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15 Jun 1980

Volume 72, Issue 12, pp. 6323-6821

Page 1 of 4 Pages Next Page | Jump to Page

Recoil 18F chemistry. XI. High pressure investigation of 1,1‐difluoroethane

Ronald G. Manning and John W. Root

J. Chem. Phys. 72, 6323 (1980); http://dx.doi.org/10.1063/1.439155 (9 pages) | Cited 4 times

Online Publication Date: 15 July 2008

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Nuclear recoil 18F reactions in CH3CHF2 have been investigated throughout the effective pressure range 0.3–190 atm. The principal reaction channel is F‐to‐HF abstraction for which the combined yield from quasithermal and energetic processes in the presence of 5 mole% H2S additive is 83.4%±0.2%. A reaction mechanism is proposed that involves the organic product forming channels F‐for‐F, F‐for‐αH, F‐for‐βH, F‐for‐ĊH3 and F‐for‐ĊHF2. The results are compared with those reported for the 18F+CH3CF3 system.
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82.30.-b Specific chemical reactions; reaction mechanisms
82.20.Hf Product distribution

Thermodynamics of sulfur chemisorption on metals. I. Alumina‐supported nickel

Jon G. McCarty and Henry Wise

J. Chem. Phys. 72, 6332 (1980); http://dx.doi.org/10.1063/1.439156 (6 pages) | Cited 31 times

Online Publication Date: 15 July 2008

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Sulfur chemisorption isosteres have been measured for nickel in powdered form and for nickel supported on two different alumina powders. The experiments were conducted in a closed‐loop gas recirculation system containing one atmosphere hydrogen. Isotherms were determined by stepwise injection of H2S aliquots into recirculating hydrogen gas and analyzing for the H2S concentration as equilibrium was approached. Isosteres were measured by varying the sample temperature and monitoring the H2S/H2 ratio in the gaseous environment. A gas chromatograph and a photoionization detector was used to measure the H2S concentration to levels below 1 ppb. As monolayer coverage is approached the H2S/H2 ratio attains the equilibrium values reported for the bulk sulfide, Ni3S2. Adsorbed sulfur is very strongly bound to the surface of nickel. The heat of formation of chemisorbed sulfur with respect to 1/2 S2(g) at 800 K is 247 kJ mol−1 more negative than the heat of formation of Ni3S2. The heat of segregation exceeds 190 kJ mol−1. The results demonstrate that the equilibrium sulfur coverage exceeds one half monolayer on Ni/Al2O3 unless the H2S/H2 ratio is less than 1 ppb even at temperatures as high as 940 K.
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68.43.-h Chemisorption/physisorption: adsorbates on surfaces
82.60.Cx Enthalpies of combustion, reaction, and formation

Picosecond study of electron ejection in aqueous phenol and phenolate solutions

Jean‐Claude Mialocq, Jack Sutton, and Paul Goujon

J. Chem. Phys. 72, 6338 (1980); http://dx.doi.org/10.1063/1.439157 (8 pages) | Cited 21 times

Online Publication Date: 15 July 2008

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The photoionization of aqueous phenol and phenolate solutions was investigated using picosecond absorption spectroscopy. Phenol and phenolate were excited in the first singlet excited state using a single picosecond pulse of 27 ps duration, at 265 nm (4th harmonic of a Nd–YAG laser). Hydrated electron formation was ascertained by following the kinetics of formation and disappearance at 630 nm with a picosecond pulse obtained by stimulated Raman scattering (SRS). Similarly to the ferrocyanide ion, the excited phenolate ion undergoes a very fast electron ejection process. The lifetime of this process is shorter than the time resolution of our experimental device. In the excited phenol molecule, hydrated electron formation is retarded, the delay between the half‐rise times of the exciting pulse and the electron absorption signal being (12±1) ps. This delay is not reduced at low pH. These observations and the high yield of eaq formation are best explained by a consecutive two photon process, the second photon being absorbed by the excited singlet state S1 and the deprotonation occuring after ejection of the electron. The kinetics of hydrated electron disappearance with NO3− as scavenger, are in excellent agreement with literature values. Using H3O+ and Cd++ to scavenge the hydrated electron, its initial formation yield remains constant, showing that Cd++ does not react with any hydrated electron precursor. This difference between our results and those obtained in picosecond pulse radiolysis shows that the precursor of the hydrated electron is not the same in the two techniques. The precursor obtained in pulse radiolysis appears to be more reactive with scavengers. A possible explanation is that after phenolate or phenol excitation, the resultant internal state of the aromatic molecule is capable of undergoing electron transfer to the adjacent solvent molecules, thus producing at least partially hydrated electrons directly.
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82.50.Kx Processes caused by X-rays or γ-rays

