Index of content:
Volume 105, Issue 21, 01 December 1996

O_{2} photoabsorption in the 40 950–41 300 cm^{−1} region: New Herzberg bands, new absorption lines, and improved spectroscopic data
View Description Hide DescriptionThe technique of cavity ring‐down (CRD) spectroscopy is particularly useful for measuring absorptions of very weak optical transitions. We have in this manner investigated the 40 950–41 300 cm^{−1} region in O_{2}, where only absorption in the O_{2}(A ^{3}Σ^{+} _{ u }–X ^{3}Σ^{−} _{ g }) 11‐0 band had been previously identified. Five new bands have been discovered in this range—the A′ ^{3}Δ_{ u }–X ^{3}Σ^{−} _{ g } 12‐0 and 13‐0 bands, the c ^{1}Σ^{−} _{ u }–X ^{3}Σ^{−} _{ g } 17‐0 and 18‐0 bands, and the A ^{3}Σ^{+} _{ u }−X ^{3}Σ^{−} _{ g } 12‐0 band. The origins of the F _{1} and F _{2} components of the latter lie only 7 cm^{−1} below the lowest dissociation limit, and 15 lines have been identified. No F _{3} levels were observed; apparently all are above the dissociation limit. The high instrumental sensitivity of the CRD technique has allowed observation of weak lines of the A–X 11‐0 band, and 12 of the 13 branches have been identified and their intensities measured. A very low upper limit has been set on the intensity of the thirteenth branch, Q _{13}. We find 107 unidentified lines in the region, the stronger ones (19) lying in the vicinity of lines of the A–X 11‐0 band. The weaker ones (88) are spread throughout the spectral region, up to and even beyond the O_{2}dissociation limit, and probably have their origin in transitions to very weakly bound O_{2} states, which may have atmospheric significance. These weaker lines have intensities that are typically 1%–5% of the strong A–X 11‐0 band lines.

The influence of high‐frequency modes on ultrashort pulse absorption initiated processes
View Description Hide DescriptionMany molecules studied by ultrashort pulse spectroscopic techniques contain modes which have both short and long periods relative to the time scale of excitation. The influence of the short period (high frequency) modes on the ultrashort pulse absorption process is usually neglected. In this paper we examine the effect of the high frequency modes on the energetics of ultrashort pulse absorption, and construct a method for calculating observables for polyatomic systems containing both high and low frequency modes. We compare our results with experimental data for the iodobenzene system.

Determination of quadrupolar and chemical shielding tensors using solid‐state two‐dimensional NMR spectroscopy
View Description Hide DescriptionThe quadrupolar and chemical shifttensors, as well as the relative orientation of the two principle axis systems, are accurately determined using a two‐dimensional nuclear magnetic resonance technique. Good agreement between experimental and simulated two‐dimensional spectra is obtained for a series of rubidium and sodium compounds at multiple magnetic field strengths. Extension of this technique to correlate the quadrupolar and dipolar interactions, as well as the incorporation of a purely isotropic dimension resulting in a three‐dimensional experiment is also discussed.

Probing three‐body intermolecular forces: Near‐infrared spectroscopy of Ar_{2}HF and Ar_{2}DF van der Waals modes
View Description Hide DescriptionFour intermolecularvibrational states of the weakly bound complexes Ar_{2}HF and Ar_{2}DF have been studied via high‐resolution infrared spectroscopy. The vibrations are accessed as combination bands built on the v=1 HF or DF intramolecular stretch. These van der Waals vibrational states correlate adiabatically with j=1 motion of a hindered HF/DF rotor, corresponding to librational motion either in, or out of, the molecular plane. The vibrational origins of the Ar_{2}HF in‐plane and out‐of‐plane bends are 4008.9665(24) and 4035.174 41(86) cm^{−1}, respectively, which are 62.374 and 88.582 cm^{−1} above the origin of the intermolecular ground state in the v _{HF}=1 manifold. For Ar_{2}DF, the in‐plane and out‐of‐plane origins are 2939.836 63(4) and 2967.101 29(5) cm^{−1}, respectively, which correspond to intermolecular bending frequencies in the v _{DF}=1 manifold of 44.852 and 72.117 cm^{−1}. Two‐dimensional angular calculations are presented which solve for the hindered rotor HF/DF eigenfunctions and eigenvalues on a pairwise additive potential constructed using a rigid Ar_{2} framework; the predicted bending frequencies reproduce the correct energy ordering of the vibrational levels, but are systematically greater than experimentally observed. Rigorous full five‐dimensional theoretical calculations of the intermolecular vibrational frequencies by Ernesti and Hutson [Phys. Rev. A 51 239 (1995)] on the full pairwise additive surface are found to be as much as 11% higher than the experimental values, indicating the presence of three‐body repulsive contributions to the true angular potential. Inclusion of conventional three‐body dispersion and induction terms can only account for a minority (≊1/3) of the observed discrepancies. The majority (≊2/3) of the vibrational shifts can be attributed to three‐body ‘‘exchange’’ effects, i.e., the strongly anisotropicinteraction of the HF/DF dipole with an exchange quadrupole formed by Ar–Ar. Inclusion of all three nonadditive terms (dispersion, induction, and exchange) improves the agreement with experiment by up to an order of magnitude.

