A HREELS study of the UV photon-induced chemistry of C6H5Cl adsorbed on Ag{111}

Surface Science Letters North-Holland

248 (1991) L279-L284

Surface Science Letter.9

A HREELS study of the UV photon-induced adsorbed on Ag( 1111

chemistry

of C,H,Cl

Y. Song, P. Gardner, H. Conrad, A.M. Bradshaw Fritz-Haber-Institut

der Max-Planck-Gesellschaft,

Faradayweg

4-6, 1000 Berlin 33, Germany

and J.M.

White of Chemistry, University of Texas, Austin, TX 78712, USA

Department Received

21 August

1990; accepted

for publication

5 March

1991

The UV photochemistry of both monolayer and multilayer C,HsCl adsorbed on Ag(ll1) surfaces has been studied using high resolution electron energy loss spectroscopy (HREELS). Photon-induced dissociation via the cleavage of the C-Cl bonds was observed as a common feature. For the monolayer, chemisorbed biphenyl appears to be formed on the surface after photolysis at 110 K and subsequent annealing to 300 K. The two phenyl rings are found to lie parallel to the metal surface. The photon-induced dissociation of multilayer CsHsCl leads, however, to photopolymerization as shown by the high thermal stability of the surface species formed and the detection of simple additive products in thermal desorption.

A number of recent studies has focused attention on the UV photochemistry of adsorbates on metal surfaces [l-5]. It has been demonstrated that under certain circumstances photon-induced fragmentation processes can in fact compete effectively with substrate quenching which is generally considered to be the dominant pathway for the decay of electronic excitations in molecules on surfaces [6]. At present, it is not clear whether the primary mechanism is direct absorption of the photon followed by dissociation (photolysis or photodissociation) or the capture of a “hot” electron, resulting from an excitation in the substrate, with subsequent dissociation. Apart from the fundamental interest in these mechanistic aspects, such processes also offer the possibility of producing novel molecular fragments adsorbed on metal surfaces. As part of a wider study of the surface photochemistry of substituted hydrocarbons, Zhou and White (ZW) have recently investigated the pho0039-6028/91/$03.50

0 1991 - Elsevier

Science Publishers

ton- and electron-induced chemistry of monochlorobenzene adsorbed on Ag{ ill} [7]. Based on TPD, XPS, UPS and work function measurements they concluded that photofragmentation via the cleavage of the C-Cl bond is the primary photochemical process and suggested that a surface phenyl species may be the direct photoproduct. This was indicated by the observation of biphenyl and AgCl as the only desorption products in TPD following photon-induced fragmentation. Clearly, more direct evidence for the presence of such an interesting surface species is required. Vibrational spectroscopy is ideally suited for this purpose [8] and there are many examples of its application to the study of adsorbed molecular fragments, indeed, also of species resulting from surface photochemical processes [9-111. In this Letter we report a HREELS study of the UV photochemistry of Ag{lll}-C,H,Cl. In order to examine the role of the substrate surface, both monolayer and multilayer systems have been investigated. In fact, it

B.V. (North-Holland)

Y. Song et al. / UVphotochemistry

appears that surface phenyl groups are not formed, but rather chemisorbed biphenyl molecules. The vacuum chamber used for this work is equipped with an EELS spectrometer [12], LEED and a quadrupole mass spectrometer suitable for TPD experiments. The Ag{lll} crystal was cleaned by repeated Ar+ sputtering and annealing at 800 K. In the EELS studies the primary beam energy was about 1 eV; the spectrometer was fine tuned for each measurement in order to achieve optimum resolution and signal-to-noise. A resolution of - 10 meV in the elastic peak was typical for most experiments. An Oriel 150 W Xe arc lamp was used as UV photon source. The light was focused only by the optics provided on the lamp; no additional lenses or filters were used. Irradiation was achieved via a MgF, window mounted on a convenient flange. A Ni-Cr-Ni thermocouple inserted into a pinhole at the back of the crystal indicated no noticeable rise in temperature during irradiation of the front surface. The adsorption of C,H,Cl on Ag{lll} has been characterized by ZW [7]. Two states were distinguished, corresponding to a chemisorbed monolayer desorbing at 230-240 K and multi-

