Pulsed ion deflection to overcome detector saturation in cryogenic ice sampling.

In 2014, we introduced a new experimental approach to study the UV photo-processing of cryogenic ices of astrophysical interest using laser ablation in a combination of ionization and time-of-flight mass spectrometry (ToF-MS). The setup, Mass Analytical Tool to Research Interstellar ICES, allowed us to detect newly formed species at low abundances. However, we found that with the increase in molecular complexity over the years, the detection of larger photoproducts was hindered by the dynamic range of detectors used. Here, we introduce a method to overcome this issue that we expect to be useful for similar applications in other research fields. The concept is based on a precisely controlled high-energy pulser that regulates the voltage across the deflection plates of the ToF-MS instrument to deflect the most abundant species and prevent them from reaching the detector. In this way, the detector sensitivity can be increased from an operating voltage of 2500 V up to 3000 V. The applicability is first illustrated in the simple case of an argon matrix, where 40Ar+ ions are deflected to increase the detection sensitivity for 40Ar2+ at m/z = 20 and 40Ar2+ at m/z = 80 by a factor 30. Similarly, it is shown that substantially larger complex organic molecules, an important species in astrochemical reaction networks, can be measured for UV irradiated methanol ice.

Reconciling spectroscopy with dynamics in global potential energy surfaces: The case of the astrophysically relevant SiC2.

SiC2 is a fascinating molecule due to its unusual bonding and astrophysical importance. In this work, we report the first global potential energy surface (PES) for ground-state SiC2 using the combined-hyperbolic-inverse-power-representation method and accurate ab initio energies. The calibration grid data are obtained via a general dual-level protocol developed afresh herein that entails both coupled-cluster and multi-reference configuration interaction energies jointly extrapolated to the complete basis set limit. Such an approach is specially devised to recover much of the spectroscopy from the PES, while still permitting a proper fragmentation of the system to allow for reaction dynamics studies. Besides describing accurately the valence strongly bound region that includes both the cyclic global minimum and isomerization barriers, the final analytic PES form is shown to properly reproduce dissociation energies, diatomic potentials, and long-range interactions at all asymptotic channels, in addition to naturally reflect the correct permutational symmetry of the potential. Bound vibrational state calculations have been carried out, unveiling an excellent match of the available experimental data on c-SiC2(A11). To further exploit the global nature of the PES, exploratory quasi-classical trajectory calculations for the endothermic C2 + Si → SiC + C reaction are also performed, yielding thermalized rate coefficients for temperatures up to 5000 K. The results hint for the prominence of this reaction in the innermost layers of the circumstellar envelopes around carbon-rich stars, hence conceivably playing therein a key contribution to the gas-phase formation of SiC, and eventually, solid SiC dust.

High-level ab initio quartic force fields and spectroscopic characterization of C2N.

While it is now well established that large carbon chain species and radiative electron attachment (REA) are key ingredients triggering interstellar anion chemistry, the role played by smaller molecular anions, for which REA appears to be an unlikely formation pathway, is as yet elusive. Advancing this research undoubtedly requires the knowledge (and modeling) of their astronomical abundances which, for the case of C2N-, is largely hindered by a lack of accurate spectroscopic signatures. In this work, we provide such data for both ground -CCN-(3Σ-) and low-lying c-CNC-(1A1) isomers and their singly-substituted isotopologues by means of state-of-the-art rovibrational quantum chemical techniques. Their quartic force fields are herein calibrated using a high-level composite energy scheme that accounts for extrapolations to both one-particle and (approximate) -particle basis set limits, in addition to relativistic effects, with the final forms being subsequently subject to nuclear motion calculations. Besides standard spectroscopic attributes, the full set of computed properties includes fine and hyperfine interaction constants and can be readily introduced as guesses in conventional experimental data reduction analyses through effective Hamiltonians. On the basis of benchmark calculations performed anew for a minimal test set of prototypical triatomics and limited (low-resolution) experimental data for -CCN-(3Σ-), the target accuracies are determined to be better than 0.1% of experiment for rotational constants and 0.3% for vibrational fundamentals. Apart from laboratory investigations, the results here presented are expected to also prompt future astronomical surveys on C2N-. To this end and using the theoretically-predicted spectroscopic constants, the rotational spectra of both -CCN-(3Σ-) and c-CNC-(1A1) are derived and their likely detectability in the interstellar medium is further explored in connection with working frequency ranges of powerful astronomical facilities. Our best theoretical estimate places c-CNC-(1A1) at about 15.3 kcal mol-1 above the ground-state -CCN-(3Σ-) species.