Surface enhanced Raman spectra: A critical review of the image dipole description

Peter R. Hilton and David W. Oxtoby

J. Chem. Phys. 72, 6346 (1980); http://dx.doi.org/10.1063/1.439158 (3 pages) | Cited 18 times

Online Publication Date: 15 July 2008

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Recent experiments have demonstrated enhancements of several orders of magnitude for Raman scattering of pyridine and other molecules adsorbed on a silver surface. One explanation which has been proposed is an image dipole model, in which a point dipole is mirrored in the metal, with the mirror image inducing an additional dipole in the adsorbed species and thus changing its effective polarizability αeff. In this paper we suggest that the large enhancements obtained from such a point dipole description result from the use of an symptotic theory at unphysically short distances. We demonstrate this through a model calculation of αeff of a hydrogen atom near a mirror surface. A point dipole approximation gives a divergent αeff near a critical distance Rc from the surface, while a more exact self‐consistent Hartree calculation (including the finite extent of the hydrogen charge cloud) gives only a slight enhancement or even a de‐enhancement near Rc.
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33.20.Fb Raman and Rayleigh spectra (including optical scattering)
68.43.-h Chemisorption/physisorption: adsorbates on surfaces

Convergence of a perturbation technique for evaluating isotopic partition function ratios

Charles E. Harvie, Philippe Bopp, and Max Wolfsberg

J. Chem. Phys. 72, 6349 (1980); http://dx.doi.org/10.1063/1.439159 (14 pages) | Cited 5 times

Online Publication Date: 15 July 2008

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Schwinger perturbation theory for partition functions is developed in a form which makes practical the numerical evaluation of higher order terms. The vibrational Hamiltonian of a molecule in the harmonic approximation, H=(1/2)∑(gijpipj+fijq iqj), has been partitioned into an unperturbed Hamiltonian (the diagonal part, terms with i≠j) and a perturbation term (the off‐diagonal part, terms with i=j). The perturbation theory technique up to fourth order is used to calculate vibrational partition functions for a number of molecules. Vibrational partition functions so calculated are employed to calculate reduced partition functions and reduced isotopic partition function ratios. The results obtained by perturbation theory are compared with those of exact calculations carried out by actually obtaining the normal mode vibrational frequencies of the vibrational Hamiltonian. The previously observed good agreement, especially for the reduced isotopic partition function ratios, between perturbation theory in second order and exact calculations, becomes usually even better in fourth order perturbation theory.
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33.20.Tp Vibrational analysis
05.20.-y Classical statistical mechanics

Molecular dynamics simulation of submonolayer krypton films on graphite

Frank Hanson and John P. McTague

J. Chem. Phys. 72, 6363 (1980); http://dx.doi.org/10.1063/1.439160 (5 pages) | Cited 19 times

Online Publication Date: 15 July 2008

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Molecular dynamics simulations of submonolayer krypton films on graphite show a first order transition from a low temperature √3×√3 registered phase to a disordered one which is fluid and not localized in the graphite wells (not lattice–gas‐like). This contrasts with simulations in the absence of a substrate ordering potential. There the transition is continuous.
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68.43.-h Chemisorption/physisorption: adsorbates on surfaces