of C,H,CI

adsorbed on Ag{lll}

surfaces

layers which are observable only below 170 K. These results have been confirmed in our own TPD experiments. Fig. 1 shows the vibrational spectra of a C,H,Cl monolayer on Ag{lll} before and after UV irradiation at 110 K as well as after subsequent flashes to higher temperatures. Only three losses are observed in the (specular) spectrum of the chemisorbed monolayer (fig. lb). On the basis of IR and Raman spectra of C,H,Cl in the liquid phase [13], the losses at 470, 740 and 3050 cm-’ are assigned to the C-Cl stretching mode, the out-of-plane C-H deformation mode (y(C-H)) and the C-H stretching mode, respectively, of molecular C,H,Cl. Off-specular experiments have shown that the C-Cl stretch and y(CH) modes are dipole-active while the excitation of the v(C-H) mode occurs largely via impact scattering. Given the absence of many in-plane vibrational modes, such as the in-plane C-H bend (6(C-H)) and the C-C stretch, and the high intensity of y(C-H) relative to v(C-H) we conclude from the surface selection rule that the chemisorbed C,H,Cl molecules are oriented with their aromatic ring planes parallel to the Ag{ 111} surface. The same adsorption geometry was sug-

x 167

x 17 -

(f)

3040 M

Cd)

ENERGY [cm~‘l Fig. 1. The UV photon-induced chemistry of monolayer C,H,Cl: HREEL spectra of (a) clean Ag(ll1); (b) after exposing to 1 L of C,H,Cl: at 200 K; (c) after 100 min full arc UV irradiation at 110 K; (d) after annealing to 300 K; (e) 400 K and (f) 773 K. Spectra (b) to (d) were taken at a crystal temperature of 110 K and spectra (a), (e) and (f) at 300 K.

Y. Song et al. / Wphotochemistry

gested by ZW [7] based on an estimation of the surface molecular density. After irradiating the C6H,Cl monolayer with full arc UV for 100 mm at 110 K, the most distinctive new feature observed in the spectrum (fig. lc) is a loss peak at 220 cm-‘. As the crystal is flashed to higher temperatures this loss peak becomes better resolved until at 773 K it disappears, corresponding to the desorption of AgCl in TPD [7,14]. We therefore assign the 220 cm-’ band to the Ag-Cl stretching mode of the surface Cl atoms produced by the photon-induced fragmentation of C,H,Cl molecules. This assignment is consistent with the high thermal stability of the surface species and with the observed low frequency of its vibrational mode. We note that apart from the 220 cm-’ band there is little difference between spectra (c) and (b). Only a broadening of the 470 cm-’ feature to higher frequency and some extremely weak bands at 995 and 475 as additional features in (c). cm-’ are apparent Spectrum (d) was obtained after annealing to 300 K, i.e. after desorption of the parent molecule. Although it looks strikingly similar to spectrum (b) of chemisorbed C,H,Cl itself, the following experiment showed conclusively that we are dealing here with a new species resulting from a photon-induced reaction: The same procedure as above was repeated but the back of the Ag{lll} crystal was exposed to the UV light. No radiation was allowed to fall on the front surface. After annealing to 300 K, no loss features were found in the EEL spectrum. Spectra (e) and (f) show the result of annealing to 400 and 773 K, respectively. Only the Ag-Cl mode is present in (e), as expected after the desorption of the biphenyl; (f) shows the clean surface following the desorption of AgCl [7,14]. In keeping with the well established photochemistry of halobenzenes in the gas [15-181 and liquid [18,19] phases, the above results can indeed be interpreted in terms of photodissociation of C,H,Cl via the cleavage of the C-Cl bond as suggested originally by ZW [7], although a “hot electron” mechanism could also be responsible. The question then arises as to whether the species characterized by the vibrational spectrum of fig. Id is that of a surface phenyl group. There is no

