Search

The authors model the absolute electron transfer (ET) rate and the vibrational quantum effects on ET rate previously observed experimentally for the ion pair complex Co(Cp){sub 2}{sup +}{vert_bar}V(CO){sub 6}{sup {minus}}. The absolute rate and vibrational rate effects cannot be predicted by the standard ET methods. In this work the authors analyze new resonance Raman, absorption, and infrared spectra and combine these results with density functional (DFT) quantum calculations of structure, vibrational modes, and solvent effects to predict absolute electron-transfer rates and vibrational quantum effects for ET. Related DFT calculations on Na{sup +}{vert_bar}V(CO){sub 6}{sup {minus}} are used to support a spectroscopic identification of the ion pair geometry. The ET is from the radical pair state reached by charge-transfer absorption of the ion pair Co(Cp){sub 2}{sup +}{vert_bar}V(CO){sub 6}{sup {minus}}. The weak coupling rate model based on the golden rule model of ET predicts absolute ET rates that are 135 times too large. From DFT calculations on Co(CP){sub 2}{vert_bar}V(CO){sub 6}, it was concluded that a small Jahn-Teller geometry change in both radicals can reduce the orbital overlap and electronic coupling in the radical pair state so that the effective coupling matrix element is much smaller than the 417 cm{sup {minus}1} inferred from themore » absorption spectrum. A new study of the electronic coupling versus geometry is required to test this suggestion versus the possibility that the weak coupling model is inappropriate for the molecule. The standard model, which emphasizes totally symmetric vibrations, also cannot explain prior experimental ET rates for quantum populations ({nu} = 0, 1, 2) in the nontotally symmetric CO stretching mode. These rate effects likely involve a fast IVR conversion from totally symmetric vibrations to IR active CO stretching motions followed by ET. The vibrational quantum effect on ET probably is caused by a breakdown in the Condon approximation, where an increase in the quantum number of vibration increases the electronic coupling matrix element. The models suggest a number of new experiments to probe the mechanism of ET in weak coupled molecules.« less

The laser induced dispersed fluorescence spectra (LIDFS) of CH 3 N radical from five different vibronic excited levels were recorded and assigned. The anharmonic constants of the ν″ 3 (CN stretching) mode were determined to be ω 0 = 1049 ± 2 cm −1 , ω 0 ξ 0 = 9 ± 0.5 cm −1 . The previous tentative assignment of 1 0 1 band in the excitation spectrum [J. Chem. Phys. 103 (1995) 4418] was confirmed by LIDFS. The lifetimes of different vibrational levels of the A 3 E state were measured and the radiative lifetimes of ν ′ = 0, ν ′ 3 = 1, ν ′ 2 = 1, ν ′ 3 = 2, ν ′ 3 = 3 and ν ′ 1 = 1 states were extrapolated as 415 ± 8, 341 ± 10, 231 ± 18, 219 ± 9, 170 ± 3 and 166 ± 7 ns, respectively.

An investigation of the photodissociation of methylazide at wavelengths from 292 nm to 325 nm is presented. Emission spectra and lifetime analysis show the existence of the triplet CH3N radical. A simple kinetic model is proposed to explain the observed fluorescence real time profile, which reflects the vibrational relaxation of hot radical at high pressures. The photodissociation dynamics of methylazide seems complex. The predominant channel is likely to produce CH2NH via concerted 1,2‐hydrogen shift and N2 extrusion process. The triplet CH3N comes from a minor spin‐forbidden channel which involves possibly strong interaction between the low‐lying excited singlet and triplet states of methylazide.

The vibrationally resolved laser‐induced fluorescence spectra of the A 3E↔X 3A2 system of CH3N radical were obtained experimentally. The dispersed spectra at different vibrationally excited states confirm that the system studied belongs to the CH3N radical. From the vibrational progressions in the excitation spectrum, following vibrational frequencies of the A 3E state were determined: ν1′=2943 cm−1, ν2′=1239 cm−1, ν3′=759 cm−1, and ν5′=1500 cm−1. In addition, the lifetimes of the A 3E state at four vibrational states (ν3′=0, 3, 4, and 6) were measured, which are, respectively 292.8±1.2, 203.4±1.2, 188.2±0.5, and 157.2±0.5 ns at a total pressure of 1.7 Torr. Our experiments show that the A 3E state is bound at least up to 4800 cm−1 and is relatively free from photochemistry.

