Volume 99, Issue 4, 15 August 1993
Index of content:

Spin–orbit and rotational autoionization in HCl and DCl
View Description Hide DescriptionThe rotationally resolved spin–orbit autoionization spectrum of HCl has been obtained in double resonance via levels of lowest angular momentum in v=0 of the 4pσ D ^{1}Π state. A self‐consistent assignment is offered for the portion of the spectrum ranging over principal quantum numbers from 13 to 18. Positions established in HCl point to series of similar quantum defect converging to the rotational ground state of DCl^{+} ^{2}Π_{1/2}. HCl features are further confirmed by comparison with earlier double‐resonance spectra obtained via the F ^{1}Δ state, and with vuv autoionization spectra observed in single‐photon transitions from the neutral ground state. Multichannel quantum defect theory calculations have been carried out, which introduce 28 parameters corresponding to the quantum defects, transition moments, and s–d l‐mixing coefficients for Rydberg states of HCl in Hund’s case (a). As output they predict positions and intensities for transitions involving all 53 channels accessible in Hund’s case (e) for l ranging from 0 to 2 and J from 1 to 2. The same parameters for both isotopes yield reasonable correspondence between experiment and theory for all the major resonances in HCl and DCl from principal quantum number 13 to 18 (about 70 resonances), with an average ΔE less than 3 cm^{−1}.

Rotationally resolved single‐photon ionization of HCl and DCl
View Description Hide DescriptionThe rotationally resolved photoionization yields of jet‐cooled HCl and DCl have been measured in the energy range between their spin–orbit split ^{2}Π ionic thresholds. For single‐photon excitation, narrow‐band vuv radiation is generated by resonant frequency mixing. The spectra are complex due to an interaction of autoionizing resonances belonging to series converging to different rotational states of the ion core. This is taken into account using the multichannel quantum defect theory in a treatment capable to handle a transition in angular momentum coupling from Hund’s case (c) to case (e). Even for Rydberg orders n≳35 the experimental data cannot be explained using a pure case (e) picture.

Raman scattering by high‐density dispersions. II. Calculation of surface‐enhanced Raman scattering from CO and benzene physisorbed on pairs of particles of Ag, Pt, Ge, and SiO
View Description Hide DescriptionIn part I, a mathematical formalism was developed for calculating Raman scattering intensities from a collection of spherical particles uniformly coated with molecules, and applied to the special case of two CO‐coated Ag particles in contact, with their centers aligned with the laser field and under parallel polarization conditions. In this paper, the additional relations required to perform the calculations for arbitrary orientation and polarization conditions are developed and used in calculations for CO and benzene physisorbed onto pairs of particles of Ag, Pt, Ge, and SiO. The large increase in Raman radiation intensity per molecule, reported in part I on going from a one‐particle to a two‐particle system, is shown here to apply only to the Raman radiation component polarized parallel to the interparticle axis and due to the incident radiation component similarly polarized. For all other components, the excitation spectra are similar for the two systems. The large increase for the parallel–parallel case is due to the process in which the primary Raman multipoles centered on one particle induce Raman dipoles on the pair of particles.

Vibrations of pyrazine and its ion as studied by threshold ionization spectroscopy
View Description Hide DescriptionThe pump–probe threshold ionizationphotoelectron spectra of pyrazine have been recorded using nine different vibrations of S _{1} as intermediate resonances. The extensive vibrational structure in these spectra of the ionic ground state have enabled the measurement of 12 of its vibrational frequencies and their assignments. Three new vibrational assignments of S _{1} are also made. MP2/6‐31G* calculations of the vibrational frequencies of the neutral ground, S _{1}, and the ionic ground state are compared with the experimental values, finding that certain vibrations of S _{1} and the ion which engage in extensive vibronic coupling are not properly determined by the calculated force field. Most vibrational frequencies are accurately reproduced, however. Variations in the complexity of the threshold ionization spectra with the level of S _{1} excitation indicate that internal vibrational relaxation is taking place at a very low energy in that state, possibly involving vibronic interactions and mixing with the triplet manifold.

