INTERPRETATION OF HIGH-SPEED RAMPED FIELD DESORPTION SPECTRA ON NO ON Pt

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INTERPRETATION OF HIGH-SPEED RAMPED FIELD DESORPTION SPECTRA ON NO ON Pt

T. Kessler, G. Abend, N. Kruse, J. Block

To cite this version:

T. Kessler, G. Abend, N. Kruse, J. Block. INTERPRETATION OF HIGH-SPEED RAMPED FIELD DESORPTION SPECTRA ON NO ON Pt. Journal de Physique Colloques, 1986, 47 (C7), pp.C7- 407-C7-412. �10.1051/jphyscol:1986768�. �jpa-00225963�

INTERPRETATION OF HIGH-SPEED RAMPED FIELD DESORPTION SPECTRA ON NO ON Pt

T. KESSLER, G. ABEND, N. KRUSE and J.H. BLOCK

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 0-1000 Berlin 33, F.R.G.

A b s t r a c t - Ramped f i e l d d e s o r p t i o n has been employed t o probe t h e b i n d i n g s t a t e s o f adsorbed species. The use of f a s t l i n e a r ramps w i t h slopes o f 107*v/m ns a l -

lows us t o combine t h i s method w i t h t i m e - o f - f l i g h t mass spectrometry. F i r s t r e - s u l t s a r e presented f o r t h e d e s o r p t i o n o f NO adsorbed a t s t e p s i t e s o f P t f i e l d e m i t t e r t i p s . We d e s c r i b e t h e general formalism f o r c a l c u l a t i n g t h e f i e l d de- s o r p t i o n r a t e s . The a p p l i c a t i o n t o t h e system NO/Pt y i e l d s considerable agree- ment between t h e o r y and experiment.

1 - INTRODUCTION

Temperature programmed d e s o r p t i o n (TPD) i s a w e l l e s t a b l i s h e d method f o r p r o b i n g t h e b i n d i n g s t a t e s o f adsorbed species (see f o r example / 1 / ) . I n a s i m i l a r manner w e l l - d e f i n e d e l e c t r i c a l f i e l d ramps can be a p p l i e d t o d i s t i n g u i s h between d i f f e r e n t ad- s o r p t i o n s t a t e s . Attempts i n t h i s d i r e c t i o n have been made by P a n i t z (1974)/2/ and Rendulic (1980)/3/. I n t h e present work we describe t h e f e a s i b i l i t y o f o b t a i n i n g mass spectra as w e l l as ramped d e s o r p t i o n spectra by u s i n g ramps w i t h much steeper slopes (approx. 20 - nm.ps). I n f a c t these ramps a r e t h e r i s i n g p a r t s o f f i e l d pulses, which v

a r e s h o r t enough t o p e r m i t t i m e - o f - f l i g h t measurements f o r a chemical a n a l y s i s o f t h e desorbed ions. D i f f e r e n t b i n d i n g s t a t e s and consequently d i f f e r e n t d e s o r p t i o n f i e l d s t r e n g t h s d u r i n g ramped f i e l d d e s o r p t i o n have been observed a l ready by Pani t z /2/, Rendulic and Leisch /3/ and Kruse e t a l . /4/. A q u a n t i t a t i v e i n t e r p r e t a t i o n o f t h e d e s o r p t i o n spectra o f NO from a P t e m i t t e r w i l l be g i v e n here.

II - EXPERIMENTAL

The experimental set-up has been described i n d e t a i l elsewhere /5/. A p l a t i n u m f i e l d e m i t t e r t i p i s mounted a t a d i s t a n c e o f 0.1 mm i n f r o n t o f t h e h o l e o f an e l e c t r o d e . A tube generator w i t h a negative feed-back produces f a s t ramps (maximum amplitude 20 kV, r e p e t i t i o n r a t e s up t o 10 kHz), which a r e a p p l i e d t o t h e e l e c t r o d e . A secocd e l e c t r o d e i s mounted 2 mm behind t h e f i r s t one and k e p t a t a constant negative po- t e n t i a l i n order t o s h i e l d t h e i o n t r a j e c t o r i e s from t h e t i m e dependent a c c e l e r a t i o n

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986768

C7-408 JOURNAL DE PHYSIQUE

f i e l d s of the ramps. The f l i g h t tube i s approximately 2 m long. For a typical ion energy of 6.7 keV the f l i g h t times a r e i n the order of microseconds, while the d i f - ference between the mass u n i t s 29 and 30 f o r example i s about 170 ns.

For a proper mass spectrum short desorption pulses a r e required. However, time de- pendent acceleration due t o r i s i n g o r decreasing voltages will change the t o t a l energy of ions. The steeper the slope of the desorption pulse, the smaller i s t h e i r energy and, consequently, t h e i r velocity. The application of very short r i s e times will cause a broadening of t h e mass signals because the l a t e r s t a r t i n g ions pass t h e earl i e r ones, due t o t h e energy deviation. In order t o avoid t h i s problem, we use longer r i s e times: In t h i s case the broadening of a peak i n t h e mass spectrum i s e a s i l y interpreted a s the s t a r t time d i s t r i b u t i o n of the ions. This i s shown in f i g . 1.

