Volume 82, Issue 10, 15 May 1985
Index of content:

Fluorescence spectra of matrix‐isolated BiS, BiSe, and BiTe
View Description Hide DescriptionLaser‐induced fluorescence spectra are reported for BiS, BiSe, and BiTe for the first time in argon matrices. A total of nine band systems have been observed, out of which eight are believed to be new low energy electronic states of these molecules. Two of these belong to BiSe, and three each to BiS and BiTe.

High resolution spectrum of Tl–Hg excimer
View Description Hide DescriptionEmission, absorption, and fluorescence spectra have been obtained from a mixture of thallium and mercury vapors. In absorption, a number of narrow band systems near the sharp and diffuse series lines of Tl, extending primarily toward longer wavelengths, were observed. In emission, bands with intensity maxima at 3808, 4296, 4585, 5225, 5385, and 6558 Å have been observed with excitation by a Tesla coil. Most of these systems were also observed in sensitized fluorescence using a cooled mercury arc, or a zinc or an aluminiumspark as a source. 7 ^{2} S of Tl appears to be the parent atomic state for all the molecular emissions in the visible. The bands at 5225 and 4296 are likely due to HgTl_{2}. The bands are 4585 and 6558 are due to TlHg. Emission and absorption spectra have been recorded with a dispersion of 1.31 Å/mm in the red and 0.873 Å/mm in the ultraviolet. Vibrational structures have been resolved for all the band systems, except for the weak 4296 band, and the molecular constants and potentials derived.

Analytic representation of the dipole oscillator‐strength distribution. II. The normalization factor for electron continuum states in atomic fields
View Description Hide DescriptionWe address ourselves to the question what analytic formula is the most suitable for fitting the dipole oscillator‐strength distribution d f/dε over a wide range of the kinetic energy ε of an electron ejected from an atom or molecule. A suitable expression will enable one to interpolate or extrapolate data reliably and to use them readily in applications such as the modeling studies in radiation physics. It is useful to distinguish two factors that together constitute d f/dε. The first factor is defined in terms of the dipole matrix element with respect to a final‐state eigenfunction whose amplitude near the origin is i n d e p e n d e n t o f ε. As we showed earlier, this factor is analytic at all finite ε except at a singularity at ε=−I, where I is the ionization threshold energy. The other factor arises from the e n e r g y‐s c a l e n o r m a l i z a t i o n of the final‐state eigenfunction, and is the object of the present discussion from several angles. First, we present a survey of numerical data for the s,p, and d states with 0≤ε≤5 a.u. for all atoms with Z≤38, evaluated within the Hartree–Slater potential model. Next, we discuss analytic properties of the normalization factor, which include its relation to the phase shift, and its behavior near a resonance. We also elucidate the connection of the continuum normalization with the bound‐state normalization. Finally, we illustrate the significance of our findings in the practical fitting of the d f/dε data in the presence of a resonance, taking the valence‐shell ionization of Ar as an example.

Multiphoton ionization spectroscopy of ClO and BrO
View Description Hide DescriptionWe report the resonance enhanced multiphoton ionization spectra of ClO and BrO free radicals between 415 and 475 nm. The observed electronic states were prepared by simultaneous absorption of three identical photons from a dye laser.Absorption of at least one additional photon induced ionization. ClO showed spectra originating from the D, E, and F states previously reported by N. Basco and R. D. Morse. The origin of the F ^{2}Σ←X ^{2}Π_{ i } bands were reassigned to ν_{0} _{0}=70 183 cm^{−} ^{1} and ν_{0} _{0}=69 868 cm^{−} ^{1}. BrO showed three new vibrational progressions originating from transitions between the X ^{2}Π_{3} _{/} _{2} state to Rydberg states with assignments of E ^{2}Σ (ν_{0} _{0}=65 003 cm^{−} ^{1}), F ^{2}Σ (ν_{0} _{0}=67 470 cm^{−} ^{1}), and an apparently inverted state, designated G (ν_{0} _{0}=70 504 cm^{−} ^{1}), that possesses orbital angular momentum greater than zero. The G‐state bands were separated by 139(±3) cm^{−} ^{1} which should approximate the magnitude of the spin‐orbit coupling constant of this excited state if it is of ^{2}Π_{ i } symmetry.

