Linewidth of H chemisorbed on W(100): An infrared study

Surface Science 161 (1985) L559-L564 North-Holland, Amsterdam

L559

S U R F A C E S C I E N C E LETTERS L I N E W I D T H OF H C H E M I S O R B E D O N W(100): AN I N F R A R E D S T U D Y D.M. RIFFE, L.M. HANSSEN and A.J. SIEVERS Laboratory of Atomic and Solid State Physics and Materials Science Center, Cornell University, Ithaca, New York 14853, USA

and Y.J. C H A B A L and S.B. C H R I S T M A N A T&T Bell Laboratories, Murray Hill, New Jersey 07974, USA Received 26 April 1985

The symmetric stretch vibration of hydrogen on W(100) at saturation is measured with high resolution by infrared spectroscopy and found to be broad (---100 cm-1). This unusually large width is thought to reflect large H - H dynamical interactions.

We report the measurement of the symmetric stretching mode (Pl) of H on W(100) at saturation coverage. Early Electron Energy Loss Spectroscopy (EELS) data [1] showed that the linewidth associated with this mode was considerably broader (120 cm -1) at saturation than at lower coverage (60 cm 1) even though both phases were later shown to be bridge bonded [2]. This intriguing observation stimulated an infrared study [3], involving the excitation of Surface Electromagnetic Waves (SEW) by laser radiation, which resulted in the measurement of a very narrow line ( = 14 cm 1) and suggested that phonon sidebands may account for the difference. The present work involves independent measurements by two distinct infrared spectroscopic techniques. The first involves another SEW study utilizing narrow band CO 2 laser sources. The second consists of conventional one-bounce grazing incidence Reflection-Absorption Spectroscopy (RAS) and covers a broad spectrum. In contrast to previous results using SEW spectroscopy [3], we find that the linewidth determined by SEWS is 115 _+ 25 cm i while the RAS data give 90 _+ 10 cm -1. The previous SEWS interpretation is shown to be incorrect because of recently discovered interference effects and reconstruction-induced infrared absorption. The origin of the linewidth is tentatively assigned to dynamical interaction between adsorbed H atoms. Prior to any adsorption measurements, the tungsten crystal (6.1 × 0.6 × 0.17 cm 3) used in the SEWS experiments was baked at 1350 K in 1 × 10 7 Torr of 0039-6028/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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D.M. Rifle et a L / Linewldth of H chemisorbed on WflO0)

02 for about 20 h (with periodic flashes to 2300 K) by electron beam heating in order to remove any C from the surface. During the experiment, 2300 K flashes are also used to remove other contaminants prior to each H 2 adsorption. LEED and thermal desorption are used to monitor surface cleanliness. Thermal contact with a liquid N 2 feedthrough allows the sample to cool to room temperature in 8 min, which keeps surface contamination below an estimated 1% of a monolayer. Transverse Magnetic (TM) radiation from a line-tunable laser (C1302 or C1202) is coupled to SEW's via a grating etched in the sample surface. The SEW's probe a length L = 5 cm of the W(100) surface before another grating couples them back to free space radiation which is detected with a HgCdTe photoconductive detector. The actual absorption measurement is performed by monitoring the SEW signal at a fixed frequency as the H 2 is adsorbed onto the W(100) surface. Any change in the surface wave attenuation coefficient c~, due to the adsorbed hydrogen, is calculated from zla=-lln(l+Jl'l,i0

]

where ,:lI is the change in initial transmitted intensity I 0. Unfortunately, apparent changes in the surface wave attenuation can arise from interference between the SEW's and free space radiation travelling parallel to the surface as was demonstrated recently in the case of a dielectric coated metal [4]. The finite resolution of the input grating used here allows coupling of the input beam to free space radiation which skims along the surface and is scattered out by the second grating into the detector. By raising the sample temperatureand thus quenching the SEW signal, we have been able to determine that the free space radiation signal is largely temperature independent and that its itensity is ~ 1% of the total detected intensity at room temperature. At most this results in a 10% correction to ~ a [5]. Other changes in the SEW attenuation, which are not related to vibrational absorption, are observed [6] as the W(100) undergoes H-induced transitions in reconstruction [7]. These transmission variations, which are the same for H or D induced reconstruction and exhibit only a weak broadband frequency dependence, are an order of magnitude larger than the vibrational absorption. In order to determine the change in a due to the H stretching mode, separate D 2 and H a adsorptions are performed at each frequency. Since the W2-D line lies outside the frequency range studied, the difference in Aa between an H and a D-saturated surface is due only to excitation of the W2-H mode. The SEW data were taken at six discrete frequencies between 880 and 1100 c m - ~ (the overall range of the two CO 2 lasers). Between three and eight D 2 and H 2 adsorptions were done at each frequency. The average difference in Aa between D 2 and H 2 runs is plotted in fig. 1. The error bars include statistical fluctuations and an estimation of systematic errors. The observed change in zla (SEWS) can be simply related to the effective charge e* and mass m* of the

