Nonthermal Channels Research Articles

Vibrational Action Spectroscopy of Solids: New Surface-Sensitive Technique.

Vibrational action spectroscopy employing infrared radiation from a free-electron laser has been successfully used for many years to study the vibrational and structural properties of gas phase aggregates. Despite the high sensitivity of this method no relevant studies have yet been conducted for solid sample surfaces. We have set up an experiment for the application of this method to such targets, using infrared light from the free-electron laser of the Fritz Haber Institute. In this Letter, we present first results of this technique with adsorbed argon and neon atoms as messengers. We were able to detect surface-located vibrations of a thin V_{2}O_{3}(0001) film on Au(111) as well as adsorbate vibrations, demonstrating that this method is highly surface sensitive. We consider that the dominant channel for desorption of the messenger atoms is direct inharmonic vibrational coupling, which is essentially insensitive to subsurface or bulk vibrations. Another channel is thermal desorption due to sample heating by absorption of infrared light. The high surface sensitivity of the nonthermal channel and its insensitivity to subsurface modes makes this technique an ideal tool for the study of surface-located vibrations.

Standard big bang nucleosynthesis with a nonthermal reaction network

We discuss a refined scenario of standard big bang nucleosynthesis (BBN) allowing for nonthermal nuclear reactions in the primordial plasma. These reactions are naturally triggered in the early Universe by fast particles $(n,p,t,^{3}\mathrm{He},\ensuremath{\alpha})$ generated in the $t(d,n)\ensuremath{\alpha}$, $d(d,n)^{3}\mathrm{He}$, $d(d,p)t$, and $^{3}\mathrm{He}(d,p)\ensuremath{\alpha}$ processes. We take into account the nonthermal channels for reactions between nuclei with $A\ensuremath{\leqslant}7$ (of the standard BBN network) and consider new processes, such as nucleon-induced breakups of fragile $d$, $^{6,7}\mathrm{Li}$, and $^{7}\mathrm{Be}$. It is shown that fast neutrons---the most abundant and hot nonthermal plasma species---can effectively maintain some reactions as the Universe cools. Depending on the type of an individual reaction, these neutrons can increase the reaction rate coefficient ${N}_{\mathrm{A}}⟨\ensuremath{\sigma}v⟩$ by a few percent (for exoergic resonant processes) and orders of magnitude (for endoergic processes, such as the breakups and some reverse reactions). However, the individual nonthermal effects prove to be insufficient to influence the whole reaction kinetics and significantly change the predictions of thermal BBN. The nonthermal corrections to element abundances are found to be 0.10% (for T), $\ensuremath{-}0.01%$ (for $^{3}\mathrm{He}$), $\ensuremath{-}0.02%$ (for $^{7}\mathrm{Li}$), and 0.36%--0.60% (for CNO elements).

The evolution of the large-scale emission in Fanaroff-Riley type I jets

Recent observations in X-rays and gamma-rays of nearby FRI radio galaxies have raised the question of the origin of the emission detected in the termination structures of their jets. The study of these structures can give information on the conditions for particle acceleration and radiation at the front shocks. In addition, an evolutionary scenario can help to disentangle the origin of the detected X-ray emission in young FRI sources, like some Gigahertz Peaked Spectrum AGNs. This work focuses on the nature and detectability of the radiation seen from the termination regions of evolving FRI jets. We use the results of a relativistic, two-dimensional numerical simulation of the propagation of an FRI jet, coupled with a radiation model, to make predictions for the spectra and lightcurves of the thermal and non-thermal emission at different stages of the FRI evolution. Our results show that under moderate magnetic fields, the synchrotron radiation would be the dominant non-thermal channel, appearing extended in radio and more compact in X-rays, with relatively small flux variations with time. The shocked jet synchrotron emission would dominate the X-ray band, although the shocked ISM/ICM thermal component alone may be significant in old sources. Inverse Compton scattering of CMB photons could yield significant fluxes in the GeV and TeV bands, with a non-negligible X-ray contribution. The IC radiation would present a bigger angular size in X-rays and GeV than in TeV, with fluxes increasing with time. We conclude that the thermal and non-thermal broadband emission from the termination regions of FRI jets could be detectable for sources located up to distances of a few 100 Mpc.

Adsorption of triethygermane on Si(100): thermal and nonthermal channels for ethyl ligand removal/dissociation

Adsorption of triethylgemane (TEG) on the Si(100) surface at 100 K has been studied using a variety of surface-sensitive spectroscopies. Thermally and electronically desorbed species were analyzed using temperature-programmed desorption (TPD) and electron-stimulated desorption (ESD) techniques. Electronically desorbed ions (H+) were analyzed using a time-of-flight technique (TOF) and time evolution studies were conducted on desorbing neutral species. Direct analysis of surface species were carried out using X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss (HREELS) spectroscopy. Evidence is offered for (1) an electron-induced channel for direct ethylene desorption via a β-hydride elimination process; (2) an electron-stimulated transfer of ethyl ligands from a Ge to a Si site; and (3) both thermal- and electron-stimulated desorption of methyl groups. Copyright © 2006 John Wiley & Sons, Ltd.

