Index of content:
Volume 87, Issue 2, 15 July 1987

The effect of dynamics on band shapes of hydrogen bonded complexes in solution
View Description Hide DescriptionWe study the IR absorption of the ν_{ s } (XH) mode of a hydrogen bonded species (XH‐‐‐Y) in solution. Our theory assumes that the dephasing of the ν_{ s } dipole transition is due to coupling to the low frequency stochastic ν_{σ} (XH‐‐‐Y) stretching mode. Our approach is novel in that it uses Kubo’s stochastic Liouville equation to include both dynamics a n d Fermi resonance coupling of the ν_{ s } levels with other intramolecular levels. Spectra assuming three bath models (Smoluchowski, Ornstein–Uhlenbeck, and Zwanzig) are compared and discussed. The theory gives good overall qualitative agreement with experiments involving moderate strength hydrogen bonds. It also demonstrates that the overall shape of the band can be analyzed in terms of parameters that affect primarily ν_{ s }, and that dynamics should be included for a realistic description of these bands.

A modified Boltzmann kinetic equation for line shape functions
View Description Hide DescriptionThe shape of an isolated spectral transition is analyzed in terms of an approximation to the Waldmann–Snider kinetic equation. This equation is written in the form of a drift and collision operator acting on a density matrix. With the use of the spherical approximation, the collision operator is subdivided into an elastic Boltzmann‐like collision term, an inelastic loss term, and a dephasing term. The Boltzmann‐like term is responsible for Dicke narrowing of spectral lines, the inelastic loss term leads to line broadening and the dephasing term may contribute both to line broadening and shifting. Simple approximations to these terms are powerful enough to account for some of the details of experimental line shapes such as asymmetrical deviations from a Lorentzian line shape.Model numerical calculations are carried out assuming classical scattering potentials of the form 1/r ^{ n } in the Boltzmann‐like term and single complex frequencies for the other two. It was found that as long as the density was scaled to give the same diffusion constant, the exact form of the scattering cross section had little effect on the final line shape.

Methyl internal rotation in partially deuterated molecular solids: The NO_{2}CHD_{2} and NO_{2}CH_{2}D systems
View Description Hide DescriptionCombining optical, neutron scattering and specific heatmeasurements, a precise determination of the parameters for the internal rotation potential energy function of the partially deuterated methyl group CHD_{2} and CH_{2}D is obtained in crystallized nitromethane. The effects of other internal motions on the internal rotation are analyzed and compared to the corresponding results for similar molecules in condensed and gas phases. The origin of the difference in the zero point energy of the symmetrical and asymmetrical rotamers of the partially deuterated derivatives is discussed.

Calculations of electric field effects on persistent spectral holes in amorphous host–guest systems
View Description Hide DescriptionThe dependence of spectral holes of dye molecules in amorphous solids on the following parameters has been calculated: the electric field strength, the angles between the hole‐burning and probing laser light fields and the external electric field, the electric dipole moment difference Δμ_{ m } between the excited and ground state of the guest molecule, the contribution Δμ* of the host matrix to the electric dipole moment difference, and the angle γ, between Δμ_{ m } and the transitiondipole moment μ_{ g e }. When the molecular dipole moment difference Δμ_{ m } dominates the spectral hole broadens or splits into two holes by the application of the electric field, depending on the angle γ and the directions of the laser light fields and the external field. With increasing values of the host‐induced electric dipole moment difference Δμ* the spectral hole shape in the electric field changes significantly if appropriate directions of the light fields and the external field are chosen. The material parameters γ, Δμ_{ m }, and Δμ* can be determined unequivocally by comparing the calculated holes both in the frequency dimension and the electric field dimension with experimental results.

