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Volume 99, Issue 9, 01 November 1993

Zero‐kinetic‐energy photoelectron spectroscopy from the Ã ^{1} A _{ u } state of acetylene: Renner–Teller interactions in the trans‐bending vibration of C_{2}H^{+} _{2} X̃ ^{2}Π_{ u }
View Description Hide DescriptionDouble‐resonance excitation via the Ã ^{1} A _{ u } state is used to record zero‐kinetic‐energy photoelectron spectra of acetylene. The analysis of these spectra leads to an improved value of 91 952±2 cm^{−1} for the adiabatic ionization potential to the C_{2}H_{2} ^{+} X̃ ^{2}Π_{ u } ionic ground state. Because the Ã ^{1} A _{ u } intermediate state has a trans‐bent geometry, transitions from it readily populate the trans‐bending vibration of the ground state ion, leading to new information about this mode and its Renner–Teller interactions. The relative intensities of the Renner–Teller components and of the rotational structure within each component also provide information on the dynamics of the photoionization process.

Ultraviolet Raman scattering from simple alcohols and a related diol as a study of low‐lying Rydberg‐valence state characteristics
View Description Hide DescriptionRelative Raman scattering cross sections of totally symmetric vibrational modes of liquid methanol, ethanol, and ethylene glycol are measured as a function of excitation wavelength in the preresonant spectral region 37 593–47 393 cm^{−1}. Raman spectra are also obtained with 50 043 and 54 198 cm^{−1} excitation, the latter energy being directly resonant with the first excited state of the alcohols. The data indicate that the first three excited electronic states (with transition energies of approximately 54 600, 62 500, and 66 000 cm^{−1}, respectively) make no significant contribution to the Raman scattered intensities and are nearly entirely Rydberg in character. The low‐lying electronic structure of ethylene glycol appears to be primarily Rydberg in character as well. The dominant C–H and O–H stretching modes gain intensity largely from valence state(s) with transition energies near 70 000 cm^{−1}, while the C–O stretching and CH_{2,3} bending modes gain intensity from even higher energy states (∼85 000 cm^{−1}).

Absorption spectrum calculations using mixed quantum‐Gaussian wave packet dynamics
View Description Hide DescriptionWe calculate the absorptionspectrum of a cluster using a computational method in which classical‐like degrees of freedom for the nuclei are described by Gaussian wave packets while the valence electrons are treated quantum mechanically. We examine the spectral features in comparison to an even simpler mixed quantum‐classical model in which the nuclear motion is treated by purely classical mechanics. Anomalous features (such as negative absorption) in the absorptionspectrum which can arise from mixed quantum‐classical methods are examined and the Gaussian wave packet nuclear dynamics is found to substantially reduce these anomalous features. This method is applied to a two‐coordinate model problem in which exact numerical results can be obtained and we find that the method works fairly well. We also apply the method to the valence electronic absorptionspectrum for a KXe_{6} cluster. The method does not suffer from the dramatic failure seen when Xe motion is treated classically. The method is used to calculate the vibrational width and the vibrational structure of the electronic absorptionspectrum.

Relaxation theory for curve‐crossing corrections to electronic absorption line shapes in condensed phases
View Description Hide DescriptionA quantum mechanical relaxation theory is developed to enable approximate computation of electronic absorption line shapes of condensed phase systems where nonadiabatic couplingeffects are important. At the simplest level, these computations require a time kernel (termed a memory kernel) which can be obtained from a sequence of wave packet propagations, each carried out on a single Born–Oppenheimer potential surface. Complications associated with the need to evolve wave packets on several nonadiabatically coupled surfaces are thereby avoided. Moreover, for many condensed phase problems the memory kernel can be computed via semiclassical techniques which rely on classical trajectories and simple Monte Carlo methods. The promise of the theory is demonstrated by numerical applications to the spectroscopic spin boson model [R. D. Coalson, J. Chem. Phys. 86, 995 (1987)], a nontrivial multimode model of electronic absorption lineshapes involving two nonadiabatically coupled excited state surfaces. The relevant quantum dynamics for the spectroscopic spin boson model can be computed exactly via path integration techniques. In this way, the accuracy of the proposed relaxation theory can be benchmarked, and the applicability of various semiclassical prescriptions for computing the memory kernel ascertained.