Channels of energy redistribution in short-pulse laser interactions with metal targets

The kinetics and channels of laser energy redistribution in a target irradiated by a short, 1 ps, laser pulse is investigated in computer simulations performed with a model that combines molecular dynamics (MD) simulations with a continuum description of the laser excitation and relaxation of the conduction band electrons, based on the two-temperature model (TTM). The energy transferred from the excited electrons to the lattice splits into several parts, namely the energy of the thermal motion of the atoms, the energy of collective atomic motions associated with the relaxation of laser-induced stresses, the energy carried away from the surface region of the target by a stress wave, the energy of quasi-static anisotropic stresses, and, at laser fluences above the melting threshold, the energy transferred to the latent heat of melting and then released upon recrystallization. The presence of the non-thermal channels of energy redistribution (stress wave and quasi-static stresses), not accounted for in the conventional TTM model, can have important implications for interpretation of experimental results on the kinetics of thermal and mechanical relaxation of a target irradiated by a short laser pulse as well as on the characteristics of laser-induced phase transformations. The fraction of the non-thermal energy in the total laser energy partitioning increases with increasing laser fluence.

Thermal and direct etching mechanisms of Si(100) with a hyperthermal chlorine beam

The etching of Si(100) with hyperthermal beams of atomic and molecular chlorine has been studied in the temperature range between 130 and 200 K. The dominant etch product, SiCl4, is evolved in two parallel reaction pathways, only one of which is thermally activated. It is shown that the nonactivated pathway, which has an appreciable rate only when hyperthermal chlorine atoms are incident upon the surface, is due to collision-induced desorption of weakly bound SiCl4. By increasing the normal component of the chlorine incident translational energy, the efficiency of converting incident Cl into SiCl4 can be increased to 0.6% in the nonthermal channel, at temperatures for which the thermal etching rate is negligible. The normal energy scaling and the observed energy threshold suggest that this process should result in highly anisotropic etching.

Nonlinear dynamics and the problem of polymer fracture

AbstractThe results of the experimental and molecular‐dynamics study of the process of mechanical degradation of polymer chain, testifying to the essential role of nonthermal channels of dissipation of elastic energy accumulated during deformation, have been discussed. High‐energy molecular products of mechanical degradation, which are able to induce a chain branching process leading to “explosive” formation of submicrocrackes, have been discovered. It has been noted that a considerable part of polymer chain energy is concentrated in localized excitations of a soliton type, which play the determining role at all the stages of mechanical degradation under consideration.

Non-dissociative laser induced desorption of molecules from dielectric surfaces

We have been concerned with the non-dissociative desorption of 1,3-cyclohexadiene, pyridine, 2-piccoline, N-methyl-aniline and methyl-pyrazine adsorbed on surfaces of LiF (100), MgF 2, quartz and surfaces coated with Ar, Xe and N 2. As an example we present our results on the non-dissociative desorption of methyl-pyrazine from quartz. We used tunable UV laser light (248–280 nm) to excite the π→π ∗ electronic transition centered at 266 nm. The dependence of the desorption cross sections on layer thickness, laser fluence and laser wavelength has been obtained by quantitative analysis of the TOP (time of flight) mass spectra of the desorbing molecules. Two different desorption channels have been found with quantum yields two orders of magnitude different from each others. A non-thermal channel has a quantum yield of 10 −4–10 −5 at 266 nm. The second channel occurs at thicker layers and higher laser fluences with quantum yields⩾10 −3. The desorption signal is proportional to the absorption coefficient.

Ultraviolet laser photodesorption of NO from condensed films: Translational and internal energy distributions

In this paper we report that ultraviolet laser induced desorption from the surface of a thin nitric oxide film proceeds via two mechanisms which are present simultaneously. One mechanism is attributed to laser induced thermal desorption while the other is due to a nonthermal, single photon process. A film of 1–2000 ML (layers) of NO condensed on a Ag(111) substrate under UHV conditions at 25–50 K was irradiated by 5 ns pulses of 220–270 nm laser light (4.6–5.5 eV) with 0.5–5 mJ/cm2 (0.1–1 MW/cm2 ) power density at the surface. Translational energies of desorbed molecules were measured from time-of-flight (TOF) spectra taken with a mass spectrometer, while the internal energy distribution of molecules desorbed in the nonthermal channel was determined by a (1+1) resonance enhanced multiphoton ionization (REMPI) probe. NO monomer in the 2Π3/2,1/2 electronic ground states was the only significant product. There were two distinct characteristic TOF components, which we associate with different desorption mechanisms. Each component had a different velocity and angular distribution, and their relative yields varied with laser pulse energy and NO layer thickness. Under conditions where both mechanisms gave comparable desorption yields, we obtained TOF distributions which were bimodal. A ‘‘slow’’ peak with an average translational energy up to 0.06 eV was Maxwellian with temperatures between 160 and 280 K and a broad angular distribution. Yield in this peak increased strongly with layer thickness and exponentially with laser pulse energy. A ‘‘fast’’ TOF peak with average energy of 0.22 eV was non-Maxwellian, with an angular distribution peaked toward normal, and yield increasing linearly with laser pulse energy. REMPI of the fast peak showed a vibrational population ratio v=3:v=2 of 0.85. A Boltzmann plot of the rotational population distribution of v=2 molecules, if fit with a single line, gave a temperature of 2500 K. We use these angular, velocity, rotational, and vibrational distributions to suggest mechanisms for the nonthermal desorption. We also discuss factors determining the relative extent of thermal and photochemical effects, which control the morphology of ablated surfaces.