13636 | Chem. Commun., 2014, 50, 13636--13644 This journal is©The Royal Society of Chemistry 2014

Cite this:Chem. Commun., 2014,

50, 13636

Highlights from Faraday Discussion 168:Astrochemistry of Dust, Ice and Gas, Leiden,The Netherlands, April 2014

John D. Thrower,*a Sergio Ioppolobc and Catherine Walshd

The morning of the 7th April 2014 sawthe arrival of around 120 astrochemistsfrom around the globe in the quiet SouthHolland town of Noordwijkerhout, justoutside Leiden. The delegates for FaradayDiscussion 168 were treated to a premierview of Holland’s famous tulip fieldsen route via coach from Schiphol airport(Fig. 1). Hosted at the NH ConferenceCentre Leeuwenhorst, FD168 was the5th Faraday Discussion in the field ofastrochemistry, following from previousdiscussions on the Chemistry of the Inter-stellar Medium in 1992, Chemistry andPhysics of Molecules and Grains in Space in1998, Chemical Evolution of the Universe in2005 and the Chemistry of Planets in 2010.Faraday Discussions have a legacy goingback 100 years and have covered a widerange of topics in physical chemistry.The field of astrochemistry is an excel-lent example of an interdisciplinary field:it brings together astronomers, chemists,and physicists working on observations,theory, and experiments to advance our

understanding of the unique and exoticchemistry that occurs in space .

The weather on arrival was better thanexpected for this typically wet, yet lush,region of the Netherlands. Upon arrival,attendees were efficiently registered andpresented with their conference materials,including a small taste of the Netherlandsin the form of a pack of stroopwafels.Scientific discussions began in earnestover a buffet lunch adjacent to the mainauditorium, before the official openingand welcome by Martin McCoustra (Heriot-Watt University, Edinburgh), the chairof the scientific committee comprisingWendy Brown (University of Sussex), RobinGarrod (Cornell University), HaroldLinnartz (Leiden University), Anthony Meijer

(University of Sheffield), and Tom Millar(Queen’s University Belfast). In additionto introducing the 10 invited speakers,he outlined the unique form of FaradayDiscussion meetings, which is somewhatdifferent to the majority of scientificmeetings. Speakers are asked to preparea paper on their research several monthsprior to the meeting. These are thenpeer-reviewed and the preprints madeavailable to the delegates several weeksahead of the discussion, allowing forpre-digestion of the material. Speakershave a strictly enforced – using the famousFaraday traffic lights – 5 minute slot topresent the key points of each paper, withextended discussions dominating theprogramme. Importantly, the discussions

Fig. 1 FD168 delegates were greeted with views of the famous Dutch bulb fields (Credit: S. Ioppolo).

a Physikalisches Institut, Westfalische

Wilhelms-Universitat, D-48149 Munster, Germany.

E-mail: john.thrower@uni-muenster.deb Division of Geological and Planetary Sciences,

California Institute of Technology,

1200 E. California Blvd., Pasadena, USAc Institute of Molecules and Materials,

Radboud University Nijmegen, P.O. Box 9010,

6500 GL Nijmegen, The Netherlandsd Leiden Observatory, Leiden University, P.O. Box 9513,

2300 RA Leiden, The Netherlands

DOI: 10.1039/c4cc90397h

www.rsc.org/chemcomm

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are recorded and form an integral part ofthe published discussion volume, becomingciteable literature in their own right.

A 50 minute introductory lecture onthe general topic of astrochemistry wasgiven by Ewine van Dishoeck (LeidenUniversity). This comprehensive lecturehelped to ‘set the scene’ for the remainderof the meeting and highlighted recentobservational advances, including resultsfrom the Atacama Large Millimeter/Submillimeter Array (ALMA) and theHerschel Space Observatory. Advances intheoretical and laboratory astrochemistryare very much driven by new observa-tions from state-of-the-art telescopes.

The opening session on ‘‘Observationson dust, ice and gas relevant to astro-chemistry’’ was chaired by Tom Millar(Queens University Belfast). KlausPontoppidan (Space Telescope ScienceInstitute, Baltimore) kicked off the pre-sentations by describing a new methodfor deriving molecular abundances throughobservations of molecular emission linesfrom protoplanetary disks. Protoplanetarydisks are remnant tori of dust and gasaround young stars within which planetsform. The ‘planet-forming’ region of disksis best probed via mid-infrared observa-tions using space-based facilities such asthe Spitzer Space Telescope. Traditionally,these observations are analysed using asimple slab model, assuming a singletemperature and density. In reality, diskshave strong temperature and densitygradients which can influence the mole-cular line emission. Pontoppidan andBlevins1 propose a more complex ‘retrieval’method in which a large parameter spaceis investigated in parallel with sophisti-cated radiative transfer. Their preliminaryresults and comparison with Spitzer datashow the model’s potential to constrain themolecular concentration and distributionwithin the planet-forming zone.

The following paper was presented byKarin Oberg (Harvard University). Oberget al.2 present new Submillimeter Array(SMA) observations of line emission fromcomplex organic molecules (COMs) towardsseveral massive young stellar objects(MYSOs). MYSOs are objects which willevolve to form massive stars or clusters ofstars. They typically have very complexspectra with a high density of spectral lines,

with contributions from many complexmolecules. The formation of complexmolecules in interstellar and circumstellarenvironments is a current ‘hot topic’ inastrochemistry, and is thought to occuron and within ice mantles on the sur-faces of dust grains. Oberg et al. mappedthe molecular line emission acrossseveral sources from COMs includingpropyne (CH3CCH), acetonitrile (CH3CN)and acetaldehyde (CH3CHO) with thegoal of looking for trends in molecularexcitation and distribution with tempera-ture (see Fig. 2). COMs such as CH3CCHand CH3CHO can form at low temperatureswith CH3CHO also having a formationroute at higher temperatures. The observa-tions support temperature-activated chemi-stry as a route to several complex organicmolecules allowing their use as a potentialevolutionary tracer of MYSOs.

