Index of content:
Volume 93, Issue 8, 15 October 1990

Quasihomogeneous line broadening of a hydrogen‐bonded polymer, investigated by picosecond infrared holeburning
View Description Hide DescriptionTransient spectral holes with lifetimes of several picoseconds are investigated in the infrared absorption band of the OH stretching vibration of the terpolymer polyvinyl butyral. A large variation of the quasihomogeneous linewidth from 7–45 cm^{−} ^{1} is determined from the measured holewidth in the temperature range 80–330 K and interpreted in terms of anharmonic coupling to a low frequency mode ν_{2}. The result ν_{2}=135±8 cm^{−} ^{1} provides strong evidence for indirect dephasing via the OH⋅⋅⋅O bridge stretching vibration. The inhomogeneous broadening of the total band is verified by the picosecond infrared holes. The inhomogeneity is related to an approximately static distribution of hydrogen bridges generated by local disorder and displays a different temperature behavior.

Hund’s coupling case sequences in resonant multiphoton transitions
View Description Hide DescriptionDifferent Hund’s coupling case sequences are considered for the n _{1}+n _{2} near resonant multiphoton rovibronic process in electric dipole allowed transitions of any spin multiplicity. The transitional path interferences strength tensor is introduced. This tensor involves a polarization and rotational dependence as well as a transitional path dependence which couples the electronic vibrational motion with the rotational structure. The intensity of a rotational line may decompose in terms of the matrix element of this tensor and a pure electronic vibrational tensor. The specificity of the coupling case sequence is found condensed in the rotational line factors which are explicitly determined for all the coupling case sequences obtained from the case (a) and case (b) coupling.

Selective multiple quantum ^{1}H nuclear magnetic resonance in chemically exchanging systems: Applications to liquid crystalline solutions
View Description Hide DescriptionMultiple quantum (MQ) proton NMRspectroscopy is applied to chemically rearranging systems. These systems include liquid crystalline solutions of s‐trioxane, which undergoes ring inversion and cyclooctatetraene (COT), which undergoes a bond shift rearrangement. The MQ spectra are recorded by a modified version of the selective detection method of Wokaun and Ernst. In this modification the phases of both the rf pulses and of the detector are cycled in concert, yielding a more efficient and potentially more accurate method for recording the MQ transitions. Experimental fourth and sixth order MQ spectra are presented for respectively the s‐trioxane and COT solutions over wide temperature ranges. The results are compared with simulated spectra calculated using exact as well as approximate expressions for the dynamic line shapes. Potential uses and advantages as well as limitations of MQ spectroscopy for studying dynamic systems are discussed.

Mode specific internal and direct rotational predissociation in HeHF, HeDF, and HeHCl: van der Waals complexes in the weak binding limit
View Description Hide DescriptionThe near‐infrared vibration–rotation spectra of the weakly bound complexes HeHF, HeDF, and HeHCl are observed in a slit supersonic expansion. The spectra correspond to simultaneous excitation of vibration and internal rotation of the H(D)X subunit within the complex. The HeHF and HeDF P/R branch transitions show J‐dependent excess linewidths, which are attributed to rapid predissociation of the excited states from intramolecular rotation–translation energy transfer. The corresponding P/R branch transitions in HeHCl are not observed despite good S/N on the Q branch, suggesting even more rapid predissociation for the upper state of this complex. The Q branch transitions for all three complexes abruptly terminate at low J, yielding lower limits to the number of bound rotational states and good estimates of the dissociation energiesD _{0}=7.1±0.1 cm^{−1} for HeHF and HeDF, and 10.1±1.2 cm^{−1} for HeHCl. In addition to isotropic intermolecular potentials, the HeHF/HeDF data yield considerable information on the potential anisotropy in the region sampled by the bound and quasibound states. The information so obtained is complementary to results from scattering studies and provides sensitive tests for refining trial potential energy surfaces.

