CF3I adsorption on Ni{100}

Vacuum/volume 38/numbers 4/5/pages 213 to 218/1988 Printed in Great Britain

0042-207X/8883.00 +.00 Pergamon Press plc

CF31 adsorption on Ni{100} Robert G Jones and Nagindar K S i n g h , Department of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, UK

The adsorption of CF31 on Ni{ 100} has been studied using LEED and AES. At 323 K CFal adsorbed dissociatively to form a p(2 x 2) consisting of 1/8 ML of iodine atoms, and 1/8 ML of CF a. Heating this surface to temperatures greater than 470 K caused CFa to desorb from the surface; the desorption was complete by 770 K. Adsorption at elevated temperatures occurred with an initial rate of adsorption three times that for 323 K adsorption, as measured by iodine uptake; CF3 desorption occurred during the adsorption process, leaving a surface of pure chemisorbed iodine of between 1/3 ML and 5/12 ML coverage, depending on the temperature used. Adsorption at low temperature caused a partial breaking of the C-I bond and the formation of a c(2 x 2) pattern. On heating this surface to 290 K complete dissociation of the C-I bond occurred and by 470 K the p(2 x 2) structure was reformed. The initial rate of adsorption at T<_323 K could be increased three-fold, by adsorption under an electron beam; this and the increased adsorption rate at elevated temperatures implied an activation energy for dissociative adsorption which could be overcome either thermally or by electron bombardment.

1. Introduction

incident beam for AES was deliberately defocused to give the same beam diameter as for LEED). The sample could be heated The adsorption of small molecules and the interactions of resistively to 920 K, or cooled by liquid nitrogen to 140 K; molecular fragments on metal surfaces is of fundamental interest. temperature measurements were made using a chromel-alumel The adsorption of CHzI 2 molecules on AI20 3 and A1 surfaces l (type K) thermocouple spot welded to the sample. The crystal was has been used for model systems for studying the photofragmencleaned using xenon ion bombardment at 2 keV for 2.4× tation and desorption processes involving electronic excitation of 10 -3 #As, followed by a few seconds annealing at 870 K. CF3I, the adsorbate. Similar studies have been carried out for CH3Br supplied by Fluorochem Ltd, was introduced directly into the adsorbed on LiF{001 }z and for CH3Br on Ni{ 111 }3. In this study . chamber using a standard leak valve. Its purity was checked with the adsorption of CF3I on Ni{ 100} is reported. CF3I was chosen a Masstorr F (Vacuum Generators) mass spectrometer, the for three reasons: first, the chemistry of iodine on Ni{100} is now principle peaks were due to CF~- at 69 amu and I + at 127 amu. well understood4-11; second, the C - I bond in CF~I is weak The mass spectrometer, which was fitted with a Faraday cup (2.3 eV ~2) relative to the C - F bond (~-5.0 eV t3) so it might be collector, was not sensitive enough to allow thermal desorption expected that dissociative adsorption will involve breaking the spectra to be taken for the rather low heating rates (a few K s - J) C-I bond, leaving the CF 3 species intact; third, a large body of obtainable experimentally. knowledge exists on the gas phase photofragmentation dynamics The pair of iodine Auger peaks at 511 eV and 520 eV 18 were of' CF3I and the potential energy curves are known for both the easily distinguished as they occurred on a fiat background. ground state and the excited molecule ~2,~*-a6. The Ni{100} Quantitative measurements of iodine coverage were usually taken surface was chosen for this initial study as it shows a reasonable using the peak-to-peak height ratio, IppfNipp, of the iodine 511 eV reactivity and was, indeed, found to dissociate the molecule. CF3I peak to the large nickel peak at 716 eV 18. Occasionally the iodine adsorption on the less reactive Cu{ 111 } will be investigated in a peak intensity was used alone, when particular care was taken to subsequent study, where molecular adsorption is more likely. ensure that the incident beam current remained constant. The fluorine peak at 650 eV 18 is intrinsically weak and lies close to the weak 663 eV TM peak of nickel. Small changes in the shape and size 2. Experimental of the low energy side of the 663 eV nickel peak indicated the The experimental equipment and the cutting and mounting of the presence or absence of fluorine, but quantitative measurements of Ni{100} sample have already been described l?. In brief, a fluorine coverage were not possible. Carbon coverages could be four-grid retarding field analyser was used for LEED and AES; for measured in a semiquantitative manner at 323 K and above using the latter a 4.0 V rms sinusoidal signal was applied to the sample the carbon 272eV is Auger peak intensity relative to the together with the usual lock-in techniques. The incident beam background. Only qualitative measurements of carbon coverage current for LEED was 12/~A, and for AES at 2 keV, 17 pA. The were possible at low temperatures because of the large temperabeam diameters for both LEED and AES were about 1 mm (the ture dependent nickel peak at 285 eV 19, which was already about 213

