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FIELD-ASSISTED PHOTODESORPTION OF IONS FROM METAL AND SEMICONDUCTOR SURFACES
S. Jaenicke, A. Ciszewski, W. Drachsel, U. Weigmann, T. Tsong, J. Pitts, J.
Block, D. Menzel
To cite this version:
S. Jaenicke, A. Ciszewski, W. Drachsel, U. Weigmann, T. Tsong, et al.. FIELD-ASSISTED PHO-
TODESORPTION OF IONS FROM METAL AND SEMICONDUCTOR SURFACES. Journal de
Physique Colloques, 1986, 47 (C7), pp.C7-343-C7-348. �10.1051/jphyscol:1986759�. �jpa-00225954�
JOURNAL DE PHYSIQUE
Colloque C7, supplément au no 11, Tome 47, Novembre 1986
FIELD-ASSISTED PHOTODESORPTION OF IONS FROM METAL AND SEMICONDUCTOR SURFACES
S. JAENICKE, A. CISZEWSKI(~), W. DRACHSEL, U. WEIGMANN(~) T.T. TSONG( 3 ) , J.R. PITTS(
), J.H. BLOCK and D. MENZEL*
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 0-1000 Berlin 33, F.R.G.
' ~ h y s i c s Department, Technical University of Munich, 0-8046 Garching, F.R.G.
Abstract - The influence of strong electric fields on the photon-stimlated desorption of small molecules from metal and Silicon surfaces has been studied. With hyai-ogen only field adsorbed H2 as well as H3 are desorbed as singly char@ ions.
Chemisorbed atomic hydrogen obviously has a photo-ionization cross-section at metal surfaces which is too small to be detected. From clean and oxidized Si surfaces H+
as well as Si-containing hydrides and oxides are photodesorbed at field strengths as l m as 6V/nm. Field adsorption of water leads to whisker-like layers, £rom which H ~ O + . nH20-clusters are photo4esorbed with rather high quantum yields. The desorp- tion spectrum exhibits a sharp onset at a wave length of 165 nm; the onset energy shifts with qecreasing field strength towards higher energies.
1. INTRODUCTION
The simultaneous interaction of a high electric field and of photons of suitable energy on an adsorbed molecule can lead to desorption of ionic species. Itio fundamen- tally different mechanisms can be considered, which we will refer to as:
a. Photon-assisted field desorption. This mechanism has been brought forward by Tsong et al /1/; it assumes that the charge transfer from the adsorbate to the metal or serniconductor substrate proceedç via electron tunnelling from an excited state of the adsorbed molecule into an empty electron level of the solid.
b. Field-assisted photodesorption. Here ionizing the adsorbate is done by electronic excitation as in the field free case. The mechanism is similar to those discussed for electron-stimulated desorption by Redhead / 2 / , Menzel and Gomer / 3 / ,
("on leave from Physical Department. University of Wroclaw. PL 50-205 Wroclaw. Poland
(2)~ermanent address
:Fraunhofer Institute for Microstructure Research, D-1000 Berlin 33. F.R.G.
(3)0n sabbatical leave from Physics Department, Pennsylvania State University. University Park.
PA 16802, U.S.A.
(4)~ermanent address : Solar Energy Research Institute. Golden. CO.. U.S.A.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986759
C7-344 JOURNAL DE PHYSIQUE
Antoniewicz /4/, Knotek and Feibelman /5/ and Guner /6/. The electric field faci- litates the remaval of the ionized species from the surface and thus increases the escape probability. According to these mechanisms, the photon energy has always to be larqer than the ionization potential of the adsorbate.
Previous experiments with tunable laser radiation in the visible and long-wave- length W have shown that mechanism (a) could only be found, so far, for the case of ethylene on silver /7/. The aim of the present investigation is to elucidate which of the models, proposed under (a) or (b), describes the reaction in other systemç.
