Transcript DISSOCIATIVE ELECTRON ATTACHMENT IN MOLECULES
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: [email protected]
Regional workshop on atomic and molecular data, Belgrade, Serbia, June 14-16, 2012
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 numerous atomic particles known for the variety of be understood. are needed to be well collective phenomena for to 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).
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 H 2 , O 2 , CO, CO 2 , N 2 O and H 2 O, 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 professor J. D. Craggs
Milan Kurepa
from postgraduate visit in the laboratory of at the University of Liverpool in 1963. Soon after this, Vladeta Urošević and swarms, IFB) and particle collisions, Vinča) entered actively in the field. (electron impact photo-excitation Branka Čobić (heavy 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.
Historic overview Present time key experimental tool - VMI Nandi et al., Rev. Sci. Instrum. (2005) 76 053107 Adaniya et al., Rev. Sci. Instrum. (2012) 83 023106 Wu et al., Rev. Sci. Instrum. (2012) 83 013108
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 molecules were O 2 , CO 2 , CCl for TCS, incorporating TEM was constructed in Belgrade in early seventies. Studied 2 F 2 , BF 3 , Cl 2 , Br 2 , SO 2 and some more.
Christophorou et al., 1984.
Total cross section measurements - H 2 case
e + H 2 (v)
→
H 2 -*
→
H + H H(n=2) + H -
15
13.93eV
This case spans over almost entire period of modern time DEA studies!
10 5 0 0
E e
H 2 "2" 2
u + H 2 "1" 2
g + H 2 X 2
u +
3.73eV
H + H H(n=1) + H EA H = 0.754 eV
H 2 X 1
g +
2 4 6
Internuclear distance [ a.u. ]
8
H D 2 2
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 (O 2 , NO, CO, H 2 ). 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 /H 2 O
: 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 (H media.
2 O, H 2 S).
• 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
Energy partition Measurement of fragment ion energy allows determination of the excited state in which neutral fragment is left.
e + AB
{AB }
A + B -
E e + E i-ex = E f-ex + E k + D - EA E K B = M A /M AB * E K
Most atoms and many radicals have positive EA.
Hall et al., Phys. Rev. A (1978) 15 599
H from H 2 O
Belić et al., J.Phys.B: (1981) 14 175
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-/O CO 2 , N 2 O, H 2 , D 2 , HCl, DCl, HF, Na 2 , CCl 4 , CCl 2 F 2 , … 2 . Later DEA in Henderson, Fite and Brackmann, Phys.Rev.
(1969)
183
157 Spence and Schulz, Phys. Rev. (1969)
188
280 Brüning et al. (1998) Chem. Phys. Lett.
292
177
Temperature dependence – H 2 case
E (4eV) + H 2 (v)
H 2 -*
H + H -
Very strong CS dependence on internal ro vibrational excitation and also isotope effect! Allan and Wong, PRL (1978)
41
1791 Very strong temperature dependence of DEA also in HCl, DCl and HF.(Allan and Wong, 1981). Theoretical CS for DEA in H 2 (v, J=0) (Horaček et al. 2004) ( o ) and DEA CSs to some molecules from Christophorou et al., 1984.
Temperature dependence – DEA to excited target D 2 H 2 14 eV H /H 2 &D /D 2 : 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 SO
70 052715.
2 *:
Kumar et al. Phys. Rev. A (2004)
O /O 2 *:
Belić and Hall, J.Phys.B (1981) 134 124707.
DEA in SO 2 *:
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 H 2 ” • C 2 H 2 second peak – C H • CS for H /CH 4 • Signal background (H /H 2 , D /D 2 )
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.
180 150 210 120 90 isotropic gas cos( ) - beam cos( ) 2 - beam 60 30
gas beam
240
e - beam
330
H 2 ; E e = 14 eV ; T = 300 K
300 270 0
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 E e .
- Wave packet propagation in local complex potential.
- Ab initio calculations of compound state parameters.