Dissociation energy of neutral and ionic benzene‐noble gas dimers by pulsed field threshold ionization spectroscopy
View Description Hide DescriptionDelayed pulsed field threshold ionization of clusters excited to high long‐lived Rydberg states is used to study their dissociation behavior. Benzene–Ar and benzene–Kr dimers are excited by resonance enhanced two‐photon ionization to Rydberg levels close to various ionization thresholds. The field ionized threshold ions are monitored and separated from the non‐energy‐selected ions in a reflecting field mass spectrometer with high mass resolution. The appearance of threshold ions at the daughter ion mass indicates the onset of a dissociation process. Daughter ions are first observed for the 16^{1}6^{1}(3/2) level of the two investigated dimers. This leads to an upper limit of the dissociation energy of benzene–Ar of 340 cm^{−1} which is probably higher than the true dissociation energy. For the first time threshold ions are observed for large internal energies of some 5 eV in the core indicating that high Rydberg states maintain their long lifetime even if the core is electronically or vibrationally excited by several eV.

Ring‐down cavity absorption spectroscopy of the very weak HCN overtone bands with six, seven, and eight stretching quanta
View Description Hide DescriptionA nonstandard, high sensitivity, absorption detection technique has been applied to the investigation of the very weak fifth, sixth, and seventh overtones of HCN at 100 Torr and 296 K. The frequency range covered is from 17 500 to 23 000 cm^{−1}. We report high resolution, absolute absorption spectra with a noise equivalent sensitivity as low as ∼2×10^{−9}/cm (recently improved to 7×10^{−10}/cm). Band origins, rotational constants, and band intensities are reported and compared with calculated values. The HCN overtone spectra in the present study are not affected by any kind of perturbation, despite the high excitation energy involved.

Excitation profile of coherent anti‐Stokes Raman scattering from the MnO^{−} _{4} ion doped in a KClO_{4} crystal at low temperature
View Description Hide DescriptionWe have performed polarized resonance coherent anti‐Stokes Raman experiments on the permanganate ion doped in potassium perchlorate single crystals at temperature T=15 K. At this temperature the m (C _{ s }) site splitting of the excited degenerate ^{1} T _{2}‐electronic level of the permanganate ion and the vibronic structure are well resolved. We report on the A’‐ν_{1} coherent anti‐Stokes Raman excitation profile which shows a strong dependence on the frequency of the pump laser. The simulation of the experimental results is performed by using the transform theory, which enables one to calculate the resonance Raman excitation profile solely from the measured absorptionspectrum. According to the well known relation between the third‐order nonlinear susceptibility and the Raman polarizability the coherent anti‐Stokes excitation profile is given by a simple product of two Raman excitation profiles which are shifted relative to each other. The linear and the quadratic electron‐phonon coupling as well as the influence of non‐Condon terms were taken into account. Since the transform theory is mode selective in case there are no mode mixing effects the results derived from the description of the Raman excitation profiles and the coherent anti‐Stokes excitation profiles are more definite. For this purpose one only needs the model parameters of the mode of interest. The simulation of the A’‐ν_{1} coherent anti‐Stokes profile shows that a small amount of non‐Condon coupling yields to a better agreement with the experiment.

Photoelectron spectroscopy of Cu^{−} _{ n } clusters: Comparison with jellium model predictions
View Description Hide DescriptionWe present a comparison of the electronic level structure of Cu^{−} _{ n } clusters with the jellium model using photoelectron spectroscopy of metal cluster anions. The spectra are recorded at an energy resolution of 30 meV using photon energies of up to 6.4 eV. We obtain a well resolved picture of the electronic structure of the 4s derived electronic states in the energy region between the localized 3d derived states and the highest occupied molecular orbital. The observed features can be assigned to the 1s, 1p, and 1d shells predicted by the jellium model if ellipsoidal distortions and effects like shake‐up processes, multiplet splittings and the s–d hybridization are taken into consideration.

