Index of content:
Volume 83, Issue 1, 01 July 1985

Optical dephasing in organic amorphous systems. A photon echo and hole‐burning study of pentacene in polymethylmethacrylate
View Description Hide DescriptionPicosecond photon echo experiments on pentacene in polymethylmethacrylate (PMMA) show that the homogeneous width exhibits a T ^{1.3} temperature dependence. This behavior is ascribed to coupling of the pentacene transition to randomly distributed low‐frequency excitations in the amorphous host. A calculation of the resulting homogeneous width is presented which uses optical Redfield theory as a starting point. From the results of this calculation we conclude that the coupling between the pentacene molecule and the host low frequency excitations is of static dipolar nature and that the density of states of these excitations varies as ω^{0.3}. Results of nonphotochemical hole‐burning experiments on the same system are also reported. Comparison of these results with the ones obtained by the photon echo method indicates, that in the hole‐burning experiments, the hole width and its temperature‐induced broadening are dominated by spectral diffusion.

Spectra and structure of organogermanes. XXII. Microwave, infrared, and Raman spectra of methylgermyl cyanide
View Description Hide DescriptionThe microwave spectra have been recorded from 18.0 to 26.5 GHz for ^{1} ^{2}CH_{3}GeH_{2} ^{1} ^{2}C^{1} ^{4}N, ^{1} ^{3}CH_{3}GeH_{2} ^{1} ^{2}C^{1} ^{4}N, ^{1} ^{2}CH_{3}GeH_{2} ^{1} ^{2}C^{1} ^{5}N, ^{1} ^{2}CH_{3}GeH_{2} ^{1} ^{3}C^{1} ^{4}N, ^{1} ^{2}CH_{3}GeD_{2} ^{1} ^{2}C^{1} ^{4}N, and ^{1} ^{2}CD_{3}GeH_{2} ^{1} ^{2}C^{1} ^{4}N of the four naturally occurring isotopes of germane: ^{7} ^{0}Ge,^{7} ^{2}Ge,^{7} ^{4}Ge, and ^{7} ^{6}Ge. Only a‐type transitions were observed and R‐branch assignments have been made for all the isotopic species in the ground vibrational state from which the rotational constants were determined. From these data the complete structural parameters were determined to be r _{0}(C_{ME}–Ge)=1.933±0.002 Å, r(Ge–C_{CN})=1.927±0.004 Å, r(C≡N)=1.155±0.004 Å, r(Ge–H)=1.521±0.001 Å, r(C–H_{ s })=1.092±0.005 Å, r(C–H_{ a })=1.092±0.005 Å, ∢CGeC=107.36°±0.60°, ∢HGeH=111.41°±0.04°, ∢HGeC_{Me}=112.92°±0.08°, ∢H_{ s }CGe=109.5°±2.0°, and <H_{ a }CGe=109.3°±0.8°, where all of the parameters are r _{ s } values except for the carbon–hydrogen distances and angles. The dipole moment components were determined from the Stark effect to be ‖μ_{ a }‖=4.20±0.14, ‖μ_{ b }‖=0.39±0.03, and ‖μ_{ t }‖=4.22±0.14 D. From the microwave splitting method, the threefold barrier to internal rotation was determined to be 1148±23 cal/mol (402±8 cm^{−} ^{1}). The infrared (3200 to 50 cm^{−} ^{1}) and Raman spectra (3200 to 10 cm^{−} ^{1}) of gaseous and solid CH_{3}GeH_{2}CN, CH_{3}GeD_{2}CN, and CD_{3}GeH_{2}CN have been obtained. Additionally, the Raman spectra of the liquids have been recorded and qualitative depolarization values have been obtained. A complete vibrational assignment is proposed based on infrared band contours, depolarization values, and isotopic shifts. These results are compared to similar quantities in some related molecules.

