DISSOCIATIVE ELECTRON ATTACHMENT IN MOLECULES - NEEDS AND CURRENT STATUS OF AVAILABLE DATA Iztok Čadež Jožef Štefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia E-mail: iztok.cadez@ijs.si From abstract: A compact historical overview of the evolution of understanding of dissociative electron attachment (DEA) in molecules as well as on the increasing body of studied molecular species will be presented. Some experimental ambiguities which persist until modern time will be emphasised and discussed. Finally, current status of DEA studies and needs as well as perspective for the future development will be given. Dissociative electron attachment in molecules is one of classical binary electron molecule collision processes which have been studied with variable intensity since late fifties. The fact that DEA is a resonant process on one side and a rearrangement process on another, make it an interesting case for the theory development and also an important process in various environments. New needs in advanced applications, more detailed modelling codes and also, the development of new experimental techniques enhanced the interest for DEA in recent years. The fields of research which are currently driving this interest for DEA are studies of free electrons in biological environment, astrochemistry (recent discovery of anions in space and planetary atmospheres), sensitive compound selective detection techniques, edge plasma in tokamaks and various technological plasmas. Regional workshop on atomic and molecular data, Belgrade, Serbia, June 14-16, 2012

- DEA in some details (TCS, PCS, I(θ),…) - Applications and needs Outline DISSOCIATIVE ELECTRON ATTACHMENT IN MOLECULES - NEEDS AND CURRENT STATUS OF AVAILABLE DATA - Introduction - Historic overview - DEA in some details (TCS, PCS, I(θ),…) - Applications and needs - Available data - Perspectives

Introduction Many types of elementary collision processes for numerous atomic particles are needed to be well known for the variety of collective phenomena to be understood. Here we will present only fragmentary, personal view on one of such processes, dissociative electron attachment, which is one channel of one kind (resonant) of one pair of collision partners (electron + neutral molecule). From abstract: A compact historical overview of the evolution of understanding of dissociative electron attachment (DEA) in molecules as well as on the increasing body of studied molecular species will be presented. Some experimental ambiguities which persist until modern time will be emphasised and discussed. Finally, current status of DEA studies and needs as well as perspective for the future development will be given. Dissociative electron attachment in molecules is one of classical binary electron molecule collision processes which have been studied with variable intensity since late fifties. The fact that DEA is a resonant process on one side and a rearrangement process on another, make it an interesting case for the theory development and also an important process in various environments. New needs in advanced applications, more detailed modelling codes and also, the development of new experimental techniques enhanced the interest for DEA in recent years. The fields of research which are currently driving this interest for DEA are studies of free electrons in biological environment, astrochemistry (recent discovery of anions in space and planetary atmospheres), sensitive compound selective detection techniques, edge plasma in tokamaks and various technological plasmas.

e + AB  {AB-}  A + B- A, B – atoms or atomic groups; Introduction e + AB  {AB-}  A + B- A, B – atoms or atomic groups; a resonant process – specific peaked energy dependence, typ. < 15 eV(!); alternative compound state decay by autodetachment – resonant electron scattering); symmetry selection rules – angular distribution of dissociating fragment; energy partition among kinetic and internal degrees of freedom; temperature dependence – DEA to excited target.

Introduction e + AB  {AB-}  A + B- Anzai et al., Cross section data sets for electron collisions with H2, O2, CO, CO2, N2O and H2O, Eur. Phys. J. D (2012) 66: 36

Historic overview First experimental evidence of DEA J. T. Tate and P. T. Smith, Phys. Rev. (1932) 39 270

Historic overview In late fifties a strong interest for DEA started Early the most active centers for DEA research: Bell Telephone Labs. – H. D. Hagstrum (1951) Westinghouse Labs. – G. J. Schulz, P. J. Chantry (1959-1968) USSR – V. I. Khvostenko, V. M. Dukel’skii, I. S. Buchelnikova (1957-) Liverpool University – J. D. Craggs (1959) NBS/JILA (G. Dunn – 1962) Lockheed Missiles and Space Comp. – D. Rapp et al. (1965) Yale University - G. J. Schulz, and his group (1967-1981) University Orsay, Paris – F. Fiquet-Fayard (1972) First theoretical approaches borrowed from nuclear science (J. N. Bardsley, A. Herzenberg, T. F. O’Malley, H. S. Taylor, Yu. N. Demkov) (1962-).

