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15 Jun 1988

Volume 88, Issue 12, pp. 7267-8019

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Helium emission spectra from doped samples of solid hydrogen and deuterium

R. L. Brooks and J. L. Hunt

J. Chem. Phys. 88, 7267 (1988); http://dx.doi.org/10.1063/1.454337 (6 pages) | Cited 12 times

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Using proton‐beam irradiation at 15 MeV, the emission spectra from 3He and 4He doped samples of solid hydrogen and deuterium have been observed from 350–750 nm at 4.2 K. Such samples simulate the conditions of solid tritium or tritiated hydrogens after about six days of helium accretion. The spectra are very similar to pure helium gas spectra acquired at 4.2 K and closer still to spectra acquired using helium gas in the upper portion of a sample cell partially filled with solid hydrogen. The helium collects as macroscopically large bubbles whose pressure has been determined. The pure 4He gas spectrum at 4.2 K contains a band near 460 nm previously assigned to an unspecified excimer–dimer radiative transition. The pressure and isotope dependence of this band is presented which lends support to that interpretation. A mechanism is proposed for the transition.
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78.40.Ha Other nonmetallic inorganics
33.20.Kf Visible spectra
34.50.Gb Electronic excitation and ionization of molecules
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

The microwave spectrum of CO in the a 3Π state. II. The submillimeter wave transitions in the normal isotope

Norbert Carballo, Hugh E. Warner, Christopher S. Gudeman, and R. Claude Woods

J. Chem. Phys. 88, 7273 (1988); http://dx.doi.org/10.1063/1.454338 (14 pages) | Cited 4 times

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Precise measurements of the frequencies of a number of new rotational transitions of a 3Π CO (v=0–4) in the range 250–500 GHz are reported. In addition earlier measurements of the J=0–1 frequencies near 90 GHz have been repeated at higher precision and extended to include the v=6 lines. The nonlinear least‐squares analysis of this microwave data combined with the lambda doublet transition frequencies of Klemperer and co‐workers was carried out for v=0–4 by direct diagonalization of a Hamiltonian matrix that included the relevant v subblock of the a 3Π state along with the subblocks for v′=0–10 of the a′ 3Σ+ state. The new frequencies make possible the determination from the rf‐microwave data set of several parameters that previously had to be assumed, and the method of treating most of the a–a′ perturbations directly in the large matrix yields parameters, especially lambda doubling constants, which vary less, but more smoothly, with J than before. The later smoothness is found to be enhanced by inclusion of an off‐diagonal, spin–spin interaction in the a–a′ perturbation Hamiltonian, as was done in published fits of FTIR spectra of the a′ 3Σ+–a 3Π system. We also discuss the theory of the perturbed Zeeman effect of a 3Π CO, along with some preliminary measurements, and report a value of the pressure broadening parameter for a typical rotational transition in this state.
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33.20.Bx Radio-frequency and microwave spectra
33.57.+c Magneto-optical and electro-optical spectra and effects
33.20.Sn Rotational analysis

The vibronic theory of resonance hyper‐Raman scattering

Y. C. Chung and L. D. Ziegler

J. Chem. Phys. 88, 7287 (1988); http://dx.doi.org/10.1063/1.454339 (8 pages) | Cited 43 times

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The intensities of vibrational resonance hyper‐Raman transitions are developed in the usual framework of vibronic theory. In analogy to linear Raman scattering, the leading terms of RHR scattering, A, B, and C, are found when the electronic transitions moments, appearing in the transition hyperpolarizability, are expanded to first order in nuclear coordinate dependence. Noncentrosymmetric molecules are predicted to be the strongest nonlinear scatterers and derive activity from the RHR A term. In contrast to the vibronic analysis of linear Raman scattering, both one and two‐photon allowed transitions contribute to RHR scattering cross section at the same order of vibronic theory (B term) for centrosymmetric molecules. RHR scattering spectra of gas phase CH3I, NH3, and CS2 illustrate A‐ and B‐term RHR activity.
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33.20.Fb Raman and Rayleigh spectra (including optical scattering)
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions

