Index of content:
Volume 106, Issue 20, 22 May 1997

Time domain modeling of spectral collapse in high density molecular gases
View Description Hide DescriptionIn many cases, the widely used matrix inversion approach to describe the spectral interference in collisionally perturbed molecular spectra is not feasible if the particular molecular interactions do not allow the sudden impact approximation (infinitely short collision duration). To overcome this problem, we present a time domain model that describes collisional broadening and narrowing phenomena without requiring the sudden approximation. The key element of the model is a Monte Carlo type sampling process to quantify the temporal autocorrelation of the molecular dipole moment. The spectrum is then obtained numerically via fast Fourier transform. The model does not require a frequencydependent relaxation operator; the finite collision duration is simply an adjustable parameter in the time domain process. Our approach, which is generally applicable to any set of transition lines, is derived from concepts of both conventional quantummechanical and semiclassical theory of line interference. Coherent transfer effects from rotationally inelastic collisions are described as randomly occurring events which affect frequency, amplitude, and phase of the sampled oscillation. Effects of vibrational dephasing are included as well. To demonstrate its feasibility, we apply the model here to the 2.7 μ absorptionspectrum of carbon dioxide diluted in high density air ( ). The successful modeling of the experimental data, especially the full collapse of and branches at ultrahigh densities, accounts for interbranch mixing and for incoherent effects. The calculations make extensive use of the new Hitran (HITEMP) molecular database. Results include revised estimates for the collision duration of with nitrogen and oxygen at room temperature.

Compositional control of rovibrational wave packets in the “shelf” state of via quantumstateresolved intermediate state selection
View Description Hide DescriptionCompositional control in the preparation of rovibrational wave packets is demonstrated in the state of gasphase molecules using ultrafast pump–probe laser spectroscopy combined with quantumstateresolved intermediate state selection. The intermediate state, from which subsequent ultrafast excitation occurs, is a stationary rovibrational level in the state of produced by cw laser excitation from the ground state. The effect that the intermediate state has on the final composition of the wave packet is investigated by comparing the transients resulting from ultrafast pump–probe excitation of two different intermediate states ( versus ). In these experiments the pump wavelength is compensated so that in each case the same state eigenstates ( ) make up the wave packet, but with different amplitudes. Theory predicts, and experiments confirm, that the relative amplitudes of the rovibrational eigenstates are strongly dependent upon the intermediate state and determine the spatial and temporal evolution of the wave packet. Evidence for this includes differences in the observed pump–probe transients and dramatically different amplitudes of the beat frequencies in the Fourier analysis of the timedomain transients. Theoretical threedimensional wave packet simulations highlight how the composition of the wave packet is used to vary its spatial and temporal evolution.

Inhomogeneous and single molecule line broadening of terrylene in a series of crystalline alkanes
View Description Hide DescriptionWe present a study of single molecule linewidth broadening of terrylene in dodecane, tetradecane, and hexadecane matrices. The Shpolskii bulk absorption spectra exhibit increase of the bandwidths and site complexity with increasing alkane chain length. Single molecule lines are broadened above the lifetime limit even at 1.6 K in all three solvents. The linewidth distributions suggest the existence of relaxing twolevel systems coupled to the molecular transitions. Spectral diffusion was observed in the forms of spontaneous and photoinduced frequency jumps. Temperature dependencies of the single molecule linewidths show a wide range of powerlaw dependencies below 3 K and, in some cases, exponentially activated behavior above 3 K. Possible origins of the line broadening are discussed.

Electric fieldgradient contributions to the chemical shifts of liquid water
View Description Hide DescriptionContributions to the gastoliquid chemical shifts of water arising from the electric field gradient of the surrounding molecules have been calculated as a function of the temperature. Since the theoreticalmodel is based on perturbation theory, this part of the chemical shift may be calculated from quadrupole shielding polarizabilities and statistical mechanical ensemble averages of external electric fieldgradients. The contributions from the electric field gradients are found to be substantial and are calculated to −9.7 ppm for the oxygen shift and 0.7 ppm for the proton shift at room temperature.

