Surface photochemical determination of the mean free path of subvacuum electrons in adsorbate films

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Surface Science 276 (1992) 325-332 North-Holland

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Surface photochemical determination of the mean free path of subvacuum electrons in adsorbate films V.A. Ukraintsev

and I. Harrison

Department of Chemistry, University of Virginia, Charlottesville, VA 22901, USA

Received 4 December 1991; accepted for publication 27 May 1992

Dissociative electron attachment (DEA) of adsorbates was used to detect photoexcited substrate electrons with energies below the vacuum level. When adsorbate layers of variable thickness were deposited on a Ptflll) surface and capped with a topmost, DEA active, detector layer of CHaBr the mean free path of substrate derived electrons could be measured. The averaged electron scattering cross section through solid Xe at 25 K was us = 4.7’:’ AZ for electrons of energy -0.6 I E, s -0.27 eV with respect to the vacuum level. The cross section in solid CHaBr at 2.5 K was rr, 2 4.7 & averaged over an approximately 0.4 eV FWHM range of electron energies centred at E, = -0.7 eV.

1. Introduction

Mean free paths of low energy electrons through solid adsorbate films are typically obtained by transmission measurements on exiting substrate photoelectrons [l] or on incoming electrons from an externally incident electron beam [2]. In both instances, the electron energies must be greater than the vacuum level of the surface. In this paper, we describe a technique which can extend the measurement of mean free paths to subvacuum electrons with insufficient energy to escape from the adsorbate surface. Attachment of an additional electron to certain molecules can lead to repulsive interactions and dissociation of the anion produced. These dissociative electron attachment processes (DEA) often occur with high cross-section in the gas phase and have recently been observed in adsorbates 131.The practice of using a topmost adsorbate layer of DEA active molecules to detect the transmission of “very low energy electrons” through adsorbate films was first impIemented by Cowin and coworkers [4]. In their experiments ultraviolet irradiation of an adsorbate covered metal substrate efficiently produced electrons of ~39-6028/92/$05.~

energy above the Fermi level. A substantial fraction of these electrons travelled to the surface where they induced detectable DEA in the topmost adsorbate layer. The energy of the most energetic electrons was varied by changing the photon energy which was maintained at values in excess of the surface work function. A quantitative description of the surface DEA of CH,Br [.5]allows us to optimise measurements of electron transmission through adsorbates for subvacuum electrons. In the gas phase, DEA of CH,Br occurs when an attaching electron induces a vertical Franck-Condon transition between the potential energy curve of the neutral molecule and the dissociative potential of its transient anion. The vertical electron affinity (VEA) for CH,Br in the gas phase is -0.46 eV, which indicates that the anionic DEA potential lies above that of the neutral. Analysis of the gas phase potential curves indicates that the FWHM of the Franck-Condon range of attaching electron energies, centered at -VEA, is 0.4 eV [6]. When DEA occurs in the adsorbed state the transient anion is stabilized by image charge interaction with the surface and electronic polarization of the surrounding adsorbates. In the experi-

0 1992 - Elsevier Science Publishers B.V. Ah rights reserved

326

IL&. Ukr~~ntseu, I. Harrison / Subvacuum electrons in adsorbate films

ments to be discussed below in which DEA of CH,Br occurs in a topmost Iayer of CH,Br resting on Xe or CH,Br films the VEA in the adsorbed state, far from the metal substrate, is 0.47 and 0.7 eV, respectively 171.In consequence, DEA in the CH,Br detector layer will occur for subvacuum electrons of energies E, = - VEA(CH, Bread,) + 0.2 eV with respect to the vacuum level. The mean free path of subvacuum electrons through solid Xe was measured in experiments employing CH~Br/Xe/CH~Br/Pt(lll) sandwich structures of variable Xe solid thicknesses. Fig. 1 gives a schematic depiction of the energetits of the surface DEA process for this system. The first, single monolayer of CH,Br deposited on the cIean Pt(ll1) surface served to Iower the macroscopic work function, @J,from 5.8 to 4.3 eV [8]. An excimer laser was used to irradiate the sandwich system with 308 nm photons of 4.03 eV energy. Photoexcited substrate electrons were produced at energies extending up to the photon Pt(l11) I CH3Br