The electronic structure of the lithium trimer anion and cation

J. L. Gole, R. H. Childs, D. A. Dixon, and R. A. Eades

J. Chem. Phys. 72, 6368 (1980); http://dx.doi.org/10.1063/1.439161 (8 pages) | Cited 36 times

Online Publication Date: 15 July 2008

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SCF and SCF–CI calculations have been performed on Li3, its anion and cation. We predict a value of 1.1 eV for the adiabatic electron affinity. The Li−3 bond energy is found to be 0.9 eV versus the 0.4 eV energy required to dissociate Li3 to dimer and atom. Thus, the bond strength of the anion considerably exceeds that of the neutral parent. The difference in the nature of electron binding to Li2 and Li3 can be understood through modification of simple MO concepts. The energy required to dissociate Li3+ to Li2++Li is found to be 1.28 eV. Hence, the bond strength of the cation also greatly exceeds that of the neutral. The Li3+ bond energy is comparable to the Li2+ dissociation energy (1.24 eV). The adiabatic ionization potential is found to be 3.95 eV. The combination of the present study and those of previous researchers indicates that the Li3 surface is weakly varying with bond angle and may be characterized by multiple minima. The current results for Li3 and Li3+ are discussed in the light of recent mass spectrometric studies of the trimer.
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31.15.A- Ab initio calculations
31.15.V- Electron correlation calculations for atoms, ions and molecules

Distribution function of the end‐to‐end vector for short polymer chains

Juan J. Freire and M. M. Rodrigo

J. Chem. Phys. 72, 6376 (1980); http://dx.doi.org/10.1063/1.439162 (6 pages) | Cited 7 times

Online Publication Date: 15 July 2008

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A method is presented for the evaluation of the end‐to‐end vector distribution function for short polymer chains. The method is based on the expansion of this quantity in terms of a spherical harmonic series and is related with a very efficient algorithm recently proposed by Fixman et al. for the calculation of high moments. Numerical results are reported for a rotational isomeric representation of short polymethylene chains. The results are compared to those obtained earlier by other authors with an alternative procedure and the advantages of the new scheme are stressed.
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36.20.Fz Constitution (chains and sequences)

Highly excited vibrational states of molecules by thermal lensing spectroscopy and the local mode model. I. CHCl3, CHBr3, CH2Cl2, CH2Br2

Howard L. Fang and Robert L. Swofford

J. Chem. Phys. 72, 6382 (1980); http://dx.doi.org/10.1063/1.439163 (8 pages) | Cited 45 times

Online Publication Date: 15 July 2008

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Visible absorption spectra (14 700–19 100 cm−1) measured with the thermal lensing spectrometer and near‐infrared absorption (5 600–14 000 cm−1) measured with a conventional spectrophotometer are reported for CHCl3, CHBr3, CH2Cl2, and CH2Br2. Relatively strong absorption peaks are identified as overtones of the C–H stretching vibrations in these compounds. The overtone spectra are analyzed in terms of the local mode (LM) model, which treats the molecule as a set of loosely coupled anharmonic oscillators localized on individual C–H bonds. Relatively less intense peaks are observed and are assigned as combinations of a local mode C–H vibration and some lower frequency (normal) mode of the molecule. Fermi resonance is seen to occur in the spectra of CHCl3 and CHBr3, resulting in anomalously high intensities for the combination bands involved. Significant anharmonic local–normal coupling constants are seen for these same molecules. These coupling constants appear to indicate the presence of strong physical coupling of the two motions involved. The importance of these coupled motions in the intramolecular relaxation of vibrational energy is suggested.
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33.20.Kf Visible spectra
33.20.Ea Infrared spectra

Ab initio study of the vibrational dependence of hyperfine coupling constants in the methyl, silyl, and formaldehyde anion radicals

Y. Ellinger, F. Pauzat, V. Barone, J. Douady, and R. Subra

J. Chem. Phys. 72, 6390 (1980); http://dx.doi.org/10.1063/1.439137 (8 pages) | Cited 42 times

Online Publication Date: 15 July 2008

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The influence of vibrational effects on the isotropic hyperfine coupling constants is studied by ab initio methods for the CH3, SiH3, and H2CO− radicals. The calculations are carried out using double‐zeta plus polarization basis sets of contracted Gaussian orbitals. A spin‐restricted SCF plus perturbative configuration interaction method including all spin adapted configurations with three uncoupled electrons is employed. The perturbation treatment is extended to second order which yields a definitive improvement of the results over previous first order calculations. A quantum mechanical treatment of the vibrational problem shows that the influence of the nuclear motion depends on the shape of the potential. The vibrational effects are found necessary to account for the experimental findings and the overall results are in very good agreement with experiment. It is found that the usual classical approach fails to reproduce the correct behavior of the splittings, especially for low temperature experiments.
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31.30.Gs Hyperfine interactions and isotope effects
31.15.V- Electron correlation calculations for atoms, ions and molecules
31.15.A- Ab initio calculations