Apparent and real values of photochemical hole‐burning parameters. Sulfonated tetraphenylporphin doped in polyvinyl alcohol
View Description Hide DescriptionThis paper presents real values of photochemical hole‐burning (PHB) parameters for dye‐doped polymer systems. The cross section for purely electronic zero‐phonon absorption σ^{*} _{0}, quantum efficiency of hole formation η and full‐width at half‐maximum of inhomogeneous broadening Δω_{ i } are determined by the least‐squares fitting method. Our sample is sulfonated tetraphenylporphin doped in polyvinyl alcohol at 20 K. The determined values are σ^{*} _{0} = (2.3 ± 0.2)× 10^{−15} cm^{2}, η=(1.6±0.6)×10^{−2}, and Δω_{ i }=(223±7) cm^{−1}. The errors are of the order of a standard deviation. The η is much greater than previously reported values. The apparent quantum efficiencies of hole formation are derived under the assumption that the lowest‐energy absorption band consists of transitions of one type. They exhibit a marked wavelength dependence, but their values are well explained quantitatively by the real values. There is no need to consider any wavelength dependence of σ^{*} _{0} and η, at least at the initial stage of burning. The purely electronic transition component is about half of the lowest‐energy absorption band. The other component is ascribed to at least two kinds of vibronic transitions of the dye. The Debye–Waller factors for these two vibronic transitions are roughly estimated to be in the order of 10^{−2}. This result means the site‐selectivity in PHB is not perfect in a wide range of the lowest‐energy absorption band.

Magic‐angle spinning nuclear magnetic resonance spectra of second‐order two‐spin systems in the solid state
View Description Hide DescriptionMagic‐angle spinning (MAS) nuclear magnetic resonance(NMR) spectra of second‐order two‐spin (AB) systems are investigated. Using average Hamiltonian theory (AHT), general expressions for the positions and relative intensities of the four allowed transitions are derived. In principle, correction terms to any order of the average Hamiltonian may be applied; however, terms up to and including third order appear to be adequate in reproducing the most important experimental features. In general, both the positions and relative intensities of the peaks are sensitive to the sample spinning frequency. Only at the high MAS frequency extreme do the MAS NMR spectra of two‐spin (AB) systems in solids correspond to those predicted using formulas derived for solution samples. Under slow MAS conditions, MAS NMR spectra of AB spin systems deviate considerably from the corresponding AB spectra in solutionNMR studies. Three general types of MAS NMR spectra are identified and their characteristic features are discussed. The theoretical expressions derived here are applied to reproduce the observed ^{31}P MAS NMR spectra of a phosphole tetramer and cis‐1,2‐bis(diphenylphosphino)ethylene. It is shown that correction terms higher than first order must be considered in order to reproduce the anomalous spinning‐frequency dependencies in MAS NMR spectra. The importance of carrying out measurements at two different applied fields is also demonstrated in the case of the phosphole tetramer.

Some mathematical properties of the absolute area of a difference spectrum
View Description Hide DescriptionDifference spectra may be used to measure wave number shifts or changes of bandwidths between two bands. Up to this date, the quantity taken into account to get the information embodied in a difference spectrum was the difference between the ordinates of the ‘‘peak’’ and the ‘‘valley.’’ A new treatment, well adapted to multichannel spectrometry and computer facilities, which uses the absolute area of the difference spectrum, has been proposed. Some mathematical properties of this quantity, especially its dependence upon the quantities of direct interest such as the wave number shift and/or the difference of bandwidth, are established, for every situation in which a spectral shift between two bands can be defined. This study has been carried out having in view Raman spectroscopy, but its results may be extended to any emission or scatteringspectroscopy.

