Investigation of metal surfaces by VUV laser based photoemission spectroscopy

Journal of Electron Spectroscopy and Related Phenomena, 68 (1994) 73 1-746 0368~2048/94/$07.00 @ 1994 - Elsevier Science B.V. All rights reserved

Investigation

of metal surfaces by

731

VUV laser based photoemission

spectroscopy

Toshiaki Munakataa) and Ikuo Kinoshitab) %?aeInstitute of Physical and Chemical Research, 2-l Hirosawu, Wake, Saitama 35141, I n c e, Science University of Tokyo, figurazaka, b)FacuZtyof Sc’e

Japan

Tokyo 162, Japan

A novel photoelectron spectrometer based on pulsed laser radiation is developed. The spectrometer realizes two features: the fast is the photoelectron spectromicroscopy by focused vacuum ultraviolet (WV) coherent radiation, the second is the two-photon photoelectron spectroscopy to probe excited states of adsorbed species. The characteristic feature of the photoelectron spectromicroscope is that it records photoelectron spectrum at every two-dimensional mesh point of the specimen. The feature allows detailed inspection of the electronic

states of a spatially inhomogeneous surface. The two-photon photoelectron spectroscopy allows observation of the occupied and normally unoccupied states of adsorbed species. The method is applied to molecularly adsorbed NO on Cu(ll1). Preliminary result of two-photon photoelectron microscopy is also reported.

I. INTRODUCTION Photoelectron spectroscopy is one of the most general methods used to study the electronic states of a solid surface. The method requires vacuum ultraviolet (WV) photons with enough energy to emit electrons from the surface. Lasers have not been frequently used as the light source, since the required photon energy is typically beyond the coverage of commercial lasers. The limitation will be overcome by two approaches; use of frequency upconverted VUV light, and use of two-photon excitation. The VUV light generated by frequency upconversion has an intrinsic advantage over conventional WV light sources in its spectral resolution, coherence, degree of polarization, and peak power. While, the two-photon excitation method provides information on normally unoccupied state. It is highly expected, therefore, that the use of the pulsed laser as a light source will provide new aspects in the photoelectron spectroscopy of a solid surface. Here, we report on our laser based photoelectron spectrometer and its application to the study of molecular interactions with metal surfaces. Its unique capabilities are outlined according-to the two aspects: (a) spectromicroscopy by focused WV light, (b) detection of normally unoccupied state. SSDI 0368-2048(94)02182-Y

The former takes advantage of the spatial coherence of the VUV beam which is easily focused to a diffraction limited spot. The latter utilizes two-step excitationlphotoemission by a ultraviolet (UV) laser beam. These two aspects are important in investigation of chemical reactions on surfaces. The aspect (a) is intended to probe chemical reactions which proceed at some selected sites of surfaces. The aspect (b) is interesting because the adsorbed molecule’s excited state plays important role in reactions, but only little is known on its nature. In addition to these two aspects, it is desirable to extend the work to spatially resolved study of normally unoccupied state. Preliminary result of the combination of (a) and (b) is also reported. 2. INSTRUMENTATION The apparatus consists of three parts: a WV laser light source, a preparation chamber and an analyzer chamber. The background pressure was typically 2*10-10 Torr in both chambers. A crosssectional view of the analyzer chamber is shown in fig. 1[ l,Z]. Briefly, the third harmonics at the 355 nm wavelength of a Q-switched YAG laser is focused into a frequency conversion cell filled with Xe gas. Further tripling of the frequency in the cell results in generation of VUV light of

132

10.48 eV photon energy (118 nm wavelength). The WV light is then separated from the input UV laser’beam by a prism made of lithium fluoride (LiF). The WV beam enters an analyzer chamber through a LiF window, and irradiates a sample surface which is formed in a preparation chamber. The LiF lens and the shield plates shown in fig.1 are used for the spectromicroscopic work described in section 3. Photoelectrons emitted from the sample surface are detected either with a time-of-flight (TOF) energy analyzer or with a hemispherical energy analyzer. In addition to the VUV laser port, the analyzer chamber is equipped with another laser port at an including angle of 30”. Dye laser light introduced through the UV grade sapphire window of the port excites the sample surface to the unoccupied states. Each laser light is linearly polarized. The directions of polarization are varied by respective double Fresnel rhomb prisms.

%_

(h

2.1 Generation of WV light The WV laser light is generated at 118.2 nm through the frequency tripling of 355 nm laser light in Xe gas. The frequency tripling is efficient in the shorter wavelength side of the Xe 5d[l%]’ resonanceline at 119.2 nm. The YAG laser (QuantaRay, DCR-1A) at 10 Hz repetition rate and 7 ns pulse durationdelivers the third harmonics at 355 mn of 8 to 10 mJ/pulse fluence. The UV laser beam is focused with a quartz lens of 30 cm focal length into a frequency conversion cell filled with Xe gas of about 10 Torr pressure. The generated WV beam is then dispersed through the LiF prism to eliminate the input UV beam. The elimination of the UV light is essential because the light is intense enough to generate photoelectrons extend over several electron volts. The transmittance of the prism is nearly independent of the light polarization, since the incidence and emergence of the laser beam are

Xe cell

tripiing _ 355 nm 3 118 nmLiF window

Dye laser ’

Fig. 1. Cross-sectional

\

/

Hemispherical energy analyzer

view of the laser photoelectron spectrometer drawn approximately to scale, Fe

sample is

mounted on a manipulator for three dimensional translation and rotation around the vertical axis. The electrons passing through the TOF tube are detected by a microchannel plate (MCP).

