Volume 106, Issue 3, 15 January 1997
Index of content:

A study of the SO molecule with photoelectron spectroscopy using synchrotron radiation
View Description Hide DescriptionThe SO molecule has been studied by photoelectron spectroscopy using vacuum ultraviolet radiation from a synchrotron as the photon source. Both constant ionic state (CIS) and photoelectron spectra have been recorded. Resonances which appear in the CIS spectra recorded for selected vibrational levels of SO^{+} X ^{2}Π in the photonenergy region hν=11.5–15.0 eV have been assigned to excitations to Rydberg states which converge to the SO^{+} a ^{4}Π and A ^{2}Π states and autoionize to SO^{+} X ^{2}Π. Also, resonances which appear in the CIS spectra recorded in the photonenergy region 15.0–16.4 eV for selected vibrational levels of SO^{+} b ^{4}Σ^{−} have been assigned to excitations to Rydberg states which converge to SO^{+} B ^{2}Σ^{−} and autoionize to SO^{+} b ^{4}Σ^{−}. Photoelectron spectra recorded at selected resonantphoton wavelengths show that autoionization can dramatically alter the intensities of the vibrational components in a photoelectron band and may allow extra vibrational structure to be observed. The additional information obtained from using this method to study shortlived molecules is discussed.

Internal coordinate Hamiltonian model for Fermi resonances and local modes in methane
View Description Hide DescriptionA vibrational model which is based on a Hamiltonian expressed in terms of curvilinear internal coordinates is applied to the overtone spectrum of methane, CH_{4}. Symmetrized internal coordinates and their conjugate momenta are used as the bending variables. The stretching part of the Hamiltonian is expressed in an unsymmetrized form. Both the kinetic operator and the potential energy function are expanded as Taylor series around the equilibrium configuration. Symmetrized local mode basis functions for the stretches and symmetrized two and threedimensional harmonic oscillator basis functions in the Cartesian representations for bending degrees of freedom are used. Only resonance couplings are taken into account. Apart from some standard diagonal contributions harmonic oscillator matrix elements have been employed. This results in a simple block diagonal Hamiltonian model. The nonlinear least squares method is used to optimize model parameters for ^{12}CH_{4}. Observed vibrational term values up to 6050 cm^{−1} are included as data. Potential energy parameters obtained from the Hamiltonian parameters agree well with a previously published anharmonic force field calculation. A unitary transformation between internal coordinate and normal coordinate representations is found to provide simple interpretations for the standard normal mode theory based spectroscopic parameters.

Non‐coincidence splitting of the 1505 cm^{−1} adenine base vibration is due to coupling to water via hydrogen bonding
View Description Hide DescriptionWe have observed a noncoincidence splitting of 1–2 cm^{−1} for the 1505 cm^{−1} adenine base vibration in poly(rA), poly(dA), 5rAMP, and 5dAMP. In double stranded polynucleotides, the noncoincidence splitting is smaller than 0.3 cm^{−1}. Noncoincidence splitting is usually associated with a transition dipole–transition dipole coupling between identical vibrational modes on different molecules of the same species, but any angular dependent intermolecular coupling mechanism may cause this effect. It is argued that resonant vibrational coupling of 1505 cm^{−1} vibrations on different adenine bases cannot be the source of the noncoincidence splitting. As the 1505 cm^{−1} base vibration does not have a large infrared absorption, it cannot be caused by transition dipole–transition dipole coupling. We propose that a coupling via hydrogen bonding of the 1505 cm^{−1} adenine base vibration to water vibrations yields the angular dependent intermolecular coupling potential.

