51 research outputs found
Thermal H/D exchange in polar ice - deuteron scrambling in space
We have investigated the thermally induced proton/deuteron exchange in mixed
amorphous HO:DO ices by monitoring the change in intensity of
characteristic vibrational bending modes of HO, HDO, and DO with time
and as function of temperature. The experiments have been performed using an
ultra-high vacuum setup equipped with an infrared spectrometer that is used to
investigate the spectral evolution of homogeneously mixed ice upon
co-deposition in thin films, for temperatures in the 90 to 140 K domain. With
this non-energetic detection method we find a significantly lower activation
energy for H/D exchange -- K -- than previously reported. Very
likely this is due to the amorphous nature of the interstellar ice analogues
involved. This provides reactive timescales ( K)
fast enough for the process to be important in interstellar environments.
Consequently, an astronomical detection of DO will be even more challenging
because of its potential to react with HO to form HDO. Furthermore,
additional experiments, along with previous studies, show that proton/deuteron
swapping also occurs in ice mixtures of water with other hydrogen bonded
molecules, in particular on the OH and NH moieties. We conclude that H/D
exchange in ices is a more general process that should be incorporated into ice
models that are applied to protoplanetary disks or to simulate the warming up
of cometary ices in their passage of the perihelion, to examine the extent of
its influence on the final deuteron over hydrogen ratio.Comment: 10 pages, 8 figures, accepted for publication in MNRA
THz time-domain spectroscopy of mixed CO2–CH3OH interstellar ice analogs
The icy mantles of interstellar dust grains are the birthplaces of the primordial prebiotic molecular inventory that may eventually seed nascent solar systems and the planets and planetesimals that form therein. Here, we present a study of two of the most abundant species in these ices after water: carbon dioxide (CO2) and methanol (CH3OH), using TeraHertz (THz) time-domain spectroscopy and mid-infrared spectroscopy. We study pure and mixed-ices of these species, and demonstrate the power of the THz region of the spectrum to elucidate the long-range structure (i.e. crystalline versus amorphous) of the ice, the degree of segregation of these species within the ice, and the thermal history of the species within the ice. Finally, we comment on the utility of the THz transitions arising from these ices for use in astronomical observations of interstellar ices
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Systematic investigation of CO<sub>2</sub> : NH<sub>3</sub> ice mixtures using mid-IR and VUV spectroscopy – part 1: thermal processing
The adjustment of experimental parameters in interstellar ice analogues can have profound effects on molecular synthesis within an ice system. We demonstrated this by systematically investigating the stoichiometric mixing ratios of CO2 : NH3 ices as a function of thermal processing using mid-IR and VUV spectroscopy. We observed that the type of CO2 bonding environment was dependent on the different stoichiometric mixing ratios and that this pre-determined the NH3 crystallite structure after phase change. The thermal reactivity of the ices was linked to the different chemical and physical properties of the stoichiometric ratios. Our results provide new details into the chemical and physical properties of the different stoichiometric CO2 : NH3 ices enhancing our understanding of the thermally induced molecular synthesis within this ice system
The structure and dynamics of carbon dioxide and water containing ices investigated via THz and mid-IR spectroscopy
Icy dust grains play a key role in the chemistry of the interstellar medium. The cumulative outcome of recent observations, laboratory studies, and astrochemical models indicates that solid-phase reaction mechanisms may dominate the formation of complex organic molecules such as amino acids and sugars in space. Consequently, the composition and structure of the icy grain mantle may significantly influence solid-phase reaction pathways. In this work, we present a new experimental setup capable of studying astrochemical ice analogs in both the TeraHertz (THz), or far-Infrared (far-IR), region (0.3–7.5 THz; 10–250 cm^(−1)) and the mid-IR (400–4000 cm^(−1)). The instruments are capable of performing a variety of spectroscopic studies that can provide especially relevant laboratory data to support astronomical observations from telescopes such as Herschel, SOFIA, and ALMA. Experimental spectra of astrochemical ice analogs of water and carbon dioxide in pure, mixed, and layered ices were collected at different temperatures under high vacuum conditions with the goal of investigating the structure of the ice. We tentatively observe a new feature in both amorphous solid water and crystalline water at 33 cm^(−1) (1 THz). In addition, our studies of mixed and layered ices show how it is possible to identify the location of carbon dioxide as it segregates within the ice by observing its effect on the THz spectrum of water ice. The THz spectra of mixed and layered ices are further analyzed by fitting their spectral features to those of pure amorphous solid water and crystalline water ice to quantify the effects of temperature changes on structure. From the results of this work, it appears that THz spectroscopy is potentially well suited to study thermal transformations within the ice
Proton and electron irradiations of CH4:H2O mixed ices
The organic chemistry occurring in interstellar environments may lead to the
production of complex molecules that are relevant to the emergence of life.
