Volume 100, Issue 4, 15 February 1994
Index of content:

Dynamical structure of water in aqueous solutions of D‐glucose and D‐galactose by low‐frequency Raman scattering
View Description Hide DescriptionLow‐frequency depolarized Raman spectra of aqueous solutions of D‐glucose and D‐galactose have been investigated in the frequency region from −250 cm^{−1} to 250 cm^{−1} at 30.0 °C as a function of concentration up to 0.04 molar ratio. The dynamical structure of water in aqueous solution is analyzed by using the reduced Raman spectrum χ‘(ν̄), which corresponds to the imaginary part of the dynamical susceptibility. The reduced spectrum is fitted with the superposition of one Cole–Cole type relaxation mode and two damped harmonic oscillator modes by a nonlinear least‐squares fitting. The effects of D‐glucose and D‐galactose on the dynamical structure of water in aqueous solution are similar. The relaxation time of hydrogen bond among water molecules becomes slower with increasing sugar concentration. The characteristic frequencies of stretching‐like and bending‐like vibrations among water molecules do not change in both D‐glucose and D‐galactose aqueous solutions.

Determination of the threefold internal rotation barrier in ArNH_{3}
View Description Hide DescriptionThe two Σ and four Π states of the weakly bound complex Ar–NH_{3} correlating to j=2, k=±1 ammonia have been observed by tunable far infrared difference frequency‐microwave sideband spectroscopy. The results have been combined with published data to determine a new angular potential energy surface for the system. The barrier to threefold internal rotation of the NH_{3} about its C _{3} axis in the complex is estimated to be 25.606(24) cm^{−1} near the minimum energy (T‐shaped) configuration. The potential also exhibits maxima at both symmetric top configurations, with energies approximately 53 and 31 cm^{−1}, respectively above that of the global minimum. The location and splitting between the symmetric and antisymmetric Σ states are indicative of a strong interaction with another pair of unobserved states, most likely the first excited intermolecular stretch built on j=1, k=±1 Ar–NH_{3}.

Spectroturbidimetry theory for determining orientation distributions of spheroidal particles in the Rayleigh–Debye–Gans and Rayleigh scattering regimes
View Description Hide DescriptionThe general direct and inverse problems of turbidity of spheroids which are monodisperse in size and shape and fall in the Rayleigh–Debye–Gans (RDG) and Rayleigh scattering regimes are solved analytically. The theory applies to single and independent scattering from nonabsorbing optically isotropic particles. From turbidity measurements at one incident direction, one wavelength, and three incident polarizations, it is shown that three functions or moments of the orientation distribution functions can be obtained for both regimes. For RDG particles, use of turbidity spectra (at multiple wavelengths) can recover more information, i.e., more moments. The complete orientation distribution function, with no empirical fits, can in principle be obtained from turbidity spectra at many incident directions for the RDG regime, but not for the Rayleigh regime. The approach used is more general, and should provide potentially much more orientation information than measurements of conservative linear dichroism.

Picosecond vibrational dynamics of several S _{1} bands in jet‐cooled p‐difluorobenzene
View Description Hide DescriptionPicosecond pump–probe threshold photoelectron spectroscopy was used to investigate the vibrational dynamics of four vibrational bands in the S _{1} state of p‐difluorobenzene in the range 2000 to 2900 cm^{−1}. In this energy region the vibrations exhibited intermediate case restricted vibrational dynamics as evidenced by observed quantum beats as well as irreversible statistical limit decay. More complete coverage of the S _{1} state was prohibited by the wavelength restrictions on both the pump and probe laser wavelengths required in the experiment. The observed restricted vibrational redistribution is in contrast to the very rapid irreversible decays inferred from room temperature, high pressure chemical timing experiments on the same molecular bands. The differences are discussed in terms of the role of molecular rotations and the possible different experimental observations.

