Index of content:
Volume 106, Issue 14, 08 April 1997

The ground and first excited singlet state torsional potentials of 2phenylindole from supersonic jet fluorescence spectra
View Description Hide DescriptionThe transition of 2phenylindole (2PI) was studied in a supersonic jet by laser induced fluorescence. The spectra are dominated by the progression of a low frequency mode assigned to the torsional motion around the bond joining the phenyl and heterocycle moieties. The torsional potentials in the and states are determined from the spectra by fitting the torsional vibrational spacings to a onedimensional potential of the form . The equilibrium conformation of 2PI in the state is nonplanar with a twist angle between two ring systems of 27° and a barrier to planarity of ∼130 . 2PI is planar in the state. Franck–Condon analyses were performed to reproduce the intensity distributions in the spectra. Large Duschinsky effects among the low frequency modes are observed to accompany the large geometry change following the excitation.

Raman spectroscopy of the ν_{1} N–H stretch fundamental in isocyanic acid (HNCO): State mixing probed by photoacoustic spectroscopy and by photodissociation of vibrationally excited states
View Description Hide DescriptionWe report the first gasphase Raman spectrum of isocyanic acid. Using stimulated Raman excitation (SRE) to prepare vibrationally excited states, we detect transitions by both photoacoustic Raman spectroscopy (PARS) and action spectroscopy. In this paper we present results on the ν_{1} N–H stretch fundamental, leaving the spectra of the N–C–O symmetric and antisymmetric stretch modes for a separate publication. The Raman spectrum shows extensive state mixing in the ν_{1} fundamental, in agreement with previous infrared work. Measurement of the effective baxis rotational constants for different mixed vibrational states in this nearprolate symmetric top limits the number of candidates for perturbing states and shows which vibrational modes participate. Double resonance photodissociation further probes the vibrational spectroscopy of isocyanic acid. The scheme is first to prepare a vibrationally excited state by SRE, then photodissociate only the molecules prepared in the first step, and finally probe the decomposition products by laserinduced fluorescence(LIF). An action spectrum, obtained by scanning the vibrational excitation laser (Stokes) wavelength with the photolysis laser wavelength fixed and the probe laser tuned to a LIF transition in one of the photofragments, is the key to unraveling the spectroscopy. The intensity differences between PARS and action spectrum transitions reveal the vibrational state mixing and provide the Franck–Condon factors for transitions to the excited electronic state.

Light scattering study of viscoelastic properties of a liquid Gallium surface
View Description Hide DescriptionHigh frequency capillary waves on a liquid Galliumsurface were studied by quasielastic light scatteringspectroscopy. The observed damping constants of the waves differ greatly from the predictions of classical theoretical treatment of the surface as that of a simple liquid. This is explained in terms of the presence of a surface layer possessing viscoelasticproperties which are extracted from the fit of experimental spectra using different theoretical forms. One of the most important results of the data analysis is that the surface dilational viscosity appears to be negative. This result implies that the widely used phenomenological theoretical model should be replaced by one taking into account processes of adsorption and diffusion.

Nonlinear electrooptical response. I. Steady state Kerr effect relaxation arising from a weak ac electric field superimposed on a strong dc bias field
View Description Hide DescriptionThe dynamic Kerr effect (excluding inertial effects) of an assembly of both polar and anisotropically polarizable molecules acted on by a strong external dcelectric field superimposed on a weak acelectric field is evaluated by starting from the Smoluchowski equation valid in the noninertial limit. The calculation proceeds by expanding the expectation value of the second Legendre polynomial which describes the Kerr effect relaxation, as a power series up to the second order in the small ac field strength, so that frequency components in ω and 2ω exist. This is accomplished using the matrix continued fraction method which allows one to express exactly the solution of the infinite hierarchy of differentialrecurrence relations for the first and second order ac responses of ensemble averages of the Legendre polynomials (relaxation functions). In order to illustrate these results, diagrams showing the behavior of the real and imaginary parts of the complex birefringencefunctions are presented. In particular, the relaxation spectra clearly indicate the differences between the first order response (equivalent to that obtained using linear response theory) and the second order one. We present also the solutions for the particular cases where only either permanent or induced dipole moments are taken into account. The calculation then radically simplifies as the matrix continued fractions now reduce to scalar ones.