Multiphoton absorption by metal–metal long distance charge‐transfer complexes in polar solvents
View Description Hide DescriptionA theory of multiphoton absorption of mixed valence compounds in polar solvents is developed. When these systems interact with a strong laser field, there exist various absorption ‘‘channels’’ which correspond to one, two, three,..., absorbed photon quanta. The probability of each channel to be switched on or off is dependent on the laser intensity, which results in strong intensity dependence of the absorptionspectrum. Bands can be eliminated from the spectrum simply by changing the laser intensity. The physical picture of channels is justified by a derivation based on a kinetic master equation for the flow of electronic population. Calculations are carried out for the case where the polar environment can be modeled by a single high frequency (‘‘quantum’’) vibrational mode plus a classical bath represented by a collection of low frequency harmonic oscillators. The spectrum is found to be only weakly sensitive to the quantum vibrational mode. Suggestions for experimental verification are made.

Isomers of SO_{2}: Infrared absorption of SOO in solid argon
View Description Hide DescriptionSulfur dioxide (OSO) isolated in solid argon at 13 K was irradiated with light at 193 nm from an ArF excimer laser. Weak absorption lines at 1006.1, 1004.7, and 739.9 cm^{−1} observed after photolysis are assigned to sulfur superoxide (SOO); the doublet near ∼1005 cm^{−1} is due to matrix site splitting. The assignments are based on results from ^{18}O‐isotopic experiments. Calculations using the B‐P86 and the B3‐LYP density‐functional methods were made for three isomers of OSO; energies, structures, vibrational wave numbers, and infrared intensities were predicted for each species. Although observed line positions are nearly identical to those predicted with theory for cyclic‐SO_{2}, experimental relative IR intensities and ^{18}O‐isotopic shifts agree with those predicted for SOO. The mechanism of formation of SOO in a matrix cage is discussed.

Through bond and through space interactions in oligo–alkoxythiophenes: A spectroscopic study
View Description Hide DescriptionThe aim of this work is to understand the molecular structuralproperties which may justify the great observed stability of doped alkoxy polythiophenes. Infrared frequency and intensity spectroscopy and Raman spectroscopy are used as probes. Suitably synthesized oligomers with increasing chain length and preassigned chemical structure have been studied together with several model molecules. The existence of through‐space and through‐bond S‐ ‐ ‐O interactions has been revealed. The extent of the topologically dependent perturbation induced by the charge injection by the oxygen atoms is detected and measured. The inter‐ and intraring delocalizations have been monitored by the observation of the softening of the strongly Raman active ‘‘amplitude mode’’ and by the red shift of the electronic spectra. The conformation of the side chains is trans planar in the solid state and collapses in a liquidlike premelting state before the melting of the crystal.

Neutron, nuclear magnetic resonance, and dielectric study of ion motion in pyridinium hexafluorophosphate
View Description Hide DescriptionExtensive investigation of polycrystalline pyridinium hexafluorophosphate and its perdeuterated analog was performed by neutron scattering,NMR, and dielectric methods in a wide temperature range. The results of different spectroscopic methods enabled us to propose a general model of both ion reorientations and to determine their activation parameters. The aging effects observed slightly change the hindering barriers and significantly affect the phase transition temperature and the pyridinium cation dynamics.

