters. The structural sensitivity of τr is manifested by different values of τr for distinct structural isomers, by the nonmonotonous size effects on τr with increasing n, and by pronounced decrease of τr with increasing temperature. For medium‐sized clusters (n=5–18), the temperature dependence of τr originates from the enhancement of atomic motion on both sides of the aromatic microsurface and the occupation of the peripheral region, while for large clusters (n≂34) two layer–one layer isomerization processes with increasing temperature result in dramatic changes of τr. Satisfactory overall agreement between theory and experiment for 9,10 dichloroanthracene⋅Arn n=1–34 heteroclusters was accomplished.
The dominating 9,10 dichloroanthracene⋅Arn structures for n=1–18, which yield agreement between theory and experimental for τr, correspond to nearly equal distribution of the rare gases on both sides of the aromatic microsurfaces at T≤20 K, while large n=34 two‐layered structures in the temperature domain 22 K≤T≤40 K account for the experimental τr result. τr serves as a useful spectroscopic probe for the interrogation of the structure and isomerization dynamics in heteroclusters.
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36.40.-c Atomic and molecular clusters
33.70.Fd Absolute and relative line and band intensities

Vacuum ultraviolet emission spectra of the helium and neon alkali ions in the range between 60–80 nm

K. Petkau, J. W. Hammer, G. Herre, M. Mantel, and H. Langhoff

J. Chem. Phys. 94, 7769 (1991); http://dx.doi.org/10.1063/1.460162 (6 pages) | Cited 10 times

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Mixtures of neon or helium with alkali vapor were excited using an argon ion beam. The emission continua observed in the vacuum ultraviolet region between 60 and 80 nm are assigned to the decay of the ionic excimers He+K, He+Na, He+Li, Ne+Rb, Ne+K, Ne+Na, and Ne+Li into the ground state consisting of a rare gas atom and an alkali ion. The binding energies of these excimers range between 0.46 and 0.85 eV. The obtained decay energies and the fine structure splittings agree with the predictions by calculations using phenomenological potentials and by ab initio calculations. Intense emission due to the inner shell excitation of K and Rb is also observed.
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33.20.Ni Vacuum ultraviolet spectra
36.40.-c Atomic and molecular clusters
33.15.Fm Bond strengths, dissociation energies

Raman scattering evidence of the incommensurability influence on stretching modes in Rb2ZnCl4

A. Hedoux, Y. Guinet, J. Lefebvre, and M. More

J. Chem. Phys. 94, 7775 (1991); http://dx.doi.org/10.1063/1.460163 (4 pages) | Cited 4 times

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Temperature dependence of stretching modes are presented for two kinds of Rb2ZnCl4 crystals which exhibit different phase transition sequences. The comparison between both sets of results points out a special behavior of the line which appears below the normal–incommensurate phase transition. Appearance of the incommensurability is observed via the temperature dependence of certain stretching lines.
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78.30.Hv Other nonmetallic inorganics
64.70.Rh Commensurate-incommensurate transitions

Resonant two photon dissociation dynamics of HD+ in intense infrared laser field

Banani Datta and S. S. Bhattacharyya

J. Chem. Phys. 94, 7779 (1991); http://dx.doi.org/10.1063/1.460164 (9 pages) | Cited 6 times

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The intensity dependent line shapes of resonant two photon dissociation process in HD+ from the 1sσg (v=6, J=0) are obtained at three different intensities (1.53×1010, 1.53×1012, and 6.12×1012 W/cm2 ) in the wavelength range 16 000–17 200 Å using the resolvent operator formalism. The discrete state v=11, J=1 (1sσg) is the intermediate resonant state. The position of the resonance on the wavelength scale depends strongly on intensity. It is shown that for sufficiently strong fields the earlier semiperturbative theory formulated in analogy with Fano’s theory of atomic autoionization breaks down near resonance due to saturation of the strongly coupled resonant transition. Also, when the dissociative transitions to both the electronic states 1sσg and 2pσu are considered simultaneously, the line shape is altered from that obtained by the superposition of the line shapes of these two transitions separately. The conditions under which a unique time independent transition rate is ill defined are discussed. For such situations the time dependent transition probabilities can be obtained from the effective Fano asymmetry parameters, and line shifts and widths of the ground and resonant states. The limiting transition rates for such cases for short and long pulse times are also shown.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.80.Wz Other multiphoton processes
82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
33.70.Jg Line and band widths, shapes, and shifts