Viviana Guzman (Institut de Radio-astronomie Millimetrique – IRAM) sub-sequently presented some excitingdetections of COMs in a photon-dominated region (PDR) for the firsttime.3 The observed species include formicacid (HCOOH), ketene (CH2CO), acetalde-hyde (CH3CHO) and propyne (CH3CCH).PDRs form the interface between darkdense molecular clouds and the irradiatedinterstellar medium, and as such are

considered to be relatively harsh environ-ments. That gas-phase COMs are able toform and survive in PDRs is of astro-chemical significance because it suggeststhat radiation processing of ice mantlesis important for building chemical com-plexity, verifying a result that has beenknown from laboratory experiments forsome decades.

In the closing paper of the day, TedBergin (University of Michigan) movedthe discussion on to more ‘local’ mattersby presenting a paper on the origin ofcarbon in terrestrial planets. Carbon isan element of extreme astrobiologicalsignificance and provides the ‘backbone’for many of the molecules necessary forlife. Bergin et al.4 discuss the variousobservational constraints on the carbonbudget in star-forming regions and extra-solar systems in comparison with theknown carbon content in the Solar System(including the Sun, the Earth, comets,and chondritic meteorites). They suggestthat much of the carbon in the pre-solarnebula (the protoplanetary disk fromwhich the Solar System formed) may havebeen in volatile form, explaining the lackof solid and refractory carbon in thecurrent Solar System. However, observationsof line emission from C18O and HD fromour nearest early Solar System analogue,

Fig. 2 Oberg et al. compare column density ratios of various species for a range of sources inthe literature (black) and several Massive Young Stellar Objects (MYSOs; red). The observed trendsindicate that complex organic formation is triggered in MYSOs as they heat up.2

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TW Hydrae, indicate that CO is not themain carbon reservoir in this particulardisk. The authors present new modelsinvestigating mechanisms for the extrac-tion of carbon from CO. They postulatethat the destruction of gas-phase CO viareaction with He+ ions creates C+ ions,which are very reactive and lead to anincrease in other carbon reservoirs, suchas complex molecules. This can onlyoccur where CO is not in ice form; hencethe carbon budget in comets and othericy planetesimals is preserved.

The first day of the conference alsohosted the first discussion session, lead-ing to lively debates on the presentedpapers. Several comments and questionsconcerning the opening lecture by Ewinevan Dishoeck arose with suggestions fornew target species and reactions for theexperimental and computational chemistrycommunity, in advance of observationalresults from new facilities (e.g., ALMA andJWST). Prof. van Dishoeck also highlightedthe need for ‘‘back to basics’’ models tostudy the role of key reactions upon whichProf. Meuwly commented on the nowrealistic use of sophisticated computa-tional methods to study reaction kineticsof simple systems. Klaus Pontoppidanwas the next to receive comments andquestions. Several in the audience wereinterested in the low CO2 abundancesderived using their protoplanetary diskobservations. Prof. Kamp suggested includ-ing a vertical abundance profile in addi-tional to a radial profile and also postulatedthat a significant molecular reservoir maybe hidden by dust. There was much interestin Ted Bergin’s paper with comments anddiscussion concerning the different carbonreservoirs in protoplanetary disks. In parti-cular, Prof. Linnartz highlighted the impor-tance of PAHs, both as a carbon reservoirand as a source of carbon for forming moresimple molecules, the so-called ‘‘top-down’’approach, a sentiment echoed by manyothers in the audience. The chemistry ofCOMs was also extensively discussed, bothin relation to carbon budget (B10�5 withrespect to H2 as estimated by Karin Oberg)and potential formation mechanisms.Dr Ellinger was the first to comment onthe importance of chemical (reactive)desorption given the exothermicity ofgrain-surface reactions, a subject that

was to be revisited over the coming days.Prof. Millar commented on the use ofisotopologues to constrain the differentpathways to COM formation. On the com-parison between models and observations,Karin Oberg stressed the need for largerobservational sample sizes (sources andspecies) and Dr Garrod also commentedon the need for well-constrained physicalmodels for meaningful comparisons. Thefirst detection of COMs in a PDR alsoprompted some discussion. Dr Cuppenand Dr Fayolle commented on the recentexperimental results on methanol photo-desorption which suggest that radicalsand ‘‘daughter’’ species are preferentiallyejected into the gas; however, Profsvan Dishoeck and Fillion remarked onthe importance of indirect desorption viaeither a ‘‘kick-out’’ mechanism or viaelectronic excitation of CO molecules. Thefirst discussion, on the whole, highlightedmany important scientific questions thatthe observational, modelling, and laboratorycommunities can realistically work towardsanswering in the coming years.

Immediately following the discussionsession, delegates made their way to theposter session. The number of posters,at 73, was impressive, allowing almostevery delegate to contribute. Many aspectsof astrochemistry, including gas phasechemistry, observations, modelling, surfacereactions and interstellar ices, were repre-sented, and stimulating discussions conti-nued over dinner.

