Volume 89, Issue 1, 01 July 1988
Index of content:

Structural relaxation mechanisms in liquid Eugenol. A depolarized light scattering study
View Description Hide DescriptionA depolarized light scattering study of liquid Eugenol, over a large temperature range including the supercooled region, is proposed. Comparisons with shear mechanical impedance measurements, obtained at lower frequencies, lead to more precise information on the viscoelastic parameters in the supercooled region. The structural relaxation process measurements by means of the photon correlation technique are compared to the dielectric and mechanical measurements. Molecular mechanisms are proposed.

The resonance Raman and visible absorbance spectra of matrix isolated Mn_{2} and Mn_{3}
View Description Hide DescriptionThe resonance Raman spectrum obtained from manganese containing argon matrices previously assigned to the dimer [M.Moskovits, D.P.DiLella, and W.Limm, J. Chem. Phys. 8 0, 626 (1984)] is shown to belong to a zero order D _{3h } trimer subject to Jahn–Teller distortions. The actual resonance Raman spectra of the dimer in both argon and krypton matrices are reported and discussed along with the visible absorption spectra. The resonance Raman of manganese dimer in krypton gives the ^{1}Σ^{+} _{ g } ground state constants of ω_{ e } =76.4 cm^{−} ^{1} and ω_{ e } x _{ e }=0.53 cm^{−} ^{1}. Trimer constants from the resonance Raman spectra in argon are (ω_{ s }−2x _{ s })=196.79 cm^{−} ^{1}, ω_{ a }=130.15 cm^{−} ^{1}, Dω_{ a } =2.95 cm^{−} ^{1}, x _{ a }=0.06 cm^{−} ^{1}, and x _{ a s }=0.79 cm^{−} ^{1}. Independent values of ω_{ s } and x _{ s } could not be determined from our spectral data.

Red and near‐infrared laser‐induced emission of S_{2} in an Ar matrix
View Description Hide DescriptionThe S_{2} emission in the red and near‐infrared regions has been reinvestigated using the laser‐induced emission technique. Four progressions of S_{2} in solid Ar were observed in the emission spectra following excitation in the UV region with a pulsed Nd‐YAG laser system. ^{3} ^{4}S‐isotopic shifts allowed the determination of spectroscopic parameters for these progressions. Among them, ν_{0} _{0}=19 757 and 15 417 cm^{−} ^{1} for the two progressions with distinct zero‐phonon lines (ZPL); ν_{0} _{0}=19 384 and 15 003 cm^{−} ^{1} for the other two with no ZPL, respectively. The progressions are assigned as the c ^{1}Σ^{−} _{ u }→X ^{3}Σ^{−} _{ g } and the c ^{1}Σ^{−} _{ u } →a ^{1}Δ_{ g } transitions of S_{2} in two matrix sites, respectively. The data yield the T _{0} values for the c ^{1}Σ^{−} _{ u } and the a ^{1}Δ_{ g } states. The relative intensities of these four progressions varied with excitation wavelengths and isotopic species.

CO(A–X) electric dipole transition moment
View Description Hide DescriptionThe internuclear dependence of the CO (A ^{1} π– X ^{1} Σ^{+} ) transition moment has been obtained for the range 1.0<r̄<1.8 Å. This was done by making laser induced fluorescencemeasurements on highly vibrationally excited CO. These data were jointly fit with fluorescence lifetime data from a previous study done by Field e t a l. The second‐order fit obtained was 〈R _{ e }(r̄)〉 =10.57(41)[1−1.177(22)r̄+0.350(13)r̄^{2}] D. This fit provides a good description of the transition moment function in that it quantitatively fits the fluorescence branching ratio data and lifetime data and agrees with the qualitative observation of CO(A–X) transition strengths.

(e, 2e) momentum spectroscopic study of the interaction of –CH_{3} and –CF_{3} groups with the carbon–carbon triple bond
View Description Hide DescriptionRelative momentum densities have been measured for the outermost occupied π orbitals of acetylene, propyne, 2‐butyne, and perfluoro‐2‐butyne using the (e, 2e) technique with an improved multiple detector spectrometer. The results, when compared with suitably averaged quantum mechanical calculations, show systematic differences between the molecules which can be interpreted in terms of wave function averages and differences using the ΔB(r) functions. A striking feature of the data is the significant momentum density at zero momentum for all of the molecules, in contrast to the calculations that all show zero momentum density. Ground state molecular distortions are discussed as a possible cause of the effect.

