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A.C. STARK SHIFT AS A PROBE TO MEASURE A LASER POWER DENSITY DISTRIBUTION AND ITS
APPLICATION TO MULTI-PHOTON IONIZATION
P. Kruit, J. Kimman, H. Muller, M.J. van der Wiel
To cite this version:
P. Kruit, J. Kimman, H. Muller, M.J. van der Wiel. A.C. STARK SHIFT AS A PROBE TO MEASURE A LASER POWER DENSITY DISTRIBUTION AND ITS APPLICATION TO MULTI-PHOTON IONIZATION. Journal de Physique Colloques, 1982, 43 (C2), pp.C2-457-C2-459.
�10.1051/jphyscol:1982241�. �jpa-00221853�
JOURNAL DE PHYSIQUE
Colloque C2, supplément au n°ll, Tome 43, novembre 1982 page C2-457
A . C , STARK SHIFT AS A PROBE TO MEASURE A LASER POWER DENSITY DISTRIBUTION AND ITS APPLICATION TO MULTI-PHOTON IONIZATION
P. K r u i t , J. Kimman, H.G. Muller and M.J. Van der Wiel
FOM Institute Voor Atom, Kruisslaan 407, 1098 SJ. Amsterdam, Holland
Résumé. - Dans le but de réaliser des expériences d'ionisation multiphotonique à une intensité bien déterminée de la lumière laser, nous avons u t i l i s é le déplacement lumineux du niveau 6s du Xénon, afin de sélectionner la région appropriée au foyer du laser. Ceci permet d'obtenir avec précision une large d i s t r i b u t i o n d'intensités à chaque puise de notre laser multi-mode. De cette façon, nous avons pu déterminer l'ordre réel de la non-linéarité dans le processus d'ionisation résonnante et la probabilité d'absorption de photons supplémentaires.
A b s t r a c t . - I n o r d e r t o perform m u l t i - p h o t o n i o n i z a t i o n experiments a t a par- t i c u l a r l i g h t i n t e n s i t y , t h e AC S t a r k - s h i f t o f the xenon 6s l e v e l i s used as a probe t o s e l e c t t h a t i n t e n s i t y from t h e range o f i n t e n s i t i e s present i n the l a s e r f o c u s . Our analysis provides an a c c u r a t e , wide-range d i s t r i b u t i o n func- t i o n o f the i n t e n s i t i e s i n our multi-mode l a s e r pulses. Using t h i s method we determined t h e real order o f n o n - l i n e a r i t y o f the resonant i o n i z a t i o n process and the p r o b a b i l i t y f o r a d d i t i o n a l photon a b s o r p t i o n .
1 . I n t r o d u c t i o n . - A well-known problem i n m u l t i - p h o t o n i o n i z a t i o n (MPI) s t u d i e s i s the i m p o s s i b i l i t y t o create e x p e r i m e n t a l l y a homogeneous l i g h t i n t e n s i t y i n order t o perform measurements under w e l l defined c o n d i t i o n s . I n most o f the work i n t h i s f i e l d only average o r peak i n t e n s i t i e s are quoted.
Here we present a new s o l u t i o n t o t h i s problem. Although only a p p l i c a b l e i n resonant m u l t i - p h o t o n i o n i z a t i o n studies i t even permits the use o f multi-mode l a s e r pulses.
2. Experimental. - The experiment i s done w i t h a Nd-Yag pumped dye l a s e r and an e l e c t r o n spectrometer o f novel design ( K r u i t and Read, 1982). The dye l a s e r produces 6 nsec pulses o f 0 . 1 - 1 . 0 mJ i n the A = 410-450 nm r e g i o n , which are focussed by a f = 8.5 mm aspherical lens t o a diameter o f about 10 urn i n t a r g e t gas pressures o f 10"6 - 1 0 "2 Pa. The m u l t i - p h o t o n e l e c t r o n s are energy analyzed on the basis o f t h e i r T.O.F. over a 50 cm long f l i g h t p a t h . By using a magnetic f i e l d t h a t diverges from 1 T i n t h e i o n i z a t i o n region t o 1 0 "3 T i n the f l i g h t t u b e , the e l e c t r o n s are p a r a l - l e l i z e d and 502 o f a l l the e l e c t r o n s are accepted.
3. Analysis and r e s u l t s . - I n f i g . l we show an e l e c t r o n energy spectrum a r i s i n g from M . P . I , o f Xe v i a a 3 photon resonance (6s) at A = 440.5 nm. The peaks a t 0.6 and 1.9 eV a r i s e from 5 PI t o the P l / 2 and P3/2 continuum, t h e peaks a t 3.4 and 4.7 eV show a d d i t i o n a l (5+1) photon a b s o r p t i o n .
F i g . 2 shows a simultaneous wavelength scan f o r the 5 PI s i g n a l and t h e (5+1) PI s i g - nal r e s p e c t i v e l y . The f a c t t h a t a t only one side o f the resonance l e v e l i o n i z a t i o n signal i s observed i n d i c a t e s the non-resonant s i g n a l i s zero and t h a t the resonance l e v e l i s s u b j e c t t o an A.C. Stark s h i f t (see also Delone e t a l . and K r u i t e t a l . ) . This s h i f t o f t h e resonance l e v e l i s i n f i r s t order approximation p r o p o r t i o n a l t o the f i e l d i n t e n s i t y :
(1) Under c e r t a i n assumptions, which w i l l be described elsewhere, ( K r u i t et a l . 1982), we know t h a t i n a measurement a t one p a r t i c u l a r detuning AA, a l l e x c i t e d states are
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982241
JOURNAL DE PHYSIQUE
ELECTRON ENERGY ( e V )
fig.
