Low-temperature synthesis of polycyclic aromatic hydrocarbons in Titan's surface ices and on airless bodies.

Titan's equatorial dunes represent the most monumental surface structures in our Solar System, but the chemical composition of their dark organics remains a fundamental, unsolved enigma, with solid acetylene detected near the dunes implicated as a key feedstock. Here, we reveal in laboratory simulation experiments that aromatics such as benzene, naphthalene, and phenanthrene-prospective building blocks of the organic dune material-can be efficiently synthesized via galactic cosmic ray exposure of low-temperature acetylene ices on Titan's surface, hence challenging conventional wisdom that aromatic hydrocarbons are formed solely in Titan's atmosphere. These processes are also of critical importance in unraveling the origin and chemical composition of the dark surfaces of airless bodies in the outer Solar System, where hydrocarbon precipitation from the atmosphere cannot occur.

Origin of alkylphosphonic acids in the interstellar medium.

For decades, the source of phosphorus incorporated into Earth's first organisms has remained a fundamental, unsolved puzzle. Although contemporary biomolecules incorporate P(+V) in their phosphate moieties, the limited bioavailability of phosphates led to the proposal that more soluble P(+III) compounds served as the initial source of phosphorus. Here, we report via laboratory simulation experiments that the three simplest alkylphosphonic acids, soluble organic phosphorus P(+III) compounds, can be efficiently generated in interstellar, phosphine-doped ices through interaction with galactic cosmic rays.

A vacuum ultraviolet photoionization study on the formation of methanimine (CHNH) and ethylenediamine (NHCHCHNH) in low temperature interstellar model ices exposed to ionizing radiation.

Methylamine (CH3NH2) and methanimine (CH2NH) represent essential building blocks in the formation of amino acids in interstellar and cometary ices. In our study, by exploiting isomer selective detection of the reaction products via photoionization coupled with reflectron time of flight mass spectrometry (Re-TOF-MS), we elucidate the formation of methanimine and ethylenediamine (NH2CH2CH2NH2) in methylamine ices exposed to energetic electrons as a proxy for secondary electrons generated by energetic cosmic rays penetrating interstellar and cometary ices. Interestingly, the two products methanimine and ethylenediamine are isoelectronic to formaldehyde (H2CO) and ethylene glycol (HOCH2CH2OH), respectively.

Synthesis of the hitherto elusive formylphosphine (HCOPH) in the interstellar medium - a molecule with an exotic phosphorus peptide bond.

The formylphosphine (HCOPH2) molecule was detected in the gas phase via isomer selective photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS). Synthesized in carbon monoxide (CO)-phosphine ices (PH3) exposed to ionizing radiation, the formation mechanism involves an initial phosphorus-hydrogen bond rupture in phosphine yielding the phosphino radical (PH2) along with atomic hydrogen, addition of the suprathermal hydrogen atom to carbon monoxide leading to the formyl radical (HCO), and recombination of both radicals to formylphosphine (HCOPH2). This molecule represents the isovalent counterpart of the ubiquitous interstellar formamide (HCONH2).

First identification of unstable phosphino formic acid (HPCOOH).

The hitherto elusive phosphino formic acid molecule (H2PCOOH) was detected for the first time in the gas phase. Theoretical calculations revealed an unexpected kinetic stability of H2PCOOH compared to the isovalent carbamic acid (H2NCOOH) although the replacement of a single nitrogen atom by phosphorus decreases the bond order from a partial double (-C[double bond, length as m-dash]N-) to a single (-C-P-) bond. This work provides a fundamental framework to explore the synthesis and stability of derivatives of carbonic acid (H2CO3), in which one or both hydroxyl groups (OH) are replaced by hydride moieties involving third row atoms.

XUV photodesorption of carbon cluster ions and ionic photofragments from a mixed methane-water ice.

The photochemical processing of a CH : DO 1 : 3.3 ice mixture adsorbed on an HOPG surface in the XUV regime was investigated using pulses obtained from the Free-electron LASer in Hamburg (FLASH) facility. Ice films were exposed to femtosecond pulses with a photon energy of hν = 40.

Free-electron laser induced processes in thin molecular ice.

Intermolecular reactions in and on icy films on silicate and carbonaceous grains constitute a major route for the formation of new molecular constituents in interstellar molecular clouds. In more diffuse regions and in protoplanetary discs, energetic radiation can trigger reaction routes far from thermal equilibrium. As an analog of interstellar ice-covered dust grains, highly-oriented pyrolytic graphite (HOPG) covered with D2O, NO, and H atoms is irradiated by ultrashort XUV pulses and the desorbing ionic and neutral products are analysed.

Site specificity in femtosecond laser desorption of neutral H atoms from graphite(0001).

Femtosecond laser excitation and density functional theory reveal site and vibrational state specificity in neutral atomic hydrogen desorption from graphite induced by multiple electronic transitions. Multimodal velocity distributions witness the participation of ortho and para pair states of chemisorbed hydrogen in the desorption process. Very slow velocities of 700 and 400  ms^{-1} for H and D atoms are associated with the desorption out of the highest vibrational state of a barrierless potential.