Y.-J. Chen

University of Southern California, Los Angeles, California, United States

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Publications (28)35.75 Total impact

  • [show abstract] [hide abstract]
    ABSTRACT: Context. Dust grains in cold circumstellar regions and dark-cloud interiors at 10-20 K are covered by ice mantles. A nonthermal desorption mechanism is invoked to explain the presence of gas-phase molecules in these environments, such as the photodesorption induced by irradiation of ice due to secondary ultraviolet photons. To quantify the effects of ice photoprocessing, an estimate of the photon absorption in ice mantles is required. In a recent work, we reported the vacuum-ultraviolet (VUV) absorption cross sections of nonpolar molecules in the solid phase. Aims: The aim was to estimate the VUV-absorption cross sections of nonpolar molecular ice components, including CH4, CO2, N2, and O2. Methods: The column densities of the ice samples deposited at 8 K were 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 found that, as expected, solid N2 has the lowest VUV-absorption cross section, which about three orders of magnitude lower than that of other species such as O2, which is also homonuclear. Methane (CH4) ice presents a high absorption near Ly-α (121.6 nm) and does not absorb below 148 nm. Estimating the ice absorption cross sections is essential for models of ice photoprocessing and allows estimating the ice photodesorption rates as the number of photodesorbed molecules per absorbed photon in the ice.Data can be found at http://ghosst.osug.fr/
    01/2014;
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    ABSTRACT: This paper reports the first measurements of solid-phase vacuum-ultraviolet (VUV) absorption cross-sections of heavy isotopologues present in icy dust grain mantles of dense interstellar clouds and cold circumstellar environments. Pure ices composed of D2O, CD3OD, 13CO2, and 15N15N were deposited at 8 K, a value similar to the coldest dust temperatures in space. 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 as the VUV source. Prior to this work, we have recently submitted a similar study of the light isotopologues (Cruz-Diaz, Muñoz Caro & Chen). The VUV spectra are compared to those of the light isotopologues in the solid phase, and to the gas phase spectra of the same molecules. Our study is expected to improve very significantly the models that estimate the VUV absorption of ice mantles in space, which have often used the available gas phase data as an approximation of the absorption cross sections of the molecular ice components. We will show that this work has also important implications for the estimation of the photodesorption rates per absorbed photon in the ice.
    01/2014;
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    ABSTRACT: Context. The vacuum-UV (VUV) absorption cross sections of most molecular solids present in interstellar ice mantles with the exception of H2O, NH3, and CO2 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, or H2S 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 H2O and NH3 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.Data can be found at http://ghosst.osug.fr/
    01/2014;
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    ABSTRACT: 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.
    The Astrophysical Journal 01/2014; 781(1):15-. · 6.73 Impact Factor
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    ABSTRACT: 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.
    The Astrophysical Journal 12/2013; 778(2):162-. · 6.73 Impact Factor
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    ABSTRACT: The total spectral weight \textit{S} of the emergent low-energy quasipaticles in high-temperature superconductors is explored by x-ray absorption spectroscopy. In order to examine the applicability of the Hubbard model, regimes that cover from zero doping to overdoping are investigated. In contrast to mean field theory, we found that \textit{S} deviates from linear dependence on the doping level \textit{p}. The slope of \textit{S} versus \textit{p} changes continuously throughout the whole doping range with no sign of saturation up to \textit{p} = 0.23. Therefore, the picture of Zhang-Rice singlet remains intact within the most prominent doping regimes of HTSC's.
    Physical Review B 08/2013; 88(13). · 3.77 Impact Factor