Theory of time‐resolved coherent anti‐Stokes Raman scattering from molecules in liquids. Effects of coherence transfer
View Description Hide DescriptionEffects of the coherence transfer induced by the molecule–heat bath interactions on the ultrashort time‐resolved coherent anti‐Stokes Raman scattering(CARS) from molecules in liquids are theoretically studied. Based on the perturbative density matrix formalism an expression for the CARS intensity is derived taking into account the coherence transfer between the Raman active vibrational transitions of two molecules in liquids. The coherence transfer constants and dephasing constants are properly incorporated with the aid of Liouville space Feynman diagrams. The structure of the coherence transfer matrix element which expresses the time evolution of the coherence between the relevant transitions is clarified by solving the Master equation with the coherence transfer and dephasing constants in the Markoff approximation. Frequency shifts of the quantum beats appear in the time‐resolved CARS as a result of the coherence transfer. A multispherical layer model is adopted in evaluating the coherence transfer effects in liquids in femtosecond time domains. Model calculations of time‐resolved CARS spectra have been carried out to demonstrate the coherence transfer effects in both short and long range coherence transfer cases. It is predicted that the quantum beats are amplified in the time‐resolved CARS spectra of molecules in liquids in a long range coherence transfer case when there exist differences in the coherence transfer constants between each spherical layer.

The B ^{1}Π_{ u } potential energy curve and dissociation energy of ^{3} ^{9}K_{2}
View Description Hide DescriptionThe ^{3} ^{9}K_{2} B ^{1}Π_{ u } potential energy curve has been determined using laser spectroscopic techniques and quantum mechanical calculations. The dissociation energy is 2407.6±0.5 cm^{−} ^{1} (0.2985±0.0001 eV) including a potential barrier of 298±10 cm^{−} ^{1} (0.037±0.0013 eV) found with its maximum at 8.08±0.05 Å (15.3±0.1 bohr). The long‐range behavior matches smoothly onto the form predicted from dispersion forces. The dissociation energy of the ground stateX ^{1}Σ^{+} _{ g }, obtained by a long‐range extrapolation of the vibrational separations, is D _{ e } =4444±10 cm^{−} ^{1} (0.5506±0.0013 eV), in agreement with recent theoretical prediction.

Depolarization ratio of the ν_{1} Raman band of liquid carbon tetrachloride in the range 0–1.1 kbar
View Description Hide DescriptionThe depolarization of the totally symmetric (ν_{1}) Raman band of liquid carbon tetrachloride has been measured at 22 °C and pressures up to the freezing pressure of 1.1 kbar using the scattering geometry Z(Y Z)Y, which causes little depolarization by the windows. The depolarization decreases linearly from 6.5×10^{−3} at zero pressure to 5.6×10^{−3} at 1.15 kbar. The change is explained as the resultant of a decrease of both the vibrational amplitude and the mean intermolecular distance under pressure.

Fine structure of triplet exciton polarons in polydiacetylene molecules
View Description Hide DescriptionTriplet states on conjugated polydiacetylene chains which are created by UV excitation are examined experimentally using ODMRspectroscopy. The observed fine structure shows that the triplet state can be ascribed to the conjugated chain rather than to the side groups and that it is localized. This leads to the suggestion of an exciton polaron. In the theoretical part the wave function of the exciton polaron is calculated using the configuration model in analogy to the description of p _{ z } radical electrons on carbene chain ends of reactive short‐chain intermediates. The total fine structuretensor is obtained by summing up the fine structuretensors of the individual configurations weighted by their probability densities. The transfer integral t of the configuration model is fitted with respect to good agreement between experimentally observed and calculated fine structure parameters. Finally, comparison between the experimentally observed ESRlinewidth and the calculated hyperfine structure splitting shows that the linewidth is motionally narrowed leading to the conclusion that the exciton polaron is mobile.