Nonlinear electrooptical response. II. Steady state dielectric relaxation in coupled fields
View Description Hide DescriptionNonlinear dielectric response of polar and polarizable molecules dissolved in a nonpolar solvent arising from a weak ac field superimposed on a strong dc bias field are established. The general case when both permanent and induced dipole moments are taken into account is considered. By expanding the expectation value of the first Legendre polynomial as a power series in the ac field strength up to the second order, we find two distinct components in ω and 2ω. Matrix continued fraction methods are used in order to express exactly the solution of the infinite hierarchy of the fiveterm differential recurrence relations for the ensemble averages of the Legendre polynomials(relaxation functions). The spectra of the real and imaginary parts of the first and second order complex dielectric response functions are presented in figures in order to show the essential differences between the linear and the nonlinear dielectric responses. The analytical expressions so obtained can be checked in so far as each component can be separated experimentally.

Spectral hole burning study of electron–phonon coupling in polymers
View Description Hide DescriptionPersistent hole burning in the transition of tetratertbutyltetraazaporphine was used to investigate the electron–phonon coupling in a broad range of polymeric solids between 5 and 30–50 K. The maxima of pseudophonon sidebands are displaced by 5–20 from the 0–0 hole. The relationships between the values and the velocities of transversal and longitudinal ultrasound waves as well as the Young’s modulus of polymers were established. At the same time compare well to inelastic neutron scattering maxima, the first boson peaks in the Raman scatteringspectra, and the characteristic modes responsible for extra specific heat and heat conductivity plateau. Mutual correlations of the molecular structure, nanoscopic, and bulk properties in glassy and partially crystalline polymers are pointed out. The quasihomogeneous hole width at fixed temperature increases when becomes smaller and the polarity of the host increases. Hole widths measured at 15 and 25 K also display a common linear relationship with total heat content of the matrix in less polar hosts. Irreversible broadening of holes as a function of excursion temperature was investigated by means of cycling. The contribution of slow irreversible broadening processes (spectral diffusion) to does not exceed 20%. The shift of holes burned at 4–8 K upon the rise of was measured. The pure thermal hole shift was calculated by taking into account the solvent shift contribution due to the density change of the matrix. This pure phononinduced shift is always bathochromic with increasing . The dependence of both the hole width and shift can be equally well fitted with the power law and a coth function. In most systems both the width and shift obey the power law with similar coefficients of 2.8±0.5 and 2.4±0.5, respectively. A consistent description of the dependence of the Debye–Waller factor, the hole shift, as well as the width in terms of an anharmonic singlemode model can be achieved for most of the polymers with the same characteristic energy (entering the coth function) which is approximately by a factor of 4 larger than The influence of crystallinity, tacticity, molecular weight, polarity, and chemical structure of the macromolecular host on the strength of electron–phonon coupling is analyzed in detail. The relative importance of the Stark effect and intermolecular dispersive forces in the dynamic modulation of electronic energy levels causing the optical dephasing is discussed.

Spectroscopy and predissociation dynamics of the state of HNO
View Description Hide DescriptionThe spectroscopy and predissociation dynamics of the state of HNO have been investigated by measurement of line positions and lifetime broadened linewidths in the cavity ringdown (CRD) spectrum. CRD spectroscopy is a technique better suited to studies of molecular predissociation than methods such as laser induced fluorescence in cases where the excited state dissociation lifetime is short compared to its fluorescence lifetime. The CRD spectrum extends well beyond the dissociation limit (we have identified transitions to rotational states lying up to 2400 above the dissociation limit of 16450 ). The lifetimedependent Lorentzian components of the line shapes of numerous rovibrational features of the CRD absorptionspectrum have been deconvoluted from the Doppler and laser line profiles to obtain lifetimes and predissociation rates for individual states. Here, the labels and denote the number of quanta of the N–H stretch, N=O stretch, and H–N–O bending vibrations, respectively. We have measured line broadening (of up to 0.3 cm^{−1}) in transitions to six vibronic states above the predissociation threshold (the 100 and 020 states, for which the higher levels are above the dissociation limit, and the 101, 030, 110, and 111 states). For three substates (100 101 and 110 ) strongly dependent transition linewidths are seen. The 100 and 101 substates show maximum transition linewidths midway through the observed spectral transitions while the linewidths for transitions involving the 110 substate increase with Linewidths also generally increase with Some lines involving the 100 state are markedly asymmetric. Linewidths for transitions to states having excitation of the bending mode (the 101 and 111 states) are larger than those for which These observations clarify the predissociation mechanism suggested by previous absorption and LIF studies. We attribute the primary predissociation mechanism to axis Coriolis coupling of state levels to discrete quasibound highly vibrationally excited levels of the ground state which in turn are coupled to the electronic ground state continuum corresponding to dissociation to Predissociation of state levels with is probably caused by axis Coriolis coupling to such quasibound levels of the ground state. The variation of predissociation rates with and for the 110 5, and 6 substates cannot be accounted for by this mechanism and we propose the onset of predissociation to the continuum of the state. Interpretation of our experimental data is assisted by calculations performed using the potential energy surfaces of Guadagnini et al. [J. Chem. Phys. 102, 774 (1995)].

