Index of content:
Volume 78, Issue 8, 15 April 1983

Collisional effects on the rovibrational transitions of the HD fundamental band
View Description Hide DescriptionThe effect of collisions with self and buffer gases, Ar, Kr, and Xe, on the linewidth,line shape, and frequency of the rovibrational transitions of HD in its ground electronic state are studied with a high‐resolution tunable laser difference‐frequency spectrometer. Collisional narrowing, manifested in sub‐Doppler width of the transitions, is observed (for the first time in HD) in both P and R branches of the infrared fundamental band. The measured line profiles and the pressure dependence of their widths are analyzed in the limits of strong and weak velocity‐changing collisions. It is shown that both models fit the data equally well in the low pressure range (below about 300 Torr). The strong collision model, however, is found to be the best fit for HD/HD collisions in the entire 10–760 Torr pressure range. In the case of HD/buffer gas collisions, a model incorporating correlations between the velocity and state‐changing collisions appears to be more appropriate. The dependence of collisional narrowing and broadening parameters on the rotational quantum number is reported and discussed.

Analysis of the 254.7 nm absorption system of carbonyl fluoride
View Description Hide DescriptionThe 230 nm absorptionspectrum of carbonyl fluoride has been recorded in the vapor phase at 25° and −78 °C at path lengths up to 48 m. In all, 170 bands were observed in the spectrum and were assigned to vibrational activity in ν_{1}′, ν_{2}′, ν_{3}′, ν_{4}′, and ν_{5}′. The presence of temperature sensitive bands attributed to ν_{4}′′ leads to the interpretation that the excited state is nonplanar. Combination differences isolating ν_{4}′′ show that the inversion doubling splitting is less than the experimental error for levels 4^{+} and 4^{−} in the Q _{4} mode of vibrational excitation. A fit of the G _{4} term values to the energy levels derived from a quadratic‐Lorentzian function yielded a central barrier of 8200 cm^{−1} and an out‐of‐plane angle of 31.8° for V(Q _{4}). The high barrier is attributed to an inductive effect from the fluorine atoms brought about by a relaxation of the symmetry from C _{2v } to C _{ s } on electronic excitation.

Mössbauer spectroscopy on high‐spin d ^{5} iron complexes: Crystal field calculations
View Description Hide DescriptionMössbauer spectra of high‐spin ferric Fe(EHPG) compounds in frozen solution have been recorded at 4.2 K in a small applied field. The spectra have been analyzed in terms of a spin Hamiltonian with S=5/2 used as an intermediate step for generating a crystal field model that has the ground sextet ^{6} A _{1} and three excited quartet states appropriately spaced in energy. The model has been found to be appropriate for g=4.3 high‐spin ferric complexes with spin Hamiltonian D values in the range 0.6–1.7 cm^{−1}. It is shown that the model can provide a three‐way correlation of the relevant data obtained by Mössbauer, EPR, and optical absorptionspectroscopy.

The OH stretching spectrum of liquid water: A random network model interpretation
View Description Hide DescriptionWe report the results of an analysis of the OH‐stretching region of the vibrational spectrum of liquid water. The random network model is realized by time‐averaged molecular dynamics configurations of ST‐2 water. The hydrogen bond network is defined using a geometric definition of bonded pairs of molecules. Force constants are calculated using empirically derived formulas relating the shifts of the intramolecular frequency and anharmonic potential energy coefficients with a hydrogen bond strength parameter. The strength parameter is related to the separation in a molecular pair, hence, it has a distribution of values defined by the molecular configuration in the model liquid. Fermi resonance between the OH stretching mode and the bending overtone is shown to be an important second‐order effect in the spectral band shape. The calculated spectra compare well with experimental results and support the contention that the spectral features are dominated by intermolecular interactions.

An improved analysis of the OH stretching spectrum of amorphous solid water
View Description Hide DescriptionWe report a calculation of the OH stretching region of the infrared and Raman spectra of amorphous solid water. The configuration space distribution of water molecules employed as a realization of the amorphous solid is a modified scaled version of the Henderson model of amorphousGe. The molecular dynamics includes, in addition to interaction between OH oscillators, local variation of hydrogen bending strength, anharmonicity of the OH stretching motion, and Fermi resonance between the overtone of the HOH bending motion and the OH stretching motion. A comparison with available experimental data is made and is used to analyze some of the shortcomings of the configuration space model and the dynamical analysis.

