Index of content:
Volume 80, Issue 7, 01 April 1984

Electronic structure of Cl_{2} from 5 to 15 eV by electron energy loss spectroscopy
View Description Hide DescriptionWe have obtained the first high resolution electron energy lossspectra for molecular chlorine over the extended energy‐loss range 5.5–14.5 eV at scattering angles between 3° and 9° for an incident electron energy of 200 eV. Spectra obtained at small scattering angles correspond closely to optical absorption spectra, the most prominent features arising from excitation of optically allowed Rydberg states, with other smaller features due to excitation of forbidden states. Our larger angle spectra show additional structures corresponding to excitation of other forbidden states and reveal a previously unobserved band. These angular scattering measurements explain some apparent differences between synchrotron radiation photoabsorption and threshold electron impact excitation spectra, and indicate some structures in the latter measurements to be mislabeled due to the presence of ‘‘hot bands.’’

EPR investigation of the intermediate spin exchange regime
View Description Hide DescriptionThe electron paramagnetic resonance spectra of di‐tert butylnitroxide (DTBN) are studied as a function of the nitroxide concentration in three hydrocarbon solvents at constant temperature. In the concentration range studied, DTBN undergoes electron spin exchange which leads to broadening, shifting, and eventual collapse of the lines forming the 19‐line protonhyperfine pattern. Particular emphasis is given to the spin exchange frequencies ω_{HE} in the region in which they are comparable with the protonhyperfine coupling constants—the intermediate spin exchange regime. Both linewidths and line shapes are found to be in full accord with theory in the intermediate exchange regime which completes the demonstration that theory and experiment are in agreement, the slow and fast exchange regimes having been studied many times in the past. Modeling the spectral lines as Lorentzian–Gaussian convolutions allows a detailed exposition of the collapse of the hyperfine structure as ω_{HE} increases and this collapse, too, is well described by theory. Bimolecular spin exchange rate constants are compared with the prediction of simple hydrodynamic theory.

The effect of thermal vibrations on extended x‐ray absorption fine structure. I
View Description Hide DescriptionThe form of the Debye–Waller factor in EXAFS is discussed in detail, and an expression is obtained for this factor in a general three‐atom system of C _{ s } symmetry. The normal modes which contribute to the Debye–Waller factors for each scattering path are dependent on the symmetry of the system. A series of model three‐atom systems with C _{2v } symmetry are studied and the Debye–Waller factors as a function of the bridging angle are discussed for each of these systems. The temperature dependence of the Debye‐Waller factor for each scattering path is also studied. In a system of C _{2v } symmetry, approximating the double and triple scattering Debye–Waller factors by the second shell single scattering factor is shown to be strictly valid only for a system close to linearity. The error due to this approximation is dependent upon the amplitude of the individual scattering paths and is shown to increase with temperature. When the single scattering contribution is unimportant, there is shown to exist a temperature where the above approximation is exact.

Time resolved studies of naphthalene mixed crystals. Fractal and Euclidian behaviors of the migration kinetics
View Description Hide DescriptionTime resolved experiments have been used in order to study the migration of triplet excitations at low temperature (T<4.2 K) in napthalene D _{8} crystals doped with 5% to 20% of naphthalene H_{8} containing a small amount of betamethylnaphthalene. The analysis of the time dependence of the fusion of two triplet excitations leads to clearly deomonstrate the fractal behavior of the exploration at short time and the Euclidian behavior at longer times. The spectral dimension has been measured for the smallest studied concentrations (c<12%); it ranges between 1.33 and 1 which is in very good agreement with a 3D percolation model. The time T _{0} at which the crossover between the fractal and Euclidian exploration occurs, has been approximately determined for the concentrations. It turns out that the 20% sample does not exhibit this fractal behavior on the studied time scale; this is also in agreement with the percolation model. We have also demonstrated that the time dependence of the fusion of two excitations gives a better insight in the energy migration processes than the time dependence of the supertrapping processes.

Theory of transport processes in dense fluids
View Description Hide DescriptionThe projection operator methods of Mori and Zwanzig are used to construct kinetic equations for the single‐particle distribution functions of a fluid mixture. The particle interactions are pair‐additive sums of hard‐core and soft, continuous potentials. The mean field kinetic equations obtained by discarding the multicomponent memory functions are identical in form to van Beijeren and Ernst’s revised Enskog equations for a hard sphere mixture, but incorporate the radial and direct correlation functionscharacteristic of the composite interaction. The 13‐moments methods is used to construct an approximate solution of the mean field kinetic equation for a simple fluid and to relate the associated, zero‐frequency transport coefficients to those of the conventional Enskog theory. Our choice of interaction parameters optimally mimicks a Lennard‐Jones 12‐6 fluid and yields excellent estimates of viscosity.