Xe Layer

energy above the Fermi level (i.e. E, 5 hv - Q, I -0.27 eV). The Xe acts as an insulator for electrons whose energy is too low to access the bottom of the Xe conduction band at J!?,= -(0.5 + 1.6/z) eV beiow the vacuum level, where z is the distance in A away from the metal surface [9]. The second term is the approximate surface image stabilization energy for electrons inside the Xe solid, -e2/4ez, where E = 2.24 is the dielectric constant of Xe. The smallest thic~ess of Xe solid used in the experiments was 2 ML. The minimum electron energy within the Xe conduction band next to the CH,Br detector iayer was estimated as E, = - 0.6 eV with z = 18 A. Hence, only photoexcited substrate electrons of energy -0.6 I E, I - 0.27 eV could propagate through the solid Xe layer to induce DEA in the detector layer of CH,Br. The adsorbed CH,Br made an excellent DEA detector for arrival of these electrons at the outer surface. The VEA(CH,Br(,,$ varies with the image potential. The DEA cross Top CH3Br Layer

CHS cg)*B%)+eir.-)_____-_l-p-

.,T-

T

Nuclear Separation, R H3C_Br (A) Fig. 1. Schematic energy levef diagram jllustrating the features relevant to DEA of photoexcited substrate electrons to CH,Br adsorbed on the top of CH,Br/Xe/CH,Br/Pt(lll) system. The CHsBr molecule is treated as a pseudo-diatomic. The potential energy curves for the gas phase molecule are dashed and the stabilized adsorbate curves are solid. Photoexcited substrate electrons may attach to adsorbed CH,Br at configurations on the DEA curve within the Franck-Condon region of overlap with the neutral’s ground state.

V.A. Ukraintseu, I. Harrison / Suboacuum electrons in adsorbate films

section peaks at E, = -CO.47 + 1.6,/z) f 0.2 eV which maintains excellent energy alignment with the bottom of the Xe conduction band and the range of incoming electron energies. Detection of DEA in the CH,Br detector layer was made by measurement of energetic CH, fragments leaving the surface.

2. Expe~mental Experiments were conducted in an UHV system equipped for LEED. Auger spectroscopy, thermal desorption spectroscopy, TDS, and photoproduct time-of-flight, TOT;, spectroscopy. Details of the apparatus and photoproduct TOF detection have been described elsewhere [7]. The CH,Br/Xe/CH,Br multilayer sandwichs were deposited on a clean Pt(lll) surface at 25 K. The top and bottom CH,Br layers were of 1 ML thickness. The Xe layer thickness was varied. Separate dosers for CH,Br and Xe were calibrated by thermal desorption spectroscopy. The CH,Br/Xe/CH,Br/Pt(lll) systems were irradiated by a 308 nm excimer laser at a fluence of less than 1016 photons/cm’ per 15 ns laser pulse. Laser induced heating of the surface was calculated to give a transient temperature rise of less than 2.5 K [lo]. The 4.03 eV photon energy remained below the surface work unction under all coverage conditions as was demonstrated by our inability to detect photoelectron current flow from the crystal under negative 300 V bias and laser irradiation. Our detectivity towards photoelectron production was checked by redirecting the laser onto the thermocouple wires leading to the Pt crystal. The lower work function of the CH,Br covered therm~ouple wire permitted escape of photoelectrons and phot~urrents were easily measured. A quadrupole mass spectrometer was used to collect TOF spectra of CH, fragments leaving the surface following DEA in the topmost, detector layer of CH,Br. The TOF spectra were accumulated on a multichannel scaler synchronized to the excimer laser pulses which produced photoexcited substrate electrons. The CH, fragments were detected at an angle ‘“9, = 9” with respect to

321

the surface normal and the laser incidence angle was 6, = 81”. The CH, TOE; spectra were transformed into translational energy distributions and integration over energy gave the angle-resolved yield of CH, fragments from DEA. The angular distribution of CH, fragments from DEA in the topmost layer of CH,Br on the surface was assumed to be independent of the thickness of the underlayers. The CH, yield at fir, = 9” was taken to be proportional to the extent of DEA in the CH,Br detector layer and hence the number of electrons which reached the surface.