Vibrational frequencies for polyatomic molecules. I. Indole and 2,3‐benzofuran spectra and analysis

William B. Collier

J. Chem. Phys. 88, 7295 (1988); http://dx.doi.org/10.1063/1.454340 (12 pages) | Cited 22 times

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A method is presented that can predict the vibrational frequencies of two and three ring polycyclic molecules to an accuracy sufficient for identifying fundamental vibrations. The scaled internal coordinate method of Pulay and co‐workers is applied to force constants calculated with the Austin Method 1 semiempirical Hamilitonian. The technique was applied to benzene, furan, and pyrrole to generate a set of scale factors that would allow the a priori prediction of the vibrational spectra for benzofuran and indole. The predicted frequencies had root mean squared deviations of 15–23 cm−1 from the observed frequencies. Using the indole and benzofuran predictions, the vibrational assignments were made and ideal gas entropies were calculated. These spectroscopic entropies agreed with the calorimetrically determined entropies.
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33.20.Tp Vibrational analysis
33.20.Ea Infrared spectra
65.20.-w Thermal properties of liquids
65.40.gd Entropy

The IR spectrum of ethylene in solid xenon: Host–guest mode coupling

Susan T. Collins, Patricia A. Casey, and George C. Pimentel

J. Chem. Phys. 88, 7307 (1988); http://dx.doi.org/10.1063/1.454341 (5 pages) | Cited 3 times

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Ethenes (C2H4, C2D4, cis‐1,2‐C2D2H2, and trans‐1,2‐C2D2H2) isolated in xenon matrices show dramatic and reversible infrared spectral changes between 12 and 50 K; most bands broaden with increasing temperature. In the xenon matrix, the C2H4 ν7 band is unusually broad, much broader than in the argon spectrum, while the corresponding C2D4 mode remains sharp. Though sharper than C2H4 ν7, many other bands are doublets in which one peak shows different temperature dependence than the other. The C2D4 ν7 band, however, is only a singlet. Among several possible explanations for the unusual breadth, the most likely is that C2H4 ν7 couples with the xenon lattice phonon modes through the librational movement of C2H4 within the cage. The doublets are attributed to two sites with which different modes and isotopes interact to varying degrees.
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33.20.Ea Infrared spectra
63.20.D- Phonon states and bands, normal modes, and phonon dispersion

On the luminescence of Cs2NaGdCl6

A. J. de Vries and G. Blasse

J. Chem. Phys. 88, 7312 (1988); http://dx.doi.org/10.1063/1.454342 (5 pages) | Cited 17 times

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The luminescence properties of Cs2 NaGdCl6 are reported. In the emission, as well as the excitation spectra of the Gd3+ luminescence, vibronic lines are present due to coupling with the ν3 , ν4, and ν6 vibrational modes of the GdCl3−6 octahedron. The magnetic–dipole character of the zero‐phonon transitions is confirmed by the long decay time of the 6 P7/2 emission. Energy migration among the Gd3+ ions is absent, due to the large distance (7.6 Å) between nearest‐neighbor Gd3+ ions. The emission from two extrinsic Gd3+ ions shows a coupling with the O–H stretching and bending modes.
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78.55.Hx Other solid inorganic materials
78.40.Ha Other nonmetallic inorganics

Spectra and structure of organophosphorus compounds. XXXIII. Microwave, infrared, and Raman spectra, structure, conformational stability, and vibrational assignment for ethylphosphonothioic difluoride

