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Volume 102, Issue 22, 08 June 1995

The nuclear quadrupole coupling constants and the structure of the para–para ammonia dimer
View Description Hide DescriptionExpressions are derived for the nuclear quadrupole splittings in the E _{3} and E _{4} (para–para) states of (NH_{3})_{2} and it is shown that these can be matched with the standard expressions for rigid rotors with two identical quadrupolar nuclei. The matching is exact only when the off‐diagonal Coriolis coupling is neglected. However, the selection rules for rotational transitions are just opposite to those for the rigid rotor. Hyperfine splittings are measured for the J=2←1 transitions in the E _{3} and E _{4} states with ‖K‖=1; the quadrupole coupling constants χ_{ aa }=0.1509(83) MHz and χ_{ bb }−χ_{ cc }=2.8365(83) MHz are extracted from these measurements by the use of the above mentioned correspondence with the rigid rotor expressions. The corresponding results are also calculated, with and without the Coriolis coupling, from the six‐dimensional vibration–rotation–tunneling (VRT) wave functions of (NH_{3})_{2}, which were previously obtained by Olthof et al. [E.H.T. Olthof, A. van der Avoird, and P.E.S. Wormer, J. Chem. Phys. 101, 8430 (1994)]. From the comparison of χ_{ aa } with the measured value it follows that the semiempirical potential and the resulting VRT states of Olthof et al. are very accurate along the interchange (ϑ_{ A },ϑ_{ B }) coordinate. From χ_{ bb }−χ_{ cc } it follows that this potential is probably too soft in the dihedral angle γ̄=γ_{ A }−γ_{ B }, which causes the torsional amplitude to be larger than derived from the experiment.

Spectroscopy of jet‐cooled YCu
View Description Hide DescriptionOptical spectra of jet‐cooled diatomic YCu have been recorded using resonant two‐photon ionization spectroscopy in a supersonic expansion of helium. The ground state is shown experimentally to be of ^{1}Σ^{+} symmetry, with a measuredbond length of r _{ e } ^{‘}=2.6197(6) Å and a vibrational frequency of 193.21(24) cm^{−1} for ^{89}Y^{63}Cu. Five excited electronic states are identified as the [10.2]^{1}Σ^{+} state, the [11.8]^{3}Π_{0+} state, the [12.0]^{3}Π_{1} state, the [14.0]^{1}Π state, and, with the help of ab initio theory, the [12.2]^{1}Π state. No evidence whatever for participation of the 3d orbitals of copper in the chemical bonding is found, and the electronic structure of YCu is found to exhibit a striking similarity to that of YH.

Photoelectron spectroscopy of FeO^{−} and FeO^{−} _{2}: Observation of low‐spin excited states of FeO and determination of the electron affinity of FeO_{2}
View Description Hide DescriptionThe photoelectron spectra of FeO^{−} and FeO^{−} _{2} are obtained at 3.49 eV photon energy. Transitions to the ground state (^{5}Δ) and three low‐lying excited states (^{5}Σ^{+}, ^{3}Σ^{+}, and ^{3}Δ) of FeO are observed. The two low‐spin excited states found at 6770 and 8310 cm^{−1} above the ground state, respectively, have not been observed before. The two Σ states, characteristic of detachment of a nonbonding electron from the FeO^{−} anion, exhibit no vibrational progressions while a vibrational progression is observed for each of the two Δ states. The two high‐spin states ^{5}Δ and ^{5}Σ^{+} are in agreement with a previous photoelectron study [P.C. Engelking and W. C. Lineberger, J. Chem. Phys. 66, 5054 (1977)]. The ^{3}Δ state has a vibrational frequency of 800 (50) cm^{−1}. The spectrum of FeO^{−} _{2} only shows one major feature with little vibrational structure at this photon energy. The electron affinity of FeO_{2} is determined to be 2.358 (0.030) eV.