^{3} ^{1}P spectral spin diffusion in crystalline solids
View Description Hide DescriptionWe give a derivation for the line shape function g(ω) involving the spin diffusion constant T _{SD}, which had earlier been given by Suter and Ernst, and by Henrichs e t a l. using different methods. There are three approximations which lead to different ways to evaluate the zero quantum line shape function g(ω). The first used by Suter and Ernst, another by Henrich e t a l., and the third which we show leads to the same equations as the latter authors. The angular dependencies of T _{SD} for ^{3} ^{1}P spectral spin diffusion in single crystals of dipotassium α‐D‐glucopyranose‐1‐phosphate dihydrate (GLP) and triphenylphosphine (TPP) were determined. We conclude that the ^{3} ^{1}P spin diffusion under the influence of extraneous protons is well described by a Gaussian zero quantum line shape [Eq. (20)] but not by a Lorentzian function. This distinguishes the present situation from the one treated by Suter and Ernst. It is also confirmed by theory and experiment that the equation for T _{SD} given by Henrich e t a l. is satisfactory when the S _{1} and S _{2} spins involved in the flip–flop process are quite distant from each other.

The visible photoabsorption spectrum of Ar^{+} _{3}
View Description Hide DescriptionThe photodissociation cross section of Ar^{+} _{3} was measured at a number of wavelengths between 1064 and 320 nm. A single broad and featureless band was observed peaking near 520 nm with a width of ≈2600 cm^{−} ^{1} and a peak cross section of ≈10^{−} ^{1} ^{6} cm^{2}. Consideration of the electronic structure of Ar^{+} _{3} indicates that the measured spectrum is equivalent to the photoabsorptionspectrum. Two ionic products, Ar^{+} and Ar^{+} _{2}, were observed in the photodissociation of Ar^{+} _{3}, indicative of at least two exit pathways and suggestive of two electronic transitions.

Vibrational branching ratios following two‐color excitation of autoionizing n p Rydberg states of H_{2}
View Description Hide DescriptionTwo‐color resonantly enhanced multiphoton ionization‐photoelectron spectroscopy (REMPI‐PES) was used to determine vibrational branching ratios following autoionization of the ungerade n pσ ^{1}Σ^{+} _{ u } and n pπ ^{1}Π_{ u }Rydberg states of H_{2}. In this two‐step experiment, one laser used to excite the two photon transition to the E,F ^{1}Σ^{+} _{ g }, v’=E2, J’=1 state, and a second laser was used to access the autoionizing Rydberg states near the H^{+} _{2} X ^{2}Σ^{+} _{ g }, v ^{+}=2 ionization limit. Electrons corresponding to the formation of H^{+} _{2} X ^{2}Σ^{+} _{ g }, v ^{+}=0 and 1 were collected and energy analyzed using a magnetic bottle electron spectrometer. In agreement with the well‐known propensity rule for vibrational autoionization, the vibrational branching ratios strongly favor the final ionic state that corresponds to the minimum change in vibrational quantum number. In general, the branching ratio into the v ^{+}=1 channel is 94%–96%, while that into the v ^{+}=0 channel is 4%–6%; however, two major deviations from this trend were observed for Rydberg states that are perturbed by the 3pπ ^{1}Π_{ u }, v=9 and 4pσ ^{1}Σ^{+} _{ u }, v=7 states. Although these low n/high v interlopers were not observed in the present work (since their ionization efficiency is near zero), interchannel coupling apparently causes their influence to be felt by nearby Rydberg states, resulting in v ^{+}=0 branching ratios as high as 18%. A number of additional studies suggested by these initial results are discussed.

Rotationally resolved electronic spectroscopy of tryptamine conformers in a supersonic jet
View Description Hide DescriptionWe have observed the high resolution fluorescence excitation spectrum of the molecule tryptamine in the gas phase. At low resolution the spectrum contains six features which have been assigned as the origins of different conformers of the tryptamine molecule. At high resolution the rotational structure in each of these features has been resolved, and the rotational structure of five of the features has been analyzed. This analysis has provided information about the geometries of the different conformers. Two conformers, labeled A and F, have the amino group of the molecule g a u c h e to the indole ring, while conformer D has the amino group nearly eclipsed by the indole ring. Conformer B has a rotational structure identical to that of conformer A, while the rotational structure of conformer E is identical to that of conformer D. It is suggested that the pairs of conformers with identical rotational structure are related to each other by rotation about the C_{α}–N bond, such a rotation moving only hydrogen atoms. Feature C consists of two overlapped conformers, and it is suggested that these conformers have the amino group t r a n s to the indole ring. The direction of the transition moment is measured for five of the conformers and is found to be identical for all conformers within the precision of the measurement. The direction of the transition moment indicates that the transition is to the ^{1} L _{ b }excited electronic state.

