Non-thermal Excitation Research Articles

A geração química de vapor em espectrometria atômica

The historical development of atomic spectrometry techniques based on chemical vapor generation by both batch and flow injection sampling formats is presented. Detection via atomic absorption spectrometry (AAS), microwave induced plasma optical emission spectrometry (MIP-OES), inductively coupled plasma optical emission spectrometry (ICP-OES) , inductively coupled plasma mass spectrometry (ICP-MS) and furnace atomic nonthermal excitation spectrometry (FANES) are considered. Hydride generation is separately considered in contrast to other methods of generation of volatile derivatives. Hg ¾ CVAAS (cold vapor atomic absorption spectrometry) is not considered here. The current state-of-the-art, including extension, advantages and limitations of this approach is discussed.

Non-LTE Calculation of the N[CLC]i[/CLC] [CSC]i[/CSC] 676.8 Nanometer Line in a Flaring Atmosphere

The Ni I 676.8 nm line is used by the Solar and Heliospheric Observatory Michelson Doppler Imager to measure the magnetic field and velocity field in the solar atmosphere. We make non-LTE calculations of this line in an atmosphere that is bombarded by an energetic electron beam. This case is associated with the occurrence of solar flares. The electron beam produces nonthermal ionization and excitation of the hydrogen atoms and redistributes the level populations. This results in an enhanced opacity near the Ni I line and an upward shift of its formation height, as well as an increased line source function. We find that the Ni I line may appear in emission in the presence of a fairly strong electron beam and preferentially in a cool atmosphere (i.e., sunspot umbrae/penumbrae). On the other hand, if there is no bombarding electron beam, the profile can hardly turn to emission even though the atmosphere may be heated to higher temperatures through other ways. This result implies that the sign reversal of the longitudinal magnetic field observed in some flare events may not be a true reversal but just an artifact associated with the production of an emission profile.

Spectroscopy of SN 1987A at 0.9-2.4 μm: days 1348-3158

We present near-infrared spectroscopic observations of SN 1987A covering the period 1358 to 3158 d post explosion. This is the first time that IR spectra of a supernova have been obtained to such late epochs. The spectra comprise emission from both the ejecta and the bright, ring-shaped circumstellar medium (CSM). The most prominent CSM emission lines are recombination lines of H i and He i, and forbidden lines of [S iii] and [Fe ii]. The ejecta spectra include allowed lines of H i, He i and Na i and forbidden lines of [Si i], [Fe i], [Fe ii] and possibly [S i]. The intensity ratios and widths of the H i ejecta lines are consistent with a low-temperature Case B recombination spectrum arising from non-thermal ionization/excitation in an extended, adiabatically-cooled H envelope, as predicted by several authors. The slow decline of the ejecta forbidden lines, especially those of [Si i], indicates that pure non-thermal excitation was taking place, driven increasingly by the decay of 44Ti. The ejecta iron exhibits particularly high velocities (4000–4500 km s-1), supporting scenarios where fast radioactive nickel is created and ejected just after the core bounce. In addition, the ejecta lines continue to exhibit blueshifts with values ∼−200 to −800 km s−1 to at least day 2000. These blueshifts, which first appeared around day 600, probably indicate that very dense concentrations of dust persist in the ejecta, although an alternative explanation of asymmetry in the excitation conditions is not ruled out.

On the Fast Fluctuations in Solar Flare Hα Blue Wing Emission

Fine temporal structures in hard X-ray and microwave emissions of solar flares have been known for many years. Recent observations with high time and spatial resolution revealed that emissions in the wings of Hα could also exhibit fast (subsecond) fluctuations. We argue that such fluctuations are physically related to the small-scale injection of high-energy electrons. We explore this through numerical calculations. The energy equation and the equations for energy-level populations in hydrogen, in particular including the nonthermal collisional excitation and ionization rates, are solved simultaneously for an atmosphere impacted by a short-lived electron beam. We determine the temporal evolution of the atmospheric temperature, the atomic level populations, and the Hα line intensity. We find that although the background Hα wing emission is mainly formed in the photosphere, the fast fluctuations are probably produced in the chromosphere, which is penetrated by ~20 keV electrons. To yield Hα wing fluctuations of amplitude comparable to the observations, a mean energy flux of ~ × 1011 ergs cm-2 s-1 is required for the electron beam, if one adopts a Gaussian macrovelocity of 25 km s-1. Such a burst contains a total energy of 1025-1026 ergs. These parameters are compatible with elementary flare bursts.