Bethe surface and Compton profile of NH3 obtained by 35 keV electron impact

A. Lahmam‐Bennani, A. Duguet, H. F. Wellenstein, and M. Rouault

J. Chem. Phys. 72, 6398 (1980); http://dx.doi.org/10.1063/1.439138 (11 pages) | Cited 27 times

Online Publication Date: 15 July 2008

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Complete inelastic electron impact spectra have been obtained for NH3 for a momentum transfer range from 0.4 to 12.5 a.u. using 35 keV electrons. These spectra were converted to relative generalized oscillator strengths (GOS), were placed onto an absolute scale by the Bethe sum rule, and higher order sum rules were used to test the accuracy of the GOS. The Compton profile (CP) of NH3 was determined from the GOS by means of the impulse approximation (IA) and it was found that the CP’s were asymmetric and shifted in respect to the free electron theoretical CP. This Compton defect was analyzed in detail and was shown to be due to the failure of the IA. The area under the CP is discussed in terms of the x‐ray incoherent scattering factor and experimental results for the valence, inner shell, and total CP’s are compared to several theoretical profiles.
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34.80.Gs Molecular excitation and ionization

Vibrational study of phase transition of solid malononitrile

N. Le Calvé, B. Pasquier, and A. Novak

J. Chem. Phys. 72, 6409 (1980); http://dx.doi.org/10.1063/1.439139 (5 pages) | Cited 13 times

Online Publication Date: 15 July 2008

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Solid phases and phase transitions of crystals of malononitrile, CH2(CN)2, have been studied by Raman and infrared spectroscopy. Among twelve Raman active lattice modes of T′ phase a soft mode has been identified and assigned to a R′b libration. The T′–F–T transitions can be considered as second order and of displacive type since the soft mode frequency decreases continuously to zero at Tc when the transition temperature is approached from either above or below. Critical exponent β∼1/2 has been determined from the soft mode frequency versus Tc‐T plot. Half‐width of the soft mode band has also been studied as a function of temperature and relaxation times have been extracted from the corresponding curves. The stable low temperature phase L has been characterized spectroscopically: unlike T′, F, T phases it contains CH...N hydrogen bonds. The behavior of its lattice frequencies with temperature indicates that L–F phase change is a first order transition.
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64.70.K- Solid-solid transitions
78.30.Jw Organic compounds, polymers

CN(A 2Πi→X 2Σ+) and CN(B 2Σ+→X 2Σ+) yields from HCN photodissociation

L. C. Lee

J. Chem. Phys. 72, 6414 (1980); http://dx.doi.org/10.1063/1.439140 (8 pages) | Cited 33 times

Online Publication Date: 15 July 2008

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The cross sections for the production of CN(A 2Πi→X 2Σ+) and CN(B 2Σ+→X 2Σ+) emissions from HCN photodissociation are measured using synchrotron radiation in the 1050–1550 Å region. The photoabsorption cross section of HCN is also measured, and the quantum yields for the emissions are determined. The spectra for the photoabsorption and photoemission cross sections are interpreted with the known excited states of HCN. Three new excited states are found from the CN emission quantum yields. It is observed that the vibrational population of the CN(B 2Σ+) photofragments produced by photodissociating HCN at Rydberg states are more vibrationally excited than those produced at other dissociative states. The photodissociation processes for producing CN emissions at various excited states are discussed.
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82.50.Hp Processes caused by visible and UV light
33.80.Gj Diffuse spectra; predissociation, photodissociation

Some relations connecting volume and enthalpy relaxation in the order parameter model of liquids and glasses

Arnold V. Lesikar and Cornelius T. Moynihan

J. Chem. Phys. 72, 6422 (1980); http://dx.doi.org/10.1063/1.439141 (2 pages) | Cited 7 times

Online Publication Date: 15 July 2008

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A general inequality connecting linear enthalpy and volume relaxation is derived in the context of the order parameter model of liquids and glasses. This inequality is shown to yield a wide variety of relationships among the quantities characterizing these relaxations.
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61.20.Gy Theory and models of liquid structure
81.05.Kf Glasses (including metallic glasses)

Intermolecule vibrational energy transfer dynamics in IR laser pumped SO2/18O2 mixtures

