Index of content:
Volume 82, Issue 11, 01 June 1985
82(1985); http://dx.doi.org/10.1063/1.448690View Description Hide Description
ESRspectra assigned to Ag3 molecules have been produced by codepositing atomic silver with excess nitrogen at temperatures close to 4.2 K. The spectra are characterized by an axially symmetric spin Hamiltonian having g ∥ =1.9933(3) and g ⊥ =1.9558(3) and, for 1 0 7Ag3, A ∥(1) =310.8(6) G with A ⊥(1) =310.1(6) G and A ∥(2) =76.0(2) G with A ⊥(2) =72.6(2) G for the apical(1) and basal(2) nuclei, respectively. The axial symmetry of the spectra is believed to imply that the trimer is rotating about one axis. There is no evidence for a pseudorotating trimer spectrum. The isotropic spin populations are ρ5s (1) =0.51 and ρ5s (2) =0.12 implying an acute angled geometry with ground state symmetry 2 A 1. This is in sharp contrast to the obtuse angled isomer (2 B 2ground state) found for Ag3 in a C6D6 matrix. For Ag3(2 A 1) there is little p character on the apical atom but a 10%–15% p hybridization on each of the two basal nuclei.
82(1985); http://dx.doi.org/10.1063/1.448691View Description Hide Description
The fluorescence lifetimes of cyclohexane in the solid, liquid, and vapor phase were measured using VUVsynchrotron radiation from the National Synchrotron Light Source at BNL. The lifetime of liquid and solid cyclohexane changes with temperature, and the decay rate is a combination of a temperature‐dependent Arrhenius‐type process and a temperature‐independent process. The results for liquid and solid cyclohexane from 59 to −49 °C can be represented by the expression 1/τ=[5.3×108+4.2×101 2 exp(−5100/1.987 T)] s− 1. The latest lifetime measurements available in the literature for cyclohexane are in good agreement with our values measured between 20 and 25 °C. The lifetime of liquid cyclohexane at a given temperature is independent of the excitation energy. However, in the vapor phase the lifetime strongly depends on the excitation energy.
82(1985); http://dx.doi.org/10.1063/1.448692View Description Hide Description
The influence of vibrational motion on bond lengths and quadrupole constants obtained from dipolar and quadrupolar solid state line shapes is considered. It is shown that such motions average both the magnitude and the orientation of the intrinsic interaction tensor. Explicit expressions for the effective coupling constants that can be conveniently evaluated using the results of a normal mode analysis are derived. When the vibrationally averaged interaction tensor is axially symmetric, it is shown that the effect of vibrational motion on relaxation can be rigorously incorporated into an effective coupling constant which is formally identical to the one that determines the line shape. Illustrative calculations for several alkanes, in both the gas and solid phases, are presented. The relative contributions of stretching and bending vibrations and the effect of anharmonicity on the effective C–H bond lengths and deuterium quadrupole constants are examined. The influence of vibrational averaging on the magnitude of the dipolar coupling is shown to be essentially independent of the nature of the molecule and its environment and to be quite small (i.e., for C–H bonds, the coupling constant decreases by 3% and hence the effective bond length increases by 1%). Vibrational averaging of the orientation of the dipolar interaction vector, on the other hand, depends on the size of the molecule and its environment because of the predominant role played by low frequency bending and torsional modes. The implication of these results for the value of the effective internuclear distance that should be used for the interpretation of the dipolar relaxation experiments is considered.
82(1985); http://dx.doi.org/10.1063/1.448693View Description Hide Description
Measurements of the nonlinear dispersion of χ( 3 ) have been performed on the surface plasmon resonance of aqueous colloidalgold. An analysis of the observed dispersion has revealed that the T 1 and T 2 lifetimes for the plasmon excitation are extremely rapid (<10− 1 4 s) but the release of heat by the particle electron/thermal reservoir is slower (>2 ps). Picosecond time‐resolved transient grating experiments on the same gold sol and organic dye solutions have confirmed that the nonradiative thermal decay of the metal particles occurs on the time scale of tens of picoseconds. The application of these methods to the study of ultrafast nonradiative relaxation mechanisms in molecular systems is discussed.
