OPTOGALVANIC PHOTODETACHMENT
SPECTROSCOPY
I. Mcdermid, C. Webster
To cite this version:
I. Mcdermid, C. Webster.
OPTOGALVANIC PHOTODETACHMENT SPECTROSCOPY. Journal de Physique Colloques, 1983, 44 (C7), pp.C7-461-C7-466.
<10.1051/jphyscol:1983745>. <jpa-00223302>
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https://hal.archives-ouvertes.fr/jpa-00223302
Submitted on 1 Jan 1983
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JOURNAL DE PHYSIQUE
page C7-461
Colloque C7, suppl6ment au n o l l , Tome 44, novembre 1983
O P T O G A L V A N I C PHOTODETACHMENT SPECTROSCOPY
I.S.
McDermid and C.R. Webster*
J e t PropuZsion Laboratory, CaZifornia I n s t i t u t e o f TechnoZogy,
4800 Oak Grove Drive, Pasadena, Cazifornia 91109, U.S.A.
Resum6 - La s p e c t r o s c o p i e optogalvanique a S t S Stendue 2 une technique nouvell e dans l a q u e l l e des e l e c t r o n s s o n t dEtachEs des i o n s n e g a t i f s formSs dans l a
dScharge e t observes en f o n c t i o n de l a longueur d'onde du l a s e r . La determinat i o n des a f f i n i t e s e l e c t r o n i q u e s des i o n s atomiques I- e t Cl- e s t d S c r i t e . Les
p o s s i b i l i t g s de c e t t e methode pour E t u d i e r l a s p e c t r o s c o p i e des i o n s n g g a t i f s
molEculaires s o n t a u s s i p r e s e n t e e s .
-
Abstract
A new e x t e n s i o n t o o p t o g a l v a n i c spectroscopy, i n which e l e c t r o n s
detached from n e g a t i v e i o n s formed i n t h e d i s c h a r g e a r e observed a s a f u n c t i o n
o f i n c i d e n t l a s e r wavelength, h a s been developed.
The d e t e r m i n a t i o n of t h e
e l e c t r o n a f f i n i t i e s of
and C l - a t o m i c i o n s i s described.
The p o t e n t i a l of
t h e technique f o r s t u d y i n g t h e spectroscopy of molecular n e g a t i v e i o n s i s a l s o
discussed.
r
1.
Introduction
The o p t o g a l v a n i c e f f e c t h a s p r o v e n p a r t i c u l a r l y s u i t a b l e f o r t h e d e t e c t i o n o f
unstable, r a d i c a l and i o n i c , a t o m i c and molecular s p e c i e s /l-3/,
especially since
t h e s e c a n o f t e n be g e n e r a t e d by t h e d i s c h a r g e i t s e l f . I n a new e x t e n s i o n t o t h e
o p t o g a l v a n i c technique /4/ w e have shown t h a t t h e r a d i a t i o n induced detachment of
e l e c t r o n s from n e g a t i v e i o n s i n t h e discharge can be used t o s t u d y t h e spectroscopy
of t h e s e n e g a t i v e ions, I n t h i s paper w e w i l l review t h e technique and i t s f i r s t
a p p l i c a t i o n t o t h e measurement of t h e e l e c t r o n a f f i n i t y of I-. New d a t a concerning
t h e e l e c t r o n a f f i n i t y of Cl' w i l l a l s o be presented a s w i l l a planned a p p l i c a t i o n
t o study t h e s p e c t r a of molecular n e g a t i v e ions.
The dc d i s c h a r g e c e l l and a s s o c i a t e d c i r c u i t r y have been d e s c r i b e d i n a previous
paper i n t h i s e d i t i o n (Webster and Menzies).
I n t h i s study t h e c e l l windows were
o f q u a r t z and a p u l s e d d y e l a s e r pumped by e i t h e r a n i t r o g e n o r a XeCl e x c i m e r
l a s e r was u s e d u s e d a s t h e e x c i t a t i o n s o u r c e . For t h e s t u d i e s o f 'I a n d C l ' t h e
c e l l was o p e r a t e d i n a f l o w i n g mode w i t h t h e c e l l p r e s s u r e and f l o w r a t e c o n t r o l l e d
by n e e d l e v a l v e s on b o t h t h e r e a g e n t i n l e t a n d vacuum pump.
