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Volume 97, Issue 11, 01 December 1992

Velocity relaxation of S(^{1} D) by rare gases measured by Doppler spectroscopy
View Description Hide DescriptionVelocity relaxation of S(^{1} D) by He, Ar, and Xe has been monitored by measuring the Doppler profile of the S(^{1} D) for variable collision partner pressures at a fixed time delay following creation of S(^{1} D) by pulsed laserphotolysis of OCS at 222 nm. The nascent S(^{1} D) has a mean speed about three times that at room temperature and an effective anisotropy parameter of β=0.5. A calculation assuming elastic hard‐sphere collisions is performed to model the process. The data are in qualitative agreement with the model in that the angular distribution relaxes more rapidly with collision number as the mass ratio between the collision partner and sulfur approaches infinity, whereas the speed distribution relaxes more rapidly as the mass ratio approaches one. Helium behaves as predicted by the hard‐sphere model with a collision cross section of σ_{col}=26±2 Å^{2}. However, the cross sections for argon and xenon are found to depend on the collision energy. The dependence allows an estimation of the following Lennard‐Jones parameters: for argon σ=3.6±0.5 Å and ε=2.5±0.5 kJ/mol; for xenon σ=3.9±0.5 Å and ε=3.9±0.8 kJ/mol.

Rotational spectrum, molecular structure, and ^{201}Hg nuclear quadrupole coupling constants of HCNHg
View Description Hide DescriptionHigh resolution rotational spectra for 21 isotopic species of the hydrogen cyanide–mercury complex have been measured in the 7–15 GHz region using a Balle/Flygare pulsed Fourier transform microwave spectrometer. Rotational constants and ^{14}N nuclear quadrupole coupling constants are (in MHz), for HC^{14}N^{202}Hg:B _{0}=1265.6627(7), D _{ J }=0.020 669(14), χ^{N}=−0.695(12), and for DC^{14}N^{202}Hg:B _{0}=1201.8338(16), D _{ J }=0.021 213(45), and χ^{N}=−0.748(19). The ^{201}Hg nuclear quadrupole coupling constants are, for HC^{15}N^{201}Hg: −5.97(13) MHz, and for DC^{15}N^{201}Hg: −6.03(24) MHz. The atomic arrangement is HCN...Hg with a zero‐point vibrationally averaged center‐of‐mass separation of 4.05 Å for the most abundant HC^{14}N^{202}Hg species. The equilibrium structure was not determined. HCNHg does not exhibit the extreme centrifugal distortion and isotopic sensitivity effects seen in ArHCN [K. R. Leopold, G. T. Fraser, F. J. Lin, D. D. Nelson, Jr., and W. Klemperer, J. Chem. Phys. 81, 4922 (1984)].

Transient hole‐burning and time‐resolved fluorescence spectra of dye molecules in solution: Evidence for ground‐state relaxation and hole‐filling effect
View Description Hide DescriptionA picosecond transient hole‐burning (THB) spectroscopy has been performed for organic dyes in solution. The THB spectra of rhodamine 640 have been found to show a time‐dependent spectral change. This phenomenon corresponds to the solvent relaxation effect observed in the time‐resolved fluorescence (TRF) spectrum. Although TRF spectrum is related only to the excited‐state relaxation, THB spectrum is affected by both ground‐ and excited‐state relaxations. Comparing with the TRF spectrum measured under the same exciting energy, we have clarified the presence of the ground‐state relaxation. Further, the THB spectrum of styryl‐8 gives an antihole between well‐separated two holes corresponding to ground‐ and excited‐state contributions. This may originate from the hole‐filling effect due to the nonadiabatic relaxation from the nonequilibrium excited state to the ground state. The analysis based on a configuration coordinate model gives an overall understanding for these phenomena.

