Index of content:
Volume 90, Issue 12, 15 June 1989

Vacuum ultraviolet laser spectra of ICl
View Description Hide DescriptionVacuum ultraviolet fluorescence excitation spectra of jet cooled ICl have been recorded between 158 and 166 nm using a VUV ‘‘laser’’ generated by four‐wave sum‐mixing in Mg vapor. An unambiguous vibrational analysis of the spectra has shown that transitions from v‘=0 of the ground state probe high vibrational levels of the first tier EO^{+} ion‐pair state between v’=245 and 360. Molecular constants obtained from both Dunham and near dissociation expansions are presented. Strong intensity modulation of the spectra was observed. Possible reasons for this effect are discussed.

An electron spin resonance study of the structure of CCl^{+} _{4} radical cation in carbon tetrachloride γ‐irradiated at low temperatures by powder and single crystal analyses
View Description Hide DescriptionThe CCl^{+} _{4} radical cation produced in CCl_{4} by γ irradiation at low temperatures was studied by electron spin resonance(ESR)spectroscopy to clarify the electronic structure. By combining an analysis of the angular dependence of ESRspectra in a single crystal plane with a spectral simulation of the powder pattern including the isotropic combinations of ^{3} ^{5}Cl and ^{3} ^{7}Cl atoms, it was found that two chlorine atoms are concerned in the radical, having different hyperfine coupling tensors of A(^{3} ^{5}Cl_{1})=11.2, 0.5, 1.3 mT and A(^{3} ^{5}Cl_{2})=6.2, 0.8, 1.8 mT. The gtensor is approximately axially symmetric and shows a large positive g _{⊥} shift (g=1.999, 2.10, 2.116), different from the rhombic gtensor of σ* dimer cations of alkyl halides and freons (g _{max}=2.04). The directions of the maximum hyperfine coupling of the two chlorine atoms Cl_{1} and Cl_{2} are close to each other and close to the g _{min} direction, though having a small deviation from the g _{min} direction by about 3°–10°. From a consideration on the g anisotropy combined with the optical absorption spectrum, the radical was suggested to be a Cl atom‐like species, in which one of the C–Cl bonds in the parent CCl^{+} _{4} is released and the atom‐like chlorine nuclei formed makes a three electron bond [(σCl⋅⋅⋅Cl)^{2} (σ*Cl⋅⋅⋅Cl)^{1}] with a Cl atom in the released CCl^{+} _{3} group. A discussion is given for the difference in the formation of the cationic species in CCl_{4} and the σ* dimer radical cation in the other alkyl halides.

Study of the ν_{6} band of CH_{3}Br by infrared laser sideband and submillimeter‐wave spectroscopy
View Description Hide DescriptionAbout 136 transitions in the ν_{6} band of CH_{3}Br have been measured using an infrared laser sideband spectrometer. The sidebands were generated by mixing the 10 μm radiation of a CO_{2} laser with the radiation of a tunable high power microwave source. Frequencies of transitions with J≤71 and K≤8 are reported. The accuracy of the measurements is estimated to be better than 10 MHz. Some high J rotational transitions in the v _{6}=1 state have also been measured with a submillimeter‐wave spectrometer. The new measurements have been combined with previous results to derive vibration–rotation parameters for the v _{6}=1 state.

