Xibin Gu

Untangling the chemical evolution of Titan's atmosphere and surface–from homogeneous to heterogeneous chemistry

In this article, we first explored the chemical dynamics of simple diatomic radicals (dicarbon, methylidyne) utilizing the crossed molecular beams method. This versatile experimental technique can be applied to study reactions relevant to the atmospheres of planets and their moons as long as intense and stable supersonic beam sources of the reactant species exist. By focusing on reactions of dicarbon with hydrogen cyanide, we untangled the contribution of dicarbon in its singlet ground and first excited triplet states. These results were applied to understand and re-analyze the data of crossed beam reactions of the isoelectronic dicarbon plus acetylene reaction. Further, we investigated the interaction of ionizing radiation in form of energetic electrons with organic molecules ethane and propane sequestered on Titans surface. These experiments presented compelling evidence that even at irradiation exposures equivalent to about 44 years on Titans surface, aliphatic like organic residues can be produced on Titans surface with thicknesses up to 1.5 m. Finally, we investigated how Titans nascent chemical inventory can be altered by an external influx of matter as supplied by (micro)meteorites and possibly comets. For this, we simulated the ablation process in Titans atmosphere, which can lead to ground and electronically excited atoms of, for instance, the principal constituents of silicates like iron, silicon, and magnesium, in laboratory experiments. By ablating silicon species and seeding the ablated species in acetylene carrier gas, which also acts as a reactant, we produced organo silicon species, which were then photoionized utilizing tunable VUV radiation from the Advanced Light Source. In combination with electronic structure calculations, the structures and ionization energies of distinct organo-silicon species were elucidated.

Reaction Dynamics of Phenyl Radicals in Extreme Environments: A Crossed Molecular Beam Study

Polycyclic aromatic hydrocarbons (PAHs)organic compounds that consist of fused benzene ringsand their hydrogen-deficient precursors have attracted extensive interest from combustion scientists, organic chemists, astronomers, and planetary scientists. On Earth, PAHs are toxic combustion products and a source of air pollution. In the interstellar medium, research suggests that PAHs play a role in unidentified infrared emission bands, diffuse interstellar bands, and the synthesis of precursor molecules to life. To build clean combustion devices and to understand the astrochemical evolution of the interstellar medium, it will be critical to understand the elementary reaction mechanisms under single collision conditions by which these molecules form in the gas phase. Until recently, this work had been hampered by the difficulty in preparing a large concentration of phenyl radicals, but the phenyl radical represents one of the most important radical species to trigger PAH formation in high-temperature environments. However, we have developed a method for producing these radical species and have undertaken a systematic experimental investigation. In this Account, we report on the chemical dynamics of the phenyl radical (C(6)H(5)) reactions with the unsaturated hydrocarbons acetylene (C(2)H(2)), ethylene (C(2)H(4)), methylacetylene (CH(3)CCH), allene (H(2)CCCH(2)), propylene (CH(3)CHCH(2)), and benzene (C(6)H(6)) utilizing the crossed molecular beams approach. For nonsymmetric reactants such as methylacetylene and propylene, steric effects and the larger cones of acceptance drive the addition of the phenyl radical to the nonsubstituted carbon atom of the hydrocarbon reactant. Reaction intermediates decomposed via atomic hydrogen loss pathways. In the phenyl-propylene system, the longer lifetime of the reaction intermediate yielded a more efficient energy randomization compared with the phenyl-methylacetylene system. Therefore, two reaction channels were open: hydrogen losses from the vinyl and from the methyl groups. All fragmentation pathways involved tight exit transition states. In the range of collision energies investigated, the reactions are dictated by phenyl radical addition-hydrogen atom elimination pathways. We did not observe ring closure processes with the benzene ring. Our investigations present an important step toward a systematic investigation of phenyl radical reactions under single collision conditions similar to those found in combustion flames and in high-temperature interstellar environments. Future experiments at lower collision energies may enhance the lifetimes of the reaction intermediates, which could open up competing ring closure channels to form bicyclic reaction products.

Characteristics and diagnostics of an ultrahigh vacuum compatible laser ablation source for crossed molecular beam experiments

We present the design and the characteristics of an ultrahigh vacuum compatible laser ablation source which can be operated in a crossed molecular beam machine at pressures as low as 10−9Torr. This unit provides intense supersonic beams of up to 3×1013cm−3 reactant species (carbon atoms, dicarbon, and tricarbon) in the interaction region of a crossed molecular beam machine. Practical delay time windows between the pulsed valve releasing the seeding gas and the laser have been determined to be between 16 630 and 16645μs, i.e., firing the laser 150–165μs after the 80μs pulsed valve trigger. The concentration of the dicarbon and tricarbon clusters can be maximized/minimized by adjusting the laser focus (a tighter laser focus leads to an enhanced atomic carbon component) and the laser power (multiphoton dissociation). We achieved stable peak velocities between 800 and 3800ms−1 and speed ratios S of up to 6.

