Volume 72, Issue 9, 01 May 1980
Index of content:

High pressure study of slow phosphorescence in La_{2}O_{2}S:Eu.
View Description Hide DescriptionThe effect of pressure has been measured on the decay rate of the slow phosphorescence in La_{2}O_{2}S:Eu. Although the trap structure is complex, the decay curves could be approximated as the sum of two exponentials for purposes of discussion. Activation energies for detrapping were extracted from the decay times. For the ’’shallow traps,’’ the activation energy increases linearly with pressure at a rate of 13 cm^{−1}/kbar. For the ’’deep traps’ the change in activation energy with pressure is ∼4 cm^{−1}/kbar at low pressure and increases to about 14 cm^{−1}/kbar by 40 kilobar. One can conclude that the ’’shallower trap’’ (thermally released) storage increases with pressure, as does the depth of the shallowest traps studied. The fraction of the holes captured by the shallowest traps also increase with pressure.

The effect of α‐hydroxyacetamide upon the formation of an alkoxy radical in α‐chloroacetamide single crystals: An ESR study
View Description Hide DescriptionThe inclusion of 0.05 to 20 mol% of α‐hydroxyacetamide in single crystals of α‐chloroacetamide has shown that the four line ESR pattern observed for x‐ray irradiated α‐chloroacetamide single crystals at 77 K is due to the alkoxy radical ⋅OCH_{2}CONH_{2}. The intensity of the four line ESR pattern is proportional to the concentration of α‐hydroxyacetamide up to 1 mol% at which point it levels off and then decreases above 5 mol%. The alkoxy radical is not observed in pure α‐hydroxyacetamide crystals.

The velocity autocorrelation function in a dilute hard sphere gas mixture
View Description Hide DescriptionThe velocity autocorrelation functions for the different species in a dilute hard sphere gas mixture are calculated by tracing the collision history of the individual hard spheres characterized by their number concentration (n _{A}, n _{B}), mass (m _{A}, m _{B}), and hard sphere diameter (σ_{AA}, σ_{BB}). Quantities related to the velocity autocorrelation function like the self‐diffusion coefficient are calculated and compared to results obtained from the Enskog approximation scheme to the Boltzmann equation.

High‐resolution one and two photon excitation spectra of t r a n s, t r a n s‐1,3,5,7‐octatetraene
View Description Hide DescriptionOne and two photon excitation spectra for the lowest energy singlet transition (2 ^{1}A_{ g }←1 ^{1}A_{ g }) of t r a n s, t r a n s 1,3,5,7‐octatetraene in n‐octane at 4.2 K(0–0 at 28 56l cm^{−1}) have been measured and analyzed. The one photon spectrum is found to be vibronically induced while the two photon spectrum is electronically allowed. The contribution of the higher lying 1 ^{1} B _{ u }←1^{1} A _{ g } transition (0–0 at approximately 32 100 cm^{−1}) to the observed two photon excitation spectrum is found to be negligible allowing vibrational overtones of the 2 ^{1} A _{ g }←1 ^{1} A _{ g } system to be observed. On the bases of frequency, intensity, and one or two photon allowedness, 128 measured lines are assigned as combinations of 4 antisymmetric and 11 symmetric normal modes.

MCSCF/CI ground state potential energy surface, dipole moment function, and gas phase vibrational frequencies for the nitrogen dioxide positive ion
View Description Hide DescriptionThe ground statepotential energy surface for the nitrogen dioxide positive ion, NO^{+} _{2} X ^{1}Σ^{+} _{ g }(Σ^{+},A _{1},A′), has been scanned with a correlated wave function to obtain directly, for the first time, the gas phase equilibrium geometry, force constants, vibrational frequencies, and dipole moment function. The wave function for this scan was constructed from a double‐zeta plus polarization one‐electron basis with a 12 configuration MCSCF determination of the orbital basis for a full valence ^{1}Σ^{+} _{ g }configuration interaction expansion. The calculated equilibrium bond length is 1.12 Å. The vibrational frequencies are computed to be ν_{1}=1514, ν_{2}=679, and ν_{3}=2614 cm^{−1} The present a b i n i t i o results differ significantly from crystalline spectroscopic studies and are, thus, the best values available for the gas phase vibrational frequencies. The dipole moment function is nonzero at the Σ^{+}, A _{1}, and A′ geometries included in the potential surface scan, and is obtained here to provide for the future a p r i o r i calculation of the infrared band intensities.

