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Volume 102, Issue 16, 22 April 1995

Frequencymodulation transient absorption spectrum of the HCCl transition
View Description Hide DescriptionThe rotationally resolved spectrum of the HCCl transition between 12 230 and 12 410 was obtained for the first time at Dopplerlimited resolution using a frequencymodulation (FM) transient absorption technique. Five major subbands 1 ←2, 2←1, and 2←3) of and one subband of were observed and assigned. In addition, the forbidden subband of was also observed. Thus far, we have assigned most (more than 70%) of the strong lines. The analysis of these subbands determines the geometry for the HCCl level: and Å while is fixed at the corresponding value of The height of the barrier to linearity was found to be approximately above the vibrationless level of the state. The rotational structure of this spectrum shows irregular perturbations. Both random and dependent (anomalous doubling) effects were observed. As in methylene, the perturbations most likely arise from RennerTeller effect between the two singlet states in combination with spin–orbit coupling between the singlet and triplet states.

Vibrational spectra of tetra‐atomic silicon–carbon clusters. II. Si_{2}C_{2} in Ar at 10 K
View Description Hide DescriptionFourier transform infrared measurements on the spectra of the products of the vaporization of silicon/carbon mixtures trapped in solid argon in concert with ab initio calculations using second order many body perturbation theory have resulted in the identification for the first time of two vibrational fundamentals, ν_{3}(b _{1u })=982.9 and ν_{4}(b _{2u })=382.2 cm^{−1}, of the rhombic ground state structure of Si_{2}C_{2}. The observed frequencies, intensities, and isotopic shifts are in good agreement with the ab initio predictions. Tentative assignments are also made for fundamentals of the linear (SiCCSi) and distorted trapezoidal isomers. The relative energies of the three isomers have been estimated at various ab initio levels.

Optical dynamics of excitons in aggregates of a carbocyanine dye
View Description Hide DescriptionResults of temperature and wavelength dependent fluorescence lifetime and accumulated photonecho experiments on the aggregate of the dye 5,5′,6,6′tetrachloro1,1′diethyl3,3′di(4–sulfobutyl) benzimidazolocarbocyanine (TDBC) in an ethylene glycol/water glass are reported. Additionally, the temperature dependent relative fluorescence quantum yield was determined. Using twocolor pump–probe spectroscopy, the intersite correlation of the frequency disorder and the size of the coherence domains were estimated. Both the phase and the population relaxation times are frequency dependent in the echo and the singlephotoncounting experiments. The dependence of the fluorescence lifetime on detection wavelength is restricted to temperatures below 80 K, indicating a temperature activated process. From our experiments and by comparison with previously published results we conclude that the dispersive nature of both the dephasing and the depopulation is caused by intraband relaxation processes. At higher temperatures this wavelength dependence vanishes due to thermalization. The temperature dependence of the relative fluorescence quantum yield of TDBC is equal to the one of pseudoisocyanine (PIC). Using a motional narrowing model for disordered molecular aggregates with consideration of intersite correlation, at 1.5 K, the twocolor pump–probe experiments indicate a very high correlation in the frequency disorder in TDBC aggregates with a correlation length of several hundred molecules. From pump–probe experiments the delocalization length of the exciton is determined to be between 30 and 45 molecules at 1.5 K.

The vibrational energy levels in acetylene ^{12}C_{2}H_{2}: Towards a regular pattern at higher energies
View Description Hide DescriptionWe have gathered 122 known experimental data concerning the vibrational energy levels of acetylene, C_{2}H_{2}, up to 12 000 cm^{−1}. We have fitted the data simultaneously to a model based on the clustering of the levels induced by a set of eight anharmonic resonances. That set is carefully built from the literature information on various isotopomers of acetylene. The model takes full advantage of the anharmonic pattern and systematically block diagonalizes the entire vibrational energy matrix. The 122 input energies are reproduced within a standard deviation of 0.74 cm^{−1} with 35 fitted parameters. Decisive support to the model is brought by using the resulting vibrational parameters and matrix eigenvectors to reproduce energies, principal rotational parameters, and relative intensity features. The cluster picture is discussed, in terms of the 7 normal modes of vibration in acetylene, according to the theoretical model developed by Kellman [M. E. Kellman and G. Chen, J. Chem. Phys. 95, 8671 (1991)].