Rydberg states of the ArCO_{2} and KrCO_{2} van der Waals molecules
View Description Hide DescriptionRelative photoionization cross sections were determined for ArCO_{2} at a wavelength resolution of 0.24 Å in the regions of both the Ar*+CO_{2} and the Ar+CO^{*} _{2}dissociation limits (690–920 Å) and for KrCO_{2} at a wavelength resolution of 0.4 Å in the region of the Kr+CO^{*} _{2}dissociation limits (690–780 Å). The dimer ionization potentials are 13.59±0.04 eV (912.5±2.5 Å) for ArCO_{2} and 13.33±0.04 eV (930.0±3.0 Å) for KrCO_{2}. Combining these values with the known value of the CO_{2}ionization potential and the known (or estimated) values of the ArCO_{2} and KrCO_{2}dissociation energies yields values for the dimer ion ground state dissociation energies of 0.20±0.04 eV for ArCO^{+} _{2} and 0.47±0.05 eV for KrCO^{+} _{2}. Rydbergstructure is observed in the photoionization cross section of ArCO_{2} corresponding to dissociation limits of the type Ar*(n d,n s)+CO_{2}; however, discrete structure is observed only near the Ar*(3d,5s)+CO_{2}dissociation limits. van der Waals structure corresponding to the higher Ar*+CO_{2} limits is diffuse and red degraded. Molecular Rydbergstructure also is observed in the photoionization cross sections of both ArCO_{2} and KrCO_{2} corresponding to dissociation limits of the type Ar+CO^{*} _{2}. Prominent van der Waals structure is observed corresponding to dissociation limits based on members of the CO_{2} Henning sharp and diffuse Rydberg series, which converge to CO^{+} _{2} B ^{2}Σ^{+} _{ u }; however, no van der Waals structure is observed corresponding to dissociation limits based on members of the CO_{2} Tanaka–Ogawa Rydberg series, which converge to CO^{+} _{2} A ^{2}Π_{ u }. Various experimental evidence indicates that neither vibrational predissociation of the excited van der Waals dimer nor vibrational predissociation of the dimer ion can account completely for the absence of van der Waals structure corresponding to the Tanaka–Ogawa series in CO_{2}.

Measurement and analysis of the infrared‐active stretching fundamental (ν_{3}) of UF_{6}
View Description Hide DescriptionHigh‐resolution spectra of the infrared‐active stretching fundamental ν_{3} of ^{2} ^{3} ^{8}UF_{6} have been obtained between 620.6 and 633.5 cm^{−} ^{1} using tunable semiconductor diode lasers. Interference from hot bands was suppressed by cooling the UF_{6} in a supersonic expansion, and useful monomer concentrations were produced with effective temperatures of <100 K. Portions of the band from P(77) to R(66) are illustrated. All transitions from the vibrational ground state have been assigned, and the Q branch has been fully analyzed. A total of 43 line frequencies and 110 frequency differences extending in J to P(77), Q(91), and R(67) has been used to fit seven spectroscopic constants. The ground‐ and excited‐state values of the rotational constantB could be individually determined, and the U–F bond length in the ground vibrational state is r _{0}=1.9962±0.0007 Å. The Q branch of ^{2} ^{3} ^{5}UF_{6} has also been analyzed and the ^{2} ^{3} ^{5}UF_{6}–^{2} ^{3} ^{8}UF_{6} ν_{3}isotope shift measured to be 0.603 79±0.000 17 cm^{−} ^{1}. The isotope shift and the Coriolis constant ζ_{3} have been used to refine the general quadratic intramolecular force field of UF_{6}, and the Cartesian displacement coordinates of both infrared‐active fundamentals are illustrated and compared with those of SF_{6}.

Time and spectrally resolved luminescence and relaxation processes in solid neon
View Description Hide DescriptionThe spectral distribution and the time evolution of the luminescence of atomic (a‐STE) and molecular type self‐trapped excitons (m‐STE) of solid neon is measured under primary excitation with a continuous or pulsed electron beam. In high resolution spectra, several new luminescence bands are established. In addition to the luminescence of excitons localized at different trapping sites, the emission of desorbing neon atoms show up, indicating a close connection between self‐trapping of excitons at the surface and desorption. The spectral and temporal behavior of m‐STE emission displays the step‐by‐step vibrational relaxation of the Ne^{*} _{2} centers. The decay curves of high vibrational levels of the m‐STE are the first direct measurement of the relaxation rates. Good agreement with recently published values obtained with a different method is found.