A variable energy photoelectron study of the valence and Hg 5d levels of Hg(CH_{3})_{2}
View Description Hide DescriptionUsing monochromatized synchrotron radiation, the gas phase photoelectron spectra of the Hg 5d and valence levels of Hg(CH_{3})_{2} have been obtained between 21 and 100 eV photon energies. Experimental valence band branching ratios have been compared with theoretical branching ratios from MS‐Xα calculations. Our results strongly suggest that the ordering of valence levels is 2a ^{‘} _{2}<3a _{1}<2e’<2e‘, rather than 2a ^{‘} _{2}<3a _{1}<2e‘<2e’ from previous theoretical calculations. For the 3a ^{’} _{1} orbital, two shape resonances are predicted at 30 and 50 eV kinetic energy. Only the one at 30 eV is observed. Interchannel coupling with Hg 5dphotoemission is important for the outermost 2a‘_{2} and 3a ^{’} _{1} orbitals. Shape resonances and interchannel coupling strongly suggest that He ii:He i cross section ratios cannot generally be used to infer orbital character. The Hg 5d intensity ratio in Hg(CH_{3})_{2} differs from that of atomic Hg in the low photon energy region. This effect is probably due to the differences in the outgoing f wave potential between Hg and Hg(CH_{3})_{2}.

A quantitative experimental study of the core excited electronic states of formamide, formic acid, and formyl fluoride
View Description Hide DescriptionOptical oscillator strength spectra of formamide (HCONH_{2}), formic acid (HCOOH), and formyl fluoride (HCOF) in the region of K‐shell excitation have been derived from electron energy loss spectra recorded under electric dipole dominated scattering conditions (>2.7 keV impact energy, small scattering angle). The observed features are assigned to promotions of 1s electrons to π*(C=O), σ*(HCX), σ*(C–X), σ*(C=O), and Rydberg orbitals. Systematic changes in the term values for the 1s→π*(C=O) transitions are related to the π donor strengths of the X substituents of the carbonyl group. Broad weak features, observed only in the carbonyl C1s and O1s spectra around 7 eV above the IP, are assigned to 1s→σ*(C=O) transitions. The positions of these features are in agreement with a previously documented correlation with bond length, as are the positions of features associated with σ*(C–N) in formamide, σ*(C–O) in formic acid, and σ*(C–F) in formyl fluoride. The oscillator strengths of the 1s→π* features in the various K‐shell spectra are compared to HAM/3 calculations and are used to estimate the spatial distributions of the π*(C=O) orbital in the (1s ^{−} ^{1}, π*) core excited states of these three substituted carbonyl species. We discuss the degree to which these derived orbital maps reflect the spatial distributions of π*(C=O) orbitals in the ground state.

Experimental and molecular dynamics study of depolarized Rayleigh scattering by O_{2}
View Description Hide DescriptionCollision‐induced effects on depolarized Rayleigh scattering (DRS) in gaseous oxygen at 300 K in the pressure range from 10 to 3000 bar have been studied experimentally. DRS spectra for four thermodynamic states at this temperature and in the pressure range from 130 to 3000 bar have also been obtained from molecular dynamics (MD) simulation. Molecular trajectories were calculated using the Lennard‐Jones atom–atom potential and the system polarizability was modeled using the first order dipole–induced dipole approximation. The frequency dependence of DRS band shapes is well reproduced by the MD results, which predict that the orientational contribution dominates the overall band shape. The agreement between experimental and calculated second, fourth, and sixth spectral moments is very good. The observed density dependence of the spectral moments is analyzed in terms of contributions from the collision‐induced (CI) polarizability component and orientational pair correlations. The second spectral moment, M _{2}, is found to decrease with increasing density at low densities and to increase at higher densities. The MD results indicate that the initial decrease in M _{2} is due primarily to slow relaxation of the CI polarizability at low densities. The increase in M _{2} at higher densities is found to be mainly due to the contribution from orientational‐CI cross correlations. MD results as well as analytical calculations, carried out for the low density limit, predict that the CI contribution to the total DRS intensity remains relatively small (≤4%) throughout the entire density range. The observed density dependence of the intensity is not consistent with these calculations. Possible sources of this disagreement are discussed.