The collision of two linear rotors: A scaling theoretical analysis of the H_{2}–H_{2} and HF–HF systems
View Description Hide DescriptionThe energy corrected sudden (ECS) scaling theory is applied to rotation–translation (R–T) and rotation–rotation (R–R) energy transfer in the H_{2}–H_{2} and HF–HF systems. The ECS scaling predictions are in very good agreement with the exact quantal values. Two physically important results are particularly transparent in the scaling analysis: (1) The well‐known effectiveness of ortho H_{2} in rotational relaxation can be traced directly to the higher tensor order interactions which are also operative in para H_{2} for j?2; and (2) transitions in which rotational quanta are conserved differ appreciably from those in which rotational quanta are not conserved. Scaling predictions, using different values for the average collision range l _{ c }, are compared to the exact quantal cross sections. The results indicate that essentially the same value for l _{ c } wil be determined by inspection of the dynamical solutions and by the scaling analysis. This point is of importance for future applications of the ECS scaling theory.

Collision‐induced energy absorption and vibrational excitation by intense laser radiation in CH_{3}F
View Description Hide DescriptionVibrational energy absorption and transfer in diatomic and polyatomic molecules from intense resonant laser pulses is studied in the regime where energy flow is determined by vibrational–vibrational (V–V) collisions. Simple theoreticalmodels for single mode (diatomic) and two mode (polyatomic) oscillators, based on coupled rate equations with the addition of a laser pumping term, are presented. By invoking quasiequilibrium (temperature) assumptions on the vibrational level populations (confirmed with computer simulations) simple expressions are derived for the absorbed energy in the single mode case; and for two modes similar formulas based on a two‐temperature model are derived and discussed in relation to the one mode model. For many cases of interest, CH_{3}F included, the influence of a second mode on the total energy absorption is insignificant, with most of the energy flowing into the mode being pumped at nearly the same rate as for a single mode oscillator. The experimental work studies the absorption of the P(32) line of CO_{2} by ^{13}CH_{3}F, and the subsequent energy transfer among the several modes of the molecule. Approximately 1/4 J over 2 to 3 μsec delivers several quanta per molecule to the vibrational degrees of dreedom. Fitting the theoreticalmodel to the measured absorption produces a V–V rate constant of γ_{ V V } =1.0±0.5 μsec^{−1} Torr^{−1} for the ν_{3} mode. A determination of the energy partitioning among the several modes, by means of a technique for measuring the absolute energy stored in each, shows that roughly 50% of the absorbed energy is stored in the ν_{3} vibration with the remainder distributed among the other modes. Detailed knowledge of the partitioning allows confirmation of a specific energy flow path for CH_{3}F.

Tandem quadrupole study of laser photodissociation of CO^{−} _{3}
View Description Hide DescriptionAbsolute cross sections for the production of O^{−} by photodissociation of CO^{−} _{3} have been determined at intervals of 1 nm or less throughout the range from 400 to 635 nm using a tandem quadrupolemass spectrometer equipped with a tunable dye laser. In addtion, the dependence of the apparent photodissociation cross section on ion source pressure, laser polarization, laser intensity, ion lens voltage, and pressure of CO_{2} in the photon–ion interaction region has been studied at selected wavelengths. The threshold for photodissociation of ground state CO^{−} _{3} is found to be 2.258±0.008 eV in excellent agreement with a recent indirect determination of a lower limit for D(CO_{2}–O^{−}) of 2.27 eV, but in disagreement with earlier photodissociation results. Studies of collisional enhanced dissociation of laser excited ions indicate that only a fraction of the ions excited between threshold and 2.6 eV dissociate unimolecularly within 50 microsecond observation time. Below 2.2 eV, two photondissociation of CO^{−} _{3} is observed, and at 581 nm (2.13 eV) saturation of one of the transitions involved is observed, indicating a photoexcitation cross section of c a. 4×10^{−17} cm^{2}. A model based on these observations is proposed which appears to account for the apparent disagreement between our results and earlier laser photodissociation work.