Therefore, in order to understand the origins of life itself, it is necessary
to probe the chemistry of carbon-bearing molecules under conditions that
simulate interstellar space. Several of these regions, such as dense molecular
cores, are exposed to ionizing radiation in the form of galactic cosmic rays,
which may act as an important driver of molecular destruction and synthesis. In
this paper, we report the results of a comparative and systematic study of the
irradiation of CH4:H2O ice mixtures by 1 MeV protons and 2 keV electrons at 20
K.We demonstrate that our irradiations result in the formation of a number of
new products, including both simple and complex daughter molecules such as
C2H6, C3H8, C2H2, CH3OH, CO, CO2, and probably also H2CO. A comparison of the
different irradiation regimes has also revealed that proton irradiation
resulted in a greater abundance of radiolytic daughter molecules compared to
electron irradiation, despite a lower radiation dose having been administered.
These results are important in the context of the radiation astrochemistry
occurring within the molecular cores of dense interstellar clouds, as well as
on outer Solar System objects.Comment: Published as an open access article in the MDPI journal Atom
Bombardment of CO ice by cosmic rays: I. Experimental insights into the microphysics of molecule destruction and sputtering
We present a dedicated experimental study of microscopic mechanisms
controlling radiolysis and sputtering of astrophysical ices due to their
bombardment by cosmic ray ions. Such ions are slowed down due to inelastic
collisions with bound electrons, resulting in ionization and excitation of ice
molecules. In experiments on CO ice irradiation, we show that the relative
contribution of these two mechanisms of energy loss to molecule destruction and
sputtering can be probed by selecting ion energies near the peak of the
electronic stopping power. We have observed a significant asymmetry, both in
the destruction cross section and the sputtering yield, for pairs of ion
energies corresponding to same values of the stopping power on either side of
the peak. This implies that the stopping power does not solely control these
processes, as usually assumed in the literature. Our results suggest that
electronic excitations represent a significantly more efficient channel for
radiolysis and, possibly, also for sputtering of CO ice. We also show that the
charge state of incident ions as well as the rate for CO production in the
ice have negligible effect on these processes.Comment: Accepted for publication in Ap
Proton and Electron Irradiations of CH4:H2O Mixed Ices
The organic chemistry occurring in interstellar environments may lead to the production of complex molecules that are relevant to the emergence of life. Therefore, in order to understand the origins of life itself, it is necessary to probe the chemistry of carbon-bearing molecules under conditions that simulate interstellar space. Several of these regions, such as dense molecular cores, are exposed to ionizing radiation in the form of galactic cosmic rays, which may act as an important driver of molecular destruction and synthesis. In this paper, we report the results of a comparative and systematic study of the irradiation of CH4 :H2O ice mixtures by 1 MeV protons and 2 keV electrons at 20 K. We demonstrate that our irradiations result in the formation of a number of new products, including both simple and complex daughter molecules such as C2H6, C3H8, C2H2, CH3OH, CO, CO2, and probably also H2CO. A comparison of the different irradiation regimes has also revealed that proton irradiation resulted in a greater abundance of radiolytic daughter molecules compared to electron irradiation, despite a lower radiation dose having been administered. These results are important in the context of the radiation astrochemistry occurring within the molecular cores of dense interstellar clouds, as well as on outer Solar System objects
Searches for Interstellar HCCSH and Hâ‚‚CCS
A longstanding problem in astrochemistry is the inability of many current models to account for missing sulfur content. Many relatively simple species that may be good candidates to sequester sulfur have not been measured experimentally at the high spectral resolution necessary to enable radioastronomical identification. On the basis of new laboratory data, we report searches for the rotational lines in the microwave, millimeter, and submillimeter regions of the sulfur-containing hydrocarbon HCCSH. This simple species would appear to be a promising candidate for detection in space owing to the large dipole moment along its b-inertial axis, and because the bimolecular reaction between two highly abundant astronomical fragments (CCH and SH radicals) may be rapid. An inspection of multiple line surveys from the centimeter to the far-infrared toward a range of sources from dark clouds to high-mass star-forming regions, however, resulted in nondetections. An analogous search for the lowest-energy isomer, H₂CCS, is presented for comparison, and also resulted in nondetections. Typical upper limits on the abundance of both species relative to hydrogen are 10^(−9)–10^(−10). We thus conclude that neither isomer is a major reservoir of interstellar sulfur in the range of environments studied. Both species may still be viable candidates for detection in other environments or at higher frequencies, providing laboratory frequencies are available
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