Intensities of forbidden pure torsional bands in S _{1}–S _{0} spectra of toluenes
View Description Hide DescriptionIn toluenelike molecules with sixfold internal rotation symmetry, three weak, Franck–Condon forbidden, pure torsional transitionsm ^{2} _{1}, m ^{3} _{0}, and m ^{4} _{1} invariably appear in S _{1}–S _{0}absorption spectra. The intensities are typically 1%–5% of the allowed bands, m ^{0} _{0} and m ^{1} _{1}. Determination of the preferred conformer in S _{1} as staggered or eclipsed relies on proper assignment of the upper state of the m ^{3} _{0} band as 3a ^{‘} _{1} or 3a ^{‘} _{2} under the molecular symmetry group G _{12}. In addition, inferences of the preferred conformer in S _{0} and D _{0}(ground state cation) from band intensities of fluorescence and threshold photoionizationspectra have also relied on the same assignment. For a set of six molecules having sixfold rotor potentials, including both –CH_{3} and –CD_{3} rotors, we present experimental relative intensities for the three forbidden S _{1}–S _{0} absorption bands. Within an adiabatic electronic representation, we show how a Fourier expansion of the three components of the S _{1}–S _{0} electric dipole transition moment predicts relative forbidden band intensities in quantitative agreement with experiment. This fixes the assignment of the upper state of the key m ^{3} _{0}transition as 3a ^{‘} _{1}, establishes the preferred S _{1} conformer as staggered for all six molecules, and places earlier inferences of the preferred conformers in S _{0} and D _{0} on solid ground.

Hyperfine‐resolved rovibrational spectrum of the X ^{2}Π state of HI^{+}
View Description Hide DescriptionA high resolution spectroscopic study of HI^{+} has been carried out for the first time. The absorptionspectrum in the inverted ^{2}Π ground electronic state has been measured with a linewidth of 0.004 cm^{−1} between 1995 and 2245 cm^{−1}. A total of 117 vibration–rotation transitions were observed with a tunable diode laser spectrometer coupled to an ac glow discharge cell employing velocity modulation. Lines were measured in the vibrational fundamental of the ^{2}Π_{1/2} spin substate and in the three lowest (v+1←v) bands of the ^{2}Π_{3/2} spin substate. A good fit to the data was obtained using a standard vibration–rotation, fine structure Hamiltonian. Equilibrium values were determined for 16 molecular parameters including the harmonic vibrational frequency ω_{ e }, the rotational constantB _{ e }, and the Λ‐doubling constants p _{ e } and q _{ e }. A review of the ground state properties of the hydrogen halide ions HX^{+} (X=F, Cl, Br, I) shows that the harmonic force constant is, to excellent approximation, a linear function of the internuclear spacing. In ten different vibration–rotation transitions of the two spin substates of HI^{+}, hyperfine splittings were observed. A total of 58 relative splittings of hyperfine components were analyzed to determine the iodine quadrupole coupling constant eQq _{0} as well as the Frosch–Foley magnetic hyperfine constants a, (b+c), and d. The results have been used to investigate the electronic properties of the ion.

Spectral line shape. I. Kinetic equation for arbitrary frequency detunings
View Description Hide DescriptionA theoretical formulation of the problem of the spectral line shape is presented using the superoperator technique. Conventional approximations, such as the long wave approximation, the binary approximation and the classical motion of molecular centers of mass, are employed. The kinetic equation for the line profile from resonance to the far wings is derived. Its main feature is that the terms prevailing at small or large frequency detunings are separated in the equation itself.