Conformational stability, barriers to internal rotation, vibrational assignment and a b i n i t i o calculations of fluoroacetyl chloride
View Description Hide DescriptionThe far infrared spectrum (375 to 35 cm^{−} ^{1}) of gaseous fluoroacetyl chloride, CH_{2}FC(O)C1, has been recorded at a resolution of 0.10 cm^{−} ^{1}. The fundamental asymmetric torsions of the more stable t r a n s (halogen atoms are t r a n s) and the high energy c i sconformations have been observed at 116.18 and 49.42 cm^{−} ^{1}, respectively, each with several upper state transitions falling to lower frequency. From these spectral data, an asymmetric potential function has been calculated and the potential coefficients are: V _{1}=43±6, V _{2}=1039±36, V _{3}=498±3, V _{4}=149±21, and V _{6}=−10±7 cm^{−} ^{1}. The t r a n s to c i s and c i s to t r a n s barriers are 1455±25 cm^{−} ^{1} (4.16±0.07 kcal/mol) and 914±24 cm^{−} ^{1} (2.61±0.07 kcal/mol), respectively, with an enthalpy difference of 541±45 cm^{−} ^{1} (1.55±0.13 kcal/mol). From studies of the Raman spectra at variable temperatures, values of 509±37 cm^{−} ^{1} (1.46±0.10 kcal/mol) and 310±8 cm^{−} ^{1} (0.89±0.02 kcal/mol) have been determined for the enthalpy difference for the gas and liquid, respectively. The conformational stability, barriers to internal rotation, and fundamental vibrational frequencies which have been determined experimentally, are compared to those obtained from a b i n i t i o Hartree–Fock calculations employing both the 3‐21G* and 6‐31G* basis sets, and to the corresponding quantities obtained for some similar molecules.

Rotational analysis of Ã ^{2} A _{1}↔X̃ ^{2} E electronic transition of the jet‐cooled methylthio radical
View Description Hide DescriptionA set of molecular parameters describing both the X̃ and Ã states of CH_{3}S, has been obtained by a joint fitting of the rotationally resolved electronic transitions observed in a free‐jet‐cooled laser‐induced fluorescence study of CH_{3}S and an earlier microwave study of its X̃ state. The present work shows that because of incomplete information, nearly all of the previously reported molecular parameters for CH_{3}S must be significantly revised. The present observations show an unusual electronic structure for the radical, characterized by a short C–S bond distance and peculiar methyl group geometry in the ground state. The C–S bond is observed to lengthen markedly in the excited Ã state.

Hexamethylbenzene as a sensitive nuclear magnetic resonance probe for studying organic crystals and glasses
View Description Hide DescriptionDeuterated hexamethylbenzene (HMB) is used as a probe molecule for ^{2}H NMR studies of the crystalline state of hexachlorobenzene and of several organic glasses. By measuring the spin–lattice relaxation and the line shape in the temperature range of 4–300 K the dynamical parameters of the molecular reorientation are investigated. For the system HMB/hexachlorobenzene, we find exponential relaxation and for the corresponding T _{1} an increase of its activation energy by a factor of 2 in comparison to the neat HMB. A homogeneous mixing of the guest and host molecules is found at least for guest concentrations up to 7%. In contrast, nonexponential spin–lattice relaxation is characteristic for all glass matrices, indicating motional heterogeneities. A log–Gauss distribution for the corresponding motional correlation times gives a good fit of the data. Its width parameter decreases linearly with temperature, while the mean correlation times are described by an Arrhenius law. The mean activation energy is reduced by a factor of about 3.5 as compared to neat HMB, demonstrating a loose packing of the molecules in the glass matrices.

An investigation of the trimethylammonium chloride molecule in the vapor phase by pulsed‐nozzle, Fourier‐transform microwave spectroscopy
View Description Hide DescriptionThe ground‐state rotational spectra of the symmetric‐top isotopomers [(CH_{3})_{3} ^{1} ^{4}N, H^{3} ^{5}Cl], [(CH_{3})_{3} ^{1} ^{4}N, D^{3} ^{5}Cl], [(CH_{3})_{3} ^{1} ^{5}N, H^{3} ^{5}Cl], and [(CH_{3})_{3} ^{1} ^{5}N, H^{3} ^{7}Cl] of a dimer formed between trimethylamine and hydrogen chloride have been observed in the vapor above heated samples of solid trimethylammonium chloride by pulsed‐nozzle, Fourier‐transform, microwave spectroscopy. Spectroscopic constants have been determined in each case and for [(CH_{3})_{3} ^{1} ^{4}N, H^{3} ^{5}Cl] the values are B _{0}=1800.4605(2) MHz, D _{ J }=0.320(10) kHz, D _{ J K }=13.59(11) kHz, χ(^{3} ^{5}Cl)=−21.625(5) MHz, and χ(^{1} ^{4}N)=−3.504(5) MHz. The observed rotational constantsB _{0} indicate that the nuclei N, H, and Cl lie along the C _{3} symmetry axis of the molecule in the order N⋅⋅⋅H⋅⋅⋅Cl with the distance r(N⋅⋅⋅Cl)=2.8164(3) Å. A comparison of the values of the nuclear quadrupole coupling constants χ(^{3} ^{5}Cl) and χ(^{1} ^{4}N) with those in related dimers and molecules leads to the evocation of some ionic character (CH_{3})_{3}N ^{+}H C̄l arising from a significant extent of proton transfer from Cl to N. This conclusion is reinforced when the values of χ(^{3} ^{5}Cl) and χ(^{1} ^{4}N) predicted on the basis of a recent electrostatic hydrogen‐bonded model (CH_{3})_{3}N⋅⋅⋅HCl of the dimer are considered. The intermolecular stretching force constants k _{σ}=84(3) N m^{−} ^{1} determined from D _{ J } has a magnitude closer to that expected in the ion‐pair limit than in the hydrogen‐bond limit.

