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Volume 87, Issue 3, 01 August 1987

Angular overlap model for the Jahn–Teller coupling constants in the orbital triplet states of d ^{5} ions: Case of Mn^{2} ^{+} in ZnS and ZnSe
View Description Hide DescriptionA theoretical determination of the Jahn–Teller coupling constants of d ^{5} ions in tetrahedral clusters is performed in the framework of the angular overlap model and the quasimolecular approximation. It is shown that the angular model correctly predicts the trend of the preponderant coupling of the ^{4} T _{1}(G) and ^{4} T _{2}(G) levels of Mn^{2} ^{+} in MnS_{4} and MnSe_{4} to the E‐vibrational effective mode. Finally, the limitations of this model are discussed. In particular, it is shown that more elaborated covalent models are needed to interpret the Jahn–Teller coupling coefficients of the ^{4} E and ^{4} A _{1} levels of d ^{5} configurations and to determine the influence of the crystal on the clusters.

Ligand field theory for the orbit–lattice coupling coefficients to E strains of d ^{5} ions in tetrahedral symmetry
View Description Hide DescriptionA molecular orbital model has been elaborated to determine the orbit–lattice coupling coefficients to strains of symmetry E of the orbital triplet states of d ^{5} ions in tetrahedral symmetry. The wave functions and energies of the monoelectronic molecular orbitals have been determined for a tetrahedral molecule MnX_{4} in a crystal and for a slightly distorted molecule of symmetry D _{2d } corresponding to a strain E _{θ}. It is shown that in D _{2d } symmetry, the splitting of the monoelectronic molecular orbitals e and t _{2} is due to the variations of the group overlap integrals and the variation of the crystal electrostatic field with respect to their values in cubic symmetry. The orbit–lattice coupling coefficients which are independent of the spin–orbit interactions, are obtained by diagonalizing the Tanabe–Sugano matrices for the orbital triplet states and then using the molecular orbitals in D _{2d } symmetry as zero‐order wave functions in a first‐order perturbation scheme. The proposed model for the orbit–lattice coupling coefficients of the orbital triplet states ^{4} T _{1} and ^{4} T _{2} at lower energy for Mn^{2} ^{+} in ZnS and ZnSe gives theoretical values which differ at most by 30% from the experimental values. It is shown that three fifths of the theoretical values for the orbit–lattice coupling coefficients are due to the variations of the group‐overlap integrals, the remaining part of the theoretical values being due to the variations of the crystal electrostatic field describing the Coulomb’s interaction in the molecular model.

The a _{2g } mode in ground state benzene
View Description Hide DescriptionHarmonic ground state frequencies for the a _{2g } mode, ν_{3}, in C_{6}H_{6}, C_{6}D_{6}, and ^{1} ^{3}C_{6}H_{6} have been determined by application of the Teller–Redlich product rule to b _{2u } mode frequencies obtained from two‐photon fluorescence excitation experiments for the D _{6h } symmetry molecules and a ^{′} _{2} frequencies from 1,3,5‐C_{6}H_{3}D_{3} determined at rotational contour resolution. The a _{2g } harmonic frequency values are 1366.6 cm^{−} ^{1} for C_{6}H_{6}, 1063.0 cm^{−} ^{1} for C_{6}D_{6}, and 1355.4 cm^{−} ^{1} for ^{1} ^{3}C_{6}H_{6}. The anharmonic correction to ν_{3} in C_{6}H_{6}, estimated from the difference in product rule and infrared frequencies is −17 cm^{−} ^{1}. The b _{2u } mode frequencies, ν_{1} _{4} and ν_{1} _{5}, respectively, have also been significantly modified from those obtained from lower resolution spectra to 1309.4 and 1149.7 cm^{−} ^{1} in C_{6}H_{6}, 1286.3 and 827.9 cm^{−} ^{1} (harmonic value) in C_{6}D_{6}, and 1269.8 and 1138.1 cm^{−} ^{1} in ^{1} ^{3}C_{6}H_{6}. The anharmonic correction for ν_{1} _{5} in C_{6}D_{6} is estimated as −4 cm^{−} ^{1}. The B _{2u }←A _{1g } electronic origin of ^{1} ^{3}C_{6}H_{6}, has been determined for the first time as 38 105.5±0.3 cm^{−} ^{1}. The B _{2u }←A _{1g } electronic origin of 1,3,5‐C_{6}H_{3}D_{3} has been redetermined to be 38 187.5±0.3 cm^{−} ^{1}, 3.5 cm^{−} ^{1} higher than previously reported.

