- Email: [email protected]
Time-resolved photoemission microspectroscopy based on fs-VUV laser light
Surface Science 507–510 (2002) 434–440 www.elsevier.com/locate/susc
Time-resolved photoemission microspectroscopy based on fs-VUV laser light T. Munakata a
a,*
, T. Masuda b, N. Ueno b, A. Abdureyim a, Y. Sonoda
a
The Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako 351-0198, Japan Graduate School of Science and Technology, Chiba University, Inage-ku, Chiba 263-8522, Japan
b
Abstract A novel photoemission microspectrometer based on focused VUV coherent radiation is presented for simultaneous realization of <100 fs time resolution, <30 meV energy resolution, and sub-micrometer lateral resolution. VUV light at 140 nm wavelength (8.9 eV photon energy) is generated by frequency upconversion of a regeneratively amplified Ti:sapphire laser of 100 fs pulse duration. The VUV light is focused on a sample surface by a Schwarzschild objective of 0.29 numerical aperture. Photoelectrons are detected by a time-of-flight electron spectrometer. The width of the Fermi edge feature of cooled (30 K) polycrystalline copper was observed to be narrower than 40 meV. The energy resolution is close to the Fourier-transform-limited bandwidth of the laser light. Two-photon photoemission with 420 nm light revealed that the lateral resolution is very close to the diffraction-limited spot diameter of 0.9 lm. The microspectrometer is also equipped with a photoemission electron microscope for time-resolved sub-100 nm imaging. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Electron microscopy; Laser methods; Photoelectron spectroscopy; Surface electronic phenomena (work function, surface potential, surface states, etc.)
1. Introduction Recently, lateral resolution of photoemission spectroscopy has been rapidly improving by use of several types of energy filtered electron emission microscopes or focused light sources [1–5]. In addition, laser-based photoemission spectroscopy facilitates femtosecond time resolution, revealing dynamics of electrons and atoms on surfaces [6– 10]. Combination of lateral and temporal resolutions is the next step to be realized. Here, we * Corresponding author. Tel.: +81-48-467-4047; fax: +81-48467-4046. E-mail address: [email protected] (T. Munakata).
present the first report of our plan to develop a time-resolved photoemission microspectrometer based on focused fs-VUV laser radiation. We focus our interest on the energy region of the valence band. Upon adsorption of molecules on a surface, adsorption-induced occupied and unoccupied states are formed in the vicinity of the Fermi level (EF ) [11]. The occupied adsorption-induced states can be measured by one-photon photoemission with VUV light of about 10 eV photon energy. The unoccupied adsorption-induced states can be probed by two-photon photoemission (2PPE) with combination of a wavelength tunable pump light and the VUV probe light. The photoemission features from surface states or adsorption-induced states frequently appear as sharp
0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 2 ) 0 1 2 8 2 - 7
T. Munakata et al. / Surface Science 507–510 (2002) 434–440
peaks whose widths are <100 meV [11,12]. Thus an energy resolution better than 30 meV is required. This energy resolution can be achieved by employing a titanium sapphire (Ti:Sa) laser of 100 fs pulse duration, whose Fourier-transform-limited bandwidth is about 18 meV. Among many selections of laser pulse width, repetition rate and wavelength, the Ti:Sa laser of 100 fs pulse duration is advantageous in stability and in wavelength tunability. VUV light can be generated by frequency upconversion. Lateral resolution can be achieved either by an image magnification of photoemission or by microfocusing of the VUV light. While the imaging method is advantageous in lateral resolution, energy resolution of 30 meV is highly difficult to achieve in a magnification electron lens system [1,4]. Thus we fix the specifications of our system as follows: (a) VUV light generated by frequency upconversion of a 100 fs Ti:Sa laser is used as the light source, (b) the photoelectron energy resolution should be better than 30 meV, and (c) lateral resolution is determined by the light spot diameter which is close to the diffraction limit. The idea is similar to our previous ns-laser based photoemission microspectrometer [2], but the present plan aims the resolutions ap-
435
proaching the values of physical limitations. Lateral resolution higher than the diffraction-limited light spot diameter is also attempted for future improvement.
