The role of surface defects in laser-induced thermal desorption from metal surfaces

Laser-induced thermal desorption of Na dimers from rough sodium surfaces adsorbed on quartz substrates has been studied. For this purpose, laser pulses with 2=532 and 1064 nm and 7 ns duration were used. The kinetic energy distribution and the integral desorption signal of the dimers were determined as a function of the laser fluence. Measurements were also performed after reducing the surface roughness by annealing at different temperatures. The fluence dependence of the integral desorption rate exhibits a plateau which follows and precedes a sharp increase. The results indicate that the Na dimers come off preferentially from special sites of low binding energy and low coordination number. Two of these sites can be distinguished. They exhibit different annealing behavior, different binding energies and can be depleted selectively by choosing the laser fluence appropriately. ~c'~ 1997 Elsevier Science B.V. Ko'words: Desorption induced by electronic transitions; Photon stimulated desorption; Surface structure, morphology, roughness, and topography; Thermal desorption A detailed understanding of laser-induced desorption and ablation is not only of scientific interest, but opens up the possibility of exploiting such reactions for a large number of applications. Besides the desorption of molecular species from many different surfaces, the removal of substrate atoms from semiconductors [1,2], metals [3-9] and insulators [10] by laser radiation has been studied. In order to investigate the underlying desorption mechanisms, measurements of the desorption rate and the kinetic energy distributions of the ejected atoms as a function of laser wave* Corresponding author. Fax: +49 561 8044518; e-mail: [email protected] 0039-6028/97/$17.00 CO 1997 Elsevier Science B.V. All rights reserved. PII S0039-6028(97)00241-0 length, angle of incidence, laser fluence and substrate temperature have been performed. It turns out that metal atoms like Au, Ag, A1. K or Na can even desorb in a non-thermal reaction [39], thermal evaporation only coming into effect for high fluences [9,11]. In addition to atoms, dimers can be removed from metal or semiconductor surfaces under irradiation with short laser pulses [11-14]. For example, Viereck et al. [12] have found that Na dimers can desorb from rough Na surfaces and that the mechanism is non-thermal. Under appropriately chosen conditions, the fraction of detached Na dimers can even surmount the fraction of desorbed Na atoms [ 11 ]. The studies carried out so far have given strong evidence that non-thermal desorption is based on J. Viereck et al. / Surface Science 383 (1997) L749 L754 )nic excitations at the surface, and can oe explained in the framework of the well-known scenario of Menzel, Gomer and Redhead [ 15, 16 ]. Atoms and dimers desorb predominately from binding sites of the metal surface which have low coordination numbers. These sites can be regarded as defects, and exhibit specific electronic properties which differ from those of extended, low-index single-crystal surfaces (see, e.g., Refs. [ l, 2, 9,12]). As a consequence, the appropriate choice of the applied laser wavelength makes possible the selective depletion of certain sites by laser-induced nonthermal desorption [9]. Other experiments have shown that defects also play an essential role in thermal desorption [1,17,18]. For example, Dickinson et al. [17] studied the role of defects on the emission of neutral atoms and molecules from NaNOa. They intentionally increased the number of these sites of low coordination number by electron bombardment, and found that new absorption centers were produced due to defect creation. As a consequence, a larger temperature rise is obtained for a given laser fluence. Thus, thermally activated rate-limiting steps are "catalytically" enhanced by producing defects. Similar observations were made for the laser-induced desorption of MgO [18]. Another example where surface defects play an essential role in promoting thermal desorption is the removal of Fe atoms from iron surfaces [19]. In a continuation of our earlier work, the present paper reports experiments with the objective of further clarifying the role of surface defects in laser-induced thermal desorption. For this purpose a rather peculiar process at first glance (i.e. the desorption of Na dimers) constitutes a particularly interesting case, since the rate of desorbed Na2 can be about four times as large as the rate of atoms removed simultaneously [9, 12]. In contrast, a fraction of only 10% of the desorbed material consists of dimers in a classical thermal desorption experiment (TPD). Obviously, defects play different roles depending on the heating rate, which can be as high as 10 9 K s1 under laser irradiation, but typically amounts to as little as several K s-1 in TPD. In the experiments described here, rough Na surfaces have been prepared by the deposition of Na atoms on a dielectric substrate held at cryogenic temperature. The deposited atoms form small particles, the surfaces of which offers a considerable and reproducible number of defects. This not only guarantees large desorption signals [3], but makes it possible to examine whether thermal desorption with pulsed laser light can be controlled such that different kinds of binding sites with low coordination number can be depleted selectively. For the unambiguous interpretation of the measurements described below, deliberate variation of the surface roughness by heat treatment turns out to be essential. The experimental arrangement has been described in detail elsewhere [9], and basically consists of an ultrahigh vacuum system with the sample, two lasers for stimulating desorption and photoionizing the desorption products, and a timeof-flight mass spectrometer. A thermal beam of Na atoms with well-defined constant flux is directed onto the quartz substrate in order to deposit a predetermined coverage of atoms. After preparation at a temperature of 80 K, the sample is irradiated with p-polarized light from a Nd:YAG laser at 2=532 or 1064 nm in order to stimulate desorption. The angle of incidence is 50 ° and the pulse duration 7 ns at a repetition rate of 9 Hz. At a distance of 21 mm in front of the substrate, the Na dimers detached from the metal surface are ionized with the light of an excimer laser operating at 2=248 nm. The corresponding photon energy of 5.0 eV lies only a little above the ionization threshold of Na 2 [20]. Therefore, "soft" ionization of the desorbed dimers is accomplished and there is only very little dissociation into monomers [ 12]. On the other hand, the photon energy used is too row for single-photon ionization of detached Na atoms. The Na~ ions generated pass the time-offlight mass spectrometer and are detected with a secondary electron multiplier. The ion signal is processed by a boxcar integrator and stored on a computer. By varying the delay time between the two laser pulses used for desorption and ionization, the time-of-flight distributions of the desorbed Na dimers are determined and subsequently converted into kinetic energy distributions [21]. The choice of Na as the material for the measurements described here is motivated by two arguments. Firstly, the dimers can be ionized with a single J. Viereck et al. / Surface Science 383 (1997) L749 -L754 laser photon, and secondly, the Na coverage can be evaporated entirely from the substrate at a temperature of only ~ 4 0 0 K after completion of a measurement. This makes it possible to use the same substrate repeatedly and ensures reproducible preparation of the sample as well as reproducible desorption signals. As an example, Fig. 1 displays a time-of-flight spectrum of Na dimers desorbed with p-polarized laser light of ) ,=532 nm. The Na coverage was 1.1 x 1 0 ~ a t o m s c m 2 , which corresponds to an average particle size of R,~=21 nm [22], The fluence of the Nd:YAG laser was set to a value of ~b=7.5 mJ cm 2. If the fluence is increased, the maximum of the time-of-flight spectrum is shifted