The rotational spectrum and properties of N_{2} ⋅⋅⋅ HCN
View Description Hide DescriptionThe ground state rotational spectra of eight isotopic species of a weakly bound dimer N(1)N(2) ⋅⋅⋅ HCN(3) formed between molecular nitrogen and hydrogen cyanide have been detected and measured by the technique of pulsed‐nozzle, Fourier‐transform microwave spectroscopy.Rotational constantsB _{0}, centrifugal distortion constants D _{ J }, and, where appropriate, nuclear quadrupole coupling constants χ_{ n }(^{1} ^{4}N) and χ(D) have been determined. For the five isotopic species containing ^{1} ^{4}N nuclei the results are: An analysis of the B _{0} values shows that the equilibrium geometry is linear, or nearly so, with the nuclei in the order shown. The D _{ J } values lead to k _{σ}=2.39 N m^{−} ^{1} for the intermolecular stretching force constant while the difference χ_{1}(^{1} ^{4}N)–χ_{2}(^{1} ^{4}N) is interpreted in terms of a transfer of 0.016 e from N(1) to N(2) when the complex is formed in the zero‐point state.

Photolysis of hydrogen and fluorine in solid argon. Matrix infrared spectra of (HF)_{2}, (HF) (DF), and (DF)_{2}
View Description Hide DescriptionArgon diluted samples of H_{2} and F_{2} were codeposited at 10 K, and little reaction was observed. Photolysis produced a strong new 3826 cm^{−} ^{1} band, in agreement with assignment to (HF)_{2} from HF codeposition experiments, weak HF monomer and trimer bands, and a sharp new 3930 cm^{−} ^{1} band for the H=HF intermediate species. Sample annealing markedly increased trimer, relative to dimer and monomer, and destroyed the intermediate species. Similar experiments were done with D_{2} and HD; the latter gave a strong 2808 cm^{−} ^{1} absorption for the more stable (HF) (DF) mixed dimer and a weak 3832 cm^{−} ^{1} band for the less stable (DF) (HF) mixed dimer. The ability of the argon matrix cage to quench substantial reaction exothermicity is demonstrated by the preferential stabilization of (HF)_{2} in these experiments.

Rotational assignment using phase relationships in optical–optical double resonance: The BaI C ^{2}Π–X ^{2}Σ^{+} system
View Description Hide DescriptionWe describe an optical–optical double resonance scheme in which a lower vibration‐rotation level is labeled. One laser is fixed in frequency and probes the population of the labeled level via the resulting laser‐induced fluorescence; a second laser is scanned in frequency through the same vibronic band excited by the probe. A double resonance signal results when the population in the labeled level is either increased or decreased by the action of the second laser. The positions and phase pattern of the double resonancespectrum reveal the J numbering of the labeled level and permit a good approximation to be made for the upper and lower state rotational constants. This information allows the J value of the labeled level to be systematically changed, permitting the spectrum to be unraveled. This technique is proven by applying it to the highly congested C–Xspectrum of the BaI molecule, for which no rotational information was previously available for any of its states.

Experimental and a b i n i t i o determination of the bending potential of HCP
View Description Hide DescriptionThe emission properties of HCP excited to the A, B, and d electronic states have been studied. Lifetimes and quenching rates have been measured. By spectrally resolving the emission spectrum, the energy of 94 vibrational levels of the ground electronic state have been measured to an accuracy of ≈5 cm^{−} ^{1}. These energy levels were fit to experimental accuracy by a rigid bender Hamiltonian thereby determining the bending potential over a range of bending angle from 0 to 100° (0–17 500 cm^{−} ^{1}). An a b i n i t i o bending potential has been computed for HCP and found to be in excellent agreement with the experimentally fitted one over the range that the experimental data span. This potential predicts that HPC has an energy maximum with respect to the bending coordinate. The bending potential decreases monotonically by about 30 000 cm^{−} ^{1} in going from HPC to HCP.

Vacuum ultraviolet laser spectroscopy. II. Spectra of Xe_{2} and excited state constants
View Description Hide DescriptionVibrationally and isotopically resolved fluorescence excitation spectra of Xe_{2} at 1300, 1485, and 1500 Å have been obtained and analyzed. They correspond to electronic transitions from the van der Waals ground state to the three lowest excited states. Molecules of Xe_{2} were formed in the v‘=0 level of the ground state using a pulsed supersonic jet, and fluorescence emission was excited by a tunable, coherent, and monochromatic source of vacuum ultraviolet radiation generated by four‐wave‐sum‐mixing in Mg and Zn vapors. For each of the three band systems, unambiguous assignment of upper‐state vibrational quantum numbering was possible, resulting in the determination of accurate upper‐state spectroscopic constants for the first time. From system I (at ∼1500 Å) with bands v’=36 to 43, constants for the A1_{ u } state of ^{1} ^{2} ^{9} ^{,} ^{1} ^{3} ^{2}Xe_{2} were found to be T ^{’} _{ e } =63 089.9(6.9), ω_{ e } =137.48(34), ω_{ e } x ^{’} _{ e } =1.1668(43), and D _{ e } =4174.4(7.5) cm^{−} ^{1}. Band system II (at ∼1485 Å) with bands v’=35 to 46, gave constants for the B0^{+} _{ u } state: T ^{’} _{ e } =63 795.6(4.2), ω_{ e } =124.85(20), ω_{ e } x ^{’} _{ e } =0.9372(24), and D _{ e } =4446.3(5.2) cm^{−} ^{1}. Finally, system III (at ∼1300 Å) and bands v’=16 to 27 yielded the following constants for the C0^{+} _{ u } state: T ^{’} _{ e } =75 881.7(1.5), ω_{ e } =49.71(10), ω_{ e } x ^{’} _{ e } =0.4222(41), and D _{ e } =1500.0 (3.4) cm^{−} ^{1}. Potential curves based on these constants were calculated for all three excited states.