A CROSSED MOLECULAR BEAM, LOW-TEMPERATURE KINETICS, AND THEORETICAL INVESTIGATION OF THE REACTION OF THE CYANO RADICAL (CN) WITH 1,3-BUTADIENE (C4H6). A ROUTE TO COMPLEX NITROGEN-BEARING MOLECULES IN LOW-TEMPERATURE EXTRATERRESTRIAL ENVIRONMENTS

We present a joint crossed molecular beam and kinetics investigation combined with electronic structure and statistical calculations on the reaction of the ground-state cyano radical, CN(X 2Σ+), with the 1,3-butadiene molecule, H2CCHCHCH2(X 1 A g), and its partially deuterated counterparts, H2CCDCDCH2(X 1 A g) and D2CCHCHCD2(X 1 A g). The crossed beam studies indicate that the reaction proceeds via a long-lived C5H6N complex, yielding C5H5N isomer(s) plus atomic hydrogen under single collision conditions as the nascent product(s). Experiments with the partially deuterated 1,3-butadienes indicate that the atomic hydrogen loss originates from one of the terminal carbon atoms of 1,3-butadiene. A combination of the experimental data with electronic structure calculations suggests that the thermodynamically less favorable 1-cyano-1,3-butadiene isomer represents the dominant reaction product; possible minor contributions of less than a few percent from the aromatic pyridine molecule might be feasible. Low-temperature kinetics studies demonstrate that the overall reaction is very fast from room temperature down to 23 K with rate coefficients close to the gas kinetic limit. This finding, combined with theoretical calculations, indicates that the reaction proceeds on an entrance barrier-less potential energy surface (PES). This combined experimental and theoretical approach represents an important step toward a systematic understanding of the formation of complex, nitrogen-bearing molecules--here on the C5H6N PES--in low-temperature extraterrestrial environments. These results are compared to the reaction dynamics of D1-ethynyl radicals (C2D; X 2Σ+) with 1,3-butadiene accessing the isoelectronic C6H7 surface as tackled earlier in our laboratories.

Design of a modular and versatile interlock system for ultrahigh vacuum machines: A crossed molecular beam setup as a case study

The design of a modular, versatile interlock system for ultrahigh vacuum machines is presented. This system can monitor the pressure (ultrahigh vacuum and high vacuum), the status of the power (power failure, power fluctuations, and scheduled power outages), the operation mode of the pumps (operation versus failure), the flow of cooling water, the humidity and temperature levels in the laboratory, as well as the concentration of toxic gases. If any of the set points is triggered, the vacuum machine is protected fully automatically. The interlock system is also interfaced to an automated paging system, thus transmitting a pager signal to the person on duty. Since the interlock system is modular in nature, it can be expanded and be adapted stepwise to incorporate additional safety and monitoring functions as needed.

Chemical dynamics of triacetylene formation and implications to the synthesis of polyynes in Titan's atmosphere

For the last four decades, the role of polyynes such as diacetylene (HCCCCH) and triacetylene (HCCCCCCH) in the chemical evolution of the atmosphere of Saturns moon Titan has been a subject of vigorous research. These polyacetylenes are thought to serve as an UV radiation shield in planetary environments; thus, acting as prebiotic ozone, and are considered as important constituents of the visible haze layers on Titan. However, the underlying chemical processes that initiate the formation and control the growth of polyynes have been the least understood to date. Here, we present a combined experimental, theoretical, and modeling study on the synthesis of the polyyne triacetylene (HCCCCCCH) via the bimolecular gas phase reaction of the ethynyl radical (CCH) with diacetylene (HCCCCH). This elementary reaction is rapid, has no entrance barrier, and yields the triacetylene molecule via indirect scattering dynamics through complex formation in a single collision event. Photochemical models of Titans atmosphere imply that triacetylene may serve as a building block to synthesize even more complex polyynes such as tetraacetylene (HCCCCCCCCH).