The C_{6}H_{6}–(H_{2}O)_{2} complex: Theoretical predictions of the structure, energetics, and tunneling dynamics
View Description Hide DescriptionA detailed theoretical study of the C_{6}H_{6}–(H_{2}O)_{2} complex is presented. We characterize the structure and energy by means of various potentials and correlated ab initio calculations. The potential surface is extremely flat but the structures obtained with the empirical potentials and ab initio optimizations agree fairly well. Transition states and corresponding reaction paths are calculated for four possible degenerate rearrangements. The splittings for these mechanisms are calculated from quantum simulations with the diffusionMonte Carlo (DMC) approach. We predict that two splittings should be observable in the spectrum. The DMC calculations also allow prediction of vibrationally averaged structures, bond energies, and rotational constants.

A highresolution vacuum ultraviolet photoionization, photoelectron, and pulsed field ionization study of CS_{2} near the (X ^{2}Π_{3/2,1/2}) thresholds
View Description Hide DescriptionHighresolution photoionization efficiency (PIE) and pulsed field ionization photoelectron (PFIPE) spectra for CS_{2} have been measured using coherent vacuum ultraviolet (VUV) laser radiation in the energy range of 81 050–82 100 cm^{−1}. The PIE and threshold photoelectron (TPE) spectra for CS_{2} in the energy range of 80 850–82 750 cm^{−1} have also been obtained using synchrotron radiation for comparison with results of the VUV laser study. The analysis of the PIE spectra reveals three Rydberg series converging to the excited CS_{2} ^{+}(^{2}Π_{1/2}) spin–orbit state. These series, with quantum defects of 1.430, 1.616, and 0.053, are associated with the [^{2}Π_{1/2}]npσ_{u} , [^{2}Π_{1/2}]npπ_{u} , and [^{2}Π_{1/2}]nf_{u} configurations, respectively. The Stark shift effect on the ionization threshold of CS_{2} has been examined as a function of dc electric fields(F) in the range of 0.65–1071 V/cm. The observed F dependence of the Stark shift for the ionization onset of CS_{2} is consistent with the prediction by the classical adiabatic field ionization formula. The extrapolation of the ionization onset to zero F yields accurate values for IE[CS_{2} ^{+}(X̃ ^{2}Π_{3/2})]. This study shows that in order to determine accurate IEs and to probe autoionizing structures for molecular species by PIE measurements, it is necessary to minimize the electric field used for ion extraction. The assignment of Renner–Teller structures resolved in the VUV PFIPE spectrum is guided by the recent nonresonant twophoton (N2P) PFIPE and theoretical studies. The analysis of the PFIPE spectrum also yields accurate values for IE[CS_{2} ^{+}(X̃ ^{2}Π_{3/2,1/2})]. Taking average of the IE values determined by VUVPFIPE, N2PPFIPE, and Stark field extrapolation methods, we obtain a value of 81 285.7±2.8 cm^{−1} for IE[CS_{2} ^{+}(X̃ ^{2}Π_{3/2})]. For IE[CS_{2} ^{+}(^{2}Π_{1/2})], we recommend a value of 81 727.1±0.5 cm^{−1} determined by the Rydberg series analysis. A theoretical simulation of the ^{2}Π_{3/2}(0_{0} ^{0}) and ^{2}Π_{1/2}(0_{0} ^{0}) VUVPFIPE band profiles reproduces the observed branching ratio of 1.9±0.3 for CS_{2} ^{+}(X̃ ^{2}Π_{3/2})/CS_{2} ^{+}(^{2}Π_{1/2}). The relative intensities of vibronic structures observed in the VUV PFIPE and TPE spectra are in agreement. Evidence is found, indicating that the strongly (Stark field induced) autoionizing Rydberg state,17pσ_{u} , which is ≈10 cm^{−1} below the IE of CS_{2}, has a minor contribution to the observed profile for the X̃ ^{2}Π_{3/2}(0_{0} ^{0}) PFIPE band.

