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Volume 101, Issue 2, 15 July 1994

Rotational spectra and structures of Rg–C_{6}H_{6}–H_{2}O trimers and the Ne–C_{6}H_{6} dimer (Rg=Ne, Ar, or Kr)
View Description Hide DescriptionRotational spectra of Rg–C_{6}H_{6}–H_{2}O isotopomers, where Rg=Ne, Ar, or Kr, have been observed with a Balle–Flygare Fourier transformmicrowave spectrometer. In these trimers the benzene is sandwiched between the rare gas and H_{2}O. Isotopic substitution and inertial analyses show that the Rg–C_{6}H_{6} distance in the trimer is reduced by about 0.01 Å for Ne, Ar, and Kr compared to the corresponding distance in the Rg–C_{6}H_{6} dimer. On the other hand, the c.m. (C_{6}H_{6}) to c.m. (H_{2}O) distance in the trimers is increased by only about 0.003 Å from its distance in the dimer. Symmetric top spectra of ^{20}Ne–C_{6}H_{6} and ^{22}Ne–C_{6}H_{6} were observed as an aid in the comparison. Hyperfine structure (hfs) and substitution analyses with HDO/D_{2}O containing isotopomers reveal that the m=0 and 1 states of H_{2}O in the trimer are virtually unchanged from those in the C_{6}H_{6}–H_{2}O dimer, including essentially free rotation. In addition, analyses are made of the root‐mean square (rms) deviation for the benzene C _{6} axis from the a axis in the dimers (∼19 deg) and trimers (∼21 deg) and of the displacement of the C _{2} axis of the water from the a axis in the trimers (∼37 deg).

The normal to local mode transition in AB_{2} triatomic molecules: The susceptibility of eigenstates to symmetry breaking perturbations
View Description Hide DescriptionWe argue that the physical significance of the normal to local mode transition in triatomic molecules AB_{2} lies in an enhanced susceptibility of eigenstates to symmetry breaking perturbations. States of local character have higher susceptibilities than states of normal character. The quantum mechanical condition for the normal to local mode transition is formulated. The issue of stability is addressed from a classical, semiclassical, and quantum mechanical point of view. The question of instability of the unperturbed quantum system is decided individually for each eigenstate. The relevance for experiments is outlined.

Potential curve of NaK a ^{3}Σ^{+} state near dissociation limit
View Description Hide DescriptionVibrational levels near the dissociation limit of NaK a ^{3}Σ^{+} state are observed with an optical–optical double resonance technique. High resolution spectra are sensitively detected with the combination of perturbation facilitated polarizationspectroscopy with frequency modulationspectroscopy. An electronically excited state, the B ^{1}Π state, is mixed with the c ^{3}Σ^{+} state through spin–orbit interaction. The transition from the X ^{1}Σ^{+} state to the a ^{3}Σ^{+} state through the B ^{1}Π state is facilitated by the perturbation by the c ^{3}Σ^{+} state. Hyperfine structures of the a ^{3}Σ^{+}(v=3−16,N=4−25) state are resolved with this spectroscopic technique and found to be independent of vibrational and rotational quantum number. The a ^{3}Σ^{+} state can only be perturbed by the X ^{1}Σ^{+} state through the hyperfine interaction. The vibrational levels (v≤16) of the a ^{3}Σ^{+} state are demonstrably not perturbed by the X ^{1}Σ^{+} state. The potential curve of the a ^{3}Σ^{+} state is determined by the near‐dissociation expansion fitting of molecular constants and the inverse perturbation analysis method. The coefficients C _{6}, C _{8}, and C _{10} of the potential function are determined to be (12.75±0.15)×10^{6} cm^{−1} Å^{6}, (2.22±0.19)×10^{8} cm^{−1} Å^{8}, and (1.100±0.061)×10^{10} cm^{−1} Å^{10}, respectively. The dissociation energy, D _{ e }, is obtained to be 207.858 ± 0.019 cm^{−1}, which is 2.8 cm^{−1} above the highest observed vibrational level (v=16).