Absorption, resonance Raman, and Raman excitation spectra of hafnium trimers
View Description Hide DescriptionWe report on the optical, resonance Raman, and excitation profile spectra of mass selected hafnium trimers in argon matrices at 14 K. The absorptionspectrum consists of four overlapping transitions in the range 605–620 nm. The Raman spectrum is too complex to be attributed to a single ground state. We may explain the observed spectrum by assuming five lowlying excited states and at 319.0, 413.4, 609.6, 642.8 (weak), and respectively. The ground state shows complex structure which may be interpreted as the result of a strong, but linear Jahn–Teller effect. Evidence is obtained for two pseudorotational progressions having states of vibronic angular momentum of based upon normal frequencies of 142.8 and This indicates a fluxional ground state with symmetry in the limit. No such effects are apparent in the lowlying excited states, although modes of are observed and in some cases geometrical information may be inferred. The state is geometrically an equilateral triangle ( symmetry). The state shows an vibration, but without an observed nontotally symmetric mode no geometrical information can be obtained. The state shows only a nontotally symmetric mode at while the state and the state both appear to have symmetry. Raman excitation profiles appear as several distinct types and correlate well with the four absorption bands at 606, 610, 615, and 619 nm.

Dibromine monoxide, The rotational spectrum and molecular properties
View Description Hide DescriptionThe rotational spectra of and in their ground vibrational states as well as in its state have been studied in selected regions between 90 and 523 GHz. Transitions involving a large range of quantum numbers, and have been observed permitting precise rotational and a large set of centrifugal distortion constants to be determined. All isotopic species as well as the excited state data were fit simultaneously. Groundstate effective and average structural parameters as well as an estimate of the equilibrium structure have been derived. The quartic distortion constants were used for a calculation of the harmonic force field. The complete quadrupoletensor has been determined. Its diagonalization reveals a largely covalent BrO bond with little bonding. The derived properties of are compared with those of related compounds such as HOBr, and HOCl.

Electronic excitation transfer in LennardJones fluid: Comparison between approaches based on molecular dynamics simulation and the manybody Smoluchowski equation
View Description Hide DescriptionMolecular dynamics simulations were performed to study the kinetics of longrange irreversible/reversible electronic excitation transfer in a LennardJones fluid where the translationaly mobile choromophores are thought to be embedded. The simulations are based on the Förster master rate equation approach which can be rederived from a stochastic Liouville formalism for excitation transfer between two identical chromophores in the weak dipole–dipole coupling regime. For energy transfer between two dissimilar partners, rate equations utilized are obtained from the first principle. The simulated kinetic results in this regime are then compared with the reactiondiffusion theoretical framework for excitation transfer. The theory is based on a manybody Smoluchowski equation for the reactant molecule reduced distribution function and makes use of a superposition approximation to truncate the hierarchy of equations. The comparison of the results show the scope and utility of the theoretical approach in the high friction limit when it is solved for the absorbing boundary condition at contact. In the low friction limit, like collisional quenching, the present reactiondiffusion formalism is found to perform poorly. When the stochastic Liouville equation in the strong dipolar coupling regime is solved combined with the molecular dynamics trajectories, the time dependent reaction probability of the donor shows oscillatory behavior and the diffusion coefficient of the medium has been found to have but little effect on this.

Vibrational spectra of hexaatomic siliconcarbon clusters. I. Linear
View Description Hide DescriptionFourier transform infrared measurements on the spectra of the products of the evaporation of silicon/carbon mixtures trapped in Ar at ∼10 K, combined with the results of ab initio calculations published earlier, and density functional theory(DFT) calculations carried out in the present work, have resulted in the detection for the first time of the linear cluster. Two vibrational fundamentals have been assigned, the stretching mode and the Si–C stretching mode The observed frequencies, relative intensities, and isotopic shifts are in very good agreement with the results of the DFT calculations and confirm the previously predicted, linear symmetric geometry for the ground state of

Wavelength and intensitydependent transient degenerate fourwave mixing in pseudoisocyanine Jaggregates
View Description Hide DescriptionThe results of transient degenerate fourwave mixing(DFWM) and “pumpprobe” spectroscopy in aggregates of 1,diethyl2,2cyanine (pseudoisocyanine, PIC) chloride at 300 K are reported. Spectral dispersion of DFWM efficiency within the band and near exciton resonance has been measured. Time response of both transient absorption changes and DFWM signal is found to be strongly dependent on pump photon fluence and wavelength. This behavior is qualitatively explained within a physical model of nonlinear optical dynamics in aggregate domains which accounts for exciton annihilation, and the effect of nonthermal phonons produced as a result of decay of twoexciton states. Intensitydependent evolution of excess dynamic disorder due to nonthermal phonons manifests itself in pumpprobe experiments as dispersiontype differential spectrum appearing at high pump intensity. The thirdorder nonlinear susceptibility of PIC aggregates has been calculated supposing highest density packing. The nonlinear figure of merit of aggregates was evaluated which is intensity dependent due to exciton annihilation and associated subsequent processes.

