Isotope effects on vibronic coupling of pyrazine

The vibronic couplings of pyrazine-d₀ and pyrazine-d₄ between the lowest electronic excited states ¹B_3u(n, π^*) and ¹B_2u(π, π^*) through the out-of-plane CH bending vibration ν_10a(b_1g) have been studied from the Raman, electronic absorption and fluorescence spectra. The isotope effects on the scattering cross section of the ν_10a Raman line, the vibrational potential in the ¹B_3u(n, π^*) state and on the frequency change of the ν_10a vibration between the ground and the lowest electronic excited states are well explained by conventional Herzberg-Teller coupling mechanism. However, the intensities of the vibronic bands in the electronic absorption and fluorescence spectra are hardly explained with this coupling mechanism.     

The calculated spectra of the azanaphthalenes

A modified INDO (intermediate neglect of differential overlap) method is used to calculate the electronic spectra of naphthalene and some mono-, di-, and tetraazanaphthalenes. The technique is capable of reproducing the better classified bands of this series within a rms error of 1000 cm⁻¹.The four lowest π-π^* bands of naphthalene are well represented; a fifth band reported at 52,600 cm⁻¹, and generally assigned ¹B_2u(π-π^*), may be ¹B_3g(π-π^*) borrowing intensity. The lowest excited singlet state calculated for quinoline is n → π^*, estimated nearly degenerate with the lowest π-π^*, while that for isoquinoline is calculated π-π^*; experimental evidence supports the π-π^* assignment in both cases, but the corresponding absorption in quinoline appears complex. Of the diazanaphthalenes examined (with two nitrogens in one benzenoid ring) all are calculated to have one n-π^* transition before the first π-π^* except phthalazine, in which two n-π^* transitions are calculated to be the lowest lying. This is in accord with experimental evidence to date, although the nature of the observation in phthalazine is reinterpreted as one ¹B₂(n-π). 1,4,5,8-tetrazanaphthalene is predicted to have a group theoretically forbidden n-π^* excited state as low as 21,000 cm⁻¹, and should prove interesting experimentally. Even though n-π^* excited states are often calculated to be the lowest lying, none of these compounds are predicted to have an “n” orbital as homo.Further interpretations of the spectra of the azanaphthalenes are made in view of the calculations. Theoretical limitations of the method employed for this study are discussed.     

Analysis of autoionizing Rydberg states in the vacuum ultraviolet absorption spectrum of HBr and DBr

The photographic absorption spectrum of HBr and DBr above the first ionization limit (800–1000 Å) has been analyzed. No analyzable rotational structure was observed, indicating that the autoionization rate is faster than the rotational period. Two Rydberg series converging to the second ionization limit (²Σ⁺) have been identified, one with n^* = 2.75, 3.74, ,14.7, and the other with n^* = 2.35, 3.38. The symmetries cannot be determined conclusively, but the most probable assignment for the first series is 4pσ 4pπ⁴nsσ (¹Σ⁺) N = 6, 7, , 18, and for the second, 4pσ 4pπ·npσ and/or npπ[¹Σ⁺, ¹Π] N = 5, 6. Assuming that the intensity maxima assigned are band origins of vibronic states, a new ionization limit can be determined for HBr⁺(²Σ⁺): T₀ = 123 373 ± 20 cm⁻¹.     

The Mechanism of Magnetically Tuned Singlet–Triplet Avoided Crossings in the ÃA2– A1 40 Band of Thioformaldehyde H2C=S

