Volume 85, Issue 5, 01 September 1986
Index of content:

An accurate and efficient decoupling approximation for temperature‐dependent multimode resonance Raman spectra
View Description Hide DescriptionWe present two advances in the use of the matrix method to generate the time‐dependent kernel for resonance Raman scattering. The first of these, a collocation method, allows a substantial decrease in computation time in addition to greater resolution of scattered profiles. Second, we have developed a decoupling approximation in which the exact method of Friesner e t a l. is used to generate the kernel at a relatively small number of time points with the remaining points obtained by fitting to the exact values a factorized expression containing both finite temperature and dephasing corrections. The approximation is exact in the zero temperature limit and is based on a sum rule which ensures exact reproduction of integrated intensities. Calculations with a wide variety of parameter values are presented and show that our approximate method is highly accurate in reproducing the exact results for a wide variety of physically realistic systems. In addition, our results for single mode systems are shown to compare favorably to those obtained using a transform‐based method.

The S _{2} ← S _{0} laser photoexcitation spectrum and excited state dynamics of jet‐cooled acetophenone
View Description Hide DescriptionThe S _{2} ← S _{0}photoexcitation spectrum of jet‐cooled acetophenone is presented. The observed homogeneously broadened linewidths indicate S _{2} lifetimes ≤0.26 ps while the measured emission lifetimes range from 540±30 ns for S _{2} 0^{0} _{0} excitation to 130±50 ns for S _{2} 12^{1} _{0} excitation. The dynamics revealed by the spectrum are discussed in terms of the known photochemical and photophysical rates of the excited states of acetophenone. Arguments are presented which identify the emitting state as a known triplet state of acetophenone.

Electron affinities of the alkali halides and the structure of their negative ions
View Description Hide DescriptionPhotoelectron spectra are reported for the MX (X̃ ^{1}Σ^{+})+e ^{−}←MX^{−}(X̃ ^{2}Σ^{+}) transitions of ten alkali halide anions at 488 nm. Adiabatic electron affinities (±0.010 eV) are determined to be 0.593 (LiCl), 0.520 (NaF), 0.727 (NaCl), 0.788 (NaBr), 0.865 (NaI), 0.582 (KCl), 0.642 (KBr), 0.728 (KI), 0.543 (RbCl), and 0.455 eV (CsCl). Fundamental vibrational frequencies, equilibrium bond lengths, and dissociation energies are also reported for the anion ^{2}Σ^{+}ground states. An observed linear correlation of electron affinities with α/r ^{2} (α=metal atom polarizability) is used to predict the electron affinities of the remaining alkali bromides and iodides, as well as related alkali salts. A simple electrostatic model for the alkali halide anions is also presented which enables the accurate (±0.1 eV) calculation of electron affinities.

Vibrational frequency shifts induced by molecular compression of pyridine in solution
View Description Hide DescriptionPressure‐induced vibrational frequency shifts are calculated for a diatomic oscillator immersed in a benign solvent, employing a simplified version of the Schweizer–Chandler model for solute–solvent interaction. The repulsive contribution is determined from the pair distribution function for hard‐sphere cavities. Interpolative evaluation of the pair distribution function is facilitated by noting that to an excellent approximation the pertinent expansion coefficients are merely linear functions of the reduced density. The treatment is applied to the quasidiatomic ring breathing vibrations of neat liquid pyridine, benzene, and toluene and to solutions of pyridine in several solvents including H_{2}O, D_{2}O, CH_{3}OH, CHCl_{3}, dimethylformamide, and toluene. The predicted pressure dependence of the ring breathing frequency is in the range ∂ν/∂P≈0.3–0.8 cm^{−} ^{1}/kbar for all these systems. The corresponding compression of the mean ring radius is in the range 0.9 to 2.0×10^{−} ^{4} Å/kbar. Especially for the associated solvents, the dominant contribution (>90%) to ∂ν/∂P comes from the effective hard‐sphere repulsion. Accurate values of the effective diameters thus can be evaluated from the observed pressure derivatives.