Molecular reorientation from NMR relaxation times in planar molecules
View Description Hide DescriptionThe spectral density J(0) for the autocorrelation function referred to an arbitrarily chosen body‐fixed coordinate system has been rederived for a planar asymmetric rotor undergoing rotational diffusive motions. The angular dependence of J(0) was found to be more complicated than in the case of coincidence of the body‐fixed frame with the principal axes of rotation. Therefore, the orientation of the principal axes of the rotational diffusiontensor can be determined from four different J(0) or 1/T _{1} values. This result disagrees with previous conclusions based upon the assumption that the same angular dependence should be expected irrespective of the relative orientation of the diffusiontensor axes. Likewise, the values of the three diffusion coefficients can be in principle determined from more than two (or three) different J(0) values for molecules with (or without) a binary symmetry axis. In practice, however, only two relations between them can usually be obtained due to the uncertainty in T _{1}measurements and to the relative insensitivity of J(0) to some correlated changes in the diffusion coefficients for moderate anisotropic molecular rotations.

Dipole properties, dispersion energy coefficients, and integrated oscillator strengths for SF_{6}
View Description Hide DescriptionDipole oscillator strength distributions (DOSDs) have been constructed for ground SF_{6} including the IR, UV, and higher energy parts of the absorptionspectrum. An adopted DOSD is used to evaluate the dipole oscillator strength sums S _{ k }, k=2,1, 1/2 (− 1/2 )−3,−4,−6,..., mean excitation energiesI _{ k }, k=2(−1)−2, dipole–dipole dispersionenergies for the interaction of SF_{6} with itself and with Li, N, O, H_{2}, N_{2}, O_{2}, NO, H_{2}O, NH_{3}, CO_{2}, CO, H, He, Ne Ar, Kr, and Xe, and the triple–dipole dispersionenergy for (SF_{6})_{3}. Pseudo‐DOSDs have been constructed for SF_{6} which facilitate the evaluation of the dispersionenergies involving SF_{6}. In most cases uncertainties in our results for the dipole properties and dispersionenergy coefficients are ≲1–3%.

Compound clusters. II. Intermetallics and phosphides
View Description Hide DescriptionCompound clusters composed of two metallic elements have been produced by allowing the elements to react in the vapor phase. The composition of the clusters could be varied by changing the relative partial pressures of the two vapors. Mass spectrometry indicates that clusters with certain well defined compositions are particularly stable. For example, in the Cs–Sn system, mass peaks corresponding to (Cs_{3}Sn_{2})^{+}, (Cs_{3}Sn_{5})^{+}, (Cs_{5}Sn_{4})^{+}, and (Cs_{5}Sn_{9})^{+} are unusually strong. Other metallic systems discussed are Pb–Cs, Pb–In, Pb–Sr, Pb–In, and In–Cs. In another series of experiments each of these elemental metallic vapors was allowed to react with phosphorus vapor. In this way the relative stability of the various phosphide clusters could be tested. For example, (Cs_{4}P_{7})^{+} and (Cs_{3}P_{4})^{+} are shown to be particularly stable. The probable structures of some of the intermetallic and phosphide clusters are presented.

Conversions over low barriers. VI. The c i s–t r a n s isomerization of monothioformic acid
View Description Hide DescriptionThe dynamics of the t r a n s⇄c i sisomerization of monothioformic acid (rotation about the HC–SH bond) is a case study for an intramolecular conversion over a significant barrier [microwave spectra indicates V _{max}(θ) =3500 cm^{−} ^{1}], where the density of vibrational states at the potential maximum is very low, ≈2.5/cm^{−} ^{1}. We recorded the NMR spectra of dilute solutions of the acid in CD_{2}Cl_{2}, and of the gas at low pressures, over the temperature range 300–170 K, to determine the exchange relaxation time and to deduce the corresponding kinetic parameters. In spite of difficulties with the preparation and maintainence of the pure material, (i) correct assignment of the NMRresonances were made; (ii) the magnitude of E ^{0} _{sol} for isomerization in solution, from k ^{∞} _{ u }, was found to be compatible with the barrier derived from microwave spectra; (iii) in the gas phase this intramolecular conversion is in the second order regime, but (iv) only an upper limit could be set on the magnitude of E ^{0} _{gas} for isomerization, which is at least 2 kcal/mol lower than E ^{0} _{sol}.