Magnetically tuned singlet–triplet perturbations in the 4¹Ã¹A₂–2¹3¹ã³A₂ system of thioformaldehyde, found in ortho-rotational states (I = 1, the two hydrogen spins parallel) have been identified as being caused by vibronic spin–orbit coupling. This perturbation mechanism has been confirmed in several avoided crossings observed in this work for para states (I = 0, hydrogen spins antiparallel) which are much stronger. Parametrization of the theory has led to a quantitative understanding of the experimental frequency-field relations, and to an accurate prediction of the rovibrational energies of the triplet state. This in turn permitted the detection of about 100 Doppler-limited 2¹3¹ã³A₂–0^{0 1}A₁ rovibronic transitions which led into fine structure states. The combined data was then used to determine a set of rotational, fine, and hyperfine triplet-state parameters, the term value T₀(2¹3¹ã³A₂) = (16 685.385 ± 0.002) cm⁻¹, and the spin–orbit vibronic singlet–triplet coupling constant, W_ST = (0.0691 ± 0.0016) cm⁻¹. A large number of frequency perturbations observed in the crossings, ranging from 2 to 300 MHz, can be explained with this single parameter.     

Measurement of the mass of the Λb baryon

In a data sample of four million hadronic Z decays collected with the ALEPH detector at LEP, four Λ_b baryon candidates are exclusively reconstructed in the Λ_b → Λ_c⁺π⁻ channel, with the Λ_c⁺ decaying into pK⁻π⁺, , or Λπ⁺π⁺π⁻. The probability of the observed signal to be due to a background fluctuation is estimated to be 4.2 × 10⁻⁴. The mass of the Λ_b is measured to be 5614±21 (stat.) ± 4 (syst.) MeV/c².     

nπ transitions in crystals of tetramethyl-1,3-cyclobutanedione-h12 and -d12

The polarized low-temperature crystal absorption spectra of tetramethyl-1,3-cyclobutanedione-h₁₂ and -d₁₂ have been measured from 25 000–40 000 cm⁻¹, and nπ^* excited states identified as follows: ³A_u with origin (-h₁₂/-d₁₂) at (25 720/25 780)cm⁻¹; ¹A_u at (27 130/27 173)cm⁻¹; two ¹B_1g states with origins near 32 000 cm⁻¹. Excitation is accompanied by distortion of the ring and a slight lengthening of the CO bonds, but the carbonyl groups remain planar. Surprisingly, CH/CD-stretching vibrations in the substituent methyl groups are active in intensity borrowing.     

Search for γγ→ηb in ee collisions at LEP 2

A search for the pseudoscalar meson η_b is performed in two-photon interactions at LEP 2 with an integrated luminosity of 699 pb⁻¹ collected at e⁺e⁻ centre-of-mass energies from 181 GeV to 209 GeV. One candidate event is found in the six-charged-particle final state and none in the four-charged-particle final state, in agreement with the total expected background of about one event. Upper limits of Γ_γγ(η_b)×BR(η_b→4 charged particles)<48 eV, Γ_γγ(η_b)×BR(η_b→6 charged particles)<132 eV are obtained at 95% confidence level, which correspond to upper limits of 9.0% and 25% on these branching ratios.     

Photoproduction of D mesons associated with a leading neutron

The photoproduction of D^*±(2010) mesons associated with a leading neutron has been observed with the ZEUS detector in ep collisions at HERA using an integrated luminosity of 80 pb⁻¹. The neutron carries a large fraction, x_L>0.2, of the incoming proton beam energy and is detected at very small production angles, θ_n<0.8 mrad, an indication of peripheral scattering. The D^* meson is centrally produced with pseudorapidity |η|<1.5, and has a transverse momentum p_T>1.9 GeV, which is large compared to the average transverse momentum of the neutron of 0.22 GeV. The ratio of neutron-tagged to inclusive D^* production is 8.85±0.93(stat.)^+0.48_−0.61(syst.)% in the photon–proton center-of-mass energy range 130<W<280 GeV. The data suggest that the presence of a hard scale enhances the fraction of events with a leading neutron in the final state.     