of C,H,CI

ahorbed

on Ag{I I I} surfaces

doubt that the biphenyl desorbing at 390-400 K is formed from the recombination of phenyl groups, otherwise more complicated product distributions might be expected, as in the case of the multi-layers reported below. However, it is still not clear whether the recombination takes place almost simultaneously with the photon-induced primary process or whether the surface phenyl species recombine only at the desorption temperature. What do the surface vibrational spectra tell us in this respect? We expect the losses at 740 and 3040 cm-’ in both figs. Id and lb to have the same origin, i.e., the y(C-H) and v(C-H) modes of an aromatic ring. Since the intensity ratio of the two modes is the same in both spectra the surface selection rule suggests that the aromatic rings of the photoproducts are also oriented in a similar way, i.e., lying down on the surface. This is also consistent with the absence in spectrum Id of many in-plane vibrational modes, such as 6(C-H) or v(C-C). The loss peak at 455 cm-’ occurs at a frequency very similar to the C-Cl mode of C,H,Cl in (b), although their relative intensities (for example, compared to the y(C-H) mode at 740 cm-‘) are different. On the basis of the medium to strong absorption band observed for biphenyl at 461 cm-’ in the solid [22] and at 487 cm-’ in the liquid [23] it would be reasonable to assign the 455 cm-’ loss feature to the out-of-plane ring deformation mode of adsorbed biphenyl. For a flat-lying species this mode is totally symmetric and thus dipole active, which indeed has been confirmed in off-specular experiments. Although it is not known exactly how a surface phenyl species, if it were to exist, would be bonded to a metal surface, one would certainly expect a metal-carbon u bond. Thus while some degree of tilt of the ring from an upright position might conceivably be possible, a nearly flat configuration would be highly unlikely. Since biphenyl is also the only desorption product at 390-400 K the balance of evidence therefore suggests that spectrum (d) in fig. 1 corresponds not to a surface phenyl group but rather to that of biphenyl itself chemisorbed on the Ag{lll} surface. The similarity of spectra (c) and (d) strongly suggests that the biphenyl is formed almost concomitantly with the photon-induced

Y. Song et al. / UVphotochemistty

primary process. We are, however, unable at this stage to offer a plausible mechanism for its formation. In the free molecule the two ring planes of biphenyl are twisted for steric reasons, but on a surface one might expect bonding via the r orbitals to dictate the conformation. In fact, an adsorption geometry with both rings parallel to the surface is consistent with the EEL spectrum. Ramsey et al. [20] have shown with angular-resolved photoemission that biphenyl directly adsorbed on Pd{llO} adopts the same geometry with both aromatic ring planes parallel to the surface. More recently, Zhou et al. [21] have shown that the desorption kinetics of biphenyl adsorbed directly on Ag{ 111} and of biphenyl produced by photon-induced fragmentation of chlorobenzene are different. In fact, the TPD for the latter case closely resembles that of biphenyl produced by the electron-induced decomposition of benzene where the evidence for surface phenyl formation is quite strong. This would seem to contradict the evidence from the vibrational spectra discussed above. In order to throw more light on the role of the metal substrate in this photon-induced surface

of C,H,-Cl a&orbed on Ag{l II) surfaces

reaction we have carried out a similar experiment with multilayers of C,H,Cl on Ag{lll}. Fig. 2a shows the spectrum of several layers of chlorobenzene which contains many features absent in the corresponding monolayer spectrum of fig. lb. By referring to literature data [13] all loss peaks can be assigned to the different vibrational modes and their combinations as indicated in fig. 2. The observation of vibrational modes polarized both in and out of the ring plane suggests a random molecular orientation in the condensed phase. We also note that the intensity ratio of the y(C-H) to v(C-H) modes is at least an order of magnitude lower in fig. 2a than that in fig. lb, justifying the use of this factor as an indicator of the molecular orientation. After 180 min of full arc UV irradiation, spectrum (b) is still dominated by the loss features attributable to condensed C,H,Cl except for a new peak at 205 cm-‘. A similar loss in fig. 1 was assigned to the v(Ag-Cl) mode associated with adsorbed Cl atoms and is believed to have the same origin here. The spectra (c) to (e) show the result of annealing to temperatures of 300,470 and 870 K i.e. above the desorption temperature for unreacted C,H,Cl. A comparison with the

x 125

III\-” 455

: 8 6

I

X5

205

Cc)