Potential energy surfaces of substituted anilines: Conformational energies, deuteration effects, internal rotation, and torsional motion
View Description Hide DescriptionMass resolved excitation spectra (MRES) are presented for a series of substituted anilines including 2‐ and 3‐methylaniline, 2‐ and 3‐ethylaniline, 2‐aminobenzyl amine, and 2‐aminobenzyl alcohol. The observed spectra show the following phenomena: nearly free internal rotation of the methyl substituent in the S _{1} state; long vibrational progressions attributed to C–C, C–N, and C–O side chain torsional motion; an inequivalence of the two amino hydrogens for both ring and side chain amino groups as determined from the spectra of deuterated species; and the existence of two conformers for 2‐aminobenzyl alcohol. Semiempirical and ab initio calculations are performed on these systems to aid in the analysis of the potential energy surfaces and in the interpretation of the experimental results.

Persistent spectral hole‐burning, luminescence line narrowing and selective excitation spectroscopy of the R lines of Cr(III) tris(2,2’‐bipyridine) in amorphous hosts
View Description Hide DescriptionThe distributions of the ^{2} E and the ^{4} A _{2} splittings of Cr(III) tris(2,2’‐bipyridine) embedded in a wide range of amorphous hosts are investigated by means of luminescence line narrowing and hole‐burning spectroscopies. A large spread of the ^{2} E splitting is observed for all amorphous matrices and this is attributed mainly to low symmetry field contributions. A simple model is proposed which rationalizes the observed non‐Gaussian distribution of R _{2} energies as a function of the selected R _{1} energy. The zero field splitting of the ^{4} A _{2}ground state also shows a significant variation in all the investigated samples. An important conclusion is that optically detected magnetic resonance and electron paramagnetic resonance measurements of transition metal complexes in amorphous hosts need to be interpreted with care. The homogeneous linewidth of the R _{1} line was measured at 1.5 K for five hosts and ranged from 80 MHz in glycerol to 460 MHz in 4:1 ethanol/methanol.

Vibrational autoionization in H_{2}: Vibrational branching ratios and photoelectron angular distributions near the v ^{+}=3 threshold
View Description Hide DescriptionVibrational branching ratios and photoelectron angular distributions are reported for photoionization of normal (the equilibrium ortho/para mixture) hydrogen in the region just above the v ^{+}=3 ionization limit. This region contains a number of vibrationally autoionizing Rydberg states, and the goal of the work was to observe experimentally the characteristic behavior associated with the decay of these states that was predicted by Raoult and Jungen [J. Chem. Phys. 74, 3388 (1981)] on the basis of an MQDT (multichannel quantum defect theory) calculation on para‐H_{2} (J‘=0). We have indeed observed that vibrational autoionization strongly favors the ionization channel representing the minimum change in the vibrational state of the ion core, as predicted. We also observed a sharp reduction in the photoelectron asymmetry of the v ^{+}=3 (and, to a lesser extent, the v ^{+}=2) ionization channel for resonant photoionization. Hence, qualitative agreement with theory is observed; however, a quantitative comparison now requires an extended calculation that includes the additional rotational levels that were populated under the present experimental conditions.