The π ← n transition of formic acid

New sharp bands of formic acid have been observed in the near ultraviolet at the long wave-length end of the previously observed diffuse band system (2250–2500 Å) by considerably extending the absorption path length. Both the diffuse and sharp bands belong to the same vibrational system which is assigned to the π^* ← n electronic transition in the carbonyl group. Extensive progressions are observed in the carbonyl stretching frequency which is greatly reduced in the excited state (fundamental ν₃′ ≈ 1080 cm⁻¹) and many intervals of about 400 cm⁻¹ are assigned to the OCO bending frequency ν₇′.A band contour analysis of the 2593 Å band shows that the molecule is nonplanar in the excited state because of the magnitude and sign of the inertial defect. From this analysis, the rotational constants for the excited state are S^‘=1.8755, B^‘0.4042, C^‘=0.3378cm⁻¹ By the plausible assumption that the important changes in the molecule are in the C=0 bond length, the OCO angle, and the nonplanarity due to the formyl hydrogen, the following excited state parameters are derived.rC=0 = 1.407A.The changes in formic acid are closely analogous to the changes in formyl fluoride as a result of the π^* ← n transition.     

Influence of octahedral tilting and composition on electrical properties of the Li0.2 − xNaxLa0.6TiO3 (0 ≤ x ≤ 0.2) series

The Rietveld analysis of ND patterns of polycrystalline Li_0.2 − xNa_xLa_0.6TiO₃ (0 ≤ x < 0.2) samples, recorded between 300 and 1075 K, shows an orthorhombic–tetragonal transformation, in which the octahedral tilting along the b axis is eliminated at ~ 773 K, but the vacancy ordering along the c axis remains. In Li rich samples, conductivity (10^− 3 Ω^− 1 cm^− 1 at 300 K) departs from the Arrhenius behaviour, decreasing activation energies from 0.37 to 0.14 eV when octahedral tilting is eliminated. Successive Maxwell–Wagner blocking processes, detected in the real part of dielectric constant plots, have been ascribed to the Li blocking at interior domains, grain-boundary and electrode–electrolyte interfaces. The substitution of Li⁺ by Na⁺ decreases the amount of vacant A-sites, decreasing several orders of magnitude the conductivity when the amount of vacancies approaches the vacancy percolation threshold (n_p = 0.27). Below the percolation threshold, Li ions only display local mobility, remaining confined into small domains of perovskites.     

Doppler-limited dye laser excitation spectroscopy of the HSO radical

The cw dye laser excitation spectrum of the vibronic transition of the HSO radical was observed between 16 420 and 16 520 cm⁻¹ with Doppler-limited resolution, 0.03 cm⁻¹. The HSO radical was produced by reaction of discharged oxygen with H₂S or CH₃SH. The observed spectra were assigned to 751 transitions of the K′_a ← K″_a = 2 ← 3, 1 ← 2, 0 ← 1, 1 ← 0, 2 ← 1, and 3 ← 2 subbands, and were analyzed to determine rotational constants, centrifugal distortion constants, and spin-rotation interaction constants with good precision. The signs of the spin-rotation interaction constants were determined for both the upper and the lower state from the observed spectra. The band origin obtained is 16 483.0252 (2.5σ = 0.0013) cm⁻¹. The molecular constants which were determined reproduce the observed transitions with an average deviation of 0.0045 cm⁻¹.     

Correlator of the quark scalar currents and Γtot(H → hadrons) at O(αs) in pQCD

We present the results of the analytical evaluation of the massless four-loop

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(α³_s) correction to the correlator of the quark scalar currents and the Higgs decay rate into hadrons. In numerical form we found (in

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scheme) that Γ (H →

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b ) = (3G_F/4

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π) M_Hm²_b (M_H) [1 + 5.667α_s/π + (35.94 − 1.359n_f) (α_s/π)² + (164.139 − 25.771n_f + 0.259n²_f) (α_s/π)³] where n_f is the number of quark flavour and α_s = α_s(M_H).     