710

3065

J-‘----

(b)

Fig. 2. The UV photon-induced chemistry of multilayer C,H,Cl: HREEL spectra of (a) a Ag(lll) surface exposed to 10 L of C,H,CI at 110 K; (b) after 180 min of full arc UV irradiation at 110 K; (c) annealed at 300 K; (d) 570 K and (e) 870 K. Spectra (a) to (d) were taken at a crystal temperature of 110 K and (e) at 300 K.

Y. Song et al. / UVphotochemistry of C, H,CI a&orbed on Ag{ll I) surfaces

corresponding spectra in fig. 1 reveal the following essential differences: the EELS features are considerably broader in fig. 2c than in fig. Id and the relative intensities are also very different. In particular, the general features of the EELS spectrum are not changed in fig. 2 by heating up to 570 K while the only loss peak observable in fig. le after annealing at 400 K was the v(Ag-Cl) stretching mode. Clearly, different products are formed after irradiation of condensed C,H,Cl compared to the photon-induced processes in the monolayer. It is known that chlorobenzene in the liquid phase polymerizes when irradiated with UV light [24,25]. The high molecular weight materials formed are too complex to be analysed completely, but a free radical mechanism is generally accepted [18]. We therefore propose that photoninduced polymerization processes account for the spectra of C,H,Cl on Ag{lll} in fig. 2. In the liquid phase, chlorobiphenyl (C,,H,Cl) has been observed as one of the major primary additive products [19,25]. Such a product has indeed been observed in our TPD experiments: a broad, prominent desorption peak for mass 188 occurs at about 250 K. The results from the multilayers thus suggest that it is the presence of the surface which effectively prevents polymerization in the monolayer and may give some clue as to the mechanism of biphenyl formation. A possible scenario could be as follows. In the case of the multilayers the phenyl radical resulting from C-Cl fission reacts with a neighbouring chlorobenzene molecule to give chlorobiphenyl. The Cl atom can then diffuse to the metal surface or react with other species. Further photon-induced fragmentation processes of this nature give rise to various, different species of higher molecular weight (“photopolymerization”). In the case of the monolayer, however, the reaction of a phenyl radical with a chemisorbed chlorobenzene molecule gives rise to a concerted process in which a further Cl atom is eliminated and a chemisorbed biphenyl molecule is formed. The overall reaction is strongly exothermic because of the high Ag-Cl bond energy. In summary, we have described in this Letter a HREELS study of the UV photon-induced chemistry in both monolayer and multilayers of C,H,Cl adsorbed on Ag{lll} surfaces. The EELS data

have given information on the chemical identity of the surface photoproducts and their molecular orientation. Together with previous TPD results [7], which have been confirmed in the present study, the following conclusions may be drawn: (1) The full arc UV irradiation of the Ag{lll}-C,H,Cl system at 110 K leads to the photon-induced fragmentation of C,H,Cl molecules through the cleavage of the C-Cl bonds. The formation of chemisorbed Cl atoms is shown by the observation of the Y(Ag-Cl) mode at - 220 cm-‘. (2) For the C,H,Cl monolayer, chemisorbed biphenyl and surface Cl atoms have been identified as the only products after irradiation at 110 K and subsequent annealing to 300 K. The biphenyl molecules are bonded with their ring planes parallel to the surface, indicating a preferential m-bonding interaction with the surface. No direct evidence has been obtained for a stable surface phenyl species in the temperature range of study. (3) In the multilayer system, photon-induced polymerization is the dominant process, resulting in the formation of high molecular weight materials which are extremely stable on the surface. This work has been supported by the Deutsche Forschungsgemeinschaft through the Sonderforschungsbereich 6-81.

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of C,H,Cl

adsorbed on Ag(Ill)

surfaces

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