A cryogenic ice setup to simulate carbon atom reactions in interstellar ices.

The design, implementation, and performance of a customized carbon atom beam source for the purpose of investigating solid-state reaction routes in interstellar ices in molecular clouds are discussed. The source is integrated into an existing ultrahigh vacuum setup, SURFace REaction SImulation DEvice (SURFRESIDE2), which extends this double atom (H/D, O, and N) beamline apparatus with a third atom (C) beamline to a unique system that is fully suited to explore complex organic molecule solid-state formation under representative interstellar cloud conditions. The parameter space for this system is discussed, which includes the flux of the carbon atoms hitting the ice sample, their temperature, and the potential impact of temperature on ice reactions. Much effort has been put into constraining the beam size to within the limits of the sample size with the aim of reducing carbon pollution inside the setup. How the C-atom beam performs is quantitatively studied through the example experiment, C + 18O2, and supported by computationally derived activation barriers. The potential for this source to study the solid-state formation of interstellar complex organic molecules through C-atom reactions is discussed.

A multifunctional setup to record FTIR and UV-vis spectra of organic molecules and their photoproducts in astronomical ices.

This article describes a new, multi-functional, high-vacuum ice setup that allows to record the in situ and real-time spectra of vacuum UV (VUV)-irradiated non-volatile molecules embedded in a low-temperature (10 K) amorphous solid water environment. Three complementary diagnostic tools-UV-visible (UV-vis) and Fourier Transform Infrared (FTIR) spectroscopy and temperature-programmed desorption quadrupole mass spectrometry-can be used to simultaneously study the physical and chemical behavior of the organic molecules in the ice upon VUV irradiation. The setup is equipped with a temperature-controlled sublimation oven that enables the controlled homogeneous deposition of solid species such as amino acids, nucleobases, and polycyclic aromatic hydrocarbons (PAHs) in ice mixtures prepared from precursor gases and/or liquids. The resulting ice is photo-processed with a microwave discharge hydrogen lamp, generating VUV radiation with a spectral energy distribution representative for the interstellar medium. The characteristics, performance, and future potential of the system are discussed by describing three different applications. First, a new method is introduced, which uses broadband interference transmission fringes recorded during ice deposition, to determine the wavelength-dependent refractive index, nλ, of amorphous solid water. This approach is also applicable to other solids, pure and mixed. Second, the UV-vis and FTIR spectroscopy of an VUV-irradiated triphenylene:water ice mixture is discussed, monitoring the ionization efficiency of PAHs in interstellar ice environments. The third and final example investigates the stability of solid glycine upon VUV irradiation by monitoring the formation of dissociation products in real time.

Atomically resolved phase transition of fullerene cations solvated in helium droplets.

Helium has a unique phase diagram and below 25 bar it does not form a solid even at the lowest temperatures. Electrostriction leads to the formation of a solid layer of helium around charged impurities at much lower pressures in liquid and superfluid helium. These so-called 'Atkins snowballs' have been investigated for several simple ions. Here we form HenC60+ complexes with n exceeding 100 via electron ionization of helium nanodroplets doped with C60. Photofragmentation of these complexes is measured by merging a tunable narrow-bandwidth laser beam with the ions. A switch from red- to blueshift of the absorption frequency of HenC60+ on addition of He atoms at n=32 is associated with a phase transition in the attached helium layer from solid to partly liquid (melting of the Atkins snowball). Elaborate molecular dynamics simulations using a realistic force field and including quantum effects support this interpretation.

Methane ice photochemistry and kinetic study using laser desorption time-of-flight mass spectrometry at 20 K.

