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Orientation of benzene molecules adsorbed on graphite as studied by penning ionization electron spectroscopy
Deparfntenr of Chemistry, Faculty of Science, 7% Uniuersrtyof Tokyo, Hongo,
Bunkjo-kw
Tokyo 113. Japan
-i
and Koichi OHNO and Yoshiya HARADA College of Genera/ Educarroti, The Unrversity of Tokyo. Meguro-ku,
To&o
I.53, Japan
Received 29 February 1984
Kmetic energy distnbuttons of the electrons ejected by Impact of He(2%) atoms from benzene adsorbed on graphtte (0001) plane at 100-140 K were measured. The intensities of the benzene o bands relative to its ?r bands increased gradually with benzene exposure_ The spectra indicate that at one-monolayer coverage the benzene molecules are nearly pamlIe to the substrate plane but the molecules on the top surface of multiple layers have large tilt angles wrth respect to the substrate plane.
1. Introduction The recent development of Penning ionization electron spectroscopy (PIES) has provided a new technique for investigating the physical and chemical properties of surfaces [l-12]. PIES gives information, complementary to other analytical methods such as ELS, ARUPS and IR, on the orientation of-molecules adsorbed on a surface. A change in the PIES intensity can be related to that in the electron density-of each individual orbital of the surface molecules. This method has been applied to studies of phase &u&ions of organic molecules [S] and those of CO adsorbed on transition~metal surfaces . [8,10]. ARUPS _ provides similar but slightly different information, because UPS -can hardly probe mohzcules~on a surface layer selectively; in -this case,. electrons knitted from _the layer are overlapped with those-emitted from in-side layers-and -often -from-the substrate as well, since W radiation can pen.trate into the_solid..In contrast, PIEScati; in-principle, probe an outer^ ,
most monolayer with high sensiti-vity, because Penning ionization is caused by collision of a molecule with an excited atom on rfzesurface. The purpose of the present paper is to study the orientations of benzene molecules adsorbed on a graphite surface by PIES. Changes in the PIES intensities with temperature and coverage were measured and analyzed. The graphite-benzene system was chosen for the-following reasons: ~ (1) This system has been studied extensively by various methods such as NMR [13,14], thermal r isotherm [15], and neutron diffraction [16,17],- but the _conclusions are -controversial as to whether benzene-molecules adsorbed on -the graphite_ basal plane (0001) at monolayer coverage are_paraUel_ [14,15,17] or perpend&ular[13,16]. It is not clear _ whether the conflicting results originate from dif_ ferences m the experimental methods and analyses _ from which the concmsions were derived or-from - those in the conditions-of adsorptions.= S(2) Benzene has orb&a& that have widely differ_ ent ~sp&al dkributions~of electrons. These aand
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0301~0104~84/$03.00 0 Elsevier’ Science Publishers B.V. (North_Holland Physics Publishing Division) .I = -_-
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H. Kuboro et al. / Orlenrotion of benzene molecules adsorbed on grophrre
T orbitals can be well resolved and assigned by PIES [18]. Therefore, benzene is a suitable molecule to be probed on a surface monolayer by measurement of PIES peak intensities_ (3) If the adsorbed benzene molecules are parallel to the graphite surface at one-monolayer coverage, as predicted by neutron diffraction [17] and
estimated to have an average thickness of several layers (see section 3.3). The PIES and UPS were measured on these specimens by He*(2 3S, 2 ‘S) metastable atoms and He1 (58.4 run) photons with normal incidence to the foil plane and under the same experimental conditions (figs. 1 and 2). A thermal He* beam
by theory [19], then PIES IS expected to change stgnificaatly with increasing coverage. because upon further deposition of benzene molecules the relative orientations of molecules are expected to approach their crystalline arrangement. where the molecules are nearly perpendicular to one another [20]_ Such a change in orientation should readily be detected by PIES. (4) The use of graphite as a substrate has the advantage that its characteristic UPS peak at 3.3 eV assigned to its conduction band can be used to estimate the film thickness by measurement of the decrease in intensity of this strong peak (see section 3.3).
