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Volume 78, Issue 11, 01 June 1983

Infrared spectra of OC–HX hydrogen‐bonded complexes in solid argon
View Description Hide DescriptionThe hydrogen‐bonded complexes OC–HX (X=F, Cl, Br) has been prepared by condensing Ar/HX and Ar/CO reagent mixtures at 12 K. FTIRspectra of the complexes are characterized by strong H–X absorptions (ν_{ s }) displaced below the isolated HX fundamental, strong C–O absorptions displaced above the isolated CO fundamental, and sharp degenerate librational modes (ν_{ l }) in the far infrared. Observation of a single degenerate ν_{ l } mode indicates linear structures for the complexes. The displacements Δν_{ s } and Δν_{CO} for the complexes decrease in magnitude in the series HF≳HCl≳HBr in direct relationship with the trend in hydrogen bonding strength for the hydrohalic acids.

Infrared spectrum of the CO_{2}–HCl complex in solid argon at 12 K
View Description Hide DescriptionCo‐condensation of CO_{2} and HCl in excess argon at 12 K produced the CO_{2}–HCl hydrogen‐bonded complex. Small shifts in the ν_{ s } and ν_{3} ^{ c } modes, the low ν_{ l } fundamental, and a very weak infrared Fermi doublet indicate a weakly bound complex. The more strongly bound CO_{2}–HF and C ^{1} ^{8}O_{2}–HF complexes gave more intense infrared Fermi doublets. The infrared intensity ratio ν_{3}/ν_{1} is 200/1 for the CO_{2}–HF complex demonstrating that the HF ligand induces finite electrical asymmetry in the CO_{2} submolecule.

Operator and algebraic methods for NMR spectroscopy. I. Generation of NMR spin species
View Description Hide DescriptionAlgebraic methods are developed for generatingNMR spin species and irreducible representations spanned by spin functions. These methods use generalized character cycle indices (S function when NMR groups are symmetric groups). A correspondence between the unitary group approach and the permutation group method for NMR is established by generatingNMR Gel’fand states using Schur functions. The generalized character cycle indices of molecules whose NMR groups are expressible as generalized wreath products are shown to be generalized NMR plethysms of Schur functions. These techniques enable generation of spin species without the knowledge of the character tables of NMR groups. We illustrate the methods developed here with several examples. The use of these techniques in generating symmetry‐adapted NMR spin functions can be found in the accompanying paper.

Operator and algebraic methods for NMR spectroscopy. II. NMR projection operators and spin functions
View Description Hide DescriptionWe outline double coset and restricted character cycle index methods which generate the equivalence classes of NMR spin functions and the irreducible representations in the representation spanned by spin functions in any equivalence class. Elegant operator methods are developed by which the projection operators of NMR groups can be obtained in terms of the projection operators of composing groups. Thus, the projection operators of NMR groups of many molecules can be obtained without the knowledge of their character tables. Projection operators thus obtained are applied on the set of spin functions in an equivalence class of NMR spin functions (rather than the whole set of spin functions) which are generated by double coset methods to obtain symmetry‐adapted NMR spin functions in the composite particle representation. Methods are illustrated with several examples of molecules containing as many as 2^{3} ^{0}NMR spin functions. The methods developed here are quite general so that they can be applied to NMR of nuclei with multispin states and they do not require the character tables of NMR groups. The techniques outlined here are illustrated with 2, 2, 3, 3‐tetramethyl butane for which the symmetry‐adapted NMR spin couplings are obtained in the composite particle representation. The NMR Hamiltonian matrix of this molecule which is of order 2^{1} ^{8}×2^{1} ^{8} is factored into 2^{6}×2^{6} matrix by the composite particle treatment. This is further factored into 8 1×1 matrices and 6 4×4, 4 2×2, and 3 8×8 matrices in the symmetry‐adapted basis.

Structure of liquid hydrogen fluoride studied by infrared and Raman spectroscopy
View Description Hide DescriptionInfrared and Raman spectra of hydrogen fluoride, deuterium fluoride, and their isotopic mixtures at various temperatures from 293 to 77 K have been recorded and analyzed in comparison with calculated data obtained from the classical G.F. matrix method applied to hypothetical structures chosen a priori. The best fit lead to the most probable structure of HF at 293 K which is a zig–zag chain with six or seven molecules having an angle of 42° with the average axis. At a lower temperature (190 K), just before solidification, the chains become longer, n=8 with an angle θ≂35 °. These structures fit well with the high dielectric constant of liquid hydrogen fluoride and lead to a calculated Kirkwood factor g _{1}=3.65 at 293 K and g _{1}=5.36 at 190 K which are in perfect agreement with known experimental values.

