Index of content:
Volume 102, Issue 8, 22 February 1995

Free jet infrared spectroscopy of (^{28}SiF_{4})_{2} in the 10 μm region
View Description Hide DescriptionThe rotation–vibration spectra of ^{28}SiF_{4} dimer have been studied near the ν_{3} band of monomer^{28}SiF_{4}. A well‐resolved parallel band has been observed 17.5 cm^{−1} below the monomer band origin. Two perpendicular bands, separated by 2.0 cm^{−1} and located 4.3 cm^{−1} above the origin for the lower component, show equidistant Q‐branch peaks as only resolved lines. The observed band structure indicates that the threefold degeneracy of the vibrational motions in SiF_{4} is all removed. A C _{2h } structure with a pair of SiF bonds in an antiparallel position is proposed.

Perturbation facilitated optical–optical double resonance spectroscopy of the 2 ^{3}Σ^{+} _{ g }, 3 ^{3}Σ^{+} _{ g }, and 4 ^{3}Σ^{+} _{ g } Rydberg states of ^{7}Li_{2}
View Description Hide DescriptionThis paper reports the experimental observation of the 2 ^{3}Σ^{+} _{ g }, 3 ^{3}Σ^{+} _{ g }, and 4 ^{3}Σ^{+} _{ g } states of ^{7}Li_{2} by cw perturbation facilitated optical–optical double resonance spectroscopy. Molecular constants and RKR potential curves have been obtained. Our experimental T _{ e } and R _{ e } for the 2 ^{3}Σ^{+} _{ g } state are 27 297.45(16) cm^{−1} and 3.0797(18) Å, respectively, and for the 3 ^{3}Σ^{+} _{ g } state are 31 043.93(53) cm^{−1} and 3.0378(19) Å, respectively. The above values are in very good agreement with theoretical calculations. Hyperfine splitting for both states has been resolved. Both states follow Hund’s case (b_{βS }) hyperfine coupling scheme. The experimental Fermi contact parameter, b _{ F }, is approximately 96±2 MHz for the 2 ^{3}Σ^{+} _{ g } state and 95.6±3 MHz for the 3 ^{3}Σ^{+} _{ g } state. These values are in good agreement with the previously obtained value 98.6±4 MHz [Li et al., J. Chem. Phys. 96, 3342 (1992)]. One level of the 4 ^{3}Σ^{+} _{ g } state has been observed and its hyperfine structure has been resolved and characterized with Hund’s coupling case (b_{βS }).

Rotational spectrum and structure of an (H_{2}O–HCN)–Ar trimer
View Description Hide DescriptionRotational transitions of the (H_{2}O–HCN)–Ar trimer have been measured at 3–17 GHz with the Balle/Flygare Mark II pulsed nozzle FT microwave spectrometer for the parent, ^{18}O, ^{13}C, ^{15}N, D_{2}O, and HDO isotopic species. The isotopomers exhibit both a‐ and b‐dipole transitions with ^{14}N hyperfine structure and all but the HDO have two sets of transitions assigned to 0_{00} and 1_{01} internal rotational states of the H_{2}O or D_{2}O. Rotational constants were determined by fitting the line centers separately for each isotopic set to the Watson Hamiltonian for an asymmetric top. A molecular mechanics for clusters (MMC) calculation of the potential energy surface and an approximate substitution analysis of the rotational constants give a nearly planar, Δ‐shaped structure which is a somewhat distorted superposition of the H_{2}O–HCN, H_{2}O–Ar, and Ar–HCN dimers. MMC also gives a barrier of ≲25 cm^{−1} to internal rotation of the H_{2}O. Factors governing the formation of trimers are discussed. The effects on trimer structure of differences in the pair interactions are found to be appreciable while the role of three‐body effects is small.

Self diffusion of nuclear spins in a porous medium with a periodic microstructure
View Description Hide DescriptionA Fourier approach is developed for evaluating the diffusion eigenstates and the diffusion propagator of a periodic, fluid filled porous medium, and is applied to the calculation of the pulsed‐field‐gradient‐spin‐echo amplitude M(k,t). The method is most effective for long times t, but works quite well down to times that are short enough so that asymptotic short time approximations of the diffusion process are still valid. The main advantage of the method is that it is applicable regardless of the value of the porosity or the shape of the periodic pore space. It is used to calculate M(k,t) for a number of examples of periodic porous media with porosities as low as 10%.