was prepared by impact of low-energy electrons (= 45 ev), and the He1 resonance line by a dc discharge lamp. The contribution of the He 2 ‘S atoms in the He* beam to PIES was estimated to be less than 10% from the relative peak heights of the first valence bands (marked 1 in fig. lb) due to the 2 ‘S and 2 ‘S atoms. Therefore, *Jle PIES peaks measured under the present experimental conditions were essentially due to the 2’S atoms. The energy distribution curves of the emitted electrons were measured with a spherical electron collector coated with colloidal graphite by an ac modulated retarding-field technique [2.22] with an energy resolution of = 0.3 eV. The zero point of the electron kinetic energy was determined by a sharp cutoff of the distribution peak. Since both PIES and UPS were reproduced in repeated measurements, they were regarded as essentially free from any surface contamination. The spectra showed no significant temperature or time dependence in the temperature range of loo-130 K. However, when the surface temperature was maintained at 140 K the PIES peaks l-5 shown in fig. 1 changed with time; they appeared prominentiy just after exposure to benzene vapor, but they disappeared in a few tens of minutes. This observation mdicated that gradual desorption of benzene molecules from the graphite surface took place at this temperature_
2. Eqerimentai
The measurements were made in a vacuum chamber with a base pressure of 2 X lo-’ Torr [3,6]. Grafoil produced by Umon Carbide. Inc. was used as an adsorbent. A neutron-diffraction study [21] showed that it was made of graphite crystallites with then basal planes preferentrally oriented parallel to the macroscopic foil plane. A Grafoil disk of 15 mm diameter was mounted on a copper block and was cleaned by heating it to 500 o C for 20 h under vacuum. The Grafoil surface. cooled to 140 K or lower, was exposed to benzene vapor by effusing the vapor into the chamber through a variable leak valve. The pressure of the chamber, as measured by an ionization gauge cahbrated with dry air, was = 5 x IO-’ Tot-r; it was held constant during the exposure, and the average thickness of the adsorbed layer was controlled by changing the exposure time. Four specimens were prepared and tested: (a) unexposed graphite, (b) exposed to benzene for 10 s, (c) for 100 s, (d) for 400 s. No quantitative measurement of the thickness was made, but from their He1 photoelectron spectra (UPS) specimen (b) was
3. Results
3.1. Spectra
and discussion for graphite
surface
The observed UPS for a clean graphite surface agrees well with those reported previously; the sharp peak at 3.3 eV is related to the high density of states of the upper conduction bands [23,24]_ The PIES has similar features; it extends beyond 15 eV, and a sharp but less intense peak appears at 3.3 eV. The PIES for graphite wilI be discussed in detail in a separate publication [25].
H_ Kubota et aL / Orientatton of k-h-ene mo&&.s
16
401
a&orbed o_ngraphite
12 Ekctron
4 8 EnergyleV
0
Fig. 1. Penning iontzation electron spectra (in arbitrary units) of benzene adsorbed on graphite by He* (2%). (a) Unexposed graphite, (b) exposed to benzene vapor ( = 5 X lo-’ Tot-r) for 10 s. (c) 100 s. and (d) 400 s. The absctssa represents ekctron kinetic energy with respect to the vacuum level of the system. The ordmate scale of (a) is 1.25 times those of (b)-(d)_
Frg. 2. Photoelectron spectra (in arbitrary units) of benzene adsorbed on graphite by HeI (58 4 nm). Samples (a)-(d) were the same as those used for measurement of the PIES shown tn fig. 1.
3.X
PIES measured in the gas phase [18]. (4) The intensities of these valence peaks change gradually from (b) to (d). This can be ascribed to changes in orientation of benzene, as discussed in section 3.3. (5) A strong peak appears at = 1.4 eV, and it is enhanced gradually from (b) to (d). This peak is assigned to scattered electrons [2,5]. The corresponding changes in the UPS (fig. 2) are the following: (6) The emission yields from specimens (a) to (d) are about 1.7, 3.4, and 4.4 times as high as that from clean graphite_ The origins of this increase in the yield wiII be discussed elsewhere 1251.(7) The UPS for the graphite substrate and the adsorbed benzene are superimposed in spectrum (b), but the graphite peak at 3.3 eV is barely visible in (c) and (d). This observation shows that the adsorbed benzene multilayers in specimens (c) and (d) are so thick that-essentially no UPS of the graphite substrate can be observed.
