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Volume 67, Issue 6, 15 September 1977

The percentage of bound–continuum character in the optical transition of trapped electron absorption bands, and the fate of quasifree electrons
View Description Hide DescriptionThe actual percentage of bound–free character in the optical transition of the trapped electron in 9.5M LiCl/D_{2}O aqueous glasses at 77 K has been measured at 694 nm and found to be 3.7%±1.8%. Consequently, about 96% of the transition probability must be to bound excited states at 694 nm, which is 100 nm (or 0.31 eV) on the red side of the band maximum. This measurement has taken account of the number of photoreleased electrons which are retrapped and of those electrons which are bleached by indirect secondary processes. It differs, therefore, from previous measurements of the quantum yield of n e t bleaching as obtained by ’’steady‐state’’ methods. The information was procured by evaluating both the quantum yield for n e t loss of trapped electrons (e ^{−} _{vis}) and the quantum yield for the production of the infrared‐absorbing electron state (e ^{−} _{ir}). The latter measurement was used to estimate the fraction of photoreleased electrons which were retrapped as e ^{−} _{vis}, by assuming that the relative probability of forming e ^{−} _{ir} or e ^{−} _{vis} was the same regardless of whether quasifree electrons were generated by photolysis or radiolysis. Samples of e ^{−} _{vis} were produced by γ radiolysis at 77 K in glasses made from various concentrations of LiCl in D_{2}O. They were bleached by 15 ns pulses of 694 nm light from a Q‐switched ruby laser and the resulting absorption changes within 100 ns were analysed at 633 and 1850 nm. For LiCl concentrations between 6.5 and 15M only 20% to 35% of the quasifree electrons which were generated by 694 nm photolysis were lost by reaction in these γ‐irradiated glasses. The distribution between trapping as e ^{−} _{vis} or e ^{−} _{ir} on the other hand was strongly dependent on LiCl concentration.

Raman studies of librational motion in N_{2}/CO and CO_{2}/N_{2}O mixed crystals
View Description Hide DescriptionThe Raman spectra of N_{2}/CO and CO_{2}/N_{2}O mixed crystals have been measured in the lattice mode region. The spectra of the CO_{2}/N_{2}O mixed crystals exhibit very definite one‐mode behavior indicating that the librational states are within the amalgamation limit. In the N_{2}/CO mixed crystals, the E _{ g } and lower frequency T _{ g } modes show one‐mode behavior while the higher frequency T _{ g } modes show two‐mode behavior. These spectra are shown to be consistent with the established assignments and the librational exciton concept when perturbations in both the site energy and the excitation exchange interactions are considered. The effect of the random sense of the molecules in pure N_{2}O and CO is also discussed from the librational exciton point of view. Two‐phonon scattering has been observed in all of the samples and is attributed to librational motion.

An a b i n i t i o calculation on the WO_{6} ^{6−} octahedron with an application to its luminescence
View Description Hide DescriptionAn a b i n i t i o calculation of the electronic structure of the WO^{6−} _{6} octahedron is presented. The influence of the surrounding lattice is simulated by point charges. The ground state and the excited charge transfer states in the electronic configurations t ^{5} _{1u }–t ^{1} _{2g }, t ^{5} _{2u }–t ^{1} _{2g }, and t ^{5} _{1g }–t ^{1} _{2g } are calculated. Vector coupling coefficients are given and the occurrence of the two electron integrals which are not Coulomb or exchange integrals is discussed. The calculated energy differences between the ground and excited states agree with experiment. The charge transfer is small (0.17 electron charge) because a strong delocalization occurs in the excited state.

Vibrational excitation in H_{2} and D_{2} electric discharges
View Description Hide DescriptionSpontaneous Raman scattering has been used to measure the effective vibrational temperature of the v=1 level as a function of input power in a self‐sustaining low pressure dcglow discharge in pure H_{2} and D_{2} gases. A nonmonotonic dependence of vibrational temperature on input specific power, yielding T _{ v }=1440–1960 °K in H_{2}, has been measured downstream of the discharge. Results of numerical calculations modeling the discharge/afterglow are presented. The agreement between theory and experiment is good and suggest that the behavior observed is caused by the effect of the temperature dependent vibrational–translation energy transfer rate.