The van der Waals rovibronic spectrum of p‐difluorobenzene–Ar up to 125 cm^{−1} intermolecular energy: Assignment and character of van der Waals modes
View Description Hide DescriptionThe van der Waals (vdW) vibronic spectrum built on the electronic 0^{0} _{0}, S _{1}(B _{2})←S _{0}(A _{1}) origin of p‐difluorobenzene–Ar is investigated with rotational resolution (Δν_{UV}=60 MHz). For the first time vdW vibronic bands are detected up to a van der Waals energy of 125 cm^{−1} and assigned by a rotational analysis of the band structure. The band origin positions of the ten detected bands display a regular behavior with moderate anharmonicities and minor influences due to Fermi resonances. Using the concept of three‐dimensional Kraitchman equations and of normalized effective planar moments characteristic data on the nuclear displacements in the two different bending coordinates are deduced from the measuredrotational constants. The fundamental vdW vibronic states at low energies can be described in terms of one‐dimensional normal modes while vdW states at higher energies display mixed mode character.

Photoionization and photodissociation of nitric oxide in the range 9–35 eV
View Description Hide DescriptionPhotoionization and photodissociation of NO has been studied using 9–35 eV synchrotron light and detection of fluorescence as well as of mass selected NO^{+}, N^{+}, O^{+}, and O^{−} ions. Rydberg series converging to all known states in NO^{+} below 24 eV have been analyzed and classified and several of them are found to be predissociated by various NO^{+} states as well as by NO ion pair states. The complex structure in the 950–1200 Å excitation region is suggested to originate from interaction between a ‘‘new’’ NO valence state at T _{0}=77 470 cm^{−1} and high vibrational levels of Rydberg series converging to the NO^{+}ground state.

Temperature dependence of persistent infrared spectral holes for SeH molecules in the GeAsSe glass series
View Description Hide DescriptionPersistent infrared spectral holes have been burned at low temperature into the Se–H stretch mode absorption band of SeH molecules in the GeAsSe‐alloy glass series. Eight different compositions have been examined by changing the relative contents of germanium and arsenic covering a range of mean coordination numbers from 2.0 to 2.6. This is the first time that the dephasing time has been examined as a function of the glass coordination number. For all samples a linear dependence between hole width and hole area is found as a function of burn time at constant intensity and temperature, which is then used to extrapolate the hole width to the limit of zero burn time. The temperature dependence of the hole width is found to follow a power law. Both the zero temperature width and the temperature exponent increase with increasing mean coordination number indicating that the vibrational lifetime and the dephasing time may depend on the microscopic topology of the glass rather than on the chemical composition.

Geometrical representation of coherence transfer selection by pulsed field gradients in high‐resolution nuclear magnetic resonance
View Description Hide DescriptionA formalism for the calculation of suitable sequences of pulsed field gradients for signal selection in high‐resolution NMR spectroscopy is presented. It is based on a geometrical interpretation of coherence transfer pathway selection by pulsed field gradients. The formalism allows the calculation of the suppression rates for undesired pathways and the determination of the most efficient sequence of pulsed field gradients. As an example, sequences for multiplicity filtered ^{13}C/^{1}H correlation experiments are calculated and analyzed.

Charge transfer induced local Frenkel exciton states in naphthalene crystals heavily doped with durene
View Description Hide DescriptionAbsorption and luminescencespectra of durene‐doped naphthalene crystals are reported for different dopant concentrations. New impurity‐induced absorption bands are interpreted as charge transfer induced local Frenkel exciton states. The results of theoretical calculations, performed (without invoking any adjustable parameters) for a series of finite model clusters are in very good agreement with experiment and support this interpretation.

Vibrational spectroscopy of polypyrrole, theoretical study
View Description Hide DescriptionTheoretical vibrational spectra of polypyrrole are presented, based on a single‐periodic‐chain model and harmonic potential in‐plane and out‐of‐plane parameters transferred from the pyrrole molecule. The infrared‐absorption intensities were included in the calculations for the first time in polypyrrole. Experimental spectra are analyzed and compared with theoretical ones. Some modes in the experimental spectra indicate the presence of nonplanar conformation. Also, Raman frequencies were compared with experimental ones. Significant effects of electron–phonon coupling are absent.

Determination of ground state populations and alignment parameters using nonresonant three‐photon absorption
View Description Hide DescriptionThe polarization dependence of the three‐photon absorption signal is described in terms of the matrix elements of the irreducible representation of the three‐photon absorptiontensor operator for an anisotropic ensemble with cylindrical symmetry probed with identical photons of linear polarization. The formalism is applicable to the extraction of alignment parameters for diatomics as well as polyatomic molecules produced in collisions of unpolarized particles or in the photodissociation with a single photon of linear polarization. Applications to product ensembles of HCl and CO are discussed.