3. Results and discussion Sophisticated models for electron transport through gases and condensed phases have been developed to extract electron mean free paths from e~eriments employing external electron beams. If electrons are injected at a source plane and collected at a parallel detection plane, the idealized experiment represents a one-dimensional multiple scattering problem. Theoretical aspects of such problems have been reviewed by Chandrasekhar [ll]. Electron transmission experiments through Xe gas [12] and Xe solid films [2] have demonstrated the utility of the theoretical methods. In these investigations, the electron flux at the detection plane was comprised of components due to the attenuated primary electron beam, any multiply reflected beams, and a significant component due to diffusion of scattered electrons to the detection plane. Attenuation of the primary beam by elastic and isotropic scattering produces a spatially varying source of diffusing scattered electrons. Solution of the transmission probiem within a two stream appro~mation requires detailed knowledge of the interface boundary conditions for the primary and scattered electrons. In consequence, the electron transmission is generally a complicated function of the mean free path and various other diffusion and reflection coefficients. In the case of subvacuum, photoexcited substrate electron transmission through multilayer films to a DEA detector layer of CH,Br, electron transport occurs under the influence of the long

328

FA. Ukraintsec;, I. Harrison / Suboacuum electrons in adsorbate films

range surface image potential. Only substrate photoelectrons initially travelling in a narrow cone of angles around the surface normal will have sufficient normal component of momentum to directly access the CH,Br detector layer. This directional “primary” beam of electrons is attenuated by scattering until it reaches the DEA detector layer. The fraction of the attenuated beam which does not undergo DEA is reflected with unit efficiency from the vacuum interface and is returned to the bulk substrate except for about lo-20% of the remaining beam which is reflected at the Pt(111) dipole layer due to the local work function jump [13] at the interface. For modest film thickness it will be shown that the return of elastically scattered electrons to the substrate can be considerably more probable than forward transmission to the DEA detector layer. Hence, in the absence of significant primary beam refIections, the electron transmission can be a relatively simple function of the mean free path and film thickness. The energy of a photoexcited substrate electron with respect to the Fermi level is E, = @ + E,, where @ is the macroscopic work function and E, is the electron energy with respect to the vacuum level. Once the electron moves outside of the substrate its energy may be expressed as E, = E, + # where E, is the electron’s kinetic energy and 4 is its potential energy relative to the Fermi level. The maximum value of E, will be the exciting photon energy, hv. The electron’s kinetic energy can be divided into components perpindicular and parallel to the surface, E, = E, cos2 6 and El, = E, sin2 6, where i) is the polar angle between the propagation direction and the surface normal. The electron potential, 4, is also equivalent to the local work function which is the work required to remove an electron from within the substrate bulk to a particular position above the surface. For uniform surface coverages, 4 is a function of z alone and hence only E I is reduced as the electron moves away from the surface. If the detector layer is situated at a distance z = L away from the surface then only substrate electrons initially photoexcited with E I 2 4(L) will be able to reach the detector layer without reflecting off the local work func-

tion potential. The range of polar propagation angles for this “primary beam” of electrons is constrained by 6 5 cos-“I[ +( L)/h~]i/~} if the maximum value of E I is hv. Let us consider subvacuum electron transmission experiments in which the photoexcited substrate electrons must have energies very close to the photon energy above the Fermi level in order to reach and undergo DEA at the detector layer. If we assume that photoexcitation of substrate electrons produces an isotropic angular distribution of electrons within the metal, then the flux of electrons leaving the surface plane is a cosine distribution tempered by the constraint above on 6. If the primary beam of electrons is directed close to the z axis, it will be attenuated exponentially according to the value of the mean free path, A. Under these approximations the primary electron flux at energy E, which leaves the substrate and directly induces DEA in the detector layer at z = L can be written as, I,,(L) 'dir(

=x4(1-

‘) =’

COS' lYmax) exp-L/h),

EF-a4 E

ew( -L/A),

F

(1)

where A is a scaling coefficient. For small film thicknesses, the scattered flux which reaches the detector layer can be much smaller than the attenuated primary flux. Only elastically scattered electrons will contribute significantly to the transmitted flux if the DEA detector layer is tuned to electrons with energies E, close to hv. The contribution to the transmitted flux from electrons which are elastically scattered from the primary beam and subsequently reach the detector layer without further scattering can be readily calculated. In order for an electron scattered at z to be able to reach the detector layer at z = L it must have sufficient E I to surmount the increasing local work function potential, E,=ET

cos* 8z#(L)

-4(z).