Proton spin‐lattice relaxation in C_{2}H_{4} and mixtures of C_{2}H_{4} with He, A, and N_{2}
View Description Hide DescriptionProton spin‐lattice relaxation times were measured in C_{2}H_{4} and mixtures of C_{2}H_{4} with He, A, and N_{2} as a fuction of density, composition, and temperature (300–600 K) in the region where T _{1}∝ρ using a 30 MHz spin‐echo spectrometer. The values of T _{1}/ρ for infinite dilution of C_{2}H_{4}, (T _{1}/ρ)_{C2H4–X} were found to be proportional to T ^{−} ^{n}, where n=1.43 for C_{2}H_{4}–C_{2}H_{4}, 0.54 for C_{2}H_{4}–He, 1.18 for C_{2}H_{4}–A, and 1.25 for C_{2}H_{4}–N_{2}. These data were analyzed, using Bloom–Oppenheim theory and assuming that the correlation time of the spin−rotation interaction in a J state is equal to the lifetime of a molecule in that J state, to obtain information on the spin‐rotation coupling constant, up to a constant, as well as the intermolecular anisotropicinteractions responsible for spin‐lattice relaxation in these systems.

The visible spectrum of jet‐cooled CClF_{2}NO
View Description Hide DescriptionThe nπ* electronic transition of CClF_{2}–N=O, around 680 nm, is reported in detail for the first time. The spectrum was studied at low temperatures by laser excitation of fluorescence in a pulsed supersonic free jet. A characteristic long progression is assigned to torsion around the C–N bond in the excited state. The dispersed fluorescence is compared with the absorptionspectrum of the static gas at 195 K. The electronic origin is located at 7047 Å, and the sharp absorption bands at longer wavelengths are assigned to torsional hot bands. Vibrational assignments of the electronic spectra are discussed. There is evidence that the g a u c h e conformer of the excited state and only one conformer of the ground state are mainly responsible for the red side of the excitation spectrum in a cold jet.

Orientational ordering and site structure for CCl_{4} in a krypton matrix
View Description Hide DescriptionInfrared studies of the ν_{3} and ν_{1}+ν_{4} modes of CCl_{4} trapped in krypton matrices show that the CCl_{4} molecules adopt two sites, one of which preserves the triple degeneracy while the other shows site symmetry splitting and a high degree of orientational ordering. At deposition temperatures above 35 K the orientational ordering of the low symmetry site is diminished as is the relative concentration of this site compared to the other. The results suggest that both cubic‐close‐packed and hexagonal‐close‐packed environments exist in these matrices. Significant differences in the ν_{3} vs ν_{1}+ν_{4} line widths have been observed and attributed to rapid V–Venergy transfer from the higher to the lower of these Fermi coupled modes.

Optical dephasing by uncorrelated phonon scattering to librations. An optical and picosecond photon echo study of a photosite of pentacene in benzoic acid
View Description Hide DescriptionWe report results of an optical and picosecond photon echo study on the zero‐phonon line of photosite I of pentacene in benzoic acid. The results show that optical dephasing in this system proceeds via uncorrelated phononscattering processes from the ground and optically excited state to singly excited librational levels. It is also shown that the data can not be consistently interpreted using coherence exchange theory. The librational frequencies and lifetimes at low temperature are determined to be 12 cm^{−} ^{1} and 18 ps in the ground state and 16.7 cm^{−} ^{1} and 2.5 ps in the singlet excited state. It is argued that population decay of librational overtones proceeds in single quantum steps down the librational ladder which excludes these modes from playing a role in optical dephasing. From a temperature‐dependent line shape study of the libronic transition in absorption, it is concluded that the low‐temperature librational lifetime is primarily due to resonant decay into acoustic phonons. At elevated temperature, cubic libration‐phonon anharmonicity dominates the librational lifetime. These lifetime shortening processes are reflected in the temperature dependent dephasing of the origin. Finally, with the dephasing model presented in this paper for photosite I, existing controversies concerning optical dephasing in the unburned origin of pentacene in benzoic acid can be resolved.

Composite pulse excitation in three‐level systems
View Description Hide DescriptionIt is demonstrated by theory and experiment that it is possible to excite coherence uniformly in three‐level systems with a wide range of anharmonicities by using a composite pulse excitation method. The theory is based on a formal analogy between the anharmonic three‐level case and the simpler system with equally spaced energy levels where composite pulse techniques are already well known to compensate for the effects of mismatch between photon energies and level separations. Computer simulations and solid‐state NMR experiments on the three‐level system of deuterium (spin 1) verify the uniformity of the excitation. The NMRpowder spectrum of deuterated polymethylmethacrylate is faithfully reproduced even using a radio‐frequency (rf) field strength weaker than the quadrupole interactions.