Two‐dimensional electron paramagnetic resonance spectroscopy of nitroxides: Elucidation of restricted molecular motions in glassy solids
View Description Hide DescriptionThe combination of concepts of two‐dimensional (2D) spectroscopy with the well‐known field step electron–electron double resonance(ELDOR) method offers a practical route to recording 2D ELDORspectra covering the full spectral range needed for electron paramagnetic resonance(EPR) of nitroxide spin labels in the solid state. The 2D ELDOR pattern provides information about molecular reorientation measured in real time, the anisotropies of electron phase, and electron spin‐lattice relaxation as well as nuclear spin‐lattice relaxation all of which are connected with the detailed geometry of the molecular reorientation. Thus, in 2D ELDOR the same electron spin probes the motional behavior over a wide range of correlation times from 10^{−4} to 10^{−12} s. An efficient algorithm for simulating 2D ELDORspectra is derived, based on analytical solutions of the spin relaxation behavior for small‐angle fluctuations and offers a means of quantitatively analyzing experimental data. As an example, the motion of nitroxide spin labels in a liquid‐crystalline side‐group polymer well below its glass transition is determined as a β‐relaxation process with a mean angular amplitude of 5° and a distribution of correlation times with a mean correlation time of 0.9×10^{−10} s and a width of 2.5 decades.

Vibrational studies on electronic structures in metallic and insulating phases of the Cu complexes of substituted dicyanoquinonediimines (DCNQI). A comparison with the cases of the Li and Ba complexes
View Description Hide DescriptionElectronic structures in metallic and insulating phases of the Li, Cu, and Ba complexes of 2,5‐R _{1},R _{2}‐DCNQI [R _{1}=R _{2}=Br (abbreviated as DBr) or R _{1}=R _{2}=CH_{3} (abbreviated as DMe); DCNQI=N,N’‐dicyanoquinonediimine; 2,5‐ is usually omitted] have been studied by observing temperature dependencies of their infrared absorption bands between 295 and 23 K. At room temperature, the wave numbers (ν̃_{ i }) of infrared absorption bands of R _{1},R _{2}‐DCNQI and its Li and Ba complexes are linearly correlated with the degrees of charge transfer (ρ) (ρ=−0.5 and −1.0e for the Li and Ba complexes, respectively). The ν̃_{ i }–ρ relationships indicate that the ρ value for the Cu complexes is −0.67e. This result is consistent with the previously established view that the Cu cations in the Cu complexes at room temperature are in a mixed‐valence state of Cu^{1.33+}. In the infrared spectrum of Cu(DBr–DCNQI)_{2} at room temperature, no electron–molecular vibration (EMV) coupling bands are observed.
Below the metal–insulator (M–I) transition temperature (T _{MI}), EMV bands grow continuously and the ordinary infrared bands observed at room temperature gradually split into three bands with decreasing temperature. Similarly, the infrared bands of Li(DBr–DCNQI)_{2} split into two bands. These splittings are due to an inhomogeneous charge distribution in the DCNQI columns produced by the freezing of charge‐density wave (CDW). The peak‐to‐peak amplitudes of CDWs in the DCNQI columns estimated by use of the ν̃_{ i }–ρ relationships are 0.08±0.04 and 0.40±0.04e, respectively, for the Li and Cu complexes of DBr–DCNQI. The state of the frozen CDW is inferred from the number of split bands. Based on the observed continuous change of the infrared spectra of Cu(DBr–DCNQI)_{2} and the discontinuous changes of other quantities such as x‐ray satellite reflections, lattice parameters, and magnetic susceptibilities, the M–I transition in Cu(DBr–DCNQI)_{2} may be described as follows: (1) above T _{MI} the charges on Cu cations (two Cu^{1+}’s: one Cu^{2+}) are dynamically averaged to +1.33e through the Cu...N≡C bridge. (2) At T _{MI} the charges abruptly localize in the order of (Cu^{1+}...Cu^{2+}...Cu^{1+}...)_{ n }. At the same time, the CDWs begin to be frozen in the DCNQI columns. (3) As temperature decreases below T _{MI}, the order of the frozen CDW develops gradually. In contrast to these changes in Cu(DBr–DCNQI)_{2}, neither EMV bands nor band splittings are observed in the infrared spectra of Cu(DMe–DCNQI)_{2} at low temperatures. Instead, almost all bands show negative absorption lobes on their low‐wave number sides and become asymmetric. This asymmetrization is due to interactions between the vibrational levels and low‐lying continuous electronic levels responsible for a broad band observed in the 1600–800 cm^{−1} region.