The photolysis of methylazide around 313 nm is studied experimentally by laser Raman spectroscopy. The emission spectrum with a resolution of 50 cm−1 is reported. Based on our experiments, the photodissociation of methylazide on its first excited singlet surface is proposed to be a concerted N2 elimination and 1,2-H shift process.

High amplitude coherent electron‐trajectories have been seen in 3D particle‐in‐cell simulations of the colliding pulse injection scheme of laser‐wakefield accelerators in the bubble regime, and explained as a consequence of laser‐driven coherent betatron oscillation in our recent paper [K. Nemeth et al., Phys. Rev. Lett. 100, 095002 (2008)]. In the present paper we provide more details on the shape of the trajectories, their relationship to the phase velocity of the laser and indicate the dependence of the phenomenon on the accuracy of the numerical representation and choice of laser/plasma parameters.

The origin of beam disparity in emittance and betatron oscillation orbits, in and out of the polarization plane of the drive laser of laser-plasma accelerators, is explained in terms of betatron oscillations driven by the laser field. As trapped electrons accelerate, they move forward and interact with the laser pulse. For the bubble regime, a simple model is presented to describe this interaction in terms of a harmonic oscillator with a driving force from the laser and a restoring force from the plasma wake field. The resulting beam oscillations in the polarization plane, with period approximately the wavelength of the driving laser, increase emittance in that plane and cause microbunching of the beam. These effects are observed directly in 3D particle-in-cell simulations.

The APS booster ring uses ramping power supplies to power the sextupole, quadrupole, and dipole magnets as the beam energy ramps up linearly to 7 GeV. Due to the circuit topology used, those supplies are unable to follow the linear ramp to the desired accuracy. The best regulation achieved is 0.5% while 0.1 % is desired. In addition to the unsatisfying regulation, those supplies are sensitive to AC line perturbations and are not able to reject AC line noises higher than a few tens of Hertz. To improve the performance, a linear MOSFET regulation system using paralleled MOSFET devices in series with the power supply is proposed. The system uses a real-time current feedback loop to force the MOSFETs to work in the linear operation mode. By using this linear MOSFET regulator, the voltage drop on MOSFETs, and hence the voltage imposed on magnets, can be regulated very quickly. As a result, the regulation of the magnet current can be improved significantly. Experiments and simulation have been performed to verify the proposed method. Results show that, with the linear regulator, the current regulation can be improved to better than 0.1%. Because of the high bandwidth of the linear regulator, it can reduce the harmonic content in the output current as well as reject the AC line disturbance.

Recent efforts at the APS have focused on reducing beam loss during storage ring (SR) injection to minimize radiation damage to the APS undulators. Reducing beam loss at injection is particularly important during top-up operation where injection occurs once every two minutes. One way to potentially improve injection efficiency is to simply reduce the emittance of the beam from the APS booster, thereby allowing the beam to be brought closer to the injection septum. Recently several low-emittance operating modes for the APS booster have been studied. The emittance is lowered from the standard value by increasing the horizontal tune in stages. The vertical tune is simultaneously decreased to minimize the required defocusing sextupole strength for chromaticity correction. Calculations indicate that up to 29% reduction in emittance is possible-a result that has been achieved in studies. A lattice with emittance 17% lower than the standard lattice has been thoroughly commissioned and has been used for routine APS top-up operation since July 2002. Results of beam measurements and comparison to calculations for various low-emittance lattices are reported in this paper.

The triggering of wave-breaking in a three-dimensional laser plasma wake (bubble) is investigated. The Coulomb potential from a nanowire is used to disturb the wake field to initialize the wave-breaking. The electron acceleration becomes more stable and the laser power needed for self-trapping is lowered. Three-dimensional particle-in-cell simulations were performed. Electrons with a charge of about 100 pC can be accelerated stably to energy about 170 MeV with a laser energy of 460 mJ. The first step towards tailoring the electron beam properties such as the energy, energy spread, and charge is discussed. (C) 2007 American Institute of Physics.