Historic overview The study of atomic collisions in Belgrade started after the return of Milan Kurepa from postgraduate visit in the laboratory of professor J. D. Craggs at the University of Liverpool in 1963. Soon after this, Vladeta Urošević (electron impact photo-excitation and swarms, IFB) and Branka Čobić (heavy- particle collisions, Vinča) entered actively in the field. Milan Kurepa (1933-2000) Soon also started very active theoretical work initiated by Ratko Janev (Vinča) after his return from Ph.D. stay in Lenjingrad (StPetersburg) and Petar Grujić after his return from Ph.D. stay at UC, London. Good seed + good soil + good “weather” conditions (environment) + good timing (goals) + dedicated work = plenty of good results!

Historic overview Later development of DEA research included detailed partial CS determination from triatomic and some bigger molecules, study of angular distribution of product anions and temperature dependence of DEA CS. After somehow lower intensity of this research in eighties and nineties new “boom” occurred in more recent time by development of COLTRIMS concept and position sensitive detection and driven by new areas of interest. Theory has been steadily developing and following new experimental findings.

Present time key experimental tool - VMI Historic overview Present time key experimental tool - VMI Adaniya et al., Rev. Sci. Instrum. (2012) 83 023106 Nandi et al., Rev. Sci. Instrum. (2005) 76 053107 Wu et al., Rev. Sci. Instrum. (2012) 83 013108

Present key experimental tool - VMI Historic overview Present key experimental tool - VMI Adaniya et al., Rev. Sci. Instrum. (2012) 83 023106

Total cross section measurements Experimental studies were initially concentrated on the relative and absolute cross section measurements for total anion production. Čadež, Pejčev and Kurepa, J. Phys. D: Appl. Phys. (1983) 16 305 Tate-Smith type apparatus for TCS, incorporating TEM was constructed in Belgrade in early seventies. Studied molecules were O2, CO2, CCl2F2, BF3, Cl2, Br2, SO2 and some more. Christophorou et al., 1984.

This case spans over almost entire period of modern time DEA studies! Total cross section measurements - H2 case This case spans over almost entire period of modern time DEA studies! e + H2(v) → H2-* → H + H - H2 D2 Isotope effect is common in DEA as atomic mass determines the speed of dissociation and therefore brunching to this channel of resonant decay. This is the most pronounced for H vs. D – 100% of mass difference! E. Krishnakumar, S. Danifl, I. Čadež, S. Markelj and N. J. Mason, PRL 106 (2011) 243201

Partial cross section measurements Coupling with fragment ion mass analysis allowed determination of partial cross section for production of particular negative ion. From: Matejčík et al. Int. J. Mass Spec. (2003) 223-224 9 Such arrangements are/were used at Yale, Innsbruck, Bratislava, Berlin, Belgrade... Braun et al., Int. J. Mass Spec. (2006) 252 234; Inter laboratory cooperation on specific target is very important and fruitful!

Angular distribution of fragment anion For diatomics very clear interpretation as AD is a mirror image of attachment probability – fast dissociation along molecular axis (O2, NO, CO, H2). From: Van Brunt and Kieffer, Phys. Rev. A (1970) 2 1293 & 1899

Angular distribution of fragment anion Charm of the experimental studies of atomic collisions is permanent development of elegant and more or less simple technical improvements! Modifying standard electron spectrometer by incorporating simple momentum filter for elimination of electrons allowed high resolution ion energy and angular measurements! O-/CO: Čadež et al., J.Phys.B. (1975), 8 L73; Hall et al., Phys. Rev. A (1978), 15 599; Schermann et al., J.Phys.E., (1978) 11 746

Angular distribution of fragment anion H-/H2O: Haxton et al., 2006 (theory); Adaniya et al., 2012 (experiment) For small polyatomics interpretation more difficult due to complicated few body motion – consequently, much less studied (H2O, H2S). For big biomolecules, interpretation is again easier due to large mass of neutral fragment – remains to be studied and it has very high importance for dense media.

