Contrasting singlet-triplet dynamical behavior of two vibrational levels of the acetylene S1 2(1)3(1)B2 polyad

Surface electron ejection by laser-excited metastables (SEELEM) and LIF spectra of acetylene were simultaneously recorded in the regions of the A1Au-X1Sigmag+ nominal 2(1)3(1)4(2) Ka=1<--00 and 2(1)3(1)6(2) Ka=1<--00 bands near 46,140 cm(-1). The upper states of these two bands are separated by only approximately 100 cm(-1), and the two S1 vibrational levels are known to be strongly mixed by anharmonic and Coriolis interactions. Strikingly different patterns were observed in the SEELEM spectra in the regions of the 2(1)3(1)4(2) and 2(1)3(1)6(2) vibrational levels. Because the equilibrium structure of the T3 electronic state is known to be nonplanar, excitation of nu4 (torsion) and nu6 (antisymmetric in-plane bend) are expected respectively to promote and suppress vibrational overlap between low-lying S1 and T3 vibrational levels. The nearly 50:50 mixed 2(1)3(1)4(2)-2(1)3(1)6(2) character of the S1 vibrational levels rules out this simple Franck-Condon explanation for the different appearance of the SEELEM spectra. A simple model is applied to the SEELEM/LIF spectra to explain the differences between spectral patterns in terms of a T3 doorway-mediated singlet-triplet coupling model.     

Permanent electric dipole moment of molybdenum carbide

High resolution optical spectroscopy has been used to study a molecular beam of molybdenum monocarbide (MoC). The Stark effect of the R(e)(0) and Q(fe)(1) branch features of the [18.6] (3)Pi(1)-X (3)Sigma(-)(0,0) band system of (98)MoC were analyzed to determine the permanent electric dipole moments mu(e) of 2.68(2) and 6.07(18) D for the [18.6] (3)Pi(1)(nu=0) and X (3)Sigma(-)(nu=0) states, respectively. The dipole moments are compared with the experimental value for ruthenium monocarbide [T. C. Steimle et al., J. Chem. Phys. 118, 2620 (2003)] and with theoretical predictions. A molecular orbital correlation diagram is used to interpret the observed and predicted trends of ground state mu(e) values for the 4d-metal monocarbides series.     

Spectral signatures of inter-system crossing mediated by energetically distant doorway levels: examples from the acetylene S1 state

We review recent research on the acetylene S(1) state that illustrates how mechanistic rather than phenomenological information about intersystem crossing (ISC) may be obtained directly from frequency-domain spectra. The focus is on the dynamically rich "doorway-mediated" ISC domain that lies between isolated spectroscopic spin-orbit perturbations and statistical-limit interactions between one singlet "bright state" and a quasi-continuum of triplet "dark states". New and improved experimental and data processing techniques permit the statistical-model curtain to be drawn back to reveal mechanistically explicit pathways via one or more identifiable, hence, manipulatable, doorway states, between a user-selected bright state and the undifferentiated bath of dark states.     

Relationship between density,viscosity and structure of GeO2 melts at low and high pressures

A correlative study of the viscosity, density and structure of GeO₂ melts has been carried out at low and high pressures. It is observed that under isothermal (1425°C) conditions the viscosity decreases from 6.0 × 10³ poise at 1 bar to 1.2 × 10³ poise at 9.5 kbar. The maximum variation in the density of quenched GeO₂ glasses is from 3.62 ± 0.02 g cm^?3 for glass formed from a liquid quenched at 1 atm and 1425°C to 3.95 ± 0.04 g cm^?3 for glass formed from a liquid quenched at 18 kbar and 1700°C.The similarity of the Raman spectra of GeO₂ melt quenched at 1 atm (1425°C) and at 18 kbar (1700°C) implies that the scattering units in GeO₂ glasses quenched at low and high pressures are the same. The intensity of the Rayleigh tail, however, decreases in glasses quenched at successively higher pressures, the structure apparently becoming more ordered with increasing pressure. The Raman spectra of the GeO₂ polymorphs with quartz and rutile structures, crystallized at 1150°C and at 4 and 6 kbar, respectively, are distinct because of the difference in Ge coordination, resulting in a large difference in the frequency and intensity of the GeO symmetric stretching mode in the two polymorphs. A comparison of the Raman spectra of GeO₂ glasses with those of crystalline GeO₂ polymorphs shows that the local ordering in GeO₂ glass is similar to that of hexagonal GeO₂ in which Ge is tetrahedrally coordinated.The decrease in the viscosity of GeO₂ melt with increasing pressure cannot be attributed to a pressure-induced coordination change [1]. More likely, there is a systematic change in the nature of the GeO bond with increasing pressure. The increase in the degree of local ordering in GeO₂ melts quenched at high pressures does explain the progressive increase in melt density.     

The permanent electric dipole moments of WN and ReN and nuclear quadrupole interaction in ReN

The high-resolution laser induced fluorescence spectra of tungsten mononitride WN and rhenium mononitride ReN have been recorded in a laser ablation/molecular beam spectrometer. The field free spectrum of the (0,0)A (4)Pi(3/2)-X (4)Sigma(1/2) (-) band system of (186)WN has been analyzed to produce B("), B('), and gamma(") values of 0.4659(2), 0.4554(2), and 0.0518(1) cm(-1), respectively. The permanent electric dipole moments mu for the X (4)Sigma(1/2) (-) and A (4)Pi(3/2) state were determined to be 3.77(18) and 2.45(3) D, respectively, from the analysis of the optical Stark effect. The (0,0)[26.0]0(+)-X0(+) band system of ReN was recorded in the presence of a variable static electric field. The ground and excited state electric dipole moments of (187)ReN were determined to be mu(X0(+))=1.96(8) D and mu([26.0]0(+))=3.53(4) D. Splittings in the field free (187)ReN spectrum were analyzed to produce (187)Re (I=5/2) nuclear electric quadrupole coupling constants e(2)Qq(0) of -0.0304(8) and 0.0328(9) cm(-1) for the X0(+) and [26.0]0(+) states, respectively. A molecular orbital correlation model is used to interpret the observation and a comparison is made to CrN and MoN.     

Higher order corrections in perturbative quantum chromodynamics

We present some techniques which have been developed recently or in the recent past to compute Feynman graphs beyond one-loop order. These techniques are useful to compute the three-loop splitting functions in QCD and to obtain the complete second order QED corrections to Bhabha scattering.