The S _{0}(1A _{ g })–S _{1}(1B _{2u }) vibronic transition in benzene: An ab initio study
View Description Hide DescriptionThe four e _{2g } false origin bands and the a _{1g } progression of the S _{0}(1A _{ g })–S _{1}(1B _{2u }) transition in benzene are simulated ab initio with the recently introduced configuration interaction singles (CIS) with 6‐31G orbitals. The ground and excited state CC and CH bond lengths are optimized and compared with the experiment; the CC bond elongation upon excitation is found to be slightly underestimated. The vibrational force fields are calculated at the stationary points of S _{0} and S _{1}. The 1A _{ g } force field is calculated at the Hartree–Fock level while the 1B _{2u } force field is calculated at the CIS level of theory. The two force fields are scaled to fit the experimental frequencies and the normal mode rotation upon excitation, i.e., the Duschinsky matrix, is obtained. In agreement with previous empirical fitting of the S _{1}(1B _{2u }) vibrational frequencies, the Duschinsky matrix is found to be nearly diagonal with the exception of the b _{2u } modes submatrix which shows a large amount of mixing. The mixing of the b _{2u } modes is larger before scaling but is subsequently reduced after scaling. The normal modes and the optimized geometries are used to calculate the amount of displacement, upon excitation, of the equilibrium position of the totally symmetric modes. This displacement causes the Franck–Condon progression and is slightly underestimated by the calculation. The intensity of the four e _{2g } false origins in the absorptionspectrum of S _{1} is calculated and the Herzberg–Teller intensities of the four bands are found to be very close to the experiment. In particular, the relative intensity of the CCC bend (ν_{6}) and CC stretch (ν_{8}) bands is nicely reproduced. This result is discussed in light of similar calculations at the semiempirical level of theory. We conclude that CIS can be of great value for the unravelling of vibronic spectra of conjugated systems.

Cooperative ultrafast nonlinear optical response of molecular nanostructures
View Description Hide DescriptionThe stationary nonlinear reflection and the time resolvedfour wave mixing signal from a molecular monolayer are calculated using Green function techniques. Cooperative resonant nonlinear response found in small aggregates suggests the existence of coherence size of order of optical wavelength. A new peak in the nonlinear reflection spectrum is predicted, which is missed by the local field approximation. For an infinite two dimensional molecular monolayer with transition dipole moments in the lattice plane, the momentum‐dependent two exciton decay rate is found to be larger than the sum of the single exciton radiative decay rates, as predicted by the local field approximation.

Rise profile of the thermal lens signal: Contribution of the temperature lens and the population lens
View Description Hide DescriptionTime profiles of the rise parts of variations of a probe beam light densities under the thermal lens (TL) experimental condition are measured with nanosecond pulsed laser excitation. The rise profile in benzene can be explained well by the probe beam expansion only by a refractive index lens due to the density fluctuation, which is induced by heat from the radiationless transition. In water, the signal is fitted well by the sum of two contributions of the refractive index change; i.e., due to the density (the thermal lens) and temperature (the temperature lens) variations. The coefficient of the temperature variation (∂n/∂T)_{ρ} is evaluated from the relative signal intensity. The contribution of the population lens is clearly observed in the rise curve of the ‘‘TL signal’’ after the photoexcitation of C_{60} in benzene.

Detection of OH^{+} in its a ^{1}Δ state by far infrared laser magnetic resonance
View Description Hide DescriptionThe spectrum associated with the J=3←2 transition of OH^{+} in the a ^{1}Δ(v=0) state has been observed by far infrared lasermagnetic resonancespectroscopy. A new microwavedischargesource enabled the detection of this spectrum, which is the first observation of the rotational spectrum of an ion in a metastable state. Assignment and least‐squares fitting of the observed transitions have determined the following molecular constants: B _{0}=494.420 388 (22) GHz, the proton hyperfine parameter a=74.84 (32) MHz, g _{ L } ^{’} = 1.000 915 (15), and g _{ r }=−0.001 815 (18), with the 1σ uncertainties of the last digits in parentheses. The relationship of these parameters to the geometric and electronic structure of OH^{+} is discussed.