Structure and vibrations of catechol(methanol)_{1} in the S _{0} and S _{1} state
View Description Hide DescriptionThe structure as well as the inter and intramolecular vibrations of the catechol (methanol)_{1} cluster are investigated both experimentally and theoretically. By using resonanttwophotonionization (R2PI) and dispersed fluorescence (DF) spectroscopy, the vibrational transitions of the S _{0} and S _{1} state are obtained. In order to find the corresponding vibrations of the S _{0} and S _{1} state, DF spectra are recorded by pumping the electronic origin and the most intense vibrations of the R2PI spectrum. According to ab initio calculations performed at the Hartree–Fock level [631G(d,p) basis], including MP2, BSSE, and ZPE (zero point energy) corrections, the most stable structure turns out to be translinear. The calculated vibrational frequencies are in close agreement to the experimental values. Since the catechol(methanol)_{1} cluster has no symmetry, all intermolecular fundamental vibrations of the S _{0} and S _{1} state spectra can be assigned. A large number of combination bands and overtone vibrations are observed in the low frequency region (<200 cm^{−1}) of the S _{0} and S _{1} state spectra. From the DF spectrum obtained by pumping the low frequency ρ_{1} vibration, it can be concluded that the catechol(methanol)_{1} cluster undergoes a geometry change in the S _{1} state. The OH…O bond turns out to be nonplanar with respect to the aromatic plane. This confirms the results reported for catechol and the catechol(H_{2}O)_{1} cluster.

Dipolar couplings and internuclear distances by doublequantum nuclear magnetic resonance spectroscopy of solids
View Description Hide DescriptionThe analysis of highresolution doublequantum nuclear magnetic resonance spinning sidebands for measuring dipolar couplings and internuclear distances in dipolar solids is described. For this purpose, the response of a dipolarcoupled spin system in a rigid solid is investigated with respect to highresolution multiplequantum experiments using rotorfrequency synchronized pulse sequences. For isolated, magnetically equivalent spin1/2 pairs, exact expressions for the doublequantum spinningsideband patterns are derived. These patterns show spinning sidebands only at odd numbers of the rotor frequency. For longer excitation/reconversion cycles, the doublequantum spinning sidebands are sensitive to changes in the internuclear distances. Using this technique, the dipolar couplings for spin pairs in doublelabeled polyethylene were measured in crystalline and amorphous domains, respectively. In the former the dipolar coupling reflects the carbon–carbon distance, in the latter it is reduced due to molecular dynamics. The possibility to use multiplequantum pulse sequences as a dipolar filter for the rigid domains is also shown.

High Rydberg spectroscopy of benzene: Dynamics, ionization energy and rotational constants of the cation
View Description Hide DescriptionIn a high resolution uvuv double resonance experiment with two Fouriertransform limited nanosecond laser pulses high Rydberg states up to were selectively excited and detected by ionization in a pulsed electric field. We were able to identify 64 Rydberg series in benzene C_{6}D_{6} and 20 series in benzene C_{6}H_{6} converging to different rotational levels of the ionic molecular core. Their assignment by application of a crosscorrelation analysis yields accurate rotational constants and a precise value for the lowest rotationless ionization energy of 74556.57(5) cm^{−1} (C_{6}H_{6}) and 74583.51(5) cm^{−1} (C_{6}H_{6}). All observed series show surprisingly low quantum defects below 0.01 and no strong local perturbations at their crossings points. This points to a weak intramolecular coupling of the series in the absence of an external electric field. In a second series of experiments the influence of external fields on the intensity, the position and dynamics of single Rydberg peaks is investigated. It is shown that the peak shape and peak position of a single Rydberg peak depends strongly on the applied electric field. Within the envelope of one Rydberg peak different decay times of the pulsed field ionization signal can be found.

Hydrogen bonding in acid–base complexes: The transhydroquinoneNH_{3} complex in its S _{0} and S _{1} electronic states
View Description Hide DescriptionWe deduce information about the geometry of the hydrogen bond between hydroquinone and ammonia from an analysis of the fully resolved fluorescence excitation spectrum of the 1:1 complex in the gas phase. The complex is planar in both electronic states, with NH_{3}forming a nearly linear hydrogen bond to one of the hydroxy hydrogen atoms of hydroquinone. The O–H…N heavy atom separation is R=2.85 Å and the barrier to internal rotation of the NH_{3} group about its C_{3} axis is V_{3} =35.5 cm^{−1} in the S_{0} state. Excitation of the complex to its S_{1} state decreases the heavy atom separation to R=2.77 Å and increases the torsional barrier to V_{3} =58.8 cm^{−1}. These changes are a direct consequence of the increased acidity of hydroquinone in its S_{1} state.

