RESONANT MULTIPHOTON PROCESSES IN LASER-INDUCED DESORPTION

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RESONANT MULTIPHOTON PROCESSES IN LASER-INDUCED DESORPTION

J. Reif, H. Nielsen, E. Matthias, E. Westin, A. Rosén

To cite this version:

J. Reif, H. Nielsen, E. Matthias, E. Westin, A. Rosén. RESONANT MULTIPHOTON PROCESSES

IN LASER-INDUCED DESORPTION. Journal de Physique Colloques, 1987, 48 (C7), pp.C7-737-C7-

740. �10.1051/jphyscol:19877181�. �jpa-00227006�

JOURNAL DE PHYSIQUE

Colloque C7, suppl6ment au n012, Tome 48, dbcembre 1987

RESONANT MULTIPHOTON PROCESSES IN LASER-INDUCED DESORPTION

J. R E I F , H.B. NIELSEN, E. MATTHIAS, E. WESTIN* and A .

ROSBN*

Fachbereich Physik, Freie Universitat Berlin, Institut fiir Atom- und Festkdrperphysik, Arnimallee 14, 0-1000 Berlin 3 3 , F.R.G.

'~nstitute o f Physics, Chalmers University of Technology and University of Gdteborg, 5-412 96 Gdteborg, Sweden

Abstract

The dve laser induced emission of oositive ions from the surface of cleaved highly transparent bariumfluoride shows a 'pronounced dependence on the exciting laser wave1 ength. Qua1 itatively, the same spectra are observed for the emission of electrons. For fixed wavelengths, the emission yields of electrons and positive ions follow higher powers of the laser intensity. Consequently, photoemission is inter- preted by resonantly enhanced two-photon (for less than -1.7 MW/cm2 1 aser intensity) respectively five-photon (for higher laser intensity) absorption. Ion desorption then follows from the combination of two holes at a fluorine site.

Even though the bandgap

transparent materials, e.g. ionic crystals, is of the order of 10 eV, visible radiation can be absorbed in such material via multiphoton processes. The effect is of special interest at the surface of.these crystals where it can lead to the emission of electrons (photoemission) and surface constituents (desorption). In many cases, laser induced desorption has been attributed to thermal effects /I/. In the present contribution, however, we provide evidence that non-ther- ma1 photoemission and the desorption of positive ions from bariumfluoride are closely related and are a consequence of resonantly enhanced multiphoton absorption.

When we irradiate the in vacuo cleaved (111) surface of BaF, under ultra-high vacuum with green dye laser light at an intensity of less than 10 MW/crn2, i.e. about two orders below the damage threshold

we observe the emission of positive ions from the surface

The analysis of these ions by means of a quadrupole mass filter shows, that they are

Ba' and (BaF)' (cf. Fig. 1).

Fiqure 1. Laser-pulse correlated emission of positive ions from BaF, (111).

(Laser at

nm, 10 MW/C~~).

MASS NUMBER

These ions are emitted simultaneously, as can be seen from a first time-of-flight spectrum

2).

Fiqure 2. Time-of-flight spectrum of positive ions, emitted from BaF, (111).

(Laser at

nm,

~W/crn~; distance 3 cm, accelerating potential -30

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19877181

C7-738 JOURNAL DE PHYSIQUE

We a l s o observe t h e emission o f e l e c t r o n s from t h e surface. From t h e a n a l y s i s o f t h e dependence o f t h e e l e c t r o n and i o n y i e l d s on t h e l a s e r i n t e n s i t y /3/, we know t h a t both a r e t h e r e s u l t o f m u l t i p h o t o n absorption. When we scan t h e l a s e r wavelength, we observe a pronounced s p e c t r a l dependence o f t h e y i e l d s (Fig. 3) which i s p r a c t i a l l y i d e n t i c a l f o r i o n s and e l e c t r o n s . I t e x h i b i t s , however, a dramatic i n f l u e n c e o f t h e azimuthal o r i e n t a t i o n o f t h e s u r f a c e w i t h r e s p e c t t o t h e l a s e r p o l a r i z a t i o n . The i n t e r m e d i a t e resonances remain i n p o s i t i o n independent o f whether two- o r five-photon i o n i z a t i o n takes p l a c e ( c f . F i g . 4). Hence, we conclude t h a t t h e resonance c o n d i t i o n

WAVELENGTH Inml 550 YO 510 550 YO 510

F i s u r e 3. Y i e l d s f o r p o s i t i v e i o n s and

- e l e c t r o n s from BaF, as a f u n c t i o n o f l a s e r

-

C 3

f j e c t i o n o f t h e e l e c t r i c f i e l d v e c t o r

z,

-

9 p a r a l l e l t o t h e

[I121

- +

t e, i s p a r a l l e l t o [il~].

