Volume 100, Issue 10, 15 May 1994
Index of content:

Infrared diode‐laser spectra of the ν_{9} and ν_{11} N–O stretching bands of N_{2}O_{4}
View Description Hide DescriptionThe rotationally resolved jet‐cooled infrared spectra of the b‐type ν_{9} (b _{2u }) fundamental band at 1757 cm^{−1} and the a‐type ν_{11} (b _{3u }) fundamental band at 1261 cm^{−1} of the N–O stretches of N_{2}O_{4} have been recorded with a diode laser. The ν_{9} band was found to be unperturbed, and it was possible to assign nearly 100% of the observed lines with a signal to noise greater than 2. In contrast, most of the K _{ a } states of the ν_{11} band were found to be strongly perturbed. A large number of strong lines (≊20%) are unassigned and presumably arise from the perturbing state as well as torsional hot band transitions. The rotational analysis yields precise spectroscopic constants for the ground vibrational state which are interpreted in terms of a planar centrosymmetric dimer with a N–N bond length of 1.756(10) Å. The observed nuclear‐spin statistical weights and near‐zero inertial defect are consistent with the planar centrosymmetric structure determined in earlier electron‐diffraction studies.

Dynamics and spectroscopy of higher bending vibrational levels of PH_{2}(Ã ^{2} A _{1})
View Description Hide DescriptionThe zero‐pressure lifetimes of the bending vibrational levels from v _{2} ^{’} = 5,6, and 7 of PH_{2}(Ã ^{2} A _{1}) have been measured and are found to be 720, 35 (^{ PQ } _{1}), 26 (^{ PP } _{1}), and 3 ns, respectively. The self‐quenching constants are (11.0 ± 0.9)×10^{−10} cm^{3} molecule ^{−1} s^{−1} for v _{2} ^{’} = 5 and (7.4 ± 1.4) [^{ PQ } _{1}] and (7.7 ± 1.4) [^{ PP } _{1}]×10^{−10} cm^{3} molecule^{−1} s^{−1} for v _{2} ^{’} = 6. Evidence of predissociation of PH_{2}(Ã ^{2} A _{1}) is shown by the sudden drop of its lifetime at v _{2} ^{’} = 6, which however should start at v _{2} ^{’} = 4. A model of indirect predissociation mechanism has been proposed. Composite emissions due to simultaneous excitation of v _{2} ^{’} = 5 and v _{2} ^{’} = 6 and resulting in biexponential fluorescence decay curves have been identified in the laser‐induced fluorescence(LIF) excitation spectrum of v _{2} ^{’} = 5 where the spectroscopic assignment concerning some rotational lines and reported in literature has also been found to likely be incorrect. Besides, the origin of some unassigned peaks recorded in the same spectrum has been found, through fluorescence decay studies, to itself be v _{2} ^{’} = 5.

Fine‐structure dependence of predissociation linewidth in the Schumann–Runge bands of molecular oxygen
View Description Hide DescriptionFine‐structure‐specific predissociationlinewidths have been calculated for the v=0–18, N=0–40 levels of the B ^{3}Σ^{−} _{ u } state of ^{16}O_{2} using a predissociationmodel which takes into account spin–orbit interactions between the B ^{3}Σ^{−} _{ u } state and the 1 ^{1}Π_{ u }, 1 ^{3}Π_{ u }, 1 ^{5}Π_{ u }, and 2 ^{3}Σ^{+} _{ u } states, and spin‐electronic and L‐uncoupling interactions between the B ^{3}Σ^{−} _{ u } and 1 ^{3}Π_{ u } states. The model parameters were optimized by comparing the calculated widths with existing measurements of fine‐structure linewidth for v=0, 14–18, and new determinations for v=1, 2, 5, 7, 9–13 which have been obtained from existing cross sections for the (v,0) and (v,1) Schumann–Runge bands using a fitting procedure which assumes unequal linewidths for the triplet fine‐structure components. The L‐uncoupling interaction is found to be important for all vibrational levels in determining the width ratios for the fine‐structure components, even at quite low rotational levels. The calculated linewidths vary from 0.03 (for v=16, N=30, F _{1}) to 4.8 cm^{−1} (for v=4, N=40, F _{3}), and good agreement is found between the measurements and the model calculations for most vibrational levels. The calculations are expected to be useful in the construction of state‐of‐the‐art models describing the photochemistry of the middle atmosphere. Greatly improved predissociationmodel parameters, in good agreement with ab initio calculations, have been obtained for the ^{3}Π_{ u } and ^{1}Π_{ u } states, while the ^{3}Σ^{+} _{ u } parameters remain the most uncertain. The model parameters resulting in the best fit to the widths also give good agreement with observed perturbations in the Schumann–Runge band origins. It is found that the actual separations between triplet fine‐structure levels consistently exceed values predicted from generally accepted spectroscopic constants for the B ^{3}Σ^{−} _{ u } state, suggesting that a reevaluation of those constants may be in order.