Correlation states of ethylene
View Description Hide DescriptionHigh resolution synchrotron photoelectron spectra (PES) of ethylene have been obtained at several photon energies in the range 30 to 220 eV. Further evidence is presented that the correlation (satellite) peak at 27.4 eV binding energy is ‘‘intrinsic’’ in nature. A new correlation peak at 21.4 eV binding energy, however, is found to be a ‘‘dynamic’’ correlation. Several PES of 1‐^{13}C‐ethylene have also been obtained and have been found to be identical to those of normal ethylene. Both of the correlation peaks are also present in the labeled species with similar photon energy behaviors. Sophisticated theoretical calculations are found to agree quantitatively with the experimental PES spectra.

Hyper‐Rayleigh scattering studies of first order hyperpolarizability of tricyanovinylthiophene derivatives in solution
View Description Hide DescriptionFirst order hyperpolarizabilities, β, for donor–acceptor substituted heteroaromatic compounds are measured by the hyper‐Rayleigh scattering technique. Hyper‐Rayleigh scattering utilizes the mechanism of number density fluctuations and enables the second harmonic generation signal of nonlinear optical chromophores to be generated as scattered light. Effects of substituting different electron donating groups to the parent heteroaromatic compounds and different solvents on the hyperpolarizability is investigated using this technique.

Electronic shells or molecular orbitals: Photoelectron spectra of Ag^{−} _{ n } clusters
View Description Hide DescriptionPhotoelectron spectra of Ag^{−} _{ n } clusters with n=1–21 recorded at different photon energies (hν=4.025, 4.66, 5.0, and 6.424 eV) are presented. Various features in the spectra of Ag^{−} _{2}–Ag^{−} _{9} can be assigned to electronic transitions predicted from quantum chemical ab initio calculations. While this comparison with the quantum chemical calculations yields a detailed and quantitative understanding of the electronic structure of each individual cluster, a discussion in terms of the shell model is able to explain trends and dominant patterns in the entire series of spectra up to Ag^{−} _{21}.

Neutral rare‐gas containing charge‐transfer molecules in solid matrices. I. HXeCl, HXeBr, HXeI, and HKrCl in Kr and Xe
View Description Hide DescriptionUltraviolet‐irradiation of hydrogen halide containing rare gas matrices yields the formation of linear centrosymmetric cations of type (XHX)^{+}, (X=Ar, Kr, Xe). Annealing of the irradiated doped solids produces, along with thermoluminescence, extremely strong absorptions in the 1700–1000 cm^{−1} region. Based on isotopic substitution and halogen dependence of these bands, the presence of hydrogen and halogen atom(s) in these species is evident. In the present paper we show the participation of rare gas atom(s) in these new compounds. The evidence is based on studies of the thermally generated species in mixed rare gas matrices. The new species are assigned as neutral charge‐transfer molecules HX^{+}Y^{−} (Y=halogen), and their vibrational spectra are discussed and compared with those calculated with ab initio methods. This is the first time hydrogen and a rare gas atom has been found to make a chemical bond in a neutral stable compound. The highest level ab initio calculations on the existence of compounds of type HXY corroborate the experimental observations. The mechanism responsible for the formation of these species is also discussed.