Spectral changes upon benzene adsorption
Only slight adsorption of benzene on the graphite surface causes drastic changes in the PIES (see fig. 1). (1) The yields of electron emission from specimens (b), (c), and (d) are about 2.1, 2.4, and 2.8 times as high as that for clean graphite (a)_ This implies that the mechanisms of electron emission from graphite and benzene surfaces are different [25]. (2) The characteristic features of the graphite spectrum shown in fig. la, i.e. the peak at 3.3 eV and the structure in the 13-16 eV region, disappear; even in the stage of specimen (b) there seem to be no substantial electronic interactions between the graphite substrate and the He* atom. (3) Instead, the peaks numbered l-5 appear in the 4-12 eV region, as shown in figs. lb-ld. They are assigned to the valence structures of the benzene molecule, because they correspond to those in the
40’
H. Kubota er aI_ / Orientation of benene molecules aa!ro&ed on graphite
(S) The relative intensities of the benzene valence structures 1-5 for specimens (b), (c) and (d) are nearly identical_ (9) The 1.4 eV peak is enhanced from (b) to (d) much more strongly than the correspondmg PIES peak. These observed differences between the UPS and PIES originate from the difference in the average probing depth [4.5]. _?.;. Film condltlons The film thickness can roughIy be estimated by compnrtson of the features of PIES with those of UPS. The contribution from graphite to the intensity of the 3.3 eV peak is estimated to be about one half of that of adsorbed benzene, in view of the reported escape depths of no more than 10 A for photoelectrons from similar molecular crystals [%I_ Accordingly. the thickness of film (b) is likely to be no more than a few monolayer coverage, because the sharp UPS peak of graphite at 3.3 eV due to the conduction band, shown in fig. 2a, is also observable in the LIPS of film (b). in constderation of the spectral changes in PIES upon benzene adsorption stated in section 3.2. film (b) is estunated to be covered with at least one or more layers of benzene. so that metastable atoms can hardly Interact with the substrate. The present results of UPS and PIES indicate that a film of 10~ coverage forms a monolayer instead of islands. This conclusion is consistent with the result of a neutron-diffraction experiment [17] made under similar sample conditions_ The thicknesses of films (c) and (d), which were prepared wtth sufftcient times of deposition. are at least several tens of layers of benzene. because the electrons emitted from the graphite substrate could not be observed even in UPS_
.?_d_Relative peak ultensities and moiecrch or~enkmons The PIES and UPS peaks with electron kinetic energies of 14 eV can be assigned to the valence structures of adsorbed benzene, as shown in figs. 1 and 2. The relative peak positions of these spectra correspond to those measured in the gas phase. On the other hand, the dependence of the relative peak intensrties on exposure time IS qurte dtfferent.
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The relative intensities of the UPS peaks are essentially identical at different exposure times, whereas those of the u peaks numbered 3-5 in the PIES increase gradually with exposure time in comparison with those of the r peaks 1 and 2. This spectral change can be explained by changes in the orientation of the molecules on the surface of the adsorbed layer with increasing thickness of the layer. The reIative intensities of the PIES peaks reflect electron densities of the relevant molecular orbitals in the range of interparticle distance where Penning ionization mainly takes place [HI. The n orbitals extend widely to both stdes of the molecular plane. whereas u orbitals are confined near the molecular plane. If the metastable atom approaches a benzene molecule perpendicularly to the molecular plane. the T bands are expected to be enhanced in the PIES more strongly than the (I bands. On the other hand. if the metastable atom approaches the molecule from the direction parallel to the plane. the CJbands may also appear in the PIES with appreciable intensities_ Therefore, we conclude that benzene molecules adsorbed on the graphite surface are essentially parallel to the substrate plane at one-monolayer coverage, but that they are tilted on the surface of a multiple layer. the degree of average tilting being dependent on the layer thickness. An example is thereby given for an application of PIES to selective and sensitive probing of the outermost layers of solid surfaces [27]_ The present results seem to be directly comparable with those obtained by Meehan et al. [17] by neutron diffraction; as remarked by these authors, other previous studies [13-161 were made under such widely different experimental conditions that no meaningful comparison seems to be feasible in regard to the molecular orientations on the surface. The conclusion reached in the present study that benzene moIecules at one-monolayer coverage lie parallel to the graphite surface is consistent with that of Meehan et al. derived from their experiments made at 100 K. On the other hand, the two conclusions seem to deviate from each other as to the molecular arrangements beyond one-monolayer coverage: Meehan et al_ observed that at a coverage up to 1.2 monolayers any excess benzene molecules above one monolayer
-.