Spectra of isotopic ice mixtures
View Description Hide DescriptionStudies of vibrational spectra of ice I and amorphousice in the stretching mode frequency range were extended to include (a) the observed effect of a full range of isotopic dilution on the infrared spectra and (b) computational modeling of the observed influence of each dilution step on the properties of vibrationally excited states and on infrared and Raman spectra. The quantum‐mechanical computational scheme included effects of frequency lowering due to hydrogen bonding, and of intra‐ and intermolecular coupling between bonds. The H_{2}O/D_{2}O mixtures can be viewed as a collection of clusters of one X_{2}O isotopomer embedded in a matrix of the other isotopomer. The properties of the vibrationally excited states and of the spectra are determined by complex interplay between the size distribution of the embedded clusters, and the inter‐ and intramolecular coupling. Vibrational excitations are delocalized over large portions of the embedded clusters. In the limit of a pure crystalline isotopomer, the excitations are delocalized over the entire system and thus proton disorder alone is insufficient to induce localization. The excitations in pure amorphousice show more pronounced localization effects at the band edges. Throughout the entire composition range, the vibrations of molecules in the low frequency regime retain symmetric stretch character, and the vibrations in the high frequency regime retain antisymmetric stretch character. The perpendicularly polarized Raman spectrum peaks in the region of the latter states. The parallel‐polarized Raman spectrum peaks in the low frequency end of the band where the states are globally symmetric, i.e., the contributions of excitations of all bonds to a state are of the same sign. The infrared spectrum extends over the entire band and follows roughly the density of states.

Dissociation and ionization of vibrationally autoionizing Rydberg states of H_{2} in a static electric field
View Description Hide DescriptionWe have studied the spectroscopic and dynamic properties of several autoionizing Rydberg states of molecular hydrogen which converge to the vibrationally excited v ^{+}=1 state of the ion, in the presence of a static electric field. Using the techniques of multiphoton ionization and time‐of‐flight mass spectrometry, we are able to separately observe the yield of dissociation and ionization after excitation of resolved Stark components of the Rydberg states. The energies of the Stark components can be modeled accurately by an energy matrix diagonalization calculation, when rotational interactions between states converging to different ion rotational states are taken into account. The dynamics of the different states in the Stark manifolds can be partly explained by assuming that they are governing by l mixing of predominantly predissociative s and d states and a dominantly autoionizing p state.

Determination of electric dipole moments and transition probabilities of low‐lying singlet states of CO
View Description Hide DescriptionTransitions from the X ^{1}Σ^{+}(v=0) ground state of the carbon monoxide molecule to the electronically excited A ^{1}Π(v=0), B ^{1}Σ^{+}(v=0), and C ^{1}Σ^{+}(v=0) states have been studied by 2‐photon laser induced fluorescencespectroscopy. Accurate molecular constants for the B and C state have been determined. The electric dipole moments for all three electronically excited states have been deduced from the observed Stark effects. The dipole moments for the A, B, and C states are found to be 0.335±0.013, 1.95±0.03, and 4.50±0.07 D, respectively. From the observed radiative lifetimes the transition probabilities of the B–A, B–X and C–B, C–A, and C–Xtransitions have been determined.

High resolution spectra of GeH_{4} v=6 and 7 stretch overtones. The perturbed local mode vibrational states
View Description Hide DescriptionHere we report the preliminary analysis of the high resolution spectra of ^{70}GeH_{4} (6000) and (7000) stretch overtone bands obtained by intracavity laser absorption spectroscopy (ICLAS). The observed P(J+1), Q(J), and R(J−1) transitions agree well with the combination difference relations, indicating that the Coriolis coupling in the degenerate states is negligible. This is an important feature for a spherical top molecule near the local mode limit. The vibration–rotational energy levels of upper states as well as those of some perturber states are obtained. The improved local mode parameters of GeH_{4} are also derived.

A laser‐induced fluorescence study of the BiF A O^{+}→X _{1} O^{+} system in the region 6550 to 7400 Å: Rotational analysis and evidence for an A O^{+} state potential maximum
View Description Hide DescriptionEighteen previously unobserved bands belonging to the BiF A O^{+}→X _{1} O^{+} system have been identified in the region between 6550 and 7400 Å. Nine bands representing v’=27–29 and v‘=40–45 have been rotationally analyzed, and the v’=27 and v’=29 states exhibit strong homogeneous perturbations. These upper state levels are also observed to predissociate, providing evidence for an A O^{+} state potential maximum which has been theoretically predicted. A limiting curve of dissociation suggests a rotationless potential maximum at 32 674±822 cm^{−1}. This energy must correspond to a potential maximum if it is to be reconciled with recent thermochemical measurements of the ground statedissociation energy,D _{0} ^{0}=3.76±0.13 eV. The new spectroscopic constants have been included in a calculation of Rydberg–Klein–Rees potential curves for both states. Franck–Condon factors calculated from these potentials are qualitatively consistent with the observed intensity distribution. Measured collision free radiative lifetimes are consistent with known A O^{+} state lifetimes for nonpredissociated states and are found to be shorter for predissociated states.