Studies of the temperature dependence of the phosphorescence spectra and decay rates of aromatic carbonyl molecules in mixed organic crystals

The temperature dependence of the phosphorescence spectra, decay rates and the S ₀ → T ₂(³ nπ*) absorption spectra were studied for aromatic carbonyl molecules in mixed organic crystals. The energy separation, ΔE_T , between the ³ ππ* lowest excited triplet (T ₁) states and the higher ³ π ^* states (T ₂) was estimated for several systems from the temperature dependence of the phosphorescence spectra and decay rates. It was found that the decay rates of the aromatic carbonyl molecules in the mixed crystal systems studied are determined by (1) thermal population to the ³ nπ* states and (2) increased radiationless transition rates at higher temperatures.     

The reaction of CH3 + O2: experimental determination of the rate coefficients for the product channels at high temperatures

The reaction of methyl radicals (CH₃) with molecular oxygen (O₂) has been investigated in high-temperature shock tube experiments. The overall rate coefficient, k₁ = k_1a + k_1b, and individual rate coefficients for the two high-temperature product channels, (1a) producing CH₃O + O and (1b) producing CH₂O + OH, were determined using ultra-lean mixtures of CH₃I and O₂ in Ar/He. Narrow-linewidth UV laser absorption at 306.7 nm was used to measure OH concentrations, for which the normalized rise time is sensitive to the overall rate coefficient k₁ but relatively insensitive to the branching ratio of the individual channels and to secondary reactions. Atomic resonance absorption spectroscopy measurements of O-atoms were used for a direct measurement of channel (1a). Through the combination of measurements using the two different diagnostics, rate coefficient expressions for both channels were determined. Over the temperature range 1590–2430 K, k_1a = 6.08 × 10⁷T^1.54 exp (−14005/T) cm³ mol⁻¹ s⁻¹ and k_1b = 68.6 T^2.86 exp (−4916/T) cm³ mol⁻¹ s⁻¹. The overall rate coefficient is in close agreement with a recent ab initio calculation and one other shock tube study, while comparison of k_1a and k_1b to these and other experimental studies yields mixed results. In contrast to one recent experimental study, reaction (1b) is found to be the dominant channel over the entire experimental temperature range.     

Determination of the Splitting of the 6a0 and 6b0 Bands of C-Hexadeuterobenzene in a Supersonic Jet

By using resonance-enhanced two-photon ionization, rotationally resolved spectra of the 6¹₀ band of ¹²C₆D₆ and (¹³C¹²C₅D₆ molecules have been obtained for the first time at a rotational temperature of 0.7 K in a pulsed supersonic beam. From the former, the values of B″ = 0.1573 ± 0.0008 cm⁻¹, B′ = 0.1508 ± 0.0008 cm⁻¹, and ξ′ = −0.412 ± 0.050 have been derived for rotational and Coriolis constants in the lower and upper levels of ¹²C₆D₆. Also, the spectra corresponding to ¹²C₆H₆ and ¹³C¹²C₅H₆ have been measured and the values B″ = 0.1892 ± 0.0008 cm⁻¹, B′ = 0.1815 ± 0.0008 cm⁻¹, and ξ′ = −0.586 ± 0.050 have been obtained for ¹²C₆H₆, in agreement with previous results. Rotational constants of ¹³C labeled benzene molecules have been geometrically deduced from the constants obtained. Experimental isotopic shifts of the vibronic origins of the 6a¹₀ and 6b¹₀ bands have been determined. There is agreement with previous ¹³C-benzene-h₆ data. The present results are −0.91 ± 0.05 and 3.09 ± 0.05 cm⁻¹ for ¹³C¹²C₅D₆ and −1.64 ± 0.05 and 2.64 ± 0.05 cm⁻¹ for ¹³C¹²C₅H₆. The splittings of vibrational modes 6b and 6a in the ¹B_2u state are 4.00 ± 0.10 cm⁻¹ for ¹³C¹²C₅D₆ and 4.28 ± 0.10 cm⁻¹ for ¹³C¹²C₅H₆.     