Approximate factorization of molecular potential surfaces. I. Basic approach
View Description Hide DescriptionThe intermediate vibrational energy regime—well above the zero point, yet below the first dissociation limit—plays an important role in many molecular processes, such as radiationless transitions or intramolecular vibrational energy redistribution (IVR). For molecules with N>6 vibrational degrees of freedom, the calculation of energy levels and spectra of the full anharmonic vibrational Hamiltonian at high energies presents a formidable problem. Here we derive an asymptotic factorization of the potential energy surface (PES) based on the global topography of molecular potentials. The resulting approximate models require only a few input parameters such as vibrational frequencies and cubic anharmonicities. They are compared to benchmark PES derived from curvilinear model potentials and Monte Carlo sampled ab initio calculations for several small molecules. Higher order couplings are found to make a significant contribution to IVR decays at long times.

A matrix isolation FT–IR and quasirelativistic density functional theory investigation of the reaction products of laserablated uranium atoms with NO, and
View Description Hide DescriptionThe reaction products of laserablated uranium atoms with nitrogen oxide reagent gases were trapped in excess argon at 6–7 K. Infrared spectra of the resulting matrices revealed the presence of several new product species. Major bands at 983.6 and 818.9 in the infrared spectra of the reaction products of U atoms with are due to the U–N and U–O stretching vibrations, respectively, of the linear NUO molecule. Weaker absorptions at 845.9 and 718.3 in similar experiments are indicative of a “T” shaped species. A weaker 1017.0 band provides evidence for the cation/anion pair . Several possible products of the reaction of uranium atoms with nitrogen oxides have been further characterized by quasirelativistic density functional calculations.

Dielectric study of phase transitions in 3nitro4chloroaniline
View Description Hide Description3nitro4chloroaniline (NCA) exhibiting a phase transition between two solid phases has been studied by frequency domain dielectric spectroscopy. Dielectric studies show that molecular dynamics of NCA changes distinctly at the freezing point (370 K) and at the transition (242 K) from the hightemperature solid phase I(Ph I) to the lowtemperature solid phase II(Ph II). In the Ph I just below the freezing point the molecules most probably perform a uniaxial fast reorientation around the pseudohexagonal symmetry axis of the benzene ring. On the other hand upon decreasing temperature in Ph I there are strong and temperaturedependent antiparallel correlations of molecular dipole moments. The correlations are of electrostatic origin and they cause a strong temperature dependence of the Kirkwood correlation factor leading to freezing out intermolecular reorientation. However, in the lower temperature range of the Ph I there is still an enhanced contribution to the static dielectricpermittivity originating most probably from anharmonic and anisotropic librations of molecules and/or from intramolecular motion of the NO_{2} group observed also by xray studies. The relaxation process observed in the Ph IPh II pretransition region behaves like a soft mode. The relaxation time shows nonArrhenius behavior in the vicinity of the Ph IPh II transition. Below this transition the intramolecular reorientation becomes frozen out leading to antiferroelectric order in the lowtemperature Ph II. The phase transition observed is discussed qualitatively in terms of the Fröhlich model of the orderdisorder transition.

Calculations of structure and IRspectrum for small clusters
View Description Hide DescriptionA new site–site intermolecular potential model for UF, featuring exchange, dispersion,electrostatic and induction terms, is presented. The new potential, with the parameters adjusted according to the observed monomer transition dipole moment and reproducing the experimental temperature dependence of the second virial coefficient, is used to determine UF cluster structures up to the hexamer and, by means of a second order line shift formalism, to calculate the corresponding IRspectra in the region of the vibrational mode (at 627.724 cm. The contributions of the various potential terms to the frequency shifts are analyzed and the leading interaction mechanism is found to be the resonant dipole–dipole coupling. The theoretical spectra are compared and interpreted against recent Fourier transform IRspectroscopy measurements.