Tuesday morning was devoted to thediscussion of ‘‘Laboratory astrochemistryof dust and ice’’ introduced by the sessionchair Wendy Brown (University of Sussex).Laboratory work is a fervid branch ofthe field and the last few decades havewitnessed a blooming of new and moresophisticated laboratory techniques whichhave allowed the investigation of inter-stellar ice formation under fully controlledconditions. However, to date, many of thesurface routes linked to the formation ofinterstellar COMs have not yet been veri-fied experimentally. Even simple processessuch as diffusion rates of reactants on andwithin an ice remain largely unquantified.Emanuele Congiu (LERMA: Universitiesof Paris and Cergy-Pontoise) opened thediscussion showing their latest results onthe efficient diffusion of O atoms at low

temperatures on water ice, silicates, andgraphite.5 They report an unexpectedcorrelation between ordered surfaces andefficient diffusion of O atoms. In all cases,the observed diffusion rates are larger thanpreviously predicted, suggesting thatO atoms can diffuse by quantum mecha-nical tunnelling at temperatures as lowas 6.5 K. The latter result has importantimplications in astrochemistry, since thereis a point in the evolution of molecularclouds where H and O atoms have compar-able abundances because of the efficientconversion of atomic H to molecular H2.If O atoms are mobile on grain surfaces atlow temperatures, then oxygen additionreactions can potentially play an importantrole in the synthesis of large organicmolecules.

Appropriately, the next paper waspresented by Stephen Price (UniversityCollege London) and considered thesingle and double addition of O atomsto propyne (CH3CCH) ice.6 The authorsreport the efficient formation of propenal(CH2CHCHO) at temperatures below 100 K.Price et al. show that, as the surfacetemperature decreases, the reaction yieldincreases to reach a maximum at 50 Kand then a minimum at 30 K. Boththe Langmuir–Hinshelwood (where bothreactive species are initially adsorbedand thermalized on the surface) and theEley–Rideal (where a thermalized surfacemolecule reacts with an incoming, andpotentially energetic, species from thegas phase) mechanisms are consideredin their model to explain the observedexperimental trend. They derive an acti-vation energy of B160 K for the reaction,CH3CCH + O. At surface temperaturesbelow 30 K, the yield of reaction risesagain indicating an alternative (barrierless)reaction mechanism, active only at lowtemperatures. Finally, Price and colleaguesalso observe the formation of an additionalsurface product (C3H4O2) corresponding tothe barrierless addition of one O atom topropenal.

The third paper, presented by YasuhiroOba (Hokkaido University) described adetailed work on the reaction kineticsand isotope effect of water formationthrough the hydrogenation/deuteration ofH2O2/D2O2 ice.7 Water is the most abun-dant species in interstellar ices and its

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formation proceeds through the hydro-genation of O/O2/O3 at low temperatures.Hydrogen peroxide is an intermediateproduct of the hydrogenation of mole-cular oxygen ice. The authors formed alayer of pure H2O2 by co-depositing O2

and H at 45–50 K. These experimentsallowed them to compare the efficiencyof the reactions H2O2 + H and H2O2 + Dat 10–30 K. Despite the large activationbarriers (B2000 K), both reactions pro-ceed at low temperatures with the reac-tion rate for H atoms determined to be45 times faster than that for D atoms at15 K. This large isotope effect indicatesthat the reactions proceed via quantummechanical tunnelling.

Markus Meuwly (University of Basel)introduced the final paper of the morningsession which provided a complementarytheoretical discussion concerning thediffusion of oxygen atoms during waterformation in amorphous interstellar ices.8

Their work supports the experimentalresults of Congiu and co-authors on thediffusion of O atoms at low temperatures.The authors present classical moleculardynamics (MD) simulations of O-atommigration in amorphous water ice thatuse physically motivated force fields.Their calculations show that hopping ofO atoms occurs on the sub-nanosecondtime scale at temperatures below 200 K(see Fig. 3) and yield estimates for diffusion

rates of B5 to 15 ns�1 and barrier heightsof B500 K or less. Therefore, this workfurther supports the hypothesis that oxygen-driven chemistry within ice mantles on thesurfaces of dust grains may play a moreimportant role in interstellar environmentsthan previously assumed.

After a short break, the subject of atom–grain interactions was extended to highertemperature environments (e.g. photondominated regions; PDRs) where atomscan form chemical bonds with, forexample, carbonaceous grains and mole-cules such as polycylic aromatic hydro-carbons (PAHs). Liv Hornekær (AarhusUniversity) described her group’s recentexperimental studies of H-atom additionto PAH molecules.9 PAHs are thought toaccount for more than 10% of the carbonbudget and are responsible for infraredemission features observed along manylines of sight in the ISM. Through thermaldesorption and mass spectrometricmeasurements, Skov et al. show evidencefor the formation of the fully hydro-genated state of coronene upon H-atomaddition to C24H12 and for abstractionreactions that lead to H2 formation. Theauthors also discuss a series of kineticsimulations aimed at extracting reactioncross-sections for both addition andabstraction reactions.