(2) The result is a limitation on the range of polar scattering angles which allow the scattered electron to reach the detector layer, 7Y
329

VIA. Ukraintsea 1. Harrison / Subr;acuum electronsin adsorbatefilms

where E, 2 +(L). If an electron is scattered isotropically at z then the probabili~, P&z>, of reaching the detector layer without suffering further collision is simply the fractional solid angle defined by eq. (3) times the usual exponential attenuation due to scattering, exp[-(L P,(z)

=

-z)/hl 2

+-/=j.

(4) 0’

The once scattered electron flux at energy E, which contributes to the detected signal is,

IS(~) =

I‘”

‘dir(Z)<(Z)S~

‘Lmin

A

(-3

where the lower limit of integration is the beginning of the multilayer film. Simulations show that for multilayer films of modest thickness and DEA detectable electrons with E, comparable to 4(L) the ratio of &,/I, can be much greater than one. Multiple scattering processes of higher order should fall off as inverse powers of this ratio. Hence, the electron flux transmitted to the DEA detector layer should be calculable in first order as I,,,“, = &,, + 1, for &,/I, > 1.

Fig. 2 gives the CH, yield from laser induced DEA of CH,Br/Xe/CH,Br/Pt(lll) structures as a function of Xe thickness in the intermediate layer. A limiting model of simple exponential attenuation of a primary electron beam served to fit the data with A = 4.3 ML. The data was also fit to the function I,,,,, = Zdir+ Z, under the approximation that the primary beam energy was at E, = hv which would allow for the maximum possible transmission of scattered flux. The mean free path was reduced to h = 3.7 ML in the

)

,,.’

0

*

4

“,

6

10

*

Xa Layer Thickness

12

14

(ML)

Fig. 2. The CH, photofragment yield for DEA of CH,Br/Xe/CH3Br/Pt(lll) sandwich structures as a function of Xe layer thickness. The heavy dashed line is an exponential fit to the data. The heavy solid line is a simulation, Itran = Zdir+ I,, based on transmission of an attenuated primary electron current and a scattered electron current, respectively.

simulation which showed that the contribution from the scattered flux was small. The actual mean free path can be taken to lie within the bounds of these two limiting values. The scattering cross section is a~ = (hn,)-‘: where h is the mean free path in monolayers and IZ, is the coverage of a monolayer. In fig. 2 the monolayer unit of coverage was established by TDS of a single monolayer of Xe on Pt(lll), a coverage of 5 X 1014 cms2 [14]. The scattering cross section for subvacuum electrons averaged over energies rz0.6 I E, 5 -0.27 is estimated as as = 4.7?$7 A. Although the scattering cross-section may be calculated with reference to arbitary monolayed units, evaluation of the mean free path in A requires knowledge of the Xe solid characteristics. Electron transmission experiments performed by Sanche and coworkers I21 for Xe deposited on Pt(ll1) at 25 K gave evidence for a somewhat disordered solid and a mean spacing between Xe layers of 6 A + 30%. If we assume that the disordered Xe solid grows with the same layer coverage as >he Xe monolayer on Pt(ll1) then A=25&1OA. The simulation of the electron transmission through Xe is shown in fig. 2 and broken down

330

VA. Ukraintseu, I. Harrison

/ Subvacuum

into its components Zdir and Z,. The local work function work was taken to vary in the Xe layer as, +=@-e2/&z+xe