Mixed semiclassical–classical approaches to the dynamics of complex molecular systems
View Description Hide DescriptionStarting with the semiclassical initial value representation (IVR) for the description of a composite molecular system consisting of a “system’’ and a “bath,’’ we show a systematic procedure whereby one can retain the semiclassical IVR description of the “system’’—thus including quantum interference and tunnelingeffects for these degrees of freedom—while neglecting these effects in the “bath’’ degrees of freedom, thus reducing them to a classical description. The zeroth order version of this mixed semiclassical–classical model is seen to be the semiclassical equivalent of the Ehrenfest model (closely related to the TDSCF approximation), but higher order versions are also developed that go beyond this level of approximation. Two levels of this theory are explicitly worked out. Numerical tests on simple but illustrative problems are used to discuss the merit of these approaches.

A stateselected study of the ion–molecule reactions O^{+} (^{2}D,^{2}P)+H_{2}O
View Description Hide DescriptionStateselected absolute cross sections for H_{2}O^{+} and OH^{+} formed by the O^{+} (^{2}D,^{2}P)+H_{2}O reactions have been measured in the centerofmass collision energy (E _{c.m.}) range of ≈0.10–30 eV. The charge transfer cross sections for O^{+} (^{2}D)+H_{2}O are significantly higher than those for O^{+} (^{4}S)+H_{2}O. This observation is attributed to the increased number of accessible exothermic product channels for O^{+} (^{2}D)+H_{2}O. While the H_{2}O^{+} cross sections for O^{+} (^{2}P)+H_{2}O are comparable to those from O^{+} (^{4}S)+H_{2}O at E _{c.m.}⩾1 eV, the H_{2}O^{+} cross sections for O^{+} (^{2}P)+H_{2}O at E _{c.m.}<1 eV are substantially lower than those for O^{+} (^{4}S)+H_{2}O. The lower H_{2}O^{+} cross sections observed for O^{+} (^{2}P)+H_{2}O are rationalized as due to further dissociation of excited charge transfer H_{2}O^{+} ions and/or the efficient competition of the OH^{+}+OH product channel. The cross sections for OH^{+} from O^{+} (^{2}D,^{2}P)+H_{2}O are significantly greater than those from O^{+} (^{4}S)+H_{2}O. The majority of OH^{+} ions from O^{+} (^{2}D,^{2}P)+H_{2}O are associated with exothermic channels corresponding to the formation OH^{+} (X ^{3}Σ^{−},^{1}Δ,A ^{3}Π)+OH. The comparison of the sum (σ_{T}) of the cross sections for H_{2}O^{+} and OH^{+} from O^{+} (^{4}S)+H_{2}O to those from O^{+} (^{2}D)+H_{2}O and O^{+} (^{2}P)+H_{2}O shows that σ_{T} ’s for O^{+} (^{4}S)+H_{2}O and O^{+} (^{2}P)+H_{2}O are comparable, whereas the σ_{T} values for O^{+} (^{2}D)+H_{2}O are greater than those for O^{+} (^{4}S)+H_{2}O and O^{+} (^{2}P)+H_{2}O. The σ_{T} values are found to conform with the 1/E _{c.m.} dependence at low E _{c.m.}’s, indicating that the ion–dipole interaction plays an important role in the formation of the longlived collision complexes. The high cross sections for H_{2}O^{+} and OH^{+} from O^{+} (^{2}D,^{2}P)+H_{2}O observed here suggest that these reactions should be included in the simulation of the H_{2}O^{+} and H_{3}O^{+} ion density data obtained in spaceborne mass spectrometric experiments.