Nigel Mason (Open University, MiltonKeynes) discussed the important role played

by secondary electrons, generated by thepassage of primary cosmic ray particlesthrough interstellar ices, in initiatingchemical transformations within the ices.10

Experiments in which model interstellarices are exposed to a beam of low energyelectrons can help to elucidate the reactionpathways initiated by this cascade. In thepaper by Mason et al., the irradiation ofmethanol and methanol–ammonia mixedices is used to exemplify this rich chemi-stry. Formed though the hydrogenation ofCO, CH3OH is a key carbon containingcomponent of interstellar ices and oftenconsidered the starting point for theformation of more complex organicspecies. CO and CO2 are observed to beformed during electron irradiation ofmethanol, while NH2 radicals are generatedfrom ammonia. Reactions between reactivespecies formed in this way can then go onto form more complex species. Also ofinterest was the discussion of the dissocia-tive electron attachment (DEA) mechanismthrough which reactive intermediatescan form efficiently at very low electronenergies, even below the dissociationenergy of the molecule.

Continuing on this theme, ChristopherArumainayagam (Wellesley College) reportedon a mass spectrometric study of thereaction products formed through the lowenergy electron irradiation of condensedmethanol (CH3OH).11 In these elegantexperiments, his group heated lowenergy electron irradiated methanol ices,detecting desorbing reaction productsusing mass spectrometry; this revealedthe presence of eleven distinct products.These range in complexity from form-aldehyde (H2CO) to 1,2,3-propanetriol(HOCH2CHOHCH2OH). The richness ofthe chemistry derived from this relativelysimple species highlights the importanceof detailed laboratory measurements inunravelling the complexities of radiationdriven ice processing.

To close the session, Belen Mate(Instituto de Estructura de la Materia)compared low energy electron and UVphoton irradiation for two specific cases:(a) the dehydrogenation of hydrogenatedamorphous carbon (HAC) dust surfaces,and (b) the destruction of glycine.12

It remains unclear whether prebioticspecies potentially formed in the ISM,

Fig. 3 Lee et al. use molecular dynamics (MD) simulations to investigate the mobility of oxygen atomsin amorphous water as a function of temperature.8

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such as simple amino acids, survive for asufficiently long time to become incorpo-rated into planetesimals in a formingplanetary system. By performing infraredspectroscopy on the irradiated samples,Mate et al. conclude that HAC is relativelystable to both forms of processing whilstglycine is destroyed far more efficiently.They find that electron irradiation domi-nates in the outer layers of the ice sampleas a result of the penetration depth,sounding a note of caution on the careneeded when using several keV electronsto simulate processes induced by thepassage of cosmic ray particles throughinterstellar ices.

In the subsequent discussion, the roleof quantum tunnelling in atomic oxygendiffusion was discussed at length.Prof. Meuwly noted that departure fromArrhenius behaviour is not necessarilysufficient to infer that tunnelling isinvolved, especially when a distributionof diffusion barriers is present. Dr Congiuconfirmed that the investigation of isotopeeffects is indeed necessary to confirm thattunnelling is at play. When Dr Garrodcommented on the fact that Eley–Ridealand hot-atom processes cannot be ruled-out a priori, Dr Congiu explained that theyactually implemented those mechanismsin their model. However, the outcomeof their study indicates that quantumtunneling diffusion becomes dominant.Dr Hama highlighted the importance ofdetermining correct surface diffusionrates, and indicated that the temperaturedependence of O-atom diffusion rateobserved in Dr Congiu’s experimentscould be explained by thermal hoppingwithout tunnelling diffusion. Dr Ioppoloand Prof. McCoustra pointed out thatappropriate laboratory techniques forcontrolling the degree of porosity andcrystallinity of H2O films are needed tostudy mechanisms such as the diffusion ofspecies on different surfaces. Prof. Herbstsuggested to Prof. Price and co-workersto look into the sub-monolayer regime tobetter understand absolute rate coefficientsof surface reactions and the mechanismsat play with the goal of extending labora-tory results to the interstellar medium. Thesame regime was used by Dr Oba to studywater formation from a pure thin layer ofH2O2. In this case, the authors studied the

water-formation reaction-scheme startingfrom one of the products of O2 hydro-genation. Although this strategy narrows thereaction pathway, a multiple formation-pathfor water ice cannot be excluded, as pointed-out by Dr Ioppolo. In the discussion ofelectron-induced processes Prof. Rawlingsindicated that experimentalists reportcross-sections and yields, while modellerswish to incorporate energy integrated rateconstants in their models. This wouldrequire knowledge of the effective flux ofsecondary electrons, something that ishighly reliant on Monte Carlo simulations,as pointed out by Prof. Arumainayagam.Dr Woods and Prof. Kamp questioned theextent to which photon and electron drivenprocesses drive the formation of COMSand the reliability of extrapolating labora-tory measurements to low flux interstellarenvironments. Prof. Mason explained thatalthough many species can be formedthrough thermal routes, experiments haveshown that non-thermal processes areequally as effective in forming COMs. Heacknowledged that the timescale limita-tions of experiments make appropriatemodelling essential while Prof. McCoustrahighlighted the flux dependent measure-ments performed by experimentalists toensure that only single-photon/particleexcitation driven processes are probed.Prof. Linnartz added that hydrogenationreactions are probably the most efficientway to form simple hydrogenated species,but the relative efficiency of electron andphoton induced processes in the formationof more complex molecules remains unclear.Prof. Arumainayagam and Prof. Masonagreed that this is an important point,although direct comparison betweenexperiments using photon and electronirradiation remains difficult.