D.M. Rifle et al. / Linewidth of H chemisorbed on W(IO0) I

I

I

I 1000

1 1t00

L561

W ( 1 O O ) - ( t x t) H SEWS

'E

;"

4

(o

b 2

_2 I 800

I 900

1200

FREQUENCY ( c m -1)

Fig. 1. Change in attuation coefficient, Act,due to W2-H symmetric stretch of saturation coverage (exposure > 4 L) at room temperature. The sohd curve is a least squares fit to the data using eq. (1).

mode, the coverage N s (in c m - 2 ) a n d the m e t a l p l a s m a frequency Up ( a s s u m i n g a D r u d e form of the dielectric c o n s t a n t [8] a n d a L o r e n t z i a n response of the v i b r a t o r [9]) as follows: A s = 8~

v 4 Nse*2 Rp m * ¢ 2

Av

(p2 v2)2nt_(Ap~,)2'

(1)

where v L a n d Av are the l o n g i t u d i n a l frequency and n a t u r a l linewidth of the o s c i l l a t o r , respectively. T h e p a r a m e t e r s o b t a i n e d to fit the d a t a are given in table 1. A s e p a r a t e e x p e r i m e n t was p e r f o r m e d using a b r o a d b a n d surface i n f r a r e d s p e c t r o m e t e r recently built to s t u d y s e m i c o n d u c t o r [10] a n d m e t a l surfaces. U s i n g a one b o u n c e grazing incidence g e o m e t r y (85 °) in c o n j u n c t i o n with a H g C d T e detector, spectra from 800 to 2500 c m 1 can be o b t a i n e d in five m i n u t e s with a noise level of AR/R = 1 × 10 -4 for a resolution of 8 c m which is sufficient to resolve the W 2 - H line. T h e W(100) s a m p l e (2.5 x 0.75 ×

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D.M. Rifle et aL / Linewidth of H ehemisorbed on W(100)

Table 1 Vibrational parameters for the W 2 - H symmetric stretch Technique

e*/e

v L (cm 1)

/~u (cm 1)

SEWS RAS EELS

0.067 _+0.01 0.053 _+0.005 0.052 [9]

1059 ! 10 1070 ___ 5 1067 [1]

115 + 25 90 + 10 120 [1]

List of parameters used in eq. (1) for SEWS, eq. (2) for RAS and extracted from refs. [1] and [9] for EELS for the W(100)-(1 x 1)H saturated surface.

0.13 cm 3) is cleaned by repeated baking at 1475 K in 1 x 10 7 Torr of oxygen ( > 10 h) and flashing in vacuum to 2150 K by electron bombardment heating. The Auger spectrum of the clean sample shows typically less than 1% monolayer of carbon and oxygen as measured by a single pass cylindrical mirror analyzer. The surface remains clean for the time of the experiment (30 min) in a base pressure of 4 x 1 0 -11 Torr. The LEED patterns can be monitored during the IR measurements without moving the sample and all the various reconstructions previously reported [7] for increasing coverage of hydrogen are observed before the saturation 1 x 1 pattern is established. The spectra were recorded by measuring the reflectivity of the clean surface and subtracting it from that of the H-covered surface as was done in previous studies [10]. As H (or D) is adsorbed the broadband reflectivity varies in a manner consistent with the SEW measurements. These large amplitude reconstruction-induced changes in reflectivity will be reported later [11]. The weak frequency dependence of these variations makes it possible to subtract them using a straight line baseline in the region of the W2-H vibration. A typical spectrum is shown in fig. 2. The raw data show an asymmetric line. The high frequency contribution actually comes from the overlap with another line, also due to H adsorption, which is under current investigation [11]. Under the same assumptions [8,9] leading to eq. (1), the change of reflectivity, for an angle of incidence 0 such that (I,p/V) 2 COS20 >7> 1, can be written: AR _ 8v 2 sin20 N~e.2

R

cos 0

m*c 2

Av

(2)

2)2(a

The dashed line is a fit using eq. (2) with the parameters listed in table 1. Reproducibility between runs falls within the uncertainty level due to baseline correction which is represented by the error bar. The good overall agreement between the new SEW data and the refection data supports the analysis used to extract the vibrational line from the SEW experiment and confirms that the early SEW measurements were erroneously interpreted. We present here a brief argument to indicate the source of error. "st, the large absorption reported is consistent with the large change of

D.M. Rifle et al. / Linewidth of H chernisorbed on W(IO0) I

t0

W(IOO)-(IX t)H RAS

L563

1

b <3

t 4

-

I

2-

S

i

I %

//

O 800

900

%%

1000

FREQUENCY

I 1t00

1200

(cm -t)

Fig, 2. Change of reflectivity, A R / R , induced by saturation (4 L) of H on W(100) at room temperature. The resolution is 8 cm ] (no smoothing). This spectrum is typical and required 1024 scans (3.5 rain) for both the H-saturated and clean surfaces.