Probability tables for small clusters of impurity atoms in sc, bcc, and fcc lattices assuming long‐range interaction
View Description Hide DescriptionPowers and coefficients of polynomials describing the concentration dependent probabilities of small clusters of impurities are presented. For singles, pairs, and triples of impurities randomly distributed in simple cubic, body‐centered‐cubic, or face‐centered‐cubic host lattices, all interaction ranges between first next neighbor (1NN) and 8NN interaction are considered (singles and pairs up to 16 NN). The change of the probability functions with increasing interaction range is described. It is noted that an additional consideration of clusters with more impurities than a triple does not essentially improve the statistical error in the case of long‐range interaction. For the three lattices, empty shells and inequivalent lattice vectors are given.

Collisional line shape for the rotational spectrum of methylcyanide: Experiments and theory
View Description Hide DescriptionA collisional line‐shape study specialized in the rotational spectrum of CH_{3}CN is presented. This system provides a test for pressure broadening and shift theories which has until now been hindered by inadequate theory and by experimental results at variance. New accurate measurements are presented which allow an explanation of the problem related to the reliability of experimental data. The thorough comparison made between measurements and an improved theory—obtained inside the frame of the Anderson–Tsao–Curnutte approximation—demonstrates the ripeness of the study of such rotational spectrum from both the experimental and theoretical points of view.

Infrared spectrum of sodium hydride
View Description Hide DescriptionThe infrared spectrum of gaseous NaH from 886 to 1245 cm^{−} ^{1} has been measured with a resolution of 0.015 cm^{−} ^{1} at temperatures between 670 to 720 °C. The v=1←0, 2←1, and 3←2 transitions have been observed and combined with rotational transitions measured by others to obtain appropriate rovibrational constants and Dunham potential constants. The Herman–Wallis intensity effect has been measured in order to estimate a transition moment of 0.31±0.05 D for the v=1←0 transition.

Anisotropic reorientational relaxation of biphenyl: Transient grating optical Kerr effect measurements
View Description Hide DescriptionSubpicosecond transient gratingoptical Kerr effect measurements have been used to evaluate the reorientation of biphenyl molecules in neat biphenyl and n‐heptane solutions. Besides an ultrafast (100 fs time scale) component associated with librational damping/dephasing, two reorientational relaxation components are observed. The slow reorientation is due to rotation around the short axes of the molecule (tumbling motion), the fast reorientation is associated with internal rotation around the central C–C bond and/or rotation of the whole molecule around its long axis (spinning motion). Whereas the tumbling motion has been observed in earlier depolarized light scattering data, the time resolvedKerr data presented here are the first ones to reveal the dynamics of the fast reorientation component and the ultrafast librational dynamics. It is shown that the diffusive reorientational relaxation must be coupled to the ultrafast librational dynamics, and implications of this coupling are pointed out.