Electron correlation effects on the excitation energies of the lowest triplet states of glyoxal
View Description Hide DescriptionIn an attempt to understand the ordering of excited states of glyoxal the ^{1} A _{ g }ground state and low‐lying triplet states ^{3} A _{ u }, ^{3} B _{ u }, and ^{3} B _{ g } have been studied with a b i n i t i o correlated wavefunctions. A double zeta basis set was used in the configuration interaction calculations, and ∼6000 Slater determinants were selected for each electronic state. The predicted CI adiabatic excitation energiesT _{ e } are 21 800 cm^{−1} (^{3} A _{ u }), 25 100 cm^{−1} (^{3} B _{ u }), and 29 300 cm^{−1} (^{3} B _{ g }). The effects of d functions and higher (than double) excitations (unlinked clusters) have also been evaluated. For the ^{3} B _{ u } state, both d functions and higher excitations raise the predicted T _{ e } value by perhaps as much as 5 000 cm^{−1}. For the ^{3} B _{ g } state, d functions raise the predicted excitation energy but higher excitations have the opposite effect. It is concluded that the excitation energy of the ^{3} B _{ u } state is dramatically increased by correlation effects, while T _{ e } for the ^{3} A _{ u } and ^{3} B _{ g } states is markedly decreased by correlation.

Laser magnetic resonance spectroscopy of the ν_{2} fundamental band of HCO at 9.25 μm
View Description Hide DescriptionThe ν_{2} bending fundamental (ν_{0}=1080.76 cm^{−1}) of the formyl radical, HCO, has been studied using CO_{2} laser magnetic resonance. Fluorine atoms from a discharge in CF_{4} were reacted with H_{2}CO to form the short‐lived HCO, which then flowed through an absorption cell located between the pole faces of an electromagnet and within the optical cavity of a CO_{2} laser. By means of the Zeeman effect, HCO, vibration‐rotation transitions were tuned through resonance with the laser lines. Numerous resonances involving levels with 1⩽N⩽7 and 1⩽K _{ a }⩽3 were assigned, and from analysis of the spectra accurate determinations of the band origin, rotational, centrifugal distortion, spin‐rotation, and Fermi interaction parameters for ν_{2} were made. Fairly large changes in the values of A, Δ_{ K }, ε_{ a a }, and η_{ a a a a } between the ground and v _{2}=1 vibrational states were observed.

The nonexponential character of the Zeeman relaxation of reorienting methyl groups in solids
View Description Hide DescriptionIf the rapid reorientations of methyl groups in solids are the dominant relaxation mechanisms, it is theoretically found that the Zeeman system is dynamically coupled to the so‐called rotational polarization system, resulting in a nonexponential Zeemanrelaxation. Though this result has been confirmed experimentally, in many samples within measuring error an exponential Zeemanrelaxation has been observed. In order to decide whether or not this is contradictory to the theory the degree of nonexponentiality has been further investigated in this paper. It is shown that the nonexponential character depends stronly on the following parameters: (i) the way in which the spin system is prepared for the observation of the Zeemanrelaxation, (ii) the number of protons not belonging to the methyl groups, (iii) the relaxation due to the intermethyl dipolar interactions, and (iv) the temperature. It is found that under specific conditions the observed relaxation can become almost exponential. The corresponding time constant, however, can differ considerably from that predicted by the usual spin temperature theory. Therefore, the interpretation of the experiments with this theory can lead to incorrect values of the different parameters characterizing the relaxation process. The various results have been illustrated with experiments on polycrystalline samples of 2,6‐dimethylphenol, methylcyanide, 2‐methylpiperazine, and 1,4‐dimethoxy‐2,6‐dimethylbenzene.

Observation of orbiting resonances in the scattering of He+Ne
View Description Hide DescriptionExperimental total cross section measurements for the He+Ne atom pairs are presented over the relative velocity range of 88 to 1200 m/sec. One orbiting resonance is observed at 160 m/sec. The data around 100 m/sec are consistent with the existence of another resonance at 88 m/sec. A comparison of experimental results to calculated cross sections gives the best fit for the HFD potential. This fit suggest that the lowest velocity resonance is from an l=3 partial wave, while the higher velocity resonance is from an l=4 partial wave.

Far‐infrared absorption by ices Ih and Ic at 4.3 °K and the powder diffraction pattern of ice Ic
View Description Hide DescriptionThe far‐infrared spectra of H_{2}O and D_{2}O ices Ih and Ic at 4.3 °K are reported. Many weak features are observed and for ice Ih are shown to correspond well with features in the published Raman spectra. Differences between the spectra of ices Ih and Ic are reported for the first time; they are due to the different crystal structures and can be understood in more detail from published calculations of dispersion curves and densities of translational vibrational states. These results and the Raman spectra provide much information with which to test calculations of the translational vibrations in ice I. Extra lines in powder diffraction patterns of ice Ic have generally been attributed to ice Ih impurity. From the measurements reported in this work it is suggested that this may not be the case.