Two‐photon photochemistry: Angular distribution of photofragments and application to isotope separation
View Description Hide DescriptionThe resonant two‐photon dissociation process in a supersonic molecular beam is investigated. Angular distributions of photofragments in such a process are derived via a density matrix formalism. Conservation of angular momentum and bound‐continuum Franck–Condon factors determine the angular distributions and the photodissociation cross sections. Criteria which the polarization state of the photon fields and the initial J state of the molecule should meet, to have the largest cross section and good photofragment collection efficiency, are discussed. Applications are foreseen in laser isotope separation.

Strong, positive‐ion hydrogen bonds: The binary complexes formed from NH_{3}, OH_{2}, FH, PH_{3}, SH_{2}, and ClH
View Description Hide DescriptionA systematic, a b i n i t i oelectronic structure analysis of the strong, positive‐ion hydrogen bond is reported. Energies and wave functions have been obtained at the 4‐31G level for the twenty‐one complexes, (B⋅⋅⋅H–A)^{+}, where A,B=NH_{3}, OH_{2}, FH, PH_{3}, SH_{2}, ClH. The A–H bond length (r _{1}), the B⋅⋅⋅A separation (R(, and the angle (ϑ) measured relative to the symmetry axis of B have been optimized. Calculated dimerization energies E _{ D } are found to be in reasonable agreement with experiment. Charge density difference plots of these complexes exhibit a remarkable similarity to the pattern of alternating charge gain and loss known for the neutral H‐bonded dimers. The proton donor is characterized by a charge gain region between A and the proton and charge loss on the proton; the electron donor by a charge loss between B and the proton. Four of the complexes (HF⋅⋅⋅H⋅⋅⋅FH)^{+}, (H_{2}O⋅⋅⋅H⋅⋅⋅OH_{2})^{+}, (HCl⋅⋅⋅H⋅⋅⋅ClH)^{+}, (HCl⋅⋅⋅H⋅⋅⋅FH)^{+}, have unusually short internuclear separations and show large charge gain around the protons. This is the first theoretical evidence of a transition from predominantly electrostatic to predominantly covalent binding in hydrogen bonding and it corroborates a recent experimental X–N study. An estimate of the amount of charge lost from the proton, Δq _{H}, has been obtained from the difference plots and is found to bear a linear relation with the dimerization energy E _{ D } for a series of complexes with a single proton donor. The inverse relation between E _{ D } and the difference in monomerproton affinities, ΔPA, reported in the literature for substituted pyridinium ions, is shown to hold as well for all A and B. Our calculated results also give a quantitative demonstration of the recently proposed inverse relationship between proton position and ΔPA. Several useful new organizing principles have been found: (a) R varies linearly with r _{2}, the B⋅⋅⋅H distance. Covalent bonding in the four complexes noted above is indicated by deviation from this line. Crystal structures taken from the literature also obey this relationship and the satisfactory agreement between experiment and calculations show that the 4‐31G basis is adequate for predicting strong H‐bond geometries. (b) R _{ N }/R?1.2, where R _{ N } is the A⋅⋅⋅B separation in neutral hydrogen bonds. (c) E _{ D } displays a smooth inverse relation to r _{2} for a sequence of complexes with a single electron donor.

