Volume 89, Issue 4, 15 August 1988
Index of content:
89(1988); http://dx.doi.org/10.1063/1.455121View Description Hide Description
It is proposed that the two Λ‐doublet levels of linear molecules with nonzero electronic orbital angular momentum be labeled Λ(A’) and Λ(A‘), e.g., Π(A’) and Π(A‘) for Π states, etc., according to the following prescription: All series of levels in which the electronic wave function at h i g h J is symmetric with reflection of the spatial coordinates of the electrons in the plane of rotation will be designated Λ(A’) for all values of J, and all those for which the electronic wave function is antisymmetric with respect to reflection will be denoted Λ(A‘). It is emphasized that this notation is meant to supplement, a n d n o t r e p l a c e, the accepted spectroscopice/f labeling and the parity quantum number. The utility of the Λ(A’)/Λ(A‘) notation is that it is of most relevance in the mechanistic interpretation of reactive or photodissociative processes involving open‐shell molecules.
89(1988); http://dx.doi.org/10.1063/1.455715View Description Hide Description
Spectra of the 2 n 0 progression in the B̃ 1 E‘–X̃ 1 A ’ 1 transition in NH3 and ND3 have been recorded at an effective resolution down to 0.008 cm− 1 at the VUV equivalent energy. Line shape analysis reveals a predissociation lifetime of 0.25(2) ns for ND3 and 6.1(7) ps for NH3 independent of rotational state and v 2. Measurement of the magnetic sensitivity of low J lines gave an electronic g value. Rotational analyses of the 2 n 0 bands with n up to 6 for ND3 and 8 in NH3 are presented and indicate some small vibrational perturbations. The observed difference between the Coriolis coupling constant ζ derived from the rotational structure and the g value indicates a mild Jahn–Teller effect.
89(1988); http://dx.doi.org/10.1063/1.455122View Description Hide Description
We have analyzed the ν3 antisymmetric stretching fundamental of the N3 radical. Azide radicals N3 were generated in a multipass cell by the reaction of Cl atoms and HN3 and detected in absorption using a Fourier transform spectrometer. Improved constants for the 000 vibrational level of X̃ 2Π g are reported along with observations of the 001 level. The frequency of the ν3 fundamental at 1645 cm−1 was found to be somewhat lower than expected.
Statistical fine structure in the inhomogeneously broadened electronic origin of pentacene in p‐terphenyl89(1988); http://dx.doi.org/10.1063/1.455123View Description Hide Description
Recently, the first observation of statistical fine structure on an inhomogeneously broadened absorption profile was reported [W. E. Moerner and T. P. Carter, Phys. Rev. Lett. 5 9, 2705 (1987)] for mixed crystals of pentacene in p‐terphenyl using laser frequency‐modulation spectroscopy. Statistical fine structure is time‐independent structure on the inhomogeneous line caused by statistical variations in the spectral density of absorbers in each frequency interval. In this work, a model and an analysis of statistical fine structure using autocorrelation techniques are presented, and the dependence of the effect for pentacene in p‐terphenyl at 1.4 K on modulating frequency, detection phase, center concentration, and position in the sample is described. Statistical fine structure provides a new method for probing inhomogeneous systems that allows investigation of the underlying probability distribution function and the determination of the homogeneous linewidth. This fine structure also represents a fundamental limit on the detectability of shallow spectral features in inhomogeneously broadened lines.
89(1988); http://dx.doi.org/10.1063/1.455124View Description Hide Description
The entire absorptionspectrum of PuF6 in the near IR and visible regions has been obtained at 0.5 cm− 1 or higher resolution. A detailed analysis of the vibronic energy levels is presented and spectroscopic assignments of all observed transitions are made. The energy levels obtained from these spectroscopic measurements of the gas phase PuF6 molecule are the most accurate values available. A comparison is shown of our assignments and results with the matrix‐isolation experiment and available theoretical studies.
89(1988); http://dx.doi.org/10.1063/1.455125View Description Hide Description
Quantitative calculations show that numerous spin‐forbidden linear optical transitions observed in trivalent rare earth ions acquire a major fraction of their intensity from hitherto neglected contributions involving spin–orbit linkages within excited configurations. Motivated by the importance of analogous linkages previously demonstrated in two‐photon absorption, we derive a general expression applicable for all l N →l N transitions which can be incorporated into a revised Judd–Ofelt analysis of observed intensities. Presenting this revised analysis of observed linear absorption intensities for Gd3 + and Eu3 +, we show that the new contribution is often comparable to standard contributions. With substantial modification of previously fitted phenomenological parameters, an improved fit to observed intensities is achieved, suggesting that reanalysis of linear intensity data for all trivalent rare earths is warranted.
