L166 Surface Science 151 (19X5) Llbh-L170

North-Holland. Amsterdam

SURFACE SCIENCE LETTERS

THE ROLE OF THE PRECURSOR STATE AND THE MUTUAL INTERACTION IN HYDROGEN ADSORPTION ON THIN IRON FILMS

E. NOWICKA *, Z. WOLFRAM, W. LISOWSKI and R. DUS

Instrtute of Physrcal Chemrstry, Polish Acaderpv of Screncer. UI. Kospmkrr 44/J?, 01-724 Wur.~w.

Poland

Received 8 August 1984; accepted for publication 19 November 1984

The stickmg probability S( 8) for hydrogen adsorption on thin iron films have been measured

within the pressure interval of lo- “‘~10 ’ Torr. over both the adsorption states & and /I&. The

role of the precursor state and the mutual interaction is shown.

It was announced that two steps in the hydrogen adsorption rate on thin iron films [1,2] and on single iron crystal [3] surfaces were observed, and two or three desorption states detected [4,5].

The existence of three adsorption states called /3S, /Ii and (~6 was recently reported [6,7]. It was found that the atomic, negatively polarized ,f3s and /?w states differ in binding energy and in the dipole moment value and that the molecular, negatively polarized (~6 state is very weakly bound. It was con- cluded [7] that the flw state appears due to induced heterogeneity in the result of mutual repulsive interaction in the adsorbed layer when the coverage

exceeds the critical value denoted 8,. This interaction influences the rate of adsorption in the /3w state. The existence of the molecular state (YE may suggest that adsorption occurs via a precursor state. Such announcement has

been reported previously [8]. The aim of the present work was to recognize to what extent the precursor

state influences the adsorption rate of hydrogen on thin iron films, and to ascertain when the process starts to be governed by the mutual interaction. For

that purpose the sticking probability S(e) has to be determined over the large pressure interval of lo-” up to - 10-l Torr, since that gives the possibility to reach a large interval of coverage at three temperatures: 78, 195 and 298 K.

Two methods were applied to determine S(e). The flow method [9] with application of the Groszkowski’s type ionization gauge with modulation, working with an emission current as low as 4 x 10e5 A [lo], was used within the pressure range 10-‘“-10-6 Torr.

l Space Research Centre, Polish Academy of Sciences, Ordona 21, 01-237 Warsaw, Poland.

0039-6028/85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

E. Nowicka et al. / Role of precursor state L167

Direct monitoring of the change of hydrogen population on the surface by means of surface potential measurements with an ultrasensitive, short response time static capacitor circuit was adopted at a pressure of lo-’ to 10-l Torr [7].

The coverage 0 is defined as:

N

e=c7 (1)

where N, is the population of hydrogens per cm* of the real area of the film

16371, Na(maxj is the maximal total population in the (/3s + pw) states reached at 78 K, which we suppose to correspond to the complete monolayer.

In both types of experiments thin iron films were deposited on Pyrex-glass maintained at 78 K using Johnson-Matthey wires grade I, and then sintered at 330 K. During the deposition the residual gas pressure did not exceed 2 x lo-” Torr. The thickness of the films was - 1000 A, and their roughness factor 18 & 2 [6,7]. Spectroscopically pure hydrogen was additionally purified by diffusion through a palladium thimble.

The dependence of the sticking probability on coverage for hydrogen adsorption on thin iron films is shown in fig. 1. Arrows marked with & indicate the coverage 8, above which the & state is formed. It can be seen that the S(e) values characteristic of the j3; state arising at the beginning of the process are in the interval 1-(1O-3-1O-5) at various temperatures. The pw state is formed with low S(8) value from (10-3-10-5) to 10e9.

The dependence S = (p(d) is not a simple one. To find to what extent the precursor state influences the kinetics of adsorption we applied Kisliuk’s model [ll] for adsorption with dissociation to describe the S = ~(8) relation in fig. 1. According to this model:

ln s(e) ~ = In (1 -e) 1 + so [ i

++&so -’ 1 I

9 (2)

where So = lim,,,S(B) and K is a parameter independent of 8. The So value can be estimated experimentally, so there is only one parameter K to fit eq. (2) to the graphs in fig. 1.

In fig. 2 one can see that for adsorption at all investigated temperatures the beginning of adsorption is very well described by Kisliuk’s model. In fig. 1 arrows marked with K indicate a coverage 8 corresponding to the limit of the validity of eq. (2). This limit is bound by the upper value of the coverage at which the rB,- state is formed [6,7].

We found that the value of the parameter K rises with increasing tempera- ture of adsorption reaching 2.9, 10.9 and 34.3 at 78, 195 and 298 K respec- tively. This may indicate a higher mobility of the adsorbate with the increase of temperature, and can also be caused by a temperature dependence in the desorption rate from a precursor over a filled site [ll].

L168 E. Nowicka et al. / Role o/precursor state

,.ld9 i 0 01 02 03 0.4 05 06 01 08 0.9 1

-e Fig. 1. Sticking probability dependence on coverage for hydrogen adsorption on thin iron film

surfaces at 298, 195 and 78 K. The arrows marked with p& indicates the beginning of the /tW state

formation. The arrows marked with K indicate the limit of the validity of Kisliuk’s model for

adsorption. Data points corresponding to the experimental results obtained with application of the

two methods are put in the graphs. The results of the classical flow method are marked: ( X) 298

K, (0) 195 K, (v) 78 K. The results of Asp measurements are marked: (a) 298 K, (0) 195 K, (v)

78 K.

The rate of adsorption of the & ad-species does not fit to the Kisliuk’s model, nor to the Langmuir’s model. It has been shown [7] that within the /I& state the adsorption rate is influenced by a repulsive interaction in the adsorbed layer forming a barrier AU. It has also been demonstrated that the Morse type potential describes well this interaction [7], leading to the equation for S(8) for the & state:

S(O) = S,,(l - 0)” exp( -AU/RT), (3)

E. Nowicka et al. / Role of precursor state L169

-5

-6

-1

-0

-9

-10

-11

-12

. a mu x

0 195K ’ 29au L

x d 0

0

x x

(v) 78 K, (0) 195 K, (X) 298 K.

where

AU=r[l-expy(l-/m)12.

S,, c and 8, are parameters that are experimentally determined while y is taken 6 as it is usually done for condensed systems. It should be emphasized that the parameter 8, is temperature dependent [6,7] increasing strongly with decrease of temperature. For that reason S(e) values are lower at higher temperatures. On the other hand the parameter c should be independent of temperature. Indeed we found its values 11.5, 13.2 and 12.3 kJ/mol at 298, 195 and 78 K correspondingly.

L170 E. Nowrcko ef al. / Role of precursor slate

-~-m- [1-exp6(1-m]*

2 L 6 8 10 12 14 16 x ld'at195K;298K

x IO-*at 78K

-2~

78K

-12.

-lb-

-16. \

Fig. 3. The examination of the validity of the Morse type potential application for the description

of the influence of short range order interaction in the adsorbate layer on sucking probability for

the PW state of hydrogen on iron.

In fig. 3 one can see that eq. (3) describes well the adsorption rate within the fiw state formation.

We conclude that hydrogen adsorption on thin iron films occurs via a

precursor state and at the beginning of the process the sticking probability is well described by Kisliuk’s equation. At high coverages, when the induced weakly adsorbed j3w state is formed, the adsorption rate is influenced by the mutual interaction between the ad-species and the sticking probability can now be described by using the Morse type potential to express the barrier for adsorption.

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