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not far from the normal of the prism surface. Actually, no marked intensity variation at the sample surface resulted from rotation of the WV polarization prior to prism incidence. The vacuum chamber for the LiF prism is designed to,be usable at different wavelengths. It consists of a doubled coaxial cylinder: the outer cylinder is connected to the frequency tripling cell, while the inner one installs the prism on the cylinder axis and is connected to the analyzer chamber. The gap between the cylinder walls is vacuum-sealed with an o-ring, thereby allowing rotation of the outer cylinder.The prism chamber is then capable of dealing with different wavelengths by rotation of the outer cylinder, and of operating at the condition of the minimum deflection by rotation of the prism itself around the cylinder axis. The dispersed UV light is blocked out by a blackened metal plate. The residual scattered light is further blocked out from entering the analyzer chamber by use of a number of aperture disks installed in the connecting arm. The number of electrons emitted by the WV light from the sample surface was typically 5*105 per laser shot, corresponding to a light fluence of about lo-” J/cm’. At a higher photon fluence, the space charge around the sample surface broadened the photoelectron spectrum. The WV fluence is several orders of magnitude smaller than that estimated from the conversion efficiency of 10v5.This is because only a part of the doughnut-shaped beam profile undergoes the frequency upconversion. The transmission of the LiF optics is also responsible for the reduced fluence.

The TOF energy analyzer was used throughout this work. Two different flight tubes, one of 48.5 cm and the other 23.5 cm total length, were employed. The tubes are 30 mm i.d. aluminum coated with colloidal graphite (Auadag), and has 10 mm apertures at the entrance and exit. The entrance is 39 mm distant from the sample surface. A cylindrical electrode of 27 mm length, with 3 mm gaps at each end, is installed at a position of 111 mm from the entrance of TGF tube. Photoelectrons emitted from the sample are accelerated towards the TOF tube which is typically biased 1.0 V from ground. The tube entrance is tapered in order to provide space for the lens mount. The central part of the flight tube is electrically isolated from both ends and works as an electron lens to efficiently increase the detection sensitivity. The electrons transmitted through the TOF tube are detected by a microchannel plate (Hamamatsu, F2221-21s). The signal from the microchannel plate is fed into a digital oscilloscope (Gould, 4072) of 100 MHz bandwidth with 8 bits vertical resolution. The electron spectrum recorded iu the oscilloscope is then fed to a microcomputer for signal processing. The intensity of the photoelectron signal was typically a few hundred millivolts at the oscilloscope, while a signal pulse height due to au impingement of a single electron was 2-4 mV. Then the number of photoelectrons was estimated to be a few hundred per laser shot at a peak of a photoelectron spectral structure.

2.2 UHV photoelectron spectrometer Two electron energy analyzers, TOF and hemispherical ones, are installed in the analyzer chamber as shown iu fig-l. Each energy analyzer is installed at an angle of 60” from the UV laser port so that the two-photon photoelectron spectrum would be measured at identical light incidence add electron detection configurations. The configuration is required in an energy calibration of the TOF energy analyzer as described later. The resolution of the hemispherical-type electrostatic analyzer, 3 cm mean radius with a four element cylindrical electron lens, was 90 meV

2.3 Calibration

of the

TOF

electron

spectrometer

The present TOF electron spectrometer is designed for recording the photoelectron spectrum at 10 Hz rate. The electron lens provided a signal intensity increase of a factor of 30 or more. Then photoelectron pulses piled up with each other giving reproducible photoelectron spectrum at every laser shot. This einzel lens caused difficulties with; (1) a decrease in energy resolution, (2) an energy dependence on transmission probability, and (3) difficulty with energy calibration. These difficulties were all examined by numerical calculation of electron

734

trajectories. In this calculation, the electrostatic field was obtained by a finite element analysis. The electron trajectory was then calculated by numerically solving the time dependent differential equations of electron motion. The calculation ~sults are summarized as follows: (1) The broadening of the TOF spectrum is less than 2 ns. Since the pulse width of the WV light is about 5 ns, this broadening is no serious problem. (2) The transmission probability increases with electron energy before rapidly decreasing after reaching a maximum at around 9 eV energy. The distortion of the photoelectron spectrum caused by the transmission probability is not a serious problem because the photon energy of our light source is smaller than 10.5 eV. (3) The acceleration of electrons in the lens makes the electron flight time shorter than in the case of using a simple field free tube. The amount of TOF shortening is a similar function of energy as the transmission probability. Since it is as large as 10 ns, it should be taken into account in the conversion of time scale to energy scale. The accuracy of the calculated time-toenergy conversion function was examined with reference to two-photon photoelectron spectra for silver (111) and copper (111) surfaces [3,4]. In the two-photon photoemission, the first photon was tuned to the resonance between the occupied surface state and the n=l image potential state and the second photon at various photon energy in a region between 1.9 and 4.0 eV was used to photoionize from this intermediate state [5]. The two-photon spectra are featured by sharp peaks and low scattered electrons, and are convenient for energy calibration. Similar spectra measured with the hemispherical energy analyzer showed that the peak positions were well reproduced from the binding energy of the image potential state reported in references [3,4]. The Fermi edge and the surface state structure measured by the WV one-photon photoemission were also convenient for energy calibration. We measured about 30 peak positions which covered the energy region from 0.5 to 5.9 eV. Then the peak positions measured with the TOF energy analyzer were compared with the calculated time-to-energy conversion function. It was found that the accuracy of the trajectory calculation was better than 0.1 eV in an energy region between