J. R. Durig, R. D. Johnson, H. Nanaie, and T. J. Hizer

J. Chem. Phys. 88, 7317 (1988); http://dx.doi.org/10.1063/1.454343 (14 pages) | Cited 5 times

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The microwave spectra of ethylphosphonothioic difluoride, CH3CH2P(S)F2, and eight isotopic species have been investigated in the region from 26.5 to 39.5 GHz. Only a‐type transitions were observed and R‐branch assignments have been made for all the isotopic species in the ground vibrational state for both the gauche and trans (methyl group trans to the P=S bond) conformers from which the rotational constants were determined. From these data the complete r0 structural parameters were determined for the gauche conformer with the values for the heavy atom parameters being: r(C–C)=1.532±0.006 Å, r(C–P)=1.800±0.007 Å, r(P=S)=1.880±0.003 Å, r(P–F)=1.555±0.005 Å, ∢CCP=112.6±0.3°, ∢CPS=119.4±0.4°, ∢CPF=102.0±0.2°, dih ∢FPCS=129.3±0.2°, and dih ∢CCPS=56.9±0.2°. The parameters of the trans conformer which differed significantly from the values for the corresponding ones in the gauche conformer were: r(C–P)=1.814±0.011 Å, r(P=S)=1.861±0.007 Å, and ∢CCP=114.8±0.2°. The infrared (3500 to 40 cm−1) and Raman (3500 to 20 cm−1) spectra of the gaseous and solid CH3CH2P(S)F2 and CD3CD2P(S)F2 as well as the Raman spectrum of the liquid have been recorded. Both trans and gauche conformers have been identified in the vibrational spectra of the fluid phases, but only the trans corformer remains in the solid state and a complete vibrational assignment is proposed for the trans conformer. The barrier to methyl rotation for the trans conformer was determined to be 808 cm−1 (2.31 kcal/mol). The asymmetric torsion for the trans conformer was observed as a series of closely spaced Q branches beginning at 73.25 cm−1 and falling to lower frequency and the gauche transitions begin at 70.82 cm−1. These transitions along with the dihedral angle for the gauche conformer have been used to obtain the potential
function for the asymmetric rotation which indicates that the trans conformer is more stable than the gauche conformer in the gas phase by 63±37 cm−1 (180±106 cal/mol). All of these results are compared with corresponding quantities for several similar organophosphorus compounds.
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33.20.Bx Radio-frequency and microwave spectra
33.20.Ea Infrared spectra
33.20.Fb Raman and Rayleigh spectra (including optical scattering)
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

Effect of nuclear hyperfine structure on microwave spectral pressure broadening

Sheldon Green

J. Chem. Phys. 88, 7331 (1988); http://dx.doi.org/10.1063/1.454344 (6 pages) | Cited 11 times

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The spectral pressure broadening formalism of Ben‐Reuven [Phys. Rev. 145, 7 (1966)] is applied to rotational transitions of a closed‐shell linear molecule with nuclear quadrupole hyperfine structure (hfs) due to a nucleus of spin I. If, as expected, nuclear spin does not affect molecular collision dynamics, generalized pressure broadening cross sections can be expressed in terms of the spin–free collisional S matrices. For the three hfs components of the lowest j=0–1 rotational transition, the line shape is a simple sum of three noninterfering Lorentzians each of which has the same width and shift as would be expected in the absence of nuclear spin. For higher rotational transitions, however, the line shape is no longer so simple; in general, each hfs component is described by a different width and shift, and collisions transfer intensity among them. Numerical results for HCN broadened by He atoms are presented using both the accurate close coupling (CC) collision formalism and also the infinite order sudden (IOS) approximation. For the case that broadening is very large compared with the hfs splittings it is shown (numerically, within the IOS approximation) that the line shape is nearly (but not exactly, except for j=0–1 as noted above) a Lorentzian with the same width as would be expected in the absence of nuclear spin.
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33.20.Bx Radio-frequency and microwave spectra
33.70.Jg Line and band widths, shapes, and shifts
34.50.Ez Rotational and vibrational energy transfer
33.15.Pw Fine and hyperfine structure

Geometry and torsional motion of biphenyl in the ground and first excited singlet state

