Absorption (logic)

About: Absorption (logic) is a research topic. Over the lifetime, 5733 publications have been published within this topic receiving 236302 citations.

How wrong is collisional Monte Carlo modeling of fast electron transport in high-intensity laser-solid interactions?

Abstract: The interaction of a high-intensity laser with a solid target generates a large current of fast electrons flowing into the target. Due to the large value of the current, the fast electrons generate significant electric and magnetic fields in the target and rapidly heat it to high temperatures. However, these effects were neglected in interpreting x-ray emission experiments, so the details of the fast electron generation that were inferred could be incorrect. This is considered, theoretically, for layered target, $K\ensuremath{\alpha}$ emission experiments, by using a hybrid Monte Carlo code that includes field generation. The code is used to model such experiments with aluminum and plastic targets, using fast electron parameters taken from experimental results for average intensities of around ${10}^{18}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\ensuremath{-}2}.$ These numerical results are then interpreted in the same manner as previous experiments, using only the Monte Carlo part of the code. The field generation leads to lower total emission and to an apparent two-temperature fast electron distribution. The laser absorption into fast electrons inferred by Monte Carlo modeling is consistently lower than the actual value. The mean fast electron energy inferred could be either higher or lower than the actual value, depending on the experimental setup and the cone angle and energy distribution used in the Monte Carlo modeling. The errors caused by neglecting the fields are, in general, greater for plastic than aluminum targets, leading to inconsistencies in results obtained by Monte Carlo modeling.

The carbon monoxide flame bands

Abstract: The carbon monoxide flame bands have been photographed under high resolution from an afterglow source. Bands in the wavelength range 3100 to 3800 $\overset{\circ}{\mathrm A}$ show a pattern which has been reproduced by calculations of the energies of high vibrational levels of the $^1\sum^+\_g$ ground state of CO$\_2$. The structure of this energy level pattern is strongly affected by extensive Fermi resonance in the $^1\sum^+\_g$ state. The spectrum is emitted by excited CO$\_2$ molecules which radiate to the ground state from the lowest vibrational level and from the $v'\_z$ = 1 level of a B$\_2$ state. This excited state lies approximately 46 000 cm$^{-1}$ above the lowest level of the ground state, and has an OCO angle of 122 $\pm$ 2$^\circ$ and a CO bond length of 1.246 $\pm$ 0.008 $\overset{\circ}{\mathrm A}$. Combination of these results with the work of other authors shows that the excited state is a $^1$B$\_2$ state, and that the carbon monoxide flame bands are associated with the weak absorption system of CO$\_2$ at 1475 $\overset{\circ}{\mathrm A}$.

Far-infrared spectroscopy on oriented films of dry and hydrated DNA

Abstract: Far-infrared measurements, between 3 and 450 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, of absorption spectra of highly oriented films of Li-DNA and Na-DNA in the temperature range 5\char21{}300 K are reported. Five low-frequency infrared-active vibrational modes are observed. The lowest infrared-active mode at 45 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for Li-DNA and 41 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for Na-DNA, as observed for the first time, is found to soften upon sample hydration. Studies of the hydration-induced absorption at the low-frequency end of the spectrum also show a pronounced absorption band at about 10 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ that is attributed to relaxation processes. Based on a simple lattice-dynamics model, the description of low-lying vibrational modes of DNA is presented. Eigenvectors of the lowest infrared-active and Raman-active modes are calculated. In addition, the model semiquantitatively describes the influence of hydration on mode frequencies.

Quadrupolar vibrational mode of silver clusters from plasmon-assisted Raman scattering

Abstract: Absorption and low-frequency Raman-scattering experiments have been performed on thin films consisting of small silver clusters embedded in a porous alumina matrix. When the Raman excitation wavelength is close to the maximum (\ensuremath{\approx}420 nm) of the Mie band (dipolar surface plasmon resonance) the Raman spectra exhibit a strong band located around 10 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, the maximum of which depends on the cluster diameter D at the maximum of the cluster-size distribution in the sample according to the approximate law ${\ensuremath{\omega}}_{\mathrm{vib}}\ensuremath{\propto}{D}^{\ensuremath{-}1}.$ The Raman band corresponds to the excitation of the quadrupolar vibration mode of the clusters, via the plasmon-phonon interaction. Moreover, the maximum of the Raman band shifts towards lower frequencies when the excitation light is shifted to the red. This feature, as well as the rather large Mie-band width, is shown to reflect the ellipsoidal shape distribution of part of the embedded clusters.

Wear, Plasticity, and Rehybridization in Tetrahedral Amorphous Carbon

Abstract: Wear in self-mated tetrahedral amorphous carbon (ta-C) films is studied by molecular dynamics and near-edge X-ray absorption fine structure spectroscopy. Both theory and experiment demonstrate the formation of a soft amorphous carbon (a-C) layer with increased sp2 content, which grows faster than an a-C tribolayer found on self-mated diamond sliding under similar conditions. The faster $$\hbox{sp}^{3} \rightarrow\,\hbox{ sp}^{2}$$ transition in ta-C is explained by easy breaking of prestressed bonds in a finite, nanoscale ta-C region, whereas diamond amorphization occurs at an atomically sharp interface. A detailed analysis of the underlying rehybridization mechanism reveals that the $$\hbox{sp}^{3}\, \rightarrow\hbox{ sp}^{2}$$ transition is triggered by plasticity in the adjacent a-C. Rehybridization therefore occurs in a region that has not yet experienced plastic yield. The resulting soft a-C tribolayer is interpreted as a precursor to the experimentally observed wear.