reflectivity occurring as the reconstruction goes from a split c(2 × 2) to a disordered state [7]. This observation indicates that the signal reported previously resulted from reconstruction changes rather than from vibrational excitation. Next, the apparently sharp "line" can be understood if one takes into account the newly discovered interference effect between the surface wave and free space radiation [4,5] which is particularly severe for the prism/waveguide geometry used in ref. [3] since free space radiation is necessarily confined near the W(100) surface. This makes the gap correction (table 1 of ref. [3]) •inappropriate since the overall interference from the free space radiation varies rapidly with the gap value. Without these corrections, the frequency depen~ dence is no larger than the error bars. In summary, the previous SEW work actually measured a reconstruction-induced change in absorption which exhibited an apparent frequency dependence due to interference effects. The magnitude of the measured vibrational linewidth is intriguing. Preliminary results from isotopic mixture experiments show that the W2-H vibrational line shifts down in frequency and narrows as the relative con-

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D.M. Rifle et al. / Linewidth of H chemisorbed on W( IO0)

c e n t r a t i o n of D increases at a c o n s t a n t total coverage [11]. The n a r r o w i n g of the linewidth u p o n D dilution rules out a purely i n h o m o g e n e o u s b r o a d e n i n g ahd suggests that pure lifetime broadening, arising from a coupling of the W 2 - H mode to either the substrate p h o n o n s [12] or to the e l e c t r o n - h o l e pair c o n t i n u u m [13], c a n n o t account for the entire linewidth. The shift in frequency indicates that there is a strong d y n a m i c a l interaction b e t w e e n adsorbed H atoms [14,15]. While a linear coupling is sufficient to account for a shift of the vibrational line, strong n o n - l i n e a r interactions could also b r o a d e n the line. The origin of such a dynamical interaction is not clear. D i p o l e - d i p o l e interaction is inconsistent with the narrowing of the W 2 - H line u p o n D dilution a n d is too weak to account for the shift. Other possibilities such as diffusion or indirect interactions mediated via the metal electrons or via the non-rigid substrate ion cores, c a n n o t be distinguished without further studies [11]. In conclusion, we have measured the infrared a b s o r p t i o n of the W 2 - H vibration by means of two i n d e p e n d e n t techniques. The linewidth is found to be consistent with early EELS experiments. Although we c a n n o t present a detailed model at this time to account for this large linewidth, it appears that it may originate from d y n a m i c a l interaction between adsorbed hydrogen. D.M. Rifle, L.M. H a n s s e n and A.J. Sievers acknowledge support from N S F u n d e r G r a n t No. DMR-81-06097 and A F O S R u n d e r G r a n t No. A F O S R - 8 1 0121F. Y.J. Chabal and S,B. C h r i s t m a n wish to t h a n k E.E. C h a b a n for critical technical assistance. Cornell Materials Science Center Report No. 5532.

References [1] A. Adnot and J.D. Carette, Phys. Rev. Letters 39 (1977) 209. [2] M.R. Barnes and R.F. Willis, Phys. Rev. Letters 41 (1978) 1729. [3] Y.J. Chabal and A.J. Sievers, Phys. Rev. B24 (1981) 2921. [4] Z. Schlesingerand A.J. Sievers, Appl. Phys. Letters 36 (1980) 409. [5] L.M. Hanssen, D.M. Rifle and A.J. Sievers, 44th Conf. on Physical Electronics, Princeton, NJ, June 1984; L.M. Hanssen, PhD Thesis, Cornell University (June 1985). [6] D.M. Rifle, L.M. Hanssen and A.J. Sievers, Bull. Am. Phys. Soc. 30 (1985) 361; D.M. Riffe, L.M. Hanssen and A.J. Sievers, to be published. [7] R.A. Barker and P.J. Estrup, Phys. Rev. Letters 41 (1978) 1307; M.K. Debe and D.A. King, Phys. Rev. Letters 39 (1977) 708. [8] In the region (~.)2 >> 1. Z. Schlesingerand A.J. Sievers, Surface Sci. 102 (1981) L29. [9] See H. Ibaeh, Surface Sci. 66 (1977) 56. Note, however, that all values of e* in that reference are too low by a factor of ~2 as pointed out in A.M. Braro, H. Ibach and H.D. Bruchmann, Surface Sci. 88 (1979) 384. [10] Y.J. ChabaL G.S. Higashi and S.B. Christman, Phys. Rev. B28 (1983) 4472. Ill] Y.J. Chabal, to be published. [12] J.C. Ariyasu, D.L. Mills, K.G. Lloyd and J.C. Hemminger, Phys. Rev. B30 (1984) 507. [13] M. Persson and B. Hellsing, Phys. Rev. Letters 49 (1982) 662. [14] B.N.J. Persson and R. Ryberg, Phys. Rev. B24 (1981) 6954. [15] C. Nyberg and C.G. Tengst~, Phys. Rev. Letters 50 (1983) 1680.