Infrared laser spectroscopy of H_{2} and D_{2} Rydberg states. I. Application of the polarization model
View Description Hide DescriptionMultistate perturbation theory has been used to assign high resolution Rydbergspectra of H_{2} and D_{2} in the infrared. The theoretical model treats interactions between the H^{+} _{2} ion core and the spatially extended case (d) Rydberg electron in terms of the multipole moments and polarizabilities of H^{+} _{2}. The long range interaction model of Sturrus, Hessels, Arcuni, and Lundeen [Phys. Rev. A 3 8, 135 (1988)] was refined to include terms up to r ^{−} ^{8} in the Rydberg electron radial matrix element. Implementation of the multipole moment–polarization matrix diagonalization model was carried out at three levels: Rotation only (‘‘pure precession’’), vibrationally extended, and full multichannel. At each level the calculations were made to order r ^{−4} (low order), r ^{−6} ( (1)/(2) V _{6}) and r ^{−} ^{8} (high order). The most generally successful model was found to be the (1)/(2) V _{6} vibrationally extended model.

Infrared laser spectroscopy of H_{2} and D_{2} Rydberg states. II. Diode laser spectra and assignment of 5g–4f, 6h–5g, and 8i–6h systems
View Description Hide DescriptionInfrared diode laserabsorption spectra of portions of the 5g–4f, 6h–5g, and 8i–6hRydberg bands of H_{2} and D_{2} have been measured at Doppler limited resolution in low pressure A. C. discharges. The spectra, arising from L uncoupled states of H_{2} and D_{2}, are assigned using an a b i n i t i opolarization model supported by intensity calculations. Details of the different implementations of this polarization model are given in the preceding paper. The most useful was the single channel vibrationally extended (1)/(2) V _{6} model which became progressively better at higher n (and L). Results of multichannel calculations for a selected set of transitions are also reported.

Fourier transform far infrared spectroscopy of the ν^{’} _{3} vibration of SiC_{2} in Ar at 10 K
View Description Hide DescriptionA Fourier transform study of the vibrational spectrum of SiC_{2} produced by vaporizing mixtures of silicon and carbon‐12 or carbon‐13 at 2900 K and quenching the products in argon at 10 K, has enabled the identification for the first time of the ν^{‘} _{3}(b _{2}) vibrational mode, which the results of an earlier matrix study had suggested should lie in the far infrared. The assignment of a frequency observed at 160.4 cm^{−1} to the ν^{″} _{3} mode is confirmed by isotopic data and supported by the predictions of a b i n i t i o calculations. Optimized force constants have been derived using the frequencies of the newly assigned mode, the previously reported, ν^{‘} _{1}(a _{1})=1741.3 and ν^{″} _{2}(a _{1})=824.3 cm^{−1} vibrations, and their values on single and double carbon‐13 substitution. Two models, cyclic and T‐shaped, are discussed for the molecule, which is of C _{2V } symmetry.

Asymmetric‐to‐centrosymmetric structure change of molecules in squaric acid crystal: Evidence for pressure‐induced change of correlated proton potentials
View Description Hide DescriptionHigh‐pressure effects on vibrational modes have been investigated in crystals of squaric acid H_{2}C_{4}O_{4} (H_{2}SQ) and its deuterated analog (D_{2}SQ) which show two‐dimensional networks of hydrogen bonds. Specific vibrational modes in Raman and infrared spectra were observed to anomalously disappear with the application of pressures above 30 kbar in H_{2}SQ, but not in D_{2}SQ, indicating that the H_{2}SQ molecule in crystal shows the asymmetric (C _{1h })‐to‐centrosymmetric (C _{4h }) structure change. This is possibly caused by pressure‐induced deformation of the correlated proton potential essentially from the double‐well to single‐well type.

Brillouin scattering on the glass former GeSBr_{2}
View Description Hide DescriptionBrillouin scatteringmeasurements were performed on the partly interrupted network glass former GeSBr_{2}. Spectra were taken in symmetrical transmission arrangement, and in backscattering, thus varying momentum transfer between 2×10^{4}/cm and 44×10^{4}/cm. The temperature range was −100 °C<T<300 °C; the glass transition occurs at T _{ g }∼−30 °C. The Brillouin spectra were carefully compared with viscoelastic theory. Strong relaxation accompanied by a dispersion of sound velocity occurs between 100 and 200 °C. A comparison of the spectra taken at different scattering angles and temperatures constitutes convincing evidence for a distribution of relaxation times (a Cole–Davidson distribution is used). The temperature dependence of the maximum relaxation time τ_{0} can well be described as Arrhenius‐like, τ_{0} varying from 10^{−11} to>5×10^{−9} s. The temperature dependences of the high frequency and the adiabatic sound velocities are derived. From the q‐dependence, a fast relaxation channel can be identified. We discuss the assumption that it is directly connected with the break up of the glass above T _{ g }. Including this term an agreement is reached between experiment and viscoelastic theory on the level of about 1% for the Brillouin line position and of about 10% for the linewidths, for the glassy, highly viscous, and (relatively) low viscosity regime.