Robert G Jones and Nagindar K Singh: CF31 adsorption on Ni{1 00}

the size of the carbon Auger peak at 323 K, but became much larger than the carbon Auger on cooling to 213 K [Figure l(a)]. The size of this 285 eV nickel peak was also sensitive to adsorbed iodine, making an accurate determination of the carbon peak height impossible at low temperatures.

0.8

/~//

04

,/

0.2

/I/

•

/

/4

(a)

/

o (b)

•

0.6 0.4

/ 28SeV,;"

dN(E) dE

[pp Nipp

.~m rail ' ,~,=]llm~ W" " •

I .~,-- LSHARP p(2x2) 0.2 } ~ "-FAINT/DIFFUSE p(2x2) ~ L STREAKS 0

)

(b) z LU

. . . . . . . .

1.2-]

carbon

OOOQ •

//%" •

• • • " • • •

•

•

1.0

Z ©

0.8

n-.< O

u

2;0

3;0 250 E/eV

300

0.6

Figure 1. (a) Full line, temperature dependent Ni peak at 285 eV for clean Ni{ 100} at 213 K; dashed line, after saturation with CF3I, showing carbon Auger peak. (b) Full line, scan over carbon Auger region after saturation with CF3I at 203 K; dashed line, after warming the surface to 290 K, showing the increase in size of the carbon Auger peak.

0.4 0.2

(e) 0

5 10 EXPOSUREx 105 tabor

3. Results 3.1. Adsorption at 323 K. The clean surface was dosed with CF3I with the electron gun switched off, then after pumping the CF3I out of the vacuum chamber, Auger scans were taken and the dosing process repeated until saturation coverage was reached. At saturation coverage, a slightly diffuse p(2 x 2) LEED pattern was observed, with carbon, fluorine and iodine present on the surface, the latter with Ipv/Nipp = 0.6 + 0.1. The sticking probability, $323, measured using the iodine Auger peak, was constant to high coverage and saturation occurred for exposures greater than 8 x 10 -5 mbar s [Figure 2(a)]. If the primary electron beam for AES (2 keV, ~- 20 A m - 2) was incident on the surface during adsorption then a slightly diffuse p(2 x 2) LEED pattern was again observed at saturation, with carbon and fluorine present on the surface and Ipp/Nipp= 0.64 + 0.1. The I pp/Nipp ratio as a function of exposure is shown in Figure 2(b), together with the carbon Auger peak intensity after background subtraction. At saturation the carbon and iodine Auger peaks were the same size as for adsorption in the absence of electrons. This, and the slightly diffuse p(2 x 2) observed for adsorption with and without electrons present, suggests that the saturated surface coverage and structure do not depend on the incident electrons. However, the rate of adsorption, using either Ipp/Nipp or the carbon Auger peak intensity, Figure 2(b), was three times faster with electrons incident, than without, with complete saturation after only 3 x 10 -s mbar s. This increased rate of adsorption will be referred to as $323.e. The diffraction patterns observed for adsorption under the primary beam for LEED (~- 130 eV, ~ 15 Am -2) are also marked in Figure 2(b), indicating that the same increase in adsorption rate occurred with an incident LEED beam as with an incident AES beam. 214

Figure 2. (a)

15

$e£

Ir,v/Niop

for CFaI adsorption on Ni{100} at 323 K with no incident electrons during adsorption. (b) Ip~Nipp (circles) and carbon Auger peak intensities (squares) for adsorption at 323 K with electrons incident on the surface during adsorption. (c) IpJNi~p for adsorption at 723 K with an incident electron beam (small circles) and at 588 K without an incident electron beam (large circles). Also shown is the carbon Auger peak intensity for 588 K adsorption.