A b i n i t i o calculations of the minimum energy path in the doublet surface for the reaction N(^{4}S)+O_{2}(^{3}Σ^{−} _{ g }) →NO(^{2}Π_{ u }) +O(^{3} P)
View Description Hide DescriptionA b i n i t i o MCSCF+CI calculations of the minimum energy path in the doublet surface for the N(^{4} S)+O_{2}(^{3}Σ^{−} _{ g }) →NO(^{2}Π_{ u })+O(^{3} P) reaction were carried out using a 4‐31 G basis set. The minimum path was calculated to pass through a bent N–O–O ^{2} A′ transition complex with a barrier of 21.7 kcal/mole. A high barrier was found for the doublet C _{2v } approach of N(^{4} S) to O_{2}(^{3}Σ^{−} _{ g }). Two distinct regions of orbital transformations, given by the MCSCF calculations were found along the minimum energy path.

Relaxation of the mercury 6 ^{3} P _{0} and 6 ^{3} P _{1} states
View Description Hide DescriptionThe relaxation of 6 ^{3} P _{0}mercury atoms excited by a laser pulse was measured for densities ranging from 2×10^{17} to 8×10^{18} cm^{−3}. The decay of the 253.7 nm resonance line was also measured for densities ranging from 5×10^{16} to 10^{18} cm ^{−3}. The decay coefficients were analyzed to yield rate coefficients for three body molecular formation, the ^{3} P _{1}→^{3} P _{0} collision rate and a collisional quenching rate for the ^{3} P _{0} state. Small loss rates due to diffusion and radiation were also observed and found to be consistent with theoretical estimates of these quantities.

Some new electronic states of matrix‐isolated diatomic bismuth
View Description Hide DescriptionAbsorption spectra of the diatomic bismuth molecule in argon matrices reveal three hitherto unreported electronic states with band system origins at 12 395, about 15 246, and about 17 736 cm^{−1}. The ω_{ e } values for the three states are, respectively, 123, 132, and 145 cm^{−1}. Laser excitations from the red into the ultraviolet produce only a single emission band system for Bi_{2} in argon or krypton; in argon the emission system origin is at 12 395 cm^{−1}. The ground state ω_{ e } value is 151 cm^{−1}, 13% smaller than for the gaseous molecule.

Transition moments between excited electronic states of N_{2}
View Description Hide DescriptionWe report the transition moments between the excited states of molecular nitrogen including their dependence on internuclear distance. These moments are calculated using the equations of motion method and can be obtained with only a slight increase in the effort needed to obtain the ground to excited statetransition moments. The transition moments along with reliable vibrational wavefunctions should be useful in the analysis of observed band intensities of N_{2}.

Absorption spectra of matrix‐isolated alkaline earth metal diatomic molecules
View Description Hide DescriptionMg, Ca, and Sr atoms were codeposited with argon and krypton at 10 K for optical absorption investigation of the van der Waals diatomic molecules. Very strong and weak structured absorptions were observed for Mg_{2} at 367 and 444 nm, respectively. An unstructured 507 nm and a strong structured 648 nm absorption were found for Ca_{2}, and a strong structured 710 nm absorption was observed for Sr_{2}. Intermediate absorptions with intermediate spacings were observed for CaMg, SrMg, and SrCa in mixed‐metal experiments. The strong absorptions are assigned to ^{1}Σ^{+} _{ u }(^{1} S+^{1} P) ←^{1}Σ^{+} _{ g }(^{1} S+^{1} S) transitions.

Rate constants for the reaction of O(^{3} P) atoms with CH_{2}–CHF, CH_{2}–CHCl, and CH_{2}–CHBr over the temperature range 298–442 °K
View Description Hide DescriptionAbsolute rate constants for the reaction of O(^{3} P) atoms with CH_{2}=CHF, CH_{2}=CHCl and CH_{2}=CHBr have been determined over the temperature range 298–442 °K using a flash photolysis–NO_{2}chemiluminescence technique. The Arrhenius expressions obtained were k _{2}(CH_{2}=CHF) =8.03×10^{−12} e ^{−(2025±200)} ^{ R T } cm^{3} molecule^{−1} sec^{−1}, k _{2}(CH_{2}=CHCl) =5.58×10^{−12} e ^{−(1325±200)/R T } cm^{3} molecule^{−1} sec^{−1}, k _{2}(CH_{2}=CHBr) =9.34×10^{−12} e ^{−(1660±200)/R T } cm^{3} molecule^{−1} sec^{−1}, with room temperature rate constants of (2.64±0.27) ×10^{−13} cm^{3} molecule^{−1} sec^{−1}; (5.98±0.60) ×10^{−13} cm^{3} molecule^{−1} sec^{−1} and (5.65±0.57) ×10^{−13} cm^{3} molecule^{−1} sec^{−1} for CH_{2}=CHF, CH_{2}=CHCl, and CH_{2} =CHBr, respectively.