Pep,

(6)

where @ is the macroscopic work function, -e2/4ez is the image potential and ,y,P,- is the polarization stabilization energy of an electron in solid Xe. For the CH,Br/Xe/Ch,Br/Pt(lll) sandwich structures, @ was taken as 4.3 eV. The polarization stabilization energy must be -0.5 eV in order to give the correct location of the bottom of Xe conduction band far from the surface. The image potential is approximately -1.6/z eV A. The local work function or the position of the bottom of the Xe conduction band with respect t: the Fermi level is 4(z) = 3.8 eV -1.6/z eV A. The distance zcale was set by taking a Xe layer spacing of 6 A. Substrate electrons photoexcited with 4.03 eV photons will only be able to access the Xe conduction band if their initial E I 2 4(L). This condition selects a subset of electrons whose angle of propagation relative to the surface normal is restricted to 6 I 17” even for the thinnest Xe solid of 2 ML. The most probable energy for DEA of the CH,Br detector layer also varies with the image potential and remains level with the bottom of the Xe conduction band. Notice that for electrons of this energy, E, = 4(L), the propagation direction must be precisely along z in order to reach the detector layer and there will be ~to signal from scattered electrons (eq. (4)). The electron kinetic energy within the Xe layer was small, E, I 0.33 eV, and hence the image potential was significant. The simulation in fig. 2 was carried out under the assumption that all the electrons had energy E, = hv in order to give an upper bound for the magnitude of the scattered, transmitted current. The lower limit of integration for Z, of eq. (5) was 6 A owing to the 1 ML of preadsorbed CH,Br. For modest Xe layer thicknesses, fig. 2 shows that scattered subvacuum electrons were a relatively insignificant component of the transmitted current. However, in the limit of large thicknesses or in cases where E, % 4(L) the scattered electron current can become important. This was likely the case in Cowin’s [4] electron

electrons in adsorbate films

transmission experiments through solid Xe using DEA of CH,Cl as a detector and photon energies in excess of the work function. In this instance, the electron transmission through Xe was essentially constant over some 65 ML of coverage (instead of the exponential decline in CH, yield witnessed here for subvacuum electrons). The electrons with energies greater than the vacuum level may have scattered and then diffused to either the substrate or detector layer interface with comparable probability if their energies were significantly higher than the local work function. While the subvacuum electrons traverse the Xe solid in the conduction band they will have kinetic energies of 0 to 0.33 eV. The detection efficiency using CH,Br DEA is calculated to peak for electrons at the bottom of the conduction band and fall to half maximum for N 0.2 eV electrons. The total scattering cross section for the detected electrons was a, = 4.7?i.7 A’. This value is smaller than those for gas phase scattering experiments [15] in which the cross section peaks at 100 A2 for zero kinetic energy electrons and drops to 5 k 3 A2 by 0.2 eV, close to the Ramsauer minimum at 0.6 eV. In the low temperature limit, theory predicts that the differential scattering cross section in the solid should become the gas-phase value multiplied by the Xe solid’s structure factor [16]. Careful electron transmission experiments through Xe films employing an external electron beam 121have shown that scattering cross sections in the solid are systematically reduced five to ten-fold with respect to those in the gas phase. Our cross section for subvacuum electron transmission through solid Xe shows a similar reduction if the measurement is considered as an average over the first 0.2 eV of electron kinetic energies in the solid Xe conduction band where the CH,Br DEA efficiency is greatest. 3.2. Electron transmission through CH,Br at 2.5 K Using the same technique, it was possible to measure the electron scattering cross section through solid CH,Br for subvacuum electrons. Electrons were transmitted through pure multilayers of CH,Br adsorbed on Pt(lll>. Fig. 3

VA. Ukraintseu, I. Harrison / Subvacuum electrons in adsorbate films

331

This made detection of subvacuum electrons in the topmost layer of CH,Br via DEA consistently optimal for electrons of energy E., = - 0.7 eV. The breadth of the DEA transition was estimated to be 0.4 eV in FWHM and hence the detectable electrons in these experiments should have energies -0.9 I E, I -0.5eV. Inelastic scattering of electrons of higher energies did not appear to significantly contribute to the detected electron flux as evidenced by the latter’s simple exponential decay with CH,Br thickness.

yield for DEA of Fig. 3. The CH, photofragment CH,Br/Pt(lll) as a function of CH,Br monolayers. The dashed line is a linear interpolation on the semilogarithmic data.