The ice photochemistry of pure methane (CH4) is studied at 20 K upon VUV irradiation from a microwave discharge H2 flow lamp. Laser Desorption Post-Ionization Time-Of-Flight Mass Spectrometry (LDPI TOF-MS) is used for the first time to determine branching ratios of primary reactions leading to CH3, CH2, and CH radicals, typically for fluences as expected in space. This study is based on a stable end-products analysis and the mass spectra are interpreted using an appropriate set of coupled reactions and rate constants. This yields clearly different values from previous gas phase studies. The matrix environment as well as the higher efficiency of reverse reactions in the ice clearly favor CH3 radical formation as the main first generation photoproduct.

Laser desorption time-of-flight mass spectrometry of ultraviolet photo-processed ices.

A new ultra-high vacuum experiment is described that allows studying photo-induced chemical processes in interstellar ice analogues. MATRI(2)CES - a Mass Analytical Tool to study Reactions in Interstellar ICES applies a new concept by combining laser desorption and time-of-flight mass spectrometry with the ultimate goal to characterize in situ and in real time the solid state evolution of organic compounds upon UV photolysis for astronomically relevant ice mixtures and temperatures. The performance of the experimental setup is demonstrated by the kinetic analysis of the different photoproducts of pure methane (CH4) ice at 20 K. A quantitative approach provides formation yields of several new species with up to four carbon atoms. Convincing evidence is found for the formation of even larger species. Typical mass resolutions obtained range from M/ΔM ∼320 to ∼400 for CH4 and argon, respectively. Additional tests show that the typical detection limit (in monolayers) is ⩽0.02 ML, substantially more sensitive than the regular techniques used to investigate chemical processes in interstellar ices.

Wavelength resolved UV photodesorption and photochemistry of CO2 ice.

Over the last four years we have illustrated the potential of a novel wavelength-dependent approach in determining molecular processes at work in the photodesorption of interstellar ice analogs. This method, utilizing the unique beam characteristics of the vacuum UV beamline DESIRS at the French synchrotron facility SOLEIL has revealed an efficient indirect desorption mechanism that scales with the electronic excitations in molecular solids. This process, known as DIET--desorption induced by electronic transition--occurs efficiently in ices composed of very volatile species (CO, N2), for which photochemical processes can be neglected. In the present study, we investigate the photodesorption energy dependence of pure and pre-irradiated CO2 ices at 10-40 K and between 7 and 14 eV. The photodesorption from pure CO2 is limited to photon energies above 10.5 eV and is clearly initiated by CO2 excitation and by the contribution of dissociative and recombination channels. The photodesorption from "pre-irradiated" ices is shown to present an efficient additional desorption pathway below 10 eV, dominating the desorption depending on the UV-processing history of the ice film. This effect is identified as an indirect DIET process mediated by photoproduced CO, observed for the first time in the case of less volatile species. The results presented here pinpoint the importance of the interconnection between photodesorption and photochemical processes in interstellar ices driven by UV photons having different energies.

Solid state chemistry of nitrogen oxides--part I: surface consumption of NO.

The role of nitrogen and oxygen chemistry in the interstellar medium is still rather poorly understood. Nitric oxide, NO, has been proposed as an important precursor in the formation of larger N- and O-bearing species, such as hydroxylamine, NH2OH, and nitrogen oxides, NO2 and N2O. The topic of this study is the solid state consumption of NO via oxygenation and the formation of NO2 and other nitrogen oxides (ONNO2 and N2O4) under conditions close to those encountered on icy grains in quiescent interstellar clouds. In our experiments nitric oxide and oxygen allotropes (O, O2, and O3) or N atoms are co-deposited under ultra-high vacuum conditions on different substrates (silicate, graphite, compact ASW ice, and gold) at temperatures ranging between 10 and 35 K. Reaction products are monitored via Fourier Transform Reflection Absorption Infrared Spectroscopy (FT-RAIRS) and Temperature Programmed Desorption (TPD) using mass spectrometry. We find that NO2 is efficiently formed in NO + O/O2/O3/N solid surface reactions. These are essentially barrier free and offer a pathway for the formation of NO2 in space. Nitrogen dioxide, however, has not been astronomically detected, contradicting the efficient reaction channel found here. This is likely due to other pathways, including regular hydrogenation reactions, as discussed separately in part II of this study.