Adiabatic output coupling of a Bose gas at finite temperatures

We develop a general theory of adiabatic output coupling from trapped atomic Bose-Einstein Condensates at finite temperatures. For weak coupling, the output rate from the condensate, and the excited levels in the trap, settles in a time proportional to the inverse of the spectral width of the coupling to the output modes. We discuss the properties of the output atoms in the quasi-steady-state where the population in the trap is not appreciably depleted. We show how the composition of the output beam, containing condensate and thermal component, may be controlled by changing the frequency of the output coupler. This composition determines the first and second order coherence of the output beam. We discuss the changes in the composition of the bose gas left in the trap and show how nonresonant output coupling can stimulate either the evaporation of thermal excitations in the trap or the growth of non-thermal excitations, when pairs of correlated atoms leave the condensate.

Near‐Infrared Line Observations of Arp 299 as a Tracer of Activity in the Nuclei

We have obtained images of the molecular hydrogen (H2) and Brγ emission lines in the interacting galaxy system Arp 299. In order to investigate the nuclear activities, we also have obtained H- and K-band spectra for three of its 2 μm continuum peaks and also J-band spectra for one of them. From the line ratios of Paγ, Paβ, Br10, and Brγ, and also those between [Fe II] 1.26 μm and [Fe II] 1.64 μm, we have successfully modeled the extinction for component A as a combination of AK = 2.6 mag region(s) and AK = 0.3 mag region(s). The extinction corrected Brγ luminosity requires 5 × 1054 s-1 of Lyman photons, which corresponds to 3 × 105 "O6 stars." The mean stellar density within its 100 pc core is comparable to those for the cores in blue compact dwarf galaxies even though the luminosity is almost 2 orders of magnitude larger. The starbursts at the nucleus of component A are probably the most dominant source for the far-infrared emission from this component. In this component, we have found large [Fe II]/H I ratios, which are at the higher end of starburst galaxies and are typical for AGNs or AGN/starburst composites. The nucleus must be in a starburst phase that includes many supernova remnants. The spatial distribution of the H2 emission for this interacting system is similar to that of the 12CO 1-0 emission mapped by Aalto et al. The H2 intensity, however, is not proportional to the CO intensity among components, because of the difference in the H2 excitation mechanism among them. We report the first possible detection of J-band H2 lines in infrared luminous galaxies in component A, which suggests a significant contribution of nonthermal excitation mechanisms.

Ultraviolet-laser induced desorption of NO from the Cr2O3(0001) surface: Involvement of a precursor state?

NO molecules interact with the Cr2O3(0001) surface to form a chemisorption bond of 1.0 eV. At higher coverages an additional more weakly bound species appears in thermal desorption spectra with a binding energy of 0.35 eV. By infrared spectroscopy the weakly adsorbed species is identified to be an unusually strong bound NO-dimer exhibiting a weak feature at 1857 cm−1 beside the chemisorbate absorption band at 1794 cm−1. Laser induced desorption experiments performed at 6.4 eV are presented with main emphasis on the high coverage regime. The desorbing molecules are detected quantum state selectively using resonance enhanced multiphoton ionization. The desorbing molecules are strongly rotationally and vibrationally excited conform with a nonthermal excitation process. The velocity distributions of single rovibronic states of desorbing NO are bimodal and exhibit a strong coupling of rotation and translation. With increasing coverages an additional channel is observed appearing in the time-of-flight spectra of v″=0 as smoothly increasing intensity at long flight times. The numeric values of these unusually long flight times are indicative for long residence times on the surface rather than small kinetic energies. The desorption efficiencies weakly depend on the concentration and vibrational state ranging from (2.0±0.3)×10−17 cm2 at low coverages to (1.0±0.4)×10−17 cm2 at high coverages for v″=0. The intensity of the desorption signal per laser pulse only increases proportional to the chemisorbate coverage. The data are interpreted assuming the dimers to act as extrinsic precursors within the desorption process.