82(1985); http://dx.doi.org/10.1063/1.448694View Description Hide Description
Fluorescence excitation and single vibronic level (SVL) fluorescence spectra of fluoranthene seeded in an Ar free‐jet are presented. The S 1 excitation spectrum builds on a strong 427 cm− 1 vibronic false origin. Fluorescence spectra exciting SVL with vibrational energy 0, 427, and 751 cm− 1 contain sharp lines, whereas with 1345 cm− 1 the fluorescencespectrum is a featureless continuum. The sharp lines have been analyzed and assigned with the help of Raman and IR data. The broad spectrum when exciting 1345 cm− 1 above the S 1 origin is ascribed to energy redistribution (IVR). The behavior of fluoranthene in light of current understanding of the IVR process is discussed.
82(1985); http://dx.doi.org/10.1063/1.448695View Description Hide Description
The influence of an internal vibrational resonance on the IR multiphoton excitation of a model molecular system is studied numerically using Floquet theory. It is shown that a classical resonance zone can bottleneck photon absorption. The bottleneck effect is easily understood in terms of the strongly perturbed phase space topology near a classical resonance zone and the corresponding distortions in quantum mechanical eigenstates and eigenvalues of the field‐free Hamiltonian.
82(1985); http://dx.doi.org/10.1063/1.448696View Description Hide Description
The products from vaporizing siliconcarbide at 2900 K and quenching in argon at 8 K have been studied using Fourier transform infrared spectroscopy. Under the assumption of various possible geometries, calculations based on two vibrations observed at 1741.1 and 824.4 cm− 1 were made to predict frequencies for SiC2 singly and doubly substituted with carbon‐13. Comparison with the spectra observed on isotopic substitution shows conclusively that these vibrations belong to SiC2, and that the molecule is cyclic (i.e., possesses C 2v symmetry) in the ground state. This geometry is in agreement with recent results from a rotational analysis of the band system at 500 nm. Force constants derived in the present study suggest that the silicon atom is singly bonded to two doubly bonded carbon atoms.
Evidence for inhomogeneous broadening in vibrational overtone transitions: Formation of 1, 3‐cyclohexadiene from c i s‐1, 3, 5‐hexatriene82(1985); http://dx.doi.org/10.1063/1.448697View Description Hide Description
By pumping the 4–0 and 5–0 C–H overtones of c i s‐1, 3, 5‐hexatriene, the rates of 1, 3‐cyclohexadiene formation are measured using a Stern–Volmer analysis. While the rate for 5–0 C–H excitation is faster than 4–0 C–H excitation, the latter is about one order of magnitude larger than that calculated from an RRKM treatment. Moreover, nonmonotonic variation in rates are found as a function of excitation wavelength within each overtone region. These observations cannot be explained by assuming pure homogeneous broadening of each overtone transition, but can be rationalized by including inhomogeneous contributions, which may originate from hot bonds in the initial state and/or from a rotational dependence of this electrocyclic reaction rate near threshold.
82(1985); http://dx.doi.org/10.1063/1.448698View Description Hide Description
A recently developed band contour model is used to analyze the gas phase spectra of rare gas hydrogen halide complexes in the region of the hydrogen halide fundamental. The sensitivity of model predicted spectra to variation of major spectroscopic parameters is illustrated for ArHCl. Published spectra from several groups for KrHCl, XeHCl, ArHBr, and XeHBr are considered. The unusual appearance of the XeHCl spectrum is attributed to the large shift of the HCl fundamental frequency −12.5 cm− 1 when complexed to Xe. Through spectral analysis the dissociation energies of these complexes were found to be 214 (KrHCl), 220 (XeHCl), 220 (ArHBr), and 300 cm− 1 (XeHBr). The uncertainty in these dissociation energies and comparison to other estimates are discussed.