0.5 and 6.0 eV. 3,PHOTOELECTRON SPECTROMKROSCOPY Photoelectron spectromicroscope has long been devised to inspect physical and chemical properties of real surfaces: polycrystalline surfaces, nonuniform adsorbates, impurities, grain boundaries, small particles, and so on. However, until recently the spatial resolution of the photoelectron spectroscopy has been limited to tens of microns. The method of photoelectron spectroscopy with improved lateral resolution will open up a new area of the surface science. Since the first attempt of photoelectron spectromicroscope by Beamson et al.[6], several instruments have been under development with the aim to achieve spatial and energy resolution at the same time [7-111. There are two classes of photoelectron spectromicroscopes, image magnification and microprobe. In the former approach, one floods the sample with photons and magnifies the source image of photoelectrons with either electrostatic or magnetic lenses. This class of microscope requires some complicated electron optics to acquire spatial and energy resolution simultaneously [7]. In the latter approach, one forms a focused light beam and rasters the sample through the illumination spot. This method is advantageous in that high energy resolution is easily obtained by using a conventional electrostatic energy analyzer. The practical problem is that the conventional incoherent WV light is not easily focused to a small spot. The method is under development by using well designed focusing optics [8-113, but tradeoff between intensity and resolution is inevitable. lf one is interested only in the work functions of samples, the light source can be replaced with a simpler W lamp, and such an approach yielded successful observation of an oscillatory chemical reaction on a platinum surface [12]. As a distinct approach, we developed a spectramicroscope by use of coherent vacuum ultraviolet (VUV) radiation as a light source [2,5]. Spatial coherence of the WV light allows focusing the beam to its diffraction limited size providing high spatial resolution. The pulsed light allows the employment of a time-of-flight (TOF)

735

type photoelectron spectrometer, which facilitates rapid recording of the photoelectron spectrum_ These capabilities were successfully realized in the present new scanning microscope. 3.1 Experimental procedure The microscopic work was performed by using the apparatus described in section 2 with some modifications. As shown in fig. 1, the coherent WV light passes through a LiF planoconvex lens of 34 mm radius of curvature. This lens focuses the light onto the sample surface, which is about 60 mm away. Threefold metal plates with apertures to allow the light to pass &rough are placed in front of the LiF lens, and are connected to the TOF tube. This electron shield is necessary to prevent distortion of the electric field by the LiF lens. The sample is mounted on a precision manipulator (Vacuum Generators, OMNI AX 600) so that the WV irradiation position can be controlled three dimensionally. The z-position, in the direction of the incident light, is manually adjusted to form the smallest light spot on the sample surface. The lateral motions in the x- and y-direction are driven from outside of the UHV chamber by individual step-motors with minimal step-increments of 1.0 and 0.5 pm, respectively. Accumulation of the photoelectron spectrum and the lateral motion of the sample are processed by a microcomputer. The digital oscilloscope trace recorded on a fixed sample point is transferred to the microcomputer and typically summed over 3 laser shots. The bundle of spectral data along a single scan in the ydirection is stored in a disk drive for each incremental step in the x-direction. The time to collect 70*50 data points was about 3 hours, and the sampling rate is mainly limited by the data transfer through the GPIB interface of the present digital oscilloscope. If photoemission intensity at a given energy is of interest, the digital oscilloscope can be replaced with a gated integrator (Stanford Research Systems, SR250) and a computer interface module (SR245). The acquisition time for this system is typically 50 min for 70*50 sampling points, and the sampling rate is then limited by the response of the step-motor controller. The hemispherical energy analyzer can

also be used in this mode with higher energy resolution. The advantage of the TOF analyzer over the hemispherical one is that photoemission intensities at two or more selected energies can be recorded for each laser shot, depending on the number of integrators used. 3.2 Evaluation of the spatial resolution In order to evaluate the spatial resolution of the present apparatus, a striped test sample was prepared [2]. Here, line shaped silver films of 300 pm width aud 200 nm thickness, each separated by 100 pm, were formed by vacuum evaporation on a crystalline silicon base. A photoelectron spectrum of the sample was measured after argon ion sputtering and annealing.

64

2

0

Electron Energy / eV Fig.2. TOP photoelectron spectra of a striped sample measured on (a) silver zone and (b) silicon substrate. The ratio of photoemission intensity A to B is used to reproduce a surface image shown in fig.3.

The spectra from the silver and silicon zones are shown in figs.2(a) and (b), respectively. The spectra were recorded from the Sample irradiation with the s-polarized light of 30” incidence. The silver spectrum is featured by low electron emission at around 3.0 eV energy (indicated by A) and a prominent d-band peak at around 2.0 eV (indicated by B). The photoemission from silicon at energy A is stronger than that from silver. The photoemission intensity I, and I at the respective energy A and B were measure CB with individual

136

gated integrators. The intensity ratio I& is then displayed on a gray scale ranging from white (Si) to black (Ag). The two dimensional image of the striped sample is reproduced in fig. 3. The silicon zone is vi$ble as a white stripe among the black silver zones. The edge line of the silicon stripe is not always sharp and straight but is rather vague and zigzags within a width of about 10 pm. The gray patches on the white silicon stripe is due to silver particles which are formed during the argon ion sputtering of the cleaning process. The value 1,/I, of a selected region of fig.3 is plotted in fig.4 against the sampling position from scanning across the zone boundary. Judging from the variation width, the spatial resolution is better than 4 pm. A less steep variation of the ratio is visible at other part of the stripe, which may be caused by roughness at the boundary. The diffraction limited spot diameter, d, of the WV light is roughly estimated from da%,, where h is the wavelength and nAthe numerical aperture of the lens. The numerical aperture of the

minimal spot of about 3.5 pm. Our experimental resolution of 4 pm is close to the minimal spot size. This shows that diffraction limited focusing is achieved in the present apparatus. The spherical aberration of the lens and/or the mechanical instability of the whole system are estimated to affect the resolution by about 2-3 W.