2. Experimental setup A schematic of the apparatus is shown in Fig. 1. The apparatus consists of a VUV light source and a measurement chamber containing a timeof-flight (TOF) electron spectrometer, a photoelectron emission microscope (PEEM) and a sample stage. The measurement chamber and a preparation chamber are evacuated, respectively, by turbo pumps and ion pumps to 10 8 Pa background pressure. The sample in the measurement chamber can be cooled to 30 K by thermal conduction from a low-vibration He refrigerator. The VUV light was generated by frequency tripling the second harmonic output of a regeneratively amplified Ti:Sa laser (Coherent, Mira 900F and RegA 900) operated at a 250 KHz repetition rate. The high repetition rate is essential in photoemission spectroscopy. Irradiation of laser
Fig. 1. Schematics of the apparatus for photoemission microspectroscopy.
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pulses inevitably generates a large number of photoelectrons within the laser pulse width. The dense electron bunch frequently causes a spacecharge effect at the sample surface and within an electron energy analyzer [2]. The space-charge effect can be reduced by employing the high-repetition laser. The wavelength was tuned to 840 nm so that its sixth harmonics was at the short wavelength region of the Xe 6s level, increasing the VUV generation efficiency. By focusing the 840 nm light with a 70 cm focal length lens into a LBO crystal of 1.5 mm thickness, the second harmonic output at 420 nm was typically 300 mW. The 420 nm light was focused into a frequency conversion cell containing Xe gas. VUV light at 140 nm was generated as the third harmonics of the input light. The transmitted light was then dispersed through a LiF prism of 20° apex angle to eliminate the input visible light. The VUV light enters the measurement chamber through a LiF window, and is then focused onto a sample surface by a Schwarzschild objective of 0.29 numerical aperture. The objective consists of a pair of aluminum-coated mirrors mounted in a Cu–Be housing. The focus point is 40 mm from the front surface of the objective. The diffraction-limited spot diameter of the 140 nm wavelength light is estimated to be 0.3 lm. This determines the goal for the lateral resolution of the present system. Photoelectrons emitted from the sample are accelerated to a TOF tube which is typically biased by 0.5–1.0 V from the chamber ground. Passing through the TOF tube of 165 mm length, photoelectrons are detected by a microchannel plate (MCP) of 40 mm diameter. The entrance aperture of the TOF tube is 15 mm from the sample. The exit aperture of the TOF tube is covered with a sheet of electroformed Ni mesh of 90% transmission to suppress stray electric field from the MCP. The signal from the MCP is fed to a fast preamp of 1.3 GHz bandwidth and is detected by a constant fraction discriminator and a picosecond time analyzer (EG&G, 9308). The time analyzer is triggered by the output of a fast photodiode whose rise time is 1 ns. A commercial PEEM (Omicron) is installed at the opposite side of the TOF analyzer. By switching the light path, a PEEM image is gener-
ated by the femtosecond light. The PEEM is used for characterization of the sample as well as for time-resolved imaging with high spatial resolution. A standard UHV manipulator (VG, OMNIAX) was modified and used as the sample stage. In order to improve the resolution of sample translation, two sets of step motors and glass scales were mounted on the translation stage of the manipulator, outside of the UHV chamber. The motor actively controls the position of the stage through feedback from the glass scale. The nominal resolution of the closed loop stage control is 0.1 lm. The stage allows reproducible observation after translation of a sample as far as 100 mm.