Photodissociation dynamics of a hydrogen molecule via superexcited states as studied through the angular‐momentum population of an excited hydrogen atom
View Description Hide DescriptionPhotodissociation dynamics of a hydrogen molecule via vibrationally excited superexcited states has been studied through the angular‐momentum population of an excited hydrogen atom with the principal quantum number 3. The population shows a strong dependence on the incident photon wavelength in the longer wavelength region and a weak dependence in the shorter wavelength region. The population seems to converge to the equal partition as the incident photon wavelength becomes shorter. It is shown that the Rosen–Zener‐type transition between the three states, not two states, at a large internuclear distance plays an important role in the dissociation dynamics. A close multilevel problem should be remarked even for a simple system such as a hydrogen molecule.

A model for the energy levels of rare gas–spherical top van der Waals complexes
View Description Hide DescriptionA model for the rovibrational energy levels of a complex formed between a rare gas atom and a spherical top molecule is developed, specifically for a tetrahedral XY _{4} molecule in its ground and its triply degenerate vibrational states. Under the assumption that the tetrahedral molecule remains undistorted upon complexation, a large amount of the tetrahedral symmetry is retained in the complex. Using symmetry adapted terms to represent the anisotropy of the intermolecular potential, the perturbations to the energy levels of the tetrahedral molecule caused by the intermolecular potential have been considered. Correlation diagrams showing the pattern of these internal rotor states between the free rotor and rigid molecule limits have been constructed.

Spectroscopy and dynamics of rare gas–spherical top complexes. The infrared spectrum of the ν_{3} band of argon–silane
View Description Hide DescriptionThe infrared spectrum of Ar–SiH_{4}, a rare gas–spherical top van der Waals complex, has been recorded in the vicinity of the silane ν_{3} triply degenerate stretching vibration ∼2189 cm^{−1}. A complex, dense spectrum is observed, and this is due to the observation of 12 bands with origins corresponding to transitions between different internal rotor states of the silane molecule within the complex. From the analysis of the rotational structure within each band, the average argon–silane separation is determined to be 4.043 Å in the ground vibrational state and 4.046 Å in the excited vibrational state with v _{3}=1. Using a model developed previously and a very simple form for the intermolecular potential, these 12 bands have been assigned and an effective anisotropicintermolecular potential for the internal rotation of the silane molecule within the complex has been determined. The vibrational anisotropy responsible for lifting the threefold degeneracy of the ν_{3} vibration is found to be fairly small, of the order of 1 cm^{−1}, while the rotational anisotropy is much larger, of the order of 90 cm^{−1}.

High resolution electronic spectroscopy of p‐toluidine. A precessing rotor model for G _{12} molecules
View Description Hide DescriptionBased on a study of the high resolution S _{1}←S _{0}fluorescence excitation spectrum of p‐toluidine (p‐methylaniline) and related G _{12} molecules, we propose that the threefold axis of the methyl group is tilted slightly with respect to the symmetry axis of the molecular frame, and exhibits a kind of precessional motion in the course of its hindered internal rotation. We derive a new Hamiltonian to describe this motion and show that it is consistent with previous modifications of the traditional torsion–rotation Hamiltonian first proposed by Wilson, Lin, and Lide [J. Chem. Phys. 23, 136 (1955)]. Applying the new Hamiltonian to the S _{1}←S _{0}spectrum of p‐toluidine, we have determined the sixfold barrier heights V _{6}(S _{0}) = ( − ) 5.6 and V _{6}(S _{1}) = ( − ) 43.9 cm^{−1}, values that are similar to those of toluene and other 4‐substituted toluenes.