A CROSSED MOLECULAR BEAMS STUDY ON THE FORMATION OF THE EXOTIC CYANOETHYNYL RADICAL IN TITAN'S ATMOSPHERE

The reaction of the dicarbon molecule (C2) in its ^(1)Σ_(g) + electronic ground state with hydrogen cyanide HCN(X^(1)Σ^+) is investigated in a crossed molecular beam setup to untangle the formation of the cyanoethynyl radical CCCN(X^(2)Σ^+) in hydrocarbon-rich atmospheres of planets and their moons such as Titan. Combined with electronic structure and rate theory calculations, we show that this elementary reaction is rapid, has no entrance barriers, and yields CCCN via successive rearrangements of the initial HC_(3)N collision complex to the cyanoacetylene intermediate (HCCCN) followed by unimolecular decomposition of the latter without exit barrier. New photochemical models imply that this radical could serve as a key building block to form more complex molecules as observed in situ by the Cassini spacecraft, ultimately leading to organic aerosol particles, which make up the orange-brownish haze layers in Titans atmosphere.

A chemical dynamics, kinetics, and theoretical study on the reaction of the cyano radical (CN; X2Σ+) with phenylacetylene (C6H5CCH; X1A1)

The chemical reaction dynamics to form o-, m-, and p-cyanophenylacetylene via the neutral-neutral reaction of ground state cyano radicals with phenylacetylene and D(1)-phenylacetylene were investigated in crossed beam experiments; these studies were combined with kinetics measurements of the rate coefficients at temperatures of 123, 200, and 298 K and supplemented by electronic structure calculations. The data suggest that the reaction is initiated by a barrier-less addition of the electrophilic cyano radical to the o-, m-, or p-position of the aromatic ring. The eventually fragmented via atomic hydrogen elimination to form o-, m-, and p-cyanophenylacetylene via tight exit transition states with the hydrogen atom being ejected almost perpendicularly to the molecular plane of the rotating complex. The overall reaction to form o-, m-, and p-cyanophenylacetylene was found to be exoergic by 89 +/- 18 kJ mol(-1) in nice agreement with the calculations. The o-cyanophenylacetylene isomer is of particular relevance as a potential building block to the formation of nitrogen-substituted didehydronaphthalene molecules in analogy to didehydronaphthalene in Titans aerosol layers--a pathway hitherto neglected by the planetary science modeling community.

Reaction Dynamics on the Formation of 1- and 3-Cyanopropylene in the Crossed Beams Reaction of Ground-State Cyano Radicals (CN) with Propylene (C3H6) and Its Deuterated Isotopologues †

Crossed molecular beams experiments were utilized to explore the chemical reaction dynamics of ground-state cyano radicals, CN(X(2)Sigma(+)), with propylene (CH3CHCH2) together with two d3-isotopologues (CD3CHCH2, CH3CDCD2) as potential pathways to form organic nitriles under single collision conditions in the atmosphere of Saturns moon Titan and in the interstellar medium. On the basis of the center-of-mass translational and angular distributions, the reaction dynamics were deduced to be indirect and commenced via an addition of the electrophilic cyano radical with its radical center to the alpha-carbon atom of the propylene molecule yielding a doublet radical intermediate: CH3CHCH2CN. Crossed beam experiments with propylene-1,1,2-d3 (CH3CDCD2) and propylene-3,3,3-d3 (CD3CHCH2) indicated that the reaction intermediates CH3CDCD2CN (from propylene-1,1,2-d3) and CD3CHCH2CN (from propylene-3,3,3-d3) eject both atomic hydrogen through tight exit transition states located about 40-50 kJ mol(-1) above the separated products: 3-butenenitrile [H2CCDCD2CN] (25%), and cis/trans-2-butenenitrile [CD3CHCHCN] (75%), respectively, plus atomic hydrogen. Applications of our results to the chemical processing of cold molecular clouds like TMC-1 and OMC-1 are also presented.

A crossed beam and ab initio study of the reaction of atomic boron with ethylene.

The reaction of atomic boron, B(2P), with the simplest alkene, C2H4, has been investigated under single collision conditions in crossed beam experiments with mass spectrometric detection. Our experimental data clearly showed that the atomic boron versus hydrogen exchange reaction led to molecule(s) of gross formula C2H3B via bound intermediate(s). According to the experimentally derived fraction of the available energy released as product translational energy, we propose that an important reaction pathways is the one leading to the borirene plus atomic hydrogen and/or the one leading to ethynylborane plus atomic hydrogen. The experimental results are accompanied by electronic structure calculations of the relevant potential energy surface and RRKM estimates of the product branching ratio. According to RRKM calculations, within the limit of complete energy randomization, the three isomers borirene, BH=C=CH2 and BH2-CCH, are all formed, with BH2-CCH being the dominant one. The discrepancies between the trend of the product translational energy distributions and the picture emerging from RRKM estimates are a symptom that a statistical treatment is not warranted for this system.