Fig. 1 - Ramped f i e l d desorption of NO from P t .

a ) l i n e a r f i e l d ramp a s a part of a desorption pulse;

b) NO' time of f l i g h t spectrum.

Flight times and mass numbers a r e calculated by referring t o an as- sumed s t a r t time a t the top of t h e f i e l d pulse. Ions which s t a r t along the f i e l d ramp a r e shifted to e a r l i e r arriva1 times.

9.2 9.6 9.9 time of flight ips f

For ^{O u r} measurements we use t h e ramped f i e l d technique t o desorb NO-molecules which are adsorbed a t the surface of a P t f i e l d emitter. Desorption of NOad begins a t 10 V/nm and i s nearly complete a t 17 V/nm a s i s shown in f i g . 1. The f i e l d pulse ampli- tude amounts t o 20 V/nm. Thus, al1 ions s t a r t along the increasing part of the f i e l d pulse. Since the slope i s constant during desorption a11 NO ions experience the same energy deviation.

I n o r d e r t o describe t h e measured i o n c u r r e n t s as a f u n c t i o n o f t h e f i e l d s t r e n g t h we use some b a s i c approaches. F o l l o w i n g t h e Gomer-Swanson mode1 /6/ o f i o n i z a t i o n we assume t h e i o n i z a t i o n t o t a k e p l a c e a t t h e i n t e r s e c t i o n o f t h e b i n d i n g p a r t o f t h e p o t e n t i a l curve o f t h e adsorbed species and t h e s t r a i g h t l i n e due t o t h e f i e l d strength. Approximating t h e b i n d i n g p a r t o f t h e p o t e n t i a l curve by a - % -term and n e g l e c t i n g t h e change o f t h e b i n d i n g energy w i t h t h e f i e l d s t r e n g t h leads t o a simple expression f o r t h e d e s o r p t i o n energy as a f u n c t i o n o f t h e f i e l d s t r e n g t h .

1

F i g . 2 - Schematic diagram o f t h e i o n i z a t i o n process. I o n i z a t i o n occurs a t t h e i n t e r s e c t i o n o f t h e p o t e n t i a l energy c u r v e o f t h e i o n , approximated by 1-@eFx a t d i f f e r e n t i n s t a n t s t, and o f t h e n e u t r a l p a r t i c l e ( d o t t e d l i n e ) . E ( F ( t ) ) = a c t i v a t i o n energy.

The a c t i v a t i o n energy f o r f i e l d d e s o r p t i o n can be w r i t t e n i n t h e f o l l o w i n g manner

E(F(t))=Eo +

9

w i t h Eo = b i n d i n g energy o f t h e n e u t r a l molecule, 1 = i o n i z a t i o n p o t e n t i a l o f t h e molecule, 0 = work f u n c t i o n o f t h e e m i t t e r , Y = f i t t i n g parameter f o r t h e b i n d i n g p a r t o f t h e p o t e n t i a l curve, F ( t ) = f i e l d s t r e n g t h .

C7-410 JOURNAL DE PHYSIQUE

Analogous t o t h e treatment o f temperature programmed d e s o r p t i o n measurements t h e de- s o r p t i o n c u r r e n t can be w r i t t e n as t h e n e g a t i v e d e r i v a t i v e o f t h e coverage

. do P

J = - = " ^{- exp}

O

w i t h j = d e s o r p t i o n c u r r e n t , a = coverage, P = i o n i z a t i o n p r o b a b i l i t y , T = r e c i p r o -

O

c a l Debye-frequency, k = Boltzmann f a c t o r , T = temperature.

By s u b s t i t u t i n g F ( t ) = D t i n equation ( l ) , equation ( 2 ) can be solved e x p l i c i t e l y , thus g i v i n g t h e i o n c u r r e n t as a f u n c t i o n o f t h e f i e l d s t r e n g t h . The parameters T ~ ,

Eo, 1 and $ a r e taken from l i t e r a t u r e /7,8,9/. The Eo value has been taken as 1.43 eV, as found by Campbell e t a l . /7/ and Kruse e t a l . /8/ f o r stepped P t surfaces.

p/ ; has been chosen t o 10 l3 s - l , which i s reasonable on account o f t h e l i t e r a t u r e values f o r ~g /7,8/ and t h e assumption of a p-value n o t t o o f a r below u n i t y . I n f a c t , the c a l c u l a t i o n o f t h e d e s o r p t i o n curve i s n o t v e r y s e n s i t i v e t o p h o . The value f o r

v i s provided from a b e s t f i t w i t h r e s p e c t t o t h e maximum o f t h e measured d e s o r p t i o n curve.