I

I

0

4clm

1

I

I

I

20 40 distance / pm

1

I

60

Fig.4. The ratio of the photoemission intensities at energies A and B (see fig.2) is plotted against the sample translation distance, The variation of the intensity ratio shows that the spatial resolution is better than 4 pm.

a

I a 4

20fim Fig.3. Image of the striped sample surface reproduced by mapping the intensity ratio A to B indicated in fig.2. The silicon substrate zone (white) is visible among silver zones (black).

present system is estimated to be 0.03 from the beam diameter 4mm4 at the LiF lens. So the VUV light of 0.118 pm wavelength forms a

3.3 Observation of defective surface area The ability of the microscope was demonstrated by an observation of a copper (111) surface. In order to create a defect rich zone, a fine scratch of a few pm depth and about hundred pm width was formed on a polished crystalline surface. Photoelectron spectrum was measured with the light irradiation and electron detection angles of 30” and O”, respectively. Typical photoelectron spectra measured at a (111) crystalline in f&5(a)

zone and at a defective

zone are shown

and (b). The photoelectron spectrum from the crystalline zone is characterized by the sharp surface state structure (A) just below the Fermi level (Ep) and the d-band structure (B). On the other hand, the spectrum for the defective zone shows no A structure, but shows broad structure C around the energy region between Ep and the onset of the d-band structure. These spectra were measured with p-polarized

Time-of-Flight/ua 240 200

16rD

65

4 3 Electron Energy I eV

280

2

Fig.5 Photoelectron spectra of a scratched Cu(ll1) surface measured on (a) crystalline zone and (b) defective zone. The spectrum for the crystalline zone is characterized by a sharp surface state structure labeled A. The d-band structure labeled B is commonly observed for (a) and (h). The broad structure between the Fermi level E and the onset of the d-band structure is charactektic of the defective zone.

light irradiation. The selection of the polarization is important, since the structures A and C become weak in s-polarization spectra. High contrast between the crystalline and defective zones is obtained only with highly polarized light irradiation. A bundle of photoelectron spectra covering two-dimensional mesh point of the surface was collected by scanning the sample with the stepincrement of 5 km in both the x- and ydirections. In order to generate an image from the bundle of spectra, the photoemission intensity at energy B was normalized to that at energy C. The image of the surface is shown in fig.6 by mapping the intensity ratio on a gray scale ranging from white indicating the (111) zone to black indicating the defective zone. The defective zone is displayed in black among the white crystalline zones. Shade of the defective zone is lighter on the boundary region than on the middle

Fig.& Lmageof the scratched copper surface obtained at 5 t,ut~steps reproduced by mapping the intensity ratio at the energies C to B indicated in fig5 The (111) crystalline zone is displayed as white. Photoelectron spectra of the segments labeled a-c are shown in fig.8.

region. The lighter shade indicates that the electronic structure changes gradually from the boundary to the middle of the scratch. It is noted that the contrast of the image arises from the electronic structure of the small surface area. Sensitivity to the electronic structure is the merit of the present microscope. The feature is to be contrasted with other microscopes which are sensitive to surface undulations or to atomic species. The photoelectron spectrum of defective zone shown in flgS@) is interesting because it is fairly different from those of low index surfaces or of polycrystalline surface. The spectrum suggests that the band structure is strongly perturbed by the mechanical stress. It seems as if the shape of the band C has some resemblance to the s-band structure of copper clusters [13]. 3.4 Oxygen adsorption on the defective surface The scratched surface was then exposed to

Fig.7. Image of the same surface area as fig.6 measured after oxygen exposure of 17 L. The gray defedive zone shows apparent narrowing as compared with fig.6. Photoelectron spectra of the segments labeled a-c are shown in figs. The barb points to a light gray segment.

oxygen gas. After an exposure of the room temperature sample to oxygen gas of 17 L (1L = 10-6Torr*sec), the surface image changed as shown in fig.7. The image shows the same surfacearea as that in fig.6. It is noted that the gray defective zone shows obvious narrowing. A higher oxygen exposure up to 300 L resulted in narrowing of the gray more extensive scratched zone. In order to see the cause of the image change, photoelectron spectra are inspected for several sampling segments. The photoelectron spectra shown in fig.8 are obtained at the segments indicated by a-c in figs.6 and 7. The indicated positions are the same for figs.6 and 7. The spectra for clean and oxygen exposed surfaces are shown in left and right columns, respectively. At the segment (a) on a clean crystalline zone, the spectrum shows sharp surface state structure as shown in the left column of fig.8(a). Oxygen exposure only slightly affected the shape of these structures as shown in the right column of figS(a). The spectrum of the segment (b), close to the scratch edge, shows complicated feature as shown in left column of fig.8.(b). After the oxygen exposure, the (b) segment spectrum changes as shown in the right column of fig.8@).