II. EXPERIrnWAL
The apparatus used has been described elsewhere /8/. In short, it consists of a field ion microscope, where the tip to screen distance also serves as the mass analyzer in the time-of-flight (t-o-f) mode. Monochromatized synchrotron radiation
(SR) was focussed ont0 the tip, and mass spectra of the desorbeci ions were recorded, taking advantage of the pulsed structure of the SR light (5 MHz at BESSY, 1 MHz at DORIS). The substrates kere shaped in the fonn of field emitter tips by chernical etching and electropolishing, following established procedures /9/. Most of the gases used were Messer Griesheim pressure cans with a nominal purity > 99.99 %.
Deutrium was supplied by Linde in a break seal flask. Water was triply-distilled and thoroughly degassed by several freeze-pump-thaw cycles. In order to separate the surface signal from the background, due to field ionization and photoionization in the gas phase, it is mandatory to work at pressures less than 2 * 1 O - ' mbar. There- fore a good background pressure is essentiaï for meaningful results. A pressure of
< 3 * 1 O-' mbar was routinely achieved 12 hours after a light bakeout using a 330 1/s turbomolecular pump and a lq. N2.cooled Ti sublimation pump. The residual gas was analysed with a Balzers QMG 311 quadropole mass spectrometer and found to consist mainly of H2 and CO with traces of H20 and a2. In order to avoid unwanted reactions on the hot filaments, gas composition and pressure measurements were only made &ter the photodesorption data had been taken.
III. RESULTÇ
Previous experiments with laser radiation /IO/ had established in most of the studied systems that for photodesorption, light energies above the ionization energy are necessary. In this case, mechanisms involving electron tunneling can be excluded If the photon energy, which leads to desorption is comparable to the ionization energy of the adsorbate, the Menzel-Gomer-Redhead (MGR) model describes the desorp- tion process most accurately. On the baçis of this model, one expects a very pro- nounced mass dependence of the desorption rate. Specifically on the system H2/W(100) Jelend and Menzel /Il/ found a H/D isotope effect as big as 150. We therefore invest-
igated the deso ption yield of H+ and D+ ions from W and Ni surfaces with wideband
radiation from 30 - 120 eV. Pure Hz and H2/D2 - mixtures (up to 60 % H2 from the re-
sidual gas) were dosed ont0 the tip, and the desorbing ions recorded as a function of temperature and field strength. Within the uncertainty of our composition deter- mination, no preferential adsorption or desorption could be found. This agrees well with another observation of Menzel, who found only a very mal1 isotope effect for H* 2 f r m ~(100), contrasting the extreme isotope sensitivity for H+ desorption. H2 +
thus is the primary species in field-assisted photo-desorption. This becmes appar- ent also in the field strength dependence of the H+, H2f and : H desorption yield /12/.
No stimulated desorption is observed belaw approx. 15 V/m. At higher field strength when field adsorption of hydrogen molecules sets in, : H is detected. If one
80-
15 20 25 30 35 40
field strength [ V I nm 1
NI; H2/DZ: 80 K; 9.0 K V
Fig. 1 - Ion yield for photostinilated desorption as a function of field strength
x x x H+
0 0 0
HZ+
O
H3+
NI. HZ/DZ. 160 K. 9 0 KY
3000
l 1 I I l2500 -
Ln
g zoo0
-1 :
1500 - -
2 ,000 - -
U
500 - -
" 00 - O CHANNEL
O 200 ZAO NI: H2/D2; 120 K; 9.0 KY
3000
- - ( ,NI; H2/D2; 80 K; 9.0 KV 3000
2500
YI O
2000
V1
2 1500
2
21000
U500
O
O 50 100 150 200 250
CllhNNEL
h P..*.,
Fig. 2 - Temperature dependence of the desorption signals: H /D from Ni. Original
traces; the peak on the right hand sight is due to stray liggt $rom the desorption
pulse.