Reactions of Rydberg states of molecular hydrogen
View Description Hide DescriptionThe Rydberg statereaction H^{*} _{2}+H_{2}→H^{+} _{3}+H+e ^{−} was studied by exciting the (X ^{2}∑^{+} _{ g })np,v=0,JRydberg states with n=30–70 by double resonance excitation via the E,F ^{1}∑^{+} _{ g },v=0,J=1 level and by detecting the product ions or electrons. The results are consistent with the reaction occurring between the ion core of the Rydberg molecule and the ground state molecule, with the Rydbergelectron acting as a spectator. Thus, these reactions can be used to provide information on the analogous ion–molecule reactions, and the possibility of using the Rydberg statereactions to study the rotational state dependence of the corresponding ion–molecule reactions is discussed.

Pyrolysis jet spectroscopy and ab initio studies of the S _{1} and T _{1} states of germanium difluoride
View Description Hide DescriptionSpectra of jet‐cooled germanium difluoride were obtained by the reaction of germanium metal and fluorine in the throat of a heated supersonic jet. Two band systems at 331–305 and 231–224 nm were observed by laser‐induced emission excitation spectroscopy. On the basis of high quality ab initio predictions of the energies, geometries and vibrational frequencies of the lower electronic states, the two band systems are assigned as ã ^{3} B _{1}–X̃ ^{1} A _{1} and Ã ^{1} B _{1}–X̃ ^{1} A _{1}, respectively. The T _{1}–S _{0} spectrum consists of a long, well‐resolved progression in the excited state bending frequency with ν_{2} ^{’} = 192.2 cm^{−1} and T _{00}=30 582.1 cm^{−1}. The S _{1}–S _{0} spectrum is a complex tangle of vibronic structure accompanied by a rising background. A partial analysis of the vibronic structure gave ν_{2} ^{’} = 159.6 cm^{−1} and T _{00}=43 860.9 cm^{−1}. The spectra are very similar to the analogous band systems of GeCl_{2}.

^{13}C nuclear magnetic resonance study of the solution‐state dynamics of benzene
View Description Hide DescriptionNuclear spin relaxation‐induced polarization transfer is utilized to investigate the reorientational dynamics of liquid benzene. The second rank reorientational correlation time associated with the overall tumbling of the molecular plane, τ_{⊥}=2.2±0.1 ps, and the motional anisotropy, τ_{⊥}/τ_{∥}=2.4±0.4, were determined at 283 K. From the quantitative measure of relaxation‐induced three spin order, the proton shielding anisotropy can be determined, Δσ_{H}[1+(3/5)η_{H}]=−5.3±1.0 ppm.

Measurement of the vibrational Zeeman effect for HCF_{3} using magnetic vibrational circular dichroism
View Description Hide DescriptionMagnetic vibrational circular dichroism (MVCD) for different vibrational modes of HCF_{3} have been measured. The vibrational Zeeman effect for the E‐symmetry v _{4} and v _{5} modes have been determined for the first time from the solution and gas phase MVCD and absorption spectra. An analysis of the vibrational Zeeman effect for the other E‐symmetry mode, v _{6}, has been derived from the v _{3}+v _{6} combination band MVCD and absorption spectra based on the vibronic coupling model. Comparisons of these results to predictions of different theoretical models have been used to analyze the sources of the observed Zeeman effect.

Study of low‐lying electronic states of ozone by anion photoelectron spectroscopy of O^{−} _{3}
View Description Hide DescriptionThe low‐lying electronic states of ozone are studied using anion photoelectron spectroscopy of O^{−} _{3}. The spectra show photodetachment transitions from O^{−} _{3} to the X̃ ^{1} A _{1}ground state and to the five lowest lying electronic states of the ozone molecule, namely the ^{3} A _{2}, ^{3} B _{2}, ^{1} A _{2}, ^{3} B _{1}, and ^{1} B _{1} states. The geometry of the ozonide anion determined from a Franck–Condon analysis of the O_{3} X ^{1} A _{1}ground state spectrum agrees reasonably well with previous work. The excited statespectra are dominated by bending vibrational progressions which, for some states, extend well above the dissociation asymptote without noticeable lifetime broadening effects. Preliminary assignments are based upon photoelectron angular distributions and comparison with ab initio calculations. None of the excited states observed lies below the ground statedissociation limit of O_{3} as suggested by previous experimental and theoretical results.