U _{ q }(4)⊇O _{ q }(4) description of diatomic molecules: Vibrational spectra
View Description Hide DescriptionQuantum‐deformed algebra (q‐DA) of the O(4) dynamical symmetry of the U(4) group is studied in respect of the vibrational spectra of diatomic molecules. It is shown that, as in the case of the U(6) group for nuclei, the q‐deformation parameter in U _{ q }(4)⊇O _{ q }(4) must also be complex for any meaningful gain in going from the classical to the q‐deformed algebra. Application is made to the ground electronic state (X ^{1}Σ^{+} _{ g }) of the H_{2} molecule.

Pulsed field ionization zero kinetic energy photoelectron spectroscopy of the vanadium dimer molecule
View Description Hide DescriptionThe technique of pulsed‐field‐ionization zero‐kinetic‐energy (PFI‐ZEKE), photoelectron spectroscopy was employed to probe the electronic structure of the V^{+} _{2} cation. Rotationally resolved PFI‐ZEKE spectra of the V^{+} _{2}ground state were obtained by two color excitation via the 700 nm A ^{3}Π_{ u }←X ^{3}Σ^{−} _{ g } system. The observation of transitions from the A ^{3}Π_{2u } state to two spin–orbit components with Ω=1/2 and Ω=3/2, confirms that the cation ground state has ^{4}Σ_{ g } ^{−} symmetry, in accordance with previous experimental and theoretical work. Striking differences were observed in the rotational selection rules for the ^{4}Σ_{ g } ^{−}←A ^{3}Π_{1u } and the ^{4}Σ_{ g } ^{−}←A ^{3}Π_{2u } transitions. The adiabatic ionization potential of V_{2} was determined to be 51 271.14(50) cm^{−1}. From an analysis of the rotational structure of the PFI‐ZEKE spectra, the following molecular constants were determined for the ^{4}Σ_{ g } ^{−} state: r _{0}=1.7347(24) Å, second order spin–orbit splitting, λ=5.248(17) cm^{−1}, spin–rotation constant, γ=0.0097(87) cm^{−1}, T _{0}=51 282.20(50) cm^{−1} (1σ error bounds).

Computer simulations of the electron spin resonance spectra of steroid and fatty acid nitroxide probes in bilayer systems
View Description Hide DescriptionMonte Carlo dynamics (MCD) techniques are used to simulate the orientational behavior and rotational motion of probe molecules in lipid bilayers. The trajectories of molecular orientations generated from the simulations are then used to calculate the order parameters and the orientational time correlation functions. The behavior of the time correlation functions is compared with the predictions of the rotational diffusion (RDM) and the compound motion (CM) models. The MCD trajectories are also used to produce electron‐spin resonance (ESR)spectra, employing a recently developed time‐domain algorithm. Two questions which have been the subject of debate in the literature are addressed. The first question concerns the discrepancy between the ability of motional models to describe ESRspectra and fluorescence depolarization measurements on rigid molecules in vesicles—while the RDM does an excellent job of fitting the former, the latter require the CM to describe them properly. It is argued that the key to resolving this lies in the fact that the ESR line shapes are sensitive to the tumbling motions of the long molecular axes as well as to rotational motions about them, while fluorescence anisotropy is blind for the latter. The rotation about the long molecular axis introduces a fast decay into the correlation functions in a way independent of the tumbling motion of the axis. The second question concerns the fidelity of reporting by fatty acid spin probes in lipid bilayers. It is shown that the motion of the bulky hydrophillic doxyl group does not, in fact, reflect the motion of the chains about it and consequently these spin probes cannot be considered good reporters for these applications.

Electronic energy transfer in concentrated micellar solutions
View Description Hide DescriptionA problem of electronic energy transfer among chromophores solubilized in a micellar solution is addressed theoretically with the emphasis on the intermicellar interactions. Using the formalism of characteristic functions, a general method is advanced for determining the distribution of distance between two molecules located in separate spherical micelles. Averaging the microscopic decay over the pair distance distribution function is the key step in relating the macroscopic observables to the underlying structure of the embedding space if the irreversible direct energy transfer between two chemically distinct species is considered. The problem of excitation transport among identical chromophores can also be formulated to a good approximation as a superposition of pairwise interactions. Analytical solutions to the distance distribution function as well as the corresponding configurationally averaged energy transfer observables are presented for several spatial arrangements of practical importance. Applications to concentrated micellar solutions and to clusters of micelles are discussed.

