T.-S. Yih

VACUUM ULTRAVIOLET EMISSION SPECTRUM MEASUREMENT OF A MICROWAVE-DISCHARGE HYDROGEN-FLOW LAMP IN SEVERAL CONFIGURATIONS: APPLICATION TO PHOTODESORPTION OF CO ICE

We report measurements of the vacuum ultraviolet (VUV) emission spectra of a microwave-discharge hydrogen-flow lamp (MDHL), a common tool in astrochemistry laboratories working on ice VUV photoprocessing. The MDHL provides hydrogen Ly-α (121.6 nm) and H2 molecular emission in the 110-180 nm range. We show that the spectral characteristics of the VUV light emitted in this range, in particular the relative proportion of Ly-α to molecular emission bands, strongly depend on the pressure of H2 inside the lamp, the lamp geometry (F type versus T type), the gas used (pure H2 versus H2 seeded in He), and the optical properties of the window used (MgF2 versus CaF2). These different configurations are used to study the VUV irradiation of CO ice at 14 K. In contrast to the majority of studies dedicated to the VUV irradiation of astrophysical ice analogs, which have not taken into consideration the emission spectrum of the MDHL, our results show that the processes induced by photons in CO ice from a broad energy range are different and more complex than the sum of individual processes induced by monochromatic sources spanning the same energy range, as a result of the existence of multistate electronic transitions and discrepancy in absorption cross sections between parent molecules and products in the Ly-α and H2 molecular emission ranges.

Vacuum-UV spectroscopy of interstellar ice analogs - I. Absorption cross-sections of polar-ice molecules

Context. The vacuum-UV (VUV) absorption cross sections of most molecular solids present in interstellar ice mantles with the exception of H2O, NH3 ,a nd CO 2 have not been reported yet. Models of ice photoprocessing depend on the VUV absorption cross section of the ice to estimate the penetration depth and radiation dose, and in the past, gas phase cross section values were used as an approximation. Aims. We aim to estimate the VUV absorption cross section of molecular ice components. Methods. Pure ices composed of CO, H2O, CH3OH, NH3 ,o r H 2S were deposited at 8 K. The column density of the ice samples was measured in situ by infrared spectroscopy in transmittance. VUV spectra of the ice samples were collected in the 120−160 nm (10.33−7.74 eV) range using a commercial microwave-discharged hydrogen flow lamp. Results. We provide VUV absorption cross sections of the reported molecular ices. Our results agree with those previously reported for H2 Oa nd NH 3 ices. Vacuum-UV absorption cross section of CH3OH, CO, and H2S in solid phase are reported for the first time. H2S presents the highest absorption in the 120−160 nm range. Conclusions. Our method allows fast and readily available VUV spectroscopy of ices without the need to use a synchrotron beamline. We found that the ice absorption cross sections can be very different from the gas-phase values, and therefore, our data will significantly improve models that simulate the VUV photoprocessing and photodesorption of ice mantles. Photodesorption rates of pure ices, expressed in molecules per absorbed photon, can be derived from our data.

Carbamic acid produced by the UV/EUV irradiation of interstellar ice analogs

Context. TeV gamma-ray emission from two massive binaries of the microquasar type, LS 5039 and LS I +61° 303, show clear variability with their orbital periods. Aims. Our purpose is to calculate the GeV and TeV γ-ray light curves from the massive binary LS 5039, which are expected in the specific inverse Compton e± pair cascade model. This model has successfully predicted the basic features of the high energy γ-ray emission from LS 5039 and LS I +61°303. Methods. In the calculations we apply the Monte Carlo code, which follows the IC e ± pair cascade in the anisotropic radiation of the massive star. Results. The γ-ray light curves and spectra are obtained for different parameters of the acceleration scenario and the inclination angles of the binary system. We find that the GeV and TeV γ-ray light curves should be anti-correlated. This feature can be tested in the near future by the simultaneous observations of LS 5039 with the AGILE and GLAST telescopes in GeV energies and the Cherenkov telescopes in the TeV energies. This model also predicts a broad maximum in the TeV γ-ray light curve between the phases ∼0.4-0.8 consistently with the observations of LS 5039 by the HESS telescopes. Moreover, we predict an additional dip in the TeV light curve for large inclination angles ∼60°. This feature could serve as a diagnostic for independently measuring the inclination angle of this binary system indicating also the presence of a neutron star in LS 5039.

SOFT X-RAY IRRADIATION OF METHANOL ICE: FORMATION OF PRODUCTS AS A FUNCTION OF PHOTON ENERGY

Pure methanol ices have been irradiated with monochromatic soft X-rays of 300 and 550 eV close to the 1s resonance edges of C and O, respectively, and with a broadband spectrum (250-1200 eV). The infrared (IR) spectra of the irradiated ices show several new products of astrophysical interest such as CH2OH, H2CO, CH4, HCOOH, HCOCH2OH, CH3COOH, CH3OCH3, HCOOCH3, and (CH2OH)2, as well as HCO, CO, and CO2. The effect of X-rays is the result of the combined interactions of photons and electrons with the ice. A significant contribution to the formation and growth of new species in the CH3OH ice irradiated with X-rays is given by secondary electrons, whose energy distribution depends on the energy of X-ray photons. Within a single experiment, the abundances of the new products increase with the absorbed energy. Monochromatic experiments show that product abundances also increase with the photon energy. However, the abundances per unit energy of newly formed species show a marked decrease in the broadband experiment as compared to irradiations with monochromatic photons, suggesting a possible regulatory role of the energy deposition rate. The number of new molecules produced per absorbed eV in the X-ray experiments has been compared to those obtained with electron and ultraviolet (UV) irradiation experiments.

