998 resultados para 1995_12130645 Optics-14
Resumo:
Polar photodissociation of a set of bromo-chloro-alkanes in the vicinity of the Br 3d core edge has been observed for the first time. It is shown that negative photoion spectroscopy is a powerful tool for investigating the various decay mechanisms of core-excited molecules. Analysis of these results indicates that the observed polar photodissociation arises from two competing spectator Auger decay processes in which the molecule can dissociate either before or after the core hole relaxation.
Resumo:
A non-adiabatic quantum molecular dynamics approach for treating the interaction of matter with intense, short-duration laser pulses is developed. This approach, which is parallelized to run on massively-parallel supercomputers, is shown to be both accurate and efficient. Illustrative results are presented for harmonic generation occurring in diatomic molecules using linearly polarized laser pulses.
Resumo:
We set out aspects of a numerical algorithm used in solving the full-dimensionality time-dependent Schrodinger equation describing the electronic motion of the hydrogen molecular ion driven by an intense, linearly polarized laser pulse aligned along the molecular axis. This algorithm has been implemented within the fixed inter-nuclear separation approximation in a parallel computer code, a brief summary of which is given. Ionization rates are calculated and compared with results from other methods, notably the time-independent Floquet method. Our results compare very favourably with the precise predictions of the Floquet method, although there is some disagreement with other wavepacket calculations. Visualizations of the electron dynamics are also presented in which electron rescattering is observed.
Resumo:
KLL dielectronic recombination resonances, where a free electron is captured into the L shell and at the same time a K shell electron is excited into the L shell, have been measured for open shell iodine ions by measuring the detected yield of escaping ions of various charge states and modeling the charge balance in an electron beam ion trap. In the modeling, the escape from the trap and multiple charge exchange were considered. Extracted ions were used to measure the charge balance in the trap. The different charge states were clearly separated, which along with the correction for artifacts connected with ion escape and multiple charge exchange made the open shell highly charged ion measurements of this type possible for the first time. From the measured spectra resonant strengths were obtained. The results were 4.27(39)x10(-19) cm(2) eV, 2.91(26)x10(-19) cm(2) eV, 2.39(22)x10(-19) cm(2) eV, 1.49(14)x10(-19) cm(2) eV and 7.64(76)x10(-20) cm(2) eV for the iodine ions from He-like to C-like, respectively.
Resumo:
14C wiggle-match dating (WMD) of peat deposits uses the non-linear relationship between 14C age and calendar age to match the shape of a sequence of closely spaced peat 14C dates with the 14C calibration curve. A numerical approach to WMD enables the quantitative assessment of various possible wiggle-match solutions and of calendar year confidence intervals for sequences of 14C dates. We assess the assumptions, advantages, and limitations of the method. Several case-studies show that WMD results in more precise chronologies than when individual 14C dates are calibrated. WMD is most successful during periods with major excursions in the 14C calibration curve (e.g., in one case WMD could narrow down confidence intervals from 230 to 36 yr).
Resumo:
We present a numerical and theoretical study of intense-field single-electron ionization of helium at 390 nm and 780 nm. Accurate ionization rates (over an intensity range of (0.175-34) X10^14 W/ cm^2 at 390 nm, and (0.275 - 14.4) X 10^14 W /cm^2 at 780 nm) are obtained from full-dimensionality integrations of the time-dependent helium-laser Schroedinger equation. We show that the power law of lowest order perturbation theory, modified with a ponderomotive-shifted ionization potential, is capable of modelling the ionization rates over an intensity range that extends up to two orders of magnitude higher than that applicable to perturbation theory alone. Writing the modified perturbation theory in terms of scaled wavelength and intensity variables, we obtain to first approximation a single ionization law for both the 390 nm and 780 nm cases. To model the data in the high intensity limit as well as in the low, a new function is introduced for the rate. This function has, in part, a resemblance to that derived from tunnelling theory but, importantly, retains the correct frequency-dependence and scaling behaviour derived from the perturbative-like models at lower intensities. Comparison with the predictions of classical ADK tunnelling theory confirms that ADK performs poorly in the frequency and intensity domain treated here.
Resumo:
We present calculations of the time delay between single and double ionization of helium, obtained from full-dimensionality numerical integrations of the helium-laser Schroedinger equation. The notion of a quantum mechanical time delay is defined in terms of the interval between correlated bursts of single and double ionization. Calculations are performed at 390 and 780 nm in laser intensities that range from 2 X 10^14 to 14 X 10^14 W /cm^2. We find results consistent with the rescattering model of double ionization but supporting its classical interpretation only at 780 nm.
Resumo:
The accuracy and reliability of popular density functional approximations for the compounds giving origin to room temperature ionic liquids have been assessed by computing the T=0 K crystal structure of several 1-alkyl-3-methyl-imidazolium salts. Two prototypical exchange-correlation approximations have been considered, i.e., the local density approximation (LDA) and one gradient corrected scheme [PBE-GGA, Phys. Rev. Lett. 77, 3865 (1996)]. Comparison with low-temperature x-ray diffraction data shows that the equilibrium volume predicted by either approximations is affected by large errors, nearly equal in magnitude (~10%), and of opposite sign. In both cases the error can be traced to a poor description of the intermolecular interactions, while the intramolecular structure is fairly well reproduced by LDA and PBE-GGA. The PBE-GGA optimization of atomic positions within the experimental unit cell provides results in good agreement with the x-ray structure. The correct system volume can also be restored by supplementing PBE-GGA with empirical dispersion terms reproducing the r-6 attractive tail of the van der Waals interactions.