Photoionization of molecular Rydberg states: H_{2}, C ^{1}Π_{ u } and its doubly excited states
View Description Hide DescriptionExplanations are offered for some anomalies seen in photoelectron spectra resulting from single‐photon ionization of unperturbed single‐configuration Rydberg states of molecules. Some anomalous behavior may be due to occurrence of Cooper minima and shape resonances. Stronger effects can be due to excitation of the ion core producing doubly excited states whose decay characteristics and their effects on photoelectron spectra are discussed. As an example, anomalous photoelectron spectra produced in resonant multiphoton ionization of H_{2} via its C state are attributed to the excitation and autoionization of the ^{1}Π_{ g }(2pσ)(2pπ) state. Formation of excited atomic hydrogen is predicted. The value of such experiments in investigating doubly excited states in general is discussed as well as the problem of preparation of ions in selected states.

Observation of Rydberg transitions from the inner valence shell of ethane
View Description Hide DescriptionThe electron impact spectrum of ethane has been examined in a region that includes ionization out of the inner valence shell. One diffuse structure and a progression of ten vibrational bands have been found in a 4 eV range below and to some degree overlapping the ^{2} A _{2u } ion threshold. Evidence indicates that the observed transitions belong to the symmetry forbidden Rydberg series (2a _{2u })^{2}→(2a _{2u }, n pσ or n pπ).

Sub‐Doppler infrared spectrum of the carbon dioxide trimer
View Description Hide DescriptionA spectrum of the carbon dioxide trimer van der Waals species has been recorded near 3614 cm^{−} ^{1} at sub‐Doppler resolution using an optothermal (bolometer‐detected) molecular‐beam color‐center laser spectrometer. A planar, cyclic structure with C _{3h } symmetry has been determined for the complex with a carbon–carbon separation of 4.0382(3) Å. The observed perpendicular band, corresponding to an in‐plane E ^{′}‐symmetry vibration of the trimer, has been attributed to a localized excitation of the 2ν^{0} _{2} +ν_{3} combination mode of a CO_{2} subunit by virtue of its small blue shift (∼0.98 cm^{−} ^{1}) from that of the isolated monomer.

Harmonic force field and Raman trace scattering intensity parameters for cyclohexane
View Description Hide DescriptionAbsolute Raman trace scattering cross sections have been measured for gaseous cyclohexane‐h _{1} _{2}, cyclohexane‐d _{1} _{2}, and the axial and equatorial isomers of cyclohexane‐d _{1} _{1}. A set of ∂ᾱ/∂S _{ j } intensity parameters were determined from the experimental cross sections. The quadratic vibrational force field required for this analysis was obtained by scaling an a b i n i t i o force field to fit well determined vibrational frequencies. The unexpected intensity ratio observed for the CH stretching bands of the two conformers of d _{1} _{1} is explained by a difference in the ∂ᾱ/∂r _{CH} value for the axial and equatorial CH bonds. The transferability of Raman intensity parameters is considered in light of recent results.

Tunable UV laser photolysis of NF_{2}: Quantum yield for NF(a ^{1}Δ) production
View Description Hide DescriptionThe UVphotodissociation of NF_{2} has been investigated from 240–270 nm, using tunable UV radiation from a frequency upconverted YAG pumped dye laser system. The absorption cross section of NF_{2}, and the photolysis quantum yield for the fragment NF(a ^{1}Δ) were measured with 0.25 cm^{−} ^{1} resolution. The NF(a ^{1}Δ) quantum yield decreases at longer wavelengths and is only 1% at 260 nm. This suggests that the first long wavelength band in NF_{2} leads primarily to ground state NF(X ^{3}∑), and that the existence of a new higher lying NF_{2} electronic state is responsible for the NF(a ^{1}Δ) production.

Time‐dependent analysis of the Hartley absorption band and resonance Raman spectra in ozone
View Description Hide DescriptionA detailed investigation is made of the short‐time dynamics involved in the Hartley absorption band in ozone and the related resonanceRaman spectra for low vibrational overtones. A quadratic expansion of the X ^{1} A _{1} and dissociative ^{1} B _{2}potential energy surfaces involved leads to an analytic calculation of three‐mode vibrational correlation functions, which are then Fourier‐transformed to obtain the absorption and Raman spectra. A type of time‐dependent perturbation theory is used to approximate the effects of including anharmonic terms in the expansion of the upper state. It is concluded that an improvement in the description of the Raman spectra will require at the least a major modification of the parabolic barrier behavior along the antisymmetric stretch coordinate and, better, a large‐amplitude‐motion treatment.

Fine structure in the [(^{2}Γ_{3}+ν_{3})←^{1}Γ_{1}] vibronic band in PuF_{6}
View Description Hide DescriptionHigh resolution spectra of the [(^{2}Γ_{3}+ν_{3})←^{1}Γ_{1}] transition from the absorption band systems near 800 nm in PuF_{6} have been recorded. Rotational lines and some fine structures have been resolved. The results support a spherical‐top configuration despite the possibility of Jahn–Teller distortion.