FORMATION OF S-BEARING SPECIES BY VUV/EUV IRRADIATION OF H2S-CONTAINING ICE MIXTURES: PHOTON ENERGY AND CARBON SOURCE EFFECTS

Carbonyl sulfide (OCS) is a key molecule in astrobiology that acts as a catalyst in peptide synthesis by coupling amino acids. Experimental studies suggest that hydrogen sulfide (H2S), a precursor of OCS, could be present in astrophysical environments. In the present study, we used a microwave-discharge hydrogen-flow lamp, simulating the interstellar UV field, and a monochromatic synchrotron light beam to irradiate CO:H2S and CO2:H2S ice mixtures at 14 K with vacuum ultraviolet (VUV) or extreme ultraviolet (EUV) photons in order to study the effect of the photon energy and carbon source on the formation mechanisms and production yields of S-containing products (CS2, OCS, SO2, etc.). Results show that (1) the photo-induced OCS production efficiency in CO:H2S ice mixtures is higher than that of CO2:H2S ice mixtures; (2) a lower concentration of H2S enhances the production efficiency of OCS in both ice mixtures; and (3) the formation pathways of CS2 differ significantly upon VUV and EUV irradiations. Furthermore, CS2 was produced only after VUV photoprocessing of CO:H2S ices, while the VUV-induced production of SO2 occurred only in CO2:H2S ice mixtures. More generally, the production yields of OCS, H2S2, and CS2 were studied as a function of the irradiation photon energy. Heavy S-bearing compounds were also observed using mass spectrometry during the warm-up of VUV/EUV-irradiated CO:H2S ice mixtures. The presence of S-polymers in dust grains may account for the missing sulfur in dense clouds and circumstellar environments.

Absolute photoabsorption cross sections of Sr I from the 5s ionization threshold to the 5p threshold

We have measured the absolute photoabsorption cross sections of Sr I from its 5s ionization threshold up to the thresholds. The spectrum includes the Sr I , , and doubly excited series which converge to the or series limits. Synchrotron radiation, from the 1 m Seya-Namioka beam line of the Synchrotron Radiation Research Center at Hsin-Chu, Taiwan, was used as the background continuum. The absolute column density was determined by measuring simultaneously the temperature distribution profiles and the total pressure in a heatpipe. Absolute cross sections were obtained using the Beer-Lambert law. The measured absolute cross section for the 5s ionization threshold was . At the most significant autoionizing resonance, around , the absolute cross section was measured as . The absolute cross sections presented here are larger than those based on saturated vapour-pressure data, and less than those based on the f-value measurements. All the absolute cross sections in this work are compared with both recent experiments and

A simple apparatus for determining column density and absolute photoabsorption cross sections

This article presents a simple measuring apparatus that is capable of measuring column density and absolute photoabsorption cross sections. The apparatus includes a simple high-temperature heatpipe furnace, an MKS Instruments Baratron, a hydrogen discharge lamp, and an optical detecting system. The heatpipe furnace provides an absorbing medium with a steady total pressure in a closed system so that the column density can be obtained using the ideal gas relation. The apparatus is designed for a working temperature range below 1000 K and a pressure range up to 10 Torr. Encountered experimental difficulties and their found solutions will be discussed in detail. In this work, the absolute photoabsorption cross section of Mg is measured near the 3s threshold using this apparatus. The absolute photoabsorption cross section result at the 3s threshold, which is 2.1±0.3 Mb, is in good agreement with hook method data (2.36 Mb).

Observation of new Rydberg series in the many-electron transition region of N2

Fluorescence excitation spectra produced through photoexcitation of N(2) using synchrotron radiation in the spectral region between 50 and 62.5 nm have been obtained with a resolution of 0.004 nm. A broadband detector (in the 115-180 nm region) was employed to monitor fluorescence originated from neutral excited atomic nitrogen fragments which are produced through direct dissociation processes and predissociation from the well-known many-electron excited Rydberg states. We have identified a new Rydberg series (2 (2)Π(g)) 4sσ, a better resolved Rydberg (D (2)Π(g)) npσ series, and also the prominent Codling series converging to the D (2)Π(g), and C (2)Σ(u)(+) states of N(2)(+), respectively. By normalizing our relative fluorescence intensities to previously measured absolute fluorescence cross-section data we obtain the cross-section data of undispersed fluorescence in the 115-180 nm region. The fluorescence quantum yields for the present photodissociative excitation processes are found to be less than 0.05. The present results may provide important data for our understanding of competitions among the various decay channels of the many-electron transition states of N(2).

Column density and temperature effects on narrow resonance structures in atomic photoionization and photoabsorption

We consider a formation of the Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) phase in a quasi-onedimensional (Q1D) conductor in a magnetic field, parallel to its conducting chains, where we take into account both the paramagnetic spin-splitting and orbital destructive effects against superconductivity. We show that, due to a relative weakness of the orbital effects in a Q1D case, the LOFF phase appears in (TMTSF)2ClO4 superconductor for real values of its Q1D band parameters. We compare our theoretical calculations with the recent experimental data by Y. Maeno’s group [S. Yonezawa et al., Phys. Rev. Lett. 100, 117002 (2008)] and show that there is a good qualitative and quantitative agreement between the theory and experimental data.

Spin-mixed doubly excited resonances in Ca and Sr spectra

We present a joint theoretical and experimental investigation to demonstrate explicitly how the combined spin-dependent interaction and the configuration interaction may affect the mixing of different spin states along various doubly excited autoionization series for Ca and Sr as energy increases across several ionization thresholds. In particular, our study has identified the inversion of energy levels between members of a number of multiplets, i.e., in contrast to the Hunds rules, due to the presence of perturber from other overlapping resonance series. We are also able to demonstrate the beginning of the breakdown of the LS coupling for resonance series corresponding to electron configurations with higher orbital angular momenta and those above the third ionization threshold.