Excited‐state structure and photochemical ring‐opening dynamics of 1,3,5‐cyclo‐octatriene from absolute resonance Raman intensities
View Description Hide DescriptionAbsolute resonance Raman cross sections are measured for 1,3,5‐cyclo‐octatriene (COT) in cyclohexane with excitation from 325 to 200 nm. These intensities and the absorptionspectrum are modeled using a fully thermalized time‐correlator theory to quantitate the excited‐state equilibrium geometry displacements along 19 Raman‐active normal modes. The resonance Raman spectra show significant intensity in low‐frequency modes corresponding to planarization of the eight‐membered ring. The 140 cm^{−1} twist‐boat planarization (Δ=4.6) and the 339 cm^{−1} ring deformation (Δ=1.6) are particularly strong. However, no intensity is observed in modes which project onto the predicted disrotatory ring‐opening motion, such as the nontotally symmetric CH_{2} twist fundamental or its overtone. Analysis of the fluorescence quantum yield (φ_{ F }=2×10^{−6}) gives an excited state lifetime on the order of ∼30 fs. These results show that ring planarization is the first step in the disrotatory ring opening of COT followed by rapid depopulation of the initially prepared state to a lower‐lying excited electronic state upon which the actual ring opening occurs. Comparison of these results with the excited‐state dynamics of other pericyclic systems suggests that pericyclic rearrangements occur only once a planar structure is established and that the bond rearrangement occurs predominantly on a low‐lying, optically forbidden excited state.

Atom‐spherical top van der Waals complexes: A theoretical study
View Description Hide DescriptionThe theory of the vibration‐rotation states of atom‐spherical top van der Waals complexes is developed. The exact close‐coupled equations are closely analogous to those encountered in atom+spherical top scattering. The structure of the coupled equations is investigated, and close‐coupling calculations of the bound states of Ar–CH_{4} are presented for two different intermolecular potentials. The role of symmetry in the complex is discussed, and the energy levels are interpreted using a model in which the CH_{4} molecule undergoes hindered rotation in the field of the Ar atom. Correlation diagrams are presented, showing how the free‐rotor levels are converted into near‐rigid vibrational energy levels as the anisotropy of the intermolecular potential increases. The effect of higher‐order anisotropic terms is investigated, and correlation diagrams are given for complexes of tetrahedral, octahedral, and icosahedral molecules. The role of monomer vibrational angular momentum is investigated.

Bipolarons and the temperature dependence of exchange in the metal trihydrides
View Description Hide DescriptionWe analyze a two‐particle, two‐site, tight‐binding model linearly coupled to a harmonic bath in order to describe the temperature dependence of exchange in the metal trihydrides. We present a variational calculation that predicts two regimes: coherent, motionally narrowed exchange, and incoherent, quenched exchange. We show for an ohmic bath that the behavior rapidly switches from unquenched to quenched at a finite temperature that scales linearly with the correlation energy U and logarithmically with both bath coupling parameters and the zero‐temperature exchange 4t ^{2}/U. We show that Zilm’s description of the temperature dependence of the spin coupling (Ref. 1) can be derived asymptotically and shows a geometric dependence of the ‘‘activation’’ energy that is absent in a finite basis approximation. The model proposed explains the temperature dependence of the exchange splittings observed by nuclear magnetic resonance for the metal trihydrides.

On the preparation and measurement of superpositions of chiral amplitudes
View Description Hide DescriptionWe examine the preparation and detection of superpositions of chiral amplitudes of a handed molecule, showing that specific sequences of phase‐controlled ultrashort light pulses enable the preparation and measurement of chiral coherences. It is found that certain choices of relative optical phase between the pulses of the preparation sequence set up left–right superpositions that would be inaccessible by tunneling dynamics alone.

