Download Anions Desorption induced by low-energy electron collisions with

© Paweł Możejko
Negative ion desorption from condensed
molecules induced by low energy
electrons
Paweł Możejko
Atomic Physic Group
Department of Atomic Physics and Luminescence
Faculty of Applied Physics and Mathematics
Gdańsk University of Technology, Poland
Department of Nuclear Medicine and Radiobiology
Faculty of Medicine
University of Sherbrooke, Canada
© Paweł Możejko
Interaction of charged particles with matter in the
nature ...
Photo by Bożena Żywicka-Możejko
© Paweł Możejko
...and laboratory
UHV conditions 10-10-10-11 Torr
Vapor deposited films on metal substrate
held at cryogenic temperatures
(17 K< T <300K)
Monochromatic electron beam produced by
electrostatic monochromator (E~ 20-80 meV)
ESD – electron stimulated desorption of anions
as a function of incident electrons energy
(yield function) and current
© Paweł Możejko
Applications
Radiation damage to biomolecules from
secondary electrons
Dielectric aging
Electron lithography
Nanolithography
Astrophysics
© Paweł Możejko
Experimental setup
10-10 Torr
Monoenergetic electron beam
E= 0-20 eV ∆E= 20-80 meV (FWHM) I0= 1 nA
Uncertainty in the film thickness ~ 50 %
Magnetic field reduced to less than 15 mG
Target temperature T~20 K (17 K < T <300 K)
© Paweł Możejko
Intrinsic and extrinsic factors in ESD
process
Dissociative electron attachment (DEA)
Dipolar dissociation (DD)
e- + AB → AB*- → A- + B
e-(E) + AB → (AB)* + e-(E’)
→ A- + B+ + e-(E’)
Intrinsic effects modify and perturb TNI
states during DEA and the electronically
excited neutral states leading to DD
Extrinsic effects influence the anion ESD
process at times before the formation of
the intermediate anion and after its decay
Intrisic – interaction between TNI and the image charge it induces in the supporting
molecular film and metal substrate
– number of decay channels available for an autodetachment
Extrinsic – electron scattering (and energy losses) prior to the formation of either the DEA
or DD intermediate species
– fragment anions interaction with their environment
© Paweł Możejko
Intrinsic, extrinsic and porosity effects in ESD of
O- from O2 condensed on different substrates
M.A. Huels, L. Parenteau, and L. Sanche,
J. Chem. Phys. 100, 3940 (1994)
Reduction in the
DEA and DD
processes !!!
For condensed molecules DEA and DD are strongly perturbed by
the surrounding medium i.e., the neighboring molecules and
molecular solid surface
© Paweł Możejko
Interpretation problem
How to differentiate between intrinsic and
extrinsic factors influencing ESD anion
yields
© Paweł Możejko
ESD of D- from deuterocarbons
condensed on Kr and H2O films
Saturated deuterocarbons: CD4, C2D6, C3D8
Unsaturated deuterocarbons: C2D2, C2D4
Substrates: clean platinum foil and multilayers of Kr and
H2O ice of varying preparation
Use of deuterated compounds permits us to discriminate
between anion yields from ice and the molecular
absorbate
Rare gas solids provide an almost perfectly inert host
surface – number and efficiency of extrinsic and intrinsic
processes are very small – reference signal !
© Paweł Możejko
ESD from pure films of condensed
hydrocarbons
Similar intensity for
saturated and unsaturated
hydrocarbons
Mainly H- observed to
desorb
CH-, CH2-, CH3-,C2H- less than 1% of the
intensity of the H- signal
P. Rowntree, L. Parenteau, and L. Sanche, J. Phys. Chem. 95, 4902 (1991)
© Paweł Możejko
ESD from pure films of deutereted
hydrocarbons
Formation of the D- occurs via
dissociative electron attachment
and dipolar dissociation processes
A single resonant maximum –
core excited resonances
No desorbing heavier polyatomic
ions were observed due to
partitioning of the kinetic energy
and isotope effects in production
of negative ions.
”D- ion desorption from condensed CD4, C2D2, C2D4, C2D6 and C3D8 molecules induced by electron impact”
P. Możejko, L. Parenteau, A. D. Bass and L. Sanche, Radiat. Phys. Chem. 68, 215 (2003)
© Paweł Możejko
ESD from saturated deuterated hydrocarbons
”Intrinsic and extrinsic factors in anion electron-stimulated desorption: D- from deuterated
hydrocarbons condensed on Kr and water ice films”
P. Możejko, A. D. Bass, L. Parenteau and L. Sanche, J. Chem. Phys. 121, 10181 (2004)
© Paweł Możejko
ESD from unsaturated deuterated hydrocarbons
”Intrinsic and extrinsic factors in anion electron-stimulated desorption: D- from deuterated
hydrocarbons condensed on Kr and water ice films”
P. Możejko, A. D. Bass, L. Parenteau and L. Sanche, J. Chem. Phys. 121, 10181 (2004)
© Paweł Możejko
Results
All D- yield functions display a threshold near 7 eV and a single
broad resonant maximum
The broad resonant like structure is attributed to D- abstraction via
DEA
e - + CnD2n+2 → CnD2n+2*- → D- + CnD2n+1
e - + CnD2n → CnD2n*- → D- + CnD2n-1
The D- yield functions for submonolayer quantities of the saturated
compounds condensed on Kr display a sharp peak in the vicinity of
9.7 eV
For unsaturated deuterocarbons the narrow Kr*- feature is absent
from the D- yield functions
Above 15 eV the ion yield is observed to increase linearly with the
incident electron energy indicating the involvement of the direct DD
process
© Paweł Możejko
Perturbation of ESD due to intrinsic and extrinsic
effects
On nonporous ice – perturbation of the ESD process have only intrinsic character –
no extrinsic !!!