Electron spin resonance studies of magnetic soliton defects in polyacetylene
View Description Hide DescriptionWe present electron spin resonance studies of the magnetic defect in undoped polyacetylene. Temperature dependences of the absorption spectra of (CD)_{ x } and (CH)_{ x } over the range 2–295 K have been obtained. The properties of the magnetic defect in fully isomerized t r a n s samples and in c i s‐rich samples are compared. The data are consistent with a description in terms of a mobile neutral magnetic defect in the t r a n spolymer and a stationary one in the c i s‐rich polymer. The spin resonancelinewidths are determined by unresolved hyperfine splittings resulting from interaction of the electron spin with many nuclear spins within the envelope of the spatially extended magnetic defect. The results of analyses of the detailed line shapes and linewidths are discussed in terms of the bond alternation domain wall (soliton) model of the magnetic defect.

Dynamics of fluctuations and spinodal decomposition in polymer blends
View Description Hide DescriptionWe discuss here some qualitative features of the concentration fluctuations in binary polymer melts A+B, A and B being long, flexible chains, with a Flory interaction parameter χ which can be positive (favoring segregation), or negative. In the one phase domain, the fluctuations of long wavelength (2π/q) are expected to relax by a reptation process. At shorter wavelengths (q R _{0}≳1, where R _{0} is the size of one chain) equilibration takes place by l o c a l a d j u s t m e n t of each chain inside a fixed ’’tube’’: the predicted relaxation rates τ_{ q } ^{−1} are proportional to q ^{6} for χ?0. (For a negative χ, opposing segregation, one expects τ_{ q } ^{−1}∼q ^{4}). In the two phase domain, where the trend towards segregation is strong (χ^{N}≫1, N being the number of monomers per chain), one should observe a very anomalous type of spinodal decomposition: in the early stages, the growing fluctuations have a wavelength comparable to the thickness of the interfacial region, and small compared with R _{0}: the growth is realized through local adjustment. During the later stages (Ostwald ripening) the size R(t) of the growing droplets should change from a t ^{1/5} behavior (when R≲R _{0}) to a t ^{1/3} behavior (in the classical limit R≳R _{0}).

Spectroscopic B _{0} constants for Rydberg states of H_{3} and D_{3}
View Description Hide DescriptionA new technique was used for deriving spectroscopic constants by reference to the known constants of a related electronic state. Applications of this method to several diatomic molecules revealed its usefulness and encouraged its use with the triatomic hydrogen molecules. SpectroscopicB _{0} constants for Rydberg states of H_{3} and D_{3} are obtained. Predicted and observed B _{0} values for the upper state of the 3d(A _{1}′)→A _{2} ^{″}(2p) band are 21.87 and 21.96 cm^{−1}, respectively.

Mössbauer studies of trimethyl and triphenyl tin chloride adsorbed on grafoil
View Description Hide DescriptionTrimethyl tin chloride [(CH_{3})_{3}SnCl] and triphenyl tin chloride [(C_{6}H_{5})_{3}SnCl] adsorbed on grafoil were studied by the Mössbauer resonance in ^{119}Sn. The temperature dependence of the Mössbauer fraction and the anisotropy ratio of the quadrupole transitions in (CH_{3})_{3}SnCl measured at two different angles supplied information on the structure of the substrate and on the anisotropy of the recoilless fraction. The relative orientation of (C_{6}H_{5})_{3}SnCl adsorbed on grafoil was found to be with the Sn–Cl axis perpendicular to the exposed basal planes of grafoil, contrary to (CH_{3})_{3}SnCl, where the molecules have their symmetry axes parallel to the grafoil plane.

Studies of rotational predissociation of van der Waals molecule by the method of complex coordinate
View Description Hide DescriptionA practical method is presented for calculating resonance energies and widths (lifetimes) of metastable states of van der Waals molecules, incorporating the use of complex coordinate transformation and square‐integrable basis functions. The utility of the method is illustrated through a study of the level widths and energies of rotationally predissociating atom–diatom model systems. Satisfactory agreement with previous works was found. Besides involving only bound state calculations and being free from imposement of boundary conditions, the method can be readily extendable to multichannel coupling problems.