Solid state chemistry of nitrogen oxides--part II: surface consumption of NO2.

Nitrogen oxides are considered to be important astrochemical precursors of complex species and prebiotics. However, apart from the hydrogenation of solid NO that leads to the surface formation of hydroxylamine, little is known about the full solid state reaction network involving both nitrogen and oxygen. Our study is divided into two papers, hereby called Part I and Part II. In the accompanying paper, we investigate the surface reactions NO + O/O2/O3 and NO + N with a focus on the formation of NO2 ice. Here, we complement this study by measurements of the surface destruction of solid NO2, e.g., NO2 + H/O/N. Experiments are performed in two separate ultra-high vacuum setups and therefore under different experimental conditions to better constrain the experimental results. Surface reaction products are monitored by means of Fourier Transform Reflection Absorption Infrared Spectroscopy (FT-RAIRS) and Temperature Programmed Desorption (TPD) techniques using mass spectrometry. The surface destruction of solid NO2 leads to the formation of a series of nitrogen oxides such as NO, N2O, N2O3, and N2O4 as well as HNO, NH2OH, and H2O. When NO2 is mixed with an interstellar more relevant apolar (i.e., CO) ice, solid CO2 and HCOOH are also formed due to interactions between different reaction routes. The astrophysical implications of the full nitrogen and oxygen reaction network derived from Parts I and II are discussed.

Porosity and thermal collapse measurements of H2O, CH3OH, CO2, and H2O:CO2 ices.

The majority of astronomical and laboratory based studies of interstellar ices have been focusing on ice constituents. Ice structure is a much less studied topic. Particularly the amount of porosity is an ongoing point of discussion. A porous ice offers more surface area than a compact ice, for reactions that are fully surface driven. In this paper we discuss the amount of compaction for four different ices--H2O, CH3OH, CO2 and mixed H2O : CO2 = 2 : 1--upon heating over an astronomically relevant temperature regime. Laser interference and Fourier transform infrared spectroscopy are used to confirm that for amorphous solid water the full signal loss of dangling OH bonds is not a proof for full compaction. These data are compared with the first compaction results for pure CH3OH, pure CO2 and mixed H2O : CO2 = 2 : 1 ice. Here we find that thermal segregation benefits from a higher degree of porosity.

SURFRESIDE(2): an ultrahigh vacuum system for the investigation of surface reaction routes of interstellar interest.

A new ultrahigh vacuum experiment is described to study atom and radical addition reactions in interstellar ice analogues for astronomically relevant temperatures. The new setup - SURFace REaction SImulation DEvice (SURFRESIDE(2)) - allows a systematic investigation of solid state pathways resulting in the formation of molecules of astrophysical interest. The implementation of a double beam line makes it possible to expose deposited ice molecules to different atoms and/or radicals sequentially or at the same time. Special efforts are made to perform experiments under fully controlled laboratory conditions, including precise atom flux determinations, in order to characterize reaction channels quantitatively. In this way, we can compare and combine different surface reaction channels with the aim to unravel the solid state processes at play in space. Results are constrained in situ by means of a Fourier transform infrared spectrometer and a quadrupole mass spectrometer using reflection absorption infrared spectroscopy and temperature programmed desorption, respectively. The performance of the new setup is demonstrated on the example of carbon dioxide formation by comparing the efficiency through two different solid state channels (CO + OH → CO2 + H and CO + O → CO2) for which different addition products are needed. The potential of SURFRESIDE(2) to study complex molecule formation, including nitrogen containing (prebiotic) compounds, is discussed.

Efficient surface formation route of interstellar hydroxylamine through NO hydrogenation. II. The multilayer regime in interstellar relevant ices.