Vibrational splitting in Si 2p core‐level photoelectron spectra of silicon molecules
View Description Hide DescriptionHigh‐resolution (∼0.1 eV) Si 2p gas phase photoelectron spectra of the following twenty‐three siliconcompounds have been recorded: SiH_{ XD } _{4−X }; Si(CH_{3})_{ X }(OCH_{3})_{4−X }; Si(CH_{3})_{ X }[N(CH_{3})_{2}]_{4−X }; SiH_{ X }[Si(CH_{3})_{3}]_{4−X } (where x=0–4), and SiH_{3}–CH_{3}, SiH_{3}–SiH_{3}, SiH_{3}–SiH_{2}–SiH_{3}, Si(CH_{3})_{3}–Si(CH_{3})_{3}, Ge[Si(CH_{3})_{3}]_{4}, and [Si(CH_{3})_{2}]_{6}. Vibrational fine structure has been resolved in the Si 2pspectra of the five SiH_{ X }D_{4−X } (x=0–4) compounds, methyl silane (H_{3}C–SiH_{3}), disilane (H_{3}Si–SiH_{3}), and trisilane (SiH_{3}–SiH_{2}–SiH_{3}). For the five mixed hydrogen/deuterium compounds and methyl silane, the Si 2p vibrational structure is determined by the totally symmetric Si–H, Si–D, or Si–C stretching vibrational mode. In contrast, the spectra of disilane and trisilane are dominated by the nontotally symmetric Si–H bending vibrations—the first example of this in core‐level photoelectron spectroscopy. This unusual vibrational effect is interpreted in terms of vibronic coupling that results from core‐hole localization in the ion states of molecules such as disilane and trisilane which have equivalent cores. In the remaining compounds the vibrational effects are not well resolved. However, the Si 2p peak widths increase in the order Si(CH_{3})_{4}≤Si[Si(CH_{3})_{3}]_{4}≤Si[N(CH_{3})_{2}]_{4}≤Si(OCH_{3})_{4}≤SiF_{4} showing that the size of the vibrational manifold increases in the same order. The Si 2pphotoelectron spectra of the series SiH_{ x }[Si(CH_{3})_{3}]_{4−x } mimic the chemical shifteffects of zero, one, two, and three hydrogens bonded to a silicon surface. Vibrational effects must now be considered for adsorbate systems such as H adsorbed on a silicon surface. The Si 2pspectra are also used to predict the overall C 1slinewidth of organic analogs in the gas phase and in polymers.

Ligand‐field splittings and core‐level linewidths in I 4d photoelectron spectra of iodine molecules
View Description Hide DescriptionHigh‐resolution I 4dphotoelectron spectra (total instrumental resolution ∼0.06 eV) of seven simple iodine molecules, ICl, IBr, I_{2}, HI, CH_{3}I, CH_{2}I_{2}, and CF_{3}I, have been recorded. Ligand‐field splitting (as measured by the asymmetric C _{2} ^{0} crystal‐field term) splits the 4d _{3/2} and 4d _{5/2} lines into a doublet and triplet, respectively. In contrast to Si 2p spectra, the I 4d spectra generally show little evidence for vibrational splitting or broadening. The C _{2} ^{0} values, like the binding energies E _{4d }, generally increase as the ligand electronegativity increases. C _{2} ^{0} also correlates well with the nuclear field gradient, eq _{ n }, as measured by Mössbauer spectroscopy or nuclear quadrupole resonance. For the first time we have been able to study the chemical dependence of the inherent lifetime width of a core hole. We show that the I 4dlinewidths generally decrease as the electronegativity of the ligand increases, and as the valence I 5p electron density decreases. We also show that there are large relaxation effects on E _{4d }, C _{2} ^{0} and especially the linewidth.

Two‐dimensional exchange nuclear magnetic resonance of powder samples. IV. Distribution of correlation times and line shapes in the intermediate dynamic range
View Description Hide DescriptionThe two‐dimensional (2D) exchange nuclear magnetic resonance(NMR) experiment is applied to study ultraslow as well as faster chain motions in amorphouspolymers in the glass transition range. Acquisition of the time domain data with a four‐pulse sequence leads to new characteristics in the corresponding ^{2}H 2D line shapes if the correlation times of the motion are in the intermediate dynamic range. From the asymmetric 2D line shapes, the width of the correlation time distribution can be determined with higher accuracy than through conventional 1D NMR methods. Experimental data are presented on two amorphous polymers—atactic polypropylene and cis‐1,4‐polyisoprene—and are analyzed in terms of isotropic rotational diffusion. Deviations from this simple model due to the presence of conformational transitions within the polymer backbone are detected. Close to T _{ g }, the mean correlation times extracted from 2D exchange NMR exhibit strongly nonArrhenius behavior usually described by the Williams–Landel–Ferry (WLF) equation. In addition, the width of the correlation time distribution is found to decrease with increasing temperature.