Effect of temperature on the infrared and sumfrequency generation spectra of adsorbates
View Description Hide DescriptionThe dephasing and energy relaxation contributions to the line width in infrared (IR) and sumfrequency generation (SFG) spectra of adsorbates are derived from the generalized master equation approach. Expression for the line shift is also obtained. The anharmonic interaction between the adsorbate and the substrate is expanded in a polynomial in terms of the adsorbate and phonon coordinates, and the dephasing is shown to be mainly due to twophonon processes, while twophonon, threephonon or fourphonon processes can contribute to energy relaxation, depending on the relative values of the adsorbate vibrational and the phonon frequencies. The temperaturedependence data of the IR absorption for C(111):H is found to be consistent with the theory, and the large line width for C(111):D can be accounted for by the efficient twophonon energy relaxation process which is not available for C(111):H due to the higher adsorbate vibrational frequency for C(111):H.

Simulation of manyspin system dynamics via sparse matrix methodology
View Description Hide DescriptionA sparsematrixbased numerical method is constructed to simulate NMR spectra of manyspin systems, including the effects of chemical exchange and/or relaxation. The associated computational demands are predicted to scale like , as the number of spins increases. This is vastly superior to the inevitable scaling of conventional Householderbased methodology. The improved scaling is verified via numerical computations of simple isomerization systems with four to nine spins. The new method is based on splitting the propagator and use of Chebyshev polynomial expansion of the exponential function.

The mobilities of ions and cluster ions drifting in polar gases
View Description Hide DescriptionThe mobility of ions drifting in polar gases is explored both theoretically and experimentally. New experimental results are presented for (i) NO^{+} ions drifting in H_{2}O (the reduced zerofield mobilityK is 0.66±0.07 cm^{2} V^{−1} s^{−1}), (ii) H_{3}O^{+}(H_{2}O)_{3} ions drifting in H_{2}O (K =0.43±0.06 cm^{2} V^{−1} s^{−1}), and (iii) NO^{+}(CH_{3}COCH_{3}) _{n} ions (n=2,3) drifting in CH_{3}COCH_{3} (K =0.041 ±0.010 cm^{2} V^{−1} s^{−1} for n=2 and K =0.050±0.015 cm^{2} V^{−1} s^{−1} for n=3). A number of theoreticalmodels for ion mobilities in polar gases are described. The models are compared with the available experimental data and a reasonable agreement is obtained. For larger cluster ions the measured mobilities are considerably smaller than the calculated values. Some possible reasons for the discrepancies are discussed, including momentum transfer outside the capture cross section, dipole–dipole interactions, ligand exchange, inelastic collisions, and the validity of Blanc’s law.

The ultraviolet photolysis of acetyl and propionyl radicals studied by infrared emission spectroscopy
View Description Hide DescriptionThe photodissociation of acetyl and propionyl radicals at 248 and 266 nm has been studied by timeresolvedFourier transform infrared spectroscopy. A vibrationally excited product was observed in the emission spectra. The vibrational temperatures of the nascent CO products were about 7400 K for the acetyl radical and 6930 K for the propionyl radical. The vibrational energy partitioning of the CO fragments fits a soft impulsive model.

Satellite hole investigation of energy transfer between two different dyes
View Description Hide DescriptionSitedependent hole filling of satellite holes has been studied by the excitation of 9amino6chloro2methoxyacridine to fill the primary zerophonon hole and satellite holes of 4,4difluoro5(2thienyl)4bora3a,4adiazasindacene3propionic acid, succinimidyl ester. The results show that intermolecular energy transfer can trigger the filling of the primary holes. More important, two different filling rates are observed for four different satellite holes. The filling rate is more rapid on the red side than on the blue side of the absorption band.