Typical o p e r a t i n g
p r e s s u r e was
l 0 0 mTorr which, w i t h a d c v o l t a g e o f 400
700 V a c r o s s t h e c e l l ,
40 uA.
A 0.1 U F c o u p l i n g c a p a c i t o r
gave d i s c h a r g e c u r r e n t s i n t h e r a n g e 1 0
allowed laser-induced a c changes i n t h e discharge impedance t o be monitored w i t h a
p r e a m p l i f i e r (PARC 113) connected t o a boxcar i n t e g r a t o r (PARC 162/165).
-
-
E l e c t r o n detachment from halogen n e g a t i v e i o n s (X")
-
i s observed, a c c o r d i n g t o
Although t h e c o n c e n t r a t i o n of n e g a t i v e i o n s i n t h e discharge i s g r e a t e r t h a n t h a t
o f f r e e e l e c t r o n s / 5 / t h e d i s c h a r g e c u r r e n t i s c a r r i e d a l m o s t e n t i r e l y by t h e s e
e l e c t r o n s because of t h e i r much g r e a t e r mobility.
S t u d i e s of t i m e and s p a t i a l l y
r e s o l v e d o p t o g a l v a n i c s i g n a l s show t h a t i t i s p o s s i b l e t o d i s t i n g u i s h very c l e a r l y
* ~ r .Webster was u n f o r t u n a t e l y unable t o come t o t h e Colloquium because of a r e s c h e d u l i n g of a n a i r b a l l o o n experiment b u t h a s s e n t t h e paper h e would have p r e s e n t e d and which i s p u b l i s h e d i n t h e Proceedings.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983745
JOURNAL DE PHYSIQUE
CATHODE
RV (-1
Figure 1. Typical time-resolved
p r o f i l e s f o r LOG s i g n a l s
generated a t discharge l o c a t i o n s
i d e n t i f i e d t o t h e l e f t of t h e
figure.
S i g n a l s have been
n o r m a l i z e d t o t h e same peak
height.
CATHODE
DARK SPACE
NEGATIVE
GLOW
FAPADAY
DARK SPACE
COLUMN
QOSlTlVE
ANODE GLOH
ANODE
Cl
o
~
r
n
m
a
o
m
r
m
TIME AFlER LASER PULSE (CS)
the photodetac;unent s i g n a l from o t h e r optogalvanic s i g n a l s a r i s i n g , f o r example,
f r o m a b s o r p t i o n by t h e n e u t r a l d i a t o m i c h a l o g e n m o l e c u l e s . F i g u r e 1 shows t h e
time-resolved p r o f i l e s of t h e LOG s i g n a l generated by a b s o r p t i o n i n t h e B-X system
of I a t d i f f e r e n t l o c a t i o n s i n t h e d i s c h a r g e /6/. It s h o u l d be n o t e d t h a t e v e n
By
f o r %he f a s t e s t of t h e s e s i g n a l s t h e r i s e t i m e i s o n t h e o r d e r o f 50 VS.
c o n t r a s t , f i e r e 2 shows t h e s i g n a l caused by photodetachment of e l e c t r o n s from I-.
The pulse corresponds t o a n i n c r e a s e i n discharge c u r r e n t and r i s e s w i t h t h e l a s e r
pulse (duration
10 n s ) , t h e o b s e r v e d r i s e and f a l l t i m e of t h e s i g n a l b e i n g
l i m i t e d by c i r c u i t response time constants.
Therefore, by c a r e f u l adjustment of
t h e a p e r t u r e w i d t h and d e l a y a f t e r t h e l a s e r p u l s e i t i s p o s s i b l e t o m o n i t o r
uniquely t h e s i g n a l s due t o t h e photodetachment.
-
PHOTODETACHMENT
SIGNAL AT 402 nrn
F i g u r e 2.
The t i m e - r e s o l v e d
optogalvani c signal f o r
p h o t o d e t a c h m e n t f r o m 1- a t 4 0 2
nm.