Spectroscopic identification of the lowest rotation–vibration levels of the (2pσ)^{2} F ^{1}Σ^{+} _{ g } state of the D_{2} molecule
View Description Hide DescriptionThe previously unobserved lowest vibrational levels of the F state of D_{2} are detected by the identification of F ^{1}Σ^{+} _{ g }–B ^{1}Σ^{+} _{ u } bands in infrared emission spectra of electric discharges recorded with a Fourier–transform spectrometer. The rotation–vibration structure of the F state, v=0–5, J=0–5, is in good agreement with accurate a b i n i t i o calculations which include the effects of tunneling in the double‐minimum potential of the E F ^{1}Σ^{+} _{ g } state. For the v=J=0 levels of H_{2} and D_{2} the absolute energy error of the a b i n i t i o calculation extrapolates, for infinite nuclear mass, to an error of the electronic F state energy of +0.7±0.1 cm^{−} ^{1}. This is consistent with Jungen’s recent accurate spectroscopic determination of the ionization potentials of the excited states of H_{2}.

Structure and ion dynamics of silver borate glasses: A ^{1} ^{0} ^{9}Ag NMR study
View Description Hide DescriptionAn analysis of the ^{1} ^{0} ^{9}Ag isotropic chemical shift δ in the glasses of the AgI:Ag_{2}O:B_{2}O_{3} system is presented. In these glasses, δ covers a range of ∼500 ppm but does not depend linearly upon the fraction of I^{−} anions X=[I]/[Ag]. This means that, when AgI is added to the borate network, structures are preferentially formed in which silver is coordinated to both I^{−} and negatively charged borate groups. We discuss also the ^{1} ^{0} ^{9}Agchemical shift range covered by the different forms of AgI and the dependence of δ upon the BO_{4} units and non‐bridging‐oxygen (NBO’s) concentrations. We analyze the relationship between spatial compositional fluctuations and NMR–MAS width of the ^{1} ^{0} ^{9}Ag line in fast silver conducting glasses. The spin‐lattice relaxation of ^{1} ^{0} ^{9}Ag and ^{1} ^{1}B, as well as some of the ^{1} ^{0} ^{9}Ag line narrowing results, are quantitatively interpreted by assuming a distribution of Ag^{+} jump times and by discussing how the different nuclear interactions are affected by the diffusion of silver. We conclude that the NMR evidence does not support the microscopic picture of cation diffusion in vitreous electrolytes suggested by the weak electrolyte theory.

Infrared spectrum of the overtone band 2ν^{0} _{5} of the hydrogen bonded complex HCN‐‐‐HF
View Description Hide DescriptionThe infrared spectrum of the overtone of the ‘‘intramolecular’’ bending vibration 2ν^{0} _{5} of the hydrogen bonded complex HCN‐‐‐HF centered at 1437.539 91(24) cm^{−} ^{1} has been obtained with a resolution of 0.006 cm^{−} ^{1}. Line assignments for this band as well as two hot bands 2ν^{0} _{5}+ν^{1} _{7}−ν^{1} _{7} and 2ν^{0} _{5} +2ν^{2} _{7}−2ν^{2} _{7} have been made, and rotational constants for all the levels involved have been determined. Despite the fact that the higher J energy levels of the observed lines of 2ν^{0} _{5} are above the dissociation energy of the complex, no predissociative line broadening is apparent.