Absolute rate constants for the reaction of O^{3}(P) atoms with allene, 1,3‐butadiene, and vinyl methyl ether over the temperature range 297–439 °K
View Description Hide DescriptionAbsolute rate constants for the reaction of O(^{3} P) atoms with allene, 1,3‐butadiene, and vinyl methyl ether have been determined over the temperature range 297–439 °K using a flash photolysis–NO_{2}chemiluminescence technique. The Arrhenius expressions obtained were k _{2}(allene) =2.04×10^{−11} e ^{−(1755±200)/R T } cm^{3} molecule^{−1} sec^{−1}, k _{2}(1,3‐butadiene) =2.26×10^{−11} e ^{−(105±200)/R T } cm^{3} molecule^{−1} sec^{−1}, k _{2}(vinyl methyl ether) =6.32×10^{−12} e ^{(75±200)/R T } cm^{3} molecule^{−1} sec^{−1}, with room temperature rate constants of (1.07±0.11) ×10^{−12} cm^{3} molecule^{−1} sec^{−1}, (1.94±0.19) ×10^{−11} cm^{3} molecule^{−1} sec^{−1} and (7.14±0.71) ×10^{−12} cm^{3} molecule^{−1} sec^{−1} for allene, 1,3‐butadiene, and vinyl methyl ether, respectively.

Sequential analysis by cooperative reactions on copolymers. II. Binary copolymers with one protecting monomer type
View Description Hide DescriptionWe develop an exact theory for the kinetics of reactions on linear binary copolymers. One of the two types of monomer units is assumed to exhibit a cooperative effect in the sense that after such a unit has reacted, the reactivities of the adjacent two monomer units are changed. Hence the reaction pattern depends upon the sequential structure of the copolymer. Both periodic and aperiodic copolymers are considered: in particular, random, Markovian, and second‐order Markovian sequential structures.

Sequential analysis by cooperative reactions on copolymers. III. Second‐order Markovian chains with arbitrary protection
View Description Hide DescriptionWe develop an exact theory for the kinetics of reactions on long linear binary copolymers. Both monomer types are assumed to exert specific cooperative effects in the sense that after a unit has reacted, the reactivities of the adjacent two monomer units are changed. Hence the reaction pattern depends upon the sequential structure of the copolymer, here assumed to be a second‐order Markov chain. Special results, previously obtained by different methods, including reaction kinetics on h o m o p o l y m e r s, are recovered. The limits of the extents of reaction for long times, t→∞,when one monomer type protects completely, are very important for sequential analysis of copolymers. We study them in detail for random, Markovian, and second‐order Markovian structures as functions of the partial protection by the other monomer type, of the initial rate constants, and of the structure parameters. Interesting effects appear.

The spin‐optimized SCF general spin orbitals. Theoretical formulation
View Description Hide DescriptionA new orbital theory is proposed, in which general spin orbitals (GSO) are introduced in the spin‐optimized (SO) SCF scheme. In this SO–SCF–GSO theory, the effective Hamiltonian for each orbital takes the form of a 2×2 matrix composed of the eigenfunctions for two‐component spinors. It is found that the GSO’s thus defined should still satisfy a general form of Koopmans’ theorem. The SO–SCF GSO’s are to be obtained by solving two sets of coupled SCF equations for the spin coupling coefficients and the linear combination coefficients for basis functions. Using an STO‐6G basis set of the double ζ quality, sample calculations have been carried out for the doublet state of the linear H_{3} system for which the bond lengths are fixed at 1.470 and 2.984 bohr. The total energy obtained is ∼3 kcal/mole lower than the values which have resulted from the SO–SCF–DODS and the spin‐extended Hartree–Fock (SEHF) GSO calculations with the same basis set. The resulting orbitals are found to be more delocalized over the entire system than those obtained by the SO–SCF–DODS theory.

Optically pumped Hg_{2} studies
View Description Hide DescriptionThe continuous emission bands of molecular mercury at 0.335 and 0.485 μm are excited in a pure mercury vapor by direct optical pumping of the weakly bound ground state molecules with a 5 nsec pulse of intense 0.266 μm radiation in order to study the radiative and collisional properties of the excited molecules at densities between 10^{14} and 10^{15} cm^{−3}. Fluorescence time history of both bands are empirically analyzed in terms of dynamics governing the excited molecules. The analysis includes a density‐dependent nonexponential decay caused by bimolecular excimer quenching with a measured rate constant of k?2±1×10^{−10} cm^{3} sec^{−1} molecule^{−1}. Because of interest in the mercury system as a potential laser medium, estimates of gain coefficients are derived from fluorescent power measurements. However, transmission measurements of the excited medium at 0.325 and 0.4416 μm show significant net loss due to excited state absorption with an estimated absorption cross section on the order of 10^{−17} cm^{2} at 0.325 μm. Energy storage limitations implied by the bimolecular excimer quenching are also examined.