Hyperfine and spin–orbit structure of the ^{4}Δ_{ i } ground state of CoO
View Description Hide DescriptionA detailed examination of the structure of the ^{4}Δ_{ i } ground state of gaseous CoO has been carried out from analysis of the electronic spectrum near 6300 Å, using intracavity dye laser and wavelength‐resolved fluorescence methods. The intracavity experiments have given the details of the hyperfine structure of the two lowest spin–orbit components at sub‐Doppler resolution, permitting the hyperfine parameters, a=0.0229_{5} cm^{−1} and (b+c)=−0.0111_{7} cm^{−1}, to be derived; from the negative value of the contact interaction the electron configuration is established as (4sσ)^{2}(3dδ)^{3}(3dπ)^{2}. The wavelength‐resolved fluorescence experiments have given the relative positions of the four spin–orbit components of the X ^{4}Δ_{ i } state to ±0.3 cm^{−1}. Based on these measurements, a pair of subbands sharing a common electronic upper level, but with the Ω=5/2 and 7/2 components of the ground state as lower levels, has been recognized. Rotational analysis of Doppler‐limited intracavity spectra of these two subbands has given an accurate value for the Ω=5/2–7/2 separation as 304.321±0.007 cm^{−1}.

Experimental and theoretical studies of the F+H_{2} transition state region via photoelectron spectroscopy of FH^{−} _{2}
View Description Hide DescriptionThe transition state region of the F+H_{2}reaction is studied by photoelectron spectroscopy of FH_{2} ^{−}. The photoelectron spectra consist of overlapping electronic bands with different angular distributions. The ground state band shows partially resolved features which differ depending on whether the anion is made from normal or para hydrogen. This dependence on the anion nuclear spin statistics implies that these features are due to progressions in bending levels of the neutral FH_{2} complex. In order to confirm this, and to determine the sensitivity of the photoelectron spectrum to the bend potential near the F+H_{2} transition state, three‐dimensional simulations of the FH_{2} ^{−}photoelectron spectrum were performed assuming various potential energy surfaces for the F+H_{2}reaction. We found that the London–Eyring–Polanyi–Sato surface proposed by Takayanagi and Sato gave better agreement than either the T5a or 5SEC surfaces. From the higher energy band, we can extract information on the F+H_{2} excited electronic states, and several approximate simulations on model surfaces for these states are presented.

Resonance enhanced multiphoton ionization spectroscopy of the NF molecule: ^{1}Σ^{+} and ^{1}Δ 3p Rydberg states
View Description Hide DescriptionTwo Rydberg states of the NF molecule, the 2 ^{1}Δ and 2 ^{1}Σ^{+} states, have been investigated by multiphoton ionization (MPI) spectroscopy. These states are observed via the two‐photon resonance enhancements they provide in the multiphoton ionization spectra of NF a ^{1}Δ and NF b ^{1}Σ^{+}. Both states are assigned on the basis of experimental evidence as Rydberg states with the dominant configuration ...1π^{4}5σ^{2}2π^{1} (3pπ)^{1}. Ab initio calculations performed at the multireference determinant configuration interaction (MRDCI) level showed that these states were of mixed Rydberg‐valence character with the Rydberg character [... 1π^{4}5σ^{2}2π^{1} (3pπ)^{1}] dominating at short bond lengths (<1.35 Å) and the valence character (... 1π^{3}5σ^{2}2π^{3}) becoming more important at longer bond lengths (≳1.35 Å). These calculations also proved useful in providing a mechanism to account for the rotational predissociation observed in the experimental spectra.

Rotational energy dispersions for van der Waals molecular clusters: Analytic descriptions for Rg_{3}, Rg_{4}, and Rg_{6}
View Description Hide DescriptionWe have obtained analytic expressions, parametric in centrifugal displacement coordinates, which provide exact classical descriptions of the rotational energy dispersions, that is, the dependence of the combined rotational and ‘‘electronic’’ (vibrational potential) energies on the rotational angular momenta, for small molecular clusters bound by van der Waals interactions modeled by pairwise additive Lennard‐Jones 6–12 potential energies. The clusters considered consist of three (equilateral triangle), four (tetrahedron), and six (octahedron) units and serve as models for small clusters of rare‐gas atoms such as argon. This work represents an extension of our recently published study of analytic rotational energy dispersions for diatomic molecules bound by harmonic oscillator, Morse, or Lennard‐Jones potentials [J. Mol. Spectrosc. 155, 205 (1992)]. A parallel set of studies were made using an angular momentum‐conserving simulation program. The physical properties of the clusters that are addressed using our results include calculation of quartic and higher‐order spectroscopic constants, location of rotational instabilities, and characterization of the ‘‘cubic’’ anisotropies for the spherical top clustersA _{4} and A _{6}. Of particular interest is the result that for each of these cluster types the preferred direction of the rotational angular momentum is parallel to a molecular fourfold axis, leading to reduced symmetries of D _{2d } for tetrahedral A _{4} and D _{4h } for octahedral A _{6}.