The photodetachment s i g n a l
showed no s i g n i f i c a n t s p a t i a l
v a r i a t i o n , except a s m a l l delay
associated with electron
mobility.
The optogalvanic-electron s i g n a l observed a s t h e l a s e r wavelength was scanned i n
t h e r e g i o n 402.50 t o 406.00 nm i s shown i n f i g u r e 3. The p o t a s s i u m L I F s p e c t r u m
i n c l u d e d i n t h i s t r a c e w a s used, t o g e t h e r w i t h e t a l o n f r i n g e s , t o c a l i b r a t e t h e
a b s o l u t e wavelength scale.
LASER WAVELENGTH (nm:Air)
Figure 3. The i o d i n e photodetachment optogalvanic spectrum near t h e 405 nm
threshold.
The potassium L I F spectrum used f o r wavelength c a l i b r a t i o n i s
shown i n t h e l o w e r t r a c e , which i s o f f s e t f r o m z e r o t o s e p a r a t e t h e two
s p e c t r a The e t a l o n f r i n g e s have been o m i t t e d from t h i s f i g u r e s i n c e they
W e r e t o o c l o s e l y spaced f o r c l e a r r e p r e s e n t a t i o n
Near t h e photodetachment t h r e s h o l d t h e optogalvanic s i g n a l was observed t o r i s e i n
However, a t s h o r t e r wavelengths t h e i n c r e a s e
accordance w i t h t h e Wigner law /7/.
i n t h e optogalvanic s i g n a l was s m a l l e r than expected from t h e p r e d i c t e d i n c r e a s e i n
t h e detachment c r o s s s e c t i o n I n o r d e r t o i d e n t i f y t h e threshold we used t h e semie m p i r i c a l method of Berry e t a 1 /a/, i.e., the point of i n f l e c t i o n of t h e observed
Figure 4. Schematic p l o t of t h e
photode tachment s i g n a l near t h e
threshold.
The t h r e s h o l d
behavior r e d i c t e d by t h e Wigner
(E-Bthr)f/2 dependence i s shown
by t h e c i r c l e s .
The b r o k e n
l i n e s i l l u s t r a t e t h e method of
Berry and co-workers /8/ used t o
determine t h e t h r e s h o l d energy
Ethr shown by t h e arrow.
404.8
405.0
405.2
LASER WAVELENGTH (nm:Air)
405.4
C7-464
JOURNAL DE PHYSIQUE
and t h e tangent t o t h e curve a t t h e point
curve was taken a s t h e upper l i m i t (E,
of i n f l e c t i o n defined t h e lower l i m i t
The threshold energy was then taken
as t h e mean of these two l i m i t s a s shown s c h e m a t i c a l l y i n f i g u r e 4. The threshold
wavelength was thus determined t o be 405.18 2 0.02 nm corresponding t o a threshold
energy f o r photodetachment of 3.0591 2 0.0001 eV.
4.
m e C l - Photodetachment Soectrum
Preliminary s t u d i e s of t h e photodetachment from Cl' and Br' atoms have been made.
The primary mechanism f o r t h e production of t h e atomic halogen negative i o n s i n t h e
low p r e s s u r e discharge i s through d i s s o c i a t i v e attachment of t h e diatomic n e u t r a l ,
The c r o s s s e c t i o n s f o r t h i s process a r e much lower f o r C 1 and Br2 than f o r I /5/
and t h u s t h e X- c o n c e n t r a t i o n s a r e much lower.
Figure 5 &ows t h e photodetaciment
s p e c t r u m of C l ' r e c o r d e d a s t h e l a s e r w a v e l e n g t h w a s s c a n n e d f r o m 344.0 t o 341.5
nm.
Although t h i s s p e c t r u m r e p r e s e n t s p r e l i m i n a r y d a t a i t i s c l e a r t h a t t h e
signal-to-noise r a t i o i s much lower than i n t h e i o d i n e spectrum.
Nonetheless, t h e
s p e c t r u m i n d i c a t e s t h e o n s e t of d e t a c h m e n t a t 342.55 f 0.25 nm.
These e r r o r
l i m i t s a l s o r e f l e c t some u n c e r t a i n t y i n t h e a b s o l u t e w a v e l e n g t h s i n c e a f u l l
c a l i b r a t i o n h a s n o t y e t been made.