Observation of highly excited bending levels in NCO(X̃ ^{2}Π)
View Description Hide DescriptionNew NCO Ã ^{2}Σ^{+}–X̃ ^{2}Π vibronic bands in the (0,v _{2},0)–(v _{1},v _{2},v _{3}) sequences, with (v _{1},v _{3})=(0,0), (1,0), (0,1), (1,1), and (2,0), are reported. These bands have been observed in laser fluorescence excitation experiments of NCO produced in the CN+O_{2}reaction, where CN was prepared by 193 nm photolysis of cyanogen. The measured bandhead wave numbers were used to derive term values for a number of NCO(X̃ ^{2}Π) vibronic levels, which were then fit to a model Hamiltonian.

Fourier transform emission spectroscopy at 13 μm: Vibration–rotation spectrum of SiS
View Description Hide DescriptionThe vibration–rotation emission spectrum of the high temperature molecule SiS was recorded with a Fourier transform spectrometer. The observation of a high resolution emission spectrum at 13 μ (750 cm^{−} ^{1}) is unusual but we believe this technique is generally applicable to high temperature species. Seven bands (1–0 through 7–6) of the main isotopic form, ^{2} ^{8}Si^{3} ^{2}S, were observed along with three bands (1–0, 2–1, and 3–2) for each of the minor species, ^{2} ^{9}Si^{3} ^{2}S, ^{3} ^{0}Si^{3} ^{2}S, and ^{2} ^{8}Si^{3} ^{4}S. Mass‐reduced Dunham parameters, including Watson’s correction due to the breakdown of the Born–Oppenheimer approximation, were derived from the data.

Two dimensional ^{3} ^{1}P nuclear magnetic resonance coherence transfer experiments under magic‐angle sample spinning
View Description Hide DescriptionIn this paper, we report on two dimensional (2D) ^{3} ^{1}P cross polarization (CP) magic angle spinning (MAS) nuclear magnetic resonance(NMR) experiments on the coupled two‐spin systems, sodium pyrophosphate decahydrate, Na_{4}P_{2}O_{7}, 1OH_{2}O (SP) and tetraphenyldiphosphine‐1‐oxide, (C_{6}H_{5})_{2}PP(O) (C_{6}H_{5})_{2} (TPPO), including antiecho (COSY), double‐quantum NMR, and zero‐quantum NMR experiments. These experiments are generalizations of the absolute mode 2D Fourier transform antiecho COSY performed under MAS condition by Kentgens, de Boer, and Veeman [J. Chem. Phys. 8 7, 6859 (1987)]. The 2D sideband intensities for these experiments on polycrystalline samples are shown theoretically to be real. There are two mechanisms of coherence transfer; homonuclear J coupling and dipolar coupling. Theory shows that the zero‐quantum signal for the coupled two spins can not be observed by using a (CP_{ x }−τ−(π)_{ x }−τ−(π/2)_{ x }−t _{1}−(π/2)−t _{2} pulse sequence, when the coherence transfer is due to J coupling. When, however, the coherence transfer is induced by the flip‐flop term of the dipolar coupling Hamiltonian, the zero‐quantum signal can be observed by that pulse sequence. The preparation time dependences of the double‐quantum and the zero‐quantum sideband patterns, are expected, when the coherence transfer is induced by dipolar coupling. The zero‐quantum signal was very weak for TPPO, while it was strong for SP. The apparent preparation time dependence of the zero‐quantum sideband pattern was observed for SP. These results suggest that the coherence transfer is mainly due to J coupling in TPPO, where the two ^{3} ^{1}P nuclei have different isotropic chemical shifts. While, on the other hand, the dipolar coupling is more important in SP, where the two ^{3} ^{1}P nuclei have the same isotropic chemical shifts but different orientations of the chemical shifttensors. The 2D sideband intensities of the antiecho COSY spectrum of TPPO were calculated, and the relative orientation of the two chemical shifttensors was determined.