1A
typical? &&&on e n a g y npecttlum
440.3 4L0.5 46M 44Q9 wavelength I n m l
Wavelength
bean06 ,the 5-phokon
P 3 / 2-
and
Xhtl2e ( 5+ 1I
P 3 / 2phoza eXe&on n i g n d
amund 440nm
formed i n t h a t r e g i o n o f space and time, where t h e l o c a l i n t e n s i t y I i s such t h a t t h e instantaneous value o f t h e AC S t a r k s h i f t has made t h e i n t e r m e d i a t e s t a t e reso- nant. Therefore t h e detuning i s a measure f o r t h e i n t e n s i t y a t which t h e e x c i t a t i o n takes place.
Our whole a n a l y s i s i s based on t h e assumption t h a t i o n i z a t i o n o u t o f the e x c i t e d s t a t e occurs i n a t i m e s h o r t compared t o f l u c t u a t i o n s i n t h e i n t e n s i t y . Only then t h e i o n i z a t i o n s i g n a l $(x,F) a t t o t a l energy i n t h e pulse F can be connected t o t h e i o n i z a t i o n r a t e W(1) a t one detuning s e l e c t e d i n t e n s i t y I = A h l a and t h e space-time volume P(1,F)dI i n which t h e i n t e n s i t y i s between I and I
+
d I :A p r o o f f o r t h i s assumption i s r e f l e c t e d i n f i g . 3 , where we p l o t t e d t h e r a t i o be- tween (5+1) P I and 5 P I as a f u n c t i o n o f detuning.
I n o r d e r t o f i n d t h e i n t e n s i t y dependence o f t h e i o n i z a t i o n r a t e and t h e i n t e n s i t y d i s t r i b u t i o n i n our l a s e r focus, we measured S(A,F) f o r a range o f values o f F and 1. To combine measurements a t d i f f e r e n t F ' s i n one d i s t r i b u t i o n f u n c t i o n we need an a d d i t i o n a l assumption which concerns t h e 1 in e a r s c a l i n g o f t h e i n t e n s i t y w i t h t o t a l p u l s e energy a t any p o i n t i n space and time: i .e.
r7
absorption one o r d e r o f magnitude d i f - f e r e n t from 5 P I as a f u n c t i o n o f i n - Because one expects a d d i t i o n a l photon
"
-
aC
-
t e n s i t y and we observe a 1 in e a r depen-dence as a f u n c t i o n o f detuning, t h e conclusion can be drawn t h a t detuning i s c o u p l e t t o i n t e n s i t y and t h a t t h e a d d i t i o n a l photon t r a n s i t i o n occurs a t t h e same i n t e n s i t y s e l e c t e d i n t h e ex- c i t a t i o n process.
When we combine t h i s r e s u l t w i t h a r e - c e n t c a l c u l a t i o n o f M. Aymar on t h e AC
_ n b n a o
P
.
0s
S
o c2 f
-
YI
-
4403 4405 U Q 7
?
S t a r k s h i f t o f t h e 6s l e v e l :vavelrngth [nml
.
-
detun~ng (-bght mtmslfy)b
a =0.27+0.1 cm-l/GWwe o b t a i n immediately t h e p r o b a b i l i t y f o r a d d i t i o n a l photon a b s o r p t i o n as a
?'mbabd%y
o w a n a l ? photon-
f u n c t i o n o f absolute l i g h t i n t e n s i t y of a b n o l r p f i o n ~ a d u n ~ a n o ~ d & u n i n g ( 2 . 8 + 1 . 4 ) ~ 1 0 - ~ p e r qn/ &at-a% g
I -z
c
0 a75-
1 N
5
aso-5 8
025-. n
LD
O
F1 1 P(I,F2)dI
=r
P(I--,
Fl)dI2 F2 (3)
This assumption seems reasonable considering our method of changing the pulse ener- gy, which i s t o r o t a t e the l i n e a r polarization i n f r o n t of a polarizer.
Substituting equation (3) i n (2) we derive:
F2 AX AX F1
-
S(X,F2) =W(-).
P(-.-,
F1)F1 a a F2 (4)
I f one plots 1ogCF.SI versus AX/F f o r each value of AX one obtains a s e r i e s of l i n e elements, each representing a part of the d i s t r i b u t i o n function P. A smooth and con- tinuous function P can be constructed by a proper choice of W a t each AX.
Figure 4 shows the best f i t f o r P(I ,F) f o r our data.
The values of W(1) obtained from the f i t of figure 4 are plotted i n figure 5 and re- present the r a t e of the 5 PI process as a function of the t r u e l i g h t i n t e n s i t y .
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DhLJcibuZLon ~un&on
06f i e
f i g k ti n t e n h i t y i n
OWLLahm docub
404a pube
lntithe n u g y
0.5 m J .The vanioun nymbod fie- p x u e n t bepahate
b&06 d a t a point6 at 6ixed
AXI o n i z d o n m t e an a dune- Lion 06 f i e
h eLocal?, momentatry fight intenb&
For AX > O . 15 nm, we f i n d the order of non l i n e a r i t y of the process:
N
E w = . w l =3.0 +0.24. Conclusion.
-
The essence of our analysis i s t h a t we take the derivative of log W(1) versus log I and not, as i s often seen i n l i t e r a t u r e , t h e derivative of log S(A,F) with respect t o log F which would only give information about the d i s t r i - bution function P .References
Kruit P. and Read F.H., t o be published i n J.Phys.E.
Kruit P . , Kimman J . and van der Wiel M.J., J.Phys .B:At.Mol .Phys .x(1981)L597-L602.
Delone G.A., Delone N.B., Piskova G . K . , Sov.Phys.JETP 35, 4(1972)672.
Kruit P . , Kiman J . , Muller H.G., van der Wiel M.J., t o be published i n J.Phys.B (1982)