Collisional dynamics of the IF B ^{3}Π(0^{+}) state. II. Electronic quenching at low pressures
View Description Hide DescriptionElectronic quenching of IF B ^{3}Π(0^{+}) was studied in a time resolved laser fluorescence experiment. IF (B) deactivation rate constants were determined as a function of vibrational quantum number for the following collision partners: He, Ne, Ar, Kr, Xe, N_{2}, F_{2}, I_{2}, O_{2}, and H_{2}O. Quenching by the noble gases and N_{2} was extremely slow with all rate constants less than 1×10^{−} ^{1} ^{4} cm^{3} molecule^{−} ^{1} s^{−} ^{1} for 3≤v’≤8. The quenching rate constants for F_{2} showed a weak dependence on vibrational quantum number, ranging from (3.4±0.5)×10^{−} ^{1} ^{2} to (5.2±0.4)×10^{−} ^{1} ^{2} cm^{3} molecule^{−} ^{1} s^{−} ^{1} for v’=3 and v’=7, respectively. Double exponential IF (B) fluorescence decays were observed with both O_{2} and H_{2}O indicating two depletion processes occurring over the lifetime of IF (B). The quenching rate constants, extracted from the decays at long times, were (1.4±0.2)×10^{−} ^{1} ^{2} cm^{3} molecule^{−} ^{1} s^{−} ^{1} for O_{2} and (7.6±1.6)×10^{−} ^{1} ^{2} cm^{3} molecule^{−} ^{1} s^{−} ^{1} for H_{2}O. At early times, the respective deactivation rate constants for O_{2} and H_{2}O were (8.3±1.4)×10^{−} ^{1} ^{2} and (2.4±0.7)×10^{−} ^{1} ^{0} cm^{3} molecule^{−} ^{1} s^{−} ^{1}. A possible quenching mechanism by O_{2} is discussed. The most efficient quencher was I_{2} with an estimated rate constant of 3.9×10^{−} ^{1} ^{0} cm^{3} molecule^{−} ^{1} s^{−} ^{1}.

An improved time‐dependent harmonic oscillator method for vibrationally inelastic collisions
View Description Hide DescriptionA quantal solution to vibrationally inelastic collisions is presented based upon a linear expansion of the interaction potential around the time‐dependent classical positions of all translational and vibrational degrees of freedom. The full time‐dependent wave function is a product of a Gaussian translational wave packet and a multidimensional harmonic oscillatorwave function, both centered around the appropriate classical position variables. The computational requirements are small since the initial vibrational coordinates are the equilibrium values in the classical trajectory (i.e., phase space sampling does not occur). Different choices of the initial width of the translational wave packet and the initial classical translational momenta are possible, and two combinations are investigated. The first involves setting the initial classical momenta equal to the quantal expectation value, and varying the width to satisfy normalization of the transition probability matrix. The second involves adjusting the initial classical momenta to ensure detailed balancing for each set of transitions, i→f and f→i, and varying the width to satisfy normalization. This choice illustrates the origin of the empirical correction of using the arithmetic average momenta as the initial classical momenta in the forced oscillator approximation. Both methods are tested for the collinear collision systems CO_{2}–(He, Ne), and are found to be accurate except for near‐resonant vibration–vibration exchange at low initial kinetic energies.

Chemical reaction network sensitivity analysis
View Description Hide DescriptionTraditional sensitivity analysis of a chemical reaction system’s periodic trajectory shows how the oscillation depends on the reaction rate constants. We develop sensitivity analysis within the framework of a stoichiometric network analysis, which is a theory that relates the dynamics of complex chemical reaction systems to feedback loops in the reaction network. The combined theory shows how the properties of a chemical oscillation are related to parameters that reflect the topology of the reaction network rather than to the rate constant parameters. The theory is applied to a Brusselator limit cycle as an example.