Ar and CH_{4} van der Waals complexes of 1‐ and 2‐fluoronaphthalene: A perturbed spherical top attached to a surface
View Description Hide DescriptionWe compare and contrast the low and high resolution S _{1}←S _{0}fluorescence excitation spectra of four van der Waals complexes, Ar–1FN, CH_{4}–1FN, Ar–2FN, and CH_{4}–2FN (where 1FN and 2FN are 1‐ and 2‐fluoronaphthalene, respectively) in the gas phase. Whereas the Ar and CH_{4} complexes exhibit comparable low resolution spectra, their high resolution spectra are significantly different. The CH_{4}–1/2FN complexes exhibit origin bands that are each split into three distinct subbands with different intensities and separations of less than 1 cm^{−1}. No such splittings are observed in Ar–1/2FN. The relative intensities of the three subbands in both CH_{4} complexes are 1:2:2. These are identical, within experimental error, to the total statistical weights of the J=0, 1, and 2 rotational levels of CH_{4}. Both Ar and CH_{4} are weakly attached to 1/2FN at a distance of ∼3.5 Å above the aromatic plane. This distance decreases slightly (∼0.1 Å) on S _{1}←S _{0} excitation. Thus, the splittings observed in CH_{4}–1/2FN are attributed to ‘‘surface‐induced’’ perturbations of the normally isotropic rotational motion of methane whose magnitudes depend on the electronic structure of the surface to which it is attached. A model is proposed that accounts for these observations. Comparison of the numerical predictions of this model with the experimental results shows that the rotational motion of the attached CH_{4} is nearly the same as that of the free molecule.

Fourier transform spectroscopy of the CO‐stretching band of C‐13 methanol in the torsional ground state
View Description Hide DescriptionThe Fourier transformspectrum of the CO‐stretching fundamental band of ^{13}CH_{3}OH has been investigated at 0.002 cm^{−1} resolution. For the n=0 torsional ground state, 35 subbands containing some 2300 lines have been identified for K values from 0 to 10 for the three torsional symmetries A, E _{1}, and E _{2} up to J values typically well over 30. The assigned transitions have been fitted to J(J+1) power‐series expansions to obtain subband origins and sets of phenomenological state‐specific parameters describing the J dependence of the subbands. These parameters reproduce the observed wave numbers to within the experimental uncertainty of ±0.0005 cm^{−1} for unblended lines, except when particularly large asymmetry shifts or perturbations due to Fermi or Coriolis resonances are present. The subband origins have been fitted to effective molecular constants for the excited CO‐stretching state and a torsional barrier of 378.7(8) cm^{−1} is found, representing a 1.3% increase over the ground state value. A wide variety of global or J‐localized perturbations has been seen, and most of the interacting states have been identified. New assignments have been obtained for far‐infrared laser lines optically pumped by the 10R(2) and 9P(12) CO_{2} laser lines.

Dynamic fluorescence Stokes shift of an acetylene‐bridged donor–acceptor compound in alcohols at low temperatures
View Description Hide DescriptionThe time‐dependent fluorescence Stokes shift of p,p’‐cyano thiomethyl diphenylacetylene in three different alcohols (1‐propanol, 1‐butanol, and 1‐octanol) at room temperature was measured using time‐correlated single photon counting and the spectral reconstruction method. These measurements were also performed with the probe in 1‐propanol at low temperatures. The observed behavior is consistent with the dynamics of solvation of a polar excited state. As expected, the average solvation time increases with decreasing temperature and is correlated with solvent bulk viscosity down to 180 K. Apparent deviations observed at lower temperatures are attributed to the finite emission lifetime (≊1 ns) of the probe. A significant fast component (<100 ps) is observed in the dynamic Stokes shift even at temperatures close to the freezing point of 1‐propanol. Different mechanisms that could result in such a fast component are discussed.

Electric field induced second harmonic generation/third harmonic generation measurements on molecules with extended charge transfer: Absorption domain and strong resonance effects
View Description Hide DescriptionAn enlarged method of performing electric field induced second harmonic generation (EFISHG) or third harmonic generation (THG) measurements of molecular hyperpolarizabilities in the strongly resonant regime is proposed. The influence of absorption coefficients and anomalous dispersion are discussed in detail and corrections proposed which require no change in the experimental setup. An alternative method of data interpretation is presented and its validity discussed for two examples of molecular structures.