Where to look for the electronic spectrum of hydrogen isocyanide, HNC
View Description Hide DescriptionThe geometrical structures and vibrational frequencies of the ground and first excited electronic states of HNC have been predicted by a p r i o r i theoretical methods. Using a standard double zeta plus polarization basis set, both the self‐consistent field (SCF) and configuration interaction with all single and double excitations (CISD) levels of theory have been employed. To allow a reasonable assessment of the reliability of the HNC theoretical predictions, analogous studies of the experimentally characterized HCN molecule are also reported. It is hoped that the HNC theoretical predictions will be of assistance in the identification of its electronic spectrum. The HNC electronic spectrum should be distinguishable from that observed by Herzberg and Innes for HCN by: (a) the prediction that the X̃ ^{1}Σ^{+}–Ã ^{1} A″ energy difference is ∼5000 cm^{−} ^{1} less for HNC than for HCN; (b) for HNC the upper state vibrational frequencies ν_{2} and ν_{3} are nearly equal (to within 100 cm^{−} ^{1}), while for HCN, the C ≡ N stretch occurs more than 50% higher in frequency than the bending mode.

The rotational spectrum and structure of NH_{3}–HCN
View Description Hide DescriptionThe microwave spectrum of H_{3}N–HCN has been measured using the molecular beam electric resonance technique. A symmetric top spectrum is observed and the following spectroscopic constants were obtained::[RW2:B _{0}(MHz):3016.756 1(24)]

π electron spin density distributions and g values in semiquinone cations
View Description Hide DescriptionIntuitive arguments suggest that the approximate linear relation between g and the π electron spin density on the oxygens observed in semiquinone anions must also be applicable to the corresponding cations. This has been substantiated by redetermining the spin density distribution in p‐benzosemiquinone cation and by demonstrating the linear relationship between g and the ^{1}H hyperfine splittingsa ^{H} _{OH} associated with the hydroxyl protons of the cations. Our estimates of the spin density on the oxygens and Q ^{H} _{OH} are substantially different from the estimates reported in the literature. The ^{1} ^{7}O hyperfine splittinga ^{O} can be accounted for by using an additional parameter Q ^{O} _{OH} (∼50 G). Such a parameter was needed to account for the a ^{O} of semiquinone anions in hydrogen bondingsolvents.

Optical Zeeman spectroscopy of the lowest doublet excited states of t r i s(acetylacetonato)chromium(III)
View Description Hide DescriptionThe three principal phosphorescent bands of Cr^{3} ^{+} (1%)/t r i s(acetylacetonato)aluminium(III) single crystals at 1.8 K, have been examined at field strengths of 0–5.7 T in various orientations. The principal g values of the corresponding doublet excited states are determined to lie in the ranges ‖g _{∥} ‖:1.4–1.7, ‖g _{⊥} ‖:1.8–2.0, in marked contrast to those of the inorganic analogue, ruby.

A molecular model for dielectric shifts in infrared spectra of pure liquid phases and vapor pressure isotope effects
View Description Hide DescriptionIt is demonstrated that the same red shift of infrared fundamentals in pure liquid phases which follows from a model of a molecule imbedded in a dielectric also occurs in a quantum mechanical molecular model of a system of three‐dimensional isotropic harmonic oscillators. The shift arises from dipole–dipole interactions which lead to a band of excited states;transitions from the ground state are permitted only to one state in the lower part of this band. The relationship of these findings to vapor pressureisotope effects is discussed.

Vibrationally resolved optical spectra of c i s,t r a n s‐1,3,5,7‐octatetraene
View Description Hide DescriptionVibrationally resolved one photonfluorescence excitation and fluorescence spectra have been measured for c i s,t r a n s‐1,3,5,7‐octatetraene in n‐alkane matrices at 4.2 K. Except for absolute intensities and the details of vibronic development, these spectra are similar to those observed for the all‐t r a n s isomer. That is, there is a weak S _{0}–S _{1} transition (0–0 at 28 645 cm^{−} ^{1} in n‐hexane) approximately 3490 cm^{−} ^{1} below the strongly allowed S _{0}–S _{2} transition at 32 134 cm^{−} ^{1}. The vibrational development of the S _{0}–S _{1} absorption and fluorescence is that of a symmetry allowed transition (this transition is symmetry forbidden in the all‐t r a n s isomer). Thus, the low intensity of the S _{0}–S _{1} transition testifies to the importance of multiple excitations in S _{1}. Vibrational modes in S _{1} are weakly coupled with S _{2} due to the small energy gap. This coupling is energy dependent. Finally, Franck–Condon factors for the S _{0}–S _{2} transition have been estimated. These Franck–Condon factors together with the vibrational structure observed for the S _{0}–S _{1} excitation and fluorescence spectra show that neither S _{1} nor S _{2} is significantly distorted with respect to the ground state. Nonetheless, c i s–t r a n s photoisomerization competes with fluorescence, even at 4.2 K.