Hoong‐Sun Im and E. R. Bernstein

J. Chem. Phys. 88, 7337 (1988); http://dx.doi.org/10.1063/1.454345 (11 pages) | Cited 51 times

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The lowest excited singlet state of biphenyl (BP) and a number of its isotopically and chemically substituted analogs have been studied by supersonic jet laser spectroscopy. The symmetry species of this excited state in BP can be classified as B+2u in the G16 extended molecular symmetry group G16 (EM). The symmetry‐allowed origin of the biphenyl ‐h10 S1←S0 electronic transition occurs at 35 268 cm−1. The frequency of the torsional motion in S1 is determined to be ∼65 cm−1. The potential parameters for this motion in S1 are V2=1195 cm−1, V4=−190 cm−1, and V6=18 cm−1. The torsional motion for the ground state (∼50 cm−1) can be described by V2=50 cm−1 and V4=−148 cm−1. The change in the dihedral angle between the two benzene rings in BP upon S0 to S1 excitation is determined to be ∼44° based on a Franck–Condon factor calculation. Several fundamentals of the molecular vibrations are assigned in the S1 state. The exciton interaction between the coupled benzene rings in biphenyl is suggested to be large (>103 cm−1).
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33.50.Dq Fluorescence and phosphorescence spectra
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
33.15.Bh General molecular conformation and symmetry; stereochemistry

Neon matrix ESR investigation of 12C+2 and 13C+2 : Confirmation of a 4Σ−g ground electronic state

Lon B. Knight, S. T. Cobranchi, and E. Earl

J. Chem. Phys. 88, 7348 (1988); http://dx.doi.org/10.1063/1.454346 (9 pages) | Cited 35 times

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The 12C+2 and 13C+2 cation radicals have been isolated in neon matrices by the photoionization of C2(g) generated by the laser vaporization of thin carbon disks. A new procedure of supporting and thermally outgassing such targets for laser vaporization ESR studies in rare gas matrices is described. The ESR results confirm the 4Σ−g ground electronic state assignment for C+2 and provide a valence orbital electronic structure characterization based on the 13C hyperfine structure (hfs). The ESR lines are extremely narrow for a high‐spin powder sample and an intense off‐angle or ‘‘extra’’ absorption feature exibits unusual hfs. Normally forbidden (ΔMS>1) transitions were detected at low magnetic fields. The magnetic parameters for 13C+2 in a neon matrix at 4 K are: D=3668(3) MHz; g⊥ =2.0018(5); g∥ =2.0023(5); ‖A∥‖ =99.4(8) MHz; and ‖A⊥‖ =95.2(5) MHz.
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33.35.+r Electron resonance and relaxation
76.30.Rn Free radicals

Determination of population, alignment, and orientation using laser induced fluorescence with unresolved emission

Andrew C. Kummel, Greg O. Sitz, and Richard N. Zare

J. Chem. Phys. 88, 7357 (1988); http://dx.doi.org/10.1063/1.454347 (12 pages) | Cited 41 times

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A method is presented for determining the population, A{0}0+, the alignment moments, A{2}q± and A{4}q±, and the orientaion moments, A{1}q± and A{3}q±, for a ground state distribution of diatomic molecules probed by 1+1 laser induced fluorescence. General expresssions are developed for all rotational branches as a function of the rotational quantum number for excitation with linearly, circularly, or elliptically polarized light. Specific expressions are evaluated for the case in which the emission is unresolved and collected independent of its polarization and for the case in which the emission is unresolved but is analyzed with a polarizer. When the emission is collected independent of its polarization, the real polarization moments, A{k}q±, cannot be independently determined and only the apparent moments, A{k}q± (app), can be measured, explicit expressions for the apparent moments in terms of the real moments are presented. However, for the case in which the excitation light is created by passing linearly polarized light through a quarter‐wave plate and the emitted light is analyzed with a quarter‐wave plate and a linear polarizer, the real alignment and orientation moments can be independently determined.
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33.50.Dq Fluorescence and phosphorescence spectra
33.80.Be Level crossing and optical pumping

Solvent perturbations on the excited state symmetry of randomly oriented molecules by two‐photon absorption