nucleate to form crystallites, whereas our present study has provided indication that the average tilting angle of benzene molecules on the outermost surface of a multiple layer increases gradually with the thickness of the layer, so that crystallites seem to be formed at much higher coverage. This difference probably originates from a difference in sample preparation: In the present study benzene molecules were adsorbed on the graphite surface at 100-130 K, whereas Meehan et al. introduced a known volume of benzene vapor onto the adsorbent at room temperature, and then the sample was cooled and annealed slowly. The relative intensities of the PIES bands also differ among the u bands. The ezs and b,, bands appear only very weakly in the PIES, whereas the e,,, b,, and ala bands have appreciable intensities. These differences can be interpreted in terms of the bonding characters of these orbitals. Since the e,,* b iU, and a,s orbitals are strongly C-H bonding, the 1s atomic orbital of hydrogen is expected to contribute significantly to these orbitals. On the other hand, the q, orbital is weakly C-H bonding and b,, strongly C-C bonding. Therefore, the C-H bonding orbitals are expected to interact with the incident metastable atom more strongly than the C-C bonding orbitals, which are localized in the C-C bond regicns and shielded from the C-H boy,‘ing orbitals. The different localizabilities of the c molecular orbitals on the relative ionization prob‘lbilities were also observed in the PIES of gas-phase molecules [28]. The present results demonstrate that PIES can probe a change ,n the molecular orientations on a solid surface, f.om which information on the spatial distribution of electrons in each molecular orbital can bz obtained_
I21T. Munakata.
T. Htrooka and K. Ku&i&u, J. Electron Speetry_ 13 (1978) 219. [3! T. Munakata, T. Hirooka and K. Kuchitsu, J. Electron spectty. 1s (1980) 51. 141 T. Munakata, K. Ohno and Y. Harada. J. Chem. Phys. 72 (1980) 2880. I51 H Kubota, T. Munakata, T. Hirooka. K. Kuchitsu and Y. Harada, Chem. Phys. Letters 74 (1980) 409. 161 H. Kubota. T. Hirooka, T. Fukuyama. T. Kondow, K Kuchitsu and A.J. Yencha, J. Electron spectry. 23 (1981) 417.
I71 A.J. Yencha.
H. Kubota. T. Fukuyama, T. Kondow and K. Kuchitsu. J. Uectron Spectty. 23 (1981) 431. ISI1 H. Conrad, G. Ertl. J. Ktippers, W Sesselmann and H. Haberland. Surface Set. 121 (1982) 161.
[91 W.
Sesselmann, H. Conrad. G. Ertl. J. Ktippers. Et Woratchek and H. Haberland. Phys. Rev. Letters 50 (1983) 4256.
WI
F. Bozso.
P11 C. Bomau.
References [I] T_ Shtbata. T_ Hirooka Letters 30 (1975)
241.
and K.
Kuchitsu.
Chem.
Phys.
Yates
Jr..
J. Arias,
H. Metiu and
R M.
C Garot.
R Nuvolone
and J. Roussel, Surface
Sci. 91 (1980) 313. WI J Roussel. C. Boiziau. R. Nuvolone and C. Reynaud, Surface Sci 110 (1981) L634. r131 B. Boddenberg and J.A_ Moreno. J. Phys. (Parts) 38 (1977) C4-52.
r141 I. Tabony.
J.W. White, J.C. Selachaume and M Coulon, Surface Sci. 95 (1980) L282. I151 Y. Kbartr. M. Coulon and L. Bonnetain, J. Chim. Phys 75 (1978) 789. [16) R. Stockmeyer and H. Storm&, Surface Sci. 81 (1979) L315.
1171 P. Meehan. T. Rayment, J.W. White, 2011.
J. Chem.
R K. Thomas, G. Bomchil and Sot. Faraday Trans. I 76 (1980)
PSI T. Munakata.
K. Kuchnsu and Y. Harada. Chem Phys Letters 64 (1979) 409. P91 L. Battezzati. C Pisani and F. Rtcca, 1. Chem. Sot. Faraday Trans. II 71 (1974) 1629. PO1 E.G. Cox. D.W J. Cnuchshank and J.A. Smith, Proc. Roy Sot. A247 (1958) 1. WI J.K. KJem& L. Passell. H. Taub. J.G Dash and A.D. Novaco, Phys. Rev. B13 (1976) 1446. H. WI T. Htrooka, K. Tanaka. K. Kuchitsu. M. Fqtbiia. Inokucht and Y. Harada. Chem. Phys. Letters 18 (1973) 390. 1231 R-F. Wtllis, B. Feuerbacher and B Fitton. Phys. Rev. B4 (1971)
The present study is supported by a Grant-inAid for Scientific Research from the Ministry of Education, Science and Culture of Japan.
J-T.
Martin, J_ Chem. Phys. 78 (1983) 4256.
2441.
1241 B. Feuerbacher
and B. Fitton,
Phys. Rev. Letters 26 (1971)
840.
1251 H. Kubota.
T. Hirooka and K. Kuchitsu. unpublished data. 1261 V-V. Grechov. Chem. Phys Letters 96 (1983) 237. 1271 K. Ohno. H. Mutoh and Y. Harada, Surface 8ci. 115 (1982) L12S_ [28) K. Ohno. H. Mutoh and Y. Harada, J. Am. Chem Sot. 105 (1983)
4555.