Z 0 W

2 C

"l 0 a

n

_I

W

0 2 (L +

U W _I W

2 3 2 4 2 3 2 L

PHOTON ENERGY lev1 WAVELENGTH lnrnl 550 530 510

F i s u r e 4. Spectral dependence o f t h e e l e c t r o n y i e l d from BaF, (111) f o r d i f - f e r e n t l a s e r i n t e n s i t i e s :

a) 3.2 x

l o 6

b) 2.8 x

lo6

l o 6

l o 6

2.3 2 L PHOTON ENERGY lev1

i s imposed on t h e l a s t two photons i n each case, i n good agreement w i t h t h e c a l c u - l a t e d d e n s i t y o f s t a t e s i n t h e bandgap o f a n o n - s t o i c h i o m e t r i c surface. Together w i t h t h e o b s e r v a t i o n o f t h e two-photon i o n i z a t i o n as such, which r e q u i r e s imp1 i c i t l y oc- cupied s t a t e s i n t h e middle o f t h e bandgap, t h i s j u s t i f i e s t h e assumption o f t h e n o n - s t o i c h i o m e t r i c composition o f t h e surface. Apparently, a t v e r y low l a s e r i n t e n s i - t i e s , which a r e always present i n t h e wings o f t h e temporal p u l s e shape, k i n d o f such a p r e p a r a t i o n can t a k e place; as can be seen from t h e a c c i d e n t a l l y measured develop- ment o f t h e s p e c t r a l dependence a t one s p e c i f i c s u r f a c e s i t e (Fig. 5 ) .

WAVELENGTH (nm

F i q u r e 5 . A c c i d e n t a l l y observed "preparation"

w i t h i n c r e a s i n g i n t e n - s i t y (a1 1 MW/cm2, ba 2 MW/cm

,

0 ments a t lower i n t e n -

s i t i e s , t h e s t r u c t u r e o f c ) p e r s i s t e d .

The apparent c l o s e r e l a t i o n s h i p between e l e c t r o n and i o n s p e c t r a makes us assume t h a t resonant m u l t i p h o t o n i o n i z a t i o n i s t h e o r i g i n f o r t h e desorption. T h i s idea i s supported by t h e r e s u l t , t h a t t h e i n t e n s i t y dependence o f t h e i o n y i e l d i s t h e square o f t h a t f o r e l e c t r o n s , i n d i c a t i n g t h a t two e l e c t r o n s must be e m i t t e d from r e l a t i v e l y neighbouring s i t e s t o c r e a t e one p o s i t i v e i o n /3/.

From t h e f a c t t h a t i o n s and e l e c t r o n s appear quasi instantaneously a t each l a s e r p u l s e and cease being e m i t t e d a f t e r t h e end o f each pulse, t o g e t h e r w i t h a l l t h e above observations, we deduce t h a t our observed phenomena are indeed p u r e l y e l e c t r o n - i c t r a n s i t i o n s and n o t r e l a t e d t o thermal energy d i s s i p a t i o n .

Acknowl edqements

T h i s work was supported by t h e Deutsche Forschungsgemeinschaft, Sfb 6, t h e Swedish N a t u r a l Science Counci 1 and t h e Swedish Board o f Technical Development.

References

/1/ see e.g. R. K e l l y , Surface Science

90,

/2/ F. Rainer, W.H. Lowdermilk, and D. Milam, NBS Special P u b l i c a t i o n 669, 157 119841 _{1 - - -} -,

/3/ J. R e i f , H.B. Nielsen, 0. Semmler, P. Tepper, E. Matthias, E. Westin, and A.

Roskn, Resonant M u l t i p h o t o n Processes i n Laser-Surface I n t e r a c t i o n , t h i s volume, p 443