Electronic orbital alignment effects in the reaction Mg*(3p ^{1} P _{1})+CH_{4}→MgH+CH_{3}
View Description Hide DescriptionWe have measured the final state resolved far‐wing action spectra for the MgCH_{4} reactive collision system. The results show a dramatic ‘‘Π‐like’’ orbital alignment preference in the reaction channel. The reactive channel action spectra for different MgH rotational states in v=0 are identical, suggesting that the reaction follows from a single approach geometry, with the product rotational distribution determined by exit channel effects. Based on these observations and molecular orbital considerations, we propose that the reaction proceeds in η_{2} approach geometry through a triangular C–Mg–H transition state.

On the selection of magnetic states in angle‐resolved photodissociation
View Description Hide DescriptionThe information contained in magnetic‐state‐selected photodissociation differential cross sections is examined by means of a quantum mechanical time‐independent theory. Motivated by recent experimental demonstration of the possibility to select molecules with respect to their magnetic‐rovibronic state and measure the energy‐resolved angular distribution following their photofragmentation, we examine analytically and numerically the sensitivity of such angular distributions to the structure and the dynamics of the studied system. It is found that magnetic‐state‐selected cross sections contain significant information with respect to both the electronic structure (the potential energy surfaces as well as the transition dipole vector) and the reactiondynamics of photoinitiated reactions, which cannot be obtained from more averaged observable. In particular we find that such cross sections provide a mapping of the transition dipole vector which couples the ground state with the excited manifold. This feature is traced to the coherent excitation of a small subset of helicity states in the absorption process. It is suggested that the information contained in these angular distributions can be appreciated and extracted by preceding the experimental measurements with theoretical analysis.

Path‐integral treatment of multi‐mode vibronic coupling. II. Correlation expansion of class averages
View Description Hide DescriptionA path‐integral approach to real‐time quantum dynamics is presented which is suitable to treat the dynamics of vibronic coupling or spin boson models. In these models the vibrational dynamics is nonseparable as a consequence of the electronic inter‐state coupling. The sum over all possible paths in electronic‐state space generated by the usual Trotter procedure is expressed in terms of single‐mode averages over classes of paths and statistical mode correlations. The averages for classes of a given length can be calculated iteratively from averages over shorter paths. This expansion is formally exact and finite for a finite number of modes. Usually only a limited number of terms has to be evaluated in order to obtain converged results. The scaling of the computational effort with respect to the number of time steps and the number of modes is given by a low‐order power law, depending on the chosen class structure and the order of the expansion. The usual time‐dependent wave‐packet propagation and the full path enumeration, which exhibit an exponential scaling behavior with respect to either the number of modes or the number of time steps, can be considered as opposite limiting cases of the correlation expansion (CE) of the path integral. The convergence of the CE is tested by application to a two‐state four‐mode model representing S _{1}‐S _{2} vibronic coupling in pyrazine, for which exact references (time‐dependent correlation functions) are available. The potential of the CE approximation for the treatment of multi‐mode problems is demonstrated by application to an extended 24‐mode vibronic‐coupling model. This model is suitable to provide a microscopic description of ultrafast optical dephasing processes in large molecules.

Orthogonally spin‐adapted single‐reference coupled‐cluster formalism: Linear response calculation of static properties
View Description Hide DescriptionAn orthogonally spin‐adapted coupled‐cluster (CC) linear response (LR) method, to calculate first‐order static properties of closed‐shell systems, is formulated and implemented. Cluster operators are truncated at single and double excitations (CCSD‐LR approach) and the explicit first‐order CCSD‐LR equations, in terms of molecular integrals and zeroth‐ and first‐order cluster amplitudes, are given. Various aspects of the computer implementation of the CCSD‐LR approach are discussed. In particular, the efficiency of the CCSD‐LR approach is compared with corresponding finite field calculations. Advantages of using the LR approach are illustrated by calculating electrostatic multipole moments for CH_{2} and HF at equilibrium and displaced geometries.