Bunkjo-kw
Tokyo 113. Japan
-i
and Koichi OHNO and Yoshiya HARADA College of Genera/ Educarroti, The Unrversity of Tokyo. Meguro-ku,
To&o
I.53, Japan
Received 29 February 1984
Kmetic energy distnbuttons of the electrons ejected by Impact of He(2%) atoms from benzene adsorbed on graphtte (0001) plane at 100-140 K were measured. The intensities of the benzene o bands relative to its ?r bands increased gradually with benzene exposure_ The spectra indicate that at one-monolayer coverage the benzene molecules are nearly pamlIe to the substrate plane but the molecules on the top surface of multiple layers have large tilt angles wrth respect to the substrate plane.
1. Introduction The recent development of Penning ionization electron spectroscopy (PIES) has provided a new technique for investigating the physical and chemical properties of surfaces [l-12]. PIES gives information, complementary to other analytical methods such as ELS, ARUPS and IR, on the orientation of-molecules adsorbed on a surface. A change in the PIES intensity can be related to that in the electron density-of each individual orbital of the surface molecules. This method has been applied to studies of phase &u&ions of organic molecules [S] and those of CO adsorbed on transition~metal surfaces . [8,10]. ARUPS _ provides similar but slightly different information, because UPS -can hardly probe mohzcules~on a surface layer selectively; in -this case,. electrons knitted from _the layer are overlapped with those-emitted from in-side layers-and -often -from-the substrate as well, since W radiation can pen.trate into the_solid..In contrast, PIEScati; in-principle, probe an outer^ ,
most monolayer with high sensiti-vity, because Penning ionization is caused by collision of a molecule with an excited atom on rfzesurface. The purpose of the present paper is to study the orientations of benzene molecules adsorbed on a graphite surface by PIES. Changes in the PIES intensities with temperature and coverage were measured and analyzed. The graphite-benzene system was chosen for the-following reasons: ~ (1) This system has been studied extensively by various methods such as NMR [13,14], thermal r isotherm [15], and neutron diffraction [16,17],- but the _conclusions are -controversial as to whether benzene-molecules adsorbed on -the graphite_ basal plane (0001) at monolayer coverage are_paraUel_ [14,15,17] or perpend&ular[13,16]. It is not clear _ whether the conflicting results originate from dif_ ferences m the experimental methods and analyses _ from which the concmsions were derived or-from - those in the conditions-of adsorptions.= S(2) Benzene has orb&a& that have widely differ_ ent ~sp&al dkributions~of electrons. These aand
_- _ --
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0301~0104~84/$03.00 0 Elsevier’ Science Publishers B.V. (North_Holland Physics Publishing Division) .I = -_-
-
:
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.t
-I
_ -
. _
_-
_ _ _--
.-
_
-
-
_
_-
400
H. Kuboro et al. / Orlenrotion of benzene molecules adsorbed on grophrre
T orbitals can be well resolved and assigned by PIES [18]. Therefore, benzene is a suitable molecule to be probed on a surface monolayer by measurement of PIES peak intensities_ (3) If the adsorbed benzene molecules are parallel to the graphite surface at one-monolayer coverage, as predicted by neutron diffraction [17] and
estimated to have an average thickness of several layers (see section 3.3). The PIES and UPS were measured on these specimens by He*(2 3S, 2 ‘S) metastable atoms and He1 (58.4 run) photons with normal incidence to the foil plane and under the same experimental conditions (figs. 1 and 2). A thermal He* beam
by theory [19], then PIES IS expected to change stgnificaatly with increasing coverage. because upon further deposition of benzene molecules the relative orientations of molecules are expected to approach their crystalline arrangement. where the molecules are nearly perpendicular to one another [20]_ Such a change in orientation should readily be detected by PIES. (4) The use of graphite as a substrate has the advantage that its characteristic UPS peak at 3.3 eV assigned to its conduction band can be used to estimate the film thickness by measurement of the decrease in intensity of this strong peak (see section 3.3).