82(1985); http://dx.doi.org/10.1063/1.448699View Description Hide Description
The energy shifts in 3d photoelectron and M4N4,5 N4,5 Auger spectra of Xe have been studied for HXeF2, XeF4, and XeF6 molecules in the gas phase. Using the Auger parameter method, the contributions of the initial state chemical and final state extra‐atomic relaxation effects to the observed shifts have been estimated. The relaxation part has found to be significant both in the binding energy (up to 1.8 eV for XeF6) and especially in the Auger shifts. The ground state shifts are thus larger than the 3d binding energy shifts reported earlier. Calculated atomic charges are also now in better agreeement with theory and other experimental values.
82(1985); http://dx.doi.org/10.1063/1.448700View Description Hide Description
In a recent paper (Az I), well‐structured T=300 K resonance Raman (RR) profiles for the 1400, 1260, 900, and 2×825 cm− 1 lines of azulene in CS2 and for the 825 cm− 1 line of azulene in methanol were reported. Previously developed transform techniques were used to (1) compute RR profile line shapes directly from measured optical absorptionspectra, and (2) extract ratios of Stokes loss parameters from the line shape scale factors. The transform analysis indicated that (1) our model assumptions (adiabatic and Condon approximations, harmonic phonons, atomic equilibrium position shifts, and small vibrational frequency shifts upon excitation to a single electronic state) are basically correct allowing for
minor modifications, and (2) any deviations from these assumptions are likely to be larger for the 900 cm− 1 mode and smaller for the 1400 and 1260 cm− 1 modes. In this paper (Az II), we report model calculations of the optical absorptionspectra, RR profile line shapes, and relative RR intensities. In these calculations, we use a recently proposed nonzero temperature multimode time‐correlator modeling procedure. Compared with the conventional sum‐over‐states method, our time‐correlator modeling procedure is superior in that (1) our optical absorptionspectra and RR profiles computed via fast Fourier transform techniques have a practically unlimitedspectral range and (2) the computing times are short for nonzero temperature multimode calculations. In our basic model, we adopt the assumptions of Az I and use seven azulene modes to obtain simultaneous good fits of the well‐structured RR profile line shapes and optical absorptionspectra. However, we find that the basic model does not account for the intensity of the 900 cm− 1 Raman line relative to that of the 1400 cm− 1 line, even though the individual profile line shape fits for these modes are very good. The basic model is therefore modified to allow mixing of the normal coordinates of these two modes. By introducing a single, relatively small mode‐mixing parameter, we obtain a good fit of the relative RR intensities in addition to simultaneous detailed fits of the optical absorptionspectra and RR profile line shapes. In an alternate approach, we modify our basic model and find that the inclusion of two relatively small non‐Condon parameters, instead of one mode mixing parameter, can also produce simultaneous detailed fits of all of our optical absorption and RR data.
A comparison of the two modified models solely on the basis of simplicity favors the mode‐mixing model, since only one extra parameter is required to modify our basic model.
82(1985); http://dx.doi.org/10.1063/1.448651View Description Hide Description
A continuous wave optically pumped iodine monofluoride laser operating on the B 3Π(0+) → X 1Σ+system is described. The laser operated on a series of rovibronic lines originating from v’=2–5. By tuning the dye laser pump source over a single rovibronic line, spectral structure was produced on the IF laser output that was analyzed and found to be consistent with hyperfine structure in IF. Hyperfine coupling constants were determined in this analysis. Implications for a chemically pumped IF laser are also presented.