Ee + Ei-ex = Ef-ex + Ek + D - EA Energy partition Measurement of fragment ion energy allows determination of the excited state in which neutral fragment is left. e + AB  {AB-}  A + B- Ee + Ei-ex = Ef-ex + Ek + D - EA EKB- = MA/MAB * EK Most atoms and many radicals have positive EA. H- from H2O Belić et al., J.Phys.B: (1981) 14 175 Hall et al., Phys. Rev. A (1978) 15 599

DEA to excited target Electron collisions with excited targets are frequent in hot media – an overview in L. C. Christophorou and J. K. Olthoff, Electron interactions with excited atoms and molecules, Advances in Atomic, Molecular and Optical Physics, vol. 44, Academic Press 2001. First observed temperature dependence of DEA studied in O-/O2. Later DEA in CO2, N2O, H2, D2, HCl, DCl, HF, Na2, CCl4, CCl2F2, … Henderson, Fite and Brackmann, Phys.Rev. (1969) 183 157 Brüning et al. (1998) Chem. Phys. Lett. 292 177 Spence and Schulz, Phys. Rev. (1969) 188 280

Temperature dependence – H2 case E (4eV) + H2(v) ® H2-* ® H + H - Very strong CS dependence on internal ro-vibrational excitation and also isotope effect! Allan and Wong, PRL (1978) 41 1791 Theoretical CS for DEA in H2(v, J=0) (Horaček et al. 2004) (o) and DEA CSs to some molecules from Christophorou et al., 1984. Very strong temperature dependence of DEA also in HCl, DCl and HF.(Allan and Wong, 1981).

Temperature dependence – DEA to excited target H2 14 eV H-/H2&D-/D2: Čadež et al., J.Phys.B. (1988) 21 3271; Hall et al. PRL (1988) 60 337, Schermann et al., J.Chem.Phys. (1994) 101 8152 Markelj and Čadež, J. Chem. Phys. (2011) 134 124707

DEA to electronically excited target Also to specific vibronic states of SO2*: Kumar et al. Phys. Rev. A (2004) 70 052715. O-/O2*: Belić and Hall, J.Phys.B (1981) 134 124707. DEA in SO2*: Krishnakumar et al., Phys. Rev. A (1997) 56 1945.

The way of experimental development Some mistakes are indispensable on the way and they contribute to the charm of scientific development! “no temperature dependence of CS in H2” C2H2 second peak – C-  H- CS for H-/CH4 Signal background (H-/H2, D-/D2) Influence of electron energy resolution, momentum transfer, target gas temperature on experimental result. Total clearness of results and perfect agreement between the theory and experiment is an ultimate goal but the quest for this goal is sometimes a way to errors.

The way of experimental development Transfer of the momentum of incident electron to the target is often overseen although it is not negligible – in modern momentum imaging it is clearly visible and normally taken into account.

DEA – theoretical description There are two energy manifolds one for the neutral target molecule and another for compound negative ion. Particle, that connects these two manifolds is electron – basically, satiation is similar to what one has in elementary particle physics! The theory describes different aspects of resonant electron-molecule collision: Energy levels of neutral molecules (common for all molecular spectroscopy). Energy levels of negative ion compound molecule (unstable!) – both real part and decay width. For both cases energy levels are function of molecular shape parameters (bond lengths and bond angles). Time evolution of compound molecule – typically on fs level. Extraction of cross sections for particular decay channel, resonant scattering and DEA.

DEA – theoretical description First theories were taken from, then, more advanced nuclear physics. Later, very sophisticated theories developed for molecular resonances: Local complex potential - resonant state dependent only on R. Non-local complex potential – resonant state dependent on R and Ee. Wave packet propagation in local complex potential. Ab initio calculations of compound state parameters.

DEA in polyatomic molecules – C2H2 DEA – theoretical description DEA in polyatomic molecules – C2H2 Recent detailed theoretical analysis of DEA in acetylene: e + C2H4  C2H- + H Chourou and Orel, Phys.Rev.A (2008) 77 042709

Applications and needs Where is DEA present? As a binary collision process in rarefied media where free electrons are present. The basic physical mechanism of DEA – resonant electron capture to a molecule and subsequent bond breaking, occurs also on surfaces and in dense media. The later relevance drives main interest for DEA in the present time!

Applications and needs DEA in rarefied media - Modelling of ionized gases (BF3, SF6, CH4, SiH4, …) Particular example – fusion plasma: Relatively small number of molecular species in edge plasma but still relevant process – H2, D2, T2, HD, HT, DT and also hydrocarbons – satisfactory data base exists (e.g. http:\\www. eirene.de – Juel Reports 3966, 4005, 4038, 4105; R. K. Janev, D. Reiter and U. Samm). New development due to ITER material mix (Be and W compounds) but in particular processes with nitrogen – N2, NH3, … and isotopologues. Besides being important for the plasma properties, it is potentially relevant to specific collision processes related to impurity transport and interaction with surfaces – deposition and desorption).