Bond polar parameters from infrared intensities in X_{2}CY molecules
View Description Hide DescriptionThe integrated gas‐phase infrared intensities in F_{2}CO, Cl_{2}CO, F_{2}CS, and Cl_{2}CS are interpreted in terms of bond polar parameters using recently proposed parametric model of IR intensities. The calculated values for the bond parameters correlate with the electronegativities of the atoms forming the chemical bonds in these molecules. It is concluded that the band intensities in the infrared spectra of these systems are largely dependent on the static charge distribution.

Microwave structure measurements on the cyclopropane–HCN complex
View Description Hide DescriptionMicrowave rotational transitions for the cyclopropane‐HCN complex were observed using the Flygare–Balle pulsed beam Fourier transformspectrometer. The 1_{11}→2_{12}, 1_{01}→2_{02}, 1_{10}→2_{11}, 2_{12}→3_{13}, 2_{02}→3_{03}, 2_{11}→3_{12}, 3_{13}→4_{14}, 3_{03}→4_{04}, 3_{12}→4_{13}, 4_{14}→5_{15}, 4_{04}→5_{05}, and 4_{13}→5_{14} transitions were observed. The rotational constants obtained are A=20 243±1344 MHz, B=1384.209±0.001 MHz, and C=1327.901±0.001 MHz. The distortion constants are D _{ J }=1.53±0.03 kHz and D _{ JK }=50.9±1.1 kHz. The nitrogen quadrupole coupling strength is eQq_{aa} =−4.38±0.01 MHz with eQq_{bb} −eQq_{cc} = 0.002±0.003 MHz. The cyclopropane carbon atoms and HCN lie in a plane. The most reasonable bond distance is obtained with the HCN hydrogen atom bonding to the midpoint between two of the cyclopropane carbon atoms. The separation of the centers of mass of HCN and cyclopropane is R _{cm}= 4.472±0.002 Å. The HCN carbon atom is R _{CC–C}=3.476±0.002 Å from the cyclopropane carbon–carbon bond. The stretching force constant is k _{ s }=0.062 mdyn/Å and the approximate bonding energy is 860 cm^{−} ^{1}.

Variation of dipole moments with triplet state energy spacings in substituted benzaldehydes
View Description Hide DescriptionRecent studies of the Stark effect on bands in the laser induced phosphorescence excitation spectrum of 2,4,5‐trimethylbenzaldehyde, isolated in a durene single crystal, revealed that the dipole moment associated with the most intense feature in the spectrum, believed to arise from the T _{2}(nπ*) state, was the same within experimental uncertainty as that for the T _{1}(ππ*) origin. This result may indicate that the T _{2}(nπ*) state is strongly coupled to a set of nearby T _{1}(ππ*) vibronic levels. In order to evaluate the effect of the density of ππ* vibronic levels on the dipole moment of the T _{2}(nπ*) state, we have made similar measurements on 2,4‐ and on 2,5‐dimethylbenzaldehyde. We have found that as the T _{1}‐T _{2}energy spacing decreases, the dipole moment of the second triplet state becomes more characteristic of an nπ* state.

Raman scattering study of pressure effects on the structure and the coordination‐induced charge redistribution in 4‐cyanopyridine
View Description Hide DescriptionThe Raman spectra in the frequency regions of the lattice and the internal modes of the crystalline CNPy and Zn(CNPy)_{2}Cl_{2} have been measured at various pressures up to 16.5 and 20 kbar, respectively. A structuralphase transition at 10 kbar in CNPy and two at 8.2 and 18 kbar in Zn(CNPy)_{2}Cl_{2} were observed. At high pressure, CNPy suffers a lattice distortion along the eigenvector of a soft phonon, and Zn(CNPy)_{2}Cl_{2} undergoes a molecular configuration distortion which leads to the softening of a rotational phonon. The charge distribution on the CNPy molecule is changed by coordination and also by external pressure and this charge redistribution has been correlated with the vibrational spectral changes.