Multichannel quantum defect theory simulation of the zero‐kinetic‐energy photoelectron spectrum of H_{2}
View Description Hide DescriptionThe multichannel quantum defect theory (MQDT) is applied to the simulation of the v ^{+}=2 band of the zero‐kinetic‐energy (ZEKE) photoelectron spectrum of molecular hydrogen and also to the photoionizationspectrum involving autoionizing Rydberg states in the region between the v ^{+}=1 and v ^{+}=2 thresholds. The results of the calculations are compared with previously reported experimental results [J. Chem. Phys. 96, 4149 (1992)]. The calculations are in very good agreement with experiment and provide quantitative confirmation of the previously proposed mechanisms for intensity perturbation.

Study of the LiHg excimer: Blue–green bands
View Description Hide DescriptionWe report for the first time the production of the LiHg excimer by a photochemical reaction of excited Li_{2} molecules with Hg(6s ^{2} ^{1} S _{0}). Excitation energy was provided by a cw ultraviolet (UV)‐argon ion laser operating in single‐line and in multi‐line mode or by pulsed XeCl excimer laser at 3080 Å. We observed bound‐free emission of LiHg in the form of diffuse bands peaking at 4450 and at 4670 Å. The latter was structured by bound–bound emission lines. Relativistic ab initio calculations for the potential energy curves of LiHg and for the relevant dipole transition moments were performed. Using these results, an interpretation of the observed emission is presented.

The ν_{1} band of ketene
View Description Hide DescriptionInfrared and Raman spectra of the ν_{1} band of ketene, H_{2}CCO, have been recorded at Doppler resolution. The infrared spectrum has been obtained with a difference frequency infrared spectrometer, and the Raman spectrum of the Q branches of this band has been recorded using a stimulated Raman spectrometer. The vib‐rotational analysis of the data is very complicated because of many crossings with other vibrational states, which can interact with v _{1}=1 through Fermi or Coriolis mechanisms. We present a discussion on global and local resonances, and we are able to extract information on the perturbing levels and on the perturbation parameters, even though the perturbing bands are not always detected in the spectrum.

A spherical top or a symmetric top? A photoacoustic titanium:sapphire ring laser spectrum of the sixth stretching vibrational overtone band system of ^{116}SnH_{4}
View Description Hide DescriptionA Doppler limited high resolution photoacoustic titanium:sapphire ring laser spectrum of the sixth stretching vibrational overtone band system of monoisotopic stannane, ^{116}SnH_{4}, has been measured in the wave‐number region 11 970–12 130 cm^{−1}. The spectrum has been rotationally analyzed both using a spherical top and a symmetric top model. The results obtained by both approaches are compared with each other. In addition, the degree of localization of the sixth stretching vibrational overtone of stannane is compared with that of the first and second stretching vibrational overtones. No perturbations due to the bending vibrations are found, which demonstrates the suitability of stannane as a model molecule for studying vibrational localization.

Spectroscopy of a chromophore coupled to a lattice of dynamic two‐level systems. I. Absorption line shape
View Description Hide DescriptionFrequency modulation of a dilute chromophore’s transition is a common mechanism of line broadening in condensed phases. In many different physical situations this frequency modulation arises from coupling of the chromophore to a collection of flipping two‐level systems. In this paper we calculate the absorption line shape for such a chromophore when the two‐level systems occupy the sites of a regular lattice. We consider both general and specific (dipolar) two‐level system/chromophore interactions. We present both exact (numerical) and approximate (analytical) expressions for the line shape as a function of the two‐level system excitation probability, p, and the flip rate. We find that when p≪1 the line shape is Lorentzian for any value of the flip rate, and that when p=1/2 the chromophore’s fluctuating frequency is described approximately by a Gaussian random process, and that therefore the seminal stochastic model of Anderson and Kubo is applicable.

