Jennifer Noble

How micron-sized dust particles determine the chemistry of our Universe

In the environments where stars and planets form, about one percent of the mass is in the form of micro-meter sized particles known as dust. However small and insignificant these dust grains may seem, they are responsible for the production of the simplest (H2) to the most complex (amino-acids) molecules observed in our Universe. Dust particles are recognized as powerful nano-factories that produce chemical species. However, the mechanism that converts species on dust to gas species remains elusive. Here we report experimental evidence that species forming on interstellar dust analogs can be directly released into the gas. This process, entitled chemical desorption (fig. 1), can dominate over the chemistry due to the gas phase by more than ten orders of magnitude. It also determines which species remain on the surface and are available to participate in the subsequent complex chemistry that forms the molecules necessary for the emergence of life.

The desorption of H2CO from interstellar grains analogues

Context. Much of the formaldehyde (H2CO) is formed from the hydrogenation of CO on interstellar dust grains, and is released in the gas phase in hot core regions. Radio-astronomical observations in these regions are directly related to its desorption from grains. Aims. We study experimentally the thermal desorption of H2CO from bare silicate surfaces, from water ice surfaces and from bulk water ice in order to model its desorption from interstellar grains. Methods. Temperature-programmed desorption experiments, monitored by mass spectrometry, and Fourier transform infrared spectroscopy are performed in the laboratory to determine the thermal desorption energies in: (i.) the multilayer regime where H2CO is bound to other H2CO molecules; (ii.) the submonolayer regime where H2CO is bound on top of a water ice surface; (iii.) the mixed submonolayer regime where H2CO is bound to a silicate surface; and (iv.) the multilayer regime in water ice, where H2CO is embedded within a H2O matrix. Results. In the submonolayer regime, we find the zeroth-order desorption kinetic parameters nu(0) = 10(28) mol cm(-2) s(-1) and E = 31.0 +/- 0.9 kJmol(-1) for desorption from an olivine surface. The zeroth-order desorption kinetic parameters are nu(0) = 10(28) mol cm(-2) s(-1) and E = 27.1 +/- 0.5 kJmol(-1) for desorption from a water ice surface in the submonolayer regime. In a H2CO:H2O mixture, the desorption is in competition with the H2CO + H2O reaction, which produces polyoxymethylene, the polymer of H2CO. This polymerization reaction prevents the volcano desorption and co-desorption from happening. Conclusions. H2CO is only desorbed from interstellar ices via a dominant sub-monolayer desorption process (E = 27.1 +/- 0.5 kJmol-1). The H2CO which has not desorbed during this sub-monolayer desorption polymerises upon reaction with H2O, and does not desorb as H2CO at higher temperature.

Diffusion measurements of CO, HNCO, H2CO, and NH3 in amorphous water ice

Context. Water is the major component of the interstellar ice mantle. In interstellar ice, chemical reactivity is limited by the diffusion of the reacting molecules, which are usually present at abundances of a few percent with respect to water. Aims. We want to study the thermal diffusion of H2CO, NH3, HNCO, and CO in amorphous water ice experimentally to account for the mobility of these molecules in the interstellar grain ice mantle. Methods. In laboratory experiments performed at fixed temperatures, the diffusion of molecules in ice analogues was monitored by Fourier transform infrared spectroscopy. Diffusion coefficients were extracted from isothermal experiments using Ficks second law of diffusion. Results. We measured the surface diffusion coefficients and their dependence with the temperature in porous amorphous ice for HNCO, H2CO, NH3, and CO. They range from 10(-15) to 10(-11) cm(2) s(-1) for HNCO, H2CO, and NH3 between 110 K and 140 K, and between 5-8 x 10(-13) cm(2) s(-1) for CO between 35 K and 40 K. The bulk diffusion coefficients in compact amorphous ice are too low to be measured by our technique and a 10(-15) cm(2) s(-1) upper limit can be estimated. The amorphous ice framework reorganization at low temperature is also put in evidence. Conclusions. Surface diffusion of molecular species in amorphous ice can be experimentally measured, while their bulk diffusion may be slower than the ice mantle desorption kinetics.