No differences were observed between the behaviour of the surfaces formed with and without an incident electron beam, when they were heated in vacuum, again both with and without an electron beam incident on the surface during the heating. On heating to 360 K the p(2 x 2) spots became sharper and by 470 K they were as sharp as the integral order spots [Figure 3(c)]. At about 620 K the centring (1/2, 1/2) beams became diffuse and streaks appeared and by 640 K the (1/2, 1/2) beams had faded. At 650 K the (0, 1/2) beams also became diffuse and by 770 K no extra beams were visible in the LEED pattern. The Auger spectra showed a loss of carbon from the surface which commenced at 470 K and was complete by 770 K, Figure 4. By 770 K fluorine had also disappeared from the surface but iodine showed no loss of coverage until temperatures greater than 870 K were reached. 3.2. Adsorption at elevated temperatures. The results for adsorption at 723 K with an AES primary beam incident on the surface, and at 588 K without an incident electron beam, are shown in Figure 2(c). In both cases it can be seen that the Ipp/Nipp ratio rises to a value of 1.2+0.1 and that the initial rate of adsorption is the same as that for 323 K adsorption with an electron beam incident on the surface, $323,e. The behaviour of the carbon Auger for adsorption at 588 K is also shown, indicating that the carbon

Robert G Jones and Nagindar K Singh." CF31adsorption on Ni{100)

ql

ql

J

•

t o

o

(a)

(d)

e

i g

(b)

(e)

.

•

U

•

I

•

O

II.

•

,

s

1

(c)

(f)

Figure 3. (a) diffuse c(2 x 2) formed by CF31 adsorption at 201 K with no electrons incident during adsorption (135 eV). (b) Sharp c(2 x 2) formed by warming (a) to 288 K (128 eV). (c) p(2 x 2) formed by heating (a) to 469 K (130 eV). (d) (2 × 3) formed by adsorbing CF~I at 723 K (135 eV) and (e) c(2 × 12) formed by adsorbing CF3I at 585 K (133 eV), both followed by cooling to 323 K in CF31 at 6.7 × 10 -s mbar. (f) (2 × 3) formed by heating (d) to 693 K in vacuum, followed by cooling (105 eV). coverage initially increased and then dropped to zero as the iodine adlayer saturated. At saturation both carbon and fluorine were undetectable on the surfaces formed at either temperature. After cooling from 723 K in the C F f l atmosphere of 6.7 x 10- 8 mbar, a slightly diffuse ( 2 × 3 ) structure was observed [Figure 3(d)], which, while cooling from 588 K, formed a more complex pattern [Figure 3(e)]. These and other experiments indicated that the electron beam had no effect on the adsorption behaviour at elevated temperatures. Heating either surface to 690 K in

vacuum, followed by cooling, formed a sharp (2 x 3) structure [Figure 3(f)]. 3.3. Adsorption at low temperatures. In the temperature range 163-201 K adsorption occurred, in the absence of an electron beam, with an initial rate mid-way between $323, o and S3z a (Figure 5). The rate of adsorption was not constant to high coverage and the Ipp/Nipp ratio saturated at 0.8 + 0.1. Both carbon and fluorine were detectable on the surface; the carbon Auger

215

Robert G Jones and Nagindar K Singh: CF31 adsorption on Ni{100}

O8

%._ tpp

~

Ntpp

•

Z ttl z

•

,e<

z O nn n-"

02

P(2x21

SHARP

~

p(2x2) p(2x2)

300

400

-- ~

EXTRA~

500

x

SPOTS

~

600

700

[ I

•

-- NO

- EXTRA SPOTS

800

t.)

900

TEMPERATURE / K

Figure 4. Ipp/Nipp(circles)and carbon Auger peak intensities(squares) vs temperature for a p(2 x 2) formed at 323 K, being heated in vacuum. The solid lines are to guide the eye. was noticeably smaller than for adsorption at 323 K and the saturated surface showed a diffuse c(2 x 2) structure [Figure 3(a)]. For adsorption in the range 163-183 K with an electron beam incident on the surface, the Ipp/Nipp ratio showed an initial rate of adsorption equal t o S 3 2 3 . e (Figure 5), however adsorption continued and was still occurring when the Ipp/Ni0p ratio was 2.3 after 14x 10 -5 mbar s. After this exposure, the carbon Auger peak was undetectable, but the fluorine Auger peak was easily observed and the LEED pattern consisted of a high background with barely discernible integral order beams. Moving the crystal showed that the large increase in iodine coverage only occurred under the electron beam. Adsorption at 203 and 215 K, with the electron beam present, caused the Ipp/Nipp ratio to saturate, with an initial rate of adsorption very similar t o 5 3 2 3 . e (Figure 5). The saturated surfaces for 203 and 215 K adsorption, for which both carbon and fluorine were detectable, consisted of a diffuse c(2 x 2) pattern. On warming the diffuse c(2 x 2), formed by adsorption at low temperatures in the absence of incident electrons, to 290 K, the iodine Auger peak remained unchanged, but the carbon Auger peak doubled in size [Figure l(b)]. By 290 K the c(2 x 2) had become quite sharp (Figure 3(b)] and dim p(2 x 2) spots had begun to appear at the (0, 1/2) positions. By 470 K a sharp p(2 x 2) was observed [Figure 3(c)] and the Auger peak intensities were the same as those observed for adsorption at 323 K. Heating the surfaces formed under an electron beam at 203 and 215 K caused the same changes in the LEED and AES. Heating the surface formed under an electron beam between 163 and 183 K caused a complex sequence of LEED patterns to evolve, due to the reaction of the solid iodine formed at these temperatures (see Discussion) with the surface. 4. Discussion