89(1988); http://dx.doi.org/10.1063/1.455126View Description Hide Description
Nuclear quadrupoleinteraction parameters have been determined at 293 and 77 K at the site of 1 8 1Ta nuclei in anhydrous HfCl4 by the method of time‐differential perturbed angular correlations. Measurements performed on the 133–482 keV γ–γ cascade in 1 8 1Ta following the β decay of 1 8 1Hf indicate that HfCl4 does not possess cubic symmetry as claimed by some researchers.
Reorientation of benzene in its crystalline state: A model case for the analogy between nuclear magnetic resonance spin alignment and quasielastic incoherent neutron scattering89(1988); http://dx.doi.org/10.1063/1.455127View Description Hide Description
The close analogy between 2H‐NMR spin alignment and 1H quasielastic incoherent neutron scattering [J. Chem. Phys. 8 4, 4579 (1986)] in determining the geometry and time scale of molecular reorientation is illustrated by an experimental example. Analysis of the final states of both methods show consistently that benzene in its (poly)crystalline state reorients by rotational jumps about the molecular sixfold symmetry axis. Emphasis is put on the quasielastic structure factor of incoherent neutron scattering, which excludes random jumps among the six orientations as the reorientation mechanism, allowing only single rotational jumps.
89(1988); http://dx.doi.org/10.1063/1.455128View Description Hide Description
The polymers of CH2Se and CD2Se were pyrolyzed at 700 °C to yield monomeric selenoformaldehyde. Laser excitation spectra of these molecules were recorded. The origin of the weak singlet–singlet Ã 1 A 2←X̃ 1 A 1 system in CD2Se was observed at 13 631.5 cm− 1 while that of CH2Se has been calculated to be at 13 555 cm− 1. The much more extensive and intense singlet–triplet ã 3 A 2←X̃ 1 A 1 systems of CH2Se/CD2Se have origins at 12 171.0/12 262.7 cm− 1. A double minimum potential energy function was used to fit the triplet state ν’ 4 inversion levels and was found to be similar to that observed for thioformaldehyde.
U l t r a high‐resolution fluorescence excitation spectrum of 1 B 1 pyrimidine in a molecular beam. Structural assignments, analysis of singlet–triplet perturbations, and implications for intersystem crossing in the isolated molecule89(1988); http://dx.doi.org/10.1063/1.455129View Description Hide Description
We have observed, and assigned, the fluorescence excitation spectrum of the 00 0 band in the 1 B 1←1 A 1 electronic transition of pyrimidine, at a resolution of ∼10 MHz. The rotational constants of the 1 B 1 state, the lowest excited singlet state, are A’=6352±3, B’=5853±3, and C’=3042.0±0.5 MHz. The magnitudes of these constants are not very different from those of the ground (1 A 1) state. However, the in‐plane a and b inertial axes in the 1 B 1 state are rotated by 90° with respect to those of the 1 A 1 state. The spectrum also exhibits numerous perturbations, evidenced by the presence of extra lines, anomalous intensities and lifetimes, and shifts of the main lines from their expected positions. The perturbations are strongly magnetic‐field dependent, demonstrating that they arise from an intramolecular coupling of the 1 B 1 state with nearly isoenergetic rovibronic levels of a lower triplet (3 B 1) state. Models are proposed to account for this behavior based on a deconvolution of the experimental spectrum and simulations of the observed Zeeman effects. The most satisfactory interpretation of the data (in the language of the zero‐order states) is obtained if it is assumed that a single rovibronic 1 B 1 level is spin–orbit coupled to one or a few 3 B 1 levels, which in turn are coupled v i a rotationally dependent Coriolisinteractions to a dense manifold of background levels, probably those of the 1 A 1 state. Because the latter coupling is small, typically less than the linewidths in the spectra, it is manifested only in a K ’ +1 dependence of the lifetimes of selected molecular eigenstates and the reduced g values required to fit the magnetic‐field dependence of their spectra.
89(1988); http://dx.doi.org/10.1063/1.455130View Description Hide Description
Electrochromism (electric‐field‐modulated absorption)spectra are reported for a thiapyrylium dye in a polycarbonate host. When the dye is homogeneously dissolved in the polymer, excitation to the first excited singlet state changes its dipole moment by approximately 6 D (2×10− 2 9 C m). When the dye is aggregated with the host polymer, the structure of the aggregate is believed to consist of layer of the dye associated with layers of the polymer. The electrochromismspectrum indicates that the aggregate possesses two (or more) excitonic states separated by ∼450 cm− 1, of which only the lower contributes to the absorptionspectrum in the absence of an electric field; application of a field mixes and shifts these states and transfers intensity between them. The near degeneracy is believed to be a consequence of weak Davydov interactions between the exciton states of pairs (or larger stacks) of dye layers.
