1,006 research outputs found
Investigation of HNCO isomers formation in ice mantles by UV and thermal processing: an experimental approach
Current gas phase models do not account for the abundances of HNCO isomers
detected in various environments, suggesting a formation in icy grain mantles.
We attempted to study a formation channel of HNCO and its possible isomers by
vacuum-UV photoprocessing of interstellar ice analogues containing HO,
NH, CO, HCN, CHOH, CH, and N followed by warm-up, under
astrophysically relevant conditions. Only the HO:NH:CO and HO:HCN
ice mixtures led to the production of HNCO species. The possible isomerization
of HNCO to its higher energy tautomers following irradiation or due to ice
warm-up has been scrutinized. The photochemistry and thermal chemistry of
HO:NH:CO and HO:HCN ices was simulated using the Interstellar
Astrochemistry Chamber (ISAC), a state-of-the-art ultra-high-vacuum setup. The
ice was monitored in situ by Fourier transform mid-infrared spectroscopy in
transmittance. A quadrupole mass spectrometer (QMS) detected the desorption of
the molecules in the gas phase. UV-photoprocessing of
HO:NH:CO/HO:HCN ices lead to the formation of OCN as main
product in the solid state and a minor amount of HNCO. The second isomer HOCN
has been tentatively identified. Despite its low efficiency, the formation of
HNCO and the HOCN isomers by UV-photoprocessing of realistic simulated ice
mantles, might explain the observed abundances of these species in PDRs, hot
cores, and dark clouds
Vacuum-UV spectroscopy of interstellar ice analogs. II. Absorption cross-sections of nonpolar ice molecules
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. The aim was to estimate the VUV-absorption cross sections of
nonpolar molecular ice components, including CH4, CO2, N2, and O2. 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. 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-alpha (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.Comment: 9 pages, 6 figures, 7 table
UV photoprocessing of CO2 ice: a complete quantification of photochemistry and photon-induced desorption processes
Ice mantles that formed on top of dust grains are photoprocessed by the
secondary ultraviolet (UV) field in cold and dense molecular clouds. UV photons
induce photochemistry and desorption of ice molecules. Experimental simulations
dedicated to ice analogs under astrophysically relevant conditions are needed
to understand these processes. We present UV-irradiation experiments of a pure
CO2 ice analog. Calibration of the QMS allowed us to quantify the
photodesorption of molecules to the gas phase. This information was added to
the data provided by the FTIR on the solid phase to obtain a complete
quantitative study of the UV photoprocessing of an ice analog. Experimental
simulations were performed in an ultra-high vacuum chamber. Ice samples were
deposited onto an infrared transparent window at 8K and were subsequently
irradiated with a microwave-discharged hydrogen flow lamp. After irradiation,
ice samples were warmed up until complete sublimation was attained. Photolysis
of CO2 molecules initiates a network of photon-induced chemical reactions
leading to the formation of CO, CO3 ,O2 , and O3 . During irradiation,
photon-induced desorption of CO and, to a lesser extent, O2 and CO2 took place
through a process called indirect desorption induced by electronic transitions
(DIET), with maximum photodesorption yields (Ypd) of 1.2 x 10-2
molecules/incident photon , 9.3 x 10-4 molecules/incident photon , and 1.1 x
10-4 molecules/incident photon , respectively. Calibration of mass
spectrometers allows a direct quantification of photodesorption yields instead
of the indirect values that were obtained from infrared spectra in most
previous works. Supplementary information provided by infrared spectroscopy
leads to a complete quantification, and therefore a better understanding, of
the processes taking place in UV-irradiated ice mantles
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