Nuclear modulation effects in ‘‘2+1’’ electron spin‐echo correlation spectroscopy
View Description Hide DescriptionThis work represents the synthesis of the 2+1 pulse sequence for the study of electron spin‐echo envelope modulation (ESEEM) with the technique of spin‐echo correlated spectroscopy (SECSY), which has previously been used to study nuclear modulation by two‐dimensional Fourier transformESR methods. This example of ‘‘pulse adjustable’’ spectroscopy, wherein the pulse width and pulse amplitude of the second pulse in a three pulse sequence are introduced as adjustable parameters, leads to enhanced resolution to the key features of the nuclear modulation that are important for structural studies. This is demonstrated in studies on (i) a single crystal of irradiated malonic acid and (ii) a frozen solution of diphenylpicrylhydrazyl in toluene. In particular, it is shown for (i) how the nuclear modulation cross peaks can be preferentially enhanced relative to the autopeaks and to the matrix proton peaks, and also how the autopeaks can be significantly suppressed to enhance resolution for low‐frequency cross peaks. For (ii) the low‐frequency ^{14}N nuclear modulation could be suppressed leaving just the high‐frequency matrix ^{1}H modulation. Additionally, the T _{2} homogeneous linewidth broadening in the f _{1} frequency direction is removed in 2+1 SECSY. These features significantly improve resolution to the modulation decay, which is the main observable utilized for distance measurements by ESEEM, compared to SECSY. A simple example of a distance measurement to matrix protons is presented for (ii). It is shown that a major advantage of the 2D format is that the full spin‐echo shape is collected, which permits one to study how the effect of the nuclear modulation upon the echo varies with evolution time t _{1} after the first pulse, and thereby to detect the important modulation features. Additionally it allows for correlating the modulation cross peaks in making spectral assignments. A detailed quantitative theory for 2+1 SECSY is also presented. In general, very good agreement between experiment and theoretical simulations is obtained.

Orientational processes in liquid nitromethane studied by depolarized light scattering and transient optical Kerr effect
View Description Hide DescriptionThe dynamics of nitromethane in the liquid phase is investigated by steady‐state (Raman and Rayleigh) and time‐resolved (optical Kerr effect)spectroscopic experiments performed at variable temperature. Both experiments show that the entire relaxation process is completed in a few picoseconds and that the dynamics can be described by assuming a biexponential function for the molecular response. The prominent dynamical feature at longer times appears to be that of a diffusionally reorienting symmetric top. The time dependence for orientational correlation at the shorter times observed in the coherent optical processes (Rayleigh and OKE) suggests that perturbative phenomena in the subpicosecond time scale (collisional and cage effects) are operative prior to the onset of the diffusional regime. Reorientational times from steady‐state coherent and incoherent light scattering experiments enable us to exclude that pair‐particle orientational correlation is effective. The same orientational activation energy was estimated from all the experiments. The reorientation times closely follow the η/T (η shear viscosity) linear dependence, conforming to the predictions of slip rather than stick Stokes–Einstein–Debye hydrodynamic theory.

Absorption spectroscopy and photodissociation dynamics of small helium cluster ions
View Description Hide DescriptionThe optical absorption of size‐selected helium cluster ions was studied via photofragmentation spectroscopy. Absorption cross sections were measured for He^{+} _{ n } (n=3, 4, 10, 21, and 30). A broad absorption peak was found, which for He^{+} _{3} is centered at ≊5.3 eV, and which with increasing cluster size shifts slightly to the red. In addition, the kinetic energy release to the ionized and neutral photofragments was measured by a time‐of‐flight technique for cluster sizes between n=3 and 10. From the energy balance the total binding energy of the He^{+} _{3} trimer ion was determined to be 2.6±0.15 eV. The results further indicate that a charged linear trimer acts as a core molecule for the sizes n=4–7. For n≳7, an additional isomer with a tetramer core is identified. The results are compared with recent ab initio calculations.

Tensor analysis of femtosecond Raman induced polarization spectroscopy: Application to the study of rotational coherence
View Description Hide DescriptionIn this paper we present a tensoranalysis of Raman induced polarizationspectroscopy (RIPS), a variation of time‐resolved rotational coherencespectroscopy (RCS), developed in our laboratory to study coherent rotational motions in fluids over a range of densities. Based on the irreducible tensor algebra, the analysis separates the fundamental molecular response in RIPS from the dependence of the signal on geometric factors (i.e., laser polarizations). Explicit formulas for the femtosecond RIPS response of symmetric and asymmetric tops are derived. The results are in good agreement with experimental RIPS spectra of low pressure CO_{2} and O_{3}. The tensoranalysis is also generalized to other types of four wave mixing experiments, allowing detailed comparison of the various frequency‐ and time‐domain pump–probe experiments.