Infrared profile of van der Waals dimers HCl–RG* (RG*=Ar, Kr, Xe) trapped in rare gas matrices
View Description Hide DescriptionThe infrared profile of the van der Waals dimers HCl–RG* trapped in a ternary matrix containing small molar ratio of hydracids (≤1/1000) and of rare gas dopants (≂1/10) is theoretically investigated. The homogeneous profile of a single dimer is determined, using a conventional cumulant expansion of the coupling Hamiltonian between a renormalized optical system described by the vibration‐orientation Hamiltonian of the trapped molecule and a renormalized bath defined by the lattice modes and the molecule and dopant center of mass motions. The influence of the ternary matrix inhomogeneities is also studied within the framework of a stochastic model which describes the interaction between the various species in the solid. A detailed analysis of the homogeneous and inhomogeneous broadenings of the infrared lines of these dimers is done on the basis of relatively accurate interaction potentials, previously determined. The comparison of the infrared profiles with recent FTIRspectroscopic data shows a fair agreement in most cases.

High‐resolution infrared overtone spectroscopy of ArHF via Nd:YAG/dye laser difference frequency generation
View Description Hide DescriptionThe first high‐resolution spectra of ArHF excited to the v _{HF}=2←0 manifold near 7800 cm^{−1} are recorded via direct infrared absorption in a slit supersonic expansion. The tunable difference frequency light is generated via nonlinear subtraction of a cw Nd:YAG laser from a tunable cw ring dye laser in temperature phase matched LiNbO_{3}, and permits continuous single‐mode access to the 1–2 μm near‐IR region. Rotationally resolved spectra are presented for the pure HF stretching overtone (2000)←(0000), as well as for combination band excitation into the Σ bend (2100)←(0000) and Π bend (2110)←(0000) internal rotor levels built on the v _{HF}=2 overtone stretch. Local perturbations in the Π bend spectrum are observed which arise from a resonant crossing of rotational levels with the (2002) van der Waals stretch and allow spectroscopic analysis of this state. Nonresonant coupling between the Σ and Π bend vibrational levels is evidenced by anomalous P branch/R branch transition intensities and is analyzed as Coriolis interactions in a tumbling, hindered rotor. The spectra reveal Doppler limited line shapes [Δν=79(11) MHz] characteristic of the temperature and geometry of the slit expansion. An upper limit of Δν_{prediss}≤2 MHz Lorentzian broadening is established, indicating an 80 ns lower limit to the predissociation lifetime. Comparison of intermolecular vibrational levels in ArHF v _{HF}=0, 1, and 2 indicates a systematic increase in both angular anisotropy and radial well depth upon excitation of the high‐frequency HF stretch. In conjunction with previous results from the v _{HF}=1 and v _{HF}=0 vibrational levels, these studies provide the necessary data for fitting an atom+diatom potential energy surface as a function of all intermolecular and intramolecular internal degrees of freedom.

Measurement of the Kerr constant for H_{2} and D_{2} gases
View Description Hide DescriptionThe Kerr electro‐optic constant of deuterium and hydrogen gases have been measured at room temperature and atmospheric pressure for λ=632.8 nm, by using a very sensitive ellipsometrycal technique. A significant difference between the Kerr constants of the two gases has been found.

Nonclassical structures in silicon‐containing molecules: The monobridged isomer of Si_{2}H_{2}
View Description Hide DescriptionA monobridged isomer of the disilyne Si_{2}H_{2}, the planar bridged‐2 form Si(H)SiH, has been detected through its millimeter‐ and submillimeter‐wave rotational spectrum. The molecules are produced in a low pressure, low power plasma in a mixture of SiH_{4} and Ar cooled at liquid nitrogen temperature. From the analysis of a total of 148 lines, a set of accurate rotational constants have been derived. Using the geometry predicted by ab initio calculations (see the preceding paper), the spectrum of Si(D)SiD has been predicted, observed, and analyzed. From the two sets of rotational constants, a preliminary molecular structure has been derived by fixing the Si_{1}–Si_{2}–bridged H angle to the ab initio value of 52.5°: Si_{1}–Si_{2}=2.119 Å; Si_{2}–bridged H=1.629 Å; Si_{2}–terminal H=1.474 Å; Si_{1}–Si_{2}–terminal H=157.5°.