Spectroscopy of a chromophore coupled to a lattice of dynamic two‐level systems. II. Spectral diffusion kernel
View Description Hide DescriptionWe consider the spectraldiffusion of a chromophore coupled through dipolar interactions to a regular lattice of flipping two‐level systems. In particular, we calculate the spectraldiffusion kernel, P(ω,t‖ω_{0}), which is the conditional probability density that the chromophore will have transition frequency ω at time t, given that it had frequency ω_{0} at time 0. At very short times we find that the spectraldiffusion kernel is Lorentzian, for any value of the two‐level system excitation probability, p. For longer times the form of the spectraldiffusion kernel depends on the value of p. We derive several approximate expressions for the spectraldiffusion kernel, all of which go to the correct equilibrium distribution of frequencies for long times. For p≂1/2, when the frequency distribution is nearly Gaussian, we find that the spectraldiffusion kernel is not at all Gaussian for short times. We compare all of our approximate expressions with numerically exact results. Motivated by certain optical spectraldiffusion experiments on individual molecules in solids, we also calculate P(Δ;t), the distribution of spectral jumps, which is the probability density that the chromophore’s frequency will change by an amount Δ in time t. In a subsequent paper we will analyze these spectraldiffusion experiments with our results.

Zero‐kinetic‐energy photoelectron spectroscopy of the hydrogen‐bonded phenol‐water complex
View Description Hide DescriptionTwo‐photon, two‐color (1+1’) zero‐kinetic‐energy (ZEKE) photoelectron spectra are presented for the 1:1 phenol‐water complex, a prototype system for hydrogen bonding between an aromatic molecule and a simple solvent. ZEKE spectra via different (intermolecular) vibrational intermediate S _{1} levels of the fully protonated complex (C_{6}H_{5}OH–H_{2}O, h _{3}) as well as the ZEKE spectrum via the vibrationless S _{1} state of the threefold deuterated complex (C_{6}H_{5}OD–D_{2}O, d _{3}) have been recorded. The spectra are rich in structure, which is mainly attributable to intermolecular vibrations of the ionic complex. Progressions of the intermolecular stretch vibration (240 cm^{−1}) in combination with different intermolecular and intramolecular vibrational levels are the dominant feature of all ZEKE spectra obtained and indicate a large change in the complex geometry along the hydrogen‐bond coordinate on ionization. Comparison between the spectrum of the d _{3} complex and the spectra via different intermediate intermolecular levels of the h _{3} complex has allowed a more detailed analysis of the intermolecular features compared to previously reported results. Finally, the vibrational assignments obtained are compared with ab initio results for the phenol‐water cation reported in the following paper in this issue.

Ab initio study of the phenol‐water cation radical
View Description Hide DescriptionThe phenol‐water cation radical has been investigated by ab initio theory using the spin‐restricted open‐shell Hartree–Fock and spin‐restricted open‐shell second‐order Mo/ller–Plesset theories with 3‐21G*(O) and 6‐31G* basis sets. The full geometrical optimization was performed for several hydrogen‐bonded structures and one hemibonded structure. Clearly, the most stable structure has been found for C _{ s } symmetry with the linear hydrogen bond between the proton of the OH group of the phenol cation radical and the oxygen of the water, and the water hydrogens pointing away from the phenyl ring. For this structure harmonic (and for some intermolecular modes anharmonic) vibrational frequencies have been computed for various isotopic complexes. The computed shifts of phenol‐localized intramolecular modes on complexation and on deuteration as well as the calculated intermolecular frequencies of the different isotopic complexes allow for an assignment of vibrational frequencies observed in the experimental zero‐kinetic‐energy (ZEKE) photoelectron spectra. Five out of a possible six intermolecular vibrations and several intramolecular modes have been assigned, including the 18b vibration which shows a strong blue shift in frequency upon complexation. Structure and properties of the phenol‐water cation radical are compared with those of the corresponding neutral complex.