C. E. Mohler and M. J. Wirth

J. Chem. Phys. 88, 7369 (1988); http://dx.doi.org/10.1063/1.454348 (7 pages) | Cited 2 times

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The polarization dependence of two‐photon absorption is used to determine the excited state symmetry of solute molecules dissolved in room temperature liquids. Perturbations of excited state symmetry are interpreted as solvent induced state mixing in the solute. Using a first‐order expansion in the zero‐order system wave functions, an expression is derived which describes the effects of solute–solvent electric dipole interactions on the symmetry of the solute excited state. Measurements on the S0→S1 0–0 band of fluorene in nonassociating polar and nonpolar liquids are fit well by the derived expression, using continuum reaction field models. Deviations from the derived expression are found when fluorene is dissolved in liquids capable of self‐association or complexation with aromatic solutes. In n‐alcohol solvents, perturbations on the excited state symmetry of fluorene are adequately accounted for using Kirkwood g factors for the solvent.
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31.70.Dk Environmental and solvent effects
33.80.Wz Other multiphoton processes
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors

Observation of ionic excimer states in the alkali halides

F. Steigerwald, H. Langhoff, and W. Hammer

J. Chem. Phys. 88, 7376 (1988); http://dx.doi.org/10.1063/1.454349 (4 pages) | Cited 12 times

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Ionic excimers, which are isoelectronic to the rare gas halides, are investigated. In a charge transfer reaction of He+ or Ne+ ions to CsF and RbF, continua in the VUV at 184.5 and 131 nm, respectively, have been observed, which show the same properties as the analogous transitions in the rare gas fluorides. The results are compared with predictions by a Rittner model. In KF a transition at 143 nm was observed which could be assigned to the transition of KF+→K+F.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
78.60.Ps Chemiluminescence
33.50.Dq Fluorescence and phosphorescence spectra

Zero‐field and high‐field electron paramagnetic resonance (EPR) of Cr3+ in some cage complexes

Steven J. Strach and Richard Bramley

J. Chem. Phys. 88, 7380 (1988); http://dx.doi.org/10.1063/1.454350 (7 pages) | Cited 5 times

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High‐field and zero‐field electron paramagnetic resonance spectra of polycrystalline [Cr diamsar]Cl3⋅H2O, [Cr diamsar H2]Cl5⋅xH2O, and 1% Cr/rac[Co diamsar H2]Cl5⋅1.5H2O were recorded (diamsar≡1,8‐diamino‐3,6,10,13,16,19‐hexaazabicyclo [6.6.6] eicosane). Combining the high‐ and zero‐field spectra made determination of the spin‐Hamiltonian parameters from powder spectra an easy matter, even with two sites in the cobalt host case. Full matrix refined parameters are given, but an iterative perturbation method was used with the actual field B rather than the central field, B0=hν/gμB, to get accurate parameters. Corrected perturbation formulas are given. For a rigid cage ligand like diamsar, there is considerable variation in the spin‐Hamiltonian parameter D and evidence that the principal axis of the corresponding interactions does not necessarily lie close to the ligand’s threefold axis.
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76.30.Fc Iron group (3d) ions and impurities (Ti-Cu)

Extraction of differential cross sections by the method of moments. I. Photodissociation

G. Bandukwalla and M. F. Vernon

J. Chem. Phys. 88, 7387 (1988); http://dx.doi.org/10.1063/1.454351 (9 pages) | Cited 4 times

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We describe a general way of extracting the center‐of‐mass frame differential photodissociation cross section from the space‐fixed fragment time‐of‐flight distributions obtained in laboratory experiments. Our approach is a variant of the method of moments and involves only elementary numerical computations. Experimental resolution and error analysis are naturally incorporated in the methodology. Several numerical examples are provided for the purpose of illustration.
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34.50.Rk Laser-modified scattering and reactions
33.80.Gj Diffuse spectra; predissociation, photodissociation