The remarkable monobridged structure of Si_{2}H_{2}
View Description Hide DescriptionInspired by the observation of a monobridged structure of Si_{2}H_{2} by Cordonnier et al. via microwave spectroscopy (see the following paper), we have reinvestigated the Si_{2}H_{2} singlet state potential energy surface using large basis sets and extensively correlated wave functions. Coupled‐cluster single, double, and (perturbative) triple excitation methods [CCSD(T)] in conjunction with a triple‐zeta 2df (TZ2df ) basis set on silicon and a triple zeta with two sets of polarization (TZ2P) basis set on hydrogen predict that the monobridged Si(H)SiH structure is indeed a minimum; in fact, Si(H)SiH is the second most stable Si_{2}H_{2} isomer, as suggested by a recent theoretical study [B. T. Colegrove and H. F. Schaefer, J. Phys. Chem. 94, 5593 (1990)]. The predicted Si(H)SiH geometrical structure—which exhibits the shortest SiSi bond distance of any molecule characterized to date—and hence the rotational constants, as well as the quartic centrifugal distortion constants are in good agreement with the experimental data. We have located transition states between these pairs of minima—disilavinylidene H_{2}SiSi and monobridged Si(H)SiH; monobridged and dibridged Si(H_{2})Si; trans‐HSiSiH and monobridged. We predict Si(H)SiH to lie 8.7 kcal mol^{−1} above Si(H_{2})Si, with the transition state between them 3.7 kcal mol^{−1} higher. H_{2}SiSi is predicted to lie 11.6 kcal mol^{−1} above Si(H_{2})Si and the transition state barrier between H_{2}SiSi and Si(H)SiH is 2.4 kcal mol^{−1} above H_{2}SiSi. Predictions of absolute 0 K heats of formation for the various structures are presented.

High resolution electronic spectroscopy of Ar⋅OH and Ar⋅OD vibronic bands
View Description Hide DescriptionLaser‐induced fluorescence spectra with resolved rotational, fine, and hyperfine structure have been observed for the U bands of the Ã←X̃ electronic transition of the Ar⋅OH/D complex. The appearance of these spectra are very different from the previously analyzedA band system of the complex. Detailed analysis of the spectra reveals that their spectral differences are caused by changes in values of molecular parameters (due to the different regions of the potential surface sampled in the Ã state) and their resulting effect upon the coupling of the angular momenta in the complex. Numerical results are given for the molecular parameters in the Ã state levels examined in these experiments and the implication of these values for the geometry and the potential surface of the complex are discussed.

A spectroscopic puzzle in ArHF solved: The test of a new potential
View Description Hide DescriptionThe perturbed (v,b,K,n)=(1210),(1113)←(0000) band of ArHF is observed in a tunable laser/slit supersonic expansion spectrometer. The (1210) level correlates with j=2 rotation of the HF within the complex and therefore provides a test of high‐order terms in a Legendre expansion of the intermolecular potential. Transitions to (1113) are observed due to intensity sharing with (1210), induced by a strong homogeneous (J‐independent) perturbation that is analyzed quantitatively. The (1113) level has three quanta of Van der Waals stretch and thus probes the radial dependence of the potential close to the dissociation limit. The vibrational and rotational assignment is made possible by predictions based on the new H6(4,3,2) intermolecular potential of Hutson [J. Chem. Phys. 96, 6752 (1992)], which agree nearly quantitatively with experiment.