H2(v) from the wall to edge plasma Sensitivity on vibrational excitation of H2 from the wall 1-D Monte-Carlo model for neutral particle transport (Kotov and Reiter, 2005) All H2 from the wall in v=0 All H2 from the wall in v=4 1017 m−3 < ne < 1020 m−3, 1eV < Te < 100 eV, 10−3 ne < nI < 10−1ne, nHo ≈ 10−3ne

Applications and needs Rarefied media – Volume H- (D-) ion sources This is a classic example of application of DEA for plasma development From : M. Bacal, Nuclear Fusion 46 (2006) S250

Applications and needs Rarefied media – Volume H- (D-) ion sources Vibrationally excited H2 are precursor for H- ion production by DEA They are produced by e-V: H2 + e (slow)  H2(X 1Sg+, v’’) + e E-V: H2 + e (fast)  H2(B 1Su+,C 1Pu)  H2(X 1Sg+, v’’) + hn Cascade: H2(X 1Sg+, v=0) + e  H2(E, F 1Sg+)  H2(B 1Su+)  H2(X 1Sg+, v’’) + hn Recombinative desorption: H + H + wall  H2(X 1Sg+ , v’’ = 1, 2) followed by the E-V excitation of the X1Sg+ state with the low v’’: H2(X 1Sg+, v’’ = 1, 2) + e (fast)  H2(B 1Su+,C 1Pu)  H2(X 1Sg+, v’’  1, 2) + hn From : M. Bacal, Nuclear Fusion 46 (2006) S250

Applications and needs Rarefied media - Sensitive gas detectors READ – Reversed Electron Attachment Detector Low energy electron attachment is very efficient to producing characteristic anions for low level pollution monitoring. From :Boumsellek and Chutjian. 1992 and Darrach et al. 1998

Applications and needs Rarefied media - Aeronomy and astrochemisty (from Earth and other planetary atmospheres to cosmology) DEA is potentially important in the environments where low energy electrons are present and neutral molecules and radicals – mainly indirect evidence from modelling. L. Campbell and coworkers have been showing the importance of accurate data on e-molecule collisions for actrochemistry modelling.

>160 Interstellar Molecules The number of molecular species observed in various regions in space is steadily increasing 2 3 4 5 6 7 8 9 10 11 H2 C3 c-C3H  C5 C5H  C6H  CH3C3N  CH3C4H  CH3C5N?  HC9N  AlF  C2H l-C3H  C4H l-H2C4 CH2CHCN  HCOOCH3 CH3CH2CN  (CH3)2CO  AlCl  C2O C3N  C4Si C2H4  CH3C2H  CH3COOH  (CH3)2O  NH2CH2COOH ? 12 C2 C2S C3O  l-C3H2 CH3CN HC5N  C7H CH3CH2OH  C6H6 CH  CH2 C3S  c-C3H2 CH3NC  NH2CH3 H2C6 HC7N  CH+  HCN C2H2 CH2CN CH3OH HCOCH3 CH2OHCHO  C8H  13+ CN HCO CH2D+ ?  CH4 CH3SH  c-C2H4O  HC11N CO HCO+ HCCN  HC3N HC3NH+  CH2CHOH  PAHs CO+ HCS+ HCNH+  HC2NC HC2CHO  C60+ CP HOC+ HNCO HCOOH NH2CHO CSi  H2O HNCS  H2CHN C5N  HCl  H2S HOCO+  H2C2O KCl  HNC H2CO H2NCN NH HNO H2CN  HNC3 NO  MgCN H2CS SiH4 NS  MgNC H3O+ H2COH+ NaCl N2H+ NH3 OH N2O SiC3 PN  NaCN SO OCS SO+  SO2 SiN  c-SiC2 SiO  CO2 SiS  NH2 CS H3+ HF SiCN SH FeO AlNC >160 Interstellar Molecules National Radio Astronomy Observatory, (http://www.cv.nrao.edu/~awootten/allmols.html (Adapted from N. J. Mason, 2010)

Role of anions - data needs for modelling Hydrocarbon anions are observed in different environments in space (e.g. Millar et al., 2007, Harada&Herbst, 2008) and detailed modelling of these requires data for various processes. Result of modeling of the time evolution of CnH and CnH- following the evaporation of methane ice as applied to explain the observations from L1527, an envelope of a low-mass star-forming region - from Harada&Herbst, 2008. Recent relevant study of DEA in H−C≡C−C≡C−H by May et al. PR A 77, 040701R (2008) and on RVE by Allan et al., PR A 83, 052701 (2011)

Planetary atmospheres – Titans in particular From: S. Atreya, Titan Workshop, Kauai, 12. April 2011 http://www.chem.hawaii.edu/Bil301/Titan2011.html Composition ≈ 97% N2 + 2% CH4 + 1% C2H2, C2H4,….Ar(?)