The ethylenedione anion: Elucidation of the intricate potential energy hypersurface
View Description Hide DescriptionAb initio molecular orbital theory has been used to study the controversial potential energy surface of the ethylenedione anion C_{2}O^{−} _{2}. Seven different basis sets, the largest being triple zeta plus two polarization functions and one set of higher angular momentum functions (TZ2Pf) in quality, were utilized in conjunction with five correlated methods, the highest‐level being coupled‐cluster theory including single, double, and perturbative triple excitations [CCSD(T)]. Equilibrium geometries and harmonic vibrational frequencies of the predicted ^{2} A _{ u } trans‐bent ground state are presented. The Renner–Teller potential energy surface resulting from the splitting of the doubly degenerate linear ^{2}Π_{ u } transition state into the nondegenerate bent ^{2} A _{ u } and linear ^{2} B _{ u }surfaces is also characterized by means of energy predictions for these three states. Several recent peak assignments in the experimental spectrum, as well as the isotopic shifts associated with them, are supported by theory. A correct description of the potential energy hypersurface is obtained only by application of large basis sets in conjunction with methods including high‐level treatment of electron correlation effects. The TZP+/CCSD(T) methodology predicts the OCC bond angle to be 146.5°.

Concentration fluctuations and coupling to stress relaxation in viscoelastic binary solutions
View Description Hide DescriptionDynamics and coupling of concentration fluctuations to longitudinal stress relaxation in a viscoelastic binary polymer solution have been analyzed using nonequilibrium thermodynamics. The calculation supplements the result previously obtained by this author using the Bearman–Kirkwood hydrodynamic equation for a multicomponent system. We have shown that the dynamics of concentration fluctuations is affected by the stress modulus of the solution only when there is a difference in the specific volume between polymer and solvent components. The present work also extends Bearman–Kirkwood’s result by further relating the partial longitudinal stress modulus of the polymer to the total longitudinal stress modulus of the polymer solution.

Molecular dynamics simulation of the wave vector‐dependent static dielectric properties of methanol–water mixtures
View Description Hide DescriptionThe static wave vector‐dependent dielectric properties of methanol–water mixtures at room temperature are investigated through molecular dynamics simulation. We report results for the transverse and longitudinal components of the permittivitytensors, as well as the short‐ and long‐ranged portions of the Kirkwood g factors at small wave vectors k, calculated from the mean squared dipole densities for mixtures of different compositions. In addition, we have calculated dipolar symmetry projections h ^{110}(r) and h ^{112}(r) of the pair distribution functions for each mixture and determined the asymptotic behavior of the second projection at large distance r. We find that h ^{110}(r) and h ^{112}(r) as well as their contributions from different molecular pairs are strongly composition dependent, but that the resulting Kirkwood correlation factor is nearly independent of composition. We obtained the dielectrictensor ε(k) over a wide range of k using Fourier–Hankel transforms of h ^{110}(r) and h ^{112}(r). Our results indicate that this real space route is an attractive alternative approach to the static, k‐dependent dielectric properties of polar liquids. The k‐dependent transverse components of the permittivities obtained this way are in good agreement with the values calculated from the mean squared dipole densities. The longitudinal permittivities, on the other hand, show a less satisfactory agreement at small wave vectors due to inaccuracies in the numerical transforms.

Time‐dependent and temperature‐dependent aspects of electron distribution functions: H, Ar, and Cs atomic gases
View Description Hide DescriptionTime‐dependent and temperature‐dependent aspects of the thermalization of electrons in atomic gases are studied by using the Boltzmann equation. H, Ar, and Cs gases were chosen for the present study because of the characteristic and significantly different dependences of their momentum‐transfer cross sections on electron energy; H has a smoothly varying cross section, Ar has a conspicuous Ramsauer–Townsend minimum, and Cs has a resonance‐like peak. The effects of these cross section shapes on electron distribution functions and degradation spectra are examined.