47 research outputs found
CO ice photodesorption: A wavelength-dependent study
UV-induced photodesorption of ice is a non-thermal evaporation process that
can explain the presence of cold molecular gas in a range of interstellar
regions. Information on the average UV photodesorption yield of astrophysically
important ices exists for broadband UV lamp experiments. UV fields around
low-mass pre-main sequence stars, around shocks and in many other astrophysical
environments are however often dominated by discrete atomic and molecular
emission lines. It is therefore crucial to consider the wavelength dependence
of photodesorption yields and mechanisms. In this work, for the first time, the
wavelength-dependent photodesorption of pure CO ice is explored between 90 and
170 nm. The experiments are performed under ultra high vacuum conditions using
tunable synchrotron radiation. Ice photodesorption is simultaneously probed by
infrared absorption spectroscopy in reflection mode of the ice and by
quadrupole mass spectrometry of the gas phase. The experimental results for CO
reveal a strong wavelength dependence directly linked to the vibronic
transition strengths of CO ice, implying that photodesorption is induced by
electronic transition (DIET). The observed dependence on the ice absorption
spectra implies relatively low photodesorption yields at 121.6 nm (Ly-alpha),
where CO barely absorbs, compared to the high yields found at wavelengths
coinciding with transitions into the first electronic state of CO (singulet Pi
at 150 nm); the CO photodesorption rates depend strongly on the UV profiles
encountered in different star formation environments.Comment: 5 pages, 2 figures, published in ApJ
UV photodesorption of methanol in pure and CO-rich ices: desorption rates of the intact molecule and of the photofragments
Wavelength dependent photodesorption rates have been determined using
synchrotron radiation, for condensed pure and mixed methanol ice in the 7 -- 14
eV range. The VUV photodesorption of intact methanol molecules from pure
methanol ices is found to be of the order of 10 molecules/photon, that
is two orders of magnitude below what is generally used in astrochemical
models. This rate gets even lower ( 10 molecules/photon) when the
methanol is mixed with CO molecules in the ices. This is consistent with a
picture in which photodissociation and recombination processes are at the
origin of intact methanol desorption from pure CHOH ices. Such low rates
are explained by the fact that the overall photodesorption process is dominated
by the desorption of the photofragments CO, CH, OH, HCO and
CHO/CHOH, whose photodesorption rates are given in this study. Our
results suggest that the role of the photodesorption as a mechanism to explain
the observed gas phase abundances of methanol in cold media is probably
overestimated. Nevertheless, the photodesorption of radicals from methanol-rich
ices may stand at the origin of the gas phase presence of radicals such as
CHO, therefore opening new gas phase chemical routes for the formation of
complex molecules.Comment: 13 pages, 2 figures, 1 tabl
Indirect ultraviolet photodesorption from CO:N2 binary ices - an efficient grain-gas process
UV ice photodesorption is an important non-thermal desorption pathway in many
interstellar environments that has been invoked to explain observations of cold
molecules in disks, clouds and cloud cores. Systematic laboratory studies of
the photodesorption rates, between 7 and 14 eV, from CO:N2 binary ices, have
been performed at the DESIRS vacuum UV beamline of the synchrotron facility
SOLEIL. The photodesorption spectral analysis demonstrates that the
photodesorption process is indirect, i.e. the desorption is induced by a photon
absorption in sub-surface molecular layers, while only surface molecules are
actually desorbing. The photodesorption spectra of CO and N2 in binary ices
therefore depend on the absorption spectra of the dominant species in the
subsurface ice layer, which implies that the photodesorption efficiency and
energy dependence are dramatically different for mixed and layered ices
compared to pure ices. In particular, a thin (1-2 ML) N2 ice layer on top of CO
will effectively quench CO photodesorption, while enhancing N2 photodesorption
by a factors of a few (compared to the pure ices) when the ice is exposed to a
typical dark cloud UV field, which may help to explain the different
distributions of CO and N2H+ in molecular cloud cores. This indirect
photodesorption mechanism may also explain observations of small amounts of
complex organics in cold interstellar environments.Comment: 21 pages 5 figure
An experimental study of the reactivity of CN- and C3N- anions with cyanoacetylene (HC3N)
International audienceThe reactions of the CN- and C3N- anions with cyanoacetylene HC3N, of special interest for the chemistry of Titan’s upper atmosphere, have been investigated by means of FTICR mass-spectrometry. Primary ions, CN- and C3N-, have been produced by dissociative electron attachment (DEA) from BrCN and BrC3N, and prepared in a clean way before reaction. Total rate constants have been measured for both reactions at 300 K and are found to be: (3.9 ± 0.5) 10-9 and (1.0 ± 0.2) 10-10 cm3.s-1 for the reaction of HC3N with CN- and C3N-, respectively. For the CN- + HC3N reaction, proton transfer is found to be the only reactive channel within our detection limits. Proton transfer is also dominant for the C3N- + HC3N reaction but the resulting ionic product being identical to the primary ion C3N-, this process is transparent for the kinetics of the C3N- + HC3N reaction and the kinetic rate retrieved corresponds to a slow and competitive detachment pathway. Yet the nature and energetics of the neutral product(s) formed through this process remain unknown. Additional experiments using isotopic products have allowed to retrieve specific rate constants associated to the proton transfer channel in the C315N- + HC3N and C3N- + HC315N reactions and the measured rates are found to be significantly lower than for the CN- + HC3N system. This decrease and the evolution of reactivity when going from CN- to C3N- and the opening of a new detachment pathway is finally discusse
Indirect Ultraviolet Photodesorption from CO:N2 Binary Ices — An Efficient Grain-Gas Process
Ultraviolet (UV) ice photodesorption is an important non-thermal desorption pathway in many interstellar environments that has been invoked to explain observations of cold molecules in disks, clouds, and cloud cores. Systematic laboratory studies of the photodesorption rates, between 7 and 14 eV, from CO:N2 binary ices, have been performed at the DESIRS vacuum UV beamline of the synchrotron facility SOLEIL. The photodesorption spectral analysis demonstrates that the photodesorption process is indirect, i.e., the desorption is induced by a photon absorption in sub-surface molecular layers, while only surface molecules are actually desorbing. The photodesorption spectra of CO and N2 in binary ices therefore depend on the absorption spectra of the dominant species in the sub-surface ice layer, which implies that the photodesorption efficiency and energy dependence are dramatically different for mixed and layered ices compared with pure ices. In particular, a thin (1-2 ML) N2 ice layer on top of CO will effectively quench CO photodesorption, while enhancing N2 photodesorption by a factor of a few (compared with the pure ices) when the ice is exposed to a typical dark cloud UV field, which may help to explain the different distributions of CO and N2H+ in molecular cloud cores. This indirect photodesorption mechanism may also explain observations of small amounts of complex organics in cold interstellar environments.Astronom
Réactivité d’ions atomiques sélectionnés dans des états doublet - quadruplet … et singulet - triplet
International audienc
Revisiting the gas-phase reactions of CH3+ with small organic molecules of interest for astrochemistry
National audienc
State-selected O2+ reactions with VUV synchrotron radiation for cold plasmas applications
International audienc
Revisiting the gas-phase reactions of CH3+ with small organic molecules of interest for astrochemistry
National audienc