was prepared by impact of low-energy electrons (= 45 ev), and the He1 resonance line by a dc discharge lamp. The contribution of the He 2 ‘S atoms in the He* beam to PIES was estimated to be less than 10% from the relative peak heights of the first valence bands (marked 1 in fig. lb) due to the 2 ‘S and 2 ‘S atoms. Therefore, *Jle PIES peaks measured under the present experimental conditions were essentially due to the 2’S atoms. The energy distribution curves of the emitted electrons were measured with a spherical electron collector coated with colloidal graphite by an ac modulated retarding-field technique [2.22] with an energy resolution of = 0.3 eV. The zero point of the electron kinetic energy was determined by a sharp cutoff of the distribution peak. Since both PIES and UPS were reproduced in repeated measurements, they were regarded as essentially free from any surface contamination. The spectra showed no significant temperature or time dependence in the temperature range of loo-130 K. However, when the surface temperature was maintained at 140 K the PIES peaks l-5 shown in fig. 1 changed with time; they appeared prominentiy just after exposure to benzene vapor, but they disappeared in a few tens of minutes. This observation mdicated that gradual desorption of benzene molecules from the graphite surface took place at this temperature_
2. Eqerimentai
The measurements were made in a vacuum chamber with a base pressure of 2 X lo-’ Torr [3,6]. Grafoil produced by Umon Carbide. Inc. was used as an adsorbent. A neutron-diffraction study [21] showed that it was made of graphite crystallites with then basal planes preferentrally oriented parallel to the macroscopic foil plane. A Grafoil disk of 15 mm diameter was mounted on a copper block and was cleaned by heating it to 500 o C for 20 h under vacuum. The Grafoil surface. cooled to 140 K or lower, was exposed to benzene vapor by effusing the vapor into the chamber through a variable leak valve. The pressure of the chamber, as measured by an ionization gauge cahbrated with dry air, was = 5 x IO-’ Tot-r; it was held constant during the exposure, and the average thickness of the adsorbed layer was controlled by changing the exposure time. Four specimens were prepared and tested: (a) unexposed graphite, (b) exposed to benzene for 10 s, (c) for 100 s, (d) for 400 s. No quantitative measurement of the thickness was made, but from their He1 photoelectron spectra (UPS) specimen (b) was
3. Results
3.1. Spectra
and discussion for graphite
surface
The observed UPS for a clean graphite surface agrees well with those reported previously; the sharp peak at 3.3 eV is related to the high density of states of the upper conduction bands [23,24]_ The PIES has similar features; it extends beyond 15 eV, and a sharp but less intense peak appears at 3.3 eV. The PIES for graphite wilI be discussed in detail in a separate publication [25].
H_ Kubota et aL / Orientatton of k-h-ene mo&&.s
16
401
a&orbed o_ngraphite
12 Ekctron
4 8 EnergyleV
0
Fig. 1. Penning iontzation electron spectra (in arbitrary units) of benzene adsorbed on graphite by He* (2%). (a) Unexposed graphite, (b) exposed to benzene vapor ( = 5 X lo-’ Tot-r) for 10 s. (c) 100 s. and (d) 400 s. The absctssa represents ekctron kinetic energy with respect to the vacuum level of the system. The ordmate scale of (a) is 1.25 times those of (b)-(d)_
Frg. 2. Photoelectron spectra (in arbitrary units) of benzene adsorbed on graphite by HeI (58 4 nm). Samples (a)-(d) were the same as those used for measurement of the PIES shown tn fig. 1.
3.X
PIES measured in the gas phase [18]. (4) The intensities of these valence peaks change gradually from (b) to (d). This can be ascribed to changes in orientation of benzene, as discussed in section 3.3. (5) A strong peak appears at = 1.4 eV, and it is enhanced gradually from (b) to (d). This peak is assigned to scattered electrons [2,5]. The corresponding changes in the UPS (fig. 2) are the following: (6) The emission yields from specimens (a) to (d) are about 1.7, 3.4, and 4.4 times as high as that from clean graphite_ The origins of this increase in the yield wiII be discussed elsewhere 1251.(7) The UPS for the graphite substrate and the adsorbed benzene are superimposed in spectrum (b), but the graphite peak at 3.3 eV is barely visible in (c) and (d). This observation shows that the adsorbed benzene multilayers in specimens (c) and (d) are so thick that-essentially no UPS of the graphite substrate can be observed.