Surface enhanced Raman scattering (SERS) from pyridine on silver‐UHV interfaces: Excitation spectra
View Description Hide DescriptionExcitation spectra of various surface enhanced Raman lines of pyridine adsorbed to evaporated silverfilms (1 L exposure) in UHV have been measured. Relative to ordinary scattering from thick pyridine layers on SERS inactive Agfilms, every vibration exhibits a resonance profile which peaks at the same Stokes frequency corresponding to 2.0 eV independent of vibration. This value is shifted to the red after additional adsorption of 10^{3} L of pyridine on the SERS active film. The ratio of corresponding Raman line intensities from ’’bulk’’ and ’’surface’’ pyridine varies differently for different vibrations with exciting frequency. The results are discussed with respect to various proposed enhancement mechanisms.

Effects of interlamellar forces on longitudinal acoustic modes of n‐alkanes
View Description Hide DescriptionEffects of the interlamellar interactions in n‐alkane crystals on the longitudinal acoustic modes (LAM), including both the fundamental and the higher overtones, were investigated on the basis of the polarized Raman spectra of n‐C_{2} _{8}H_{5} _{8}, n‐C_{3} _{0}H_{6} _{2}, n‐C_{3} _{4}H_{7} _{0}, and n‐C_{3} _{6}H_{7} _{4} crystallized in the orthorhombic II form (a double layer polytype of the monoclinic form). The LAM‐1 bands of this crystal modification split into a doublet consisting of the (cc) and (bc) polarization components separated by about 6 cm^{−} ^{1}. The frequency gaps of the LAM‐m bands were found to decrease with m (approximately proportional to m ^{−} ^{1}). Theoretical considerations on the correlation field splitting and the scattering intensities of the LAM bands led to the conclusion that the observed band splitting was caused by the intermolecular force acting between the successive monomolecular layers. The intrinsic value of the Young modulus of the n‐alkane molecules free from the external forces in the crystalline state and the strength of the interlayer force were evaluated from the observed frequencies of the two components of LAM using a simple dynamical model. By comparing the Raman spectra of n‐C_{2} _{8}H_{5} _{8} and n‐C_{3} _{0}H_{6} _{2} between the triclinic and orthohombic II modifications, it was concluded that the effect of the lateral force on the LAM was not so significant as that of the longitudinal force.

The far‐infrared spectrum of ice Ih in the range 8–25 cm^{−} ^{1}. Sound waves and difference bands, with application to Saturn’s rings
View Description Hide DescriptionThe absorptionspectrum of single crystals of ice Ih with the light traveling perpendicular or at 5° to the c axis has been measured in the frequency range 8–25 cm^{−} ^{1} at 80, 100, 150, and 202 K. At high frequencies, the absorptivity is proportional to the fourth power of the frequency, and is caused by fundamental hypersonic waves having wavelengths of about 100 Å. At low frequencies, it is probably caused by difference bands from the translational lattice vibrations. The theory of the intensity of binary combination bands of ionic crystals has been specialized to the low‐frequency difference bands. Ice Ih has been assumed to have the same absorptivity as ice Ic, and ice Ic has been represented by an ordered crystal that resembles sphalerite. The model fits the measured absorptivity of ice Ih assuming that the difference bands arise mostly from the strong first‐order band centered at ∼230 cm^{−} ^{1}. The acoustic branches do not contribute significantly. The absorptivity, extrapolated to 3.0 cm^{−} ^{1} and interpolated to 85 K by means of the theory, was used to determine the mean thickness of ice in Saturn’s rings from the measured thermal component of the brightness temperature of the rings. If the slab model is used and scattering is neglected, the mean thickness is 30±10 cm.