Role of anions - data needs for modelling V. Vuitton et al., Negative ion chemistry in Titan’s upper atmosphere, Planetary and Space Science 57 (2009) 1558–1572 - The Electron Spectrometer (ELS), revealed the existence of numerous negative ions in Titan’s upper atmosphere. Up to 10,000 amu/q, two (three) distinct peaks at 22 ± 4 and 44 ± 8 (and 82 ± 14 ) amu/q, Ionospheric model of Titan including negative ion chemistry. DEA mostly to HCN initiate the chain of reactions. - Radiative electron attachment is fast for bigger carbon chain molecules as for C6H but very slow for light ones. - Anions from thermal energy electron capture – not taken into account. - Data for DEA are used (CH4,C2H2, estimate for C4H2 and C6H2. - Ion pair production by photons (but not by electrons) - Photo-detachment, cation-anion recombination, anion-neutral associative detachment. - Proton transfer is very efficient (e.g. H- + C2H2 →C2H- + H2). - Polymerization (e.g. C2nH- + C2H2 → C2n+2H- + H2).

Low energy H- yield from DEA to small hydrocarbons Potential relevance of DEA to small hydrocarbons stimulated an experimental study of the low energy H- ion yield from some small HCs. Two processes contribute: - Dissociative electron attachment for Ee <15 eV Polar dissociation (ion pair production) for Ee > 15 eV. Activity within COST CM0805 – The Chemical cosmos Čadež, Rupnik and Markelj, Eur. Phys. J. D (2012) 66: 73

Applications and needs Dense media and surfaces Similar resonant states exist in molecules incorporated in dense media but their properties (energy, symmetry and lifetime) are modified: - by substrate if adsorbed on the surface - by the close neighbor molecules (thick layers, clusters, in the bulk). Different scenarios occur regarding released anion from DEA - it can be emitted out of the system (e.g. condensed layers) or can induce further reactions.

DEA at surfaces – dense layers Group of R. E. Palmer at the University of Birmingham: molecule manipulation by STM at room temperature Selective dissociation of chlorine atoms from individual oriented chlorobenzene molecules adsorbed on a Si(111)- 7x7 surface at room temperature. Proposed two electron mechanism: first electron (b) excites C-Cl wag vibrations (c) and second electron (d) induce dissociation of C-Cl bond. Free Cl sticks to the surface (e). Some public titles following the paper in Nature (Google): - “Quantum electron “submarines” help push atoms…” - (New Scientist) - “Nano-surgeons break the atomic bond (The Telegraph)” - “Birmingham Scientists Witness the Birth of an Atom” Sloan and Palmer, Nature 434 (2005) 367

DEA at surfaces – dense layers Group of L. Sanche, Univ. of Sherbrooke, Quebec, Canada Group of R. Azria, A. Lafosse… , UPS, Orsay, France Lafosse et al., Phys. Chem. Chem. Phys. 8 (2006) 5564–5568 D- from amorphous ice at 190 K Simpson et al., J.Chem.Phys. 107 (1997) 8668

Radiation Damage Thymine DEA in biomolecules Electron driven rections Aspect. Radiation Damage/radio therapy (From E. Illenberger, 2007)

DEA in biomolecules F. Martin, P. D. Burrow, Z. Cai, P. Cloutier, D. Hunting, and L. Sanche, PRL 93 (2004) 068101

Data – production and needs Data collection, evaluation and recommendations Needs Users Modelling Data formatting DATABASE Sensitivity analysis

Data – production and needs Experiments of “light” (more individual work) - new processes - basic properties - benchmark cases - new exp. methods Experiments of “fruit” (more collective work) - choice of subject - application of methods - data production Interpretation of data Theory - In-depth explanation of processes - development of models - data production and model evaluation Data evaluation Collection from all available sources, new and old. Evaluation of applied methods and claimed accuracy. Recomdation of best data to be used. Feedback with data producer. Recommendations for new measurements or calculations. Data “shaping” Formation of standardised data bases Appropriate data formats Accessibility Data usage Modelling of complex processes, new technological procedures, processes in other sciences. Sensitivity analysis Feedback to data producers.