The absorption spectra of small nickel clusters via photodissociation: Ni_{ n }Ar_{ m }→Ni_{ n }+mAr
View Description Hide DescriptionThe photodissociation action spectra of Ni_{ n }Ar_{ m } (n=3,4,5, and 7) have been recorded from 306 to 674 nm. The spectra are continuous, with cross sections which increase monotonically into the ultraviolet. It is observed that the photodissociationspectra recorded for Ni_{3}Ar_{ m }, Ni_{4}Ar_{ m }, and Ni_{5}Ar_{ m } are independent on m, indicating a negligible influence of the weakly bound argon atoms on the absorption spectra of the underlying clusters. The oscillator strengths increase monotonically with increasing cluster nuclearity (n) in accord with the Thomas–Reiche–Kuhn sum rule. The results are discussed from both molecular and classical electrodynamic points of view.

Maximal entropy spectral fluctuations and the sampling of phase space
View Description Hide DescriptionAn analytical functional form for the distribution of intensities in an absorptionspectrum is derived. Deviations from the purely statistical ‘‘Porter–Thomas’’ distribution are shown to be directly related to finite time information on the dynamics in phase space. The predicted distribution is wider than the purely statistical one with a higher proportion of very low intensity transitions. The derivation is based on a maximum entropy form of the spectrum. The constraints used are the values of the survival amplitude at finite number of times. The amplitude is obtainable as the Fourier transform of an observed spectrum or as the result of a dynamical computation. The optimal choice of the time points which characterize the spectrum, is discussed and a numerical algorithm is provided. Extensive spectral fluctuations occur when more than one time scale is needed to characterize the dynamics. This separation of time scales is also manifested as a clump structure in the spectrum of maximal entropy. The formalism also provides the distribution of line spacings and the ‘‘correlation hole’’ in the time autocorrelation function is discussed as an illustration.

Molecular‐beam spectrum of the 970 cm^{−1} Fermi triad of CF_{3}CH_{3}
View Description Hide DescriptionA tunable microwave‐sideband CO_{2} laser is used with an electric‐resonance optothermal spectrometer to investigate the infrared spectrum of CF_{3}CH_{3} near 970 cm^{−1}. A Fermi‐coupled triad of states is observed, resulting from the interactions of 2ν_{6}+ν_{11} and ν_{5}+ν_{12} with the fundamental vibration, ν_{10}, which is assumed to carry the oscillator strength in this region. The high resolution (∼3 MHz) of the spectrometer allows the observation of tunneling splittings associated with the ν_{6} torsional vibration. These splittings are used to identify the torsional character of the states observed. At the normal‐mode level the ν_{10} and ν_{5}+ν_{12} states are found to be nearly degenerate and interacting by an anharmonic matrix element of ∼3 cm^{−1}. The lower‐energy component of this diad exhibits torsional splittings of up to 400 MHz due to an anharmonic coupling of 0.70 cm^{−1} with the lower energy 2ν_{6}+ν_{11} state which has an intrinsic tunneling splitting of ∼800 MHz. A fourth state, 3ν_{6}+ν_{12}, which has a still larger zeroth‐order tunneling splitting, may also be affecting the torsional splittings of the observed states. The present investigation illustrates the utility of using resolved torsional splittings to unravel complex vibrational couplings in molecules.

Dielectric relaxation study of hexamethylphosphoramide‐water mixtures using time domain reflectometry
View Description Hide DescriptionThe dielectric relaxation studies of hexamethylphosphoramide‐water mixtures have been carried out at different temperatures using time domain reflectometry technique in the frequency range of 10 MHz to 10 GHz. A calibration method based on the least squares fit has been suggested to determine the complex permittivity at the high frequency region for the aqueous solutions. The excess dielectric, Kirkwood correlation factor and thermodynamic properties in the mixture have been determined. These properties suggest a Cage‐type structure of hexamethylphosphoramide molecules around water molecules. The structure has a tendency to break down in the presence of water.