Marsha Isack Lester and George W. Flynn

J. Chem. Phys. 72, 6424 (1980); http://dx.doi.org/10.1063/1.439142 (13 pages) | Cited 4 times

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Intermolecule vibrational energy sharing in SO2/18O2 mixtures has been observed by monitoring fluorescence emission from the SO2 ν3 state following pulsed CO2 laser excitation of the ν1 mode. The rate constant for the vibrational energy crossover step from the SO2 ν3 state to the v=1 level of 18O2 has been determined to be 5.74 (±0.91) msec−1 (Torr 18O2)−1 or approximately 1400 gas kinetic collisions. The present results are compared with an earlier ultrasound investigation of vibrational relaxation in the SO2/16O2 system. Theoretical probabilities of intermolecule energy transfer due to short‐range repulsive forces and long‐range dipole–quadrupole forces have been calculated for the SO2/18O2 and SO2/16O2 systems. Comparison of experimental and theoretical results suggests that both short‐range and long‐range forces contribute to the intermolecule vibrational energy process in SO2/18O2 mixtures while only short‐range forces appear to be important in the SO2/16O2 exchange. A general technique has been developed for determining the error in kinetic rate constants from the experimental uncertainty of the measured fluorescence eigenvalues.
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34.50.Ez Rotational and vibrational energy transfer
82.20.Rp State to state energy transfer

Lifetimes and transition moments among excited electronic states of BaO

Yen Chu Hsu, Bruce Hegemann, and J. Gary Pruett

J. Chem. Phys. 72, 6437 (1980); http://dx.doi.org/10.1063/1.439143 (8 pages) | Cited 7 times

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The lifetime of the BaO C 1Σ+ state is investigated for several vibrational and rotational levels at 1 Torr and under molecular beam conditions. A collision free radiative lifetime of 10.5±1 nsec is observed for the v=0 level. Dispersed fluorescence from v=0, 1, and 2 to various low lying electronic states is measured and used to obtain electronic dipole transition moments of : μ2C–X=9.9 D2, μ2C–A=16.8 D2, μ2C–A′=9.8 D2, μ2C–b=3.1 D2, μ2A–X=3.2 D2, μ2A′–X=0.14 D2.
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33.70.Fd Absolute and relative line and band intensities
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
31.50.Df Potential energy surfaces for excited electronic states

Rotationally inelastic collisions of LiH with He. I. Ab initio potential energy surface

David M. Silver

J. Chem. Phys. 72, 6445 (1980); http://dx.doi.org/10.1063/1.439144 (7 pages) | Cited 34 times

Online Publication Date: 15 July 2008

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The diagrammatic many‐body perturbation theory is applied through third order in the correlation energy to the interaction potential between He and a rigid LiH molecule. The ab initio calculations are used to derive an analytic representation of the potential surface in terms of orthogonal polynomials. Several different basis sets are employed to demonstrate the sensitivity of the energies to the computational techniques. The resulting potential surfaces are highly anisotropic with respect to the LiH center‐of‐mass and allow for a weak binding (∼7 meV) of the He to the Li end of the LiH axis.
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34.50.Ez Rotational and vibrational energy transfer

Rotationally inelastic collisions of LiH with He. II. Theoretical treatment of the dynamics

Eugene F. Jendrek and Millard H. Alexander

J. Chem. Phys. 72, 6452 (1980); http://dx.doi.org/10.1063/1.439145 (10 pages) | Cited 42 times

Online Publication Date: 15 July 2008

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Integral cross sections for the rotational excitation of LiH (j=0,1, and 2→j′) in collisions with He have been determined at collision energies of 0.2057, 0.3057, and 0.4057 eV. The rigid‐rotor collision dynamics were solved within the coupled states (CS) approximation. The ab initio potential surface described in the preceding article was used. Sizeable inelastic flux occurs even for large changes in the rotational quantum number, which is probably a reflection of the extreme anisotropy in the potential surface. The dependence of the cross sections on j′, the final rotational quantum number, is not monotonic but displays a pronounced oscillatory structure which is a sensitive function of collision energy. Use of the infinite order sudden approximation results in a disappearance of this oscillatory structure as well as a pronounced lowering in the actual magnitudes of the inelastic cross sections. By contrast excellent agreement with the CS j=1,2→j′ cross sections is provided by sudden limit scaling relations with the CS j=0→j′ cross sections as input. Neither the exponential‐gap nor power‐law parametrizations provide an adequate fit to the CS integral cross sections. The rotationally inelastic cross sections obtained with an electron‐gas potential surface are significantly smaller, because the surface is less repulsive. Thermally averaged cross sections are determined for comparison with the results of the experiment described in the following article.
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34.50.Ez Rotational and vibrational energy transfer

