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