MPI photoelectron spectroscopy of u n g e r a d e excited states of acetylene: Intermediate state mixing and ion state selection
View Description Hide DescriptionThree photon resonant, four photon (3+1) ionization spectroscopy and photoelectron spectroscopy have been used to study the u n g e r a d eexcited states of acetylene in the energy range from 74 500 to 90 000 cm^{−} ^{1}. Sharp bands from the n R (π^{3} _{ u } n sσ_{ g }) and ^{1}Φ_{ u } (π^{3} _{ u } n dδ_{ g }) Rydberg series dominate the MPI spectrum. A large number of Rydberg and valence states which are prominent in VUV absorption spectra are absent or weak in MPI studies. These weak bands are only observable under high power conditions, which suggests that nonradiative decay is rapid enough to depopulate these states before ionization occurs. The photoelectron results provide further insight into the nature of the excited states.Ionization through the sharp bands occurs via Δν=0 Franck–Condon transitions, resulting in ions in a single vibrational state.Ionization through bands which are mixed results in complicated ion vibrational distributions including excitation of both c i s and t r a n s bends.

Spectroscopic study of the E ^{1}Σ^{+} _{ g } ‘‘shelf’’ state in ^{7}Li_{2}
View Description Hide DescriptionThe E ^{1}Σ^{+} _{ g } ‘‘shelf ’’ state of the ^{7}Li_{2} molecule was investigated using a pulsed optical optical double resonance technique. The measurements cover the vibrational levels in the range 0≤v≤29 including the shelf region around 13≤v≤15. Molecular constants have been determined. Using an inverted perturbation approach, an effective potential energy curve has been generated within the adiabatic approximation whose quantum mechanical energyeigenvalues reproduce all the measured term values to within 0.113 cm^{−} ^{1} for the range 0≤v≤23 and 0≤J≤47.

Ag_{7} cluster: Pentagonal bipyramid
View Description Hide DescriptionThe Ag_{7} metal cluster was formed in a solid neon matrix at 4 K. The identity of the cluster was established via X‐band electron‐spin‐resonance (ESR)spectroscopy, and its structure was found to be a pentagonal bipyramid (D _{5h } symmetry) with a ^{2} A ^{‘} _{2} ground state. Thus its properties are completely analogous to its Group IA alkali‐metal counterparts.

High resolution infrared spectra of ethylene clusters
View Description Hide DescriptionInfrared predissociationspectra have been measured for ethylene clusters near the ν_{7} (950 cm^{−} ^{1}) monomer band using CO_{2} lasers. One of the CO_{2} lasers was piezoelectrically scanned over a 240 MHz range around each rotational lasing transition, to obtain high resolution spectra of the ν_{7} band. The resulting spectra showed fine structure with widths of the order of 20 MHz, representing a lower limit on the predissociation lifetime of around 10 ns. The fine structure is shown to be due to ethylene dimers, and the broad background is attributed to a superposition of inhomogeneous contributions from larger clusters.

A study of the vibration–rotation and fine structure energy levels of the NSe radical by laser magnetic resonance
View Description Hide DescriptionVibration–rotation and fine structure transitions of the NSe radical in its X ^{2}Π state have been studied by CO_{2} laser magnetic resonance at 11 μm. Transitions involving the five most abundant naturally occurring isotopes of selenium have been detected. These data were all analyzed together by invoking the appropriate isotopic relationships between molecular parameters. The vibrational band origin for N^{8} ^{0}Se is determined to be 944.542 16(9) cm^{−} ^{1} and the spin–orbit coupling constant Ã is 891.891 32(11) cm^{−} ^{1}. From a consideration of the likely sign of the lambda doubling parameter (p+2q), it is suggested that B electronic state may be ^{2}∑^{+} rather than ^{2}∑^{−}.