On the theory of multiple scattering for a nonpolar fluid composed of chiral molecules
View Description Hide DescriptionA statistical theory of multiple scattering is given for a nonpolar fluid composed of chiral molecules. The theory describes through the correlation function the scattering intensities to all orders. The circular intensity differential due to the single scattering is discussed. The results show that the scattering intensities orthogonal to the direction of propagation of the incident light are slightly different when the light is right and left circularly polarized, while the intensities parallel to the direction of propagation are same.

Large paramagnetic anisotropy of NaMg[Cr(oxalate)_{3}]⋅9H_{2}O
View Description Hide DescriptionThe magnetic susceptibilities of single crystals of NaMg[Cr(oxalate)_{3}]⋅9H_{2}O are reported over the temperature interval 40 mK–4.2 K. Large anisotropy is observed due to the large zero‐field splitting of the ^{4} A _{2} ground state; the parameters that fit the data are 2D/k =−2.45(3) K, g _{∥}=1.985(5), g _{⊥}=2.035(10), and an exchange correction of z J/k =0.006(1) K.

Laser photoelectron spectroscopy of CrH^{−}, CoH^{−}, and NiH^{−}: Periodic trends in the electronic structure of the transition‐metal hydrides
View Description Hide DescriptionThe laser photoelectron spectra of CrH^{−}, CoH^{−}, and NiH^{−} and the analogous deuterides are reported. The spectra are interpreted using a qualitative description of the electronic structure for the hydrides. This model is used to assign off‐diagonal transitions in the photodetachment to low‐spin states of the neutrals, and diagonal transitions to high‐spin states of the neutrals. These data are used to identify the high‐spin states of CoH and NiH; several other states of CrH, CoH, and NiH are also identified. Periodic trends in the bond lengths, vibrational frequencies, and electronic excitation energies for the MnH through NiH molecules are examined. Electron affinities are reported for CrH (0.563±0.010 eV), CoH (0.671±0.010 eV), and NiH (0.481±0.007 eV), and the corresponding deuterides.

CO_{2} laser–microwave double resonance spectroscopy of NH_{3}: Precise measurement of dipole moment in the ground state
View Description Hide DescriptionCO_{2} laser–microwave double resonance (LMDR) spectroscopy with an intense electric field was applied to the NH_{3} molecule. Using infrared transitions in the ν_{2} band for pumping, 76 and 37 LMDR signals originating from 25 and 16 inversion transitions in the ground vibrational state were observed for ^{1} ^{4}NH_{3} and ^{1} ^{5}NH_{3}, respectively. The applied electric field was calibrated with a precision of 10 ppm by a new method using the infrared–infrared double resonance signals of PH_{3}, which have been calibrated against the OCS LMDR signals. The observed signals were analyzed to yield the precise dipole moment with rotational dependence, which was expanded as μ_{0}+μ_{ J } J(J+1) +μ_{ K } K ^{2}+⋅⋅⋅. The coefficients determined are, with one‐standard‐deviation uncertainties in the parentheses, μ_{0}=1.471 932(7) D, μ_{ J }=1.8736(34)×10^{−} ^{4} D, and μ_{ K } =−3.5550(47)×10^{−} ^{4} D for ^{1} ^{4}NH_{3}, and μ_{0}=1.471 964(11) D, μ_{ J } =1.9244(77)×10^{−} ^{4} D, and μ_{ K } =−3.5903(98)×10^{−} ^{4} D for ^{1} ^{5}NH_{3}. The absolute accuracy of the dipole moment is, however, limited to 50 ppm as determined by the uncertainty of the OCS dipole moment. The effective dipole moments for the J,K=1, 1 and 1,0 levels of ^{1} ^{4}NH_{3} calculated from the present coefficients are consistent with the results of a molecular beam electric resonance experiment. The μ_{ J } and μ_{ K } coefficients are interpreted in terms of Watson’s θ^{αβ} _{γ} constants, resulting in a good agreement between the observed and theoretical values. The consistency between the two isotopic species is also confirmed.

Internal state distributions of CO from HNCO photodissociation
View Description Hide DescriptionThe internal state distributions of CO produced by photodissociation of HNCO at 1930 (230.1 nm) and at 10 200 cm^{−} ^{1} (193.3 nm) in excess of the dissociation energy are determined from multiphoton ionization spectra of the CO fragment measured under collision‐free conditions. The rotational state distribution of the CO produced at the lower photolysis energy is characterized by a temperature of (491±23) K. The rotational state distribution of CO produced by photodissociation at the higher photon energy in not well characterized by a temperature. This latter distribution has maximum population near J=37, extends beyond J=65, and accounts for ∼20% of the available energy in excess of that necessary to rupture the HN–CO bond. An impulsive dissociation model assuming that dissociation occurs from an excited HNCO complex containing a nonlinear N–C–O configuration accounts for the average CO rotational excitation while phase‐space theory does not agree with the observed distributions. Fitting a semiclassical model to the data using a logically constructed potential energy surface and a ground‐state‐dependent function provides a useful parametrization for the impulsive dissociation. Although not absolute, this analysis suggests that the dissociation occurs directly on a repulsive excited statepotential energy surface.