A photoionization study of the vinyl radical

J. Berkowitz, C. A. Mayhew, and B. Ruščić

J. Chem. Phys. 88, 7396 (1988); http://dx.doi.org/10.1063/1.454352 (9 pages) | Cited 28 times

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The photoionization spectrum of vinyl radical is reported, from its observed threshold to 1160 Å. Two methods of preparation have been employed; (a) the abstraction reaction of F atoms with C2H4, and (b) the pyrolysis of divinyl mercury at 1200 K. In both experiments, relatively sharp autoionization structure is observed, and interpreted as a Rydberg series converging to the excited 3A″ state of vinyl cation. The analysis leads to an adiabatic ionization energy of ∼10.7 eV for this state, with a structure similar to that of vinyl radical but with an increased C–C distance. The observed ionization threshold for the ground state of vinyl cation is 8.59±0.03 eV with the F atom reaction, and 8.43±0.03 eV with the pyrolysis method. The lower value in the latter experiment is interpreted as a hot band. The relatively low value of the photoionization cross section near threshold implies a large geometry change between vinyl radical and ground state vinyl cation. A progression in the in‐plane C–H bending vibration is indicated in the photoionization spectrum; it is quite possible that the vibrational 0–0 transition lies one quantum lower than our detected limit. With this bracketed adiabatic ionization potential and the appearance potential of C2H+3 (C2H4), a C–H bond energy in ethylene of 107–110 kcal/mol (0 K) is deduced.
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33.80.Eh Autoionization, photoionization, and photodetachment
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy

Theoretical study of structures and energies of acetylene, ethylene, and vinyl radical and cation

Larry A. Curtiss and John A. Pople

J. Chem. Phys. 88, 7405 (1988); http://dx.doi.org/10.1063/1.454303 (5 pages) | Cited 53 times

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A systematic study of the structures and energies of acetylene, ethylene, and vinyl radical and cation using high levels of ab initio molecular orbital theory, including correlation effects beyond fourth order perturbation theory, is presented. In agreement with previous theoretical studies, the present theory predicts that the bridged form of C2H+3 is more stable than the classical form with an energy difference of 3.1 kcal/mol (including zero point energies). The bond dissociation energy of C2H4,D0(C–H), is calculated to be 110.2 kcal/mol in agreement with the recent photoionization study of Berkowitz et al. and in significant disagreement with several previous experimental studies. Finally, triplet vinyl cation is studied for the first time and is calculated to be 54.5 kcal/mol higher in energy than the singlet ground state.
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31.15.V- Electron correlation calculations for atoms, ions and molecules
33.15.Fm Bond strengths, dissociation energies
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
33.15.Bh General molecular conformation and symmetry; stereochemistry

Surface crossings and predissociation dynamics of methyl iodide Rydberg states

D. J. Donaldson, Mark S. Child, and Veronica Vaida

J. Chem. Phys. 88, 7410 (1988); http://dx.doi.org/10.1063/1.454304 (8 pages) | Cited 21 times

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We report the results of a calculation of the adiabatic surface crossings between the B and C Rydberg states of CH3I and the dissociative A state. The bound potentials are derived empirically; the dissociative states are based on recent empirical and ab initio results. The surface crossings predissociate the B state at an energy below that of v=1 in the C–I stretching mode, and near the classical turning point for v=2 of the methyl umbrella mode. Our model predicts that at least two dissociative potentials are of importance in the B state predissociation. These results are in agreement with experimental predictions. Classical trajectory results suggest that the predissociation might exhibit mode specificity. The formation of methyl iodide dimers shifts the surface crossings to higher energy, which allows the absorption to v=1 of the C–I stretch and v=2 of the umbrella mode to be observed. This result is also in agreement with experiment.
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31.50.Df Potential energy surfaces for excited electronic states
33.80.Gj Diffuse spectra; predissociation, photodissociation

Nitric oxide photofragment rotational distribution from UV dissociation of cyclopentadienylnickel nitrosyl

Savas Georgiou and Charles A. Wight

J. Chem. Phys. 88, 7418 (1988); http://dx.doi.org/10.1063/1.454305 (6 pages) | Cited 7 times

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The rotational state distribution for NO produced by dissociation of (η5–C5H5)NiNO near 225 nm has been obtained by [1+1] resonance‐enhanced multiphoton ionization. The distribution is approximately characterized by a Boltzmann distribution at T=900±200 K. The relative populations of the two spin–orbit components of the ground 2Π manifold are also consistent with this temperature. No preference is observed for populating either of the lambda–doublet components, even at high rotational quantum numbers. Comparison of the results with statistical models shows that the state distribution is significantly colder than expected. One plausible explanation for this is that the parent molecule dissociates to produce cyclopentadienylnickel fragments in an excited electronic state.
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82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
82.20.Rp State to state energy transfer

