Absorption (logic)

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

E(B-V), N(H I) and N(H_2)

Abstract: We consider the structure of the N(H I) - E(B-V) relationship when H I is measured in the 21 cm radio line and \EBV\ is defined by far-IR dust-derived measures. We derive reddening-dependent corrections to N(H I) based on interferometric absorption measurements over the past 30 years that follow a single power-law relationship $\int \tau(H I) dv = 14.07 ~\kms\ $\EBV$^{1.074}$ at 0.02 $\la$ \EBV\ $\la$ 3 mag. Corrections to 21cm line-derived H I column densities are too small to have had any effect on the ratio N(H I)/\EBV\ $= 8.3 \times 10^{21}\pcc$ mag$^{-1}$ we derived at 0.015 $\la$ \EBV\ $\la$ 0.075 mag and \absb\ $\ge$ 20\degr; they are also too small to explain the break in the slope of the N(H I) - \EBV\ relation at \EBV\ $\ga$ 0.1 mag that we demonstrated around the Galaxy at \absb $\ge 20$\degr. The latter must therefore be attributed to the onset of \HH-formation and we show that models of \HH\ formation in a low density diffuse molecular gas can readily explain the inflected N(H I)- \EBV\ relationship. Below \absb\ = 20\degr\ N(H I)/\EBV\ measured at 0.015 $\la$ \EBV\ $\la$ 0.075 mag increases steadily down to \absb\ = 8\degr\ where sightlines with small \EBV\ no longer occur. By contrast, the ratio N(H I)/\EBV\ measured over all \EBV\ declines to N(H I)/\EBV\ $= 5-6 \times 10^{21}\pcc$ mag$^{-1}$ at \absb\ $\la 30$\degr, perhaps providing an explanation of the difference between our results and the gas/reddening ratios measured previously using stellar spectra.

Excited-State Relaxation of Ag 8 Clusters Embedded in Helium Droplets

Abstract: Neutral silver clusters ${\mathrm{Ag}}_{N}$ are grown in ultracold helium nanodroplets. By exploiting a strong absorption resonance recently found for ${\mathrm{Ag}}_{8}$, first photoelectron spectra of this neutral species are recorded. Variation of the laser photon energy reveals that direct vertical two-photon ionization is hindered by rapid relaxation into the lower edge of a long-living excited state manifold. The analysis of the dynamics gives a precise value of $(6.89\ifmmode\pm\else\textpm\fi{}0.09)\text{ }\mathrm{eV}$ for the vertical ionization potential of ${\mathrm{Ag}}_{8}$. The influence of the helium matrix on photoemission is discussed.

The Absorption of One-Half Centimeter Electromagnetic Waves in Oxygen

Abstract: The absorption of electromagnetic waves in the one-half cm wave-length range has been measured for ${\mathrm{O}}_{2}$ and ${\mathrm{O}}_{2}$-${\mathrm{N}}_{2}$ mixtures as a function of pressure. The apparatus employs a klystron oscillator, crystal-rectifier frequency-multiplier, wave guide absorption path, and crystal detector. The measured values are in agreement with the theory of Van Vleck both as regards the absolute value of the absorption (which is as great as 67 db/km at the band center for pure ${\mathrm{O}}_{2}$ at a pressure of one atmosphere) and the dependence on pressure.

Thermal decomposition of methanol in shock waves

Abstract: The thermal decomposition of methanol behind reflected shock waves with 1372 < T{sub 5} < 1842 K and 1.81 {times} 10{sup {minus}5} < {rho}{sub 5} < 2.15 {times} 10{sup {minus}5} mol/cm{sup 3} was studied by IR laser kinetic absorption spectroscopy and gas-chromatographic analysis of reaction products. The absorption profiles and product yields were satisfactorily modeled with a 26-reaction mechanism. The initial rate of decrease of IR absorption intensity and the product yields were very sensitive to the rate constants of the reactions CH{sub 3}OH + M = CH{sub 3} + OH + M, CH{sub 3}OH + H = CH{sub 3} + H{sub 2}O, CH{sub 3}OH + H = CH{sub 2}OH + H{sub 2}, and CH{sub 2}OH + M = CH{sub 2}O + H + M. The rate constant expression for the last reaction was derived.

High-precision limit on variation in the fine-structure constant from a single quasar absorption system

Abstract: The brightest southern quasar above redshift $z=1$, HE 0515$-$4414, with its strong intervening metal absorption-line system at $z_{abs}=1.1508$, provides a unique opportunity to precisely measure or limit relative variations in the fine-structure constant ($\Delta\alpha/\alpha$). A variation of just $\sim$3 parts per million (ppm) would produce detectable velocity shifts between its many strong metal transitions. Using new and archival observations from the Ultraviolet and Visual Echelle Spectrograph (UVES) we obtain an extremely high signal-to-noise ratio spectrum (peaking at S/N $\approx250$ pix$^{-1}$). This provides the most precise measurement of $\Delta\alpha/\alpha$ from a single absorption system to date, $\Delta\alpha/\alpha=-1.42\pm0.55_{\rm stat}\pm0.65_{\rm sys}$ ppm, comparable with the precision from previous, large samples of $\sim$150 absorbers. The largest systematic error in all (but one) previous similar measurements, including the large samples, was long-range distortions in the wavelength calibration. These would add a $\sim$2 ppm systematic error to our measurement and up to $\sim$10 ppm to other measurements using Mg and Fe transitions. However, we corrected the UVES spectra using well-calibrated spectra of the same quasar from the High Accuracy Radial velocity Planet Searcher (HARPS), leaving a residual 0.59 ppm systematic uncertainty, the largest contribution to our total systematic error. A similar approach, using short observations on future, well-calibrated spectrographs to correct existing, high S/N spectra, would efficiently enable a large sample of reliable $\Delta\alpha/\alpha$ measurements. The high S/N UVES spectrum also provides insights into analysis difficulties, detector artifacts and systematic errors likely to arise from 25-40-m telescopes.