Supernova 1996L: evidence of a strong wind episode before the explosion

Observations of the Type II supernova (SN) 1996L reveal the presence of a slowly expanding (v∼ 700 km s-1) shell at ∼ 1016 cm from the exploding star. Narrow emission features are visible in the early spectra superposed on the normal SN spectrum. Within about two months these features develop narrow symmetric P Cygni profiles. About 100 d after the explosion the light curve suddenly flattens, the spectral lines broaden and the Hα flux becomes larger than that expected from a purely radioactive model. These events are interpreted as signatures of the onset of the interaction between the fast moving ejecta and a slowly moving outer shell of matter ejected before the SN explosion. At about 300 d the narrow lines disappear and the flux drops until the SN fades away, suggesting that the interaction phase is over and that the shell has been swept away. Simple calculations show that the superwind episode started 9 yr before the SN explosion and lasted 6 yr, with an average M˙M=10-3 M⊙ yr-1. Even at very late epochs (up to day 335) the typical forbidden lines of [O i], Ca ii], [Fe ii] remain undetected or very weak. Spectra after day 270 show relatively strong emission lines of He i. These lines are narrower than other emission lines coming from the SN ejecta, but broader than those from the circumstellar material (CSM). These high-excitation lines are probably the result of non-thermal excitation and ionization caused by the deposition of the γ-rays emitted in the decay of radioactive material mixed in the He layer.

Heating in the Lower Atmosphere and the Continuum Emission of Solar White‐Light Flares

Observationally, there is a small fraction of solar white-light flares (WLFs), the so-called type II WLFs, showing an increased visible continuum but no significant Balmer jump and less strong chromospheric line emission in comparison with type I WLFs. The classical point of view, that the flare energy is initially released in the corona and then transported downward, can hardly explain WLFs of this kind. In this paper we explore the possibility that type II WLFs originate from a deeper layer. Assuming an in situ energy release, in particular in the form of high-energy particles, in a region around the temperature minimum, the continuum emission is computed in different time stages during the flare evolution. At first, nonthermal excitation and ionization of hydrogen atoms caused by bombarding particles result in a decline of the visible continuum. Later on, the lower atmosphere is gradually heated through radiative transfer, mitigating the continuum decline. In the final stage, when the particle bombardment stops while the atmosphere still keeps a heated state, we obtain a positive continuum contrast without an obvious Balmer jump. This meets the condition required for type II WLFs. The presence or absence of a continuum decline in the early stage of the flare provides a diagnostic tool for nonthermal processes in the lower atmosphere.

Measurements of Suprathermal Electrons in Hohlraum Plasmas with X-Ray Spectroscopy

Nonthermal excitation of atomic states by suprathermal electrons was observed for the first time within inertial confinement fusion hohlraum plasmas. The nonthermal excitation process results in the simultaneous emission of the $\mathrm{Ly}\ensuremath{-}\ensuremath{\alpha}$ transition and the $K$-shell satellite series (lithiumlike through carbonlike) which were observed on temporally resolved x-ray emission spectra. A quantitative analysis with a time-dependent collisional-radiative non-Maxwellian model shows that these spectra can be used to obtain temporally and spatially resolved measurements of the suprathermal electron fraction in indirectly driven inertial confinement fusion targets.

Late Spectral Evolution of SN 1987A. II. Line Emission

Using the temperature and ionization calculated in our previous paper, we model the spectral evolution of SN 1987A. We find that the temperature evolution is directly reflected in the time evolution of the lines. In particular, the IR catastrophe is seen in the metal lines as a transition from thermal to nonthermal excitation, seen most clearly in the [O I] λλ6300, 6364 lines. The good agreement with observations clearly confirms the predicted optical to IR transition. Because the line emissivity is independent of temperature in the nonthermal phase, this phase has a strong potential for estimating the total mass of the most abundant elements. The hydrogen lines arise as a result of recombinations following ionizations in the Balmer continuum during the first ~500 days and later as a result of nonthermal ionizations.

The 1.05- m feature in the spectrum of the Type Ia supernova 1994D: He in SNe Ia?