The observation of stimulated emission in the 119 to 149 nm range from HD excited by picosecond 193 nm radiation
View Description Hide DescriptionIntense stimulated emission in the 119 to 149 nm region is observed from HD excited by two quanta at 193 nm. For certain transitions, the conversion efficiency approaches 1% so that peak powers of ∼10 MW are produced. Electron collisions with the HD (E,F) level are found to efficiently transfer population to the HD(C) state which produces stimulated emission on several C → X Werner band lines in the vicinity of 120 nm. The data on the pressure dependence of the stimulated signals strongly support the conclusion that, in comparison to H_{2}, HD exhibits a large isotopic dependence on the cross section for rotationally inelastic electron collisions in electronically excited states. The rate constant for this rotationally inelastic process is estimated as ∼2×10^{−} ^{7} cm^{3}/s. This is the first observation of an isotopically sensitive electron collisional process involving an electronically excited level.

Electric dipole moments of HF–C_{2}H_{2}, HF–C_{2}H_{4}, and HF–C_{3}H_{6}
View Description Hide DescriptionElectric dipole moments of HF–cyclopropane, HF–ethylene, and HF–acetylene have been determined using the molecular beam electric resonance technique. The dipole moments of HF–C_{3}H_{6}, HF–C_{2}H_{4}, and HF–C_{2}H_{2} are 2.5084(28), 2.3839(45), and 2.3681(28) D, respectively. The induced dipole moments of these systems are discussed, along with that of HF–benzene which has been previously determined. The induced dipole moments do not scale simply with hydrocarbon polarizabilities. Moreover, they do not correlate well with the frequency shifts of the HF submolecule stretching vibration measured in matrix isolation studies.

Radiative decay of multiply excited core hole states in H_{2}O
View Description Hide DescriptionA high resolution x‐ray emission spectrum of water vapor has been recorded, using a 7 keV electron beam and a 10 m grazing incidence spectrometer. The spectrum is interpreted in terms of dipole transition moments between CI wave functions built from separately optimized molecular orbitals. The principal features of the spectrum are reproduced considering transitions in the singly and doubly ionized species, giving rise to main lines and Wentzel–Druyvesteyn satellites, respectively.

Application of the superposition model to calculate the zero‐field splittings in molecules
View Description Hide Descriptionb ^{0} _{2} =D zero‐field splitting parameters are calculated for the GdF_{3} and MnX_{2} (X=F, Cl, Br) molecules using the superposition model with intrinsic parameters obtained from crystal measurements and interatomic distances from gas‐phase electron diffraction, where available.

Photoabsorption cross section of acetylene in the EUV region
View Description Hide DescriptionUsing synchrotron radiation as a continuum background, the absolute total absorption cross section of C_{2}H_{2} has been measured using a double ionization chamber. The cross sections range from 2.5 to a maximum of 35 Mb in the 175–740 Å region. Two new Rydberg series have been identified and apparently converge to the B̃ ^{2}Σ^{+} _{ u } state of C_{2}H^{+} _{2} at 18.71 eV. The observed Rydberg states are tentatively assigned to (2σ_{ u })^{−} ^{1} n sσ_{ g } ^{1}Σ^{+} _{ u } and (2σ_{ u })^{−} ^{1} n dσ_{ g } ^{1}Σ^{+} _{ u }, respectively. The present cross section data have been used in the analysis of various sum rules including the TRK sum rule.

A study of the quantal time delay matrix in collinear reactive scattering
View Description Hide DescriptionThe Eisenbud–Wigner time delay matrix is used to study the dynamics of reaction close to vibrationally adiabatic barrier energies. Maxima in the time delay are predicted and are found to be in excellent agreement with vibrationally adiabatic barrier energies determined by quantized pods. The actual time spent in the vicinity of the barriers is estimated by separating out the free particle time. This ‘‘real time’’ is then used to analyze the validity of the adiabatic and sudden approaches to reactive scattering in the 3D H+H_{2} and D+H_{2}reactions.