A near‐wing correction to the quasistatic far‐wing line shape theory
View Description Hide DescriptionA new representation is introduced in which the rapidly varying time‐dependent part of the time displacement operator can be factored out and the remaining part, which varies with time more slowly, can be expanded in the usual perturbational fashion. The lowest order approximation leads to the far‐wing quasistatic line shapetheory developed previously, whereas the next order approximation, related to the noncommutation of the Liouville operators describing the unperturbed absorber and bath molecules and the interaction between them, leads to a near‐wing correction. Explicit expressions are derived for both the corrections to the spectral density and the statistical band‐average line shape function assuming an anisotropic dipole–dipole interaction. Detailed computations for the case of self‐broadened H_{2}O are carried out for the line‐shapes and the corresponding absorption coefficients for several temperatures and for frequencies to 10 000 cm^{−1}. From these results, we conclude that the near‐wing corrections generally increase the line shape function between 10 and 200 cm^{−1}, and that this increase is more important for lower temperatures than for higher ones. This in turn leads to increased absorption nearer the band centers, especially for lower temperatures, and thus to improved agreement between theory and experiment.

Spectral line shapes of damped quantum oscillators: Applications to biomolecules
View Description Hide DescriptionWe present a full quantum mechanical treatment, using the quantum fluctuation–dissipation theorem, which is useful in describing the absorption line shape of a system composed of damped vibrational (harmonic) oscillators that are linearly coupled to an electronic excitation. The closed form expressions obtained from the model predict optical line shapes that are identical to standard treatments at high temperature or in the absence of damping. However, at low temperature, quantum corrections become important and the model predicts a skewed optical line shape that reflects the condition of detailed balance and differs significantly from the ‘‘Brownian oscillator’’ model of Yan and Mukamel [J. Chem. Phys. 89, 5160 (1988)]. We also find that quantum effects become observable in the line shape of the overdamped oscillator only when k _{ BT }/ℏω_{0}≲ω_{0} /γ <1, which effectively depresses the temperature for crossover into the quantum regime. In Appendix D we discuss how the time correlator expressions derived for the line shapeanalysis can also be used to describe chemical reactions in the presence of quantum damping. The fact that the transition temperature for quantum behavior is depressed in the presence of strong damping may explain why the ‘‘classical’’ Arrhenius expression is often found to hold, even at temperatures where k _{ BT }<ℏω_{0}. Finally, we explore the consequences of introducing a classical control variable (corresponding to slow conformational motions of a biomolecule), which is coupled to the optically active vibrational mode(s) of the embedded chromophore. This leads to a modulation of the Stokes shift and optical coupling in the system and results in a type of inhomogeneous broadening that has both a Gaussian and non‐Gaussian component. The non‐Gaussian broadening is found to be consistent with the highly skewed inhomogeneousline shape of deoxymyoglobin.

Dynamics in supercooled glycerol by high resolution stimulated Brillouin gain spectroscopy
View Description Hide DescriptionWe have used high resolution stimulated Brillouin gain spectroscopy to probe the dynamics of glycerol over the temperature ranges 146 to 305 K and 401 to 534 K, which include both the supercooled liquid and glass regimes. The high resolution and large spectral range of the technique have allowed us to resolve Brillouin peaks at low temperatures (146 K) with widths as narrow as 13 MHz and shifts as large as 17.3 GHz. A comparison of the observed Brillouin shifts and linewidths with predictions based on previous work at lower frequencies indicates that the main dispersion in our data arises from the primary (α) structural relaxation processes. However, this comparison also reveals that additional relaxation processes, perhaps associated with the secondary (β) processes, contribute to the Brillouinlinewidths both above and below the glass transition. Our results also show a distinct kink in the temperature dependence of the speed of sound at 187 K, the glass transition temperature.

The vibronic structure of the S _{0}↔S _{1} and S _{0}↔S _{2} transitions in simple oligomers of thiophene
View Description Hide DescriptionWe present a quantum‐chemical study of the electronic spectra of the oligomers of thiophene. Geometries and vibrational force fields of the ground and excited electronic states are obtained by an updated version of the semiempirical quantum consistent force field/π electron method implemented to describe sulphur atoms and by ab initio Hartree–Fock and configuration interaction singles methods. The displacement parameters of totally symmetric modes are then obtained and used to model the vibrational structure of the electronic spectra. The contribution of sulphur atoms to the description of the excited state is predicted to be negligible both by ab initio and semiempirical methods which, conversely, indicate a close similarity of thiophene oligomers and polyenes. Based on the results of the simulated spectra a reassignment of some of the bands is proposed. It is shown that mode mixing upon excitation, and not large frequency changes, are responsible for the different Franck–Condon structure of the absorption and emission spectra. In addition, a vibronic coupling mechanism analogous to that active in simple polyenes is identified. It accounts for the ‘‘apparent’’ frequency increase of the most active a _{ g } mode upon excitation to the 1B state.