The thermal reactivity of HCN and NH3 in interstellar ice analogues

HCN is a molecule central to interstellar chemistry, since i t is the simplest molecule containing a carbon-nitrogen bond and its solid state chemistry is r ich. The aim of this work was to study the NH3 + HCN→ NH + CN − thermal reaction in interstellar ice analogues. Laborator y experiments based on Fourier transform infrared spectroscopy and mass spectrometry were performed to characterise the NH + CN − reaction product and its formation kinetics. This reaction is purely thermal and can occur at low temperatures in interstellar ices without requiring non-thermal processing by photons, electrons or cosmic rays. The reaction rate constant has a temperature dependence of k(T ) = 0.016 +0.010 −0.006 s −1 exp( −2.7±0.4 kJ mol −1 RT ) when NH3 is much more abundant than HCN. When both reactants are diluted in water ice, the reaction is slowed down. We have estimated the CN − ion band strength to be ACN − = 1.8±1.5×10 −17 cm molec −1 at both 20 K and 140 K. NH + CN − exhibits zeroth-order multilayer desorption kinetics wit h a rate of kdes(T ) = 10 28 molecules cm −2 s −1 exp( −38.0±1.4 kJ mol −1

Hydrogenation at low temperatures does not always lead to saturation: the case of HNCO

Context. It is generally agreed that hydrogenation reactions dominate chemistry on grain surfaces in cold, dense molecular cores, saturating the molecules present in ice mantles. Aims. We present a study of the low temperature reactivity of solid phase isocyanic acid (HNCO) with hydrogen atoms, with the aim of elucidating its reaction network. Methods. Fourier transform infrared spectroscopy and mass spectrometry were employed to follow the evolution of pure HNCO ice during bombardment with H atoms. Both multilayer and monolayer regimes were investigated. Results. The hydrogenation of HNCO does not produce detectable amounts of formamide (NH2CHO) as the major product. Experiments using deuterium reveal that deuteration of solid HNCO occurs rapidly, probably via cyclic reaction paths regenerating HNCO. Chemical desorption during these reaction cycles leads to loss of HNCO from the surface. Conclusions. It is unlikely that significant quantities of NH2CHO form from HNCO. In dense regions, however, deuteration of HNCO will occur. HNCO and DNCO will be introduced into the gas phase, even at low temperatures, as a result of chemical desorption.

Diffusion of molecules in the bulk of a low density amorphous ice from molecular dynamics simulations

The diffusion of molecules in interstellar ice is a fundamental phenomenon to take into account while studying the formation of complex molecules in this ice. This work presents a theoretical study on the diffusion of H2O, NH3, CO2, CO, and H2CO in the bulk of a low density amorphous (LDA) ice, while taking into account the physical conditions prevailing in space, i.e. temperatures below 150 K and extremely low pressure. This study was undertaken by means of molecular dynamics simulations. For CO2 for which no experimental data were available we conducted our own experiments. From our calculations we show that, at low temperatures, the diffusion of molecules in the bulk of a LDA ice is driven by the self-diffusion of water molecules in the ice. With this study we demonstrate that molecular dynamics allows the calculation of diffusion coefficients for small molecules in LDA ice that are convincingly comparable to experimentally measured diffusion coefficients. We also provide diffusion coefficients for a series of molecules of astrochemical interest.

The electronic spectra of protonated PANH molecules

Aims. This study was designed to examine the viability of protonated nitrogen-substituted polycyclic aromatic hydrocarbons (H+PANHs) as candidates for the carriers of the diffuse interstellar bands (DIBs). Methods. We obtained the electronic spectra of two protonated PANH cations, protonated acridine and phenanthridine, using parent ion photo-fragment spectroscopy and generated theoretical electronic spectra using ab initio calculations. Results. We show that the spectra of the two species studied here do not correspond to known DIBs. However, based on the general properties derived from the spectra of these small protonated nitrogen-substituted PAHs, we propose that larger H+PANH cations represent good candidates for DIB carriers due to the expected positions of their electronic transitions in the UV-visible and their narrow spectral bands.Aims. This study was designed to examine the viability of protonated nitrogen-substituted polycyclic aromatic hydrocarbons (H + PANHs) as candidates for the carriers of the diffuse interstellar bands (DIBs). Methods. We obtained the electronic spectra of two protonated PANH cations, protonated acridine and phenanthridine, using parent ion photo-fragment spectroscopy and generated theoretical electronic spectra using ab initio calculations. Results. We show that the spectra of the two species studied here do not correspond to known DIBs. However, based on the general properties derived from the spectra of these small protonated nitrogen-substituted PAHs, we propose that larger H + PANH cations represent good candidates for DIB carriers due to the expected positions of their electronic transitions in the UV-visible and their narrow spectral bands.