Infrared laser‐induced post‐pulse dissociation of CF_{2}HCl and CF_{2}Cl_{2} under high pressure and fluence conditions
View Description Hide DescriptionThe unimolecular decomposition of the halogenated methanes CF_{2}HCl (one main channel) and CF_{2}Cl_{2} (two main channels) in the focused beam of a pulsed CO_{2} laser under high pressure and fluence conditions (p=100 Pa–2 kPa; Φ=5–200 J/cm^{2}) was studied by a special laser‐induced fluorescence(LIF) technique, permitting spatially resolved fragment concentration measurements in the focal region. Considerable amounts of CF_{2} product were formed after the end of the laser pulse. In the one‐channel‐dissociation case of CF_{2}HCl LIFmeasurements of the CF_{2} yield distribution Y(z,r) can be related to the spatial distribution of the average absorbed energy in the parent molecules. Only part of the absorbed energy is consumed by multiphoton dissociation, while most reactant molecules remain highly vibrationally excited in the focus volume far into the double cone. Using the long‐lived CF_{2} also as a probe for measuring the rotational, translational, and vibrational temperatures, the redistribution of the internal energy in the molecules and fragments involved is monitored. The post‐pulse production of CF_{2} is shown to be caused by the energy pooling v–v transfer mechanism, while contributions of pyrolytic and gas dynamic processes are of little importance.

The kinetics of the bimolecular A+B→0 reaction in condensed matter: Effects of non‐equilibrium charge screening
View Description Hide DescriptionThe kinetics of the bimolecular A+B→0 reaction between charged reactants is studied in two dimensions, i.e., on a surface. The theory is based on the Kirkwood superposition approximation for three‐particle densities and the self‐consistent treatment of the electrostatic interactions defined by the non‐uniform spatial distribution of similar and dissimilar reactants. Special attention is paid to pattern formation and many‐particle effects arising from reaction‐induced formation of loose domains containing similar reactants only. It is shown that the critical exponent α characterizing the algebraic concentration decay law, n(t)∝t ^{−α}, differs strongly between symmetric (D _{ A }=D _{ B }) and asymmetric (D _{ A }=0) reactant mobilities. This effect is abnormal from the point of view of standard chemical kinetics. It arises directly from the specific spatial distribution in the system as in ‘‘raisins A in a dough B.’’ At long reaction times the asymptotics of the interaction potentials is of non‐equilibrium type at large relative distances. The accumulation kinetics in the presence of a permanent source is studied. Results of the microscopic formalism are compared with a previous mesoscopic theory.

Theory of continuum mediated two‐photon ionization, with applications to CH_{3}I
View Description Hide DescriptionTheory of two‐photon ionization via intermediate dissociative states is developed. The theory is applied to the one‐color two‐photon ionization of CH_{3}I for which the ^{3}Q_{0} and ^{1}Q_{1} dissociative states serve as intermediate resonances. Both CW and transient ionization spectra are calculated. The computed methyl iodide CW ionization line intensities are in profound disagreement with line intensities derived from two‐photon zero kinetic energy electrons (ZEKE) experiments. The discrepancy may be viewed as evidence that ZEKE line‐intensities are not simply proportional to the population of the ionic‐core states. This extra dependence on the ionic‐core states, most likely due to the existence of external ions, can be accounted for by comparing our calculations to the observed ZEKE line intensities. The dynamics revealed by ultrashort pulsed two‐photon ionization is also studied. In methyl iodide, we find that pulses capable of revealing ‘‘real‐time’’ dissociative dynamics must be considerably shorter than 50 fs.