Quasielastic gamma‐ray scattering from polydimethylsiloxane in benzene solutions
View Description Hide DescriptionQuasielastic gamma‐ray scattering of 46.5‐keV Mössbauer photons by polydimethylsiloxane has been studied at room temperature as a function of dilution in benzene. The high energy resolution of this novel technique allowed the separation of the scattering signal into a narrow component associated with stiff motions along the polymer chain backbone and a quasielastic component associated with softer side group motions. The narrow component disappears upon dilution in benzene while the intensity of the quasielastic component grows proportionately. This result is interpreted as a softening of the backbone normal modes upon dilution.

Optical dephasing in solution: A line shape and resonance light scattering study of azulene in isopentane and cyclohexane
View Description Hide DescriptionResults of a line shape and resonance light scattering study of the S _{1}←S _{0} and S _{2}←S _{0} electronic transitions of azulene in isopentane and cyclohexane are reported. The results are analyzed using two different non‐Markovian master equations that make different assumptions about the statistical properties of the bath. For both these origin transitions we find that the solution dynamics fall in the so‐called intermediate modulation regime. If exponential decay is assumed for the bath correlation function we obtain a correlation time of the bath of 25 fs for the S _{1}←S _{0} transition and of 13 fs for the S _{2}←S _{0} transition at room temperature. From the frequency dependence of the ratio of fluorescence to Raman yields of the S _{1}←S _{0} transition we calculate an excited state lifetime of 1.4±0.2 ps using the parameters of the bath derived from the line shape analysis, and irrespective of which master equation is used.

The structure of NH_{3}⋅H_{2}S and free internal rotation effects
View Description Hide DescriptionThe structure of NH_{3}⋅H_{2}S has been determined from microwave and radiofrequency spectroscopy of this complex and its deuterated isotopomers, using molecular beam electric resonance techniques. Rotational constants,electric dipole moments and nitrogen quadrupole coupling constants were obtained from the spectra. The molecule was found to have a linear, hydrogen bonded structure with the ammonia as the proton acceptor. The N⋅⋅⋅S distance is 3.639 Å, the root‐mean‐square angular deviation of the NH_{3} axis from the N–S axis is 24.6° and the H_{2}S C_{2} axis is 40.5° from the N–S axis. Although the molecule is an asymmetric rotor, first‐order Stark effects were observed for K=1 rotational levels. These Stark effects are caused by torsional oscillations which are essentially ammonia monomer free internal rotation. Similar effects were observed for NH_{3}⋅H_{2}O and are reported here.

Theory of resonance Raman scattering and fluorescence from strongly vibronically coupled excited states of polyatomic molecules
View Description Hide DescriptionA theoretical description of secondary emission from complex absorption bands of isolated polyatomic molecules is developed. The strong non‐Born–Oppenheimer coupling associated with conical intersections of the multidimensional excited‐state potential‐energy surfaces is included in a fully microscopic manner by solving the time‐dependent Schrödinger equation for appropriate model systems incorporating the most relevant electronic states and vibrational modes. The effect of the large number of remaining vibrational modes and of the weaker coupling with additional electronic states is modeled by phenomenological relaxation terms (lifetime broadening and pure dephasing) in the framework of the density‐matrix formalism. Explicit eigenstate‐free expressions for absorption, resonance Raman, and fluorescence spectra are derived via density‐matrix perturbation theory. The computational feasibility of the resulting mixed microscopic/phenomenological theory is demonstrated for a simple three‐mode model of the vibronic coupling of the S _{1}(nπ*) and S _{2}(ππ*) states of pyrazine. The effect of excited‐state vibronic coupling and ultrafast S _{2}→S _{1} internal conversion on resonance Raman and fluorescence spectra is analyzed on the basis of these model calculations.