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    ABSTRACT: The formation of ice mantles on pre-cometary dust grains was simulated experimentally under ultrahigh vacuum conditions. An ice mixture containing H2O, CO, CO2, CH3OH, and NH3 was deposited at 8 K. The ice layer was either irradiated by UV or X-rays, or warmed up in a controlled way. The ice was monitored by infrared spectroscopy in transmittance and the species in the gas phase were detected by quadrupole mass spectroscopy (QMS). The temperature programmed desorption (TPD) of a complex ice mixture with up to five molecular components can aid to interpret the data collected by mass spectrometers on board cometary missions like Rosetta during the flyby. The irradiation experiments led to the formation of many photo-products. We will focus on those made by irradiation of ices containing H2S to study the formation of the detected sulfurbearing species in comets, such as S2.
    09/2012;
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    ABSTRACT: The quasiparticle relaxation dynamics in a single (001) Ca-doped YBCO (Y0.7Ca0.3Ba2Cu3O7−δ ) superconducting thin film was probed by using the femtosecond time-resolved spectroscopy. The (001) Y0.7Ca0.3Ba2Cu3O7−δ thin film was prepared on a (100)STO substrate by pulsed laser deposition. Through controlling the oxygen pressure and the annealing temperature within quartz tube, the doping level (0.08<p<0.22) in the phase diagram could be widely tuned from the overdoped region to the underdoped region with varying the hole concentration(p) in a single sample. In the overdoped region, two different components (positive/negative) were obviously identified in the transient reflectivity curves (ΔR/R). The negative component in ΔR/R emerges at T<T c and quickly defeated by the development of the positive component. The dramatic change in the positive component of ΔR/R arises well below T c. However, this anomaly change does not appear in the underdoped region. Only the positive ΔR/R was clearly observed at whole temperatures. These results indicate that the scenario of high-T c superconductivity in the overdoped region would be different from the underdoped region. KeywordsCuprate superconductors–Ultrafast spectroscopy–Nernst effect
    Journal of Superconductivity and Novel Magnetism 01/2011; 24(1):515-517. · 0.70 Impact Factor
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    ABSTRACT: The quasiparticle relaxation dynamics in optimally doped (001) YBa2Cu3O7−δ thin films were measured at various temperatures by white light pump-probe technique. The 2D ultrafast spectroscopy with broadband (∼200 nm) and high time-resolution (∼9 fs) was performed by the non-collinear optical parametric amplifier (NOPA). The relaxation time and the amplitude of the transient transmissivity changes (ΔT/T) are wavelength-dependent. The maximum ΔT/T appears at around 625 nm (∼1.98 eV) which corresponds to the transition between the Fermi level and the UHB (upper Hubbard band). Furthermore, the oscillations with ∼150, ∼320, ∼500 cm−1 have been clearly observed in the Fourier transformed graph of the 2D ultrafast spectroscopy, which are consistent with the phonon modes in Raman-scattering spectra.
    Journal of Superconductivity and Novel Magnetism 01/2011; 24(1):519-521. · 0.70 Impact Factor
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    ABSTRACT: An H2O:CO2:NH3 = 1:1:1 ice mixture, used as a model mixture for cometary and interstellar ices, was irradiated with ultraviolet (UV)/extreme ultraviolet (EUV) photons in the broad 4–20 eV (62–310 nm) energy range at 16 K. The desorbed species were detected in situ by mass spectrometry during photo-irradiation, and a quartz microbalance was used as a substrate to measure the mass of material remaining on the surface. The total mass desorption for this H2O:CO2:NH3 = 1:1:1 ice mixture at 16 K was measured to be 1.8 × 10−18 μg photon−1, which is comparable to the 1.5 × 10−18 μg photon−1 measured for pure H2O ice irradiated under the same conditions. The main desorbed species produced during the photolysis of the ices were H2, , OH•, CO, and O2, along with the starting components H2O, NH3, and CO2. We also tentatively assigned minor mass peaks to larger species such as OCN•/OCN−, HNCO, CH4, H2CO, CH3OH, and HCOOH. This result supports the scenario in which complex organic molecules can be formed in cometary and/or astrophysical ices and desorbed to the gas phase, and helps to better understand the photochemical processes occurring at the surface of Solar System icy bodies such as comets, as well as in cold astrophysical environments such as star-forming regions and protostars.
    Advances in Space Research 01/2011; · 1.18 Impact Factor
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    ABSTRACT: The comprehensive study of the temperature dependent x-ray absorption spectroscopy (XAS) reveals a dynamical spectral weight $\alpha$ in YBa$_2$Cu$_3$O$_y$ (YBCO). Large spectral weight changes for both the Upper Hubbard band and the Zhang-Rice band due to dynamics of holes are experimentally found in the underdoped regime. A large value of $\alpha \geq 0.3$ is indispensable to describing XAS of YBCO with the conservation of states. The value of $\alpha$ is linearly proportional to the pseudogap temperature in the underdoped regime, but becomes smaller as the doping level goes to the undoped limit. Our results clearly indicate that the pseudogap is related to the double occupancy and originates from bands in higher energies. Comment: 10 pages; 3 figures; references updated