,I O-ADSORBED

CLEAN

(a>

lb)

Cb)

& 65

Electron

I

’’

‘I.1

2

Energy /

eV

,

.

.

1’1’1’1

3

I

’

I

6543 2 Electron Energy / eV

’

Fig, 8. Photoelectron spectra of segments labeled by a-c in figs. 6 and 7. The lefi and right columns are for the clean surface (figd) and the O-adsorbed surface (fig.‘l), respectively.

It resembles that of the segment (a). The change of the spectral feature appears as narrowing of the scratch in the surface image. In the middle of thescratch, the structures between E, and the dband loses intensity by the oxygen exposure as is seen in left and right spectra of fig.8(c). The stability of the crystalline segment (a) is reasonable because the oxygen adsorption probability is quit low at the (111) surface. While at the defective segment (c), high oxygen adsorption resulted in suppression of the photoemission intensity. At the segment (b), a boundary of crystalline and defective zone, photoelectron spectrum for clean surface shows complicated features. This suggests that the segment (b) is a mixture of crystalline facets and defective aggregate. After the oxygen exposure, photoemission from oxygen adsorbed defective aggregate is reduced. Dominant photoemission from the crystalline facets resulted in the fig.8(b) right column spectrum which looks like that of the crystalline surface. As another interpretation of the image change, contribution from adsorption induced step interactions may be considerable. Such a discussion awaits for further experimental investigation with higher spatial resolution. Combination with other microscopic method may be interesting. 4, SPECTROSCOPY OF UNOCCUPIED STATES

NORMALLY

The study of excited electronic states of simple molecules on solid surfaces has become an area of active research, because the excited states play important roles in photoreaction and photodesorption. However, little information is available’ on the excited states of adsorbed molecules. Two-photon photoemission is a promising method to probe the excited states [3,4,14]. In the two-photon photoemission, the first photon is used to populate a normally unoccupied state and the second photon is used to photoionize from this excited state. The method realizes advantages in energy resolution and in polarization selective excitation. On the other hand, its sensitivity is strongly dependent on the lifetime of the excited state. Two-photon photoemission from adsorbed molecule is less

effective than that from clean surfaces, since adsorbed molecule’s excited-state lifetime is quite short. As a result, no two-photon photoemission has been reported for adsorbed molecules on metal surfaces. Here, we report on a measurement of the twophoton photoemission from molecularly adsorbed NO on Cu(ll1) [15]. Adsorption of NO on the Cu(ll1) surface has been studied by various methods. However, the excited state of adsorbed been known as yet. A NO has not photodesorption study suggested desorption from a photoexcited state [16], though details of the excited state were not reported. We determined the energy levels of occupied and unoccupied states of NO adsorbed on Cu(ll1). 4.1 Experiment Two pulsed laser light sources were employed for photoelectron spectroscopy, i.e., the VW light of 10.48 eV photon energy was used for the one-photon photoemission and a tunable UV light for the two-photon photoemission. The former was described in section 2, and the latter was the frequency doubled output of a dye laser pumped with the second harmonic output of the YAG laser. For both lights, the pulse duration was about 5 ns and the diameter of the light beam was about 4 mm. The incidence angles for the VUV and UV lights were 30” and 60”, respectively. The energy distribution was measured for electrons emitted in the normal to the sample surface. The two-photon photoelectron spectrum was measured with W photon fluence of about 0.31.5 mJ/cm’ depending on the photoemission efficiency. Space charge effect &as carefully avoided throughout the measurement. In the present situation to record weak photoelectron signals from the NO induced states, the flight tube was biased 3 V from ground to efficiently collect photoelectrons. The energy resolution was 0.4 and 0.2 eV at energies of 3 and 1 eV, respectively. The signal-to-noise ratio was mainly determined by the fluctuation of laser light in power as well as in temporal/spatial modes. The two-photon photoemission intensity averaged over several laser shots was proportional to the square of the W fluence as expected.

740

The electron kinetic energy is given with reference to the vacuum level of the clean surface. Then, the binding energies are given with reference to the Fermi level so that they are not affected by the work function change due to NO adsorption. The sample was cleaned by repeated cycles of Art sputtering (500 V, 2-4 pA) and annealing at 750 K The temperature was measured with a chromel-alumel thermocouple attached to the sample surface. Surface cleanliness was confirmed by a sharp low energy electron diffraction (LEED) pattern and by a sharp photoelectron spectrum due to the occupied surface state as shown in fig.S(a). Exposure to NO was carried out at 130 K by flowing NO gas through the chamber with continuous pumping by a turbo molecular pump. Since the temperature of 130 K is close to the thermal desorption threshold [16], the sticking probability may be very low. No ordered overlayer was produced as judged by the LEED pattern. 4.2 Exposure dependence Two-photon photoelectron spectra measured with 4.33 eV photon are shown in fig.9 for the clean and NO adsorbed Cu(lll) surfaces. The two-photon photoelectron spectrum for the clean Cu( 111) measured with p-polarized light (fig.g(a)) shows a sharp surface state structure S that originated from the occupied surface state via the image potential state as intermediate. No photoemission occurs with s-polarization as shown in fig.9@). The two-photon photoelectron spectra are in accordance with the reported data [3]. A feature of our spectrum is that no scattered electrons are visible around zero electron energy. Introduction of NO gas by 0.3 L exposure resulted in remarkable intensity reductions of the two-photon photoemission. The spectrum in fig.g(c) was measured with a laser fluence 3 times higher than in the case of fig-g(a) and displayed expansion at 5 times. When the intensity of the surface state structure is normalized to the square of the laser fluence, its intensity is only about l/50 of that in fig.g(a) for the clean surface. It is noted that a new structure indicated by N

