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ELECTRON DETACHMENT OF ANIONS BY ATOMS AND MOLECULES
Nelson V. de Castro FariaDIRECTOR
Institute of PhysicsFederal University of Rio de Janeiro
Presentation
The work I did in the Nineties ...Electron detachment of negative ions.
New method: systematic studies. Experimental results.Simple model.
Working after turning 70 years old ...LaCAM (Laboratory of Atomic andMolecular Collisions).
The work I did in the Nineties ...
BRIEFLY...
Last results (cluster source of Lyon closed in 2001)
.
...then I moved from PUC to the Federal University of Rio de Janeiro!!!
Electron detachment of negative ions:
New method: systematicExperimental results.Simple model.
Accelerator
pressure calibration procedure
strip
B
P Lk T
π =
Molecular Flow
⇓
.
0 1 2 3 4 5 6 7 8 9 100
1
2
3
4
5
6
7
8
Pres
são
no S
tripp
er (1
0-3 T
orr)
Π (1 0 16 cm -2)
0 5 10 15 20 25 30 35 40 450.0
0.1
Valores Experimentais e Curva Teórica
Fraç
ão d
e H
0
Pressão (10-7 Torr)
Attenuation curves
3 0CFI I e σπ−=
0 5 1 00. 0
1. 0
2. 0
I= I0 e - σd Π
Cor
rent
e (u
nida
des
arbi
trár
ias)
Π ( 10 1 5 c m -2 )
Cross sections and
Anions with same electronic configuration (C, Si and Ge) colliding with
He, Ne and Ar
Anions of the 2nd and 3rd periods colliding with He, Ne and Ar
0.0 0.5 1.0 1.5 2.00.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
He
Ne
Ar
Li B C O F Al Si S Cl
σ
/ Σi (1
0-15 c
m2 )
v (a.u.)
Negative clusters colliding with He, Ne and Ar
0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,80
2
He
Ne
Ar
σ(10
-15 cm
2 )
v(u.a)
C1- C2
- C3- C4
-
Si1- Si2
- Si3- Si4
- Al1-
Al2- Al3
- Al4-
Several anions species colliding with molecular N2
0.0 0.2 0.4 0.6 0.8 1.0 1.20
1
2
3
40.0 0.2 0.4 0.6 0.8 1.0 1.2
0.0
0.2
0.4
0.6
0.8
1.0
N2
Bn
-
Al
n-
Al - Al2
- Al3
- Al4
- B - B
2
-
σ x
ki
v(u.a.)
B r -
I -
F -
Cl -
σ /
π (
r_ P+ r_ A
)2
0. 0 0. 5 1. 0 1. 50. 0
0. 2
0. 4
0. 6
0. 8
1. 0
1. 2
0 .0 0 .5 1 .0 1 .50
5
1 0
1 5
2 0
2 5
3 0
Col
isão
Elá
stic
a e
Inel
ástic
aσ
(u.a
rb.)
H e
N e
A r
Det
achm
ent
v (u .a .)
N eH e
A r
Does a negative ion behave like an free electron?
Electron s
Electron s
Anion s
Anion s
g(v-V) -
velocity
distribution
of the
anion´s
outermost
electron in the
atomic
anion reference
frame → Fourier transform
of the
negative
ion
wave
function
in coordinate
space
(for example: 3P state
of Al-, 2P state
of C-
and
2P state
of Si-)
σeletron (V) –
electron impact cross section for Ar
σeleconv
(v)
–
convoluted
anion´s
outermost
electron
cross section
for Ar
Semi-classical Model
( ) ( ) bvv eleconvanion +=σσ
Experimental destruction cross sections of Si-
n with n=1-4 and semi-classical model
0,0� 0,3� 0,6� 0,9� 1,2� 1,5�0,0�
0,5�
1,0�
1,5�
2,0�
2,5�
3,0�
3,5�
4,0�
4,5�Si�
4�
_�
Si�3�
_�
Si�2�
_�
Si �_�
�
�
v(a.u.)�
� �(1
0�-15� cm
�2� )�
eleconv�
Experimental destruction cross of negative clusters with N=2 and semi-
classical model
0,0� 0,3� 0,6� 0,9� 1,2� 1,5�1,0�
1,5�
2,0�
2,5�
3,0�
3,5�
Al�2�
_�
C�2�
_�
Si�2�
_�
�
�
v (a.u.)�
�(1
0�-1
5� cm�2� )�
Geometrical Interpretation
( )2. effAr RRb +=πRAr = 0.67A
Reff = effectice radius
Anion b (Å)2
Reff (Å) Rgeom (Å) EA (eV)
C−
6.0 0.7 0.66 1.26
C−2
9.7 1.1 0.63 3.3C−
3 13.5 1.4 1.28 1.95C−
4
16.5 1.6 1.23 3.7
Al−
9.0 1.0 1.21 0.43Al−2
13.5 1.4 1.43 1.46Al−3
18.5 1.8 1.51 1.89Al−4
21.5 1.9 1.93 2.20
Si−
10.0 1.1 1.06 1.39Si−2
17.0 1.7 1.12 2.2Si−3 23.0 2.0 1.40 2.18Si−4 27.0 2.3 1.92 2.0
Collision Chamber
T O F
OFmTq
≈
START
STOP
Gás0 V
Vext+ 400 V
Vext-
400 VAnel
-
100 VTubo
-
1300 VChanneltron
-
3100 V
0.0 0.5 1.0 1.50. 0
0. 2
0. 4
0. 6
0. 8
1. 0
1. 2
0.0 0.5 1.0 1.50
5
10
15
20
25
30
Col
isão
Elá
stic
a e
Inel
ástic
aσ
(u.a
rb.)