Hydroxylamine (NH(2)OH) is one of the potential precursors of complex pre-biotic species in space. Here, we present a detailed experimental study of hydroxylamine formation through nitric oxide (NO) surface hydrogenation for astronomically relevant conditions. The aim of this work is to investigate hydroxylamine formation efficiencies in polar (water-rich) and non-polar (carbon monoxide-rich) interstellar ice analogues. A complex reaction network involving both final (N(2)O, NH(2)OH) and intermediate (HNO, NH(2)O·, etc.) products is discussed. The main conclusion is that hydroxyl-amine formation takes place via a fast and barrierless mechanism and it is found to be even more abundantly formed in a water-rich environment at lower temperatures. In parallel, we experimentally verify the non-formation of hydroxylamine upon UV photolysis of NO ice at cryogenic temperatures as well as the non-detection of NC- and NCO-bond bearing species after UV processing of NO in carbon monoxide-rich ices. Our results are implemented into an astrochemical reaction model, which shows that NH(2)OH is abundant in the solid phase under dark molecular cloud conditions. Once NH(2)OH desorbs from the ice grains, it becomes available to form more complex species (e.g., glycine and ß-alanine) in gas phase reaction schemes.

Water formation by surface O3 hydrogenation.

Three solid state formation routes have been proposed in the past to explain the observed abundance of water in space: the hydrogenation reaction channels of atomic oxygen (O + H), molecular oxygen (O(2) + H), and ozone (O(3) + H). New data are presented here for the third scheme with a focus on the reactions O(3) + H, OH + H and OH + H(2), which were difficult to quantify in previous studies. A comprehensive set of H/D-atom addition experiments is presented for astronomically relevant temperatures. Starting from the hydrogenation/deuteration of solid O(3) ice, we find experimental evidence for H(2)O/D(2)O (and H(2)O(2)/D(2)O(2)) ice formation using reflection absorption infrared spectroscopy. The temperature and H/D-atom flux dependence are studied and this provides information on the mobility of ozone within the ice and possible isotope effects in the reaction scheme. The experiments show that the O(3) + H channel takes place through stages that interact with the O and O(2) hydrogenation reaction schemes. It is also found that the reaction OH + H(2) (OH + H), as an intermediate step, plays a prominent (less efficient) role. The main conclusion is that solid O(3) hydrogenation offers a potential reaction channel for the formation of water in space. Moreover, the nondetection of solid ozone in dense molecular clouds is consistent with the astrophysical picture in which O(3) + H is an efficient process under interstellar conditions.

Water formation at low temperatures by surface O2 hydrogenation I: Characterization of ice penetration.

Water is the main component of interstellar ice mantles, is abundant in the solar system and is a crucial ingredient for life. The formation of this molecule in the interstellar medium cannot be explained by gas-phase chemistry only and its surface hydrogenation formation routes at low temperatures (O, O(2), O(3) channels) are still unclear and most likely incomplete. In a previous paper we discussed an unexpected zeroth-order H(2)O production behavior in O(2) ice hydrogenation experiments compared to the first-order H(2)CO and CH(3)OH production behavior found in former studies on hydrogenation of CO ice. In this paper we experimentally investigate in detail how the structure of O(2) ice leads to this rare behavior in reaction order and production yield. In our experiments H atoms are added to a thick O(2) ice under fully controlled conditions, while the changes are followed by means of reflection absorption infrared spectroscopy (RAIRS). The H-atom penetration mechanism is systematically studied by varying the temperature, thickness and structure of the O(2) ice. We conclude that the competition between reaction and diffusion of the H atoms into the O(2) ice explains the unexpected H(2)O and H(2)O(2) formation behavior. In addition, we show that the proposed O(2) hydrogenation scheme is incomplete, suggesting that additional surface reactions should be considered. Indeed, the detection of newly formed O(3) in the ice upon H-atom exposure proves that the O(2) channel is not an isolated route. Furthermore, the addition of H(2) molecules is found not to have a measurable effect on the O(2) reaction channel.

Water formation at low temperatures by surface O2 hydrogenation II: The reaction network.