Electronic fine structure transitions and rotational energy transfer of NO(X ^{2}Π) in collisions with He: A counterpropagating beam study
View Description Hide DescriptionThe collision dynamics of NO(X ^{2}Π) with He is investigated at a collision energy of 147 meV using the method of counterpropagating pulsed molecular beamscattering. One‐dimensional product velocity distributions are determined through ion time‐of‐flight analysis. State specific detection of NO is achieved through (2+1) resonance enhanced multiphoton ionization detection via the E ^{2}Σ^{+} intermediate state. Calculated frequencies and line strength factors for the two‐photon transition enable the extraction of state resolved degeneracy averaged integral and differential cross sections. The overall behavior of the integral cross sections for fine structure conserving and changing transitions, weighted by the degeneracy of the final state, is well described by two different exponential energy gap laws. Differential cross sections for both types of transitions exhibit very different rotational rainbow structures. For the multiplet changing transition, the scaling law suggests an increased energy transfer efficiency while the rotational rainbow structure indicates a larger effective anisotropy. Therefore, the dynamics at the probed collision energy is clearly dominated by fine structure changing collisions when magnetic sublevel specific cross sections for a specified energy transfer are compared.

The reaction of CCl_{3} radicals and Cl atoms with hydrogen molecules
View Description Hide DescriptionA small amount of H atoms are observed as products of the reaction of Cl atoms generated by photodissociating Cl_{2} at 351 nm in the presence of H_{2}. A much larger H atom concentration is detected when a mixture of H_{2} and CCl_{4} is photodissociated at 193 nm with an average kinetic energy of 6.5±0.5 kcal/mol. The reactant is shown to be the vibrationally hot CCl_{3} radical. In the reaction of CCl_{3} and Cl atoms with HD the atomic H/D product ratios are 1.47±0.16 and 0.59±0.11, respectively. The former result is in accord with the theory of Johnson et al. [J. Chem. Phys. 94, 2749 (1991)] that for kinematic reasons the D end of the HD molecule is more reactive unless the HD molecule is strongly rotating. The F and Cl atoms which give H/D ratios less than 1 when reacting with HD are special cases for which higher J states of HD are still reactive.

Density functional approach to the solvent effect on the dynamics of nonadiabatic electron transfer reactions
View Description Hide DescriptionA density functional approach is proposed for calculating the activation free energy in a nonadiabaticelectron transferreaction where the molecular nature of the solvent is incorporated in a self‐consistent manner. The overall electron transferrate constant is obtained by taking into account the static as well as dynamic aspects of solvent effects. Illustrative numerical results are presented for both the activation barrier and the electron transferrate constant.

Nonadiabaticity and intramolecular electronic energy transfer in the photodissociation of 1‐bromo‐3‐iodopropane at 222 nm
View Description Hide DescriptionThe photodissociation of 1‐bromo‐3‐iodopropane (1,3‐C_{3}H_{6}BrI) at 222 nm is studied with crossed laser‐molecular beam experiments. Irradiation at this wavelength excites an n(Br)→σ*(C–Br) transition which promotes the molecule to an approximately diabatic excited statepotential energy surface which is dissociative in the carbon–bromine bond. This surface intersects an approximately diabatic surface of n(I)→σ*(C–I) character at extended C–Br distances; this surface is dissociative in the carbon–iodine bond. Crossings from the surface initially accessed to the intersecting surface correspond to intramolecular excitation transfer from the carbon–bromine to the carbon–iodine bond. The incidence of such transfer and hence of carbon–iodine bond fission depends upon the strength of the off‐diagonal potential coupling of the two diabatic states. These experiments test the dependence of the coupling and consequent energy transfer upon the separation distance of the C–Br and C–I chromophores. The data show C–Br fission dominates C–I fission by a ratio of 4:1 and determine the center‐of‐mass translational energy distributions and angular distributions of these processes. The measured anisotropy parameters are β(C–Br)=1.6±0.4 and β(C–I)=0±0.2. A third photofission process, IBr elimination, also contributes to the observed signal. The results of the study of C–Br and C–I fission are compared to previous studies on similar molecules to understand how the branching depends on the relative positioning of the C–Br and C–I chormophores.

Reaction‐path potential and vibrational frequencies in terms of curvilinear internal coordinates
View Description Hide DescriptionWe present a general formulation that allows physically intuitive curvilinear internal coordinates to be used for the calculation of potential energy expansions and generalized normal‐mode vibrational frequencies in reaction‐path calculations. The reaction path is defined, as usual, as the minimum‐energy path in the mass‐scaled Cartesian coordinate system, and curvilinear coordinates are used for vibrational frequency calculations at nonstationary points. The method is well adapted for use in variational transition state theory with semiclassical multidimensional tunneling (VTST/MT) approximations to calculate thermal rate constants. We present VTST/MT calculations for five reactions, H+H_{2}→H_{2}+H, O+H_{2}→OH+H, CH_{3}+H_{2}→CH_{4}+H, H+O_{2}→HO_{2}, and Cl+HBr→HCl+Br, to illustrate the use of the new curvilinear coordinates, and we compare the results to calculations employing rectilinear coordinates. We make detailed comparisons not only of the calculated rate constants but also of the vibrationally adiabatic ground‐state potential energy curves and bound‐state vibrational frequencies as functions of the reaction coordinate.