Upon the application of cumulant analysis to the interpretation of quasielastic light scattering spectra
View Description Hide DescriptionMonte Carlo simulations were used to study how noise limits the interpretation of quasielastic light scatteringspectra with cumulant analysis. The effects of signal‐to‐noise ratios, channel spacing, line shape, and the truncation order of the cumulant series were all examined. The underlying spectra included narrow and broad unimodal and bimodal forms, as expected for ternary polymer:polymer:solvent mixtures. The upper bound on the usable number of cumulants is determined by the signal‐to‐noise ratio, the spectral nonexponentiality, and the number and spacing of the spectrometer channels. Upper limits on the acceptable degree of spectral nonexponentiality—below which the cumulant expansion is useful—can be stated in terms of the measured second cumulant. Use of logarithmically rather than linearly spaced correlator channels greatly increases the region over which cumulant fits are useful.

Vibrational predissociation in the CO_{2} dimer and trimer and rare gas–CO_{2} complexes
View Description Hide DescriptionVibrational predissociationlinewidths for the CO_{2} dimer and trimer and the Ne–CO_{2} and Ar–CO_{2} complexes have been resolved using a bolometer‐detected (optothermal) molecular‐beam color‐center laser spectrometer. Observations were made on the pair of vibrations near 3715 and 3613 cm^{−} ^{1} corresponding to the ν_{1}+ν_{3}/2ν^{0} _{2}+ν_{3} Fermi diad of CO_{2}. Homogeneous linewidths of from ∼0.5 to ∼22 MHz (FWHM) were measured for these related complexes, with Ne–CO_{2} exhibiting both the broadest and the sharpest lines for the upper and lower bands, respectively. Ar–CO_{2} and (CO_{2})_{2} showed mode‐independent intermediate predissociation rates while only the lower band of (CO_{2})_{3} could be found. The results indicate that V→V energy transfer processes are the dominant predissociation channels with symmetry selection or propensity rules and specific resonances playing a role.

Velocity modulation infrared laser spectroscopy of negative ions: The (011)–(001) band of azide (N^{−} _{3})
View Description Hide DescriptionWe have measured 43 transitions centered at 1972 cm^{−} ^{1} in the (011)–(001) bending hot band of the azide ion (N^{−} _{3}) using diode laser velocity modulation spectroscopy of an NH_{3}/N_{2}O discharge. The data, ranging from P(32) to R(40), were fit to a standard l‐type doubling Hamiltonian through quartic terms. The l splittings in the spectrum were unusually large compared with similar molecules. Intensity measurements indicate that the vibrational and rotational degrees of freedom are equilibrated, unlike the case for isoelectronic NCO^{−}. Additional lines of the ν_{3} fundamental have also been measured, which further refine the values of the ground state parameters.

Microwave and infrared studies of acetylene dimer in a T‐shaped configuration
View Description Hide DescriptionRadio frequency,microwave, and infrared spectra have been obtained from molecular beams of acetylene dimer. The geometry of the isomer studied here is T shaped, with the centers of mass of the monomers separated by 4.38 Å. The equilibrium structure appears not to have C _{2v } symmetry and the vibrationally averaged structure has the ‘‘top’’ of the T rotated about its center of mass by 27°. The electric dipole moment of this complex is 0.28 D.

Microwave spectrum, structure, mercury‐201 quadrupole coupling constant, and dipole moment of methylmercury cyanide
View Description Hide DescriptionThe microwave spectrum of CH_{3}HgCN has been measured for the ground vibrational state and the lowest bending mode ν_{1} _{0}. The rotational constants of 25 isotopic species have been measured and used to calculate the following substitution structure: Hg–C (methyl)=2.0563(1), Hg–C(cyanide)=2.0369(2), C–N=1.1570(2), C–H=1.098(10) Å, and HCH angle=109.1(3)°. The average zero‐point structure has also been estimated. The nuclear quadrupole coupling constant of ^{2} ^{0} ^{1}Hg (I=3/2) has been determined as e Q q=−1313.8(13) MHz, along the axis of the symmetric top for the vibrational ground state. The dipole moment has been determined from Stark effectmeasurements on the l‐type doublets of V _{1} _{0}=1 as 4.69(4) D. The l‐type doublet constant was measured as q _{1} _{0}=2.794(11) MHz.