clean S-PO1

I

Q

A

11

(d

Qs

65432

N

Ah

“V .,

0.31 P-PO1

+

N

1 Electron Energy / eV

0

Fig.9. lko-photon photoelectron spectra for clean (a,b) and NO-adsorbed Cu(lll) (c,d) measured with 4.33 eV photon. The spectra (c) and (d) are about 50 times expanded compared to (a) (see text). The structures due to the surface state and to the NO-induced state are indicated by S and N. The open triangle shows two-photon photoemission from the Fermi level.

appears in fig.9(c). Increase of exposure from 0.3 to 0.6 L results in no marked intensity increase or reduction as is seen in fig.g(d). At a higher exposure the structure N broadens out and its intensity reduces. These spectral features in fig.9 are unchanged during experiment for several hours. The one-photon photoelectron spectrum with 10.48 eV photon was similar to that in ftgd(a) for clean surface. Introduction of NO gas resulted in intensity decrease of the surface state structure by about half. No significant change was observed for the d-bandstructure. Heating of the 0.3 L exposure surface to room temperature reproduces the one- and twophoton spectra of the clean surface. This result indicates that the adsorption of NO is molecular in the present condition, The NO adsorption on

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fig-lo(a). The surface state structure and the NO induced structure are indicated by S and N. The remarkable features of the p-polarization spectra are summarized as follows: (a) The peak positions of the NO induced structure shift to higher energy as the photon energy hv increases. The peak positions are plotted in fig. 11 against the photon energy. (b) The low energy cutoff of the NO induced structure, indicated by open circles in fig.10, shifts to higher energy with hv and becomes ambiguous at hv > 4.2 eV. The cutoff energy is also plotted in fig.11. (c) At hv > 4.2 eV, the NO induced structure is broadened. Its contour shows round headed shape forming somewhat of a shoulder on the low energy side, and its tail extends to zero electron energy. The spectra measured with s-polarized light (fig.10 (b)) shows no surface state structure. The weak emission below E, is attributed to direct two-photon photoemission from the bulk states of copper.

Cu(ll1) at high exposure is known to be dissociative, resulting in adsorption of oxygen and nitrogen containing species which do not desorb at room temperature. Actually, when similarly heated to room temperature, the surface of an exposure higher than 0.6 L didn’t return to the clean surface. This suggests that the degradation of the N band at an exposure higher than 0.6 L is probably attributed to the dissociative adsorption. 4.3 Photon energy dependence

of the twophoton spectrum The two-photon photoelectron spectrum for

the surface of 0.3 L exposure is measured at a photon fluence of 0.7-1.5 mJ/cm’ for photon energy in the region between 3.7 and 4.5 eV. Typical spectra measured with p- and s-polarized light irradiation are shown in figs.lO(a) and 10(b), respectively. The photoemission is more intense with p-polarization than with s-polarization. The spectra in fig.lO(b) are displayed with an expansion of three times compared with those in :a> p-pal.

S

(b) s-pal.

N

N

photon

energy 4.47 eV

4.34 eV

4.12 eV

4.07 eV

3.84 eV 5432

1

Electron Energy / eV

0543

2

1

0

Electron Energy / eV

Fig.10. Typical two-photon photoelectron spectra for NO-adsorbed Cu(ll1) measured at various photon energies with p- (a) and s-polarized (b) light irradiation. The spectra in @) are expanded three times compared to those in (a). The metal surface state and the NO-induced structures are labeled by S and N, respectively. The low energy cutoff of the N structure and two-photon photoemission from E, are indicated by open circle and triangle, respectively.

742

intermediate state, the final electron energy follows the relation, +0

low energy

E,=hv

cutoff

+ E,,

where E, is the energy of the intermediate state above E,. The process C leads to a electron energy independent of hv. The energies Ei and E are to be determined by plotting the measure3 electron energy against the photon energy.

0.0 [

I

3.8

I

I

4.0

I

I

4.2

I

I

I

4.4

photon energy / eV Fig.11. Go-photon photoelectron energies are plotted against photon energy. The solid and open marks are for p- and s-polarized light irradiation, respectively. The top straight line of slope 2 shows the process A (see text and fig. 12) due to the fixed initial state of 2.3 eV belowEp. The bottom line of slope 1 shows the process B fromthe fixedintermediate state 1.3 eV above E,.

4.4 Discussion Two-photon excitation mechanisms have been classified as follows [3]: (A) excitation of the intermediate state from an occupied surface state followed by ionization with a second photon, (B) excitation into an upper state, relaxation into the fixed intermediate state, and subsequent ionization, and (C) energy pooling via collisions between two electrons in intermediate states. The processes A and B are schematically shown in fig. 12. Characteristics of these processes appear in the hv dependence of the electron kinetic energy. In the process A with the intermediate state broader than the initial state, the final energy of electron Er follows the relation, Ef = 2hv - Ei,

(1)

where E. is the binding energy of the initial state. The energies E and E. are to be measured with respecft to I$. For the process B with ionization from a fixed

(A)

(B)

Fig.12. Schematics of photoemission processes. Process A shows the two-photon process without notable relaxation in the intermediate state. In process B, photoemission occurs from a fixed intermediate state formed by relaxation.