Phase space bottlenecks and rates of no‐barrier fragmentation reactions into polyatomic molecules
View Description Hide DescriptionAn expression of the microcanonical unimolecular rate for an arbitrary transition state surface in phase space is derived and applied to fragmentation reactions into polyatomic molecules without potential barrier. The transition state which has a ‘‘point of no return’’ property in unimolecular dissociation is defined as an interfragment bottleneck in phase space. The fragmentation rate based on the interfragment bottleneck in phase space is compared with the rate based on the transition state defined in configuration space. The rate derived from the flux which crosses the interfragment bottleneck by intermode energy transfer is found to be smaller than the rate derived from the Rice–Ramsperger–Kassel–Marcus or phase space theory by an approximate factor (s+r/2)W̃/E, where E is the total energy and W̃ is the magnitude of the coupling energy between the reaction coordinate and the s‐dimensional vibrational and r‐dimensional rotational modes of the fragments. Phase space theory grossly overestimates the rate of fragmentation of small molecules with small W̃ in the high energy range, because the theory does not take into account the slow process of intramolecular energy redistribution.

Energy redistribution in cluster–surface collision: I_{2} ^{−} (CO_{2})_{ n } onto silicon surface
View Description Hide DescriptionFragmentation of I_{2} ^{−}(CO_{2})_{ n } (n=1−30) by its collision on a siliconsurface was investigated by measuring the fragment anions and their translational energy parallel to the surface (surface–parallel translational energy) in a tandem time‐of‐flight mass spectrometer equipped with a collision chamber evacuated down to ∼10^{−8} Pa. At the collision energy (per I_{2} ^{−}) of 50 eV and the incident angle of 26° with respect to the surface normal, the distributions of the surface–parallel translational energies of the fragment anions from a given parent cluster anion were found to obey the one‐dimensional Maxwell–Boltzmann distribution with the same translational temperature, T _{ s } ^{∥} The results show that the cluster anion and its neighboring surface atoms reach quasiequilibrium before the fragment anions leave the surface. A general increasing trend of T _{ s } ^{∥} (6000–12 000 K) with n is interpreted as an increasing extent of cluster–impact heating with n, while the reduction of T _{ s } ^{∥} in the 13≤n≤∼19 range is attributable to efficient transmission of the I^{−} and I_{2} ^{−} translational energies to the CO_{2}solvent cage. The effective volume and pressure of I_{2} ^{−}(CO_{2})_{ n } colliding on the surface were estimated; at n=10, the volume and the pressure were 100 nm^{3} and 10 MPa, respectively.

Feasibility of using photophoresis to create a concentration gradient of solvated molecules
View Description Hide DescriptionThe objective of this work is to estimate the feasibility of creating a measurable concentration gradient of molecules in a solvent by a laser driven photophoresis process. The molecules are dissolved in a suitable solvent that is not significantly absorbing at the applied radiation frequency. The molecule is anisotropic, or ideally propeller shaped, and has an appropriate transition dipole capable of driving rovibrational motion. The polarization of the laser can be taken as rotating slowly with the molecules. The resulting torque driven hindered rotation imparts a forward thrust, and thus creates a net flow of the molecules which can set up a concentration gradient in a finite cell. The relevant physical parameters are estimated with the aid of instantaneous normal mode and molecular dynamics simulation methods on a prototype system, and the results indicate that a detectable concentration gradient may be established. A practical issue is to treat heating and resultant mixing or turbulence in the medium. Laboratory experiments are needed to further explore the photophoresis process.

Ab initio study of the He(^{1} S)+CH(X ^{2}Π) interaction
View Description Hide DescriptionPotential energy surfaces for the ^{2} A′ and ^{2} A″ states of the He(^{1} S)–CH(X ^{2}Π) complex were calculated using supermolecular unrestricted Mo/ller–Plesset perturbation theory and analyzed via the relevant perturbation theory of intermolecular forces. It has been found that the two states are distinctly different. The potential energy surface (PES) of the A″ state has only a single and relatively deep minimum of D _{ e }≊335 μE _{h} for the T‐shaped geometry, at R=5.0 a _{0} and Θ=100°. The position of this minimum is determined by the exchange repulsion which is substantially reduced at this geometry. The minimum is unusually deep for a complex of He, and it can be viewed as an example of an incipient chemical bond. In contrast, the A′ state’s PES represents a typical van der Waals interaction which is characterized by two similarly deep minima. The shape and location of these minima are determined primarily by the anisotropy of the dispersion component. The first minimum occurs for the collinear He–C–H arrangement, at R≊7.5 a _{0}, and Θ=0°, and is 55 μE _{h} deep. The second minimum has a troughlike form which joins the region between R=7.5 a _{0}, Θ=140° and R=8.0 a _{0}, Θ=180°. The lowest point is approximately 54 μE _{h} deep and occurs at R=7.5 a _{0} and Θ=140°.