Absorption spectra of cold dilute solid solutions
View Description Hide DescriptionInfrared absorption spectra have been obtained for some compounds trapped in crystalline solids by freezing liquid Xe, Kr, Ar, or CH_{4}solutions. The optical quality of the solid solutions is good, and they have been cooled to ∼80 K in 1.35 cm sample thicknesses to study the absorption in fundamental vibrational bands of the solutes. In the cases discussed, the bands are narrow, with observed full widths at half‐maximum absorbance 0.05–0.30 cm^{−1} greater than the instrumental resolution (0.18–0.29 cm^{−1}). The spectra appear to be free of ‘‘multiple site’’ and solute aggregate absorptions.Spectra displaying isotropic splitting in bands of natural BCl_{3}, SeF_{6}, OsO_{4}, TiCl_{4}, and MoF_{6} are presented, and band frequencies are compared with some results obtained in evaporative matrices, in the gas phase, and in liquidsolutions. For this comparison we have obtained some spectra of SeF_{6} and BCl_{3} gas.

An electron bombardment procedure for generating cation and neutral radicals in solid neon matrices at 4 K: ESR study of ^{1} ^{4}N_{2} ^{+} and ^{1} ^{5}N_{2} ^{+}
View Description Hide DescriptionA new generation technique for matrix isolated cations and neutrals involving the electron bombardment of the matrix sample during deposition is presented. Use of the method for the study of ion‐neutral reactions and other types of matrix experiments is discussed. Results for the gas phase electron bombardment of (CH_{3})_{2}O are compared with electron bombardment of (CH_{3})_{2}O conducted during neon deposition. The relative amounts of N and N_{2} ^{+} radicals trapped in neon matrices have been measured as a function of electron energies. An ESR analysis of ^{1} ^{4}N_{2} ^{+} and ^{1} ^{5}N_{2} ^{+}generated by electron bombardment in neon matrices at 4 K yields: g _{iso}=2.0004(2); A _{iso}(^{1} ^{4}N)=104.1(6) MHz and A _{iso}(^{1} ^{5}N)=146.1(6) MHz. The ESR results are used to estimate the extent of s‐p hybridization of N_{2} ^{+} and a comparison with theoretical calculations and several other 13 electron diatomic radicals is made.

Stark‐tuned Lamb‐dip spectroscopy of the ν_{4} and 2ν_{2} bands of ^{14}NH_{3}
View Description Hide DescriptionResults are presented of Stark‐tuned Lamb‐dip measurements in the 6 μm ν_{4} and 2ν_{2} bands of ^{14}NH_{3} obtained using an isotopic CO laser with an intracavity Stark cell. Some 230 lines from the isotopes^{12}C ^{16}O, ^{13}C ^{16}O, ^{12}C ^{18}O, and ^{13}C ^{18}O were investigated with fields up to 30 kV/cm. 71 coincidences with Stark‐tuned NH_{3} lines, involving several hundred Lamb dips, were assigned and analyzed. The positions of these lines relative to the CO laser lines are reported with a typical accuracy of 5–20 MHz. Model calculations of Lamb‐dip spectra are also given that include power and pressure broadening and the collisionally transferred resonances. These are directly compared with observed spectra, from which we deduce values for the pressure broadening coefficients and the collisional transfer efficiency. Nuclear hyperfine structure is resolved on several lines, and from fitting one of these we obtained the value e q Q=−4.83±0.3 MHz for the nuclear quadrupole coupling constant in the s2ν_{2} (J=10, K=9) state. We see numerous examples of weak ‘‘forbidden’’ transitions ΔM _{ J }=0, ±2. These arise from the nuclear quadrupole coupling, which mixes M _{ J } states.

Sub‐Doppler infrared–infrared double resonance spectroscopy of NH_{3} using Stark tuning and a diode laser
View Description Hide DescriptionDouble resonance experiments on NH_{3} are described in which a CO laser pumps a 6 μm ν_{4} transition while a tunable diode laser probes a 10 μm ν_{2} transition having a common lower level. Four different combinations of pump–probe transitions are studied. The CO laser is Lamb‐dip stabilized on the pump transitions, which are tuned into coincidence with it using a precision intracavity Stark cell. The pump and probe beams overlap colinearly in the Stark cell. The double resonance signals appear as narrow transmission peaks on the diode laser scans. The narrowest observed widths are ≂3 MHz (FWHM), a large portion of which is due to unresolved hyperfine structure. An analysis of the various broadening mechanisms indicates that the diode laser contributes less than 1 MHz to the widths. Resonances due to velocity‐preserving but state‐changing collisions are seen. Asymmetries between co‐ and counterpropagating linewidths are shown to arise from a combination of field inhomogeneity and coherent narrowing effects. The data are recorded using a computer‐aided, rapid‐scan, digital signal averaging technique.