The effect of reagent rotation in the reaction OH( j)+H2( j′)→H2O+H

Judith A. Harrison and Howard R. Mayne

J. Chem. Phys. 88, 7424 (1988); http://dx.doi.org/10.1063/1.454306 (10 pages) | Cited 34 times

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Classical trajectory calculations on the reaction OH( j)+H2( j′)→H2O+H have been carried out with j≤40 and j′≤15 on both the Schatz–Elgersma [Chem. Phys. Lett. 73, 21 (1980)] and the Rashed–Brown [J. Chem. Phys. 82, 5506 (1985)] potential energy surfaces. When there is no rotation in the OH, then a plot of reaction cross section, SR( j′) resembles that for an atom–diatom system: Just above threshold, rotation decreases reactivity for small j′, but increases it for high j′; at higher translational energies this trend is less obvious, but still present. When j′=0, then SR( j) is a complicated function, decreasing for low j, then climbing to a maximum, finally decreasing once more at very large values of j. We have also carried out calculations with isotopically substituted H in OH, and show that these effects scale as the mass of the hydrogen isotope. We show that this behavior is due to artifacts in both the potential surfaces. Using a simple model we are able to rationalize this behavior. Using this same model, together with a potential in which the artifacts are absent, we predict that rotation in OH will decrease reactivity rather less than was reported in previous trajectory studies [Schatz, J. Chem. Phys. 74, 1133 (1981); Rashed and Brown, J. Chem. Phys. 82, 5506 (1985)].
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Rp State to state energy transfer
82.20.Fd Collision theories; trajectory models

Dynamics of energy flow from CH overtone excitations: Theoretical and experimental studies of CH3C≡CH

P. Hofmann, R. B. Gerber, M. A. Ratner, L. C. Baylor, and E. Weitz

J. Chem. Phys. 88, 7434 (1988); http://dx.doi.org/10.1063/1.454307 (14 pages) | Cited 27 times

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A study has been made of the vibrational energy flow mechanisms and time scales pertaining to the overtone stretch excitations of methyl and acetylenic CH stretches in propyne. Classical trajectories are used to interpret the experimental data for the overtone linewidths, as well as to analyze the role that individual modes play in determining energy flow. The full anharmonic potential surface for these calculations, including all modes, has been developed from spectroscopic and structural information, including the linewidth data. The principal results are: (1) The trajectory calculations show a localization transition, corresponding to a switch over from normal‐mode behavior for CH3 excitations up to v≅3 to a local‐mode CH excitation within the CH3 moiety for excitations of v≳6, with transition behavior for v=4,5. (2) The acetylenic CH shows local‐mode behavior from v=1. Extremely long lifetimes are found for the excitations of this mode, and the trajectories indicate that the experimental width is predominantly rotational. (3) The rocking and deformation modes are dominant receiving modes in the relaxation of the methyl stretch. (4) A shorter lifetime is calculated for the v=6 vs the v=5 or v=7 overtones of the methyl C–H stretch. Experimental results are qualitatively consistent with this prediction. The origin of this shorter lifetime is a band of resonances between the stretch excitation and combinations of rocking, deformation, and pseudorotation modes. (5) CH3 internal rotation figures importantly in the relaxation of some levels (v=5, 8 of CH3) where it ‘‘closes the energy gap’’ for achieving resonant energy transfer. (6) For v=8 of the methyl CH, some direct energy transfer to both C–C☒C stretching modes is seen. The switching on of the stretches as receiving modes is a consequence of sufficiently strong interactions between the excited H and the C–C☒C chain, which take place at these high vibrational energies. (7) Evidence is found for long distance ‘‘through‐space’’ energy transfer due to long‐range dipole–dipole forces. This transfer occurs from the acetylenic to the methyl CH stretches. This result is illustrated for the v=2 excitation of the acetylenichydrogen, and constitutes a direct demonstration of intramolecular long‐distance, through‐space v–v energy transfer. These results demonstrate the potential importance of large amplitude modes such as rocking and deformation as initial receiving modes for vibrational energy from excited CH overtones. On the time scale probed here (∼1 ps), despite the availability of many degrees of freedom, the transfer process is dominated by specific energy transfer channels and by the specific behavior of individual modes, rather than by statistical considerations, which will certainly prevail on longer time scales.
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33.20.Ea Infrared spectra
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