Supplementary information on the near-infrared spectroscopic data of the infrared camera (IRC) onboard AKARI

We have investigated the on-orbit properties of the spectroscopic data taken with NIR channel of the Infrared Camera (IRC) onboard AKARI during the phases 1, 2 and 3. We have determined the boundary shape of the aperture mask of NIR channel by using the spectroscopic data of uniform zodiacal background emission. The information on the aperture mask shape is indispensable in modeling and subtracting the spectroscopic background patterns made by the diffuse background emission such as zodiacal emission and the Galactic cirrus emission. We also have examined the wavelength dependency on the profile of the point spread function and its effect on the spectroscopic data. The obtained information is useful, for example, in reducing the spectroscopic data of a point source badly affected by bad pixels and in decomposing the overlapping spectra of sources that are aligned in the dispersion direction with a small offset the cross dispersion direction. In this paper, we summarize the supplementary knowledge that will be useful for the advanced data reduction procedures of NIR spectroscopic datasets.

Reactivity in interstellar ice analogs: role of the structural evolution

Context. The synthesis of interstellar complex organic molecules in ice involves several types of reactions between molecules and/or radicals that are usually considered to be diffusion controlled.Aims. We aim to understand the coupling between diffusion and reactivity in the interstellar ice mantle using a model binary reaction in the diffusion-limited regime.Methods. We performed isothermal kinetic laboratory experiments on interstellar ice analogs at low temperatures, using the NH3:CO2:H2O model system where reactants NH3 and CO2 have a low reaction barrier and are diluted in a water-dominated ice.Results. We found that in the diffusion-limited regime, the reaction kinetics is not determined by the intrinsic bulk diffusivity of reactants. Instead, reactions are driven by structural changes evolving in amorphous water ice, such as pore collapse and crystallization. Diffusion of reactants in this case likely occurs along the surface of (tiny) cracks generated by the structural changes.Conclusions. The reactivity driven by the structural changes breaks the conventional picture of reactant molecules/radicals diffusing in a bulk water ice. This phenomenon is expected to lead to a dramatic increase in production rates of interstellar complex organic molecules in star-forming regions.

Inhomogeneity of the amorphous solid water dangling bonds

Amorphous solid water (ASW) is one of the most widely studied molecular systems because of its importance in the physics and chemistry of the interstellar medium and the upper layers of the Earths atmosphere. Although the global structure of this material, i.e. the bulk and the surface, is well characterised, we are far from having an overall understanding of the changes induced upon chemical or physical perturbation. More specifically, the behaviour of the surface and the immediate sublayers upon mid-infrared irradiation must be understood due to its direct effect on the adsorption capacities of the ASW surface. Small molecules can accrete or form at the surface, adsorbed on the dangling OH groups of surface water molecules. This behaviour allows further reactivity which, in turn, could lead to more complex molecular systems. We have already demonstrated that selective IR irradiations of surface water molecules induce a modification of the surface and the production of a new monomer species which bonds to the surface via its two electronic doublets. However, we did not probe the structure of the dangling bands, namely their homogeneity or inhomogeneity. The structure and orientation of these surface molecules are closely linked to the way the surface can relax its vibrational energy. In this work, we have focussed our attention on the two dH dangling bonds, carrying out a series of selective irradiations which reveal the inhomogeneity of these surface modes. We have also studied the effects of irradiation duration on the surface reorientation, determining that the maximum photoinduced isomerisation yield is ∼15%.