Deuterium relaxation of a partially oriented methyl group. III. Order director fluctuations and solute reorientation in a nematic solution of acetonitrile‐d _{3}
View Description Hide DescriptionDeuteron spin‐lattice relaxation rates (R _{1Z }) and relaxation rates of quadrupolar order (R _{1Q}) have been determined between 13 and 47 °C at 38.4 MHz, for a 10.5 mol % of acetonitrile‐d _{3} in Merck Licristal phase 5, and selective and nonselective measurements of spin‐lattice relaxation have been carried out at 4.6 and 9.2 MHz over the same temperature range. The data can be satisfactorily explained by a combination of fast, anisotropic reorientation (corrected for effects of static ordering) and hydrodynamic fluctuations in the orientation of the nematic director. Large values of the rotational anisotropy were derived from the spin‐lattice relaxation data, and confirmed by 38.4 MHz measurements of the temperature dependence of the relaxation rate R _{2}(Z Q) of a zero quantum coherence. The results obtained here are discussed in terms of recent related ^{13}C relaxation studies of acetonitrile in nematic solution.

An EPR study of the reaction between poly(p‐phenylene sulfide) and electron‐acceptor dopants
View Description Hide DescriptionEPR spectra for electron‐acceptor doped poly(p‐phenylene sulfide) are reported. The g values obtained from these spectra correspond to an electron density centered around a divalent sulfur radical cation R–S–R. This assignment is supported by corresponding EPR data from an oligomeric model compound φ–S–φ–S–φ (φ=phenyl). The g value of the sulfur‐based radical cation of doped poly(p‐phenylene sulfide) distinctly contrasts the nearly free electron g values obtained from EPR measurements on poly(p‐phenylene) and its oligomer p‐terphenyl.

Theoretical studies of H_{2}–H_{2} collisions. IV. Ab initio calculations of anisotropic transport phenomena in para‐hydrogen gas^{a)}
View Description Hide DescriptionThe temperature dependence of the effective Waldmann–Snider cross sections determining the Senftleben–Beenakker effects of viscosity and heat conductivity has been studied for pH_{2} gas between 10 and 200 K. From ab initio nonspherical potentials of H_{2}–H_{2}, scattering matrices have been determined in close‐coupling calculations. From these, the elements of the scattering amplitude matrix have been obtained and used as input quantities for the evaluation of the various Waldmann–Snider collision integrals. The results of these first ab initio numerical calculations of anisotropic transport coefficients show excellent agreement of calculated and measured effective cross sections, especially for the most recent improved version of the interaction potential. In addition, it has been shown that the polarization production cross sections are quite sensitive to the potential anisotropy.

Excimer laser photolysis studies of translational‐to‐vibrational energy transfer in collisions of H and D atoms with CO
View Description Hide DescriptionTranslational‐to‐vibrational excitation of carbon monoxide is observed as a result of collisions of high energy H and D atoms with CO. The fast atoms are produced by excimer laserphotolysis of H_{2}S, D_{2},S, HCl, HBr, or HI at 193 or 248 nm. Detection of time and wavelength‐resolved infrared fluorescence is used to quantify the CO vibrational state excitation. The CO (v=1−6) state distribution from H (H_{2}S, 193 nm, E _{cm}=2.3 eV) +CO collisons is 0.74±0.15, 0.15±0.01, 0.08±0.01, 0.01±0.01, 0.02±0.01, and 0.01±0.01. The corresponding state distribution from D (D_{2}S, 193 nm, E _{cm}=2.2 eV)+CO is 0.79±0.19, 0.13±0.01, 0.05±0.02, 0.02±0.02, 0.01±0.01, and 0.01±0.01. Rotational excitation is at least as significant as vibrational excitation, based on estimates of the total energy transfer.Measurements of the relative vibrational excitation efficiency as a function of initial H atom energy show that the fraction of translational energy converted to CO vibration increases by more than 300% as the initial H atom energy is increased from 1.0 to 3.2 eV. Good qualitative agreement is found between the experimental results and classical trajectory calculations carried out for collinear collisions of H and D atoms with CO using a simple repulsive interaction potential.

A uniform WKB approximation for spheroidal wave functions
View Description Hide DescriptionUniform WKB wave functions are developed for prolate spheroidal wave functions which arise when the Schrödinger equation is given in prolate spheroidal coordinates. A detailed numerical test is presented for angular spheroidal wave functions. It shows that the approximate uniform WKB wave functions are excellent in accuracy.