Effective collision cross sections for SF6 from nuclear magnetic relaxation

Cynthia J. Jameson and A. Keith Jameson

J. Chem. Phys. 88, 7448 (1988); http://dx.doi.org/10.1063/1.454308 (5 pages) | Cited 9 times

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The thermal average cross sections for transfer of rotational angular momentum in SF6 on collision with Ar, Kr, Xe, N2, CO, HCl, CO2, CH4, CF4, and SF6 have been determined from pulse measurements of 19F nuclear spin relaxation times in binary mixtures of gases at 260–400 K.
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34.50.Ez Rotational and vibrational energy transfer
33.25.+k Nuclear resonance and relaxation

Formation and dissociation of negative ion resonances in methanol and allylalcohol

Alexander Kühn, Heinz‐Peter Fenzlaff, and Eugen Illenberger

J. Chem. Phys. 88, 7453 (1988); http://dx.doi.org/10.1063/1.454309 (6 pages) | Cited 24 times

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Dissociative electron attachment by methanol, different deuterated methanols, and allylalcohol is studied including an analysis of the translational excess energy release of the ionic fragments. Although the energetic threshold for negative ion formation (OH−, O−) is near 2 eV, these ions are generated in methanol only within a prominent resonance at 10.5 eV. OH− and OD− formation is characterized by hydrogen scrambling in the temporary parent ion while the formation of CH3O− (and the deuterated analog) proceeds directly (no hydrogen scrambling). Only O− is generated with considerable translational excess energy (1.0±0.2 eV). In contrast to methanol, allylalcohol captures electrons at 1.7 eV to form OH− and the enolate anion (M–H)−. This resonance is interpreted as a Π radical anion comparable to that known in ethylene.
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34.80.Gs Molecular excitation and ionization

Spatially inhomogeneous structures in a one‐dimensional array of Brussellators

A. Lahiri and S. S. Ghosal

J. Chem. Phys. 88, 7459 (1988); http://dx.doi.org/10.1063/1.454310 (9 pages) | Cited 5 times

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Considering a one‐dimensional array of diffusively coupled Brussellators, we examine the bifurcations of the steady homogeneous solution to steady but spatially inhomogeneous structures. Several important aspects of such bifurcations are related to branching (see below) of the fixed point of a four‐dimensional conservative map. In addition to the usual period doubling (‘‘wavelength doubling’’ in our terminology) observed in case of two‐dimensional maps, the four‐dimensional map is seen to exhibit a new type of branching, namely, giving rise to an invariant curve in phase space. In case of subthreshold branching of either type, the steady homogeneous solution bifurcates to either a ‘‘wavelength‐two’’ or a quasiperiodic solution. In case of superthreshold branching, on the other hand, bifurcations involving more complicated spatial and temporal behavior are possible. Other relevant questions are dealt with.
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82.40.Bj Oscillations, chaos, and bifurcations

Transition rates in a bistable system driven by external dichotomous noise

Ivan L’Heureux and Raymond Kapral

J. Chem. Phys. 88, 7468 (1988); http://dx.doi.org/10.1063/1.454311 (10 pages) | Cited 21 times

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A study of dichotomous noise‐induced transitions is presented for a simple one‐dimensional system exhibiting deterministic bistability between two steady states. A phenomenological rate law for the kinetics of such transitions is derived and the corresponding rate coefficient is evaluated. Critical slowing down for such transitions is shown to be possible and an asymptotic scaling form for the rate coefficient is derived. Finally, memory effects and the breakdown of the phenomenological rate law due to the magnitude of the noise correlation time are discussed.
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82.40.Bj Oscillations, chaos, and bifurcations
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