Photoinduced charge transfer in bichromophoric molecules in the gas phase
View Description Hide DescriptionWe have studied a series of bichromophores of the form A‐(CH_{2})_{ n }‐D where A is a 9‐anthryl group, D is either a N‐methylanilino group or a N‐methyl‐p‐methoxyanilino group, and n=1,2,3, or 4. The fluorescence excitation and emission spectra of these molecules have been observed in a supersonic jet and in a variety of solvents. In the gas phase, in the n=1 and n=3 molecules, and in an extended conformer of the n=4 molecules interactions are weak resulting only in a slightly redshiftedanthracenespectrum. No redshifted emission was observed in these cases. For the n=2 molecules and a second conformer of the n=4 molecules we observed a redshifted emission in the gas phase. This emission is assigned as coming from a charge transfer(CT) state or exciplex. On the basis of the excitation and emission spectra we observed two different pathways to reach the CT state. For the molecules with donor N‐methyl‐N‐alkylaniline, excitation took place to the locally excited (LE) state followed by relaxation to the CT state. For the molecules with donor N‐methyl‐N‐alkyl‐p‐methoxyaniline, the CT state was low enough to interact with the ground state forming an intramolecular electron–donor–acceptor (EDA) complex. From this EDA ground state direct excitation to the CT state occurred. In the molecule with n=2 and donor N‐methyl‐p‐methoxyaniline the EDA interaction is very weak allowing excitation to the LE state as well as to the CT state. This set of molecules exhibits the full range of spectroscopic behavior expected in bichromophores.

Magneto‐infrared spectra of the Si_{2}, Ge_{2}, and Sn_{2} molecules in rare‐gas matrices
View Description Hide DescriptionAbsorption spectra of Si_{2}, Ge_{2}, and Sn_{2}, observed between 50 and 5000 cm^{−1} in neon and argon matrices at 4 K, were assigned to the lowest ^{3}Π_{ u }←X ^{3}Σ_{ g } ^{−} electronic transition. In all three cases, the Franck–Condon envelope of vibrational bands allowed T _{00}, ω_{ e } ^{’}, ω_{ e } ^{’} x _{ e } ^{’} to be determined. In the Si_{2} molecule [near Case (a)] T _{00}=313 cm^{−1} and each vibrational band was split into Ω triplets (0,1,2) in the upper state. A Zeeman effect on these lines was detected in magnetic fields up to 4 T. As the spin–orbit coupling becomes larger in Ge_{2} and Sn_{2}, the 1_{ u }←X0^{+} _{ g } transition [case (c)] was observed, and the corresponding parameters in the upper state determined. For Sn_{2} the magnetic‐dipole 1_{ g }←X0^{+} _{ g } transition was also detected and identified by the splitting and broadening of the band in magnetic fields up to 4 T. The results are discussed in the context of the ab initio calculations and other experimental results on these molecules.

Anomalous complex shift of low‐frequency out‐of‐plane vibrations in aniline‐M van der Waals complexes (M=He, Ne, Ar)
View Description Hide DescriptionFluorescence excitation vibronic spectra of aniline‐M (M=He, Ne, Ar) van der Waals (vdW) complexes have been measured in the spectral region from 300 to 500 cm^{−1} above S _{1}−S _{0} origin. Anomalous behavior of the vdW shifts are observed for two out‐of‐plane vibrational modes. The resulting vdW vibrational shifts for the ν_{16a } vibration are as large as 26.5 cm^{−1}. Similarity with shifts which have already been reported for other aromatic molecules, like pyrimidine [H. Abe, Y. Ohyanagi, M. Ichijo, N. Mikami, and M. Ito, J. Phys. Chem. 89, 3512 (1985)] and tetrazine [D. V. Brumbaugh, J. E. Kenney, and D. H. J. Levy, J. Chem. Phys. 78, 3415 (1983); P. M. Weber and S. A. Rice, J. Chem. Phys. 88, 6120 (1988)] are pointed out.