Photophysics of atomic magnesium isolated in solid methane and perdeuteromethane. I. Optical absorption of an impurity species in a quantum solid89(1988); http://dx.doi.org/10.1063/1.455131View Description Hide Description
Optical absorptionspectra recorded at 12 K for the 1 P–1 S transition of atomic magnesium isolated in solidmethane and perdeuteromethane exhibit significant differences in their spectral positions and bandwidths. The greater blue shift in CD4 relative to CH4 was found to be unchanged upon sample annealing. Shifts in the spectral position of the resonance transition in the two solids are discussed in the context of mass related differences in the lattice parameters of these light solids which is a manifestation of their behavior as quantum solids. From the blue shift observed in CD4 , the isotope having the smaller cage size, it is proposed that repulsive forces dominate in the interaction of the atomic magnesium guest species with its host. The greater bandwidth of the absorption profile in CH4 , relative to CD4 is discussed in terms of a Jahn–Teller model where the effective frequency of the phonon modes coupling with the electronic transition of the optically active impurity are higher in the former solid.
Photophysics of atomic magnesium isolated in solid methane and perdeuteromethane. II. Temperature dependence of steady state and time‐resolved luminescence89(1988); http://dx.doi.org/10.1063/1.455132View Description Hide Description
The visible emission resulting from photoexcitation of the first allowed singlet resonance transition of atomic magnesium isolated in the solid methanes is analyzed with spectral and time resolution. Two emission bands are observed in both solid methane (CH4) and solid perdeuteromethane (CD4) at 516 and 560 nm. The measured radiative lifetimes of the two bands are 12 and 19 ms, respectively, and independent of isotopic variation of the host. On the basis of their spectral position and radiative lifetimes, the emission features are assigned to the spin‐forbidden 3 P–1 S intercombination band of atomic magnesium.Emission spectra recorded at various temperatures in the range 12–33 K were found to show reversible temperature dependence whereby the intensity of the higher energy 516 nm band decreased monotonically with a concomitant increase in that of the lower energy 560 nm band. Time‐resolved measurements of the two bands in Mg/CH4 and Mg/CD4 show that the radiative lifetime of the 516 nm band decreases with increasing temperature while that of
the 560 nm band remains constant but showing a rise time component. The magnitude of this rise time is correlated with that of the decreasing lifetime of the 516 nm band. With a combination of the temperature dependence observed in the radiative lifetimes and spectral intensities of the 516 and 560 nm emission bands in the Mg/CH4 and Mg/CD4 systems, dynamical information is extracted which allows for the creation of a simple kinetic model and an identification of the mechanism giving rise to this temperature dependent phenomenon. The experimental data is found to exhibit linear behavior on an Arrhenius plot indicating an activated process of population interconversion between the emitting levels. In solid methane the barrier in this activated process is 177 and 125 cm− 1 in solid perdeuteromethane. The origin of this process is discussed in terms of (a) structural phase transitions of the solid methanes and (b) the effect of strong Jahn–Teller coupling between phonon modes of the solid methane host and the spin triplet electronic state of the atomic magnesium guest yielding symmetry‐distinct minima on this surface. The lengthened values of the measured radiative lifetimes in the solid methanes, indicate population equilibration amongst the total degeneracy of the triplet state J sublevels of atomic magnesium in these solids. Because of the extremely low temperatures at which the experiment is conducted, extensive orbital reduction factors are implied to achieve such equilibration conditions. This is a manifestation of very strong Jahn–Teller coupling, a necessary condition for the stabilization of symmetry‐distinct minima on the triplet spin state surface and the mechanism proposed to explain the presence of the temperature dependent emission features.