4,4 a Energy levels of adsorbed NO The plot of peak position of the NO-induced structure with respect to hv lies on a straight line of slope 2 as shown in fig.11; thereby confirming that the band originates from a fixed initial state through the process A. The energy of the initial state is determined by using eq.(l) to be 2.3 2 0.1 eV below Ep. The initial state is attributed to the occupied state of the adsorbed NO since it is observable only for the NO-adsorbed surface. It is not attributed to the bulk d-band which has a large state density at higher binding energies. The occupied NO state is shown in fig.13 by a solid bar. The process A is enhanced by a broad excited state which is mediated by photo-excitation from the occupied NO state. This is because if the NO induced structure was produced through nonresonant two-photon process with virtual

143

energy / eV

Evat

Cu(ll1) k/, =0

metal NO induced surf ace state state

Fig.13. Energy level diagram of the NO-adsorbed Cu(lf1). The hatched area for NO-induced state shows the broad unoccupied NO-induced state. Relaxation to bulk states becomes significant at the extended hatched area at 1.9 eV above E,. intermediate states, the contour of the NOinduced structure should be independent of the excitation wavelength. Electrons below the peak energy are due to photoemission after relaxation in the excited state. The low energy cutoff of the NO-induced structure at hv x 4.2 eV indicates that the relaxation terminates at a bottom of the excited state. As shown in fig.11, the low energy cutoff shows linear dependence of slope 1 on photon energy. This indicates that the process B involving a fixed intermediate state is the mechanism for the cutoff formation. By using eq. (2) the fixed intermediate state is determined to be 1.3 %0.1 eV above E . This bottom energy is shown in fig.13 by a sofId bar at the bottom of the hatched area of excited NO state. The low energy cutoff becomes vague and extends to zero kinetic energy at hv > 4.2 eV_ The contour of the NO induced structure becomes broader (fig.lO), and its peak position of ppolarization spectrum deviates from the straight line of slope 2 (fig.11). This deviation suggests that another relaxation channel, which does not terminate at the above mentioned bottom level,

opens up at the high energy part of the excited NO state. Such a relaxation is allowed by coupling of ihe excited NO state with unoccupied bulk states of copper. The opening of the new relaxation channel is located at 1.9 eV above E, where electron is excited by 4.2 eV photon from the occupied NO state 2.3 eV below E, The coupling of the excited NO state with bulk states is shown in fig.13 by an extended hatched area of the excited NO state. The spectra in fig.10 also show that the binding energy of the occupied metal surface state is unchanged by the NO adsorption. The spectra also shows that the image potential state is broadened by the NO adsorption. The broadened image potential state is shown in fig. 13 by a hatched area for metal surface state. 4.4 b Comparison with previous works One-photon photoemission studies have shown that the highest occupied energy level of NO adsorbed on metal surfaces are located at 2-3 eV below EF [17-191. The binding energy of 2.3 eV determined here correlates well with these results. It is noted that use of the two-photon photoemission method is more suitable for the determination of the binding energy. Since the method is selectively sensitive to states localiid at surfaces, the resultant structures are separated from bulk structures. Whereas in the onephoton photoemission method, binding energies are to be determined from small shoulders on intense bulk structures. The unoccupied energy level of NO adsorbed on Cu(ll1) has not been known. The inverse photoemission studies for NO adsorbed on metal surfaces have shown that the lowest unoccupied levels are located at 1.6-1.8 eV above I$ [20,21], being slightly higher than the present value. The comparison is made with reservation since the energy levels for a Cu(ll1) surface need not be similar to those of other metals. Furthermore, the inverse photoemission probes a state formed by addition of one electron. Such a state may be different from the excited state examined in the present case. ln the photodesorption work on a NO covered Cu(ll1) surface, So et al. [16] concluded that direct adsorbate excitation is the principal mechanism for photodesorption at hv > 3.5 eV_

The threshold energy for the direct adsorbate excitation is in close correspondence with the energy difference of 3.6 eV between the occupied and unoccupied NO states determined in the present work. The correspondence, however, awaits for further investigations because we observed no photon induced changes of the surface. 4.4 c Polarization dependence of the NO induced structure Information on symmetry characters of the electronic states of the adsorbed NO should be involved in the polarization dependent spectra shown in fig.10. The light electric field at the surface for p-polarization is estimated to be about 5 times greater than for s-polarization. As a result, if the transition moment between the occupied and unoccupied NO states is normal to the surface, the NO induced structure for ppolarization should be 25 or more times larger than that for s-polarization. The present result in fig.10 shows that the p-polarization spectra are only about 3 times larger than the s-polarization. Hence, the transition moment is considered to be strongly tilled. If the transition moment is parallel to the molecular axis, we expect that configuration of NO adsorption is highly tilted. The assumption of the parallel transition is reasonable if both the occupied and unoccupied states are derived from the 2x orbital of NO molecule.