Time‐dependent theory of Raman scattering for systems with several excited electronic states: Application to a H^{+} _{3} model system
View Description Hide DescriptionThe time‐dependent formulation of Raman scatteringtheory is used to study how nonadiabatic interactions affect the Raman spectrum of a model H^{+} _{3} system, which has two excited electronic states. We start with a formula derived by Heller which gives the Raman scattering cross section as the Fourier transform (over time) of a time‐dependent overlap integral. The latter is calculated with a method proposed by Fleck, Morris, and Feit, and extended to curve crossing by Alvarellos and Metiu. In performing these calculations we are especially interested in displaying effects typical of systems having more than one upper state. If the incident laser populates two electronic states there are several ways (i.e., excite to state one and emit from state two, excite to state one, and emit from state one, etc.) by which the Raman process can reach a given final state, and this leads to quantum interference. This interference is manifested in the Raman cross section as approximate selection rules controlling which final states can be reached through the Raman process. These selection rules depend on the relative orientation of the transition dipoles that radiatively couple the ground electronic state with the excited electronic states. The magnitude of the nonadiabatic contribution to the Raman emission, e.g., the contribution from absorbing to state one and emitting from state two, can be determined from the polarization dependence of the Raman emission if the transition dipoles have neither parallel nor antiparallel relative orientation.

Transient photophysical hole‐burning spectroscopy of the hydrated electron: A quantum dynamical simulation
View Description Hide DescriptionResults for the time‐dependent adiabatic eigenspectrum of an electron in water evolving in dynamic equilibrium have been obtained via quantum molecular dynamics simulation and used to evaluate the results expected from time‐resolved transient optical hole‐burning experiments. The dependence on excitation frequency and pulse length have been explored. The calculated results indicate that a relatively broad hole is created, but that, for ultrashort pump–probe time delays (≤100 fs) and comparably short pulses, the shape is distinctly different from the equilibrium spectrum. A slower component in the spectral evolution is also present, but appears likely to be difficult to distinguish experimentally. The shape of the absorption deficit is characteristic of the inhomogeneously broadened 1s, 2p‐type electronic state structure found previously to underlie the equilibrium spectrum, and distinguishes between this description and a number of proposed alternatives. With pulse durations comparable to the best now available, the phenomenon appears experimentally accessible.

Ionization yields, total absorption, and dissociative photoionization cross sections of CH_{4} from 110 to 950 Å
View Description Hide DescriptionAbsolute absorption and photoionization cross sections of methane have been measured with an accuracy of about 2% or 3% over most of the wavelength range from 950 to 110 Å. Also, dissociativephotoionization cross sections were measured for the production of CH^{+} _{4} , CH^{+} _{3} , CH^{+} _{2} , CH^{+}, and C^{+} from their respective thresholds to 159 Å, and for H^{+} and H^{+} _{2} measurements were made down to 240 Å. Fragmentation was observed at all excited ionic states of CH_{4}.

Similarity transference of molecular parameters. II. The bond distances, force constants and polar tensors of HC_{3}N and HC_{5}N
View Description Hide DescriptionThe similarity transference procedure is extended to the calculations of the geometries and vibrational frequencies of HC_{3}N and HC_{5}N. Similarity models are constructed using the experimental and STO‐3G molecular orbital values of the bond distances and force constants of the HCN, C_{2}H_{2}, CH_{3}CN, C_{4}H_{2}, CH_{3}CCH, and C_{2}N_{2} reference molecules. Simple linear regression calculations based on these similarity models and using STO‐3G values of the bond distances and force constants of HC_{3}N result in estimates of the experimental bond distances and frequencies which have root mean square errors about an order of magnitude smaller than the one for the molecular orbital values. Similarity models for the bond distances and vibrational frequencies and intensities of HC_{5}N are also reported.

Spectra, radiative lifetimes, and band oscillator strengths of the A ^{1}Π–X ^{1}Σ^{+} transition of BH
View Description Hide DescriptionWe report new spectroscopic information on the A ^{1}Π–X ^{1}Σ^{+} transition of BH including the observation of the 0–1 and 1–2 bands using laser induced fluorescence(LIF) techniques. Ratios of Einstein coefficients, band oscillator strengths and transition probabilities have been obtained for the 0–1 compared to the 0–0 band and the 1–0 and 1–2 compared to the 1–1 band. These ratios indicate that the emission observed occurs predominantly within the diagonal elements. Additionally, the radiative lifetimes of the v’=0, 1, and 2 levels have been measured to be 127±10, 146±12, and 172±14 ns, respectively. Using the ratios above and the experimental lifetimes, we have obtained Einstein emission coefficients and band absorption oscillator strengths. These values are compared to several calculations from the literature and in some instances large differences are seen.