Rotationally inelastic collisions of LiH with He. III. Experimental determination of state‐to‐state cross sections

Paul J. Dagdigian and Bruce E. Wilcomb

J. Chem. Phys. 72, 6462 (1980); http://dx.doi.org/10.1063/1.439146 (4 pages) | Cited 45 times

Online Publication Date: 15 July 2008

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State‐to‐state rotationally inelastic cross sections for the scattering of 7LiH with helium have been determined by the use of electric quadrupole rotational state selection and dye laser fluorescence detection. Results for the incident j=1 state and final states j′=0, 2–15 at an average collision energy of 0.32 eV are reported. As a result of the finite angular acceptance of the laser detection zone and the observed dependence of the cross sections on the scattering path length l, the present values are lower limits to the absolute integral σj=1→j′ values. Because of the theoretical simplicity of the LiH–He system, comparison with accurate quantum calculations for the cross sections (see Papers I and II of this series) has been possible. Calculations using the coupled states dynamical approximation with an ab initio potential surface reproduce well the qualitative behavior of the experimental cross sections. Comparison with results from simpler theoretical models suggests that the accuracy of the potential energy surface employed is crucial for an adequate theoretical description of the dynamics.
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34.50.Ez Rotational and vibrational energy transfer

A method to reduce the number of two electron integral transformations in a second order multiconfigurational Hartree–Fock procedure

Poul Jørgensen, Preben Albertsen, and Danny L. Yeager

J. Chem. Phys. 72, 6466 (1980); http://dx.doi.org/10.1063/1.439147 (8 pages) | Cited 8 times

Online Publication Date: 15 July 2008

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See Also: Erratum

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In standard multiconfigurational Hartree–Fock (MCSCF) theory, a two electron integral transformation is performed before application of the equation defining the MCSCF approach in each iterative cycle. It is shown how the two electron integral transformation may be replaced by an approximate orbital transformation introduced directly into the equation that defines the second order MCSCF approach. In this way, the number of two electron integral transformations required to obtain a set of MCSCF orbitals is reduced considerably. Numerical examples for the 3Σ−g, 1Δg, and 1Σ+g states of O2 indicate that an accuracy of 10−6 a.u. in the total energy may be obtained by carrying out 2–3 two electron integral transformations, which is about half the number of transformations required to obtain the same accuracy in the second order MCSCF approach. An accuracy of 10−10 a.u. is obtained after one further iteration is carried out with a second order MCSCF scheme.
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31.15.A- Ab initio calculations

Photoionization of atomic silver between 17 and 41 eV

M. O. Krause

J. Chem. Phys. 72, 6474 (1980); http://dx.doi.org/10.1063/1.439148 (5 pages) | Cited 15 times

Online Publication Date: 15 July 2008

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Photoelectron spectra of free silver atoms were measured at six energies of the He and Ne discharge lamps. Spectra were obtained at 90 ° and 54.7 ° in relation to the photon beam. The four D term levels were resolved and their branching ratio was found to be nearly statistical and to vary slowly with energy. Spectra indicate the presence of an autoionization resonance near 16.8 eV. The partial photoionization cross section of the 5s level is about 5% of that of the 4d level. The atomic 4d cross section rises slowly above threshold in qualitative agreement with the trend of the atomic potential, and it displays an energy dependence similar to that reported for metallic silver.
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32.80.Fb Photoionization of atoms and ions
33.60.+q Photoelectron spectra

Laser induced fluorescence of Se2 and Te2 in rare gas matrices

V. E. Bondybey and J. H. English

J. Chem. Phys. 72, 6479 (1980); http://dx.doi.org/10.1063/1.439149 (6 pages) | Cited 17 times

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The laser induced fluorescence excitation and resolved emission spectra of Se2 and Te2 suggest that these species are only insignificantly perturbed by the solid medium. The spectroscopy of both molecules in the matrix is governed by efficient nonradiative transitions among numerous low lying electronic states and fast vibrational relaxation. Several new electronic states of these species are observed and characterized.
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33.50.Dq Fluorescence and phosphorescence spectra
33.50.Hv Radiationless transitions, quenching