The first session after lunch, chaired byRobin Garrod (Cornell University), coveredastrophysical modelling. Modelling is oftenrequired to both guide and interpretastrophysical observations. Anthony Jones(Institut d’Astrophysique Spatiale, France)returned to the theme of carbon, by pre-senting a paper discussing the ‘‘cycling ofcarbon into and out of dust’’ in the transi-tion regions between the diffuse ISM andmolecular clouds.13 In the ISM, carbon ispresent not only in ionic and molecularform, e.g., C+, C, and CO, but also in

refractory form. These, sometimes nano-scale, particles are composed of smallaliphatic and aromatic hydrocarbons andcan undergo chemical processing by UVradiation. Jones et al. discuss and modelthe different production and destructionmechanisms for carbon-rich dust acrossthe conditions found in the ISM. Theirresults suggest that in low extinctionenvironments, small carbonaceous grainsmay consist of aromatic-rich carbonmaterials whereas in higher extinctionregions, the carbonaceous material ismore aliphatic in nature, leading to avariation in the optical properties of dustgrains across the ISM.

The next paper changed tack, bring-ing the discussion back to ice formationand chemistry. Herma Cuppen (RadboudUniversity Nijmegan) presented resultsfrom a microscopic kinetic Monte Carlomodel developed to investigate the forma-tion of water on interstellar grains viagrain-surface reactions. Water, in bothice and gas form, is prevalent throughoutmany astrophysical environments.14 It isnow generally understood that water formsprimarily on grain surfaces; however, theexact mechanism for its formation and itschemistry remains an active area ofresearch. Lamberts et al. explore the roleof reaction exothermicity on the formationof water and related species, such as, OH,O2H, and H2O2. They find that allowingreaction products to convert their excessenergy into translation motion or desorptionhelps to build a more compact ice. For theexothermic reaction H + O2H - OH + OH,translational motion of the two productscan help explain the detection of OH inlaboratory experiments and correspondinglack of H2O2.

Appropriately, the next paper was pre-sented by Berengere Parise (University ofCardiff) on a deep search for gas-phaseH2O2 in numerous and diverse astro-physical environments using the AtacamaPathfinder Experiment (APEX) telescope.15

The motivation was the first detection ofgas-phase H2O2 in space, towards thecloud core, r Oph A. H2O2 is a moleculeof interest given its role in water chemistry.Parise et al. report no additional detectionsof H2O2 toward any of their sources with atypical upper limit to the abundance ofr10�11 with respect to H2. Their chemical

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model shows that the production of gas-phase H2O2 is very sensitive to temperatureand reaches an appreciable abundanceonly in a narrow range between 20 and30 K. They suggest that r Oph A is a specialsource in which the bulk of the materiallies within this temperature range. Thatthis source has a higher temperature maybe related to its external environment: it isexternally irradiated by nearby massivestars. Parise et al. suggest observations ofadditional externally heated sources mayhelp prove this hypothesis.

The next paper, presented by JonathanRawlings (University College London), dis-cussed a new mechanism for the productionof gas phase COMs at low temperatures.16

It is believed that COMs are formed on orwithin ice mantles on dust grains. However,the recent detection of COMs in low tem-perature (10 K) environments has promptedmany to revisit this mechanism. Rawlingset al. propose that gas phase COMs areformed via three-body gas-phase reactionsfollowing complete sudden sublimation ofthe ice mantle following an ‘explosion’ eventtriggered by the recombination of trappedhydrogen and radicals. They use thisparadigm to model the formation ofC2H4O2 isomers (methyl formate, glycol-aldehyde, and acetic acid) as wellas the simplest amino acid, glycine(CH2NH2COOH). Their model can explainthe observed abundances of the C2H4O2

isomers provided the necessary fraction ofprecursor radicals are released from ice.They also predict a maximum fractionalabundance of B10�10 relative to H2 forgas-phase glycine, in line with the con-strained upper limits.

The final paper was presented byCatherine Walsh (Leiden University) andcontinued the theme of COM formation,this time, in protoplanetary disks.17 Walshet al. present calculations of the abundanceand distribution of COMs along stream-lines in both isolated and externally irradi-ated protoplanetary disks. This latteraspect is important because the Sun maynot have been born in isolation. Walshet al. find that chemical complexity canincrease along the accretion flow in iso-lated protoplanetary disks if only simpleices are injected. For the irradiated disk,COMs are unable to form via thermalgrain-surface chemistry; hence the initial

composition of the ice mantle is critical.COMs more complex than methanol donot survive transport through the diskdue to thermal and radiation processing,indicating that dynamical timescales arevery important. Also, complex moleculespresent in the planet-forming zone mayhave an interstellar origin provided thespecies are injected with a sufficientlyhigh abundance. Planetesimals in irradi-ated disks may be composed of moresimple ices, given the degree of thermaland radiation processing en route to theplanet-forming zone.

The final discussion session of the dayopened with a debate on the formation ofdust in the ISM. Among the questionsraised by Robin Garrod was the role of Oand N atoms in carbon-dust formationwhich, according to Anthony Jones, canaid dust production and produce specieswhich may help resolve the origin of DIBs(diffuse interstellar bands). The proposedmodel also revises current estimates fordust masses in the ISM downwards(prompted by a question from Prof.van der Tak). A topic highlighted for thefuture, was the role of charged grains ondust formation in the ISM (Prof. DaanSchram). The discussion moved on toconsider the specific parameters adoptedin the kinetic Monte Carlo models pre-sented by Herma Cuppen, which allowedthe authors to clarify the setup andmethods adopted. The simulations supportthe non-detection of gas-phase H2O2 in allbut a single astrophysical environment aspresented in the paper by Berengere Parise.Points raised for consideration on thislatter issue included a measured ratecoefficient for the reaction of H2O2 withOH (Ms Caravan) which may have anappreciable rate coefficient at low tem-peratures, and the use of isotopologues,e.g., HOOD (Prof. Linnartz). COM forma-tion via gas-phase recombination reactionsfollowing the explosive release of grainmantle material through radical recombi-nation reactions by Jonathon Rawlingsgenerated a lively debate. Several audiencemembers had reservations highlightingfactors such as H-atom concentrationwithin the ice (Robin Garrod), survival ofradicals following explosion (Eric Herbst),and the long lifetimes of molecular cloudscompared with laboratory timescales