First consider the reaction of CF3I with Ni{ 100} at 723 K. If the assumption is made that dissociative adsorption occurred by cleavage of the C-I bond, the CF 3 group remaining intact, then the adsorption reaction is CF3I(g )--*I(~)+ CF3(s) , 216

(1)

where g and s refer to gas and surface, respectively. Both carbon and fluorine were undetectable on the surface at saturation and the carbon coverage was observed to increase and then decrease during adsorption [Figure 2(c)]. Both imply the desorption of CF3, as either the radical or as C2F6, during adsorption, 2CF3(s)-+2CF3(g) or C2F 6.

(2)

The saturated surface consisted entirely of iodine, which formed a reasonably sharp (2 × 3) LEED pattern on cooling. Previous studies of iodine adsorption on Ni{100} 4 have shown that the (2 x 3) has a coverage of 1/3 ML, and that this is the stable structure formed by heating a surface of higher iodine coverage to about 700 K in vacuum. Reaction at the lower temperature of 588 K also led to a surface of pure iodine, exhibiting a more complex LEED pattern with the matrix notation4

(, ot:) -- 1

cot

where c~~ 40 °. This structure belongs to a family of structures (of which the (2 x 3) is also a, member) formed by heating a higher surface coverage of iodine to about 500 K 4. For c~= 39.8 ° the structure comes into coincidence with the Ni{ 100} substrate to form a c(2 x 12) with a coverage of 5/12 ML of iodine. So for reaction at these two temperatures, the iodine coverage was determined by the temperature at which adsorption occurred. The adsorption behaviour was the same irrespective of the presence of an electron beam on the surface, showing that thermal activation at these temperatures was sufficient for rapid adsorption. The initial rate of adsorption at 323 K in the presence of an electron beam, $323,~, was the same as that for adsorption at temperatures > 588 K, however, in the absence of an electron beam, the initial rate was 1/3 of this value. The surfaces formed at 323 K, both with and without incident electrons were the same consisting of iodine, carbon and fluorine and exhibiting a p(2 x 2) LEED pattern. It follows that adsorption at 323 K occurred dissociatively, reaction (1), but at this lower temperature only one in three of the incident molecules dissociated, and the CF 3 fragment remained on the surface. The effect of the electron beam

Robert G Jones and Nagindar K Singh:

CF31adsorptionon Ni{100)

-2.2 -1.8 10-

S

•

#

. J ~l

v

05- ~

_

"~ .

w

-1.4

@ A

"I"0 IP----~p

-0.6 -0.4 ,0.2

I

I

5

10

EXPOSUREx 105

0

rnbQr se¢ Figure 5. Iodine Auger peak intensities (linear left hand scale) and [pa/Nipp (non-linear right hand scale) for CF3I adsorption at 178 K in the absence oJ incident electrons (circles). Also shown is the adsorption bebaviour at 178 K (squares), 203 K (lozenges) and 215 K (triangles) with electrons incident on th~ surface during adsorption.

was to increase the rate of adsorption, and this may be attributed to an electronic, rather than a thermal effect in the following way. The same increase was found for both the L E E D beam and the primary AES beam, where there was a ten-fold difference in energy. The L E E D beam deposited a m a x i m u m of 3.4 x 10-3 W in a l - m m dia spot, insufficient to raise the temperature under the beam by more than a few tens of K at most. Reaction (1) may now be modified to

(b)

(a)

\

CF3I(g)*--*CFaI(phys)--*CF3(s)+ I(s ) t h e r m a l a c t i v a t i o n

(la)