Deuterium spin relaxation and spectral densities in the liquid crystal 4‐n‐pentyloxybenzylidene‐4’‐n‐heptylaniline‐2, 3, 5, 6,‐d 482(1985); http://dx.doi.org/10.1063/1.448652View Description Hide Description
We report on the measurements of the deuterium Zeeman and quadrupolar spin‐lattice relaxation times and the deuterium spin–spin relaxation time in the nematic, smecticA and C phases of a ring‐deuterated 4‐n‐pentyloxybenzylidene‐4’‐n‐heptylaniline. The spectral densities of motion J 0(0), J 1(ω0), and J 2(2ω0) at ω0/2π=14.73 MHz were separated from the above relaxation times. A simple ‘‘rotational diffusion in a cone’’ model was used to account for the observed increase in J 1(ω0) and the decrease in J 2(2ω0) in the nematic phase as the temperature was decreased. The model of rotational diffusion in a restoring potential was also examined. Our data can be understood qualitatively using the strong collision model or semiquantitatively using the cone model. The restricted rotational diffusion in a cone model supports the molecular reorientations being in the intermediate motion regime (ω0τ≂1). Additional information on J p (ω) such as its frequency dependence are required to check the various assumptions used in the above motional models.
Absorption spectra for collinear (nonreactive) H3: Comparison between quantal and classical calculations82(1985); http://dx.doi.org/10.1063/1.448653View Description Hide Description
Absorption spectra for the collinear (nonreactive) H+H2→H♯ 3 →H+H2 are calculated quantum mechanically, using the Siegbahn–Liu–Truhlar–Horowitz (SLTH) a b i n i t i o potential and a model H* 3 surface as the ground and excited H3 surface, respectively. They are compared to classical spectra previously computed by Mayne, Poirier, and Polanyi using the same potential energy surfaces [J. Chem. Phys. 8 0, 4025 (1984)]. The spectra are calculated at several collision energies and for both H+H2 (v=0) and H+H2 (v=1). The quantal and classical spectra are shown to agree with respect to basic features and trends. Nevertheless, the two sets of spectra differ considerably in their overall appearance because of some purely quantum aspects of the H+H2 system.
Radiation damage in thiophene derivatives: ESR and ENDOR studies of x‐irradiated 5‐nitrothiophene 3‐carboxaldehyde single crystals82(1985); http://dx.doi.org/10.1063/1.448654View Description Hide Description
The crystal structure of 5‐nitrothiophene 3‐carboxaldehyde was determined. X‐irradiation of these crystals, at 77 K, leads to the formation of a radical pair (D=−875 MHz, E=49 MHz) which exhibits hyperfine interaction with two protons. By using the structural information of the crystal structure together with the ESRtensors, it is possible to show that this pair is composed of an allyl‐type radical and of a σ radical produced by a C–H homolytic scission. This pair disappears at 155 K. At 290 K a new species is created; the ENDOR analysis shows that this new radical results from a radical addition at the carbon bearing the NO2 group.
82(1985); http://dx.doi.org/10.1063/1.448655View Description Hide Description
The photoabsorption cross sections of C3H6 isomers (cyclopropane and propylene) and C4H8 isomers (1‐butene, iso‐butene, c i s‐2‐butene, and t r a n s‐2‐butene) have been measured in the wavelength region from 300 Å to the ionization potential of each molecule (about 1200–1400 Å) using an ionization chamber with four ion‐collection plates. The obtained photoabsorption cross sections show a maximum value at about 700–800 Å for each molecule. In the wavelength region which is shorter than about 500 Å the cross sections are almost the same among the isomers and equal to the sum of the cross sections for the constituent atoms. In the longer wavelength region, however, the cross sections have different peaks and shoulders depending on an isomer, i.e., on its molecular structure. The obtained cross section values, together with previously reported ones in the wavelength region outside of the present experiment, satisfy the Thomas–Kuhn–Reiche sum rule within experimental errors.