The structures and dipole moments of Ar–PF_{3} and Kr–PF_{3}
View Description Hide DescriptionThe complexes of PF_{3} with Ar and Kr, were studied by Fourier transformmicrowave spectroscopy. The force constants and amplitudes of vibration for the van der Waals modes of the complexes and the average moments of inertia and structural parameters were estimated from the centrifugal distortion constants. The distance (R _{c.m.} )_{ave} between the rare‐gas atom and the center of mass of PF_{3} is 3.959 Å for the Ar complex and 4.077 Å for Kr while the angle (θ_{c.m.} )_{ave} between the R _{c.m.} vector and the C _{3} axis of the PF_{3} is 69.30° and 67.25°, respectively. The dipole moments of both complexes and of free PF_{3} were determined. The induced dipole components estimated for the rare gas using electric fields from a b i n i t i o calculations of PF_{3} agree with the experimental values for a conformation with the rare gas over a PF_{2} face. The PF_{2} face conformation is also consistent with the observed and a b i n i t i o estimates of the ^{83} Kr nuclear quadrupole coupling constant for the ^{83} Kr–PF_{3} species.

Nondipole light scattering by partially oriented ensembles. II. Analytic algorithm
View Description Hide DescriptionWe consider the elastic single scattering of light from an ensemble of identical particles with nonrandom orientational distribution. The particle is modeled as an arbitrary rigid array of N dipole polarizabilities, and the polarizabilities within each particle interact through the retarded dipole–dipole tensor. We consider the scattering ensemble to be an ‘‘optical element’’ in the Müller formalism; thus its polarized scattering properties, both dipole and nondipole, are specified completely by a four‐by‐four Müller matrix M(ψ), where ψ is the scattering angle. In a previous work on orientationally random ensembles, the slow convergence of the nondipole elements suggested that they might be particularly sensitive to small orienting forces. This was confirmed by numerical calculations, which, however, also demonstrated the need for a faster analytic averaging algorithm. In this paper we extend the uniform distribution analytic algorithm to cases in which the orientation of the particles is specified by a general nonuniform distribution. We then specialize the general formula to the case of axially nonrandom orientation, such as might be caused by an external static electric fieldE operating on a permanent electric dipole of the model particle. We present an explicit algorithm for M(ψ,E) for this special case, together with an estimate of its computability. For models with just a few subunits, it should be computable by a desktop machine with four megabytes of memory. For more realistic models using 1000 subunits, the job can easily rise into the supercomputer range.

Electronic spectra of isolated cations in supersonic jets by mass‐selected ion‐dip spectroscopy. Cations of benzene, p‐difluorobenzene, and 1,3,5‐trifluorobenzene
View Description Hide DescriptionThe electronic transitions of the cations of benzene, p‐difluorobenzene (p‐DFB), and 1,3,5‐trifluorobenzene (1,3,5‐TFB) have been measured by mass‐selected ion‐dip spectroscopy which utilizes the dissociation of a parent cation in an excited state. This spectroscopy was successfully applied to the vibrational level selected cation in the ground electronic state which was prepared by 1+1 REMPI (resonant enhanced multiphoton ionization) of the neutral molecule in a supersonic jet. For all the cations, the spectra due to the transition from the ground state to the excited π,π state were observed. Ion‐dip spectra having sharp vibrational structures were found for p‐DFB and 1,3,5‐TFB cations, while a broad spectrum was observed for a benzene cation. It was also found that the vibrational structure of the ion‐dip spectrum of the 1,3,5‐TFB cation is quite different from that of the fluorescence excitation spectrum. The assignments of the ion‐dip spectra and dissociation mechanisms of the excited cations will be discussed.

Examination of CD_{3} vibrational structure by resonance Raman spectroscopy
View Description Hide DescriptionThe application of the resonance Raman spectroscopic method to the deuterated methyl radical is reported. Spectroscopic constants for CD_{3} are compared with previously reported values for CH_{3}. The experimental and theoretical molecular physics of the methyl radical are discussed.