ִ intrinsic perturbation of DEA due to a transfer of the excess electron from the
deuterocarbon anion to either an electron state of (H20)n in the film or an
neighboring molecule
ִ Intrinsic perturbation of DD for unsaturated molecules due to intrinsic
modulation of the autoionization of neutral excited state prior to dissociation
On porous ice – porosity of ice introduces extrinsic effect connected with
postdissociation interaction – DEA and DD suppressed be a factor of 2 relative to
nonporous ice
© Paweł Możejko
Application in Radiation damage to DNA
Radiation affects DNA in two ways:
Induces damage that compromises cell
function (an origin of cancer)
Induces cell death (good in context of
radiotherapy)
Low-energy secondary electrons are produced
in large quantities by ionizing radiation
A complete description of the mechanisms
responsible for such damage must involved
LEE studies on small molecular „sub-units”
of DNA, viz., H2O, DNA bases, and sugarphosphate backbone analogues
© Paweł Możejko
LEE interactions with DNA
Very low energy threshold for single and
double strand breaks (SSB and DSB)
DSB seems to be a single stage
(one-electron-hit) process
Highly structured dependence on energy
Resonances
At low electron energies
HCNOH-, O- (or NH2-)
OCN- and CH- from
B. Boudaiffa, P. Cloutier, D. Hunting,
Thymine have been
M.A. Huels, L.Sanche,
observed
Science 287 (2000) 1658
Abdoul-Carime et al.,
Radiat. Res.155 (2001) 625
© Paweł Możejko
New things to do
Low energy electron (LEE) damage to
DNA and its components
Reactive scattering within DNA
Cross section measurements
Cross section calculations
© Paweł Możejko
Calculations (simple or more
sophisticated)
P. Możejko and L. Sanche, Radiat. Phys. Chem. 73 (2005) 77-84
P, Możejko and L. Sanche, Radiat. Environ. Biophys. 42 (2003) 201-211
© Paweł Możejko
New experiments
Low energy electron (LEE) damage to tetrahydrofuran in the
presence of N2O
LEE damage induced in films of the deoxyribose analogues like
3-hydroxytetrahydrofuran and α-tetrahydrofurfuryl alcohol in the
presence of O2
Real living cell contains large amount of water !!!
Measurements of LEE induced damage to the deoxyribose
analogues (tetrahydrofuran etc) coadsorbed with water and
molecular oxygen
© Paweł Możejko
ESD from condensed multilayers films of
deoxyribose analogues
Only H- observed to desorb
Below 15 eV H- produced
by DEA
Yields similar for I, II and III
so H- probably from furan
ring.
D. Antic, L. Parenteau, M. Lepage and L. Sanche,
J. Phys. Chem. B 103 (1999) 6611
D. Antic, L. Parenteau and L. Sanche,
J. Phys. Chem. B 104 (2000) 4711
© Paweł Możejko
Reactive Scattering of 1-5 eV O- in Films of Tetrahydrofuran
e- + O2(3Σg-) → O2* -(2Πu , 2Σu+, 2Σg +) →
O-(2P) + O(3P or 1D)
e-(E) + O2 → O2* + e-(E’<E) → O- + O+ + e-(E’<E)
O- + C4H8O → [OC4H8O]*- → OH- + C4H7O
M. A. Huels, L. Parenteau and L. Sanche,
J. Phys. Chem. B 108 (2004) 16303
© Paweł Możejko
LEE induced damage in the THF + N2O
films
O- is produced via
dissociative electron
attachment (DEA) to
condensed N2O molecules:
e- + N2O → (N2O)- →N2 + O.
In the pure THF film case, His created via DEA to THF:
e- + C4H8O → (C4H8O)- →
H- + other fragments.
We have did not observe Oanions desorbed from the
pure film of THF.
OH- anionic fragment is
produced through reactions
of O- with THF molecule e.g.
O-+C4H8O→
→OH- + C4H7O
(or other fragments).
P. Możejko, L. Parenteau A.D. Bass and L. Sanche (2005), to be
published
© Paweł Możejko
3-hydroxytetrahydrofuran (C4H8O2)
and α-tetrahydrofurfuryl alcohol (C5H10O2)
condensed onto O2 films
The resonant structures in O- desorption signal
from multilayer of O2 in vicinity of 5-15 eV are due
to DEA to O2 molecules
e-+O2 → O2-(2Πu,2Σ+g,u) → O-(2P)+O(3P,1D).
None OH- are desorbed from pure films of
3-hydroxytetrahydrofuran and α-tetrahydrofurfuryl
alcohol
OH- is created in the reactions involving O- and
C4H8O2 and C5H10O2 interactions, respectively:
O-+C4H8O2 → OH- + other fragments,
O-+C5H10O2 → OH- + other fragments.
P. Możejko, L. Parenteau, A.D. Bass and L. Sanche
(2005), to be published
© Paweł Możejko
Acknowledgements
Paweł Możejko kindly acknowledges
financial support from CIHR in the form of
a Post-doctoral Fellowship.
Contributing Scientists:
Léon Sanche
Andrew D. Bass
Luc Parenteau
© Paweł Możejko
After hours ...
Photo by Bożena Żywicka-Możejko