Radiative processes following laser excitation of the A ^{2}Σ^{+} state of PO
View Description Hide DescriptionLaser induced fluorescence in the (0,0) band of the A ^{2}Σ^{+}–X ^{2}π system of the PO radical (∼2470 Å) has been used to study the radiative properties of the A state. A laser excitation scan of the (0,0) band and a fluorescence scan of the emission are given. Fluorescence from the B ^{2}Σ^{+} state to the X state was observed (∼3250 Å) when the A state was pumped by the laser. The branching ratio for emission from the A state to the lower B and X states was indirectly determined. The A state was found to have a very short free radiative lifetime, 9.68±0.47 ns. In the absence of quenching, the excited state decay is found to be primarily due to radiative processes. Upper limits were determined for the quenching rates of Ar and He carrier gases. Relative intensities of emission of the v’=0 progression in the A–X system were also measured. These intensities were used to determine the electronic transition moment function in the region of the equilibrium internuclear distance.

Lattice vibrations and infrared absorption of ice Ih
View Description Hide DescriptionLattice dynamics calculations have been carried out for ice Ih using the SPC rigid‐molecule effective pair potential whose parameters were fitted to the properties of water. We employ a periodically replicated nonpolar sample of 128 molecules whose constituent molecules are arranged according to the ice rules. Long range dipole–dipole interactions are handled using the Ewald method. The calculated density of states in the translational and librational regions accords well with available experimental data. However, the frequency of the longitudinal optic mode is a little low as is the position of whole librational band. Although polarization (induction) effects are not explicitly included in the calculation of the vibrational frequencies and their associated eigenvectors, they are invoked to evaluate the quantities which determine the infrared absorption, namely, derivatives of the crystal dipole moment with respect to the normal coordinates. The overall features of the infrared spectrum for translational modes are well described by including induction effects due to the molecular dipole and quadrupole fields. The same approach is used to calculate the dielectric constant and the energy loss spectrum. The latter calculations lead to a value of 10 cm^{−} ^{1} for the longitudinal optic–transverse optic mode splitting.

Einstein coefficients and transition moment variation for the NO(A ^{2}Σ^{+}–X ^{2}Π) transition
View Description Hide DescriptionBranching ratio measurements for the NO γ bands excited by energy transfer from metastable nitrogen molecules show that the electronic transition moment for the NO(A ^{2}Σ^{+}–X ^{2}Π) transition varies by about 40% over the r‐centroid range of 1.13–0.97 Å. Combining this transition‐moment variation with radiative lifetime measurements provides a complete set of Einstein coefficients for NO(A–X) transitions from v’=0–2.

Perturbed rotational structure in the 18 373 cm^{−} ^{1} band of the fluorescence excitation spectrum of jet‐cooled chromyl fluoride
View Description Hide DescriptionThe 18 373 cm^{−} ^{1} band in the fluorescence excitation spectrum of jet‐cooled CrO_{2}F_{2} has been recorded with 0.007 cm^{−} ^{1}linewidth. The stronger lines have been assigned to a type c transition, consistent with a B _{1}←A _{1} vibronic transition analogous to that in CrO_{2}Cl_{2}. Ground‐state rotational constants,A″=0.148, B″=0.130, and C″=0.120 cm^{−} ^{1}, agree well with electron diffraction predictions. The B _{1}excited state is significantly perturbed. Evidence includes (i) a decreasing fluorescence yield with increasing J′, such that signals were not observed for J′>3, (ii) the appearance of ‘‘extra’’ lines in the spectrum, leading to perturbing levels strongly mixed with levels of the B _{1} state, and (iii) anomalous effective rotational constants for the B _{1} state.

Polarized Raman spectra of thin films. I. The case of oblique incidence for uniaxially oriented samples
View Description Hide DescriptionIn the case of oblique incidence (when the incident and the scattered beams are not perpendicular) it is shown that if the electric vector is parallel to the scattering plane the Raman spectra have a ‘‘complex’’ polarization—the Raman intensities are obtained after adding the scattered amplitudes and squaring their sum. However, if the samples are uniaxially oriented around a Z axis, simplification occurs, due to the average over the φ rotation angle (and if necessary over ψ, which is the rotation around the z axis of the molecule) and now it is the intensities which can be added.