(Mr Fedoseev). Much of the debate sur-rounded the observable signature of sucha mechanism. Proposed methods includedusing the relative abundances of structuralisomers, e.g., C2H4O2 (Dr Ellinger), ortargeted searches from COMs in CO ice-poor regions, because the mechanism formethanol production is related to CH4

and not CO (Jonathon Rawlings). Thefinal debate centred on COM formationin protoplanetary disks where the impor-tance of dynamics on the chemistry washighlighted (Dr Semenov), as was theimportance of a complete treatment ofbulk ice chemistry as well as surfacechemistry (Prof. Kaiser), and the use ofsensitivity analyses from grain surfacechemistry to identify critical parameters(Dr Heays). A potential topic for the futurewas suggested by Anthony Jones and con-cerned the influence of the presence of ahydrocarbon-rich material at the ice–graininterface in protoplanetary disks whichCatherine Walsh agreed may help buildchemical complexity in disks. This discus-sion nicely highlighted the need forongoing dialogue between the modellingand laboratory communities.

After the conclusion of the afternoonsession, a short break was provided inthe programme before the attendees wereinvited to begin the evening’s festivities withpre-dinner drinks. Places were then takenfor the three course conference dinner,after which Andrew Mount (University ofEdinburgh), chair of the Faraday StandingCommittee on Conferences, spoke on thehistory of the Faraday Discussion series andintroduced the tradition of the FaradayLoving Cup, which then made its wayaround the room. FD168 also saw thejoint award of the Skinner Prize forbest student poster to Rebecca Caravan(University of Leeds) for her posterdescribing ‘‘Chemical mechanismsoperating at very low temperatures toenhance the rates of gas phase reactionsrelevant to interstellar environments’’,and Robert Frigge (University of Munster)for his poster on ‘‘Femtosecond laser-induced desorption of atomic and mole-cular hydrogen isotopes from graphite’’.

The final session, on Wednesdaymorning, was chaired by Harold Linnartz(Leiden University) and centred on‘‘New directions in solid and surface

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astrochemistry’’, a perfect ending for thisinteresting Faraday Discussion. CorneliaJager (Max Planck Institute for Astronomy)reviewed the first paper, entitled ‘‘Coldcondensation of dust in the ISM’’.18 Thispaper shows the condensation of complexsilicates with pyroxene composition attemperatures between 10–20 K by accre-tion and reaction of molecules and atomson cold surfaces. The final condensatesare efficiently formed at low temperaturesand are fluffy aggregates consisting ofsmall nanometre-sized amorphous grains.In space, gaseous precursors of theseinterstellar-relevant condensation pro-cesses can come from the erosion of dustby supernovae shocks. In the laboratory,the same species were formed by ablationof silicate materials with a pulsed Nd:YAGlaser. Embedded in solid neon matrices,these species were further studied usingspectroscopy in the UV/VIS and IR ranges.The authors note that a band at 10 mm ofthe low temperature siliceous condensatesresemble the well-known and observed10 mm feature due to interstellar silicates.

The second paper was presented bySergio Ioppolo (California Institute ofTechnology) and focused on THz time-domain and mid-IR spectroscopy ofinterstellar-relevant ices at different tem-peratures.19 With the primary intent ofproviding the scientific community withan extensive THz ice database in supportof international astronomical observations(i.e., Herschel Space Telescope, SOFIA, andALMA), Ioppolo et al. have investigated thecomposition and structure of some of themost abundant interstellar ice analogues(i.e., water, carbon monoxide, and methanol).THz spectra of simple ices were thencompared to those from more complexinterstellar-relevant molecules that sharethe same functional groups, includingformic acid and acetic acid, as well asacetaldehyde and acetone. The authorsfind that, since THz frequencies aredominated by inter- and intramolecularforces, THz spectra are sensitive to reversi-ble and irreversible transformations withinthe ice, such as thermal processing of crys-talline and amorphous ices, respectively.

The third and final paper before themorning break was presented by RalfKaiser (University of Hawaii at Manoa).Kaiser and co-workers have recently built

a new ultra-high vacuum (UHV) system tostudy the formation of COMs in ice atlow temperatures.20 Their paper showsconclusive evidence for the formation ofglycolaldehyde (HOCH2CHO) in methanoland methanol–carbon monoxide ices uponexposure to ionizing radiation at 5.5 K(see Fig. 4). Glycolaldehyde is the simplestmonosaccharide sugar detected in numerousastrophysical environments and is alsoastrobiologically relevant. In this work,glycolaldehyde ice is unambiguously identi-fied using infrared spectroscopy and resultsare constrained by gas-phase data acquiredwith a single photon-ionization reflectrontime-of-flight (ReTOF) mass spectrometer.The authors discuss three possible forma-tion reaction pathways involving thereaction of formyl radical (HCO) with ahydroxymethyl radical (CH2OH), both formedin situ via energetic processing of ices.