III - RESULTS AND DISCUSSION

The ramped f i e l d technique has been a p p l i e d t o desorb NO from P t f i e l d e m i t t e r sur- faces a t room temperature. The r e s u l t s a r e p l o t t e d . i n f i g . 3. The NO' d e s o r p t i o n t r a c e i s t o be understood i n terms o f v a r y i n g d e s o r p t i o n p r o b a b i l i t i e s and surface concentrations o f NOad. I n c r e a s i n g NO' i n t e n s i t i e s a r e due t o i n c r e a s i n g i o n i z a t i o n p r o b a b i l i t i e s , decreasing i n t e n s i t i e s a r e evidence f o r a d e p l e t i o n o f t h e adsorbed 1 ayer.

F i g . 3 - NO' d e s o r p t i o n c u r r e n t as a f u n c t i o n o f t h e f i e l d s t r e n g t h d u r i n g a f i e l d ramp o f 20

"

1 * 1 0 ~ ~ s - ~ , 1-$=3.6 eV, y =0.4 nmOev

1 kHz, corresponding t o a f i e l d f r e e a d s o r p t i o n p e r i o d o f 1 ms each. Thus, f o r a steady gas pressure o f 1 . 3 * 1 0 - ~ Pa, NOad coverages a r e l e s s than 1 % o f a monolayer.

The step d e n s i t y on our t i p surface amounts t o several percent. I n i t i a l a d s o r p t i o n m a i n l y occurs a t t e r r a c e s i t e s . Since we expect t h e t e r r a c e s i t e adsorbate t o be mo- b i l e a t 300 K ( t h e TPD peak i s observed a t 340 K), d i f f u s i o n and t r a p p i n g a t s t e p s i t e s take p l a c e before t h e f i e l d ramps s t a r t . We conclude t h a t o u r data r e f e r pre- dominantly to d e s o r p t i o n o f NO' from steps.

Obviously, t h e measured curve i s broader than t h e c a l c u l a t e d one. During a measure- ment, which c o n t a i n s t h e summed r e s u l t s o f about one m i l l i o n d e s o r p t i o n ramps i n t h e multichannel analyser, a change o f t h e p r o p e r t i e s o f t h e observed surface may a l s o cause a double-peak s t r u c t u r e o r a peak-broadening i n t h e d e s o r p t i o n spectra. Such e f f e c t s have been observed and c o u l d be excluded i n t h i s experiment. We suggest d i f - f u s i o n i n t o t h e monitored area t o occur d u r i n g t h e f i e l d ramps. Distances o f several pm can be t r a v e l l e d i n times o f some 100 ns. Such d i f f u s i n g molecules a r e consequent- l y i o n i z e d l a t e r and may cause t h e observed t a i l . B i n d i n g s t a t e s w i t h d i f f e r e n t bind- i n g energies should produce more than one d e s o r p t i o n peak, s i m i l a r t o thermal desorp- t i o n spectra. However, under our experimental c o n d i t i o n s , o n l y step s i t e s a r e covered by NOad, due t o t h e v e r y low coverage. Recent measurements f o r NO on p a r t l y oxygen covered P t showed e s s e n t i a l l y two d e s o r p t i o n peaks. D e t a i l s w i l l be published e l s e - where.

I V - CONCLUSIONS

We presented a method t o d e s c r i be ramped f i e l d d e s o r p t i o n q u a n t i t a t i v e l y . Though t h e r e a r e s t i l l some problems o f i n t e r p r e t a t i o n , we showed how t o use t h i s method t o probe t h e b i n d i n g s t a t e o f adsorbed species a t v e r y low coverages. Future a p p l i c a - t i o n s w i l l be t o d i s t i n g u i s h d i f f e r e n t s t a t e s on account o f t h e i r d e s o r p t i o n f i e l d strengths, as has been q u a l i t a t i v e l y shown e a r l i e r /4/.

Acknowl edgement

This work was p a r t i a l l y supported by t h e Sonderforschungsbereich ( S f b 6 ) a t t h e F r e i e U n i v e r s i t a t B e r l i n .

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REFERENCES

/1/ Petermann, L.A., i n : Progress i n Surface Science, Vol. 3, P a r t 1, S.G. Davison ed., Pergamon Press 1972, p . 1.

/2/ Panitz, J.A., J. Vac. Sci. Technol . ¹⁶⁽³⁾ (1979) 868.

/3/ Rendulic, K.D., and Leisch, M., Surf. Sci. (1980) 1.

/4/ Kruse, N., Abend, G., and Block, J.H., Z. Phys. Chem. 144 (1985) 1.

/5/ Block, J.H., and Czanderna, A.W., i n : Methods and Phenomena, Vol. 1, Methods o f Surface Analysis, A.W. Czanderna ed., E l s e v i e r S c i e n t i f i c Publ. Comp. 1975, p . 379.

/6/ Gomer, R., and Swanson, L.W., J. Chem. Phys. (1963) 1613.

/7/ Campbell, C.T., E r t l , G., and Segner, J., Surf. Sci. - 115 (1982) 309.

/8/ Kruse, N., Kessler, T., Abend, G . , and Block, J.H., J , de Physique C9,12

(1984) 227.

/9/ CRC Handbook o f Chemistry and Physics, 60th E d i t i o n (1980) E-77.