A P Cygni profile with absorption at 1.05 μm was observed in three pre-maximum J-band spectra of the Type Ia supernova (SN) 1994D. The feature was not present in two post-maximum spectra. The line was attributed to He 10830 ... or MgII 10926 ..., based on a local thermodynamic equilibrium (LTE) treatment. The detection of He in the ejecta of a SN Ia would be useful for determining the pre-SN evolution and the explosion mechanism of SNe Ia. In this paper, synthetic spectra are presented for both the He and Mg models. The population of the He levels has been computed in non-local thermodynamic equilibrium (NLTE), including non-thermal excitation and ionization effects resulting from the deposition of γ-rays from the decay of 56Ni and 56Co. The J-band feature in the pre-maximum spectra can be reproduced either assuming the presence of a narrow shell, between 10000 and 12500 km s−1, containing about 0.01 M⊙ of He, or increasing the abundance of Mg by about a factor of 5 with respect to the W7 value, implying a Mg mass of about 0.08 M⊙ above 10000 km s−1. Both models are in good agreement with the optical spectrum. In particular, a strong HeI 10830-... line does not imply a strong 5876-... line, because the departure coefficients of the 2p and 2s levels of HeI differ by about an order of magnitude. Unfortunately, neither model is able to reproduce the sudden disappearance of the J-band feature in the post-maximum spectra. Possible explanations are discussed.

Thermal and photon-induced desorption of NH3 and ND3 from Ag{111}

Thermal desorption of NH3 and ND3 from Ag{111} reveals that adsorption is reversible with three desorption peaks at 145, 120, and 104 K. Irradiation of ammonia with 193 nm photons results in the desorption of molecular ammonia from the surface and in subsequent temperature programmed desorption there is no evidence for photodissociation. Below 3 monolayers (MLs) the cross section for photodesorption is less than 1×10−20 cm+2 for both isotopes, and above 3 ML the desorption yield increased linearly with coverage. Time-of-flight distributions were recorded as a function of coverage (3–50 ML) and characterized by two channels with average translational temperatures of 3150 and 600 K. The average flux-weighted kinetic energy of both channels is independent of laser pulse energy and the yield increases linearly with laser power, suggesting a nonthermal excitation mechanism. For multilayers, we propose a photodesorption model involving the direct excitation of ammonia.

The Type IIB Supernova 1993J

A non-LTE study of the Type IIb supernova 1993J in the galaxy M81 taking into account nonthermal ionization and line blocking effects is carried out. Hydrodynamical models and theoretical spectra clearly show that nonthermal ionization and excitation dominate after the second maximum, at day ∼ 30, and play a decisive role in reproducing both the smooth tail of the light curve and the emergence of helium lines in the spectrum, similar to those observed. Based on our model of supernova 1993J, we predict that the light curves of Type Ib supernovae should be subject to nonthermal ionization and excitation at earlier times than even those for supernova 1993J. In our model, the outburst of supernova 1993J is interpreted as the explosion of a ∼ 4 M⊙ red supergiant, which underwent core collapse and left a neutron star in a binary system. The progenitor is supposed to have a helium core mass of ∼ 3 M⊙, corresponding to a ∼ 13 M⊙ main-sequence star. Supernova 1993J adds evidence to the scenario that Type Ib supernovae originate from moderately massive stars on the main sequence that have lost their hydrogen envelopes in interacting binary systems.

Low energy electron-induced chemistry of biacetyl on Ag(111)

The electron-induced chemistry of one monolayer (1 ML) of biacetyl (CH3COCOCH3) on Ag(111) has been studied using temperature programmed desorption (TPD), x-ray photoelectron spectroscopy, and Auger electron spectroscopy. Monolayer biacetyl has a sharp saturable parent peak at 180 K, with a tail toward higher temperatures that is attributed to defect sites. No thermal decomposition occurs, confirming that Ag(111) is inert with respect to the breaking of C—C, C=O, and C—H bonds. Nonthermal excitation pathways by which the surface chemistry of biacetyl may be directed were explored by irradiating 1 ML of biacetyl with 50 eV electrons. During irradiation, CO, CH3, ketene (H2O=C=O), and C2H6 desorb. After irradiation, the parent TPD peak area drops with little change in position or shape. The initial total cross section for electron-induced decomposition of biacetyl is 8.0±0.2×10−17 cm+2. As products accumulate, this drops to 4.0±0.2×10−17 cm+2. The cross section has a threshold around 8 eV and rises smoothly with increasing energy. The ionization of biacetyl is proposed as the initiation step.