State‐to‐state differential cross sections for rotationally inelastic scattering of Na_{2} by He
View Description Hide DescriptionState‐to‐state differential cross sections for rotational transitions of Na_{2} in collisions with He are measured in the electronic and vibrational ground state at thermal collision energies using a new laser technique. Single rotational levels j _{ i } are labelled by modulation of their population via laser optical pumping using a dye laser. The modulation of the fluorescence induced by an Ar^{+} laser tuned to the level j _{ f }=28 is proportional to the cross section for collisional transfer j _{ i }→j _{ f } and is detected at the scattering angle ϑ. A single optical fiber and a fiber bundle provide a flexible connection between the detector and the laser and photomultiplier, respectively. Transitions as large as Δj=20 are observed. At small angles elastic scattering is dominant, but rotationally inelastic processes become increasingly important at larger scattering angles. Rotational rainbow structure causing a steep onset of the cross section with the scattering angle ϑ (at fixed Δj) or a sharp cutoff with Δj (at fixed ϑ) is found. Preliminary results on rotational energy transfer in v=1 indicates that vibrational motion of the molecule favors larger rotational quantum jumps. semiclassical picture for the scattering of a hard ellipsoid gives a

The polarizability anisotropy of I_{2} from laser assisted molecular beam spectroscopy
View Description Hide DescriptionThe polarizabilityanisotropy of I_{2} has been measured in a molecular beammagnetic resonance experiment using laser induced fluorescence for state selection and detection. Stark fields of up to 16 kV/cm produced 50 kHz frequency shifts and α_{∥}−α_{⊥}=6.69±5% Å^{3}.

Molecular pair effects and the breakdown of Onsager’s dielectric theory
View Description Hide DescriptionThe source of breakdown of Onsager’s dielectric constant formula for simple polar fluids of high dielectric constant is identified as a purely electrostatic two cavity effect. An estimate of this effect yields a Kirkwood g factor of the order of the dipolar strength parameter 4π/3 ρu ^{2}β.

X‐ray photoemission studies of atom implanted matrices: Ni in carbon
View Description Hide DescriptionNickel, in low concentration, has been implanted in carbon foil. Both the Ni 2p _{3/2} and the L _{3} M _{45} M _{45} Auger levels are shifted 0.9 eV relative to nickel foil, while the valence band level is shifted by 1.2 eV. The characteristic energy loss peak associated with the core and valence levels of bulk nickel is observed on the core and valence levels of implanted nickel. An analysis is presented to show that the characteristic loss peak is consistent with 3d→4s or 4s→4p shake up processes. Model‐based spectra are consistent with an initial state configuration... 3d ^{9}4s ^{1} for implanted nickel atoms.

Nonlinear laser spectroscopy in nitric oxide studied through VUV harmonic generation
View Description Hide DescriptionTwo and three‐photon spectra have been obtained by single and double resonance enhancements of the elements of the third‐order nonlinear susceptibility (χ^{(3)}), which describe four‐wave sum mixing into the vacuum ultraviolet (VUV). Results are reported for nitric oxide, in which the A ^{2}Σ^{+}−X ^{2}Π transition is utilized for two‐photon resonant enhancement in χ_{3ω} ^{(3)}. Relative intensities of the recorded rotational structure agreed well with the predictions of theory based on multiphoton transition probabilities, with the exception of cases where there was accidental double resonant enhancement of χ^{(3)}, occurring through both two and three‐photon transition probabilities. Finally, in studies designed to explore double resonance effects in χ^{(3)} _{(2ω1+ω2)}, two lasers were used to probe simultaneous contributions from the A ^{2}Σ^{+} and M ^{2}Σ^{+} states of NO. Signals were further enhanced by a factor of 10^{3} to values as large as 10^{−4} of the input dye laser intensity.