Gianfranco Vidali (Syracuse University,New York) opened the second part of the‘‘New Directions’’ session by describinghis group’s recent application of modifiedrate equations to extracting kinetic para-meters from temperature programmeddesorption measurements.21 Such measure-ments are widely used to assess the strengthof binding between molecules and grain

surfaces through determination of thedesorption energy. Usually, the desorptionkinetics are analysed by assuming a singledesorption step, that may include a distri-bution of adsorption energy sites, andassuming that molecules do not diffuseacross the surface prior to desorption.For systems where such diffusion doesoccur, significant redistribution of adsor-bates during the sample annealing maylead to a significant error in the deriveddesorption energy distribution. He et al.present a modified rate equation for thermaldesorption that includes a diffusion termand demonstrate its application to severalsystems that exhibit desorption both withand without diffusion.

Photodesorption is a frequently invokedmechanism by which molecules can bereturned to the gas phase in low tem-perature environments where thermaldesorption is negligible. Although desorptionyields have been obtained for simplemolecule (e.g. CO) desorption underirradiation from broadband VUV lamps,the wavelength dependence, and under-lying chemical physics remains unclear.Jean-Hugues Fillion (Universite Pierre etMarie Curie, Paris) outlined a series ofpioneering measurements performed at

Fig. 4 Maity et al. have developed a combined infrared and time-of-flight (ToF) system to investigatemolecule formation in ices triggered by ionizing radiation. A and B show the mass distribution ofproducts formed as a function of temperature for CH3OH and CH3OH–CO ices respectively.20

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the SOLEIL synchrotron.22 Fillion et al.investigate the desorption of CO and CO2

from CO2 ice irradiated with photons inthe range 7–14 eV. Both species showdesorption yields that closely follow thephoton absorption cross section of CO2

above 11 eV. Pre-irradiation of CO2 pro-duces a reservoir of CO molecules thatcan enhance CO2 desorption throughan indirect mechanism involving initialphoton absorption by CO molecules,extending the wavelength range over whichCO2 is desorbed. Such measurements aretherefore crucial to understanding non-thermal desorption in mixed interstellarices, where indirect processes are likely tobe important.

The final discussion paper was pre-sented by Helmut Zacharias (Universityof Munster) who moved the discussion ofphoton induced ice processing to theextreme ultraviolet (XUV). In a series ofunique measurements at the Free-electronlaser in Hamburg (FLASH) facility, hisgroup have used femtosecond XUV pulsesto probe the photon induced reactiondynamics in simple ices (D2O, NO), as wellas the desorption of chemisorbed H atoms

from graphite.23 A schematic of theirexperimental setup is shown in Fig. 5.The application of femtosecond pulses toinvestigate ultrafast dynamics provides apotentially exciting new approach forprobing ice chemical processes.

The final discussion session coveredsome of the recent developments in astro-chemical research. Prof. Kamp opened thediscussion asking Dr Jager to elaboratemore on the Mg-poor nature of theirlaboratory dust grains compared to theones observed in space. Soon the discus-sion moved on to some detailed aspectsof Cornelia Jager’s work including thehigh efficiency of the cold condensationprocess in laboratories versus space(Mr Rab), and the possibility of includingO3 in the experiments to test whetherolivine rather than pyroxene would beformed, as suggested by Prof. Plane.Finally, Cornelia Jager remarked thenon-equilibrium conditions of the con-densation process as studied in theirexperiments that most likely explain thenon-formation of olivine. The next seriesof questions from Prof. van der Tak,Dr Pontoppidan, Prof. van Dishoeck,

and Prof. Linnartz were addressed toDr Ioppolo, who showed that THz-icefeatures can potentially be detected inthe ISM and can give information on thetemperature and structure of interstellarices. During the discussion session,Prof. Kaiser had the opportunity to revealsome of the details of their new experi-mental apparatus that comprises numerousin situ and gas-phase probing techniques(e.g., infrared spectroscopy, Raman spectro-scopy, ultraviolet-visible spectroscopy,reflection time-of-flight mass spectro-metry and photoionization). The benefitsand limitations of a variety of techniquesin elucidating reaction mechanisms forthe formation of more complex specieswere discussed. As chair of the session,Prof. Linnartz introduced the new ‘‘labastro’’website (www.labastro.eu) that aims to high-light the laboratory astrochemistry researchbeing undertaken in Europe and provideinformation on the groups involved aswell as an overview of the infrastructureavailable to the community at a Europeanlevel. In addition, Prof. Mason mentionedthe importance of database projects suchas the international virtual observatoryproject (www.ivoa.net) and VAMDC (www.vamdc.eu). Such initiatives provide a valu-able resource to the community in termsof collating data and the development ofprotocols for data validation and assess-ment. The importance of fully exploitingsolid-state laboratory spectra to gain moredetailed physical insights, rather thansimply generating vast catalogues of spectrawas also discussed. Sergio Ioppolo agreedwith Prof. McCoustra’s comment on theneed to combine laboratory data andhigh-level computational calculations tounderstand the nature of the collectivemodes of the condensed phase. Prof.Dulieu asked Prof. Zacharias and Prof.Fillion if they had any insights as to thesharing of the incident photon energyduring photo-induced desorption processes,as well as the exact role of the substrate.Prof. Fillion explained that vibrationalrelaxation within the ice matrix appearsto compete with the desorption channeland that the nature of the substrate has astrong effect on quenching of photo-desorption, resulting from differences inadsorbate–substrate binding. Prof. Zachariasagreed, highlighting the difference in

Fig. 5 Siemer et al. have used the Free Electron Laser in Hamburg (FLASH) facility to measure theultrafast dynamics of XUV photon induced processes occurring in interstellar ices.23

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mechanisms between single ice layers,where the underlying substrate plays acentral role in the energy transfer process,and thick ice layers where excitation ofvibrational motion within the ice latticebecomes important.