Rotational state distributions of NH(a ^{1}Δ) from HNCO photodissociation
View Description Hide DescriptionWe have examined the photofragmentation HNCO→NH(a ^{1}Δ)+CO using radiation at wavelengths shorter than 230 nm. Nascent NH(a ^{1}Δ) shows relatively little rotational excitation, accounting for less than 12% of the energy in excess of the dissociation energy. The rotational state distributions evidence less population in high rotational states than predicted by statistical theories but more than expected on the basis of a simple impulsive dissociation. A semiclassical impulsive model that describes photoproduct rotation as developing during fragmentation successfully describes the rotational state distributions of NH(a ^{1}Δ) produced by HNCO photodissociation over a wide range of wavelengths. The success of this model in describing the NH rotational state distributions and previously measured CO rotational state distributions suggests that the excited statepotential energy surface may be repulsive with minima in HNC and NCO bond angles each near 120°.

Valence photoionization of hydrogen fluoride with synchrotron radiation
View Description Hide DescriptionPhotoionization from hydrogen fluoride has been studied with synchrotron radiation in the 30–130 eV photonenergy range. Relative photoelectric cross sections have been obtained for the 1π (ionizationenergy of 16.1 eV) MO, the 3σ MO (19.9 eV ionizationenergy), and in some cases the inner 2σ valence MO (I.E.=39.6 eV). Comparison is made with other measurements such as quasiphotoionization results and with one‐center calculations. Angular distribution asymmetry parameters have also been measured for the 1π and 3σ MO’s and compared with those of Ne2p. In the inner valence region, satellites have been observed; this is in accord with most theoretical calculations which predict the breakdown of the one‐particle description in this region.

Isotopic exchange in gamma‐irradiated mixtures of C_{2} _{4}H_{5} _{0} and C_{2} _{4}D_{5} _{0}: Evidence of free radical migration in the solid state
View Description Hide DescriptionIrradiation of a 1:1 co‐melted mixture of tetracosane‐h _{5} _{0} and tetracosane‐d _{5} _{0} in the solid state resulted in isotopic exchange in very high yield [G(H–D)=25]. Exchange was not observed for a similar isotopic mixture irradiated in the liquid phase. A mathematical model, which operates by iterative computer calculations, has been constructed to simulate the complex changes in isotopic peak patterns which arise in the irradiated solid mixture. The simulations show that the observed isotopic exchange is not random, but rather that an individual molecule which has once undergone an isotopic exchange has an enhanced probability of undergoing additional exchanges. The results are very consistent with the occurrence of a localized chain‐reaction process specific to the solid phase, and they provide evidence supporting the hypothesis that free radical transport in solid organics occurs extensively by a repetitive ‘‘hydrogen‐hopping’’ mechanism involving successive hydrogen abstractions. It can be estimated that the average number of H–D exchanges occurring per radical pair, from formation to termination, is 14.3.

A collinear quantal study of vibrational predissociation and prereaction of van der Waals molecules
View Description Hide DescriptionIt was recently suggested that vibrational excitation of van der Waals molecules could lead to prereaction instead of predissociation. A mechanism for vibrational prereaction based on tunneling of a light atom is proposed and tested. The effect of van der Waals wells on the reactivity of collinear MuD_{2}, ClHBr, and ClHCl systems is studied. We find that vibrational prereaction is sensitive to the ratio of tunneling and vibrationally nonadiabatic interaction. If tunneling dominates, prereaction will take place. This is the case for the ClHBr and ClHCl systems. The interplay between reaction probability and photodissociation cross sections is studied. We conclude that vibrational prereaction will lead to an increase in reaction probability and an increase in the formation of products in photodissociation. This study suggests that vibrational prereaction could be observed at least in principle in light atom transfer systems.

Suppressed kinetics of short range ordering at low temperatures
View Description Hide DescriptionThe kinetics of short range ordering in a binary alloy were studied by Monte Carlo simulation. Unlike the kinetics of ordering by the direct interchange of atoms, at low temperatures there is a marked suppression of the kinetics of ordering when ordering occurs by the vacancy mechanism. Large activation energies are associated with this suppression of kinetics. These activation energies suggest that certain heterogeneities in the atomic arrangements on the lattice behave as traps which the vacancy cannot easily escape at low temperatures. Such traps were observed during simulations of the ordering process.