Water is abundantly present in the Universe. It is the main component of interstellar ice mantles and a key ingredient for life. Water in space is mainly formed through surface reactions. Three formation routes have been proposed in the past: hydrogenation of surface O, O(2), and O(3). In a previous paper [Ioppolo et al., Astrophys. J., 2008, 686, 1474] we discussed an unexpected non-standard zeroth-order H(2)O(2) production behaviour in O(2) hydrogenation experiments, which suggests that the proposed reaction network is not complete, and that the reaction channels are probably more interconnected than previously thought. In this paper we aim to derive the full reaction scheme for O(2) surface hydrogenation and to constrain the rates of the individual reactions. This is achieved through simultaneous H-atom and O(2) deposition under ultra-high vacuum conditions for astronomically relevant temperatures. Different H/O(2) ratios are used to trace different stages in the hydrogenation network. The chemical changes in the forming ice are followed by means of reflection absorption infrared spectroscopy (RAIRS). New reaction paths are revealed as compared to previous experiments. Several reaction steps prove to be much more efficient (H + O(2)) or less efficient (H + OH and H(2) + OH) than originally thought. These are the main conclusions of this work and the extended network concluded here will have profound implications for models that describe the formation of water in space.

A zero-gravity instrument to study low velocity collisions of fragile particles at low temperatures.

We discuss the design, operation, and performance of a vacuum setup constructed for use in zero (or reduced) gravity conditions to initiate collisions of fragile millimeter-sized particles at low velocity and temperature. Such particles are typically found in many astronomical settings and in regions of planet formation. The instrument has participated in four parabolic flight campaigns to date, operating for a total of 2.4 h in reduced-gravity conditions and successfully recording over 300 separate collisions of loosely packed dust aggregates and ice samples. The imparted particle velocities achieved range from 0.03 to 0.28 m s(-1) and a high-speed, high-resolution camera captures the events at 107 frames/s from two viewing angles separated by either 48.8 degrees or 60.0 degrees. The particles can be stored inside the experiment vacuum chamber at temperatures of 80-300 K for several uninterrupted hours using a built-in thermal accumulation system. The copper structure allows cooling down to cryogenic temperatures before commencement of the experiments. Throughout the parabolic flight campaigns, add-ons and modifications have been made, illustrating the instrument flexibility in the study of small particle collisions.

Fermi interaction between the nu1 and the nu2+4 nu(s) bands of Ar...DN2(+).

Two rotationally fully resolved vibrational bands have been assigned unambiguously to the linear deuteron bound Ar...DN(2) (+) complex by using ground state combination differences. The ionic complex is formed in a supersonic planar plasma expansion optimized and controlled by a mass spectrometer and is detected in direct absorption using tunable diode lasers and applying production modulation spectroscopy. The band origins are located at 2436.272 cm(-1) and at 2435.932 cm(-1) and correspond to the nu(1) band (NN stretch) and to the nu(2)+4 nu(s) combination band (DN and intermolecular stretch), respectively. The two bands overlap strongly and the large intensity of the combination band is explained in terms of a Fermi interaction. This interaction perturbs the observed transitions, particularly for low J values. Least-squares fitting yields values for the Fermi interaction parameters of F(0)=0.332 cm(-1) and F(J)=-0.001 46 cm(-1) and results in accurate rotational constants. These are discussed both from an experimental and a theoretical point of view.

Differential cross sections for collisions of hexapole state-selected NO with He.

The first measurements of differential inelastic collision cross sections of fully state-selected NO (j=12, Omega=12, epsilon= -1) with He are presented. Full state selection is achieved by a 2 m long hexapole, which allows for a systematic study of the effect of parity conservation and breaking on the differential cross section. The collisionally excited NO molecules are detected using a resonant (1+1') REMPI ionization scheme in combination with the velocity-mapped, ion-imaging technique. The current experimental configuration minimizes the contribution of noncolliding NO molecules in other rotational states j, Omega, epsilon--that contaminates images--and allows for study of the collision process at an unprecedented level of detail. A simple method to correct ion images for collision-induced alignment is presented as well and its performance is demonstrated. The present results show a significant difference between differential cross sections for scattering into the upper and lower component of the Lambda-doublet of NO. This result cannot be due to the energy splitting between these components.