Photophysics of atomic magnesium isolated in solid methane and perdeuteromethane. III. Evidence for a kinetic isotope effect in the competitive process of atomic magnesium insertion into a C–H bond89(1988); http://dx.doi.org/10.1063/1.455133View Description Hide Description
The formation of the C–H bond insertion product methylmagnesium hydride (CH3MgH) and the simultaneous emission of atomic triplet magnesium are observed following photoexcitation of the first allowed singlet resonance transition of atomic magnesium isolated in solid methane matrices at 12 K. Isotopic variation of the solid methane hosts produces observable differences in the relative branching ratios into the photophysical (atomic triplet emission) and photochemical (insertion product formation) channels. In solid perdeuteromethane (CD4), the intensity of the atomic emission is approximately five times that in solid methane (CH4) while the rate of formation of the insertion product shows the opposite behavior in the two solids. No singlet atomic magnesium emission is observed in the solid Mg/methane systems and the rise time of the atomic triplet emission is deduced to occur on a time scale of less than 10 ns. A simple model derived from spin and orbital correlations between reactants and products is presented which considers the effect of the low symmetry of an insertive reaction coordinate in the approach geometry of atomic magnesium to methane yielding the linear product CH3MgH. Using this model, the absence of the singlet atomic emission is explained in terms of the attractive nature of the singlet surface with respect to the formation of a bent, inserted intermediate. The observation of an enhanced rate of atomic magnesium intersystem crossing is thought to occur as a result of the symmetry‐induced participation of the repulsive triplet surface in the process leading to the linear insertion product. The model also suggests an origin for the observed kinetic isotope effects. Differences in the observed behavior of the 1 P state of atomic magnesium in gas‐phase and solid‐phase quenching experiments (explicitly the formation of fragmented products only with no unreacted atomic triplet in the former case and the formation of the insertion product with intense atomic triplet emission in the latter) are discussed in relation to the presence of efficient relaxation pathways in the solid phase and the absence of such pathways in the single‐collision conditions of the gas‐phase experiments.
89(1988); http://dx.doi.org/10.1063/1.455134View Description Hide Description
Complexes between s‐tetrazine and acetylene have been prepared in a supersonic free jet and their fluorescence excitation spectra have been observed. Analysis of the rotational structure in the electronic spectra of the complexes was used to determine the geometry of the complexes. Four bands due to complexes were observed at shifts of +15, +80, +201, and +208 cm− 1 with respect to the origin of uncomplexed tetrazine at 18 128 cm− 1. The bands at +15 and +80 cm− 1 were assigned to two different isomers of a complex containing one molecule of tetrazine and one molecule of acetylene. The +15 band was due to a planar structure where the proton of acetylene forms a hydrogen bond with the nitrogen lone pair of tetrazine and the π cloud of the acetylene interacts with the hydrogen atom of the tetrazine ring. The +80 cm− 1 band is due to a stacked complex with the acetylene molecule above and parallel to the plane of the tetrazine. In this isomer, the axis of the acetylene bisects the N–N bonds of the tetrazine. The band at 208 cm− 1 is due to a complex consisting of two acetylene molecules and one tetrazine molecule with one acetylene above and parallel to the tetrazine ring and the other at the side of but perpendicular to the acetylene ring. The band at +201 cm− 1 is assigned to a larger complex containing more than two acetylene molecules, but further analysis of this band was not possible.
89(1988); http://dx.doi.org/10.1063/1.455135View Description Hide Description
Complete resonance Raman spectra, including absolute cross sections, have been measured for CS2 in cyclohexane using four excitation wavelengths from 223 to 204 nm, on resonance with the strongly allowed S 3←S 0 electronic transition. Absolute intensities have also been measured in CS2 vapor using 200 nm excitation. These total Raman cross sections, together with the Rayleigh cross sections calculated from the Kramers–Kronig transform of the absorptionspectrum, are used to determine the homogeneous linewidth Γ for the electronic transition under the assumption that the solvent induced broadening may be partitioned into a static, inhomogeneous part and a very rapidly modulated, homogeneous component. Γ is found to be ∼13 cm− 1 in the vapor and ∼200 cm− 1 in cyclohexane solution, indicating that homogeneous broadening is responsible for much of the increase in spectral breadth between vapor and solution phases. Direct modeling of the solution phase absorptionspectrum and the intensities of the lower lying Raman transitions gives a best fit to the absolute cross sections with a somewhat smaller Γ of ∼115 cm− 1, probably due to the approximate nature of the model employed for the highly anharmonic excited statepotential surface. The possible importance of solvent ‘‘memory’’ effects (finite solventcorrelation time) on the electronic spectral broadening is discussed.
89(1988); http://dx.doi.org/10.1063/1.455084View Description Hide Description
The mean dipole moment derivatives of the carbon polar tensors of the halogenated methanes are shown to be linearly related to the carbon partial charges calculated using the equalization of the electronegativity principle. The relationship is quantitative and allows the calculation of mean dipole derivatives of carbon atoms from valence state ionization potentials and electron affinities of the atoms in the halogenated methanes. The methyl group electronegativity in ethane calculated from the carbon mean dipole derivative is in excellent agreement with the previously reported value obtained from electronic transition data. A b i n i t i o charge, charge flux, and overlap contributions to the carbon mean dipole derivatives are shown to be useful in the interpretation of the relation between the derivatives and the partial charges.