Polarized Raman spectra of thin films. II. Apparent anomaly of polarization with uniaxially oriented samples
View Description Hide DescriptionWith uniaxially oriented thin films it happens that the two polarized Raman spectra given by the E _{∥} excitation are not alike as could be expected. It is shown that because the exciting beam is slanted inside the film, two components of the electric field are to be considered which give the two ‘‘complex’’ polarized Raman spectra:I ^{∥} _{ X }=(α^{2} _{ X X } cos^{2} r +α^{2} _{ X Z } sin^{2} r)E ^{2} _{0} and I ^{∥} _{ Y }=(α^{2} _{ Y X } cos^{2} r +α^{2} _{ Y Z } sin^{2} r)E ^{2} _{0}. With uniaxially oriented samples the intensities can be added, as shown in part I. The values of α^{2} _{ X Z }=α^{2} _{ Y Z }, obtained from the spectra are small in agreement with the calculated values. As these terms decrease when the tilting angle θ of the chains decreases in the case of properly oriented samples, the second terms of I ^{∥} _{ X } and I ^{∥} _{ Y } are negligible compared with the first ones, even if cos^{2} r is small (when the exciting beam is nearly perpendicular to the scattered beam). Therefore, contrary to expectations, the observed spectra are more similar to the (X X) and (X Y) spectra than to the (Z X) and (Z Y) spectra. The discussion has been carried out with respect to the ν(CH_{2}) vibrations of the acyl chains in phospholipid films and completed with the comparison of the ν(C–C) relative intensities.

Inverted perturbation approach (IPA) potentials and adiabatic corrections of the X ^{1}Σ^{+} state of the lithium hydrides near the dissociation limits
View Description Hide DescriptionLaser‐induced fluorescence(LIF)spectra of the A–X system of the LiH isotopes, excited by the UV lines of an argon and a krypton ion laser, were observed and analyzed. Three long LIF series of different LiH isotopes were taken with a 3.4 m high resolution spectrograph. The results were combined with previous experimental data to study the breakdown of the Born–Oppenheimer approximation in the X ^{1}Σ^{+} electronic ground state of the isotopic LiH molecules. Using a quantum mechanical variational method, potential energy curves of the four LiH isotopes were extended almost up to the dissociation limits.

Fluorescence of cyclohexane vapor: Pressure dependence and quenching studies
View Description Hide DescriptionThe pressure dependence of the fluorescence lifetime of cyclohexane vapor excited at wavelengths between 168.5 and 175.0 nm has been investigated. Due to the unusual dependence of the lifetime on cyclohexane pressure, a simple Stern–Volmer type of extrapolation cannot be used to obtain the collisionless lifetime at any of the excitation energies employed. Also, fluorescence decay curves fit a single‐exponential function only under certain conditions of pressure and excitation wavelength. The rate constants for quenching of the excited state(s) by O_{2} at excitation wavelengths of 172.0 and 169.5 nm are (6.1±0.1)×10^{−} ^{1} ^{0} and (7.9±1.4)×10^{−} ^{1} ^{0} cm^{3} molecule^{−} ^{1} s^{−} ^{1}, respectively. The difference between these rate constants is not significantly larger than the experimental uncertainty. At 172.0 nm, the rate constant for quenching by Xe is (7.1±0.3)×10^{−} ^{1} ^{2} cm^{3} molecule^{−} ^{1} s^{−} ^{1}. At this same excitation wavelength, quenching by He, Ar, and N_{2} is extremely small.

Optical Hartmann–Hahn resonance and the spatial correlation of inhomogeneous broadening in molecular solids
View Description Hide DescriptionWe propose the use of optical Hartmann–Hahn resonance between guest molecules in a solid in order to study the extent of spatial correlation of site energies in the inhomogeneous distribution. The correlation length and the form of the interaction which mediates the resonance between guest molecules can be determined from the time development of fluorescence.