Infrared spectroscopy of carbo‐ions. IV. The A ^{2}Π_{ u }–X ^{2}Σ^{+} _{ g } electronic transition of C^{−} _{2}
View Description Hide DescriptionThe infrared spectrum of the A ^{2}Π_{ u }←X ^{2}∑^{+} _{ g } electronic transition of C^{−} _{2} has been observed under high resolution and analyzed. Three bands (v’←v)=(0,0), (1,1) and (0,1) have been observed; the first two bands were observed by using the difference laser frequency system in the frequency range of 3960–3780 cm^{−} ^{1} and the last by using a diode laser in the frequency range of 2210–2120 cm^{−} ^{1}. A gas mixture of 50 mTorr of acetylene and 7 Torr of He was used for the ac discharge in an air‐cooled and a water‐cooled multiple inlet–outlet discharge tube. The simplicity of the optimum gas mixture suggested that C^{−} _{2} is produced directly by simple dissociative electron attachment of acetylene. Altogether 103 absorption lines have been observed and accurately measured. Most of them are P, Q, and R form branches of allowed F _{1}↔F _{1}, F _{2}↔F _{2} transitions although some forbidden F _{1}↔F _{2} transitions and O and S transitions have also been measured. The hot bands (1,1) and (0,1) have been observed with intensity which is less than that for the (0,0) by a factor of only 2 or 3, in spite of the fact that the Franck–Condon factors of these transitions are lower than that for the (0,0) by 2.0 and 3.2, respectively, indicating that the vibrational temperature of C^{−} _{2} in the He plasma is extremely high. All observed transitions are simultaneously fit to the formula for a ^{2}Π_{ u }←^{2}∑^{+} _{ g } transition. The formulation of Brown and Watson has been used for the Hamiltonian for the ^{2}Π_{ u } state. Molecular constants for C^{−} _{2} in the two electronic states have been determined from the least‐squares fitting. The constants are compared with those of other species isoelectronic to C^{−} _{2}. A short discussion is given about the astrophysical implication of the spectrum.

Rotational spectrum and structure of the linear HCN trimer
View Description Hide DescriptionMicrowave rotational spectra have been observed for 22 isotopic species of an HCN, hydrogen‐bonded trimer with the pulsed nozzle, Fourier transform method using the Flygare/Balle Mark II spectrometer. The ^{1} ^{4}N nuclear quadrupolehyperfine structure was analyzed and the interaction constants and line centers determined. The line centers were fitted to obtain ground vibronic state rotational constants. For the normal isotopic species of (HCN)_{3}, B _{0} was found to be 469.3073(1) MHz and D _{ J }, 82.6(1) Hz. The quadrupole coupling constants χ(n) are −4.049(2), −4.251(2), and −4.375(1)MHz for n=1, 2, and 3, respectively, in HCN(1)HCN(2)HCN(3). The trimer has a linear or very near linear equilibrium structure. The B _{0}’s are insensitive to the position and torsional oscillations of the central HCN but they determine the outer HCNs quite accurately. An isotopic substitution method gives R, the c.m. distance between the outer HCN’s, to be 8.790 Å in the 14–14–14 species. A slightly smaller value 8.788 Å is obtained from a fit of the B _{0}’s which includes the effects of isotopic substitution on the H/D–C and C–N bond lengths in the monomer. The distance between c.m.(1) and c.m.(2) may be somewhat shorter (∼0.01 Å) than that between c.m.(2) and c.m.(3). The average c.m. separation in the trimer (4.395 Å) is halfway between that of the dimer (4.447 Å) which is known to be linear and the infinite linear chains in the solid (4.34 Å). It is shown that the smaller quadrupole coupling constants in the trimer compared to the monomer are caused by a combination of torsional and charge redistribution effects. The torsional vibrations have average angular displacements of 12.60° and 8.55° for HCN(1) and HCN(3). The charge redistribution effect upon χ in the trimer (6.5%) is about 1.8 times that in the HCN dimer. The dipole moment of the trimer was found to be 10.6 D, a value enhanced by 1.8 D over the vector sum of the three vibrating monomers (8.83 D), an enhancement about 1.7 times that of the dimer.