Dynamics of hydrogen atom abstraction in the reaction: Product energy disposal and angular distributions
View Description Hide DescriptionEnergy and angular distributions for the hydrogen abstraction reaction, exothermic by 0.26 eV, and a prototype ionic pathway for methaneoxidation in hydrocarbon flames have been studied in a crossed molecular beam experiment at collision energies of 0.34, 0.44, and 0.64 eV. At the two lower collision energies, two mechanisms contribute to the differential cross section: In the first, low impact parameter rebound collisions form sharply backwardscattered products, while in the second, larger impact parameter collisions produce a broad distribution of forward scattered products. We suggest that the first group of products is formed by collisions with hydrogen atoms oriented essentially along the relative velocity vector and proceeding through a nearcollinear geometry, while the second group corresponds to collisions with one of the three offaxis hydrogens. The products are formed on average with 65% of the total available energy in product internal excitation. The product kinetic energy distribution shows structure that correlates with excitation of the umbrella bending mode of . At the highest collision energy, the product angular distribution shifts entirely to the forward direction, suggesting that the low impact parameter collisions are no longer important in the reactive process. At this energy, the average product internal excitation corresponds to 59% of the total available energy. The data suggest that the majority of product internal excitation resides in the umbrella bending mode of , with OH in its ground vibrational state.

Scatteringangle resolved product rotational alignment for the reaction of Cl with vibrationally excited methane
View Description Hide DescriptionWe have applied the experimental technique of core extraction [W. R. Simpson et al., J. Chem. Phys. 103, 7299 (1995)] combined with resonanceenhancedmultiphoton ionization (REMPI) with a polarized laser beam to probe the angularmomentum alignment of the HCl product of the reaction of Cl with vibrationally excited . The core extraction method permits us to distinguish products scattered in different directions in the centerofmass frame, and thus we are able to determine the rotational alignment for various product scattering angles for individual quantum states (a stateresolved threevector correlation). For the forwardscattered HCl(, ) we observe a large positive rotational alignment. This positive velocityangularmomentum correlation is interpreted to be the result of the angular momentum of the HCl product being directed in the plane perpendicular to the lineofcenters force in a simple hardsphere scattering model.

Electronic to vibrational energy transfer between and
View Description Hide DescriptionWe have investigated the electronictovibrational (EV) energy transfer between electronically excited rubidium atoms (Rb ) and hydrogen molecules . We have used the CARS(Coherent AntiStokes Raman Scattering)spectral technique to probe the internal state distribution of collisionallypopulated molecules. Both scanned CARSspectra and activated CARSspectra reveal that during EV energy transfer processes molecules are produced only at the =1 and =2 vibrational levels. From scanned and activated CARSspectral peaks two possible population ratios are obtained. Through shape simulations of the timeresolvedCARS profiles under a simple kinetic model, the actual population ratio is unambiguously determined to be of 0.59 (σ=0.05). This ratio indicates that the molecules produced by the EV energy transfer process are 37% populated at the =1 vibrational level and 63% at =2, and that the efficiency of the EV energy transfer is 0.489 for the Rb − system and 0.481 for the − system, coincident with the highest EV transfer fraction 0.489 under the impulsive model and a collinear collision geometry.

Electrostriction effects on electron transfer reactions in solution. I. Adiabatic regime
View Description Hide DescriptionThe outersphere unimolecular electron transferreactions,, in polar solvents are studied theoretically. By employing a recent theory of fluctuating cavities [H. J. Kim, J. Chem. Phys. 105, 6818, 6833 (1996)], we construct a nonequilibrium free energy formulation that accounts for both the solvation and electrostriction aspects of the changing solute dipole moment during the reaction. With a twostate diabatic description for the solute electronic structure, we obtain a twodimensional free energy surface in terms of the cavity radius and a solvent coordinate that gauges the nonequilibrium solvent orientational polarization The equilibrium values for the reactant and product states are found to differ significantly, indicating that the cavity reorganization plays an important role in the reactionfree energetics and dynamics. The reaction mechanism and rate constant are analyzed via the solutionphase reaction path, which is found to vary markedly with the relative time scales of the and motions. To be specific, the charge transfer proceeds in a concerted fashion with active participation of both and when their characteristic times are comparable. By contrast, a stepwise mechanism results when there is a large mismatch between the and time scales. In the slow cavity limit where is much slower than the reorganization through the cavity size adjustment precedes the barrier crossing that occurs exclusively along the coordinate. In the opposite fast cavity regime where is mainly responsible for activation and deactivation, it is found that the saddle point on the free energy surface is no longer ratedetermining due to the nonlinear residual entropic effects. A brief comparison with other reaction systems involving charge shift in solution is also made.