Scattering off an ellipsoid: A semiclassical theory
View Description Hide DescriptionThe WKB approximation is developed for scattering of an atom by an ellipsoidal particle and elasticscattering cross sections are calculated by using the WKB phase shifts in the prolate spheroidal coordinates and uniform WKB approximations for angular spheroidal wave functions. The calculation of cross sections requires the solution of an elliptic boundary condition and is quite simple compared to the calculation in the close coupling theory in the spherical coordinate representation. A version of close coupling theory is briefly discussed in the prolate spheroidal coordinates.

Arbitrary order functional sensitivity densities for reaction‐diffusion systems
View Description Hide DescriptionThe concept of functional sensitivity analysis in the study of diffusion processes in a chemically reacting environment is reviewed and extended to include the notion of arbitrary order functional sensitivity densities. A new perspective on its physical and mathematical basis is offered through a detailed discussion of the motivation behind the formal procedure adopted. The arbitrary order functional sensitivity densities (defined as arbitrary order functional derivatives of a chemical species concentration with respect to the space and time dependent parameters of the system) are shown to obey certain differential equations that can be solved with the help of a Green’s function. This Green’s function is the inverse of a differential operator that defines an associated linear system in the application of Lyapounov’s linearized stability theory to (in general nonlinear) reaction‐diffusion systems. It is found that the Green’s function can be regarded as a concentration response function which allows for an interpretation of the sensitivity densities as generalized response functions. Finally, an illustration of the use of those techniques is provided by applying them to the case of a linear reaction‐diffusion system.

Rotational and vibrational‐rotational relaxation in collisions of CO_{2}(01^{1}0) with He atoms
View Description Hide DescriptionRotational and vibrational‐rotational relaxation of CO_{2}(01^{1}0) in collisions with He atoms is studied theoretically. Cross sections and rate coefficients have been calculated using a vibrational close‐coupling, rotational infinite‐order sudden method, together with an ab initiopotential energy surface. Comparisons with previous calculations and experiments on rotational relaxation in He+CO_{2}(00^{0}1) are made. The rotational relaxation cross sections are found to be insensitive to the vibrational dependence of the He–CO_{2} potential. Transitions between even and odd rotational states of the (01^{1}0) level have relatively small cross sections. Interesting oscillating structures are predicted for the rotational dependence of the cross section distributions for transitions involving the (01^{1}0) level.

Time‐resolved resonance fluorescence studies of O(^{1} D _{2}) yields in the photodissociation of O_{3} at 248 and 308 nm
View Description Hide DescriptionThe yield of electronically excited oxygen atoms O(^{1} D _{2}) following excimer laserphotolysis of ozone at 248 and 308 nm was directly measured using time‐resolved resonance fluorescence emission at ≊130 nm. The result obtained at 248 nm is in good agreement with a previous resonance fluorescencemeasurement in which the yield of ground‐state O(^{3} P _{ J }) was determined in the presence of gases which either quenched or reacted with O(^{1} D _{2}). The relative yield determined at 308 nm 0.79±0.02 is the first such determination in the region of weak Hartley band absorption near the thermochemical threshold for production of O(^{1} D _{2})+O_{2}(a ^{1}Δ_{ g }). When taken in conjunction with the previously measured relative yields of O(^{1} D _{2}) at this wavelength, this result suggests that the yield of electronically excited oxygen atoms at 300 nm is 0.96±0.02.

Atom–molecule collisions at very low energies: A correlation function approach
View Description Hide DescriptionAn extension of the neutron diffraction formalism to low energy atom–molecule scattering including all recoil effects is presented. The nature of the atom–molecule interaction suggests that the diffraction model should be qualitatively correct in the low collision energy regime for vibrational relaxation. The customary impulsive collision assumptions which lead to the use of the Born approximation are replaced by more general transforms which account for possible distortion of the wave function by the potential. Model calculations for a simple vibrational deexcitation process are given.

Initial states of maximal entropy in formal scattering theory
View Description Hide DescriptionFor a precollision state which is not sharply defined, not all details of the dynamics can be obtained by examination of the final state. A version of formal collision theory which takes full advantage of the inherent loss of detail is provided. Only the relevant aspects of the dynamics need then be computed and an explicit identification of the relevant details is provided by a generalized intertwining theorem. Several explicit examples where this point of view can be implemented are worked out.