Absolute HCO concentration measurements in methane/air flame using intracavity laser spectroscopy
View Description Hide DescriptionIntracavity laser absorption spectroscopy was used to measure the absorption spectra of a premixed, flat methane/air flame at a total pressure of 30 Torr. The spectra were measured in the spectral range of A flat flame burner was placed inside the cavity of a broadband dye laser pumped by a cw argonion laser. The spectrum of the laser output was measured by a high resolution spectrograph (with a spectral resolution of 0.003 nm). The spectrum of HCO radicals transition) was measured with a high signaltonoise ratio at different positions above the burner, providing the first quantitative measurement of the absolute concentrations of the HCO radical in flames. The linewidths of the individual rotational lines in the spectrum can be closely fitted by the equation where and The rotational temperature of the HCO radicals was evaluated from the spectra, but the error and the data scatter are relatively high since the lines with a high rotational quantum number are strongly superimposed with lines from different branches. The “hot band,” which can be assigned to the transition (0,0,1)–(0,9,1), was observed in spectra measured at high temperature. The value is evaluated from the position of this “hot band.” The concentration profile of the HCO radical has a maximum value of about which is in reasonable agreement with computer simulation results, when the uncertainties of the absorption cross section and of the rate constants for HCO reactions are taken into account. The relatively strong lines of the radical spectrum (the transition) were also recorded in the studied wavelength range. The spectra of these two radicals can be measured simultaneously which is advantageous in combustion diagnostics.

Dopplerfree twophoton absorption spectroscopy of the transition of transglyoxal
View Description Hide DescriptionDopplerfree twophoton absorptionspectra of the transition of transglyoxal have been measured by means of twophoton absorptionspectroscopy with counterpropagating light beams of identical photons within an external cavity. The relative energies of transition lines are measured with accuracy better than 0.0001 . Rotational lines are fully resolved, and 1809 lines of the transition are assigned for . Rotational constants of the and states are determined by a leastsquares fitting of eigenvalues of the reduced rotational Hamiltonian to energies of the assigned lines. Energy shifts, intensity anomalies, and line splittings are observed for several lines. When an external magnetic field is applied, remarkable changes are observed for these lines. They are identified as originating from perturbations between the and states, which become appreciable when perturbing levels are close in energy. Splittings into three lines are observed for strongly perturbed levels of , and these splittings are identified as the hyperfine splitting caused by mixing of the state.

Depolarized dynamic light scattering from three low molecular weight glass forming liquids: A test of the scattering mechanism
View Description Hide DescriptionDepolarized dynamic light scattering (DDLS) experiments are performed on the glass forming materials ortho terphenyl (OTP), bismethylmethoxyphenylcyclohexane (BMMPC), and bismethylphenylcyclohexane (BMPC). Depolarized spectra are obtained for the bulk liquids and for solutions of varying concentration in Effective optical anisotropies of the molecules are obtained from the solution spectra. A detailed analysis of the DDLS spectra, as well as a comparison with the results of quasielastic neutron scattering (QENS) experiments, indicates that the DDLS spectra in the bulk liquids result from at least two physical mechanisms: (i) The low frequency part of the spectrum, in the frequency range of the α peak, arises mainly from the molecular reorientation mechanism. The integrated intensity of this contribution is proportional to the square of the effective molecular anisotropy. (ii) At higher frequencies (but still below the Boson peak), the predominant contribution to the DDLS spectrum arises from interactioninduced scattering. The intensity of this contribution increases with increasing temperature (decreasing density) in the way expected for the cancellation effect. The implications of these results for the comparison of DDLS and QENS experiments and for the comparison of the data to the predictions of mode coupling theories of glass forming liquids are discussed.