A comparison of photoelectron spectroscopy and two‐photon ionization spectroscopy: Excited states of Au_{2}, Au_{3}, and Au_{4}
View Description Hide DescriptionPhotoelectron spectra of Au^{−} _{ n } with n=2–4 are reported. Due to the relatively high photon energy used in our experiment (hν=6.424 eV) and the energy resolution of about 50 meV, various transitions into excited states of the neutral clusters are resolved. It is demonstrated that photoelectron spectra can serve as a map of the electronic states of a cluster, while the high resolution of the resonant two‐photon ionization (R2PI) method gains information about the symmetry of the states. The comparison with similar data of Ag^{−} _{ n } clusters indicates the influence of relativistic effects and the large spin–orbit splitting for Au.

Investigation of the ground vibrational state structure of H^{35}Cl trimer based on the resolved K, J substructure of the ν_{5} vibrational band
View Description Hide DescriptionThe high resolution rovibrational IR spectrum of the antisymmetric H^{35}Cl stretching ν_{5} vibrational band in the (H^{35}Cl)_{3} isotopomer of the trimer has been measured with a tunable infrared diode laser supersonic jet spectrometer. One of the most prominent features of the spectrum is a series of strong lines each formed with ^{ pP } or ^{ rR } transitions, with resolved K, J substructure developing between two adjacent ^{ pP } or ^{ rR } heads. Analyzing the spectrum of (H^{35}Cl)_{3} as the perpendicular band of an oblate symmetric rovibrator, the following molecular parameters have been obtained: ν_{0}−C’ζ=2809.776 98(6) cm^{−1}, B‘=6.804 14(55)×10^{−2} cm^{−1}, B’=6.859 43(55) ×10^{−2} cm^{−1}, C’–C‘=2.737(78)×10^{−4} cm^{−1}, D _{ JK } ^{‘}=−8.40(38)×10^{−7} cm^{−1}, D _{ JK } ^{’}=−8.14(38)×10^{−7} cm^{−1}, D _{ J } ^{‘}=4.26(16)×10^{−7} cm^{−1}, D _{ J } ^{’}=4.16(16)×10^{−7} cm^{−1}, D _{ K } ^{‘}=4.32(22)×10^{−7} cm^{−1}, D _{ K } ^{’}=4.16(22)×10^{−7} cm^{−1}. A relationship among the centrifugal distortion constants establishes that the geometry of (H^{35}Cl)_{3} is consistent with a dynamically averaged planar ground vibrational state. The centers of mass of the H^{35}Cl components are separated by 3.693(1) Å in this structure. Ab initio methods were also used to estimate the splitting due to the tunneling motions between clockwise (cw) and counterclockwise (ccw) identical structures. Both theoretical and experimental evidence indicate that the (HCl)_{3} complex has a planar equilibrium structure with no observable tunneling probability between the cw and ccw forms, thus the molecular symmetry group of the complex is proposed to be C _{3h }(M).

Dipole‐induced dipole light scattering in supercooled liquids near the liquid–glass transition
View Description Hide DescriptionThe integrated intensity produced by the dipole‐induced dipole (DID) light scattering mechanism in a Lennard‐Jones fluid is investigated for different points of the phase diagram corresponding to the normal and undercooled liquid at zero pressure. The exactly computed intensity is compared to the full Kirkwood superposition approximation (KSA) and its so‐called Stephen approximation. The latter gives one or two orders of magnitude too large results, while the former is in much better agreement with the exact computation up to large reduced density values. On the basis of the mode coupling theory of the glass transition, it is argued that, in the vicinity of this transition, in real glass forming isotropic molecular liquids for which DID is the only light scattering mechanism, both a dynamical extension of the KSA and its Stephen approximation should yield dynamical spectra proportional to the real ones.