ESR and a b i n i t i o theoretical studies of the cation radicals ^{1} ^{4}N^{+} _{4} and ^{1} ^{5}N^{+} _{4}: The trapping of ion–neutral reaction products in neon matrices at 4 K
View Description Hide DescriptionThe ^{1} ^{4}N^{+} _{4} and ^{1} ^{5}N^{+} _{4} molecular cation radicals have been generated by the ion–neutral reaction N^{+} _{2} +N_{2} and isolated in solid neon matrices at 4 K for detailed ESR(electron spin resonance) investigation. Both photonionization at 16.8 eV and electron bombardment (50 eV) were used in conjunction with the neon matrix trapping technique to produce the N^{+} _{4} dimer cation. The ESR results clearly show that N^{+} _{4} is linear and has a ^{2}Σ_{μ}ground electronic state. The magnetic parameters in neon are: g _{∥}=2.0016(4) and g _{⊥}=1.9998(2); A _{∥}(^{1} ^{4}N)=311(1) MHz and A _{⊥}(^{1} ^{4}N)=264(1) MHz for the central atoms and ‖A _{∥}‖=10.4(5) MHz and ‖A _{⊥}‖=20.4(1) MHz for the outer or terminal ^{1} ^{4}N atoms. Electronic structure information for N^{+} _{4} was obtained from the ESR results and compared with a b i n i t i o CI calculations. The unpaired electron resides primarily on the inner nitrogen atoms with significant 2p _{σ} and 2s character. Orbital characters obtained from the commonly applied free atom comparison method (FACM) were compared with the results of a Mulliken type spin population analysis conducted on the calculated wave function. The calculated nuclear hyperfine parameters (Atensors) showed reasonable agreement with experiment except for the very small A _{iso} parameter for the outer nitrogen atoms. Benchmark calculations employing large basis sets were conducted for the free nitrogen atom; these efforts demonstrate the difficulty in computing the A _{iso} parameter when inner shell effects are important.

Electronic absorption spectroscopy of molecular ions in plasmas by dye laser velocity modulation: The A←X system of N^{+} _{2}
View Description Hide DescriptionA computer controlled velocity modulation dye laserspectrometer is used to measure the A ^{2}Π_{ u }←X ^{2}Σ^{+} _{ g } (7,3) band of N^{+} _{2}. The Doppler‐limited absorption spectra are fit to a model Hamiltonian to obtain precise parameters for the states. The ion rotational temperature was determined to be 461±11, and the translational temperature was estimated to be near 1200 K.

Multiphoton ionization of vinylchloride, trifluoroethylene, and benzene at 193 nm
View Description Hide DescriptionThe multiphoton ionization–fragmentation pattern of vinylchloride, trifluoroethylene and benzene has been measured at 193 nm using time‐of‐flight mass spectrometry and gas chromatographicanalysis. Relative cross sections for multiphoton ionization and the laser power dependence have been determined for the three species at 193 nm at low laser fluence (<10 mJ/cm^{2}) and relative cross sections for multiphoton ionization of benzene have been obtained at 193 and 248 nm. The dominant fragmentation and ionization channels in the two substituted ethylenes are discussed.

A comparison of local and global single Gaussian approximations to time dynamics: One‐dimensional systems
View Description Hide DescriptionThe detailed calculation of the dynamics of a chemical system is usually not considered due to the size and cost of the computation. It is thus useful to examine various approximation methods. Such methods first need to be tried out on simple systems, like one‐dimensional motion. Here two approaches to approximating the solutions of the Schrödinger and von Neumann equations by single time‐dependent Gaussians are explored and contrasted, explicitly for one‐dimensional barrier penetration. The first approach, in which no tunneling occurs, is local in nature and characterized by an expansion of the equations of motion to second order about the average position of the Gaussian wave packet or about the average position and momentum of the Gaussian Wigner function. This approach was first introduced by Heller [E. J. Heller, J. Chem. Phys. 6 2, 1544 (1975)]. Here both Heller’s approach and a more general truncation method are considered. Indeed tunneling can be incorporated if second‐order terms in the quantal von Neumann equation are included. However, the resulting dynamics is unstable for kinetic energies where the exact wavepacket (and Wigner function) splits into nonnegligible parts that represent reflection and transmission. In contrast, the second approach is a global method which is obtained by applying appropriate closure approximations to the equations of motion for the first‐ and second‐order position and momentum expectation values. This method allows tunneling and is stable at all kinetic energies. It is also possible to approximate the global equations in such a way that the local equations are obtained.