The rovibrational spectrum of hydroxylamine: A combined high resolution experimental and theoretical study
View Description Hide DescriptionThis paper reports the rovibrational spectrum of hydroxylamine recorded by interferometric Fourier transform spectroscopy with a resolution of up to close to the Doppler limit at room temperature, from up to the visible range of the spectrum. Detailed rotational analyses for 32 bands include all nine fundamentals and numerous overtones up to Approximate absolute band strengths, band centers and vibrational assignments are presented for a total of 72 bands up to the OH stretching overtone. The spectra are interpreted in terms of multidimensional vibrational calculations with potential and dipole surfaces constructed by multidimensional spline interpolation from more than ab initio points. The full three dimensional treatment of the torsioninversion problem reveals well separated time scales for the two processes with a cis potential well supporting localized wave functions for the zero point and fundamental levels. Up to five dimensional normal coordinate models are employed for the analysis and dynamic interpretation of the complete vibrational spectrum up to Good quantitative agreement between observed spectra and results from ab initio calculations is obtained with a simple harmonic scaling procedure without any further empirical refinement. The comparison of various coupling schemes reveals an efficient path for the coupling between the OH and manifolds mediated through the OH bending mode. The implications for the effective homogeneous broadening at high energies are discussed.

Photon correlation spectroscopy of interacting and dissociating hemoglobin
View Description Hide DescriptionExperimental and theoretical analysis of the effect of both intermolecular interactions and dissociation on the diffusionalproperties of carbon–monoxide bovine hemoglobin in solution are reported here. When performing accurate photoncorrelation spectroscopy measurements versus protein concentration, even on dilute solutions, one finds that the first cumulant diffusion coefficient of the macromolecule has a relevant dependence upon pH (5⩽pH⩽9.5), ionic strength (10–100 mM), and, particularly, on hemoglobin concentration mg/ml). The results cannot be interpreted by considering the occurrence of either proteindissociation or intermolecular interactions only. As a consequence a simple theoretical expansion of the first cumulant diffusion coefficient, to first order in concentration, is derived here with the inclusion of protein interactions and dissociation. A fit procedure based on this expression leads to a good description of the dissociation and an accurate estimate of the native protein charge and of its single particle diffusion coefficient in the full range of solution parameters.

Dipole and quadrupole plasmon resonances in large sodium clusters observed in scattered light
View Description Hide DescriptionWe study the dependence of the optical properties of clusters as a function of cluster size. The Mie theory is used to describe intensities of light scattered by growing clusters in directions orthogonal to the direction of the incident light beam. We assigned the maxima in scattered intensities to secondary fields due to plasmons excited in clusters with appropriate radius. The maxima in measured intensities of scattered light in “perpendicular” and “parallel” polarization geometry are attributed to excitation of dipole plasmon when cluster radius approaches 55 nm and of quadrupoleplasmon when cluster size approaches 118 nm, respectively.

Protoncoupled electron transfer reactions in solution: Molecular dynamics with quantum transitions for model systems
View Description Hide DescriptionA general minimal model for protoncoupled electron transfer (PCET) reactions in solution is presented. This model consists of three coupled degrees of freedom that represent an electron, a proton, and a solvent coordinate. Altering the parameters in this model generates a wide range of PCET dynamics. This paper focuses on three model systems corresponding to three different mechanisms: a concerted mechanism in which the proton and electron are transferred simultaneously, a sequential mechanism in which the proton is transferred prior to the electron, and a sequential mechanism in which the electron is transferred prior to the proton. The surfacehopping method ‘molecular dynamics with quantum transitions’ (MDQT) is applied to these model systems. The proton and electron coordinates are treated quantum mechanically, and the solvent coordinate is treated classically. Thus the adiabatic quantum states are twodimensional wavefunctions that depend on both the electron and the proton coordinates. The MDQT method incorporates nonadiabatic transitions between these mixed proton/electron adiabatic quantum states. The MDQT simulations presented in this paper provide insight into the fundamental physical principles and the dynamical aspects of PCET reactions.Nonadiabatic effects are shown to play an important role in determining the rates and mechanisms of PCET reactions. This represents the first application of MDQT to a system in which both a proton and an electron are treated quantum mechanically.