The microwave spectrum of the SiN(^{2}Σ^{+}) radical
View Description Hide DescriptionThe microwaveabsorptionspectrum of the SiN radical has been detected in a dc glow discharge in a SiCl_{4} or SiH_{4}/N_{2} mixture. The three rotational transitions N=2←1, 3←2, and 4←3, have been observed in the frequency region 87 to 175 GHz by using a source‐frequency modulation microwave spectrometer. The rotational constant, the centrifugal distortion constant, the spin‐rotation coupling constant, and the magnetic hyperfine coupling constants b and c and the electric quadrupole coupling constant of the nitrogen nucleus have been precisely determined. The electronic structure of the SiN molecule has been briefly discussed using the magnetic hyperfine coupling constants and also the quadrupole coupling constant [3.05(23) MHz] of ^{1} ^{4}N.

Observation of the 1 ^{2}Π_{ u } ← X ^{2}Σ_{ g } ^{+} system in Cs_{2} ^{+}
View Description Hide DescriptionPredissociation of vibrational levels in the 1 ^{2}Π_{ u } state of Cs_{2} ^{+} has been observed by monitoring Cs^{+} photofragments resulting from absorption of laser radiation by the X ^{2}Σ_{ g } ^{+}ground state.Predissociation of the 1 ^{2}Π_{ u } state is induced by interaction with the repulsive 1 ^{2}Σ_{ u } ^{+} state and leads to the production of photofragments Cs^{+}+Cs in their ground electronic state with center‐of‐mass separation energies around 1 eV. The dominant features of the 1 ^{2}Π_{ u } ← X ^{2}Σ_{ g } ^{+} band system are confined to the wavelength range between 7700 and 8100 Å. A lower limit for the bond energy of the 1 ^{2}Π_{ u } state, measured relative to its dissociation limit Cs^{+}(^{1} S _{0})+Cs(^{2} P _{1/2}) is found to be 0.39±0.1 eV. The spin‐orbit splitting in the 1 ^{2}Π_{ u } state is measured to be 280 cm^{−} ^{1}.

Time resolved polarization spectroscopy: Level kinetics and rotational diffusion
View Description Hide DescriptionTime resolvedpolarizationspectroscopy is a powerful and sensitive technique for the study of dynamics in the liquid phase. Using this technique, it is possible to obtain information about various relaxation processes in solution, including the motions of molecules in different electronic states, and the transitions between these states. Since the experiment typically measures both electronic relaxation and molecular reorientation, the observed signal can have a complicated form which represents a coupling of these two effects in a way that depends on the particular scheme of decay processes which are present in a given system. We present a general algorithm for deriving the form of the experimental signal for an arbitrary scheme for systems exhibiting these phenomena, assuming that the molecular motion is described by asymmetric rotational diffusion. Several examples are presented and experimental results interpreted using the derived formulas, including cases where (1) the excited statediffusiontensor differs from that of the ground state, (2) decay of the excited state occurs to levels that do not return to the ground state, (3) the principal contribution to the signal results from birefringence, (4) there are several intermediate excited states which undergo rapid relaxation, and (5) two overlapping absorption bands create an initial condition with more than one excited state populated. Application of this formalism to fluorescence depolarization experiments is also discussed.

A theory of visible/UV multiphoton processes in molecules. I. Resonance effect on three‐photon absorption
View Description Hide DescriptionUsing a transition operator formalism, an expression for three‐photon transitions is derived. The transition operator formalism makes it possible to take into account the effects of population decays and pure dephasings occurring in the relevent vibronic states on the cross sections of the resonance multiphoton processes. It is shown that for a two‐photon resonant (2+1) process, the cross section can approximately be divided into two terms, one is called sequential term which cannot be taken into account by using the lowest order time‐dependent perturbation theory, and the other which can be derived from simultaneous three‐photon cross section under constraint of a resonant condition for the second photon absorption. Assuming the adiabatic approximation for the molecular states, analytical expressions for the cross sections for the three‐photon absorptions are derived in the displaced harmonic oscillator model of the molecules. An analytical expression taken into account the temperature effect is given for the simultaneous three‐photon absorption cross section.