DEA – theoretical description DEA in polyatomic molecules – C 2 H 2 Recent detailed theoretical analysis of DEA in acetylene: e + C 2 H 4 C 2 H + 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 (BF 3 , SF 6 , CH 4 , SiH 4 , …) Particular example – fusion plasma: - Relatively small number of molecular species in edge plasma but still relevant process – H 2 , D 2 , T 2 , HD, HT, DT and also 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 – N 2 , NH 3 , … 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).
H 2 (v) from the wall to edge plasma
Sensitivity on vibrational excitation of H 2 from the wall 1-D Monte-Carlo model for neutral particle transport (Kotov and Reiter, 2005)
10 0
10 0 10 -1 10 -2 10 -3 10 -4
H 2 v=0 1 2 3 4 5 6 7 8 T e =2.0eV
n e =10 14 cm -3 n H2 =1.0 cm -3 573K at wall 9
10 -5 0 1 2 3 Distance from wall (cm) 4 All H 2 from the wall in v=0
10 -1 10 -2 10 -3 10 -4 10 -5 0 T e =2.0eV
n e =10 14 cm -3 n H2 =1.0 cm -3 573K at wall H 2 v=0 3 2 1 4 5 6 9 8 7 1 2 3
Distance from wall (cm)
4
All H 2 from the wall in v=4
10 17 m
−
3
< n
e
<
10 20 m
−
3 , 1eV
< T
e
<
100 eV, 10
−
3
n
e
< n
I
<
10
−
1
n
e ,
n
Ho
≈
10
−
3
n
e
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 H 2 are precursor for H ion production by DEA
They are produced by • e-V: • E-V : H H 2 2 • Cascade: + e (slow) + e (fast) H 2 (X 1 g + , v’’) + e H 2 (B 1 u + H 2 (X 1 g + , v=0) + e • Recombinative desorption: ,C H 1 2 P u ) (E, F H 2 (B 1 u + ) 1 H 2 (X 1 g + , v’’) + h n g + ) H 2 (X 1 g + , v’’) + h n H + H + wall followed by the E-V excitation of the X 1 g + H 2 (X 1 g + , v’’ = 1, 2) state with the low v’’: H 2 (X 1 g + , v’’ = 1, 2) + e (fast) H 2 (B 1 u + ,C 1 P u ) H 2 (X 1 g + , v’’ 1, 2) + h n 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.
The number of molecular species observed in various regions in space is steadily increasing
2
H 2 AlF NaCl OH PN SO SO+ SiN SiO SiS CS HF AlCl C 2 CH CH + CN CO CO + CP CSi HCl KCl NH NO NS SH FeO
3
C 3 C 2 H C 2 O C 2 S CH 2 HCN HCO HCO + HCS+ HOC+ H 2 O H 2 S HNC HNO MgCN MgNC N 2 H+ N 2 O NaCN OCS SO 2 c-SiC2 CO 2 NH 2 H 3 + SiCN AlNC
4
c-C 3 H l-C 3 H C 3 N C 3 O C 3 S C 2 H 2 CH 2 D+ ?
HCCN HCNH+ HNCO HNCS HOCO+ H 2 CO H2CN H 2 CS H 3 O + NH 3 SiC 3
5
C 5 C 4 H C 4 Si l-C 3 H 2 c-C 3 H 2 CH 2 CN CH 4 HC 3 N HC 2 NC HCOOH H 2 CHN H 2 C 2 O H 2 NCN HNC 3 SiH 4 H 2 COH +
6
C 5 H l-H 2 C 4 C 2 H 4 CH 3 CN CH 3 NC CH 3 OH CH 3 SH HC 3 NH + HC 2 CHO NH 2 CHO C 5 N >160 Interstellar Molecules
7
C 6 H CH 2 CHCN CH 3 C2H HC 5 N NH 2 CH 3 HCOCH 3 c-C 2 H 4 O CH 2 CHOH
8
CH 3 C 3 N HCOOCH 3 CH 3 COOH C 7 H H 2 C 6 CH 2 OHCHO
National Radio Astronomy Observatory,
9
CH 3 C 4 H CH 3 CH 2 CN (CH 3 ) 2 O CH 3 CH 2 OH HC 7 N C 8 H
(http://www.cv.nrao.edu/~awootten/allmols.html
10
CH 3 C 5 N?
(CH 3 ) 2 CO NH 2 CH 2 COOH ?