CF3Itg)e-*CF3I(phrs)--*CF3(s) + I(s ) e l e c t r o n i c a c t i v a t i o n , ( l b ) where it as assumed that CF3I can physisorb (phys) into a :relatively long lived state above the surface, in which either ~Ihermal or electronic activation can occur for dissociation. The possibility of gas phase cracking of CF3I may be rejected as the enhanced adsorption was only observed directly under the electron beam, while fragmentation of CF3I on the gun filament, tbllowed by desorption and focusing of either I - or C F 3 on to the sample, would have given very different iodine or carbon coverages, which were not observed. The (2 x 3) structure of 1/3 M L coverage, for which Ipp/Nipp = 1.2 + 0. I, allows a calibration of the Ipd~lipp ratio. F o r the p(2 x 2) formed at 3 2 3 K , Ipr,/Nipp = 0.6 -1-0.1, giving a coverage of -'---0.16 ML. The structure of the p(2 x 2), and the iodine coverage, nnay be explained by a (2 x 2) structure in which either iodine atoms or C F 3 species occupy the lattice points. Such a structure would have a coverage of I/8 M L each for the iodine and the C F 3. Figure 6(b) shows a possible arrangement for the p(2 x 2) in which the iodine and the C F 3 are located in the four-fold surface sites, the C F 3 being orientated with the carbon downwards. The slight sharpening of the p(2 x 2) L E E D pattern on heating shows that at 3123 K, the surface was not quite at thermodynamic equilibrium. Further heating to temperatures greater than 600 K caused the L E E D pattern to deteriorate and the carbon to leave the surface, both being a consequence of C F 3 desorption as in reaction (2). The iodine coverage remained constant until temperatures greater than 873 K were reached, when desorption commenced; this is consistent with an iodine coverage of less than 0.2 M L 4. Adsorption at T < 2 0 1 K in the absence of electrons led to an

Figure 6. (a) Possible adsorption geometry for the diffuse c(2 x 2) formed by CF3I adsorption on Nit100} at 178 K. The iodine and the carbon of CF 3 are positioned over four-fold sites, and the plane containing the fluorine atoms is tilted at 45° to the horizontal. (b) Possible structure for the p(2 x 2) formed by adsorption at 323 K. The iodine atoms and the CF 3 groups occupy the (2 x 2) lattice positions randomly. The CF 3 is shown with the carbon towards the nickel surface. The spheres represent van der Waals radii, the dotted circle shows the position of the carbon in CF 3.

initial rate of adsorption which was between 8 3 2 3 , e and $32 a. The L E E D structure formed at saturation was a diffuse c(2 × 2), consisting of iodine, carbon and fluorine. The diffuse c(2 × 2) suggests real space separations for adsorbed I and C F 3 species of 3.52 A, but at such a distance the C F 3 species would have to be tilted relative to the surface to enable them to adsorb next to the iodine atoms [Figure 6(a)]. An alternative way of looking at this is that the adsorbed species is a CF3I molecule with a very stretched C - I bond. The adsorbed CF3I 'molecule' would sterically hinder adsorption into adjacent sites, and so a disordered c(2 x 2) structure, with an iodine coverage well below the ideal 0.25 ML, would result. In Figure 6(a) it can be seen that a fluorine atom lies almost vertically above the carbon atom in the tilted C F 3 group, thus blocking the carbon Auger emission, which would explain the small carbon Auger peak for low temperature