The challenge of summarizing thepapers discussed during the meeting fellto Eric Herbst (University of Virginia)who provided an entertaining overviewof the research presented. In addition totying together the various strands ofastrochemistry, he also reminded theattendees of the continuing advancesin gas-phase astrochemistry that werereported in several interesting posters,despite not being as well representedthrough the papers. To conclude, he setthe scene for the future, looking forwardto further discussion meetings. Observa-tional astrochemistry is entering a newera with the ongoing construction ofALMA and the launch of JWST in 2019.It is clear that astrochemists across theboard will be kept busy in the years tocome, providing plenty of substance forfuture discussions.

To round off the meeting, MartinMcCoustra (Heriot-Watt University,Edinburgh) thanked the local organisersfrom Leiden and the staff of the RSC forhosting and organizing such a fascinat-ing and stimulating discussion meetingbefore inviting the attendees to meet for

a final lunch together before departure.The Faraday Discussion 168 volumeincluding all of the general discussionsis published by the Royal Society ofChemistry.

Acknowledgements

J.D.T. acknowledges support from theUniversity of Munster. S.I. acknowledgessupport from a Niels Stensen Fellowshipand a Marie Curie Fellowship (FP7-PEOPLE-2011-IOF-300957). C.W. acknowledges sup-port from the Netherlands Organisation forScientific Research (NWO, program number639.041.335).

References1 K. M. Pontoppidan and S. Blevins, Faraday

Discuss., 2014, 168, 49.2 K. I. Oberg, E. C. Fayolle, J. B. Reiter and

C. Cyganowski, Faraday Discuss., 2014, 168, 81.3 V. V. Guzman, J. Pety, P. Gratier, J. R.

Goicoechea, M. Gerin, E. Roueff, F. Le Petitand J. Le Bourlot, Faraday Discuss., 2014,168, 103.

4 E. Bergin, L. I. Cleeves, N. Crockett andG. Blake, Faraday Discuss., 2014, 168, 61.

5 E. Congiu, M. Minissale, S. Baouche,H. Chaabouni, A. Moudens, S. Cazaux,G. Manico, V. Pirronello and F. Dulieu,Faraday Discuss., 2014, 168, 151.

6 H. J. Kimber, C. P. Ennis and S. D. Price,Faraday Discuss., 2014, 168, 167.

7 Y. Oba, K. Osaka, N. Watanabe, T. Chigaiand A. Kouchi, Faraday Discuss., 2014,168, 185.

8 M. W. Lee and M. Meuwly, Faraday Discuss.,2014, 168, 205.

9 A. L. Skov, J. D. Thrower and L. Hornekær,Faraday Discuss., 2014, 168, 223.

10 N. J. Mason, B. Nair, S. Jheeta andE. Szymanska, Faraday Discuss., 2014,168, 235.

11 M. D. Boamah, K. K. Sullivan, K. E.Shulenberger, C. M. Soe, L. M. Jacob, F. C.Yhee, K. E. Atkinson, M. C. Boyer, D. R.Haines and C. R. Arumainayagam, FaradayDiscuss., 2014, 168, 249.

12 B. Mate, I. Tanarro, M. A. Moreno, M. Jimenez-Rodondo, R. Escribano and C. J. Herrero,Faraday Discuss., 2014, 168, 267.

13 A. P. Jones, N. Ysard, M. Kohler, L. Fanciullo,M. Bocchio, E. Micelotta, L. Verstraete andV. Guillet, Faraday Discuss., 2014, 168, 313.

14 T. Lamberts, X. de Vries and H. M. Cuppen,Faraday Discuss., 2014, 168, 327.

15 B. Parise, P. Bergman and K. Menten,Faraday Discuss., 2014, 168, 349.

16 J. M. C. Rawlings, D. A. Williams, S. Vita,C. Cecchi-Pestellini and W. W. Duley, Fara-day Discuss., 2014, 168, 369.

17 C. Walsh, E. Herbst, H. Nomura, T. J. Millarand S. Widicus Weaver, Faraday Discuss.,2014, 168, 389.

18 G. Rouille, C. Jager, S. A. Krasnokutski,M. Krebsz and T. Henning, Faraday Discuss.,2014, 168, 449.

19 S. Ioppolo, B. A. McGuire, M. A. Allodi andG. A. Blake, Faraday Discuss., 2014, 168, 461.

20 S. Maity, R. I. Kaiser and B. M. Jones,Faraday Discuss., 2014, 168, 485.

21 J. He and G. Vidali, Faraday Discuss., 2014,168, 517.

22 J.-H. Fillion, E. C. Fayolle, X. Michaut,M. Doronin, L. Philippe, J. Rakovsky,C. Romanzin, N. Champion, K. I. Oberg,H. Linnartz and M. Bertin, Faraday Discuss.,2014, 168, 533.

23 B. Siemer, S. Roling, R. Frigge, T. Hoger,R. Mitzner and H. Zacharias, FaradayDiscuss., 2014, 168, 553.

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