Quantum dynamics of a model system with a conical intersection
View Description Hide DescriptionWe study the quantum dynamics of a twoelectronic state model system with a conical intersection involving two vibrational degrees of freedom, x and y. The two diabatic surfaces are displaced only along x, while the interstate coupling is linear in y and contains Gaussian damping factors along x and y. The role of the initial momentum and of the difference of the slopes at the conical intersection is discussed. In particular, the analysis of the numerical results shows that the diabatic transition at the conical intersection is accompanied mainly by a massive flow of vibrational quanta from the x to the yoscillator of the same electronic state. Furthermore, we propose a simplified timedependent monodimensional model in which the xoscillator is assumed to move as an independent classical oscillator, while the yoscillator is described quantummechanically. Despite the crude approximations involved, this simplified model reproduces quite well the exact results for the transition probabilities at the first passage through the conical intersection and candidates itself as a useful tool for treating multidimensional systems with low computational effort.

A theoretical study of solid hydrogens doped with atomic oxygen
View Description Hide DescriptionStructure and reaction dynamics in solid H_{2}/D_{2}doped with O(^{3} P, ^{1} D, ^{1} S) is investigated through simulations based on accurate ab initiopotential energy surfaces. The ab initio calculations are performed at MCSCF level, with neglect of spin–orbit interactions. The dynamical simulations rely on nonadditive effective potentials, taking into account the anisotropy of the open shell atom by using diabatic representations for the globally fitted potential energy surfaces of O–H_{2}. The ground state of the dopedsolid is well described as O(^{3} P) isolated in paraH_{2}(J=0) since the atom–molecule interaction anisotropy is not sufficient to orient H_{2}. O(^{3} P) atoms radially localize the nearestneighbor shell, and lead to a linear increase in the density of the solid as a function of impurity concentration. The dopedsolid is stable at cryogenic temperatures, with a free energy barrier for recombination of next nearestneighbor O(^{3} P) atoms of 120 K. The solid state O(^{1} D)+H_{2}reaction is considered in some depth. While in high symmetry sites the reaction is forbidden, even at 4 K, thermal fluctuations are sufficient to promote the insertion reaction.

Reversible diffusioncontrolled reactions between two species in a fluid
View Description Hide DescriptionA model suspension of diffusing spherical particles with an internal degree of freedom taking two discrete values, or equivalently a twocomponent suspension with reactions between species, is studied. The internal degrees of freedom of neighboring particles are coupled by an Ising type interaction of short range. The dynamics of the Ising variables is described by a master operator of Glauber form. On a macroscopic level small deviations of the two densities from their equilibrium averages obey transport equations with memory character. The memory kernel is derived from the Mori formalism. The twobody contribution to the cluster expansion of the memory function is obtained analytically at zero wavevector. The spectrum of the memory function shows a marked dependence on the presence or absence of an external field, the ratio of time scales for transitions between states and for translational diffusion, and on the antiferromagnetic or ferromagnetic character of the Ising interaction.

Transient reactions in a binary suspension of diffusing spheres
View Description Hide DescriptionTransient diffusioncontrolled reactions in a binary suspension of equalsized spheres are studied on the basis of the timecorrelation function of concentration fluctuations in thermal equilibrium. The transient effects are characterized by a memory function, which is calculated in an Enskog type approximation from the nonequilibrium distribution function of a pair of diffusing and converting spheres. In Ising model language, the explicit calculation is performed for hard spheres with a square well Ising interaction and with a magnetic field present. The memory function exhibits appreciable dependence on the parameters of the model.

A high resolution energyselected kinetic energy release study of the process : Heat of formation of
View Description Hide DescriptionUsing the newly constructed photoelectronphotoion coincidence apparatus associated with the chemical dynamics beamline at the advanced light source, we have performed a high resolution energyselected kinetic energy release measurement for the dissociative photoionization process . After taking into account the centerofmass kinetic energy release, the thermochemical threshold for this process is determined to be eV. This value yields and kcal/mol for the heats of formation at 0 K for and , respectively.