Electron paramagnetic resonance and relaxation study of copper (II) and silver (II) in CsCdF_{3} single crystals
View Description Hide DescriptionCopper and silver, respectively, were introduced into single crystals of CsCdF_{3}. Our detailed electron paramagnetic resonance(EPR) study showed that both elements enter the Cd lattice site—copper as Cu^{2+}, silver as Ag^{+}, which then was converted into Ag^{2+} by x raying the corresponding samples. Cu^{2+} and Ag^{2+} were shown to present in their ground state a pseudostatic Jahn–Teller effect. Motional effects were observed in the respective EPR spectra and studied in some detail for Cu^{2+} as they are seen over a wide temperature range. Predictions of a stochastic Kubo model [J. Phys. Soc. Jpn. 9, 935 (1954)] were compared with the temperature dependent linewidths of the motionally averaged EPR spectrum. A power law (T ^{ n } with n≂1.9) was determined for the temperature dependence of the reorientation frequency between 30 and 90 K.

Static and dynamic properties of multiple light scattering
View Description Hide DescriptionWe have examined the onset and evolution of multiple scattering of light on a series of latex dispersions as a function of increasing volume concentration φ of particles. We have shown that using vertically polarized incident light, the static scattered intensity becomes progressively depolarized, with increasing φ. The polarization of scattered light is completely random in the limit of strong multiple scattering. The spectra of decay times of dynamic light scattering display a region of oligo scattering at intermediate φ where both the single and multiple scattering components can be dynamically identified. For φ≳0.03 the limit of diffusive transport of light is attained. The obtained results confirm that our earlier measurements of dynamic light scattering on systems exhibiting critical opalescence are not influenced by multiple light scattering.

Spectroscopic analysis of jet‐cooled AlCu
View Description Hide DescriptionDiatomic AlCu has been interrogated using resonant two‐photon ionization spectroscopy in a supersonic expansion of helium. The ground state is shown to be X ^{1}Σ^{+}, deriving from the 3s _{Al} ^{2}3d _{Cu} ^{10}σ^{2} configuration, in agreement with theoretical predictions. The closed‐shell nature of this molecule results in a low density of electronic states, allowing the chemical bonding and electronic structure to be investigated in detail. Five excited electronic states have been observed and characterized, leading to a potential energy diagram based solely on experimental results. Constants experimentally determined for AlCu include a ground statebond length (r _{0}) of 2.3389±0.0004 Å, a dissociation energy,D _{0} ^{○}, of 2.315±0.012 eV, and an ionization potential of 7.065±0.014 eV.

Spectroscopic studies of jet‐cooled AlNi
View Description Hide DescriptionResonant two‐photon ionization spectroscopy has been used to interrogate diatomic AlNi produced by laser vaporization of a 1:1 alloy target in a supersonic molecular beam of helium. Although a large density of states in this molecule prohibits a concise elucidation of its electronic structure, the presence of discrete transitions has allowed several bands to be rotationally resolved. From the analysis of these bands the ground state has been determined as X ^{2}Δ_{5/2}, originating from the 3s _{Al} ^{2}3d _{Ni} ^{9}σ^{2} configuration, and the bond length has been measured as 2.3211±0.0007 Å. The dissociation energy and ionization potential of AlNi have also been determined as D _{0} ^{○}(AlNi)=2.29±0.05 eV and I.P.(AlNi)=6.95±0.09 eV, respectively.

Structure of zinc phthalocyanine by gas phase electron diffraction
View Description Hide DescriptionGas phase electron diffraction data have been successfully recorded for zinc phthalocyanine. Molecular structure parameters have been determined by least square fitting a geometrically consistent model of C _{4v } symmetry to the data. The results agree well with previous x‐ray diffraction studies of crystals. However, the zinc atom was found to be slightly out the plane of the molecule in contrast with the x‐ray results.