This corresponds t o a n energy t h r e s h o l d of
3.6184 2 0.0025 eV.
344 0 343.5
343.0
342.5
342.0
341.5
LASER WAVELENGTH (nm)
Figure 5.
The c h l o r i n e photodetachment optogalvanic spectrum.
A t f i r s t i t would seem t h a t o t h e r compounds w i t h l a r g e r e l e c t r o n attachment c r o s s
s e c t i o n s , such a s halogenated polyatomic molecules, might provide b e t t e r sources of
Cl' i o n s .
I t h a s been n o t e d , f o r example, t h a t m u l t i p l e h a l o g e n s u b s t i t u t i o n
g r e a t l y i n c r e a s e s t h e d i s s o c i a t i v e attachment c r o s s s e c t i o n and t h u s t h e l a r g e s t
However, n e a r t h e
c r o s s s e c t i o n s a r e f o u n d f o r m o l e c u l e s s u c h a s CC14 /g/.
t h r e s h o l d p h o t o d e t a c h e d e l e c t r o n s h a v e no e x c e s s k i n e t i c e n e r g y and s i n c e t h e
m a g n i t u d e of t h e d i s s o c i a t i v e a t t a c h m e n t c r o s s s e c t i o n f o r C C 1 4 f o r e l e c t r o n
e n e r g i e s < 1 eV i s very l a r g e , these e l e c t r o n s a r e e f f i c i e n t l y scavenged and t h u s
cannot c o n t r i b u t e t o t h e optogalvanic signal.
5.
.
.
P o s s i b l e F i e l d and Environmental E f f e c t s o n t h e E l e c t r o n A f f i n l k y
De t e r m i n a t i o m
I n t h e l a s e r o p t o g a l v a n i c t e c h n i q u e r e p o r t e d h e r e t h e s i m p l e , compact, and
i n e x p e n s i v e d i s c h a r g e t u b e r e p l a c e s t h e complex i o n g e n e r a t i o n , h a n d l i n g and
d e t e c t i o n apparatus of t h e crossed-beam method. However, t h e energy r e s o l u t i o n i n
t h e optogalvanic measurements could be blurred due t o t h e i n f l u e n c e s of t h e complex
discharge environment.
C o n t r i b u t i o n s t o t h e inhomogeneous l i n e w i d t h i n a d c
d i s c h a r g e i n c l u d e b o t h t h e Doppler e f f e c t , due t o t h e m o t i o n o f t h e i o n s i n t h e
e l e c t r i c f i e l d , and S t a r k b r o a d e n i n g e f f e c t s due t o t h e p r e s e n c e o f f l u c t u a t i n g
microfields.
I n a d d i t i o n t o broadening, t h e photodetachment t h r e s h o l d energy may
be s h i f t e d by b o t h Doppler a n d f i e l d e f f e c t s . The i n f l u e n c e of t h e s e f a c t o r s i l l
determining t h e e l e c t r o n a f f i n i t y of I- i n t h i s work i s now considers,.
The e s t i m a t e d p o t e n t i a l g r a d i e n t i n t h e d i s c h a r g e , away f r o m t h e c a t n o d e f a l l
r e g i o n , i s 10 V cm'l and t h u s t h e m o b i l i t y o f I' i o n s i n a n e n v i r o n m e n t of 0.1
Torr I2 i s c a l c u l a t e d t o be o n the o r d e r o f 105 cm S-'.
While t h i s d r i f t v e l o c i t y
i s almost a n o r d e r of magnitude l a r g e r than t h e average molecular v e l o c i t i e s i n t h e
discharge, t h e t r a n s v e r s e geometry of t h e l a s e r probe e n s u r e s t h a t t h e component of
t h e d r i f t v e l o c i t y i n t h e l a s e r beam d i r e c t i o n i s very s m a l l .