Off‐resonant transient birefringence in liquids
View Description Hide DescriptionOff‐resonant transient birefringence measurements are analyzed using a reduced equation of motion for the ground state density matrix, which is expanded using an effective Hamiltonian. Assuming that the pump field is weak, we express the polarization relevant for the birefringence signal in terms of a convolution of the tensorial polarizability response function with the external fields. The homodyne‐detected birefringence signal is directly compared with the coherent Raman signal. The relationship between off‐resonant birefringence and spontaneous Raman experiments is discussed. By expanding the polarizability in powers of the nuclear coordinates and applying the Brownian oscillator model to the coordinate response function, we separate the birefringence signal into intra‐ and intermolecular coordinate response functions. Off‐resonant transient birefringences of acetonitrile, chloroform, dimethylsulfoxide, and a series of alcohols were measured. The data are transformed to the frequency domain by using a model independent analysis method. The spectra are discussed in the context of various models for the distribution of intermolecular modes (spectral density) in liquids.

Millimeter‐wave spectrum of CH_{3}CD_{3} in the three lowest torsional states
View Description Hide DescriptionThe pure rotational spectra of CH_{3}CD_{3} in the three lowest torsional states has been observed using a mm‐wave spectrometer. A total of 87 rotational frequencies were measured between 230–363 GHz for J=7←6 to 11←10 in v _{6}=0, 1, and 2, where v _{6} is the torsional quantum number. For the lowest two torsional states, the spectra have the classic form expected for a symmetric top (with no internal rotation) in the ground vibrational state. For v _{6}=2 and for a given (J+1)←J, a markedly different splitting pattern is observed as a result of the (K,σ) dependence of the effective rotational constantB̂, where σ labels the torsional sublevels. In order to identify the individual features in the (v _{6}=2) spectrum, an assignment procedure was developed which is based on the fact that the ratio of moment of inertia of the top about the molecular symmetry axis to that of the whole molecule about the same axis is to a very good approximation 1/3. The torsion–rotation Hamiltonian discussed earlier in connection with CH_{3}SiH_{3} [N. Moazzen‐Ahmadi et al. J. Mol. Spectrosc. 119, 299 (1986)] was used to analyze the rotational frequencies along with the molecular beam anticrossing data and the origin of the torsional fundamental. Several constants which characterize the J‐dependence of the energy levels were determined. Effective values for the barrier height Ṽ_{3} and the shape parameter Ṽ_{6} associated with the first‐order correction in the Fourier expansion of the potential function were obtained. The effect of redundancies on the interpretation of the measurements is discussed.

Large CO_{2} clusters studied by infrared spectroscopy and light scattering
View Description Hide DescriptionLarge CO_{2} clusters were formed by introducing room temperature gaseous mixtures of CO_{2} in argon into a cryogenic cell at 77 K. Rapid cooling of each mixture resulted in a highly supersaturated CO_{2} concentration, giving rise to homogeneous nucleation and thus cluster formation [F. F Abraham,Homogeneous Nucleation Theory, Advances in Theoretical Chemistry, Supplement 1 (Academic, New York, 1974), and references therein]. Experimental results will be presented here for CO_{2} in argon dilutions of 1:10^{4}, 1:2×10^{5}, and 1:10^{6}. Light scattering and infrared absorption techniques have been combined to estimate an average cluster radius of 0.20 μm for the 1:10^{4} dilution sample, and an upper limit in cluster radius of 0.10 μm for the 1:2×10^{5} dilution sample. Therefore, the higher dilution CO_{2}:Ar mixtures led to the formation of smaller cluster sizes. Infrared structure in the ν_{3}‐asymmetric stretching region of the clusters will be discussed.
The quantum mechanical exciton model and the classical Mie model are only partially successful in explaining these experimental observations. Weak absorption features have been assigned to the naturally abundant ^{13}C^{16}O_{2} and ^{12}C^{16}O^{18}O isotopes. The infrared structure attributed to these minority isotopes is relatively invariant with cluster size formed, and can be explained by the exciton model. This analysis suggests that, whatever overall shape the clusters have assumed, the CO_{2} molecules within the clusters have separations and orientations like those in the bulk crystal. Infrared spectra were collected at regular intervals over a period of 4 h for each sample. A monotonic decrease in the integrated infrared absorbance of the clusters with time was observed, with a characteristic half‐life of 65, 180, and 230 min for the 1:10^{4}, 1:2×10^{5}, and 1:10^{6} dilution samples, respectively. The average cluster radii obtained from light scattering and infrared absorption measurements have been used to calculate a sedimentation half‐life of 60 and 180 min for the 1:10^{4} and 1:2×10^{5} dilution samples, respectively, which agreed well with the observed disappearance times. A cluster radius of 0.09 μm for the 1:10^{6} dilution experiment is consistent with its sedimentation half‐life of 230 min.