Two color photoionization spectroscopy of jet cooled aniline: Vibrational frequencies of the aniline X̃ ^{2} B _{1} radical cation
View Description Hide DescriptionPhotoionization‐efficiency spectra of free jet expansion cooled aniline in the ^{1} B _{2}excited state have been obtained by the technique of two color photoionizationspectroscopy. The adiabatic ionization potential of rotationally cooled (<5 K) aniline is determined to be 62 265±18 cm^{−} ^{1}. By pumping a variety of vibronic transitions, photoionization‐efficiency curves are obtained from discrete vibrational levels in the ^{1} B _{2} state. The spectra generally show a strong propensity for vertical (Δv=0) ionization. As a result, it has been possible to determine the vibrational frequencies of the ground state aniline ^{2} B _{1} radical cation with good precision for six vibrational modes. Vibrational progressions observed in the threshold curves support these assignments as well as allowing the observation of two additional modes in the ion which do not manifest themselves in the Ã(^{1} B _{2})←X̃(^{1} A _{1}) electronic spectrum. The magnitude of these vibrational frequencies compared to those known for the ground and first excited state of aniline suggest that the ^{2} B _{1} ionic state is more rigidly planar than the ^{1} B _{2}excited state.

Photodissociation of bromine azide. II. Production of Br_{2}(D′ ^{3}Π2_{ g })
View Description Hide DescriptionThe quenching of N_{2}(A ^{3}Σ^{+} _{ u }) metastables by Br_{2}(X ^{1}Σ^{+} _{ g }) results in the generation of Br_{2}(D′ ^{3}Π2_{ g })→A′(^{3}Π2_{ u }) emission. Although this energy exchange is near resonant, its rate constant is only (5±2)×10^{−} ^{1} ^{3} cm^{3} molecule^{−} ^{1} s^{−} ^{1}. Comparison of this result with the known rate for N_{2}(A ^{3}Σ^{+} _{ u }) quenching by I _{2} indicates the importance of Franck–Condon factors in these energy transfer processes. In light of this result, the generation of intense Br_{2} D′→A′ emission from the UVphotolysis of BrN_{3} is modeled as a Franck–Condon favored process in which Br_{2}(A′ ^{3}Π2_{ u } or A ^{3}Π1_{ u }) is excited to the D′(^{3}Π2_{ g }) state by collisions with N_{2}(A ^{3}Σ^{+} _{ u }) metastables. Br_{2} (A or A′) is generated as a product of the reaction of bromine atoms, produced as a photodissociation fragment, with BrN_{3}. The rate constant for this process is k _{5}=(3.0±0.5)×10^{−} ^{1} ^{1} cm^{3} molecule^{−} ^{1} s^{−} ^{1}. The rate constant of the energy transfer process is inferred to be k _{6}≥1.5×10^{−} ^{1} ^{0} cm^{3} molecule^{−} ^{1} s^{−} ^{1}. A second mechanism producing Br_{2}(D′ ^{3}Π2_{ g }) is evident in the time profile of the D′→A′ emission. This mechanism, which produces prompt D′→A′ emission, does not involve excitation by collisions with N_{2}(A).

Two angle dependent reactive infinite order sudden approximation
View Description Hide DescriptionThe reactive infinite order sudden approximation is redeveloped in a manner in which the initial and final arrangement internal angles γ_{λ} amd γ_{ν} enter as independent quantities. The analysis follows parallel to that due to Khare, Kouri, and Baer except that matching of the wave function from different arrangements is done in a manner such that no single γ_{ν} angle is associated with a particular γ_{λ} angle. As a consequence, the matching surface parameter B _{λν} does not occur.

Localized Gaussian wave packet methods for inelastic collisions involving anharmonic oscillators
View Description Hide DescriptionWe examine several methods of implementing the time‐dependent Gaussian wave packet method of Heller for collinear atom–molecule collisions involving anharmonic vibrators. We show that reasonably accurate results can be obtained with a procedure involving uncoupled frozen Gaussians with phases evaluated along classical trajectories. Although this method is much more accurate than the standard quasiclassical trajectory method, it involves about the same computational effort.