Photoinduced desorption of potassium atoms from a two dimensional overlayer on graphite
View Description Hide DescriptionWe present an experimental and theoretical investigation of K atom desorption from the basal plane of graphite at 83 K induced by low energy photons (3–6 eV). The 2D potassium overlayer is characterized by low energy electron diffraction(LEED), highresolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), and work function measurements. At monolayer coverage (5.2×10^{14} atoms cm^{−2}), the dependence of the cross section on photon energy has a threshold at ℏω≈3.0 eV and rises up to a maximum of 1.8±0.4×10^{−20} cm^{2} at 4.8 eV. The coverage dependence of the photoyield reflects the existence of two phases of adsorbed K, dilute ionized photoactive and closepacked photoneutral, respectively. The observed photodesorption is a singlephoton, nonthermal event, consistent with a substratemediated mechanism. The desorption results from attachment of optically excited hot electrons to the empty 4s state of ionized potassium. The theory predicts in this case a Gaussian line shape of the photoyield vs photon energy. Fitting the model parameters to the experimental data, we determine (i) the energy and slope of the excited state potential energy curve, and (ii) the position and width of the potassiuminduced 4s resonance. The present findings combined with other available data for potassium on graphite are used to construct 1D potential energy curves along the surface normal for K^{+} and K^{0}. The calculated cross sections for s and ppolarized light are in qualitative agreement with the measurements.

Potential energy surfaces for polyatomic reactions by interpolation with reaction path weight: CH_{2}OH^{+}→CHO^{+}+H_{2} reaction
View Description Hide DescriptionAn improved algorithm to construct molecular potential energy surfaces for polyatomic reactions is presented. The method uses the energies, gradients, and Hessians, which can be obtained from ab initio quantum chemical calculations. The surface is constructed by interpolating the local quadratic surfaces with reaction path weights. The method is tested with a fiveatom reaction system for which an analytic potential energy surface has been reported together with classical trajectory results. An excellent agreement is achieved for energy partitioning in products obtained by trajectory calculation on the original analytic and interpolatedsurfaces. Reduction of error caused by the use of the reaction path weight is explained.

Potential energy surface and wave packet calculations on the Li+HFLiF+H reaction
View Description Hide DescriptionIn this work an analytic fit of previous ab initio points [Aguado, Suárez, and Paniagua, Chem. Phys. 201, 107 (1995)] on the potential energy surface of the LiFH system is presented and the reactiondynamics is studied using a timedependent treatment based on local coordinates. Threedimensional wave packet calculations performed for zero total angular momentum indicate that the reactivity for the HF reactant in its ground vibrational state is quite low, in contradiction with previous dynamical calculations using different potential energy surfaces. The differences with previous potential energy surfaces are further analyzed using a bidimensional approach. Finally, the effect of the initial vibrational excitation of the HF reactant on the reactivity is studied using the bidimensional approach.

Electronimpact ionization cross sections of atmospheric molecules
View Description Hide DescriptionA theoretical model for electronimpact total ionization cross sections, which has been found to be reliable for a wide range of molecules, is applied to molecules of interest to atmospheric science. The new theory, the binaryencounterBethe (BEB) model, combines the binaryencounter theory and the Bethe theory for electronimpact ionization, and uses simple theoretical data for the ground state of the target molecule, which are readily available from molecular structure codes. Total ionization cross sections of 11 molecules, CS, CS, COS, CH, HS, NH, NO, NO, O, S, and SO, are presented for incident electron energies from threshold to 1 keV with an average accuracy of 15% or better at the cross section peak. We also found that the use of vertical ionization potentials (IPs) rather than adiabatic IPs for the lowest IPs significantly improves BEB cross sections between the threshold and cross section peak for molecules whose adiabatic and vertical IPs are different by 1 eV or more (CH and NH. The BEB cross sections are presented in a compact analytic form with a small number of constants, making the cross sections suitable for modeling applications.