Exchange formalism applied to one‐dimensional triplet excitons: Observation of restricted and temperature‐dependent k to k′ scattering in the zero‐field optically detected magnetic resonance spectrum of 1,2,4,5‐tetrachlorobenzene

W. G. Breiland and M. C. Saylor

J. Chem. Phys. 72, 6485 (1980); http://dx.doi.org/10.1063/1.439150 (13 pages) | Cited 6 times

Online Publication Date: 15 July 2008

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Zero‐field optically detected magnetic resonance (ODMR) spectra of the D–‖E‖ transition in 1,2,4,5‐tetrachlorobenzene excitons have been obtained for a variety of temperatures (1.2–4.2 °K), microwave powers (0.1–5000 mW), and modulation frequencies (25–1000 Hz). These data have been fit to a theoretical model for band‐to‐band transitions of one‐dimensional triplet excitons that explicitly incorporates the effects of k to k′ scattering, microwave power, and interaction of the band with a thermal bath. It is shown that a proper interpretation of the data can be made only if one considers the actual observables used in monitoring the zero‐field spectrum. Analysis of the data indicates that low‐temperature k to k′ scattering is restricted to k′ states that lie within roughly one third of the total bandwidth from k. This restricted or nonuniform range is temperature independent below 2.2 °K, but increases above this temperature. At temperatures below 1.35 °K, the ODMR line shape depends on the microwave modulation frequency used. Explicit consideration of the actual distribution of one‐dimensional chain lengths and potentially chain‐length dependent effects that occur in a real crystal are also discussed, and questions concerning the validity of an exciton ’’lifetime’’ are raised.
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33.40.+f Multiple resonances (including double and higher-order resonance processes, such as double nuclear magnetic resonance, electron double resonance, and microwave optical double resonance)
71.35.-y Excitons and related phenomena

On band shapes of electronic transitions in the multimode weak coupling limit

P. M. Champion and A. C. Albrecht

J. Chem. Phys. 72, 6498 (1980); http://dx.doi.org/10.1063/1.439151 (9 pages) | Cited 34 times

Online Publication Date: 15 July 2008

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The influence of multidimensional Franck–Condon (FC) active vibrational subspace on the band shapes of electronic transitions is explored in the linear weak coupling limit. Given a constant (weak) coupling coefficient, computer calculated band shapes are seen to fall into three classes depending on the number of active vibrational modes, N. For large N, the electronic band of a polyatomic molecule will have a relatively weak zero–zero absorption compared to the ’’side‐band’’ absorption, which arises from a rapidly increasing density of states (DOS). For small N, a normal, weak FC progression is seen in which the peak absorption is in the zero–zero transition. The implications of the DOS/FC model are further discussed in relation to previous work. Calculations are presented to emphasize the effect of the multidimensional subspace on the relationship between the absorption band shape and the resonance Raman excitation profile.
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33.70.Jg Line and band widths, shapes, and shifts
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.20.Fb Raman and Rayleigh spectra (including optical scattering)

Infrared photodissociation of polyatomic ions

M. J. Coggiola, P. C. Cosby, and J. R. Peterson

J. Chem. Phys. 72, 6507 (1980); http://dx.doi.org/10.1063/1.439152 (6 pages) | Cited 25 times

Online Publication Date: 15 July 2008

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Dissociation of CF3I+, CF3Br+, and CF3Cl+ ions resulting from the absorption of a single 10 μ IR photon by highly vibrationally excited parent ions under collision‐free conditions is reported. In each case, the only observed products are CF3++X. Dissociation yields as a function of photon wavelength for CF3I+ and CF3Br+ peak sharply at ∼950 cm−1 corresponding to a strong absorption frequency in the molecular ion. Product ion translational energy distributions are measured for CF3I+ and CF3Br+ photodissociation, and indicate that the energy release is ≪hν. The observed dissociation can be interpreted as resulting from the unimolecular decomposition of ions, initially formed with a total energy within 900–1000 cm−1 of the disociation limit, that absorb a single infrared photon. Thus, this process corresponds directly to the final step in IR multiphoton dissociation of polyatomic ions.
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82.50.Hp Processes caused by visible and UV light
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.80.Wz Other multiphoton processes
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