Deexcitation electron spectroscopy of core‐excited NO as a function of excitation energy
View Description Hide DescriptionDeexcitation electron spectra of core‐excited NO have been measured at several excitation energies in the N 1s→2π and O 1s→2π resonances. The nitrogen spectra exhibit significant variation with excitation energy; the oxygen spectra vary only slightly. Sensitivity to excitation energy occurs because each resonance represents the overlap of three transitions to ^{2}Σ^{+}, ^{2}Δ, and ^{2}Σ^{−} states, and each of these excited states decays to a unique set of levels in the final‐state ion. We have analyzed all spectra by taking into account excitation energy, lifetime‐vibrational interference, and the ordering and splitting of the core‐excited levels. Good agreement between calculated line shapes and experiment occurs if it assumed that the level ordering is ^{2}Δ, ^{2}Σ^{−}, ^{2}Σ^{+} for core‐excited nitrogen and ^{2}Σ^{−}, ^{2}Δ, ^{2}Σ^{+} for core‐excited oxygen. Photoexcitation data for oxygen have been analyzed to determine the energies of these states 531.7, 532.7, and 533.7 eV. The deexcitation spectrum from the ^{2}Δ state of nitrogen core‐excited NO to the ground state of NO^{+} has been analyzed using the theory of lifetime‐vibrational interference to give a lifetime width for the core‐excited state of 146 meV. A similar analysis for the deexcitation of the oxygen core‐excited state is less conclusive, but is consistent with a lifetime width of 180 meV.

Calculated rotational spectrum of Ar...CO from an ab initio potential energy surface: A very floppy van der Waals molecule
View Description Hide DescriptionUsing the ab initio potential of Shin et al. (to be published), we have calculated the bound states and infrared absorptionspectrum of the van der Waals complex Ar...CO. The results show that Ar...CO cannot be treated as a quasirigid rotor, nor as a molecule with a free internal rotor. In particular, a transition to the first excited van der Waals bending level is predicted to be present in the spectrum, and its frequency varies with Ω (the projection quantum number of the total angular momentum onto the intermolecular axis going from the center of mass of CO to the Ar atom). It is also shown that, although the spectrum cannot be analyzed by the use of a rigid rotor model, rotational ‘‘constants’’ can still be defined for each value of Ω. This is consistent with the available experimental data and the predicted bending excitation can account for unassigned transitions in the infrared spectrum of this complex. Finally, a sensitivity analysis of the calculated spectrum with respect to the potential anisotropy has been performed.

Polarization of atomic photofragment fluorescence for excitation along a Fano profile: A quantum‐mechanical study
View Description Hide DescriptionA quantum‐mechanical description of the polarization of the fluorescence from photofragments, obtained by excitation along a Fano profile, is presented. For excitation on a Fano profile, dissociation can occur via two indistinguishable pathways, i.e., dissociation via the resonance and via direct excitation to the continuum. This gives rise to quantum interferenceeffects, which result in an asymmetric profile in the photon absorptionspectrum. The effect of the interference on the polarization of the photofragments fluorescence is investigated. The polarization as a function of the excitation energy varies asymmetrically around the resonant excitation energy. However, in contrast to intuition, the polarization changes in a symmetric way around an energy which is shifted from the resonant excitation energy. Simple formulas to calculate the polarization for different rotational and electronic transitions are provided.

Theoretical study on the ground and excited states of the chromate anion CrO^{2−} _{4}
View Description Hide DescriptionThe symmetry adapted cluster (SAC) and SAC‐configuration interaction (SAC‐CI) theories are applied to the calculations of the ground and excited states of the chromate ion CrO^{2−} _{4}. Electron correlations are very large for this molecule and work to relax the charge polarization of the Cr–O bonds in the ground state. The experimental spectrum of CrO^{2−} _{4} is well reproduced by the present calculations, which is the first ab initio study of the excited states including electron correlations. All of the observed peaks are assigned to the dipole allowed transitions to the ^{1} T _{2}excited states. Furthermore, many kinds of forbidden transitions are calculated in the lower energy region. Both allowed and forbidden transitions are characterized as the electron‐transfer excitations from oxygen to metal. In comparison with the previous theoretical studies, the present SAC‐CI results are in good agreement with experiment and give reliable assignments of the spectrum. We also compare the electronic structures and spectra of CrO^{2−} _{4}, MoO^{2−} _{4}, MnO^{−} _{4}, RuO_{4}, and OsO_{4}, which have been studied by the SAC and SAC‐CI methods.