Hyper‐quasielastic light scattering of liquid CCl4 due to the rotational motions of molecules and the intermolecular interactions89(1988); http://dx.doi.org/10.1063/1.455085View Description Hide Description
Hyper‐quasielastic light scattering of liquid carbon tetrachloride CCl4 is measured in the temperature range between −9 and 68 °C. The spectra consist of two different Lorentzians, narrow and broad ones. By the measurements of the polarization ratio and the temperature dependence of the spectra, the narrow Lorentzian spectrum is found to arise from the rotational Brownian motion of a CCl4 molecule. This spectrum is inactive for both Raman and infrared measurements but is only active for the hyper‐Raman measurement. The broad Lorentzian spectrum mainly comes from the temporary dipole moment induced by the intermolecular interactions between the molecules. The intermolecular interaction becomes strongest around 40 °C. The temperature dependences of the half‐width and the integrated intensity of the narrow Lorentzian are strongly affected by the intermolecular interactions.
Spectroscopy and photophysics of refractory molecules at low temperature. II. The green systems of ZrS89(1988); http://dx.doi.org/10.1063/1.455086View Description Hide Description
The diatomic molecule ZrS has been observed by laser induced fluorescence in a supersonic molecular beam following reaction of laser‐vaporized zirconium atoms with ≈2% carbonyl sulfide (OCS) added into a flow of He carrier gas. Two band systems have been observed in the 413–700 nm region. The R heads of their band origins lie at 20 218.9 and 20 208.3 cm− 1. The former is tentatively ascribed to the E 1Σ+–X 1Σ+ transition; the latter remains unassigned but is shown to borrow all of its intensity from the former through a simple intensity borrowing mechanism. A vibrational analysis leads to the following molecular constants for the ground state: ω e =548.4 cm− 1, ω e x e =1.6 cm− 1. Franck–Condon factors for several vibronic transitions are also reported. Intrinsic radiative lifetimes of several vibrational levels of both upper states have been measured, and confirm that the two states perturb each other. A limited deperturbation analysis has been carried out. Vibrational constants, mixing coefficients, interaction matrix elements, and other spectroscopic parameters are reported.
Electronic and structural characteristics of five poly membered heterocycles (polythiophene, polypyrrole): An ultraviolet and x‐ray photoelectron spectroscopy study89(1988); http://dx.doi.org/10.1063/1.455087View Description Hide Description
The electronic and structuralproperties of electrochemically synthetized five poly membered heterocycles [polypyrrole (PP), poly‐N‐methylpyrrole (PNMeP), polythiophene (PT), polybithiophene (PBT), and poly‐3‐methylthiophene (PMeT)] in their doped conducting and undoped semiconducting states have been determined by x‐ray photoelectron spectroscopy(XPS) and UVphotoelectron spectroscopy (UPS). A systematic evolution of the π bonding orbitals is observed by going successively from thiophene to bithiophene and to terthiophene which leads finally to the formation of an intrinsic π bonding band for the undoped polythiophene, located at 1.05 eV below the Fermi level E f. This π bonding band is also detected with PP, PBT, and PMeT in good agreement with theoreticalband structure calculations and implies a long range order along the polymeric chains. In contrast, PNMeP valence spectra exhibit broad features connected with the existence of structural defects and/or torsion angle between the monomeric units. Shake‐up satellites are generally observed on the high binding energy side of the C1s, S2p (or N1s) core levels related to π→π* transitions. The S/C (or N/C) ratio is always very close to the predicted theoretical value which means that the structural architecture of the polymeric backbone is very similar to the monomer one. Upon ClO− 4doping, the π bonding band is extended up to ∼0.2 eV below E f.
Simultaneously, the bonding band intensity decreases, and the C1s, S2p, (or N1s) core level peaks become broad and asymmetric (PP, PMeT, PT). These evolutions are completely reversible during the doping–undoping processes, independent of the anion used (BF− 4,SO3CF− 3) and of the thickness of the film (20–1000 Å). These modifications are dramatically reduced with PNMeP although its doping level is as high as 30%. These UVphotoelectron spectroscopy and x‐ray photoelectron spectroscopy results combined with the near edge x‐ray absorption fine structure(NEXAFS)characteristics give insight into the conduction mechanism inside these conducting polymers: Appearance of a metallic‐like behavior due to first the extraction of electrons from the π bonding band, and second, an extension of the π and π* bands towards E f narrowing the band gap from 2.1 eV in the undoped state to 0.3–0.4 eV in the doped form.