Numerical quantum propagation with time‐dependent Hamiltonian
View Description Hide DescriptionA numerical method was proposed to propagate the quantum system with a time‐dependent Hamiltonian. The propagator is composed of the time‐independent‐Hamiltonian‐type multisplit forms, which becomes possible by realizing that the evolution operator has a close link with the time‐shift operator. The present algorithm can achieve a high order of accuracy through the recurrence of multisplit forms with predetermined coefficients, and it has the advantage that only the Hamiltonian itself at different time instants is involved, while other currently available schemes need to perform either a time integration or a time differential of the Hamiltonian. This essential difference has made our proposal highly competitive. A one‐dimensional model system is considered to verify the accuracy and efficiency of the numerical scheme.

A density functional study of small copper clusters: Cu_{ n } (n⩽5)
View Description Hide DescriptionDensity functional calculations have been performed for small copper clusters, Cu_{ n } (n≤5), using the linear combination of Gaussian‐type orbitals density functional theory (LCGTO‐DFT) approach. The calculations were of the all‐electron type and local and nonlocal functionals were used. For each case, of both neutral and charged systems, several isomers have been considered in order to determine the lowest energy structures. The Jahn–Teller effect in Cu_{3} and Cu_{4} has been examined in detail. Bond lengths, equilibrium geometries, harmonic frequencies, adiabatic and vertical ionization potentials, adiabatic electron affinities, and binding energies are in reasonable agreement with experimental data, as well as with other theoretical results.

Hückel‐type semiempirical implementation of a variational method for determining electronic band gaps
View Description Hide DescriptionA previously derived [J. Chem. Phys. 103, 7645 (1995)] general variational principle, for determining the hardness (or band gap) of electronic systems, is applied to π‐electron systems by a straightforward simple parameterization of the hardness kernel. For conjugated polyenes and annulenes, the hardness(band gaps) are essentially the same as (although no identical with) those predicted by the Hückel method. In contrast with the Hückel method, one need not assume that the total electronic energy is a sum of one‐electron energies. Comparisons are made with Nalewajski’s Charge Sensitivity Analysis.

Ab initio based effective Hamiltonians for long‐range electron transfer: Hartree–Fock analysis
View Description Hide DescriptionAn ab initioelectronic structure method is developed to describe electron transfer in large systems. The method is based on a molecular fragment effective Hamiltonian approach. The strategy pieces together results of ab initio quantum chemistry calculations on overlapping molecular segments in order to build an effective Hamiltonian that describes the long‐range electronic interactions. This is accomplished by constructing fragment effective Hamiltonians that properly describe the electronic propagation characteristics of each fragment (computed at the ab initio Hartree–Fock level in an appropriate basis set). The fragment effective Hamiltonian is projected onto the valence orbital space of each fragment, and a relatively well‐localized set of effective interactions is obtained. Combining these projected fragment Hamiltonians allows the construction of a valence effective Hamiltonian for the entire system. We find that the fragment Hamiltonian matrices constructed in this way are transferable between donor–acceptor systems with homologous electron‐transfer bridges. The overall strategy of fragmentation and construction of valence effective Hamiltonians could enable ab initio quality computations of long‐range tunneling interactions in macromolecules. We demonstrate the use of the method in a series of electron‐transfer model systems of modest size.

Scattering of gases from aligned liquid crystals: Collision‐induced loss of order at the gas–liquid interface
View Description Hide DescriptionAn atomic beam of helium is scattered from the surface of an aligned nematic liquid crystalfilm (4′‐pentyl‐4‐cyanbo biphenyl, 5CB). Collisions of atoms with the surface initiate a transient loss of order in the film. The magnitude of this change depends on the momentum direction of the atomic beam with respect to the liquid crystal alignment director. The effect is significantly more pronounced when the incoming beam is perpendicular to the director than when it is parallel. This anisotropy may be due to a more efficient alignment‐changing torque exerted on the 5CB molecules by perpendicular collisions with He.