82(1985); http://dx.doi.org/10.1063/1.448656View Description Hide Description
The absorption cross section for the lowest‐lying, bound excited state (4 2Γ) of Xe2Cl has been measured at several wavelengths in the ultraviolet (UV). Temporally isolating the excited Xe2Cl species in Xe/Cl2 gas mixtures has been accomplished by producing XeCl molecules in the B state by photoassociation [Xe+Cl+ℏω → XeCl(B)] at λ=308 nm. Collisional mixing of the lowest lying ion pair states of the excimer subsequently forms the XeCl species in its C state. Following the formation of Xe2Cl* from XeCl(B,C) by a three‐body collision, the long radiative lifetime of the trimer (>200 ns) relative to those for the XeCl B and C levels is exploited to ensure that the triatomic molecule is the predominant species when the experiments are performed. A second UV laser pulse depletes the Xe2Cl(4 2Γ) population by photoexcitation [as evidenced by the sudden suppression of the 4 2Γ → 1 2Γ blue‐green (λ∼485 nm) fluorescence] and the absorption cross section is determined from the dependence of the degree of fluorescence suppression on the laser intensity. Contrary to the known Xe+ 21(1/2) u → 2(1/2) g UV absorption profile, Xe2Cl(4 2Γ) exhibits significant absorption at wavelengths as low as 193 nm, whereas the dimer ion absorption at 248 nm is only a fraction of its peak (λ∼340 nm) value. Photoionization of excited Xe2Cl at wavelengths below ∼275 nm may be responsible for the cross sections measured for photon energies as large as 6.4 eV.
Surface enhancement factors for Raman scattering at silver electrodes. Role of adsorbate–surface interactions and electronic structure82(1985); http://dx.doi.org/10.1063/1.448657View Description Hide Description
The integrated Raman band intensities for internal vibrational modes of various adsorbates at roughened silverelectrodes are combined with surface concentrations obtained electrochemically to yield surfaceRaman scattering cross sections. These data together with the corresponding bulk‐phase Raman intensities provide estimates of the degree of enhancement of the vibrational bands at the silversurface with respect to the bulk medium, i.e., the ‘‘surface enhancement factors,’’ SEF. For surface‐attached thiocyanate, isothiocyanate‐bridged Cr(III) complexes, and also for Cr(III) and Co(III) hexaammines that are electrostatically attracted to chloride‐coated silver, SEF values are obtained that are relatively insensitive to both the vibrational mode and the adsorbate, even though substantial (up to 500 fold) variations are observed in the Raman cross sections. This finding applies to stretching modes involving nonbridging ammine groups as well as to bridging thiocyanate modes, although the frequencies of the latter, but not of the former, are perturbed strongly by the silversurface. Comparisons were also made between Raman cross sections, surface enhancement factors, and excitation profiles for pyrazinepentaammineosmium(III) and (II) to explore the influence of electronic resonance since the Os(II) form displays an intense charge‐transfer band in the vicinity of the laser wavelengths (460–650 nm) employed. A pronounced peak in the Os(II) excitation profile, absent for Os(III), is obtained that is markedly (∼100 nm) red shifted for the adsorbed vs bulk complex.
82(1985); http://dx.doi.org/10.1063/1.448658View Description Hide Description
When matrix isolated Cr2 molecules are pumped with radiation corresponding to the x 1Σ g → A 1Σ u absorption a new absorption appears belonging to a metastable species of Cr2, whose lifetime ranges between 0.2 to 0.7 s according to the matrix used and the matrix site pumped. A two‐color experiment produced the resonanceRaman spectrum of this state which in solid argon has vibrational constants ω e =78.6 and ω e X e =0.4 cm− 1. The new state is interpreted as being either a member of the ground statemanifold of states with high spin multiplicity, or more likely a long‐bond form of Cr2 corresponding to the molecule trapped in the outer minimum of a double minimum ground state potential. If so the lifetime observed is the inverse rate of photon‐assisted tunneling from the outer to the inner minimum.
82(1985); http://dx.doi.org/10.1063/1.448659View Description Hide Description
Photoelectron spectra produced by the laser multiphoton ionization of CH3I show a strong band due to absorption of an additional photon above the ionization threshold and resembling either inverse bremsstrahlung or ‘‘above threshold ionization.’’ The spectra are ascribed to a process of capture of Coulomb‐retarded photoelectrons to form transient CH3I− ions which are then photodetached before, during and after dissociation to CH3+I− products. Both CH− 3 and I− ions are observed. Several aspects of the process are discussed in terms of a potential energy diagram.