EFFECTIVE He* DEEXCITATION ENERGIES IN PENNING. IONIZATION SPECTROSCOPY OF CO ADSORBED ON Ni. D. LOVRI(~, B. GUMHALTER. Institute of ...
Surface Science 189/190 (1987) 59-63 North-Holland, Amsterdam
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EFFECTIVE He* DEEXCITATION ENERGIES IN P E N N I N G IONIZATION S P E C T R O S C O P Y OF CO A D S O R B E D O N Ni
D. LOVRI(~, B. G U M H A L T E R Institute of Physics of the University, 41001 Zagreb, Yugoslavia
K. H E R M A N N lnstitut fiir Theoretische Physik der Technischen Universitiit Clausthal, D-3392 Clausthal-Zellerfeld, Fed. Rep. of Germany
G. ERTL and K. W A N D E L T Fritz-Haber-Institut der Max-Planck.Gesellschaft, D-IO00 Berlin 33, Germany Received 30 March 1987; accepted for publication 13 April 1987
In Penning ionization spectroscopy of adsorbates the effective excitation energy stored in the metastable noble gas beam atoms depends on their interaction with the target. In the present contribution we discuss and calculate the dependence of the effective H e * deexcitation energy in collisions with CO adsorbed on Ni and Pd which have been studied recently. The calculations demonstrate that both short and long range components of the interaction between H e * (He) and the CO adsorption complex are needed to establish the relevant magnitude of the change of the effective deexcitation energy observed experimentally.
In surface Penning ionization spectroscopy [1-4] beams of metastable He* (ls 1, 2sl; 21S or 23S) atoms of thermal energy are directed onto clean or adsorbate covered surfaces. Upon impact the deexcitation process He* --* He occurs and the energy released in the process is sufficient to eject electrons from the occupied valence levels of the target. An analysis of the kinetic energy and the intensity of the emitted electrons provides information on the density of the occupied states of the system studied. In this respect the Penning spectroscopy is as a method equivalent to UV photoemission (UPS), leaving the system in the same final ionized state. The main advantage of the Penning spectroscopy over UPS is its absolute surface or adsorbate sensitivity. In UPS, however, the energy hv brought in by the photon beams is fixed.solely by the choice of the light source and corresponds to the true excitation energy. On the other hand, the excitation energies E * characteristic of beams of free He* atoms (20.6 and 19.8 eV for He* singlet and triplet states, respectively) are modified in the presence of surfaces and adsorbates by the He* and He 0039-6028/87/$03.50 9 Elsevier Science Pubfishers B.V. (North-Holland Physics Publishing Division)
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D. Loorid et al. / H e * deexitation energies in Penning spectroscopy
interactions with the target. This leads to an effective He* deexcitation energy c * available for surface Penning ionization which, in general, differs from the value of E * and depends on the specific properties of each particular adsorption system studied. Therefore the interpretation of the experimental data requires an estimate of the change AE* = E * - , *
(1)
of the He* deexcitation energy for the system investigated. In the present contribution we shall report on the results of the calculations of c* pertinent to Penning spectroscopy of CO adsorbed on Ni and Pd surfaces. These adsorption systems are characteristic of strong nondissociative CO chemisorption and their electronic structure has been studied extensively both experimentally, using different techniques, and theoretically. The energetics of the deexcitation of a He* atom colliding with an adsorbate is described within the Born-Oppenheimer approximation as a Franck-Condon transition between the initial and final state total potential energy curves which appear as functions of the He*(He)-target separation R (see inset in fig. 1). These curves arise from the electronic configurations of the entire system before and after the deexcitation event. The upper curve Vi* (R) describes the initial state interaction between the excited He* atom with the neutral adsorbate and the underlying surface in their ground states. The lower curve Vf+(R) represents the final state interaction of a neutral He atom in the ground state with the adsorbate covered surface in a relaxed ionized state. As regards the target, this state is equivalent to a final electronic state as would be obtained in valence photoemission. Both energy curves comprise long range attractive van der Waals (vdW) and short range predominantly repulsive overlap induced components which depend on the electronic configurations of the He atom and the adsorption complex. Under these premises the effective He* deexcitation energy which may be converted into the energy of the ejected electron is given by , * = V~*( R ) -
V~+(R)
(2)
and depends therefore on the distance at which the F r a n c k - C o n d o n transition between Vi* and Vf+ takes place. The corresponding transition rate F(R), which is the lifetime of the excited quasimolecule He* + adsorption complex, can within the Born-Oppenheimer approximation be considered as a local quantity [5] decaying very fast with the increase of R. In our calculations of Vi*(R ) and Vf§ applicable to the case of the Penning spectroscopy of CO adsorbed on Ni we have assumed oxygen-head-on collisions of He* with a single adsorbed CO molecule. The long range component of Vi* (R) comprises the He*-adsorbate vdW interaction as a sum of a gas phase-like component and two surface mediated ones which all fall off a s R - 6 [ 6 - 8 ] , and the He *-surface vdW attraction which falls off as Z -3
D. Lovri~ et aL / He * deexitation energies in Penning spectroscopy
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V
(eV) 0,2
0963 0
--
~...2T.__~__
L
J
__
.