illustrates the laser induced yield of CH, fragments from DEA in the topmost CH,Br layer as a function of multilayer thickness. The initial behavior of the CH, fragment yield with coverage is not well understood but is believed to be connected to quenching of the DEA process for CH,Br in close proximity to the metal surface. The decline of the CH, yield after 4 ML is a simple exponential. If the quenching is assumed to be constant over this range of coverage far from the surface the exponential fit provides an electron mean free path of 4.25 ML. Without knowledge of the CH,Br conduction band energetics we are unable to simulate the scattering contribution to the signal. Nevertheless, the exponential decay of the experimental data suggests the scattering contribution should be small and the mean free path calculated above may serve as an upper bound. The monolayer unit in fig. 3 was based on TDS of a first monolayer of CH,Br on Pt(ll1). Full CH,Br coverage on Pt(ll1) is estimated as 5 X 1014 cmp2 [5]. The scattering cross section of the subvacuum electrons through solid CH,Br is estimated as us 2 4.7 A2. The work function of the CH ,Br/Pt(lll) multilayer system is taken to be 4.3 eV. The photon energy of 4.03 eV allows for a maximum electron energy of E, = - 0.27 eV. The large dielectric constant of 9.82 for CH,Br significantly reduces the magnitude of the surface image potential in the solid.

4. Conclusions The surface photochemical technique described above may become an attractive method by which to characterize subvacuum electron transmission through solids. Strengths of the method for modest film thicknesses and when the DEA detectable electron energies are close to the exciting photon energy are: (a) a well defined experimental geometry, (b) a collimated primary beam of electrons leaving the substrate close to the surface normal (c) an apparent elimination of transmitted signal from scattered electron flux due to the substrate image potential (d) simple extraction of the mean free path. Variation of the metal substrate, adsorbate and detector layers, and photon energies offer some versatility to the method. At photon energies significantly higher than the DEA electron detection energies it is likely that the scattered electron flux will become a significant contributor to the transmitted flux and many of the strengths of the method are lost. Nevertheless, under favourable conditions the photochemical technique may offer a relatively simple and perhaps unique method by which to measure the mean free path of subvacuum electrons through solids.

Acknowledgement

Acknowledgement is made to the Jeffress Trust and the Donors of the Petroleum Research Fund, administered by the American Chemical Society for support of this research.

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VIA. Ukraintsev, I. Harrison / Subvacuum electrons in adsorbate films

References [l] G. Schwenter, Phys. Rev. B 14 (1976) 5490. [2] G. Bader, G. Perluzzo, L.C. Caron and L. Sanche, Phys. Rev. B 26 (1982) 6019. [3] X.-L. Zhou, X.-Y. Zhu and J.M. White, Surf. Sci. Rep. 13 (1991) 73. [4] T.L. Gilton, C.P. Dehnbostel and J.P. Cowin, J. Chem. Phys. 91 (1989) 1937. [5] V.A. Ukraintsev, T.J. Long and I. Harrison, J. Chem. Phys. 96 (1992) 3957. [6] L.G. Christophorou, J.G. Carter, P.M. Collins and A.A. Christodoulides, J. Chem. Phys. 54 (1971) 4706. [7] V.A. Ukraintsev, T.J. Long, T. Gowl and I. Harrison, J. Chem. Phys. 96 (1992) 9114. [8] J.M. White, in: Chemistry and Physics of Solid Surfaces,

Vol. 8, Eds. R. Vanselow and R. Howe (Springer, New York, 1990) p. 29. [9] Z. Ophir, B. Raz and J. Jortner, Phys. Rev. Lett. 33 (1974) 415. [lo] J.L. Brand and S. George, Surf. Sci. 167 (1986) 341. 1111 S. Chandrasekhar, Radiative Transfer (Clarendon, Oxford, 1950). [12] P.J. Chantey, A.V. Phelps and G.J. Schultz, Phys. Rev. 152 (1966) 81. [13] S. Luryi, in: High Speed Semiconductor Devices, Ed. S.M. Sze (Wiley-Interscience, New York, 1990) p. 406. [14] A. Cassuto, J.J. Ehrhardt, J. Cousty and R. Riwan, Surf. Sci. 194 (1988) 579. [15] L.S. Frost and A.V. Phelps, Phys. Rev. A 136 (1964) 1538. 1161 J.M. Ziman, Principles of the Theory of Solids, 2nd ed. (Cambridge University Press, Cambridge, 1972).