Absolute transition probability measurement of valence‐shell electronic structure of CF_{4} and CCl_{4}: Bethe surfaces, and dipole‐dominated preionization‐edge valence and Rydberg states
View Description Hide DescriptionAbsolute generalized transition probabilities or generalized oscillator strengths (GOSs) of valence‐shell electronic transitions of CF_{4} and CCl_{4} have been determined as functions of energy loss and momentum transfer (i.e., the Bethe surfaces) using angle‐resolved electron energy loss spectroscopy(EELS) at an impact energy of 2.5 keV. A low‐lying feature at 7.2 eV was observed for CCl_{4} and was attributed predominantly to electronic transitions from the Cl 3p nonbonding orbitals (2t _{1}, 7t _{2}, and 2e) to a C–Cl σ* antibonding orbital (7a _{1}), based on the result of a single‐excitation configuration interaction excited‐state calculation. The experimental GOS profile of this low‐lying feature was found to have a shape characteristic of a mixture of dipole‐allowed and nondipole transitions with relative maxima at momentum transfers of 0 and ∼0.9 a.u., respectively. GOS profiles of other low‐lying discrete transitions below the first ionization edge at 12.6 and 13.7 eV in CF_{4} and at 8.7 and 9.7 eV in CCl_{4} were also determined and found to have secondary minima and maxima, in addition to the strong maximum at zero momentum transfer. These excitation features were assigned mainly to Rydberg transitions originated from the nonbonding highest occupied molecular orbitals (HOMOs) with t _{1} symmetry and the second HOMOs with t _{2} symmetry. The extrema in the GOS profiles have been discussed by considering the spatial overlap of the initial‐state and final‐state orbital wave functions. Tentative assignments for the rest of the valence‐shell energy‐loss features of CF_{4} (5–200 eV) and CCl_{4} (5–150 eV) were also inferred from the term values reported previously.

Vibrational autoionization in H_{2} above the v ^{+}=3 ionization limit
View Description Hide DescriptionWe report multichannel quantum defect theory calculations of vibrational branching ratios and photoelectron angular distributions for photoionization of an ortho/para equilibrium mixture of H_{2} at 300 K, between wavelengths 762.5 and 765.0 Å. We compare our results with the recent synchrotron measurement of these observables by Dehmer et al. [J. Chem. Phys. 97, 7911 (1992)], and the total photoionization cross sections measured by Dehmer and Chupka [J. Chem. Phys. 65, 2243 (1976)]. Except in the region of autoionizing resonances which compete with a dissociation channel, semiquantitative agreement between calculations and experiment is obtained for branching ratios and angular distributions for the v ^{+}=0–2 levels, and for the v ^{+}=3 branching ratio. However, the level of agreement between the present calculations and the synchrotron measurements is significantly worse than that obtained for the total photoionization cross section.

Pariser–Parr–Pople force field for π‐electrons: Raman and infrared shifts of trans‐polyacetylene
View Description Hide DescriptionThe transfer integrals t(R) and Coulomb potential V(R) of π‐electron Hamiltonians H _{ e } define linear electron–phonon (e–ph) coupling constants t’(R) and V’(R) for the equilibrium structure. We generalize linear response (LR) theory for Raman and ir shifts due to π‐electron delocalization in Hamiltonians with arbitrary t(R) and spin independent V(R). π‐electron contributions ΔF _{ ij } to the force field of trans‐polyacetylene (PA) are obtained in the symmetry coordinates S _{ i }, with i=1–5, for k=0 phonons. We compare ΔF _{ ij } for Hückel chains with alternating transfer integrals t(1±δ) and for Pariser–Parr–Pople (PPP) models with hydrocarbon parameters derived from π–π* spectra to a phenomenological ΔF _{ ij } for trans‐PA and its isotopes. An exponential rather than linear t(R) is found. The molecular PPP potential V(R) accounts quantitatively for π‐electron coupling to CCC bends and for the length dependence of the Raman shifts of finite polyenes. The dominant but not exclusive π‐electron coupling remains the dimerization coordinate singled out in previous treatments, with substantially larger t’(R) than V’(R) contributions in the PPP force field. We comment on extensions of LR theory to PA models with interacting π‐electrons and several electronic susceptibilities.

Vibrational spectroscopy of NO^{+}(H_{2}O)_{ n }: Evidence for the intracluster reaction NO^{+}(H_{2}O)_{ n }→H_{3}O^{+}(H_{2}O)_{ n−2} (HONO) at n ≥ 4
View Description Hide DescriptionInfrared spectra of mass‐selected clusters NO^{+}(H_{2}O)_{ n } for n=1 to 5 were recorded from 2700 to 3800 cm^{−1} by vibrational predissociationspectroscopy. Vibrational frequencies and intensities were also calculated for n=1 and 2 at the second‐order Mo/ller–Plesset (MP2) level, to aid in the interpretation of the spectra, and at the singles and doubles coupled cluster (CCSD) level energies of n=1 isomers were computed at the MP2 geometries. The smaller clusters (n=1 to 3) were complexes of H_{2}O ligands bound to a nitrosonium ion NO^{+} core. They possessed perturbed H_{2}O stretch bands and dissociated by loss of H_{2}O. The H_{2}O antisymmetric stretch was absent in n=1 and gradually increased in intensity with n. In the n=4 clusters, we found evidence for the beginning of a second solvation shell as well as the onset of an intracluster reaction that formed HONO. These clusters exhibited additional weak, broad bands between 3200 and 3400 cm^{−1} and two new minor photodissociation channels, loss of HONO and loss of two H_{2}O molecules. The reaction appeared to go to completion within the n=5 clusters. The primary dissociation channel was loss of HONO, and seven vibrational bands were observed. From an analysis of the spectrum, we concluded that the n=5 cluster rearranged to form H_{3}O^{+}(H_{2}O)_{3}(HONO), i.e., an adduct of the reaction products.