Visible and uv spectra of a polydiacetylene with a side group conjugated to the main chain
View Description Hide DescriptionThe polarized reflection spectra of single crystals of PDA‐CPDO [poly‐1‐(N‐carbazolyl)penta‐1, 3‐diyn‐5‐ol], which are π conjugated between the side groups and the main chain, have been measured in the photon energy region from 1.38 to 32 eV for the first time with a polarized synchrotron radiationsource.Absorption spectra have been calculated using the Kramers–Kronig relation. Transitions in the visible absorptionspectrum, which have been previously attributed to an interband transition in a similar polydiacetylene, have been found to be highly dichroic with respect to the direction along the polymer backbone. The absorptionspectrum in this region reveals two broad (ΔE≳0.4 eV) peaks at 1.9 and 2.7 eV with an almost equal absorption coefficient of 7.5×10^{4} cm^{−1}. The spectral features in the range from 3 to 8 eV result from electronic transitions of the carbazolyl side groups. A single broad (ΔE∼10 eV) absorption band observed at 18 eV is due to either transitions of σ electrons to higher σ* or π* states of the carbazolyl group, or to ionization processes.

Transitions induced in a double minimum system by interaction with a quantum mechanical heat bath
View Description Hide DescriptionA quantum mechanical treatment of a double minimum system interacting with a heat bath is presented for the purpose of interpreting experimental data on transfer kinetics in condensed hydrogen‐bonded systems. The model describes the transfer motion in one or two dimensions. The heat bath is represented by a set of harmonic oscillators and the interaction by a term linear in the system coordinates and in the bath coordinates. Extending an earlier random field approach, the present treatment consistently accounts for the quantum nature of the total system. With crystalline benzoic acid dimer used as an example, the master equation for the populations of the energy levels of the hydrogen transfer motion is derived. Transition probabilities consistent with the principle of detailed balance are obtained, based on a representation with explicit off‐diagonal tunnel interactions for pairs of states localized on different sides of the barrier and with diagonal terms describing the rearrangement of the heat bath as a consequence of the tunneling motion. The activation of the double minimum transfer process with increasing temperature is related to the excitation of the local vibrations in the two potential wells.

The fine and magnetic hyperfine structure of ^{8} ^{7}SrF in its X ^{2}Σ^{+} state
View Description Hide DescriptionA molecular beam rf–optical double resonance experiment was performed on ^{8} ^{7}SrF in its naturally occurring abundance ratio. The natural occurring abundances of the strontiumisotopes are ^{8} ^{6}Sr (9.8%), ^{8} ^{7}Sr(7.02%) and ^{8} ^{8}Sr (82.5%). Numerous magnetic dipole allowed transitions between ρ‐doublets in the X ^{2}Σ^{+} state were measured to an accuracy of 3 kHz. The observed spectra were analyzed in terms of an effective Hamiltonian which includes the magnetic hyperfine and electric quadrupole interactions arising from the ^{8} ^{7} Sr (I=9/2) and ^{1} ^{9}F (I=1/2) nuclei. The extracted spectroscopichyperfine parameters were interpreted in terms of a simple molecular orbital picture for the electronic nature of the X ^{2}Σ^{+} state. A comparison is made to previous results for the more abundant ^{8} ^{6}SrF and ^{8} ^{8}SrF isotopic forms.

Predissociation linewidths of the (3,0)–(11,0) Schumann–Runge absorption bands of ^{1} ^{8}O_{2} and ^{1} ^{6}O ^{1} ^{8}O in the wavelength region 180–196 nm
View Description Hide DescriptionPredissociationlinewidths of the (3,0)–(11,0) Schumman–Runge bands of ^{1} ^{8}O_{2} and ^{1} ^{6}O ^{1} ^{8}O in the wavelength region 180–196 nm have been obtained from the published measurements of the absolute absorption cross sections of Yoshino e t a l. [Planet. Space Sci. 3 6, 1201 (1988); 3 7, 419 (1989)] and spectroscopic constants of these molecules of Cheung e t a l. [J. Mol. Spectrosc. 1 3 1, 96 (1988); 1 3 4, 362 (1989)]. The linewidths are determined as parameters in the nonlinear least‐squares fitting of calculated to measured cross sections. Predissociation maxima are found at upper vibrational levels with v’=4, 7, and 10 for ^{1} ^{8}O_{2} and for ^{1} ^{6}O ^{1} ^{8}O. Our predissociationlinewidths are mostly greater than previous experimental values for both isotopic molecules.