He
Ne
Ar
Det
achm
ent
v (u.a.)
NeHe
Ar
- Ramsauer-Townsend effect ????
ElectronsElectrons
AnionsAnions
- Can a negative ion behave like an anti-proton?
- Shape Resonance (N2 )
Some open questionsremain...
WORKING AFTER TURNING 70...
Negative ion trap.Twin atoms.
MechanicalMechanical AnalogAnalog of of thethe IonIon TrapTrap
PaulPenning
⎧⎨⎩
- E
x
e-
- E
x
íone-
Experimental set Experimental set upup
TwinTwin atomsatoms
Laboratoire Aimé CottonUniversité Paris Sud – Orsay
Jacques RobertGabriel Rahmat
IF-UFRJ-Rio de JaneiroCarlos Renato CarvalhoAline MedinaGinette JalbertWania Wolff
Experimental set Experimental set upup
SternStern--Gerlach Gerlach interferometerinterferometer
LaCAM
Laboratory Director:Nelson de Castro Faria-Ph.D. (1970-Univ. de Montreal)-Full Professor
Director for Operations:Wania Wolff-Dr.(1992-UFRJ)- Associate Prof.
Permanent Staff:Eduardo Chaves Montenegro-Dr. (1980-PUC-Rio)- Full ProfessorLuiz Felipe de S. Coelho-Ph.D. (1981-Univ. de Sussex)-Associate Prof.Ginette Jalbert de Castro Faria-Dr. (1985-PUC-Rio)- Associate Prof.Carlos Renato de Carvalho-Dr. (1986-PUC-Rio)- Associate Prof.Wilma Machado Soares Santos-Dr. (1996-UFRJ)- Associate Prof.Stenio Dore de Magalhães-Dr. (1997-UFRJ)- Associate Prof.Marcelo M. Sant’Anna-Dr. (1997-PUC-Rio)-Associate Prof.Antonio Carlos Fontes dos Santos-Dr.(1999-PUC-Rio)-Associate Prof. Maria Helena Pedra Martins-M.Sc. (1985-UFRJ)-Assistant Prof.Alexandre Braga Rocha–Dr.(2000-UFRJ-)- Associate Prof.Paulo Soares Carrilho-Dr. (1980-UFRJ)- Associate Prof.
THE END
THANK YOU
OBRIGADO
V(r)
Vefet.
E
r
2
( 1)2
l lmr+
E r
Shape Resonance (N2 )
Radial equation
2 2
, ,2 2
( 1)( ) ( ) ( )2 2 E l E l
d l lV r U r EU rm dr mr
⎡ ⎤+− + + =⎢ ⎥
⎣ ⎦
.efetV
Shape Resonance depends on the SHAPE of the potential
Resonance
The incident partcle stay “trapped”
Ramsauer-Townsend
2 22
2 2
( ) 22 c o s 2 ( ) 2
i k a q k s e n q aR i ek q q a i q k s e n q a
− −=
− +
22 2
22 c o s 2 ( ) 2
i k a k qT ek q q a i q k s e n q a
−=− +
2 02
2 ( )m E Vq +=
2 2 2
0 28nE V
maπ
= − +
1,2,3...n =Resonance 2qa nπ=R = 0 Max Transmission
V(x)E
-a a x
V0
No reflection
Does a negative ion behave like an anti-proton?
0
2
4
6
8
10
0
2
4
6
8
10
1,0 1,1 1,2 1,3 1,4 1,5 1,6 1,7 1,8 1,9 2,00
2
4
6
8
10
Ionização Direta
Perda Simples
σ +
+ (1
0 -18 cm
2 )
Perda Dupla
v (a.u.)
0
5
10
15
20
0
5
10
15
20
1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.00
5
10
15
20
Ionização Direta
Perda Simples
H
e ++ / H
e + (%
)Ionização com Perda Dupla
v (a.u)
X - + He ---> Xq + He (+ ou ++) + n e X = B: TrianglesX = O: Squares X = F: Círcles