Infrared inter-Rydberg emission spectra as a possible probe of molecular hydrogen in astrophysical environments

The possibility of observing excited molecular hydrogen in certain astrophysical environments via its infrared high- l inter-Rydberg spectra is discussed. Astrophysical environments in which non-thermal excitations of H 2 Rydberg states occur include the aurorae of the Jovian planets and their dayglows, and sunspots. Spectra of those high- l Rydberg transitions which appear most likely to be observed by infrared astronomy are presented using the multipole-polarisation model already applied to laboratory observations of high- l H 2 spectra.

Freeze out, IR-catastrophes and Non-thermal Emission in SNe

Freeze out effects and the IR-catastrophe are discussed for SN 1987A and for Type la SNe. We show that the light curves of the optical lines in SN 1987A provide strong evidence for the IR-catastrophe. We also argue that most optical lines are dominated by non-thermal excitation after ∼ 800 days. The level of this emission is set mainly by the total mass of the elements. Models of the [OI]λλ6300 — 64 light curve show that an oxygen mass of ∼ 1.5Mʘ is needed. Light curve models for Type la SNe display a sharp decrease in the optical flux as a result of the IR-catastrophe at ∼ 500 days, producing UBV-photometry inconsistent with observations of SN1972E by Kirshner & Oke (1975).

Thermal and Low-Energy Electron-Driven Chemistry of Biacetyl on Ag(111)

The thermal and electron-induced chemistry of biacetyl (CH3COCOCH3) on Ag(111) has been studied using temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy. No thermal decomposition of biacetyl occurs, confirming that Ag(111) is inert with respect to the breaking of C−C, CO, and C−H bonds. There are five molecular biacetyl desorption peaks in TPD180, 178, 174, 188, and 215 K. The peak at 180 K is attributed to monolayer adsorption, and its saturation peak area is used to scale other TPD biacetyl peaks. The peak at 188 K is assigned to multilayers and that at 215 K to desorption from defect sites. The peaks at 174 and 188 K are discussed in terms of coverage dependent reorientation and bilayers. Nonthermal excitation pathways by which the surface chemistry of biacetyl may be directed were explored by irradiating 1 ML of biacetyl with 50 eV electrons. During irradiation, CO, CH3, ketene (H2CCO), and C2H6 desorb. After irradiation, five new post-irradiation TPD peaks appear. These are identified as H2 at 210 K, CH4 at 235 and 315 K, H2CCO at 240 K, and reaction-limited CH3COCOCH3 at 440 K. XPS shows C(a) and O(a) remain on the surface after heating to 700 K.

Heat transport and nonthermal excitations in vibrational hopping systems.

We investigate a system of high-frequency (optical) phonon modes which are localized due to disorder and interact with the low-frequency (acoustic) phonon modes which are extended and effectively act as a heat bath for the optical modes. Fourth-order anharmonicity is included in the subsystem of the optical modes and taken into account within a mean-field approximation. The thermal conductivity due to hopping of the optical modes is lowered by increasing the anharmonicity, but its temperature dependence is not affected. Furthermore, considering the system under the influence of an energy pump it turns out that the occupation of the localized modes is influenced by the anharmonicity in such a way as to screen off disorder.

Simultaneous determination of hydride forming elements by furnace atomic nonthermal excitation spectrometry (FANES)

Different approaches for the simultaneous determination of arsenic, antimony, bismuth, selenium and tellurium by furnace atomic nonthermal excitation spectrometry (FANES) are described. In the direct analysis of microvolumes of solutions, the results could be improved by working with Pd or Ir as chemical modifiers for the stabilization of the analytes during the thermal pre-treatment. Using Ir as a modifier, absolute detection limits of 0.116 ng (As), 0.062 ng (Bi), 0.044 ng (Sb), 0.316 ng (Se) and 0.016 ng (Te) could be realized. The combination of hydride generation and in situ enrichment of the analytes onto a permanent Ir-treated inner surface of the graphite tube of the FANES source has been used as pre-concentration technique. The conditions for all steps, hydride generation, pre-reduction, in situ enrichment, atomization and excitation have been studied. Using compromise conditions for the simultaneous determination of all the elements using a sample volume of 20 ml, relative detection limits of 0.032 μg l −1 As 3+, 0.015 μg l −1 Bi 3+, 0.012 μg l −1 Sb 3+, 0.047 μg l −1 Se 4+ and 0.009 μg l −1 Te 4+ are obtained. The influence of hydride forming elements on the determination of each of the elements by the technique of vapour generation and in situ enrichment is studied. The results are validated using standard reference materials.