Numerical comparison of generalized surface hopping, classical analog, and self‐consistent eikonal approximations for nonadiabatic scattering
View Description Hide DescriptionThree recently proposed semiclassical methods for nonadiabaticscattering are numerically compared for a one‐dimensional curve crossing model. The generalized surface hopping approach has been found to be in excellent agreement with quantum results over a wide range of energies. The simpler variant of the self‐consistent eikonal method follows the quantum results well at low energies. At higher energies it still follows the correct overall trend of decreasing transition probability, but it does not quantitatively reproduce the oscillations in the transition probability. The classical analog technique provides relatively good agreement with the quantum results over the entire range of energies examined. It is found in the application of this last method that there are typically four to eight stationary phase contributions at each energy and it is crucial to correctly describe the interference between these terms. It is also necessary to include uniform and analytically continued contributions at many energies, complicating the application of the method.

Effect of reactant ion internal and translational energy on the rate constants of the charge exchange reactions: CO_{2} ^{+}+O_{2}→O_{2} ^{+} +CO_{2} and O_{2} ^{+}+O_{2}→O_{2}+O_{2} ^{+}
View Description Hide DescriptionThe rate constants of the charge exchangereactions, CO_{2} ^{+}+ O_{2}→O_{2} ^{+}+CO_{2} and O_{2} ^{+}+O_{2}→O_{2}+O_{2} ^{+} have been studied as a function of both the internal and kinetic energies of the reactant ion in a tandem ICR spectrometer. Primary ions with known internal energies are formed in the source of the tandem by charge transferreactions whose energy partitioning has already been determined. The rate constant of the CO_{2} ^{+}+O_{2}reaction is found to be 4.4×10^{−} ^{1} ^{1} cm^{3} s^{−} ^{1} for ground state primary ions and increases by a factor of 2.9 when CO_{2} ^{+} ions have 1.4 eV of internal energy; this reaction is also found to be much less sensitive to the kinetic energy of the parent ion than to its internal energy. The rate constant for the symmetric charge exchangereaction O_{2} ^{+}+O_{2} has been determined for internal energies corresponding approximately to the v=0, 3, and 9 vibrational levels of the O_{2} ^{+}ground electronic state and is found to increase with internal energy, at least at low collision energy.

Energy partitioning in atom–radical reactions: The reaction of F atoms with NH_{2}
View Description Hide DescriptionAn extension of the low‐pressure infrared chemiluminescence technique has allowed the measurement of energy partitioning in the atom/radical reactions: F+NH_{2}→HF+NH, F+ND_{2}→DF+ND. A complete numerical model of the experiment is described in detail including its parametrization. This model allows the unambiguous determination of the primary energy distribution of the above reactions. These reactions give inverted product energy distributions, in contrast to the isoelectronic F+OH→HF+O reaction. The inverted primary energy distribution for F+NH_{2}/ND_{2} indicates a direct abstraction mechanism. A b i n i t i o quantum chemical computations on some features of the relevant potential energy surfaces support this direct abstraction route. An energetically accessible transition state, having approximately zero barrier, is found on the triplet surface which directly correlates reagents and products. The geometry of this triplet transition state is also suggestive of strong HF vibrational excitation. Abstraction on the triplet surface provides an alternative pathway to reaction on the lowest singlet surface, which contains a deep potential energy well corresponding to NH_{2}F.

Laser‐induced charge transfer in the CH^{6} ^{+} quasimolecule
View Description Hide DescriptionThe charge transfer cross section is calculated for C^{6} ^{+}+CH(1s) collisions, through photon assisted 5gσ–6hσ, 5gσ–4fσ, 5gσ–4fπ, and 5gσ–4dσ transitions. The theory developed by Copeland and Tang, and ourselves, is employed, and the validity of the approximations used is tested. The four processes considered have widely different characteristics with regards to the laser wavelength needed, the collisiondynamics and the applicability of back‐of‐the‐envelope estimates based on the Landau–Zener approximation. We point out the relevance of those processes to the impurity diagnostics of magnetically confined fusion plasmas and to the development of short wavelength lasers.

Nuclear‐motion corrections to Born–Oppenheimer barrier heights for chemical reactions
View Description Hide DescriptionWe use diatomics‐in‐molecules theory to estimate the nuclear‐motion contributions to atom–diatom potential energy surfaces for reactive systems, with special emphasis on the saddle point region. We examine the reactions of H, O, F, and Cl with H_{2} and H with Cl_{2}. Nuclear‐motion corrections at the saddle point are in the range 0.007–0.07 kcal/mol for cases with one or two hydrogens and classical barrier heights in the range 0.7–3 kcal/mol and are about 0.2 kcal/mol for cases with two hydrogens and classical barrier heights in the range 8–12 kcal/mol. For F+H_{2} with parameters such that the predicted nuclear‐motion correction is 0.03 kcal/mol at the saddle point, the correction is as large as 0.2 kcal/mol elsewhere on the surface. Isotopic substitution of D for H changes the classical barrier height by 0.003–0.1 kcal/mol for the cases studied.