11
HC 9 N
12
C 6 H 6
13+
HC 11 N PAHs C60 +
(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 C n H and C n H 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% N 2 + 2% CH 4 + 1% C 2 H 2 , C 2 H 4 ,….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
taken into account.
is fast for bigger carbon chain molecules as for C 6 H but very slow for light ones.
- Anions from
thermal energy electron capture
– not - Data for
DEA
are used (CH 4 ,C 2 H 2 , estimate for C 4 H 2 and C 6 H 2 .
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 H 2 ).
Polymerization
(e.g. C 2n H + C 2 H 2 → C + C 2n+2 2 H H 2 →C + H 2 ).
2 H +
Low energy H yield from DEA to small hydrocarbons
0,6 0,5 0,4 0,3 0,2 0,1 0,0 10 15 Electron energy [ eV ] 20 0,25 0,20 0,15 C 2 H 2 0,10 0,05 0,00 5 10 Electron energy [eV] 15 0,2 0,1 0,0 0,4 5 C 3 H 8 10 15 Electron energy [ eV ] 0,3 0,2 0,1 0,0 5 10 15 Electron energy [eV]
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 Sloan and Palmer, Nature
434
(2005) 367 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”
DEA at surfaces – dense layers • Group of L. Sanche, Univ. of Sherbrooke, Quebec, Canada • Group of R. Azria, A. Lafosse… , UPS, Orsay, France
D from amorphous ice at 190 K
Simpson et al., J.Chem.Phys.
107
(1997) 8668 Lafosse et al., Phys. Chem. Chem. Phys.
8
(2006) 5564 –5568
DEA in biomolecules
Radiation Damage Electron driven rections
Thymine
(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 production Data collection, evaluation and recommendations
Needs
Users Modelling
Sensitivity analysis
Data formatting DATABASE
Data production 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 – production and needs
Data evaluation Data “shaping”
Collection from all available sources, new and old.
Formation of standardised data bases Evaluation of applied methods and claimed accuracy.
Appropriate data formats Recomdation of best data to be used.
Accessibility Feedback with data producer.
Recommendations for new measurements or calculations.
Data usage
Modelling of complex processes, new technological procedures, processes in other sciences.
Sensitivity analysis Feedback to data producers.
Current activities
List of laboratories actively participating in present DEA research
Sherbrooke, Canada ( Léon Sanche, biomolecules, surfaces, experiment, theory) Lincoln, Nebraska ( Belfast ( Innsbruck ( nanodroplets) Paul Burrow, Gordon Gallup, experiment; Ilya Fabrikant, theory) Davis & Berkeley, CA ( Ann Orel, Tom Rescigno, Bill McCurdy : theory; H. Adaniya : DEA experiment – COLTRIMS) Tom Field; Gleb Gribakin, ToF DEA, biomolecules; theory) Paul Scheier, Tilmann Märk, Stefan Denifl, biomolecules, collisions in He Fribourg (Michael Allan) Berlin ( Eugen Illenberger, biomolecules) Open University, Milton Keynes ( Nigel Mason, Jimena Gorfinkiel, experiment, theory) Bratislava, Slovakia ( Štefan Matej č ik) 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)
Available data
List is too long to be presented here – only examples:
Diatomic:
H 2 , O 2 , CO, NO, S 2 , Cl 2 , Br 2 , HF, HCl, HBr
Triatomic:
HCN H 2 O, CO 2 , CS 2 , H 2 S, O 3 , SO 2 , N 2 O, NO 2 ,
Small polyatomic:
SF 6 CH 4 , NH 3 , BF 3 , C 2 H 2 , CCl 2 3 F 2 , C 6 H 2 6 , 5
Big molecules:
C 60 , HCOOH, C 2 H 5 NO 2 , uracil, glicine, HFFA (CF
3 ) 2 C=N-N=C(CF molecular clusters 3 ) 2 ), tymine, various
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 • S Matejcik, A Kiendler, P Cicman, J Skalny, P Stampfli, E Illenberger, Y Chu, A ¨ atmospheric relevance: oxygen and ozone, Plasma Sources Sci. Technol. 6 (1997)