Vibrational mode selectivity in the unimolecular decomposition of CH_{2}NNO_{2}
View Description Hide DescriptionVibrational mode selectivity has been investigated for the unimolecular decomposition of methylene nitramine (MN), CH_{2}NNO_{2}, by using classical trajectories computed on a (previously reported) potential‐energy surface (PES) that is based on ab initio results. The PES allows for the two known primary decomposition pathways: (I) N–N bond scission to form H_{2}CN and NO_{2} and (II) concerted molecular elimination giving HONO and HCN. Of particular interest in this study is the influence of selective vibrational mode excitations on the rates and branching of the decomposition via the competing reaction channels, which have barrier heights that differ by only 2 kcal/mol. Rates were computed for 57.7 kcal/mol above the zero‐point energy for initial conditions corresponding to overtone excitations of each of the 15 normal modes. In each case, zero‐point energy was assigned to each of the normal modes, and then one of the modes was assigned 57.7 kcal/mol excitation energy giving a total of 85.0 kcal/mol. One calculation was done at this energy with initial conditions corresponding to a microcanonical (statistical) distribution in which the energy was randomly distributed among all of the vibrational modes. Comparisons of the mode selective results with this statistical rate show that there is substantial enhancement of the decomposition rates for the two reactions for excitation of three of the normal modes (those with frequencies of 275, 442, and 753 cm^{−1}).
Excitation of the other 12 modes yielded rates in accord with the statistical rates for the 2 reactions. The eigenvectors of the normal modes of the molecule that give mode selectivity in the reaction rates project onto the reaction coordinates for the two pathways. One other mode (with frequency 606 cm^{−1}) also projects strongly onto the reaction path for II), but there is no enhancement of the rate when it is excited. However, we found that energy transfers out of this mode to other modes not coupled to either reaction coordinate. The remaining 11 modes do not project to any significant extent onto either reaction coordinate. The results show that projection of a vibrational mode onto the reaction coordinate is necessary for mode selective reaction, but is not sufficient since energy transfer into the reaction coordinate must compete with energy flow to other modes that do not project onto (i.e., are uncoupled from) the reaction coordinates.

Quantum scattering calculations for vibrational and rotational excitation of CO by hot hydrogen atoms
View Description Hide DescriptionCollision cross sections were calculated for vibrational and rotational excitation of CO by H atoms at collisional kinetic energies from 0.7 to 1.9 eV. The BBH [J. M. Bowman, J. S. Bittman, and L. B. Harding, J. Chem. Phys. 85, 911 (1986)] potential energy surface was used and collision dynamics were treated within the quantum coupled states approximation, which is shown to be quite accurate for this system, and also using the infinite order sudden approximation for the rotational degree of freedom, which is shown to be less accurate than expected. Results are compared with experimental data and with quasiclassical trajectory values.

Quantum dynamics of a planar model for the complex forming OH+CO→H+CO_{2} reaction
View Description Hide DescriptionThe quantum dynamics of three and four degrees‐of‐freedom planar models of the OH+CO→H+CO_{2} reaction are discussed. These computationally intensive calculations, which are carried out on a scalable parallel computer, illustrate the role of HOCO reaction intermediates or scattering resonances. The results are contrasted with previous two and three degrees‐of‐freedom quantum results, as well as with two, three, four, and six degrees‐of‐freedom quasiclassical trajectory calculations. While our quantum calculations are restricted to total angular momentumJ=0, it is possible to estimate the thermal rate constant using a J‐shifting approximation, and to make comparison with experiment and previous full‐dimensional classical trajectory results.