    09/2010;
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    ABSTRACT: Time-domain spectroscopy, which probes the dynamics of the electronic states near Fermi surface that are associated with superconductivity, has proven to be a powerful method for providing insights into the fundamental nature of both pseudogap and superconducting gap. This study shows that the unique T-x phase diagram with the time-evolving ultrafast dynamics of Y1-xPrxBa2Cu3O7-delta can be used to identify clearly the Nernst, pseudogap, and superconducting regions. All of the orders appear together at a specific delay time, such as 1.2-3 ps, after pumping by an ultrashort pulse. These discoveries suggest that the Nernst effect, the pseudogap and even superconductivity may have the same physical origin.
    Physical review. B, Condensed matter 01/2010; 82.
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    ABSTRACT: In this work, we use ultrafast optical pump-optical probe spectroscopy to probe the polaron dynamics in La <sub>0.7</sub> Ca <sub>0.3</sub> MnO <sub>3</sub> (LCMO) thin films. The temporal evolution in transient reflectivity change ΔR/R exhibits two relaxing components: a fast component with a time constant of subpicosecond and a slow component with time constant ranging from tens of picoseconds to hundreds of picoseconds. The amplitude of the fast component, though has been seldom discussed before, exhibits the similar temperature dependence with that of the resistivity and the neutron scattering intensity due to nanoscale correlated polarons. The results strongly suggest that the fast photoinduced reflectivity change may have been due to the photoexcitation and trapping process of correlated Jahn–Teller polarons in the paramagnetic and ferromagnetic phases and, thus, implies the presence of electronic inhomogeneity in LCMO manganites.
    Journal of Applied Physics 03/2009; · 2.21 Impact Factor
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    ABSTRACT: In this work, we show that the irradiation of naphthalene (C10H8), the smallest polycyclic aromatic hydrocarbon (PAH), mixed in a H2O+NH3 ice mixture with 4–20 eV (62–310 nm, i.e. in the ultraviolet/extreme-ultraviolet ranges) photons at 15 K leads to the formation of an organic residue where many amino acids were identified. However, the distribution of these amino acids is different from what was reported in previous laboratory experiments where ice mixtures containing other sources of carbon (CO, CO2, CH4 and CH3OH) were irradiated with ultraviolet photons, indicating that amino acids can be formed via several mechanisms. This result also implies that naphthalene, and probably other PAHs, constitute a non-negligible source of interstellar carbon likely to form organic molecules after photolysis, supporting a scenario where molecules of biological interest could be formed in many different astrophysical environments before being delivered to the early Earth by meteorites.
    Monthly Notices of the Royal Astronomical Society 02/2008; 384(2):605 - 610. · 5.52 Impact Factor
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    ABSTRACT: We have carried out photon irradiation study of naphthalene (C10H8), the smallest polycyclic aromatic hydrocarbon (PAH) in water and ammonia ice mixtures. Photons provided by a synchrotron radiation light source in two broad-band energy ranges in the ultraviolet/near extreme ultraviolet (4–20 eV) and the extreme ultraviolet (13–45 eV) ranges were used for the irradiation of H2O+NH3+C10H8 = 1:1:1 ice mixtures at 15 K. We could identify several photo-products, namely CH4, C2H6, C3H8, CO, CO2, HNCO, OCN−, and probably quinoline (C9H7N) and phenanthridine (C13H9N). We found that the light hydrocarbons are preferably produced for the ice mixture subjected to 4–20 eV photons. However, the production yields of CO, CO2, and OCN− species seem to be higher for the mixture subjected to EUV photons (13–45 eV). Therefore, naphthalene and its photo-products appear to be more efficiently destroyed when high energy photons (E > 20 eV) are used. This has important consequences on the photochemical evolution of PAHs in astrophysical environments.
    Asia Oceania Geosciences Society annual meeting; 06/2007
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    ABSTRACT: C(T) of NaxCo0.99Mn0.01O2 · yH2O is studied to reveal more details on the nature of the doped Mn ions and the Tc suppression. The extra specific heat contribution in NaxCo0.99Mn0.01O2 · yH2O manifests the induced local magnetic moment due to Mn doping. The absence of the superconducting transition peak in C(T) of NaxCo0.99Mn0.01O2 · yH2O suggests a reduced superconducting volume fraction.