Equilibrium structures and approximate HF vibrational red shifts for Ar_{ n }HF (n=1–14) van der Waals clusters
View Description Hide DescriptionThis paper presents a theoretical study of the size evolution of equilibrium structures and approximate HF vibrational red shifts for Ar_{ n }HF van der Waals clusters, with n=1–14. Pairwise additive Ar_{ n }HF intermolecular potential energy surfaces were constructed from spectroscopically accurate Ar–Ar and anisotropic Ar–HF potentials. The latter depend on vibrational excitation of the HF monomer. The global and energetically close‐lying local minima of Ar_{ n }HF, n=1–14, for HF v=0 and v=1, were determined using simulated annealing followed by a direct minimization scheme. For Ar_{ n }HF clusters with n≤8, the lowest‐energy structure always has HF bound to the surface of the Ar_{ n } subunit. In contrast, for n≥9, the global minimum of Ar_{ n }HF corresponds to HF inside a cage. Ar_{12}HF has the minimum‐energy configuration of an HF‐centered icosahedron, which appears to be unusually stable. Size dependence of the HF vibrational red shift in Ar_{ n }HF (n=1–14) clusters was investigated by means of a simple approximation, where the red shift was represented by the energy difference between the global minima of a cluster obtained for HF v=0 and v=1, respectively. The approximation reproduced rather accurately the experimentally determined variation of the Ar_{ n }HF red shift with the number of Ar atoms, for n=1–4, although it overestimated their magnitude. For larger Ar_{ n }HF clusters, 4<n≤14, a nonmonotonic, step‐like dependence of the red shift on the cluster size is predicted, which can be interpreted in terms of changes in the minimum‐energy cluster geometries. The predicted red shift for the icosahedral Ar_{12}HF, where the first solvation shell is full, is 44.70 cm^{−1}, which is only 5.4% higher than the experimental HF vibrational red shift in an Ar matrix, of 42.4 cm^{−1}.

Kinetics of optically detected magnetic resonance of single molecules
View Description Hide DescriptionVarious time‐resolved experiments on optically detected magnetic resonance(ODMR) of single pentacene molecules in p‐terphenyl at 1.8 K are described and discussed with the help of a model based on the optical‐microwave Bloch equations. Intersystem crossing rates for this system are deduced from the ODMRspectra, from the fluorescencecorrelation function, which varies nonmonotonically with the microwave power, and from the analysis of the fluorescence recovery transients. We also discuss the enhancement of the ODMR effect on using fluorescencephotons from single molecules as a time base to trigger the microwaves in fluorescence recovery experiments.

The accuracy of the quasiclassical Landau–Lifshitz formula for matrix elements and its application to the analysis of the intensities of vibrational overtone transitions
View Description Hide DescriptionIntensities of vibrational overtone transitions have been analyzed on the basis of the quasiclassical Landau–Lifshitz formula including the pre‐exponential factor derived previously. The accuracy of this formula is shown to be very high, on the order of a few percent. An unusual feature of the Landau–Lifshitz formula is the prediction that the overall rapid decrease in intensity with overtone number is highly sensitive to the behavior of the analytical continuation of the molecular potential into regions where the amplitudes of the wave functions are negligible. The dipole moment enters the slowly varying prefactor, which in general superimposes only small modulations on the above steep intensity falloff. However, this prefactor may go through zero, in which case the intensities of one or two transitions nearest the prefactor’s zero are anomalously low. These results imply that knowledge of the analytical continuation of the potential is crucial for extracting meaningful information about the dipole moment from overtone intensities.