Thermally stimulated luminescence studies of x‐irradiated L‐alanine:Cr^{3} ^{+} single crystals
View Description Hide DescriptionThermally stimulated luminescence studies of x‐irradiated L‐alanine:Cr^{3} ^{+} have been conducted in the interval 10–300 K. Glow peaks were observed at 42, 60, 72, 148, and 208 K. The 148 K peak has been previously reported, while the one at 208 K was of insufficient intensity to study. Also, the 60 K peak was difficult to analyze due to overlap with the neighboring peaks. Detailed analyses of the 42 and 72 K peaks yielded, respectively, the following parameters: E=24 meV, s=5.3 s^{−} ^{1}, and l=1; E=180 meV, s=2.4×10^{1} ^{0} s^{−} ^{1}, and l=2.8, where l is the kinetics order. Identical emission was observed from each peak, characterized by a maximum at 445 nm with FWHM equal to 0.605 eV. A tentative model is presented to explain these results in terms of detrapping and deexcitation via the excited singlet and triplet states.

Laser‐induced fluorescence spectrum of the CCN radical with an Ar^{+} laser
View Description Hide DescriptionThe laser‐induced fluorescence(LIF)spectrum of the CCN radical was observed with an Ar^{+} laser. The laser line of 465.8 nm coincided with two rovibronic transitions: R _{1} (23.5) and ^{ s } R _{2} _{1} (12.5) for A ^{2}Δ_{ i }:(010)Φ−X ^{2}Π_{ r }:(010)Δ. The LIFspectrum consisted of the resonant series: A ^{2}Δ_{ i }:(010)Φ−X ^{2}Π_{ r }: (v _{1} v _{2} v _{3})Δ, and the collision‐induced bands: A ^{2}Δ_{ i }:(010)Π−X ^{2}Π_{ r }:(v _{1}1v _{3})Σ^{+}, Σ^{−}. The analysis yielded the vibrational and the vibronic parameters of the ground electronic stateX ^{2}Π_{ r }.

Rotational Zeeman effect in ArHCl and ArDF
View Description Hide DescriptionThe molecular g values and magnetic susceptibilityanisotropies of ArH^{3} ^{5}Cl, ArH^{3} ^{7}Cl, and ArDF have been measured using pulsed Fourier transformmicrowave spectroscopy carried out in a Fabry–Perot cavity located in the bore of a superconducting solenoid magnet. The measured magnetic parameters for these three molecules, obtained from rotational Zeeman splittings of K=0 R‐branch transitions are The relationship between these measurements and the signs of the electric dipole moments, the molecular quadrupole moments, and the anisotropy in the second moments of the electronic charge distribution is discussed in detail, with particular emphasis on the effects of the large zero point bending motions present in these molecules. The relationships between the measuredZeeman parameters and the properties of the individual rare gas and hydrogen halide subunits is treated. We conclude that the contribution of the rare gas atom to the measuredg _{⊥} values is negligibly small, but that the rare gas atom apparently does make a measurable contribution to the magnetic susceptibilityanisotropies of at least the HF containing systems.

A structural interpretation of the vibrational spectra of binary fluorohafnate glasses
View Description Hide DescriptionBinary glass samples have been prepared in the HfF_{4}–BaF_{2} system, containing between 62–73 mol % HfF_{4}. Their infrared absorption and polarized Raman spectra have been studied. The glass transition and crystallization temperatures have also been measured. The vibrational modes have been assigned and the results were compared with those previously obtained for ZrF_{4}–BaF_{2} glasses. The cation isomorphic substitution of Zr by Hf allowed us to make a more definite vibrational assignment. The structures of the two glass systems were found to be similar, though not identical. The structural interpretation of the vibrational spectra was consistent with the occurrence of sixfold coordinated Hf atoms, probably forming a network of octahedral chains cross linked by Ba–F ionic bonds, with the degree of bridging increasing with the Hf content of the glasses. Substitution of Ba by Sr did not affect the structure. Due to the large F/Ba ratio of ∼10, there is an overlap of neighboring chains that may lead to sharing of a single nonbridging fluorine atom between two Hf atoms belonging to neighboring chains. The existence, on the average, of two such F atoms for each Hf may raise its effective coordination number from 6 to ∼8, when determined from an x‐ray radial distribution function analysis, as recent results show. Vibrational spectroscopy, however, appears to be able to effectively distinguish which is the parent Hf atom to which every fluorine is bonded by a predominantly covalent, directional and shorter bond, as part of a highly symmetric octahedral coordination shell. In this respect, vibrational spectroscopy (particularly Raman) appears to be a more sensitive structural tool than x‐ray diffraction.