Spectral changes upon benzene adsorption
Only slight adsorption of benzene on the graphite surface causes drastic changes in the PIES (see fig. 1). (1) The yields of electron emission from specimens (b), (c), and (d) are about 2.1, 2.4, and 2.8 times as high as that for clean graphite (a)_ This implies that the mechanisms of electron emission from graphite and benzene surfaces are different [25]. (2) The characteristic features of the graphite spectrum shown in fig. la, i.e. the peak at 3.3 eV and the structure in the 13-16 eV region, disappear; even in the stage of specimen (b) there seem to be no substantial electronic interactions between the graphite substrate and the He* atom. (3) Instead, the peaks numbered l-5 appear in the 4-12 eV region, as shown in figs. lb-ld. They are assigned to the valence structures of the benzene molecule, because they correspond to those in the
40’
H. Kubota er aI_ / Orientation of benene molecules aa!ro&ed on graphite
(S) The relative intensities of the benzene valence structures 1-5 for specimens (b), (c) and (d) are nearly identical_ (9) The 1.4 eV peak is enhanced from (b) to (d) much more strongly than the correspondmg PIES peak. These observed differences between the UPS and PIES originate from the difference in the average probing depth [4.5]. _?.;. Film condltlons The film thickness can roughIy be estimated by compnrtson of the features of PIES with those of UPS. The contribution from graphite to the intensity of the 3.3 eV peak is estimated to be about one half of that of adsorbed benzene, in view of the reported escape depths of no more than 10 A for photoelectrons from similar molecular crystals [%I_ Accordingly. the thickness of film (b) is likely to be no more than a few monolayer coverage, because the sharp UPS peak of graphite at 3.3 eV due to the conduction band, shown in fig. 2a, is also observable in the LIPS of film (b). in constderation of the spectral changes in PIES upon benzene adsorption stated in section 3.2. film (b) is estunated to be covered with at least one or more layers of benzene. so that metastable atoms can hardly Interact with the substrate. The present results of UPS and PIES indicate that a film of 10~ coverage forms a monolayer instead of islands. This conclusion is consistent with the result of a neutron-diffraction experiment [17] made under similar sample conditions_ The thicknesses of films (c) and (d), which were prepared wtth sufftcient times of deposition. are at least several tens of layers of benzene. because the electrons emitted from the graphite substrate could not be observed even in UPS_
.?_d_Relative peak ultensities and moiecrch or~enkmons The PIES and UPS peaks with electron kinetic energies of 14 eV can be assigned to the valence structures of adsorbed benzene, as shown in figs. 1 and 2. The relative peak positions of these spectra correspond to those measured in the gas phase. On the other hand, the dependence of the relative peak intensrties on exposure time IS qurte dtfferent.
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_
The relative intensities of the UPS peaks are essentially identical at different exposure times, whereas those of the u peaks numbered 3-5 in the PIES increase gradually with exposure time in comparison with those of the r peaks 1 and 2. This spectral change can be explained by changes in the orientation of the molecules on the surface of the adsorbed layer with increasing thickness of the layer. The reIative intensities of the PIES peaks reflect electron densities of the relevant molecular orbitals in the range of interparticle distance where Penning ionization mainly takes place [HI. The n orbitals extend widely to both stdes of the molecular plane. whereas u orbitals are confined near the molecular plane. If the metastable atom approaches a benzene molecule perpendicularly to the molecular plane. the T bands are expected to be enhanced in the PIES more strongly than the (I bands. On the other hand. if the metastable atom approaches the molecule from the direction parallel to the plane. the CJbands may also appear in the PIES with appreciable intensities_ Therefore, we conclude that benzene molecules adsorbed on the graphite surface are essentially parallel to the substrate plane at one-monolayer coverage, but that they are tilted on the surface of a multiple layer. the degree of average tilting being dependent on the layer thickness. An example is thereby given for an application of PIES to selective and sensitive probing of the outermost layers of solid surfaces [27]_ The present results seem to be directly comparable with those obtained by Meehan et al. [17] by neutron diffraction; as remarked by these authors, other previous studies [13-161 were made under such widely different experimental conditions that no meaningful comparison seems to be feasible in regard to the molecular orientations on the surface. The conclusion reached in the present study that benzene moIecules at one-monolayer coverage lie parallel to the graphite surface is consistent with that of Meehan et al. derived from their experiments made at 100 K. On the other hand, the two conclusions seem to deviate from each other as to the molecular arrangements beyond one-monolayer coverage: Meehan et al_ observed that at a coverage up to 1.2 monolayers any excess benzene molecules above one monolayer
-.
nucleate to form crystallites, whereas our present study has provided indication that the average tilting angle of benzene molecules on the outermost surface of a multiple layer increases gradually with the thickness of the layer, so that crystallites seem to be formed at much higher coverage. This difference probably originates from a difference in sample preparation: In the present study benzene molecules were adsorbed on the graphite surface at 100-130 K, whereas Meehan et al. introduced a known volume of benzene vapor onto the adsorbent at room temperature, and then the sample was cooled and annealed slowly. The relative intensities of the PIES bands also differ among the u bands. The ezs and b,, bands appear only very weakly in the PIES, whereas the e,,, b,, and ala bands have appreciable intensities. These differences can be interpreted in terms of the bonding characters of these orbitals. Since the e,,* b iU, and a,s orbitals are strongly C-H bonding, the 1s atomic orbital of hydrogen is expected to contribute significantly to these orbitals. On the other hand, the q, orbital is weakly C-H bonding and b,, strongly C-C bonding. Therefore, the C-H bonding orbitals are expected to interact with the incident metastable atom more strongly than the C-C bonding orbitals, which are localized in the C-C bond regicns and shielded from the C-H boy,‘ing orbitals. The different localizabilities of the c molecular orbitals on the relative ionization prob‘lbilities were also observed in the PIES of gas-phase molecules [28]. The present results demonstrate that PIES can probe a change ,n the molecular orientations on a solid surface, f.om which information on the spatial distribution of electrons in each molecular orbital can bz obtained_
I21T. Munakata.