The torsional Raman spectra of partially deuterated ethane molecules. I. One conformer derivatives: CH3CD3, CH3CH2D, CH3CHD2, CH2DCD3, and CHD2CD3

J. M. Fernández‐Sánchez and S. Montero

J. Chem. Phys. 94, 7788 (1991); http://dx.doi.org/10.1063/1.460165 (13 pages) | Cited 3 times

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The torsional Raman spectra of one‐conformer partially deuterated derivatives of ethane, CH3CD3, CH3CH2D, CH3CHD2, CH2DCD3, and CHD2CD3 have been experimentally investigated with an improved conventional Raman spectrometer. The recorded spectra have been interpreted with a rotational–torsional effective Hamiltonian and a simple model for the torsional dependence of the molecular polarizability. The effective threefold torsional potential parameters for CH3CD3 (V3=997.72 cm−1, V6=8.45 cm−1 ) have been taken from the recent literature, whilst the following parameters were obtained for the other derivatives: CH3CH2D, V3=1007 cm−1, V6=7.3 cm−1; CH3CHD2, V3=1003 cm−1, V6=8.5 cm−1; CH2DCD3, V3=993 cm−1, V6=8.2 cm−1; CHD2CD3, V3=995 cm−1, V6=6.7 cm−1. The estimated accuracy of these parameters is of the order of ±2 cm−1.
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33.20.Fb Raman and Rayleigh spectra (including optical scattering)

Temperature effects in positronium formation and lifetime in solutions of nonpolar organic solvents

D. A. Diehl and D. M. Schrader

J. Chem. Phys. 94, 7801 (1991); http://dx.doi.org/10.1063/1.460166 (9 pages) | Cited 3 times

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The temperature dependence of the positron annihilation lifetime (PAL) spectra of several solutions are examined. Four solvents are investigated: n‐hexane, cyclohexane, squalane, and an equivolume mixture of n‐hexane and squalane. Solutes in the study are nitrobenzene, nitrocyclohexane, and hexafluorobenzene. Arrhenius plots of lifetimes are made from 10 °C to 50 °C. The temperature dependence of the PAL spectra of nitrobenzene solutions in a magnetic field varying up to 15 kG are also presented. Anomalous behavior is observed for the solvents n‐hexane and squalane, both neat and with nitrobenzene as a solute. In addition, it is found that the behavior of neat and nitrobenzene solutions of the n‐hexane–squalane mixture is not intermediate between those involving n‐hexane and squalane separately. We conclude that there is a change in the mechanism for positronium formation from n‐hexane to squalane which is not accounted for by changes in free volume. Nitrocyclohexane is found to enhance positronium formation in squalane. This is the first example of antiinhibition by an aliphatic solute known to the authors.
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36.10.Dr Positronium
82.20.Hf Product distribution
82.30.-b Specific chemical reactions; reaction mechanisms

Laser‐induced fluorescence measurements of rotationally resolved velocity distributions for CO+ drifted in He

Christian P. Lauenstein, Michael J. Bastian, Veronica M. Bierbaum, Stephen M. Penn, and Stephen R. Leone

J. Chem. Phys. 94, 7810 (1991); http://dx.doi.org/10.1063/1.460167 (9 pages) | Cited 14 times

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Measurements of ion‐velocity distributions of CO+ in a He buffer gas are presented as a function of an applied electric field. The distributions are obtained by single frequency, laser‐induced fluorescence from various initial rotational states with the laser beam propagating parallel and perpendicular to the drift velocity vector. All distributions are well represented by a Maxwellian for the observed E/N range of 0–13 Td. The reduced mobilities, calculated from the shift of the mean velocity as a function of electric field, increase from 18.7±1.0 cm2 V−1 s−1 at very low fields to 26.4±0.7 cm2 V−1 s−1 at 13 Td. From the width of the Doppler profiles, translational ‘‘temperatures’’ are calculated, which are compared to simple attractive and repulsive Maxwell models as a function of the field. The measured values disagree with the predictions, which are well established for atomic ion systems. The differences are discussed in terms of rotationally inelastic energy transfer in the collisions, which is predicted by kinetic theory models. This argument is strengthened by the fact that even though the rotational states rapidly equilibrate, measurements on different lines yield higher temperatures for higher rotational levels. Finally, the small influence of selective quenching of the electronically excited CO+ on the Doppler profiles is demonstrated by measuring effective lifetimes as a function of the applied drift field.
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34.50.Ez Rotational and vibrational energy transfer
51.50.+v Electrical properties (ionization, breakdown, electron and ion mobility, etc.)
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