    Physica C Superconductivity 01/2007; · 0.72 Impact Factor
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    ABSTRACT: Impurity effects on NaxCoO2·yH2O are studied by Mn doping. Spectroscopic evidence reveals that the doped Mn ions occupy the Co sites and the valence is +4. The magnetic local moment induced by Mn doping was observed and . The induced local moment could lead to magnetic pair-breaking and is likely to be responsible for the Tc suppression by Mn impurities in NaxCo1-zMnzO2·yH2O.
    Journal of Magnetism and Magnetic Materials 01/2007; 310(2). · 1.83 Impact Factor
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    ABSTRACT: To shed light on the symmetry of the superconducting order parameter in NaxCoO2•yH2O , the Mn doping effects are studied. X-ray absorption spectroscopy verifies that the doped Mn impurities occupy the Co sites and are with a valance close to +4 . Impurity scattering by Mn is in the unitary limit that, however, does not lead to strong Tc suppression. This absence of the strong impurity effects on Tc is inconsistent with the simple picture of a sign-changing order parameter. Coexistence of the s -wave superconducting order parameter and the nodal one is proposed to reconcile all the existing experiments and has been implied by the specific heat experiments.
    Physical review. B, Condensed matter 01/2007; 76(9).
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    ABSTRACT: Context.Carbamic acid (NH$_2$COOH) is the smallest amino acid, smaller than the smallest proteinaceous amino acid glycine. This compound has never been observed in the interstellar medium (ISM). Previous experiments where ice mixtures containing H$_2$O, CO$_2$ and NH$_3$ were subjected to 1-MeV proton bombardment showed that carbamic acid is formed in a stable zwitterionic (NH$_3^+$COO$^-$) form.Aims.In the present work, we have carried out irradiations of ice mixtures containing H$_2$O, $^{12}$CO$_2$/$^{13}$CO$_2$ and NH$_3$ with ultraviolet (UV)/extreme ultraviolet (EUV) photons provided by a synchrotron source in the 4–20 eV range, and compared the results with those obtained for energetic protons.Methods.Infrared (IR) spectroscopy and mass spectrometry were used to identify the formed photo-products and monitor their evolution in the ices at 15 K and during the warming up to room temperature in the formed residues.Results.We identified the IR absorption features of HNCO, OCN$^-$, CO, NH$_4^+$ and NH$_2$CHO at low temperature in the ices, and features assigned to carbamic acid in the residues around 250 K. Finally, we conclude that under our experimental conditions, unlike what was obtained after bombardment with energetic protons, carbamic acid may be formed in the neutral form, and propose some photochemical pathways leading to its formation.
    http://dx.doi.org/10.1051/0004-6361:20066631. 01/2007;
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    ABSTRACT: An experiment where a H 2 O:CO 2 :NH 3 = 1:1:1 ice mixture was irradiated using the ultraviolet/extreme ultraviolet (UV/EUV) light provided by a synchrotron beam in the broad 4–20 eV (62–310 nm) range at 16 K is presented here. The main originalities of the present work are the composition of the starting ice mixture, since it did not contain any organic compound, in particular no methanol (CH 3 OH) nor methane (CH 4) as for previous similar experiments, and the photon energy range. Several amino acids were produced: nine were identified of which seven could be quantified, and some others tentatively identified using high-performance liquid chromatography (HPLC). This result shows that it is possible to form complex organics such as amino acids from the irradiation of ice mixtures contain-ing C-, H-, O-and N-atom bearing compounds, whatever the organic/inorganic nature of these compounds. Only the distribution of the formed amino acids is different from previous experiments. This discrepancy may be due to the starting mixture composition and/or the different energy range used for the irradiation. These two parameters are discussed in regard of their implications for the formation of amino acids in the laboratory and in astrophysical environments.
    Advances in Space Research 01/2007; 40. · 1.18 Impact Factor

Publication Stats

24 Citations
134 Downloads
822 Views
35.75 Total Impact Points

Institutions

  • 2014
    • University of Southern California
      Los Angeles, California, United States
  • 2005–2013
    • National Chiao Tung University
      • • Department of Electrophysics
      • • Institute of Physics
      Hsin-chu-hsien, Taiwan, Taiwan
  • 2011
    • The University of Electro-Communications
      Edo, Tōkyō, Japan
  • 2007–2011
    • National Central University
      • Department of Physics
      Taoyuan City, Taiwan, Taiwan