217

Robert G Jones and Nagindar K Singh: CF31adsorption on Ni{1 00} adsorption. O n w a r m i n g to higher temperatures, m o v e m e n t of the C F 3 species becomes possible, such that it can eventually lie flat on the surface [Figure 6(b)] so t h a t the c a r b o n a t o m is no longer shielded by the fluorine, a n d its Auger peak intensity increases. The increase in intensity occurred at a m u c h lower t e m p e r a t u r e t h a n the f o r m a t i o n of the p(2 × 2), indicating m o v e m e n t of the C F 3 g r o u p in its site prior to the surface rearranging into the p(2 × 2) structure. Finally, a d s o r p t i o n at low t e m p e r a t u r e s in the presence of an electron b e a m occurred with the high rate of initial adsorption, $323. e. F o r a d s o r p t i o n at T_< 183 K, the c o n t i n u e d a d s o r p t i o n of iodine under the b e a m m a y be explained by electrons cracking the CF3[, a n d the iodine t h e n a d s o r b i n g as solid 12 (for T < 185 Ks). F o r a d s o r p t i o n at the higher t e m p e r a t u r e s of 203 a n d 215 K, saturation occurred a n d a diffuse c(2 x 2) formed which was the same as for a d s o r p t i o n with n o electrons, as these temperatures were too high for solid iodine formation. R a t h e r interestingly, a physisorbed molecular iodine layer of a b o u t 0.3 M L has been observed on a c ( 2 x 2 ) c h e m i s o r b e d iodine surface in the t e m p e r a t u r e range 185-226 K 5. T h e 203 K a d s o r p t i o n curve in Figure 5 rises significantly higher t h a n the 215 K curve, suggesting the f o r m a t i o n of a similar molecular iodine layer on the previously c h e m i s o r b e d I a n d C F 3 layer. If $323, e is a p p r o x i m a t e l y 1.0, then the p h y s i s o r b e d molecule must reside on the surface for a sufficient time for a n incident electron to hit it a n d activate it for dissociatative adsorption. N o cross sections are available for CF3I , but if a value of -~ 10 ~ 2 is assumed for electrons in the energy range 100--2000 eV, then with an incident current density of ~-20 A m - 2 the physisorbed state must have a lifetime of ~-8 x 10 -2 s, which at 323 K c o r r e s p o n d s to a physisorption energy of ~- 70 kJ m o l - 1. This is a rather large figure when c o m p a r e d with the heat of v a p o u r i z a t i o n of CF3I, 22.4 kJ m o l - ~ (ref 20), but if the molecule were orientated in the physisorbed state, together with the large polarizability of b o t h the metal surface a n d the molecule, a n d the dipole m o m e n t of the molecule (I D2°), it is not u n r e a s o n a b l e to expect an interaction energy of this magnitude.

218

Acknowledgements We would like to t h a n k D r Ivan Powis for suggesting CF3I as an adsorbate, a n d for useful discussions concerning the dissociation of CF3I in the gas phase. N a g i n d a r K Singh would also like to t h a n k the Association of C o m m o n w e a l t h Universities for the a w a r d of a studentship.

References i T J Chuang and K Domen, J Vac Sci Technol, A5, 473 (1987). 2 E B D Bourdon, P Das, I Harrison, J C Polanyi, J Segner, C D Stanners, R J Williams and P A Young, Faraday Discuss chem Soc, 82, 343 (1986). 3 E P Marsh, F L Tabares, M R Schneider and J P Cowin, J Vac Sci Technol, AS, 519 (1987). '~ R G Jones and D P Woodruff, Vacuum, 31,411 (1981). 5 R G Jones, C F McConville and D P Woodruff, SurfSci, 127,424 (1983). 6 C Somerton, C F McConville, D P Woodruff and R G Jones, Vacuum, 33, 858 (1983). 7. C Somerton, C F McConville, D P Woodruff and R G Jones, SurfSci, 136, 23 (1984). 8 C F McConville and D P Woodruff, SurfSci, 152/153, 434 (1985). 9 R G Jones, S Ainsworth, M D Crapper, C Somerton, D P Woodruff, R S Brooks, J C Campuzano, D A King, G M Lamble and M Prutton, Surf Sci, 152/153, 443 (1985). 10 R G Jones, S Ainsworth, M D Crapper, C Somerton and D P Woodruff, SurfSci, 179, 425 (1987). 11 R G Jones, S Ainsworth, M D Crapper, C Somerton and D P Woodruff, SurfSci, 179, 442 (1987). l 2 G N A Van Veen, T Bailer, A E DeVries and M Shapiro, Chem Phys, 93, 277 (1985). 13 F A Cotton and G Wilkinson, Advanced Inorganic Chemistry. John Wiley and Sons, London (1966). 14 V N Bagratashvili, S I Ionov, G V Mishakov and V A Semchishen, Chem Phys Lett, 115, 144 (1985). 15 S Hennig, V Enge[ and R Schinke, J chem Phys, 84, 5444 (1986). 16 K G Low, P D Hampton and I Powis, Chem Phys, 100, 401 (1985). 17 R G Jones and A W L Tong, SurfSci, 188, 87 (1987). x8 p W Palmberg, G E Riach, R E Weber and N C MacDonald, Handbook of Auger electron spectroscopy. Physical Electronics, Minnesota, USA (1972). t9 G E Baker and H D Hagstrum, J Vac Sci Technol, 11,284 (1974). 20 M Stacey, J C Tatlow and A G Sharpe (Editors), Advances in Fluorine Chemistry, Vol 4, p 171. Butterworths, London (1965).