ArF laser flash photolysis of hexafluorobenzene vapor: Formation of hot molecules and their collisional relaxation
View Description Hide DescriptionTime‐resolved absorption spectra of hexafluorobenzene vapor have been observed in the time range of 0–2 μs after pulsed ArF laser (193 nm) excitation. The absorptionspectrum observed at the time t=0 is attributed to HFB* (S _{0}) (hot hexafluorobenzene with an internal energy of 639 kJ/mol). This transient absorptionspectrum can be simulated as a part of the spectrum of the S _{3}(^{1} E _{1u })←S _{0} transition at 3050 K on the basis of a modified Sulzer–Wieland model. The collisional deactivation of HFB* molecules is explained in terms of an energy transfermodel which assumes that the energy removed per collision depends upon the internal energy.

Collisional energy transfer and macroscopic disequilibrium. Application to azulene
View Description Hide DescriptionThe theory of macroscopic observables, which allows a model‐independent analysis of collisional relaxation, is applied to selected high‐quality experimental data on infrared fluorescence (IRF) from collisional deactivation of azulene excited to ∼30 000 cm^{−} ^{1} by single‐photon absorption from a 337 nm laser. Particular attention is paid to obtaining a secure relation between the experimental IRF intensity signal and the azulene bulk‐average vibrational energy 〈〈y〉〉. The azulene system turns out to be special in two respects: the initial population distribution immediately following the laser shot can be reasonably approximated by a delta function distribution, and the time decay of 〈〈y〉〉 is a simple exponential over almost half the energy range. Under these conditions, it can be shown that the initial value of the macroscopic bulk‐average energy transfer observable 〈〈ΔE〉〉 is identical to the microscopic per collision average 〈ΔE〉, which, in the azulene case, obeys the linear sum rule, i.e., is linearly dependent on excitation energy. These conclusions, which are free of assumptions concerning the nature of the transition probability, as well as the actual numerical values obtained, are in substantial agreement with the results of an earlier analysis of the data, which used a different approach based on the same microcanonical relationship connecting fluorescence intensity and vibrational energy.

Excitation of continuum radiation in collisions of (1) electrons and (2) metastable argon atoms with H_{2} and D_{2}
View Description Hide DescriptionMeasurements have been made of the spectral distribution of continuum radiation arising from collisions of low energyelectrons and of metastable (^{3} P _{0,2}) argon atoms with hydrogen and deuterium molecules. The electron impact spectra, a(^{3}Σ^{+} _{ g }, v’=0) → b(^{3}Σ^{+} _{ u })+hν, are in good agreement with the calculations of James and Coolidge. The spectra originating from Ar*+H_{2}, D_{2} were obtained using crossed supersonic beams. In both cases, an onset at 0.065 ± 0.009 eV relative collision energy was observed. For deuterium, this onset occurs below the energy required to produce D_{2}(a ^{3}Σ^{+} _{ g }) . Calculations of the spectral distributions when compared to experiment showed that, at 0.07 eV, the observed radiation arises from the first two vibrational levels of ArD* in the ratio (v’=0)/(v’=1)=4. For hydrogen at this energy, both product channels are present in the ratio ArH*/H^{*} _{2}=2 . At higher energy, both product channels are found in both systems with D^{*} _{2} predominating in the case of D_{2} and ArH* in the case of H_{2}.