Lifetimes of Rydberg states in zero‐electron‐kinetic‐energy experiments. I. Electric field induced and collisional enhancement of NO predissociation lifetimes
View Description Hide DescriptionLifetime measurements are reported for high principal quantum number (n=40–125) Rydberg states of the NO molecule, prepared using state‐selective double resonance excitation with a narrow bandwidth laser. The influence of (i) the application of a dcelectric field, and (ii) interactions of the Rydberg states with surrounding ions, were investigated. It is demonstrated that the presence of dcelectric fields can lead to enhancements in the lifetimes due to l‐mixing, and the conclusion is reached that under the conditions existing in a typical zero‐electron‐kinetic‐energy (ZEKE) photoelectron spectroscopy experiment, the Rydberg states involved are excited in a regime where l‐mixing is expected to be significant. It is observed that l‐ and m‐mixing collisional interactions provide a further mechanism to stabilize the optically prepared Rydberg states, beyond the limits which can be achieved by l‐mixing alone.

Lifetimes of Rydberg states in zero‐electron‐kinetic‐energy experiments. II. Electric field induced and collisional enhancement of Xe autoionization lifetimes
View Description Hide DescriptionLifetime measurements are reported for high principal quantum number (n=43–87) autoionizing Rydberg states of the Xe atom, prepared using state‐selective excitation with a narrow bandwidth laser. It is demonstrated that the lifetimes can be increased by l‐mixing in dcelectric fields, as well as by l,m‐mixing through the presence of surrounding Xe^{+} ions. The influence of l,m‐mixing interactions on the peak intensities in zero‐electron‐kinetic‐energy (ZEKE) photoelectron spectroscopy experiments is discussed, and a possible method for increasing the applicability of ZEKE experiments is suggested. In addition, experimental evidence is given for spin–orbit state changing Xe*(n)–Xe collisions.

The generation of CH(X ^{2}Π,v‘,N‘) fragments in the photolysis of CH_{2}(1 ^{3} B _{1}) radicals
View Description Hide DescriptionGround state methylene, CH_{2}(1 ^{3} B _{1}), was shown to dissociate into CH(X ^{2}Π, v‘, N‘)+H fragments by excitation via the first triplet absorption continuum around 200 nm. Triplet CH_{2} was generated in the 351 nm photolysis of ketene. CH(X) fragments were formed in the 193 nm photolysis of these radicals and were detected by laser induced fluorescence at around 430 nm. The relative populations of the v‘=0 and 1 vibrational states, and the rotational and Λ‐doublet levels of these states were characterized. These results are compared with those given in the literature by ab initio and dynamics calculations for the absorption and dissociation of triplet CH_{2}. Further comparisons are made with the photofragmentations H_{2}O→OH(X ^{2}Π)+H and NH_{2}→NH(A ^{3}Π)+H.

The HeCl_{2} potential: Atom–atom and ab initio compared to experiment
View Description Hide DescriptionTwo forms for the HeCl_{2} potential are compared to the available experimental data. First, an atom–atom form that incorporates the recently measured anisotropic He–Cl potential is used. The anisotropy of this potential is slight, and its strengths and weaknesses are similar to previous potentials in which the He–Cl interaction was treated as isotropic. In particular, the fit to the scattering data is poor. Second, a fit to ab initio points calculated using Mo/ller–Plesset perturbation theory to fourth order was performed. The resulting potential is much more anisotropic than any potential previously proposed and tested for HeCl_{2}. This potential fits the rotationally resolved excitation spectra as well as do previous empirical potentials, and is consistent with certain features of the total differential scattering data with which previous potentials were not. Although the ab initio potential has a global minimum in the linear configuration, the probability distribution of the ground vibrational level still maximizes in the perpendicular configuration, accounting for the good fit to the rotationally resolved spectrum. We conclude that noble gas–halogen potentials are much more anisotropic than previously believed, and we suggest several experiments that could help to confirm this anisotropy.