NMR spectra of dilute spins under slow sample‐spinning and homonuclear decoupling
View Description Hide DescriptionDilute‐spin spectra under decoupling between abundant spins and slow sample spinning at general angles are discussed, and experimentally observed with an example of calcium formate. Spectra obtained at off‐magic angles break up into a train of characteristic patterns different in shape from each other. The center pattern was found to be especially sensitive to the mutual orientation between the chemical shift and dipolar coupling tensors. Each sideband observed at the magic angle shows splitting due to the ^{1} ^{3}C–^{1}H indirect coupling. The absolute sign of the coupling constant is determined to be positive from the J‐split sideband pattern.

Vibrational predissociation spectroscopy of binary HF–base complexes
View Description Hide DescriptionThe vibrational predissociationspectra of N_{2}–HF, acetylene–HF, ethylene–HF, and cyclopropane–HF in the HF stretching region have been measured using a molecular beam apparatus equipped with a mass spectrometer and a LiNbO_{3}optical parametric oscillator(OPO). The spectra are not dominated by homogeneous broadening. The HF stretching frequencies of N_{2}–HF, C_{2}H_{2}–HF, C_{2}H_{4}–HF, and C_{3}H_{6}–HF are 3918.2(1), 3793.4(2), 3781(1), and 3761(1)cm^{−} ^{1}, respectively. Individual rotational lines have been resolved for the N_{2}–HF, C_{2}H_{2}–HF, and C_{2}H_{4}–HF complexes using the OPO in high resolution (0.06 cm^{−} ^{1} FWHM) mode. The lower limits to the predissociation lifetime obtained from the widths of these lines vary from 35 to 88 ps. An upper limit to these lifetimes of ∼2 μs was obtained from the observed depletion of the molecular beam.

Rydberg states of NO trapped in rare gas matrices
View Description Hide DescriptionSeveral new progressions consisting of broadbands have been observed in the absorption spectra of NO trapped in rare gas matrices. They are identified as the blue shifted transitions to the n=3 (A ^{2}Σ^{+},C ^{2}Π,D ^{2}Σ^{+}) and n=4 (E ^{2}Σ^{+},K ^{2}Π,M ^{2}Σ^{+}) Rydberg states of NO in Ne matrices and to the n=3 Rydberg states in Ar, Kr, and Xe matrices. Linewidths (50 to 180 meV) and matrix shifts (0.3 to 1.2 eV) decrease from neon to xenon matrices. For each matrix, the shifts decrease according to the symmetry of the excited state orbital, in the sequence sσ–pσ–pπ. An attempt to apply the quantum defect formula with values, for the Rydberg constant and the ionization potential of the matrix isolated molecule taken from literature, fails to describe the vertical transition energies of n=3 and n=4 states in neon matrices. On the other hand, using the adiabatic energies of 3sσ and 4sσ states to generate a series shows that the vertical transitions of n≥5 states lie in the ionization continuum. This could explain the nonobservation of n>4 states in Ne matrices. In the heavier matrices, this model predicts n=4 states but their existence is not borne out by experiment. The Wannier model also fails to describe these transitions.

Spin uncoupling in the 6s Rydberg states of methyl iodide. Rotational subband structure in one‐photon absorption
View Description Hide DescriptionMolecular constants of electronic states, all origins as well as some vibronic bands, derived from the lowest‐energy Rydberg configuration of methyl iodide, are reported. Values of the internal angular momentum parameter ζ_{eff}, which is a measure of the strength of Coriolis and vibronic coupling, indicate that the spin of the Rydberg electron is effectively uncoupled from the molecular axis in the pure electronic states and that dynamic interactions between electronic, rotational, and vibrational motions completely quench the internal angular momentum in some vibronic levels. It is concluded that the parameters ζ_{eff}, as obtained from one‐photon absorption spectra, are approximately those of the alternate levels, ^{2} E _{3/2} and ^{2} E _{1/2}, of the CH_{3}I^{+} ion‐core ground configuration X̃.