C7-346 J O U R N A L D E PHYSIQUE
increases t h e f i e l d s t r e n q t h f u r t h e r , t h e H+ s i g n a l grows a t t h e expence of t h e H: , presumably by f i e l d d i s s o c i a t i o n of t h e molecular ion. Neither on W nor on N i d i d w e f i n d a d e t e c t a b l e r a t e of isotope exchange a t 80 K. Increasing t h e temperature t o 120 K v i r t u a l l y suppresses t h e desorption s i g n a l (Fig.2): upon cooling back t o 80 K , t h e composition o f t h e desorbed species is almost i d e n t i c a l t o t h a t a t t h e beginning On Ni, no mass 3 is detected. s h m i n q unambiauously t h a t no HD has been formed.
On S i and oxidized S i surfaces, l a r g e r s i g n a l s were e x p c t e d s i n c e r e n e u t r a l i z a t i o n of t h e ions should be slower, due t o t h e lower concentration of f r e e e l e c t r o n s . Indeed, Ca. 10 t i m e s more i n t e n s e s i g n a l s were recorded. Also, H+ was found already a t l m f i e l d s t r e n g t h , and a t r o m temperature. Obviously, a Si-H o r S i 0 4 s u r f a c e bond can be broken p h o t o l y t i c a l l y , leading t o t h e desorption of H+. The H+-photode- sorption threshold near 10 e V could be v e r i f i e d by i n s e r t i n g a L i F - f i l t e r i n t h e SR beam. Unfortunately, t h e monochromator uçed a t BESSY had too low a photon f l u x i n t h i s range t o e s t a b l i s h t h e desorption threshold more c l e a r l y .
I n t h e search f o r a system with higher desorption y i e l d w e succeeded with H20-layers grown on a f i e l d emitter i n t h e s t r o n g e l e c t r i c f i e l d . I n t h e photodesorption mass s p e c t r a , w e observed protonated water c l u s t e r s s i m i l a r t o those seen by Tsona and Liou /l3/ and e a r l i e r by Beckey /14,15/ and Schmidt /16/. A t high f i e l d , H30' is t h e most abundant species. A t low f i e l d , hawever, H30+*2H20 is most frequently observed though c l u s t e r s up t o H30+*16H20 a r e observed. A l 1 t h e s e c l u ç t e r s show a threshold a t a wavelength near 165 nm, s q g e s t i n g a common e x c i t a t i o n s t e p f o r a l 1 t h e c l u s t e r s . This confirms t h e mode1 of water whiskers (Fig. 31, which was derived by Anway /17/ on t h e b a s i s of appearance energy measurements. Also, we want t o point out t h a t t h e observed desorption spectrum c l o s e l y resembl= t h e adsorption spectrum of l i q u i d water /18/ measured by P a i n t e r e t a l . a t t h e t r i p l e point.
Pinancial support by t h e BMFT ( P r o j e c t No. 05242 GZP) is g r a t e f u l l y acknowledged.
REFERENCES
/1/ Tsong, T.T., Block, J.H., Nagasaka, M. and Vimanathan, B., J. Chem. Phys., (1 976) 2469.
/2/ Redhead, P. A., Can. J. Phys., (1 964) 886.
/3/ Menzel, D. and Gomer, R., J. Chem. Phys., (1964) 3311.
/4/ Antoniewicz, P.R., Phys. Rev., B21 (1980) 3811. -
/5/ Knotek, M.L. and Feibelman, P.J., Phys. Rev. Lett., (1978) 964.
/6/ Gomer, R. i n : Desorption Induced by E l e c t r o n i c Transitions DIET 1, N.H. Tolk, M.M. T r a m , J.C. Tully and T.E. Madey (eds.), Springer Heidelberg, New York, 1983, p. 40.
/7/ Nishigaki, S., Drachel, W. and Block, J.H., Surface Science, (1979) 389 /8/ Drachsel, W., Weigmann, U., Jaenicke, S. and Block, J . H . in: Desorption Induced
by Electronic Transitions DIET II, W. Brenig and D. Menzel (&S.), Springer
Serieç i n Surface Science, Vol. 4 , Springer Verlag Heidelberg, 1985, p. 245
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