T. Htrooka and K. Ku&i&u, J. Electron Speetry_ 13 (1978) 219. [3! T. Munakata, T. Hirooka and K. Kuchitsu, J. Electron spectty. 1s (1980) 51. 141 T. Munakata, K. Ohno and Y. Harada. J. Chem. Phys. 72 (1980) 2880. I51 H Kubota, T. Munakata, T. Hirooka. K. Kuchitsu and Y. Harada, Chem. Phys. Letters 74 (1980) 409. 161 H. Kubota. T. Hirooka, T. Fukuyama. T. Kondow, K Kuchitsu and A.J. Yencha, J. Electron spectry. 23 (1981) 417.
I71 A.J. Yencha.
H. Kubota. T. Fukuyama, T. Kondow and K. Kuchitsu. J. Uectron Spectty. 23 (1981) 431. ISI1 H. Conrad, G. Ertl. J. Ktippers, W Sesselmann and H. Haberland. Surface Set. 121 (1982) 161.
[91 W.
Sesselmann, H. Conrad. G. Ertl. J. Ktippers. Et Woratchek and H. Haberland. Phys. Rev. Letters 50 (1983) 4256.
WI
F. Bozso.
P11 C. Bomau.
References [I] T_ Shtbata. T_ Hirooka Letters 30 (1975)
241.
and K.
Kuchitsu.
Chem.
Phys.
Yates
Jr..
J. Arias,
H. Metiu and
R M.
C Garot.
R Nuvolone
and J. Roussel, Surface
Sci. 91 (1980) 313. WI J Roussel. C. Boiziau. R. Nuvolone and C. Reynaud, Surface Sci 110 (1981) L634. r131 B. Boddenberg and J.A_ Moreno. J. Phys. (Parts) 38 (1977) C4-52.
r141 I. Tabony.
J.W. White, J.C. Selachaume and M Coulon, Surface Sci. 95 (1980) L282. I151 Y. Kbartr. M. Coulon and L. Bonnetain, J. Chim. Phys 75 (1978) 789. [16) R. Stockmeyer and H. Storm&, Surface Sci. 81 (1979) L315.
1171 P. Meehan. T. Rayment, J.W. White, 2011.
J. Chem.
R K. Thomas, G. Bomchil and Sot. Faraday Trans. I 76 (1980)
PSI T. Munakata.
K. Kuchnsu and Y. Harada. Chem Phys Letters 64 (1979) 409. P91 L. Battezzati. C Pisani and F. Rtcca, 1. Chem. Sot. Faraday Trans. II 71 (1974) 1629. PO1 E.G. Cox. D.W J. Cnuchshank and J.A. Smith, Proc. Roy Sot. A247 (1958) 1. WI J.K. KJem& L. Passell. H. Taub. J.G Dash and A.D. Novaco, Phys. Rev. B13 (1976) 1446. H. WI T. Htrooka, K. Tanaka. K. Kuchitsu. M. Fqtbiia. Inokucht and Y. Harada. Chem. Phys. Letters 18 (1973) 390. 1231 R-F. Wtllis, B. Feuerbacher and B Fitton. Phys. Rev. B4 (1971)
The present study is supported by a Grant-inAid for Scientific Research from the Ministry of Education, Science and Culture of Japan.
J-T.
Martin, J_ Chem. Phys. 78 (1983) 4256.
2441.
1241 B. Feuerbacher
and B. Fitton,
Phys. Rev. Letters 26 (1971)
840.
1251 H. Kubota.
T. Hirooka and K. Kuchitsu. unpublished data. 1261 V-V. Grechov. Chem. Phys Letters 96 (1983) 237. 1271 K. Ohno. H. Mutoh and Y. Harada, Surface 8ci. 115 (1982) L12S_ [28) K. Ohno. H. Mutoh and Y. Harada, J. Am. Chem Sot. 105 (1983)
4555.