Quantum dynamics of many‐atom systems by the classically based separable potential (CSP) method: Calculations for I^{−}(Ar)_{12} in full dimensionality
View Description Hide DescriptionA recently developed method for time‐dependent quantum simulations of large systems on short time scales is applied to the dynamics following electron photodetachment from the clusters I^{−}(Ar)_{2} and I^{−}(Ar)_{12}. The problem is treated in full dimensionality, incorporating all vibrational degrees of freedom, by the classically based separable potential (CSP) approach. This is essentially an approximate time‐dependent quantization of classical dynamics: Classical molecular dynamics is used to generate effective, single mode separable time‐dependent potentials for each degree of freedom. The quantum dynamics is then propagated separately for each mode, using the effective potentials that implicitly include effects such as energy transfer between the modes. In the current application of the CSP method we calculate properties relevant for the interpretation of spectroscopies, such as correlation functions of wave packets, as well as time‐dependent atom–atom distribution functions, pertinent to future diffraction experiments using ultrafast pulses. The insight obtained from the quantum dynamics of these clusters is discussed. In particular, light is thrown on the differences in the dynamics associated with the system landing on the three different electronic surfaces of the neutral I(^{2} P)⋅(Ar)_{ n } system.

Negative ion chemistry of SF_{4}
View Description Hide DescriptionA selected ion flow tube was used to conduct an extensive study of negative ion–molecule reactions of SF_{4} and SF^{−} _{4}. Rate constants and product ion branching fractions were measured for 56 reactions. The reactions bracket both the electron affinity of SF_{4} (1.5±0.2 eV or 34.6±4.6 kcal mol^{−1}) and the fluoride affinity of SF_{3} (1.84±0.16 eV or 42.4±3.2 kcal mol^{−1}). These results may be combined to give the neutral bond energy D(SF_{3}–F)=3.74±0.34 eV or 86.2±7.8 kcal mol^{−1}, independent of other thermochemical data except for the accurately known electron affinity of F. The heat of formation of SF^{−} _{4} is derived from the electron affinity of SF_{4}: Δ_{ fH }(SF^{−} _{4})=−9.2±0.3 eV or −212.9±7.5 kcal mol^{−1}. Lower limits to EA(SF_{2}) and EA(SF_{3}) are deduced from observation of SF^{−} _{2}(35%) and SF^{−} _{3}(65%) ion products of the reaction S^{−}+SF_{4}. Rapid fluoride transfer from both SF^{−} _{2} and SF^{−} _{3} to SF_{4} places upper limits on the electron affinities of SF_{2} and SF_{3}. The combined results are 0.2 eV≤EA(SF_{2})≤1.6 eV and 2.0 eV≤EA(SF_{3})≤3.0 eV. We review the status of measurements of EA(SF_{ n }), n=1–7.

Final state‐selected spectra in unimolecular reactions: A transition‐state‐based random matrix model for overlapping resonances
View Description Hide DescriptionFinal state‐selected spectra in unimolecular decomposition are obtained by a random matrix version of Feshbach’s optical model. The number of final states which are independently coupled to the molecular quasibound states is identified with the number of states at the dividing surface of transition state theory(TST). The coupling of the transition state to the molecular complex is modeled via a universal random matrix effective Hamiltonian which is characterized by its resonance eigenstates and provides the correct average unimolecular decay rate. The transition from nonoverlapping resonances which are associated with isolated Lorentzian spectral peaks, to overlapping resonances, associated with more complex spectra, is characterized in terms of deviations from a χ^{2}‐like distribution of the resonance widths and the approach to a random phase‐distribution of the resonance scattering amplitudes. The evolution of the system from a tight transition state to reaction products is treated explicitly as a scattering process where specific dynamics can be incorporated. Comparisons with recently measured final state‐selected spectra and rotational distributions for the unimolecular reaction of NO_{2} show that the present model provides a useful new approach for understanding and interpreting experimental results which are dominated by overlapping resonances.