List of laboratories actively participating in present DEA research Current activities List of laboratories actively participating in present DEA research Sherbrooke, Canada (Léon Sanche, biomolecules, surfaces, experiment, theory) Lincoln, Nebraska (Paul Burrow, Gordon Gallup, experiment; Ilya Fabrikant, theory) Davis & Berkeley, CA (Ann Orel, Tom Rescigno, Bill McCurdy : theory; H. Adaniya : DEA experiment – COLTRIMS) Belfast (Tom Field; Gleb Gribakin, ToF DEA, biomolecules; theory) Innsbruck (Paul Scheier, Tilmann Märk, Stefan Denifl, biomolecules, collisions in He nanodroplets) Fribourg (Michael Allan) Berlin (Eugen Illenberger, biomolecules) Open University, Milton Keynes (Nigel Mason, Jimena Gorfinkiel, experiment, theory) Bratislava, Slovakia (Štefan Matejčik) University of Podlasie, Poland (Janina Kopyra, electron transport) Prague, Charles University (Jiří Horáček, Martin Čížek, Karel Houfek (+ Wolfgang Domcke), theory) Orsay (Robert Abouaf, Roger Azria, Ann Lafosse, surfaces) London (JonathanTennyson, R-matrix theory) Island (Oddur Ingólfsson, experiment) Tata Institute, Mumbai (E. Krishnakumar, S. V. K. Kumar, V. Prabhudesai, experiment: velocity slice imaging) Hefei, China (S. X. Tian, B. Wu, experiment: velocity slice imaging) (adapted from M. Allan, ICPEAC, 2011)

List is too long to be presented here – only examples: Available data List is too long to be presented here – only examples: Diatomic: H2, O2, CO, NO, S2, Cl2, Br2, HF, HCl, HBr Triatomic: H2O, CO2, CS2, H2S, O3, SO2, N2O, NO2, HCN Small polyatomic: CH4, NH3, BF3, C2H2, CCl2F2, C6H6, SF6, many chloro- and fluorocarbons, CH3CN, N2O5 Big molecules: C60, HCOOH, C2H5NO2, uracil, glicine, nitrotoluene, cyclopentanone, tetrahydrofuran, HFFA (CF3)2C=N-N=C(CF3)2), tymine, various molecular clusters

Resonances (and DEA) in E&B field. Perspectives More data on DEA to excited molecules (both, ro-vibrational and electronic) are needed. Angular distribution of ions from DEA to larger molecules and experiments on oriented targets. Resonances (and DEA) in E&B field. Applications in future might be related to well defined time scale of e-impact induced molecular breakdown. DEA in dense media is a separate field of research of high importance with its own new experimental and theoretical development.

Collaborations on DEA and acknowledgement Milan Kurepa Ratko Janev Aleksandar Stamatović Vlada Pejčev Florance Fiquet-Fayard Richard Hall Catherine Schermann Nada Djurić Will Castleman Sabina Markelj Nigel Mason E. Krishnakumar

Some references R. E. Palmer and P. J. Rous, Resonances in electron scattering by molecules on surfaces, Rev. Mod. Physics 64 (1992) 383 S Matejcik, A Kiendler, P Cicman, J Skalny, P Stampfli, E Illenberger, Y Chu, A Stamatovic and T D M¨ark, Electron attachment to molecules and clusters of atmospheric relevance: oxygen and ozone, Plasma Sources Sci. Technol. 6 (1997) 140 A. CHUTJIAN, A. GARSCADDEN, J.M. WADEHRA, ELECTRON ATTACHMENT TO MOLECULES AT LOW ELECTRON ENERGIES, Physics Reports 264 (1996) 393-470 Savin et al. (14authors) The impact of recent advances in laboratory astrophysics on our understanding of the cosmos, Rep. Prog. Phys. 75 (2012) 036901 M. Bacal, Physics aspects of negative ion sources, Nucl. Fusion 46 (2006) S250–S259 L.G. Christophorou , D. Hadjiantoniou, Electron attachment and molecular toxicity, Chemical Physics Letters 419 (2006) 405–410