Reaction on the potential energy surface: Dependence on translational and internal energy and on isotopic composition, 93–1313 K
View Description Hide DescriptionRate constants have been measured for the reactions of with and as a function of ionneutral average centerofmass kinetic energy, at several temperatures over the range 93 K–565 K using a selected ion flow drift tube apparatus. For the reaction we also report measurements made using a hightemperatureflowing afterglow (HTFA) instrument over the temperature range 300 K–1313 K. The rate constants are found to have a very large isotope effect, with the rate constant a factor of 15 higher than the rate constant at 93 K. The rate constants generally have a minimum with respect to temperature and except for the highertemperature data for where the rate constants show only an increase with increasing kinetic energy. The data indicate that increasing rotational temperature decreases the rate constants and that rotational energy behaves similarly to translational energy. Single excitations of bending and twisting vibrations have a negligible effect on the rate constant. Either the stretching vibrations or overtones of the bending vibrations increase the rate constants. If the stretches are responsible for the increase in the rate constants, the derived rate constant for a single quantum of stretch excitation is a factor of 6 larger than the rate constant for The rate constants are approximately equal to the averages of the rate constants for the pure isotopes. The product branching ratio shows no dependence on rotational temperature or lowfrequency vibrations. A theoretical study of the minimum energy reaction path was performed to help elucidate the reaction dynamics. The minimum energy reaction surface was characteristic of the standard double minimum pathway for ion molecule reactions. The height of the central barrier was found to be close to the energy of the reactants and varied with isotopic substitution. Conformationally different transition states are found for these isotopic reactions.Theoretical studies at the QCISD(T) level of theory find distinct transition states corresponding to The transition state barriers increase in the order and in agreement with experimental reaction rates. The main features of the reactivity are explained by the characteristics of the reaction surface.

A quasiclassical trajectory study of H+CO Angular and translational distributions, and OH angular momentum alignment
View Description Hide DescriptionWe present a quasiclassical trajectory study of the H+COreaction dynamics, with emphasis on product angular and translational distributions, and OH angular momentum alignment. A new potential surface has been developed for this study, based on modifications of a previously developed full dimensional empirical HCOpotential surface. The modifications include correcting errors that cause the HO⋅⋅⋅CO dissociation barrier to be too loose, and adjusting the depth of the HCO minimum and the heights of several barriers, in order to bring them into agreement with their best estimates determined from ab initio calculations. We compare cross sections, energy partitioning, and mechanistic information calculated using the unmodified and modified surfaces with experimental results. Results from the modified surface improve the comparison with experiment for the product OH energy partitioning, but the product CO internal excitation is still high. The translational distributions have the same shape as measured distributions, and the average translational excitation matches some experiments but is lower than others. The angular distribution calculated at 2.6 eV on the modified surface is in good agreement with experimental results, showing both forward and backward scattered peaks, with a slight preference for backward scattering. By studying the average lifetime of the HOCO collision complex, we find that the lifetime is comparable to the rotational period so that there is considerable forward scattering (half rotation) and backward scattering (full rotation). The OH product angular momentum alignment indicates no preference for polarization of the OH rotational angular momentum vector. This result–an essentially isotropic distribution–agrees within the experimental uncertainty for measurements of OH polarization dependent differential cross sections and centerofmass frame alignment parameters, but not with OH

Collision induced deactivation of the bending mode vibrational level of the excited and ground electronic states of by rare gases
View Description Hide DescriptionCollisional deactivation of the vibrational level of the bending mode by rare gases has been studied for both the excited and ground electronic states of Quenching constants have been determined. While a nonSSH behavior has been observed in the dependence of the relaxation probability upon the mass of the collision partner in the ground state denoting a possible predominance of an intramolecular energy transfer process, the cross sections fit quite well the Parmenter and coworkers’ potential well depth correlation rule for both the electronic states indicating that their interactions with the quenchers occur for both of them under the influence of long range attractive forces.

Quantum control of in the gas phase and in condensed phase solid Kr matrix
View Description Hide DescriptionWe present experimental results and theoretical simulations for an example of quantum control in both gas and condensed phase environments. Specifically, we show that the natural spreading of vibrational wavepackets in anharmonic potentials can be counteracted when the wavepackets are prepared with properly tailored ultrafast light pulses, both for gas phase and for embedded in a cold Kr matrix. We use laser induced fluorescence to probe the evolution of the shaped wavepacket. In the gas phase, at 313 K, we show that molecular rotations play an important role in determining the localization of the prepared superposition. In the simulations, the role of rotations is taken into account using both exact quantum dynamics and nearly classical theory. For the condensed phase, since the dimensionality of the system precludes exact quantum simulations, nearly classical theory is used to model the process and to interpret the data. Both numerical simulations and experimental results indicate that a properly tailored ultrafast light field can create a localized vibrational wavepacket which persists significantly longer than that from a general nonoptimal ultrafast light field. The results show that, under suitable conditions, quantum control of vibrational motion is indeed possible in condensed media. Such control of vibrational localization may then provide the basis for controlling the outcome of chemical reactions.