Sulfur dioxide: Rotational constants and asymmetric structure of the state

Eight bands of the 2350 Å system of sulfur dioxide have been rotationally analyzed as A-type transitions of a prolate asymmetric rotor, confirming that the electronic transition is ¹B₂ ← ¹A₁[2b₁(π^*) ← 1a₂(π)]. The electronic energy and rotational constants of the 0-0 band are, in cm⁻¹: These constants correspond to the average structure r₀ = 1.560 Å and θ₀ = 104.3°. However, the vibrational structure can only be satisfactorily accounted for on the hypothesis of a double-minimum potential in the antisymmetrical stretching coordinate Q₃, the energies of the fundamental levels in the three modes of the B₂ state being: (100), 960 cm⁻¹; (010), 377 cm⁻¹; and (001), 220 cm⁻¹ The (001) level is not observed in the spectrum but can be calculated from the distortion constants and inertial defect of the rotational analysis: the level (002) = 561 cm⁻¹, obtained directly from the vibrational structure, establishes that there is strong, positive anharmonicity in the first three levels of this vibration, as required by the assumption of a double-minimum potential function. Preliminary values are reported for the barrier to the symmetrical configuration, V/hc 100 cm⁻¹, and for the difference in bond distances in the equilibrium configuration, Δr0.12 Å. Coon and his co-workers have previously considered the possible asymmetry of this state but the Q₃ inversion barrier obtained by them, 656 cm⁻¹, is much higher than in the present work, and reasons for this are discussed.     

Observation of χb production in the exclusive reaction ′→γγχb→γγ→γγ(eeorμμ−)

We report on the observation of 1 ³P_J (χ_b) production in the reaction ′→γχ_b→γγ→γγ(e⁺e⁻ or μ⁺μ⁻). The data were recorded with the nonmagnetic CUSB detector at the Cornell Electron Storage Ring, CESR. We observe 124 γγ events with either an electron or muon pair in the final state. In the γγ correlation plot about 40% of the events cluster around (120, 430) MeV.     

Relativistic calculations of the spectroscopic properties of low-lying states of ICI

Relativistic quantum calculations which include spin-orbit interactions and correlations were carried out for the low-lying states of ICl. Spectroscopic properties (R₃, ω_e, T_e) were calculated for these states. Based on the energies and wave functions both the absorption and emission spectra of ICl in the region below 45000 cm⁻¹ were interpreted. These calculations confirm the predissociation of the B0⁺ state and the existence of a second minimum (B′0⁺) in the 0⁺(II) state. Properties of the 0⁻(I)(³Π_0⁻) state which is yet to be observed were also predicted. The calculated properties for the 2(I)(³Π₂) state are in very good agreement with the properties obtained by the very recent characterization of the A′ state by optical three-photon resonance. The continuous and diffuse absorption spectra of ICl in the region below 45000 cm⁻¹ were interpreted and assigned to the appropriate electronic transitions.     

The strong coupling constants of excited positive parity heavy mesons in light-cone QCD

We calculate the strong coupling constants g_p^**p^*π, where P^** (D^**, B^**) is the 1⁺ p-wave state, in the framework of the light-cone QCD sum rules, and using these values ofg_p^**p^*π , we compute the hadronic decay widths forD^** → D^* π and B^** → B^* π.     

The lowest triplet state of Zn porphin

The dynamics of populating and depopulating the phosphorescent triplet state of Zn porphin in an n-octane matrix is investigated by microwave induced delayed phosphorescence experiments at 1·2K. Values for the absolute rates of decay of the three spin levels are given and also the relative steady-state populations, radiative decay rates and probabilities for entry through intersystem crossing. Further, it is established that the phosphorescence is inplane polarized. The results are compatible with the decay modes expected for a lowest vibronic state that deviates from D ₄ symmetry by combined action of the Jahn-Teller effect and crystal field anisotropy.