Vibrational energy transfer in HBr–C_{2}D_{2} mixtures
View Description Hide DescriptionInfrared fluorescence from HBr and C_{2}D_{2}(ν_{3}) has been measured following irradiation of HBr–C_{2}D_{2} mixtures with a pulsed HBr laser. The fluorescence decay from both species yields the total rate constant for deactivation of HBr(v=1) by collisions with C_{2}D_{2}, k=(1.6±0.1)×10^{4} s^{−} ^{1} Torr^{−} ^{1}. The exponential rise of C_{2}D_{2}(ν_{3}) fluorescence is characterized by the rate constants k=(3.4±0.2)×10^{4} s^{−} ^{1} Torr ^{−} ^{1} for collisions with both C_{2}D_{2} and HBr, and k=(5.5±2.3)×10^{3} s^{−} ^{1} Torr^{−} ^{1} for collisions with added Ar. A model developed to fit the observations implies that ν_{3} is populated indirectly by transfer from another, unidentified, level of C_{2}D_{2}, and that the exponential rise constants refer to transfer from ν_{3} to this level.

Vibrational excitation of CH_{3}F by collisions with ‘‘hot’’ H and D atoms
View Description Hide DescriptionHot hydrogen atoms have been produced by photolysis of HBr, HI, and H_{2}S at 193 nm, and HI at 248 nm; D atoms were produced by photolysis of D_{2}S at 193 nm. Vibrational excitation of CH_{3}F in collisions with these hot atoms has been observed by detection of infrared emission at 3.3 μm (ν_{1}, ν_{4}) and 9.6 μm (ν_{3}). Relative probabilities for excitation of these modes have been determined, and the ν_{3}/ν_{1}, ν_{4} ratio is much larger than is predicted by simple impulsive models. The ‘‘isotope effect,’’ i.e., relative excitation by H atoms compared to D atoms, is 1.4 for both vibrational modes.

Spin‐dependent electrochemical kinetics at a semiconducting photocathode
View Description Hide DescriptionWe present experimental evidence that the electrochemical rate at a semiconducting photocathode can be affected by the relative spin orientations of the electrons in the semiconductor and of the redox species in the electrolyte. Here the spin orientations are acted upon independently by combination of a large magnetic field and optical pumping of the photoelectrons with circularly polarized light. The effect can be qualitatively understood in terms of the classical models for electrochemical transfer at a semiconducting photoelectrode. The results show that the spin polarization may not be drastically affected upon the electrochemical transfer and may provide a probe for the electrochemical interface.

Dynamics of excitation transfer in solution: Short range reactive and caging effects
View Description Hide DescriptionIn this paper, we analyze the kinetics of excitation transfer when short‐range effects due to an additional reactive interaction (exchange) and to solvent cage perturbations of the relative molecular motion are introduced. We show that the short‐range solvent caging influence on the observable transfer length can be brought to direct observation only in reactive systems undergoing fast molecular motion (slow reaction limit). In contrast, considering resonant electronic excitation transfer, a strong exchange mechanism (slow diffusion regime) is required to optimize its efficiency in the competition with the long‐range dipolar potential. In this case, we show how to measure the range and the frequency factor of the exchange interaction, which are usually unknown parameters.

Translational energy dependence of Ar^{+}+XY→ArX^{+}+Y (XY=H_{2},D_{2},HD) from thermal to 30 eV c.m.
View Description Hide DescriptionCross sections for the reactions of Ar^{+} with H_{2}, D_{2}, and HD to form ArH^{+} and ArD^{+} are measured using a new guided ion beam tandem mass spectrometer which affords an experimental energy range from 0.05 to 500 eV laboratory. The apparatus and experimental techniques are described in detail. Cross sections for H_{2} and D_{2} are found to be nearly identical over this entire energy range when compared at the same barycentric energy. The total HD cross section is the same as H_{2} and D_{2} at low energies, but differs significantly above 4 eV c.m., where product dissociation becomes important. The intramolecular isotope effect for reaction with HD exhibits a reversal at low energy, favoring the deuteride product below ∼0.14 eV c.m., and surprising nonmonotonic behavior at energies above 5 eV c.m. In all these systems, a new feature at higher energies is observed. This is interpreted as the onset of a product channel having an energy barrier of 8±1 eV. The room temperature rate constant derived from the data for the reaction with H_{2} is (9.5±2)×10^{−10} cm^{3} s^{−1}, in good agreement with the literature. Analysis of the data indicates an activation energy of between 2 and 15 meV at room temperature. The results are compared to previous experimental determinations and to theoreticalreaction models.