The electric dipole moment of (NH_{3})_{2} for G: ‖K‖=1
View Description Hide DescriptionFrom the results of Stark measurements on the (NH_{3})_{2} van der Waals complex formed in a molecular jet expansion, it was possible to determine the electric dipole moment for the G: ‖K‖=1 state. The partially quenched inversion in the complex gives rise to quadratic Stark effect. We find an electric dipole moment of ‖μ‖=(0.10±0.01) D in the ground state and an upper limit of 0.09 D for the dipole moment in the excited state. These small values give evidence that for the G: ‖K‖=1–1 states, the ‘‘antiparallel’’ (cyclic) structure is more likely than the hydrogen bonded one.

Theory of continuum‐Raman spectroscopy with pulses
View Description Hide DescriptionA uniform theory of Raman scattering and resonance fluorescence from an intermediate dissociative manifold for excitation with pulses is developed. It is shown that transient effects lead to the appearance of additional terms, not included in the Kramers–Heisenberg formula. The role of ‘‘true’’ Raman scattering vs ‘‘resonance fluorescence’’ in contributing to the observed signal is elucidated. It is shown that the relative importance of these two processes is strongly dependent on the pulse parameters and the spontaneous emission lifetimes. ‘‘True’’ Raman is shown to dominate at very short times, during the rise of the pulse. At longer times resonance fluorescence sets in and dominates the observed signal. The important implications of the above to the time‐averaged excitation‐emission spectrum (the emission signal as a function of the excitation wavelength), with real ns pulses is discussed. The present study explains recent experiments showing rich structure in the excitation emission spectrum of IBr [I. Levy et al., J. Chem. Phys. 96, 1858 (1992)].

A test of different rotational Raman linewidth models: Accuracy of rotational coherent anti‐Stokes Raman scattering thermometry in nitrogen from 295 to 1850 K
View Description Hide DescriptionRotational Raman linewidths calculated from three different models have been used in temperature measurements by rotational coherent anti‐Stokes Raman scattering (CARS)—a semiclassical ab initio model, the modified exponential energy gap model (MEG), and the energy corrected sudden scaling law (ECS). Experimental rotational CARSspectra were generated, using the dual‐broadband approach, in pure nitrogen at atmospheric pressure in a heat pipe in the temperature range from 295 to 1850 K. Below 1500 K, the temperatures evaluated using the ECS linewidths agreed with the heat‐pipe temperatures to within 20 K. Above 1500 K, the errors in the evaluated temperatures increased steeply for all linewidth models, reaching errors of several hundreds of Kelvins at 1850 K. This behavior of the evaluated temperature is probably caused by the uncertainty in the values of the rotational Raman linewidths for high rotational states at high temperatures. This work therefore illustrates that rotational CARS can be used for experimentally studying Raman linewidths and in particular their dependence on temperature and rotational quantum number. The influence of different experimental parameters on the evaluated temperatures is discussed, and the spectral synthesis program is presented.

A comparative pressure tuning hole burning study of protoporphyrin IX in myoglobin and in a glassy host
View Description Hide DescriptionWe measured the behavior of spectral holes under isotropic pressure changes as a function of burn frequency. We compared a protein sample, namely protoporphyrin IX substituted myoglobin in a glycerol/water glass with a sample where the protoporphyrin IX was directly dissolved in a host glass. The differences are remarkable—holes in the pure glass behave as expected for a homogeneous isotropic material. It is the nonlinear frequency dependence of the pressure shift where the deviation of the protein sample is most obvious. These observations signal a correlation between the structures of the dye probe and the structures of the apoprotein. They further show that global parameters of the apoprotein, such as the isothermal compressibility, depend strongly on the associated conformational substates and are subject to unexpected large variations.