5. MICROSCOPY OF EXCITED STATES In the former section, the excited state induced by adsorbate was measured without spatial resolution. We thought that the Cu(ll1) surface was uniformly covered with NO molecules. But molecular adsorption is not necessarily uniform at some conditions. For example, islands of adsorbate shows complicated spatiotemporal evolution on a well prepared Pt surface [12]. Such an island formation of micrometer size has not been well studied as yet. Moreover, since real catalytic materials are not single crystals, spatially resolved investigation of the excited state is desirable. This leads to a development of a spectromicroscopy based on

two-photon photoelectron spectroscopy. Here, we will report a preliminary work on microscopic observation of the image potential state of a copper surface. 5.1 Experimental’procedure The experimental procedure is similar to that described in section 3. The VW generation cell and the prism chamber were removed and the second harmonic output of the dye laser was introduced from the port. The UV light was focused by a combination of a quartz lens and the LiF lens which is the same as that used to focus the WV light. Then the spot diameter was about 7w. The sample was again the Cu(ll1) disk on which a fine scratch was formed. The wavelength was tuned to 280 nm which is close to the resonance between the occupied surface state and the lowest image potential state [3]. The power of the light was typically 5 #/pulse, corresponding the flueace of 10 J/cm’. Such a high power density was necessary to keep enough signal-tonoise ratio. The signal intensity measured by the TOF energy analyzer was comparable to that in sections 3 and 4. 5.2 Result The two-photon photoelectron spectra at the crystalline and defective zones are shown in figs.l4(a)-(c). A sharp structure and a broad low energy structure are labeled by A and B. The structure A is due to the two-photon photoemission from the occupied surface state enhanced via the image potential state. The structure B originated from scattered electrons in the bulk. The surface state structure A loses intensity with the s-polarized light irradiation in accordance with the selection rule. While, the low energy structure B is insensitive to the light polarization. The spectrum for crystalline zone fig.l4(a) should be much the same as that in fig.g(a), but slight differences are detected. The surface state structure is about two times wider than that of fig.9 and its peak position is slightly shifted to higher energy. The broad structure B is absent in fig.g(a), but is visible even in fig.l4(a). These differences are due to the high light power density.

1

I I,,,

I

I

I

I

64 2 0 Electron Energy / eV Fig. 14. Two-photon photoelectron spectra measured at various segment of the scratched Cu(111) surface.

The two-photon photoelectron spectrum of defective zone frg.l4(b) shows weak surface state structure A and intense low energy structure B. The low intensity of the surface state structure is reasonable for the defective area. Such a spectrum as fig.14 @) is often observed for rough surfaces. The structure B is enhanced by the high light power density employed iu this experiment. The intensity of the low energy structure becomes very large at some positions as shown in fig. 14(c). Such a highly intense low energy structure is not a typical case of the twophotoelectron spectrum for clean surfaces. Some special conditions should have been created on the surface. The image of the surface on the basis of the two-photon photoemission was measured as in section 3: the two-photon photoelectron spectrum was measured at every two-dimensional mesh point of the surface, and the intensity ratio of A to B was calculated. The image in fig. 15 was

I 50&N

Fig.15 Image of the surface based on two-photon photoemission. The (111) zone is displayed as white and defective area as gray. The area surrounded by a dotted line shows the intense structure B as fig.l4(c). Photoelectron .spectra of the segments a-c are shown in fig.14.

reproduced by mapping the intensity ratio A/B from white for crystalline zone to black for defective zone. The photoelectron spectra in fig.14 were measured at the segments labeled a-c in fig.15 The surface state structure A is sharp and intense on the white regions. Among the white crystalline zones, dark defective zones are visible in fig.15. The highly intense structure 3 as fig.l4(c) is observed on the black zone surrounded by a white dotted line in fig.15. The zone is a highly defective island. On two liueshaped regions of lighter shade, two-photon photoelectron spectrum shows features as fig.l4(b). The shaded lines are the defective zones formed around the scratch. The shaded line-shaped zones are connected to the highly defective island. The image in fig.15 suggests that the highly defective island is composed of metal particles which are formed when we scratched the surface. Scattering of incident photon as well as

746

photoelectrons may result in the highly intense low energy structure as shown in fig. 14(c). The image in fig.15 shows that the image potential state is selectively observed at crystalline surface. This demonstrates the feasibility of the spectromicroscopy of excited state. The high laser power density caused the slight spectral broadening. The problem is not very serious at the present stage, and it will be overcome by optimizing the fluence of pump and probe laser lights. The spatial distribution of the excited state will give much information on chemical reactions on the surfaces. Moreover, as noted in section 4.3, adsorbed molecules are more selectively observed in the two-photon photoemission than in the one-photon photoemission. We expect that the two-photon photoelectron spectromicroscopy will become a powerful tool to probe adsorbed molecules. 6. Summary The presented apparatus for photoelectron microscopy combined high spatial resolution with high energy resolution. If desired, the spatial resolution of sub-micrometers regime is feasible by employment of higher numerical aperture optics. It is also noted that the focused light irradiation is non-destructive, even for adsorbed molecules. Since the present microscope allows for detailed inspection of electronic structure, it is highly suitable for spatially resolved investigations of adsorbed phases. The two-photon photoelectron spectroscopy is an another feature of the present apparatus. We observed for the first time the occupied and unoccupied states of adsorbed NO. Polarization dependence of the two-photon photoemission suggests that NO molecule is adsorbed in highly tilted configuration. Further information on the symmetry of the states will be obtained from two-color experiment in which the polarizations of the pump and probe lights are controlled independently. The above two features of the apparatus can be combined as the two-photon photoelectron spectromicroscopy. Such an extension is on progress and will become a powerful tool for the investigation of chemical reactions on surfaces.

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