/
-0.2
i -OA
-0.6
>Vi~-E ~.
/
/
/
-0.8
01
k_..--------,'~-- R
R=d+l.242 ctB Fig. 1. Full curves: initial state ( V i * - E * ) and final state (Vt+ ) potential energy curves for head-on collision of H e * (23S) and He atoms with CO adsorbed on Ni(001), respectively, d measures the distance between the centres of helium and oxygen atoms. Vt+ corresponds to ionization of the l~r level of adsorbed CO. Dashed curves: same potentials corresponding to He * and He interaction with CO in the gas phase. Inset: illustration of the qualitative behaviour of Vi* and Vf+ on larger scale.
where Z is the H e - s u r f a c e distance [9]. In b o t h interactions we have taken into account the positions proper of the corresponding vdW reference planes [7,9]. The long range part of Vf+(R) manifests itself as a n attraction between the g r o u n d state He a n d ionized CO ( H e - C O + ) a n d is again comprised of the gas phase-like and surface mediated v d W c o m p o n e n t s which are all proportional to R - 6 for large R, the H e - s u r f a c e v d W interaction p r o p o r t i o n a l to Z - 3 , and the attraction between the hole in one of the CO valence orbitals (4a, l~r or 5(~) a n d the n e u t r a l He atom in the g r o u n d state, which is proportional to R - 4 .
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D. Lovri6 et al. / H e * deexitation energies in Penning spectroscopy
The calculations of the strengths of all these different types of vdW potentials are based on the pseudo-oscillator strengths finn and the corresponding excitation energies r which define the dynamic polarizability a(o~) of He*, He, CO and CO +. Their values are available either from literature [10] or can be obtained from an intrapolation of the functionals involving a(~0) by using their known or calculated values to obtain those pertinent to the electronic structure and boundary conditions typical of the present problem. In the calculations of the surface mediated vdW potentials we have made use of the realistic surface response function characteristic of Ni [8,11]. The role of the chemisorption effect in the enhancement of the adsorbate polarizability due to the dynamic polarizability of the electronic charge in the 2~'* derived resonances of adsorbed CO has been treated as outlined in refs. [8,12]. In ~+ this enhancement depends also on the specific orbital (40, 1,r or 50) out of which the electron has been emitted since the hole left behind affects the subsequent relaxation of the 2,r* derived resonances in CO + in each of these cases differently. The short range component of the potentials Yi* and ~§ have been obtained from total energy calculations on linear clusters C O - H e * and CO+-He using the ab initio Hartree-Fock LCAO method with extended Gaussian basis sets [13]. Hence, the effect of the chemisorptive CO-substrate bonding on the potentials ~* and ~§ appears in the present treatment only in their long range components. A more detailed study of the chemisorption effect on the strengths of the short range potentials will be left for future studies based on extended substrate clusters. However, as the Franck-Condon transitions are likely to take place in the region of the potential well of ~+(R) where the vdW interactions are dominant (see fig. 1 and below), the present results should also demonstrate the effect of the presence of a substrate specificity in the variation of c* with respect to the strength of the adsorbate-substrate chemisorption bonding. Fig. 1 shows the calculated values of the potentials Vi*(R ) - E * and Vf+(R) with a hole in the 1,r orbital for a head-on collision of He* atoms with the CO/Ni(001) adsorption complex for which the adsorption geometry is known [14]. The potential well of Vr+(R) is rather shallow and much farther outside the surface than that of Vi*(R), in contrast to the picture implied in the literature on molecular Penning ionization [5]. Since even the turning point corresponding to He motion in ~+(R) is, for a typical initial He* kinetic energy of 63 meV, still outside the well minimum of Vi*(R), the deexcitation process He* --, He (vertical Franck-Condon transition) may take place only in the region d > 4 a B in which the vdW interactions make dominant contributions to V~*(R). Hence, the change of the effective deexcitation energy A E * (eq. (1)) is limited to the values below 0.9 eV, and in the region of the minimum of Vf§ one may estimate that AE * = 0.3 eV. The experimentally observed value of AE* = 0.5 eV for a chemically similar C O / P d ( l l l ) system
D. Lovri6 et al. / He * deexitation energies in Penning spectroscopy
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[2] indeed falls within this interval. This justifies our starting assertion on the importance of various chemically and physically induced contributions to the polarization interactions which we have singled out as the d o m i n a n t ones in o u r c a l c u l a t i o n s of the i n t e r a c t i o n p o t e n t i a l s Vi* ( R ) a n d Vf+ ( R ) . A s a g e n e r a l result it c a n b e c o n c l u d e d that A E * will for a d s o r b e d species be larger t h a n for free particles d u e to the effects of e n h a n c e d a d s o r b a t e polarizabilities discussed, w h i c h is i n a g r e e m e n t with e x p e r i m e n t a l e v i d e n c e
[21.
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