Gas phase kinetics of the reactions of NaO with H_{2}, D_{2}, H_{2}O, and D_{2}O
View Description Hide DescriptionThe gas phase reactions of the NaO radical with H_{2}, D_{2}, H_{2}O, and D_{2}O were studied in a flow tube reactor at room temperature. The reaction of NaO with H_{2} has two exothermic product channels, NaOH+H and Na+H_{2}O. Both channels were observed and the Na formation channel produces some Na in the 3^{2} P state. The rate constants for the abstraction channel for H_{2} and D_{2} reactants are (2.6±1.0)×10^{−} ^{1} ^{1} and (1.1×0.4)×10^{−} ^{1} ^{1} cm^{3} molecule^{−} ^{1} s^{−} ^{1} at 296±2 K. The reaction of NaO with H_{2}O was shown to be second order and the products are assumed to be NaOH and OH. The rate constants for H_{2}O and D_{2}O reactants are (2.2±0.4)×10^{−} ^{1} ^{0} and (1.2±0.2)×10^{−} ^{1} ^{0} cm^{3} molecule^{−} ^{1} s^{−} ^{1} at 298±1 K. The measured NaO+H_{2}O rate constant is compared to the predicted collision rate constant from a model based on the large attractive dipole–dipole force between NaO and H_{2}O. The role of these reactions in mesosphericNa chemistry is briefly discussed.

Theoretical studies of electron transfer in metal dimers: XY^{+}→X^{+}Y, where X, Y=Be, Mg, Ca, Zn, Cd
View Description Hide DescriptionThe electronic matrix element responsible for electron exchange in a series of metal dimers was calculated using a b i n i t i owave functions. The distance dependence is approximately exponential for a large range of internuclear separations. A localized description, where the two nonorthogonal structures characterizing the electron localized at the left and right sites are each obtained self‐consistently, is found to provide the best description of the electron exchange process. We find that Gaussian basis sets are capable of predicting the expected exponential decay of the electronic interactions even at quite large internuclear distances.

Determination of cluster binding energy from evaporative lifetime and average kinetic energy release: Application to (CO_{2})^{+} _{ n } and Ar^{+} _{ n } clusters
View Description Hide DescriptionThe binding of a molecule within a cluster may be determined within the context of a simple statistical model from the measurements of cluster evaporative lifetime and average kinetic energy release. If the final product density of states is approximated by a Kassel distribution, the expression for lifetime can be inverted to give the binding energy. Stringent demands are placed upon the accurate determination of kinetic energy release, but only moderate demands are placed upon the accuracy of the lifetime or model parameters. The method is illustrated with positively chargedcarbon dioxide and argon clusters. Binding energy determinations agree with previous results that CO_{2} molecules are bound within the cluster with an average of 156±26 meV. The binding energy for Ar atoms decreases, from 150 meV within clusters of size 6, to 60 meV for clusters of size 22. The ±16% average error for the argon clusters represents mostly the uncertainty in the kinetic energy measurements.

A dynamical ‘‘white spot’’ on the potential energy surface: The close interaction region of the collinear hydrogen transfer reaction F+DBr→FD+Br
View Description Hide DescriptionThe strong FDBr interaction region located in the corner of the LEPS modelpotential energy surface is diagnosed as dynamical white spot. Although energetically accessible, it is dynamically unapproachable by the vast majority of reactive classical trajectories, quantum waves represented by S‐matrix propagation, or resonance wave functions. The quantum and classical derivations and explanations yield important consequences of the dynamical white spot: In particular, quantum chemical evaluations of this region of the potential energy surface are rendered unneccessary because its dynamical effects on bi‐ and unimolecular reactivity are insignificant.