Quasiparticle exciton representation of frequency dispersed optical nonlinearities of conjugated polyenes
View Description Hide DescriptionThe frequency dispersion of χ^{(3)} of conjugated polyenes is calculated using equations of motion which provide an anharmonic (exciton)oscillator picture. Quantum confinement of the relative electron–hole motion is shown to play an important role in determining the magnitude of χ^{(3)}. The nature of the two‐photon resonance observed in two‐photon absorption and third harmonic generation is discussed, and the electroabsorption spectrum is calculated for a broad range of polyene sizes with up to 160 double bonds.

Real‐time dynamics of clusters. II. I_{2}X_{ n } (n=1; X=He, Ne, and H_{2}), picosecond fragmentation
View Description Hide DescriptionIn this second paper (II) of a series, we report our picosecond time‐resolved studies of the state‐to‐state rates of vibrational predissociation in iodine–rare gas (van der Waals) clusters. Particular focus is on the simplest system, I_{2}He, which serves as a benchmark for theoretical modeling. Comparisons with I_{2}Ne and I_{2}H_{2} are also presented. The results from measurements made in real time are compared with those deduced from linewidthmeasurements, representing a rare example of a system studied by both methods under identical conditions and excited to the same quantum (v ^{’} _{ i }) states. The discrepancies are discussed in relation to the origin of the broadening and preparation of the state. The rates as a function of v ^{’} _{ i } display a nonlinear behavior which is examined in relation to the energy‐gap law. The measured absolute rates and their dependence on v ^{’} _{ i } are compared with numerous calculations invoking classical, quantum, and semiclassical theories. In the following paper (III in this series), the cluster size of the same system, I_{2}X_{ n }, is increased (n=2–4) and the dynamics are studied.

Real‐time dynamics of clusters. III. I_{2}Ne_{ n } (n=2–4), picosecond fragmentation, and evaporation
View Description Hide DescriptionIn this paper (III) we report real‐time studies of the picosecond dynamics of iodine in Ne clusters I^{*} _{2}Ne_{ n }(n = 2–4) → I^{*} _{2} + nNe. The results are discussed in relation to vibrational predissociation (VP), basic to the I_{2}X systems, and to the onset of intramolecular vibrational‐energy redistribution (IVR). The latter process, which is a precursor for the evaporation of the host atoms or for further fragmentation, is found to be increasingly effective as the cluster size increases; low‐energy van der Waals modes act as the accepting (bath) modes. The reactiondynamics for I_{2}Ne_{2} are examined and quantitatively compared to a simple model which describes the dynamics as consecutive bond breaking. On this basis, it is concluded that the onset of energy redistribution is observed in I_{2}Ne_{2}. Comparison of I_{2}Ne and I_{2}Ne_{2} to larger clusters (n=3,4) is accomplished by introducing an overall effective reaction rate. From measurements of the rates and their dependence on v ^{’} _{ i }, the initial quantum number of the I_{2} stretch, we are able to examine the dynamics of direct fragmentation and evaporation, and compare with theory.

Studies of molecular Rydberg states by Schwinger variational‐quantum defect methods: Application to molecular hydrogen
View Description Hide DescriptionAn ab initioelectronic structure technique has been developed to study highly excited states of molecules by combining Schwinger variational methods of collision theory with generalized quantum defect theory. The technique exploits methods of scattering theory to study the region of highly excited Rydberg levels below and across ionization thresholds for molecules. The reaction matrix K, which describes the interaction of the Rydberg electron with the ionic core, is found at arbitrary negative electron energies by employing an unbounded Coulomb Green’s function in the Lippmann–Schwinger equation for the electronic wave function. Quantal conditions are imposed to obtain discrete molecular energy levels, associated Rydbergwave functions, and quantum defect functions, all as a function of the internuclear distance. Results within the static‐exchange approximation for the ^{1,3}Σ^{+} _{ u }(1σ_{ gn }σ_{ u }) and ^{1,3}Π_{ u }(1σ_{ gn }π_{ u }) Rydberg states of H_{2}, for n=2–20 and R=1.2–5.0 a _{0}, are presented and discussed.