Reinvestigation of the acetylenic C–H stretching fundamental of propyne via high resolution, optothermal infrared spectroscopy: Nonresonant perturbations to ν_{1}
View Description Hide DescriptionWe present the high resolution spectrum of the ν_{1} fundamental of propyne near 3335 cm^{−1} obtained using a very warm free jet expansion in our optothermal detection spectrometer. By using a high concentration sample expanded at low backing pressures we have been able to observe transitions for K values up to K=6. The additional data available allow us to reinvestigate this vibrational band. We find an unusual perturbation pattern in this band where the individual subbands (rovibrational transitions for a single K value) appear to be completely unperturbed at the level of precision of our data (7.5 MHz), but the subband origin orderings are perturbed through nonresonant interactions. Attempts to account for the subband ordering using a two‐state anharmonic interaction are unsuccessful indicating that the perturbations are of multistate origin. This type of nonresonant perturbation to the subband origins of symmetric top molecules should be a common feature of symmetric tops with large Arotational constants. As a result of this investigation we conclude that the previously reported value of α^{ A }, determined from a very cold expansion where only K=0 and K=1 were observed, is not a measure of the true (unperturbed) value of this constant. This conclusion is also supported by force field calculations presented here that use an empirical harmonic force field augmented by diagonal anharmonicities for the hydride stretches. These calculations, which reproduce measured values of α^{ A } and α^{ B } for lower energy bands quite successfully, also show that the previous determination of α^{ A } is too large and must be dominated by perturbation contributions. We have also measured the weak Fermi resonant band ν_{3}+2ν^{0} _{9} which acquires its intensity through interaction with ν_{1}. Again we find an anomalous subband ordering like that observed in ν_{1}.

Sub‐Doppler, infrared laser spectroscopy of the propyne 2ν_{1} band: Evidence of z‐axis Coriolis dominated intramolecular state mixing in the acetylenic CH stretch overtone
View Description Hide DescriptionThe eigenstate‐resolved 2ν_{1} (acetylenic CH stretch) absorptionspectrum of propane has been observed for J’=0–11 and K=0–3 in a skimmed supersonic molecular beam using optothermal detection. Radiation near 1.5 μm was generated by a color center laser allowing spectra to be obtained with a full‐width at half‐maximum resolution of 6×10^{−4} cm^{−1} (18 MHz). Three distinct characteristics are observed for the perturbations suffered by the optically active (bright) acetylenic CH stretch vibrational state due to vibrational coupling to the nonoptically active (dark) vibrational bath states. (1) The K=0 states are observed to be unperturbed. (2) Approximately 2/3 of the observed K=1–3 transitions are split into 0.02–0.25 cm^{−1} wide multiplets of two to five lines. These splittings are due to intramolecular coupling of 2ν_{1} to the near resonant bath states with an average matrix element of 〈V ^{2}〉^{1/2}=0.002 cm^{−1} that appears to grow approximately linearly with K. (3) The K subband origins are observed to be displaced from the positions predicted for a parallel band, symmetric top spectrum.
The first two features suggest that the coupling of the bright state to the bath states is dominated by parallel (z‐axis) Coriolis coupling. The third suggests a nonresonant coupling (Coriolis or anharmonic) to a perturber, not directly observed in the spectrum, that itself tunes rapidly with K; the latter being the signature of diagonal z‐axis Coriolis interactions affecting the perturber. A natural interpretation of these facts is that the coupling between the bright state and the dark states is mediated by a doorway state that is anharmonically coupled to the bright state and z‐axis Coriolis coupled to the dark states.Z‐axis Coriolis coupling of the doorway state to the bright state can be ruled out since the ν_{1}normal mode cannot couple to any of the other normal modes by a parallel Coriolis interaction. Based on the range of measured matrix elements and the distribution of the number of perturbations observed we find that the bath levels that couple to 2ν_{1} do not exhibit Gaussian orthogonal ensemble type statistics but instead show statistics consistent with a Poisson spectrum, suggesting regular, not chaotic, classical dynamics.