The Doppler e f f e c t
t h e r e f o r e h a s a n e g l i g i b l e e f f e c t o n t h e p o s i t i o n of t h e t h r e s h o l d , and i t s
a p p a r e n t w i d t h , which i s c a l c u l a t e d t o be 0.02 cm",
i s much s m a l l e r t h a n t h e
I t i s p e r t i n e n t t o compare t h e Doppler s h i f t s
l a s e r l i n e w i d t h of 0.6 cm'l.
e x p e c t e d i n t h e o p t o g a l v a n i c e x p e r i m e n t w i t h those observed i n t h e crossed-beam
e x p e r i m e n t s i n which t h e i o n s a r e a c c e l e r a t e d t o h i g h k i n e t i c e n e r g i e s . While,
l i k e t h e t r a n s v e r s e g e o m e t r y of t h e o p t o g a l v a n i c e x p e r i m e n t , t h e crossed-beam
geometry minimizes the s h i f t , t h e angular divergence of t h e beams ( i o n and l a s e r )
i s u s u a l l y s u f f i c i e n t t o i n t r o d u c e s i g n i f i c a n t Doppler s h i f t s t o t h e o b s e r v e d
photodetachment threshold.
..
-
I n t h e low p r e s s u r e discharge used i n t h e optogalvanic experiment t h e negative i o n s
under study a r e not i n a n i d e a l f i e l d - f r e e environment. Rather, i n a d d i t i o n t o t h e
discharge f i e l d discussed above, each i o n i s surrounded by an atmosphere of charged
The motions of t h e s e s p e c i e s
s p e c i e s described by t h e Debye-Hiickel p o t e n t i a l /TO/.
cause t h i s p o t e n t i a l t o f l u c t u a t e thereby causing a broadening o f the
p h o t o d e t a c h m e n t t h r e s h o l d due t o t h e S t a r k e f f e c t . The c o n t i n u u m l i m i t i s more
s e n s i t i v e t o S t a r k e f f e c t s t h a n t h e ground s t a t e .
Such b r o a d e n i n g h a s been
o b s e r v e d i n a b s o r p t i o n s t u d i e s of shock-produced p l a s m a s / l l / , e v i d e n c e d by t h e
o b s e r v a t i o n o f a 1 5 cm" + e d t a i l n t o t h e t h r e s h o l d . The m a g n i t u d e of t h e S t a r k
b r o a d e n i n g i n t h e dc d i s c h a r g e employed h e r e c a n be e s t i m a t e d f r o m a c o m p a r i s o n
w i t h t h e shook-plasma experiments.
The c r i t i c a l f a c t o r governing t h e i n t e n s i t y of
Electron d e n s i t i e s of 1o7
and a
t h e m i c r o f i e l d s i s t h e i o n number d e n s i t y n.
negative i o n t o e l e c t r o n r a t i o of 102 have been measured /12/ i n t h e Faraday dark
s p a c e and p o s i t i v e column o f a n i o d i n e glow d i s c h a r g e under c o n d i t i o n s c l o s e 1
s i m i l a r t o those used i n t h i s sork. The negative i o n c o n c e n t r a t i o n of n = l o g cm'
i s s i x o r d e r s of magnitude l e s s than t h a t e s t i m a t e d f o r t h e shock-produced plasma.
The e f f e c t o f S t a r k b r o a d e n i n g i n t h e o p t o g a l v a n i c d i s c h a r g e i s t h e r e f o r e s m a l l
compared t o o t h e r broadening mechanisms.
3
For photodetachment (photoionization) of a n e u t r a l species, f i e l d enhancement can
cause l a r g e s h i f t s i n t h e threshold energy.
This is because t h e photodetachment
continuum i s bounded by Rydberg l e v e l s which c a n be e f f e c t i v e l y f i e l d i o n i z e d .
T h i s i s n o t t h e c a s e f o r a n e g a t i v e i o n s i n c e t h e Rydberg s t a t e s a r e a b s e n t .
Furthermore, t h e Debye-Hiickel p o t e n t i a l a f f e c t s a l l s t a t e s by t h e same, constant
amount and t h e r e f o r e w i l l not change t h e threshold energy.
The method of a n a l y s i s employed i n c l u d e s t h e wavelength c a l i b r a t i o n uncertainty,
and t h e (semiempirical) procedure used f o r t h e t h r e s h o l d d e t e r m i n a t i o n i n c o r p o r a t e s
a l l broadening f a c t o r s .
