David A. Dows

Infra-red spectroscopic detection of bridging cyanide groups

Abstract Infra-red spectroscopic studies in the cyanide stretching region provide a means of distinguishing between terminal and bridging cyanide groups in various complex cyanides. The distinction lies in the observation that bridging cyanide groups exhibit a higher absorption frequency than do terminal cyanide groups. The results are explained on the basis of a simple vibrational analysis of bridged versus non-bridged systems.

Kramers–Kronig Dispersion Analysis of Infrared Reflectance Bands*

A procedure is presented for the use of the Kramers–Kronig dispersion relations in treatment of normal-incidence infrared reflection spectra to yield optical constants of crystals. The crux of the practical problem, treatment of unobserved wing regions in the integrations, is discussed in detail. Definitive methods of picking optimum upper and lower limits for integration over actual data and for fitting artificial wings are discussed.The method is tested on a reflection spectrum generated by a model involving several damped harmonic oscillators. It is shown how a region involving a single band (but flanked by “unobserved” bands) can be treated, and how accurately the method reproduces the theoretically known optical indices. It appears feasible to obtain the indices of absorption and of refraction to ±0.005 and ±0.002, respectively, given accurate data on reflectance. Integrated band intensities are reproduced to within a few percent.

Laser‐induced rate processes in gases: Dynamics of polyatomic systems

A model is proposed to describe the collision‐free dynamics of a polyatomic molecule driven by an intense laser field. The evolution in time of stable and quasistable (chemically reactive) energy level populations driven on and off resonance are given, and analytic estimates are derived for the effects of dynamic power broadening (a–c Stark effect), mode and level degeneracies, and rapid unimolecular decay on the efficiency of laser pumping.

Intermolecular Coupling of Vibrations in Molecular Crystals

The connection between intermolecular potential energies and the unit cell vibrational frequencies in molecular crystals is summarized in Sec. I. Details of the treatment of potentials which can be written as functions of intermolecular atom‐atom distances are developed. The frequencies of vibration of methyl chloride in the crystal are treated by the methods discussed in Sec. II. Dipole‐dipole interactions are insufficient to explain the observed splittings. For vibrations which are primarily hydrogen motions, an intermolecular hydrogen‐hydrogen repulsion potential accounts satisfactorily for the observed effects.

A model for laser isotope separation in SF6

We propose a model for the effect of CO2 laser radiation in decomposing and separating isotopes of SF6. Analytic expressions for oscillator strengths are derived for discrete‐to‐discrete, discrete‐to‐continuum, and continuum‐to‐continuum energy level transitions. Loss in oscillator strength as the system is driven up the manifold is compensated for by a cooperative power broadening that contains an explicit dependence on level degeneracies. Isotope selectivity and yield at low pressures are computed as a function of laser intensity, and a threshold in yield is found at approximately 14 MW/cm2.

Pressure dependence of the lattice frequencies of anthracene and naphthalene

Abstract The pressure dependence of the Raman lattice modes of anthracene and naphthalene molecular crystals has been studied in the range 0–10 kbar, using hydrostatic pressure and laser excitation. Gruneisen parameters in the range 3–6 have been observed. Calculations of the elastic constants and the pressure dependence of the lattice frequencies have been carried out using an intermolecular potential of the atom-atom type, with parameters derived from other crystalline properties. Agreement with experimental data is good.

Electric‐Field‐Induced Spectra: Theory and Experimental Study of Formaldehyde

An experiment is described which measures the modulation, produced by an applied electric field, of the absorption coefficient of the rovibronic spectrum of a gaseous sample. Phase‐sensitive detection provides for measurement of 0.01% modulation at a spectral resolution of 0.1 cm− 1 in the near ultraviolet. A theoretical development of the electric‐field‐induced spectral line shape and intensity as an expansion in powers of the applied field is presented. The important molecular parameters are the ground‐state and excited‐state dipole moments and rotational constants; measurement of the excited‐state moment is discussed. The dependence of line intensity on quantum numbers is predicted to be such as to emphasize transitions to states of low angular momentum; observation of this emphasis is of importance in assigning spectra. The correspondence between field‐induced and absorption spectra in a single experiment lends great confidence to assignments of specific absorption features. Experimental study of form...

Intermolecular Coupling of Vibrations in Molecular Crystals. III. Intermolecular Potential and Crystal Structure of Ethylene

Intermolecular force constants and vibrational frequency splittings are calculated for ethylene and deuteroethylene on the assumption of certain types of intermolecular potentials. The results of a calculation based on intermolecular hydrogen repulsions as the mechanism of vibrational coupling are in reasonable agreement with the observed band splittings. To a limited extent electrostatic (dipole) interactions are of importance.In the calculations detailed consideration of the as yet uncertain crystal structure were necessary, and the results lend support to the P21/n space group for ethylene at 65°K.The hydrogen repulsion parameter obtained from analysis of the data was found to be consistent with a theoretically obtained result and with values obtained from independent experimental studies. A certain amount of information about the variation of repulsive potential with distance is now available from crystal studies.

Reflection Spectra and Absolute Infrared Intensities in Pure Liquids: Benzene, Chloroform, Bromoform, Carbon Disulfide, and Carbon Tetrachloride

Author Institution: Department of Chemistry, University of Virginia; Department of Chemistry, University of Southern California

Spectroscopic Study of the Phase Transition in Crystalline Adamantane

The infrared spectrum of crystalline adamantane (tricyclo‐3,3,1,1‐decane, C10H16) has been studied at temperatures below and above the first‐order phase transition at 208°K and as a function of temperature through the transition. Splittings of the vibrational transitions and appearance of gas‐phase‐forbidden transitions below 208°K are in agreement with theoretical predictions. A hysteresis is observed in the spectroscopic characteristics over a range of a few degrees around the heat‐capacity peak occurring at the transition temperature. Calculations of lattice energy, frequency splittings, and barriers to molecular rotation have been carried out using central force functions between nonbonded atoms derived from physical properties of a number of crystals. The splittings, heat of sublimation, transition energy, and crystal structural parameters are in semiquantitative agreement with experiment. The vapor pressure of adamantane in the temperature range 312–366°K has been measured, and the heat of sublimati...