In scattering polarized electrons (P1 = 44%) by 9Be at an energy of 300 MeV at angles 115 ° ~< 0 ~< 145 ° a parity violating asymmetry of Xcorr = ( 3.5 _+ 0.7 _+ 0.2) × 10-6 was measured. After correction for finite electron polarization and background we deduce an experimental asymmetry of Acx = ( -9.4 + 1.8 _+ 0.5) × 10 6. The quoted errors indicate the statistical and the systematic uncertainties, respectively. The asymmetry, which is dominated by the quasielastic cross section, is interpreted in terms of model-independent electron-nucleon coupling constants of the weak neutral current. The error limits in the sector of axial vector coupling constants have been improved by a factor of 3 over previous results. A model-dependent analysis for the Weinberg angle yields the result sin 2 0 w = 0.221 + 0.014 + 0.004.
Femtochemistry / Wavepacket Dynamics / Proton TransferUltrafast pump-probe experiments with a time-resolution of 30 fs have been carried out to explore the non-radiative relaxation dynamics of electronically excited 1,8-dihydroxyanthraquinone (DHAQ) in polar liquid solution. The results are discussed in terms of a Lippincott-Schroeder double-minimum potential along the proton-transfer reaction coordinate for the ground (S0) and first excited singlet states (S1) of DHAQ. The 400-nm pump-visible probe data reveal a strongly Stokes-shifted stimulated emission due to the proton-transferred 1,10-quinone configuration in the S1-state. A dominant fraction of this stimulated emission appears instantaneously implying that cross-well excitation directly from the ground-state 9,10-quinone form into the excited-state 1,10-quinone configuration takes place. A smaller fraction of the stimulated emission appears delayed with a time constant of approximately 300 fs. This component may be due to proton-transfer in the S 1 -state following optical excitation. Further, a transient absorption is observed on the red edge of the linear absorption spectrum of ground-state DHAQ due to higher-lying electronic states presumably from the 1,10-configuration. Periodic modulations of the transient absorption due to wavepacket motion in the 9,10-quinone form are partially consistent with previous fluorescence excitation spectra of the molecule taken under jetcooled conditions.
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