The s h a r p r i s e o b s e r v e d i n t h e p h o t o d e t a c h m e n t c r o s s
s e c t i o n (see Figs. 3 & 5) a t t e s t s t o t h e absence of s i g n i f i c a n t broadening effects.
The e l e c t r o n a f f i n i t y of i o d i n e was t h e r e f o r e determined t o be,
and f o r c h l o r i n e we f i n d
JOURNAL D€ PHYSIQUE
C7-466
The agreement between t h e s e v a l u e s and those from previous s t u d i e s /9,13/ s u g g e s t s
t h a t t h e m a g n i t u d e o f t h e e r r o r s o u r c e s i n t h e o p t o g a l v a n i c method have been
a s s e s s e d correctly.
6.
Photodetac&ncnt SDectroscoDv f o r t h e Studv of Molecular Negative Ion9
I n a r e c e n t p a p e r /14/ S c h u l z e t a 1 have d e s c r i b e d t h e a p p l i c a t i o n o f t h r e s h o l d
We suggest t h a t t h e i o n
photodetachment spectroscopy t o t h e study of ON- and OD-.
beam s o u r c e i n t h i s t y p e o f e x p e r i m e n t c o u l d be r e p l a c e d by a n o p t o g a l v a n i c
d i s c h a r g e c e l l . I n t h e s t u d i e s of OH' t h e p h o t o d e t a c h m e n t w a s a s i n g l e p h o t o n
p r o c e s s . The s p e c t r u m a p p e a r s a s a s e r i e s o f a b r u p t r i s e s o n t o p o f a g e n e r a l l y
r i s i n g s i g n a l and r e f l e c t s t h e i n c r e a s e s i n t h e photodetachment c r o s s s e c t i o n w i t h
both i n c r e a s i n g photon energy and t h e opening of new detachment channels.
Apart
from the c o n s t a n t l y r i s i n g background, t h e d i f f e r e n t i a l of t h e observed spectrum
c o r r e s p o n d s t o t h e a b s o r p t i o n s p e c t r u m o f t h e n e g a t i v e i o n and i n f o r m a t i o n
r e g a r d i n g t h e r o t a t i o n a l and v i b r a t i o n a l s t r u c t u r e can be e x t r a c t e d i n t h e normal
way.
~~c-
Many molecular negative i o n s have s t a b l e and a c c e s s i b l e e l e c t r o n i c s t a t e s below t h e
x2z+ t r a n s i t i o n
photodetachment threshold. For example t h e 0-0 band of t h e
of C: l i e s n e a r 18450 cm".
A second photon w i t h t h i s energy i s s u $ f i c i e n t t o
p h o t o d e t a c h t h e e l e c t r o n f r o m t h e B s t a t e a n d t h u s t h e B-X t r a n s i t i o n c a n be
s t u d i e d by means of two-photon resonant detachment /15/.
I n t h i s case t h e spectrum
w i l l appear a s a normal s e r i e s of peaks r a t h e r than s t e p s a s i n t h e s i n g l e photon
process.
Also, i n a n a n a l o g o u s t e c h n i q u e t o m u l t i - c o l o u r r e s o n a n t i o n i z a t i o n
spectroscopy two o r more photons of d i f f e r e n t e n e r g i e s can be used.
S i n c e p h o t o d e t a c h e d e l e c t r o n s c a n be m o n i t o r e d unambiguously i n a n optogalvanic
e x p e r i m e n t , a n d s i n c e t h e l o w p r e s s u r e d c d i s c h a r g e can produce s i g n i f i c a n t
c o n c e n t r a t i o n s of n e g a t i v e i o n s , o p t o g a l v a n i c photodetachment spectroscopy holds
g r e a t p r o m i s e f o r t h e s p e c t r o s c o p i c s t u d y o f both a t o m i c and m o l e c u l a r n e g a t i v e
ions.
The r e s e a r c h d e s c r i b e d i n t h i s p a p e r w a s p e r f o r m e d a t t h e J e t P r o p u l s i o n
Laboratory, C a l i f o r n i a I n s t i t u t e of Technology, under c o n t r a c t w i t h t h e National
Aeronautics and Space Administration.
7.
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