3. Results 3.1. TOF electron energy analyzer The photoemission spectrum of a clean polycrystalline copper sample is shown in Fig. 2. Here, the LiF prism and the Schwarzschild objective were not used. The stepwise feature due to the Fermi edge is observed at a energy of 4 eV, and the d-band peak, at 1.8 eV. The features below 1 eV are due to 2PPE by the input 420 nm light. A magnified view around EF is shown in the inset. The 10–90% width of the rising edge is 100 meV at room temperature and 40 meV for the sample cooled to 30 K. Taking into account the width of the Fermi–Dirac distribution at 30 K (10 meV) and the spectral width of 100 fs pulsed light (18 meV), the energy resolution of the present TOF analyzer is estimated to be 34 meV. In actuality, we believe the resolution to be better than 20 meV, because the tail of the unfocused VUV light irradiated a part of the sample holder, making the flight length slightly ambiguous. Overall time resolution of the TOF system is estimated by observing the reflected VUV light from the sample. The full width at half maximum of the reflected light signal was 0.2 ns. This width is mainly determined by the rise times of the MCP, preamp, and PIN photodiode. From the TOF at EF , 200 ns, the resolving power of the TOF system is estimated to be 1000, corresponding to 8 meV resolution at 4 eV energy. Thus we expect that the
T. Munakata et al. / Surface Science 507–510 (2002) 434–440
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Fig. 2. TOF photoemission spectrum of polycrystalline copper. The magnified view around EF is shown in the inset.
bandwidth of the VUV light can be measured with the present system. By optimizing the Xe pressure, the intensity of the VUV light can be increased by orders of magnitude. Fig. 3 shows spectra measured at different VUV intensities. The intensity of the 2PPE component at 430 ns TOF is the same for all the spectra. The Fermi edge feature should have the appearance of a step function, but photoemission intensity below EF rapidly decreases as the VUV intensity increases. This deformation of the TOF spectrum is caused by the dead time of the time analyzer. Within the pulse pair resolution of 50 ns, the time analyzer fails to count a significant number of electron signals. The electron at a TOF of T1 can be counted when no electron is detected in the time span from (T1 Dpair ) to T1 , where Dpair is the pulse-pair resolution. Thus the spectrum is deformed to be the decreasing function of TOF. The counting error was simulated by randomly generating photoelectron pulses at TOF’s larger than EF . The thin line overlapping the topmost spectrum is the result of the simulation. The sim-
ulation reproduces the decay of the electron signal after the sharp rising edge as well as a small hump (marked by an arrow) at 50 ns delay from the EF feature. The hump is due to a partial recovery of the dead time. Thus a more efficient counting system is highly desirable. For a laser with a low repetition rate such as 10 Hz, analog measurement with an oscilloscope may be more advantageous than the fast counting system, although the linearity and dynamic range of analog measurement cannot be as high as those of the counting system. We examined the space charge effect caused by pulsed light irradiation. The space-charge effect results in the broadening and shift of the photoemission spectrum. The effect occurs both at the sample surface and inside of the energy analyzer. Since the width of the EF feature shown in Fig. 2 is not very sensitive to the VUV intensity, the spacecharge effect at the surface is not severe. On the other hand, a broadening of the spectrum was observed by exchanging the TOF analyzer with a hemispherical energy analyzer (VG, 100 AX) of 100 meV energy resolution. A count rate of
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trum measured with the hemispherical energy analyzer becomes broader as the counting rate exceeds 104 Hz. From a comparison with the result obtained with the TOF analyzer, the broadening is probably caused by the space-charge effect within the hemispherical energy analyzer. The TOF analyzer, in which no electron focusing is employed, is less sensitive to the space-charge effect than the hemispherical analyzer. The TOF energy analyzer is advantageous not only in simultaneous analysis of electrons with different kinetic energies but also in reducing the space-charge effect. 3.2. Lateral resolution
Fig. 3. Photoemission spectra measured with different VUV intensities. The VUV intensity increases from (a) to (d) by increasing the Xe pressure. The thin curve overlapping trace (d) is the result of simulation of the counting error.
3 104 Hz was obtained at the d-band peak under conditions similar to those in Fig. 3(d). The spec-
The lateral resolution of the present system is measured by focusing 420 nm laser light on a striped sample. For comparison, a PEEM image of the sample measured with 210 nm (fourth harmonics of Ti:Sa) light is shown on the right side of Fig. 4. Stripes of Pd were formed on a silicon base with a 5.9 lm period. The Pd stripes are visible as bright lines, and the Si base, as dark lines. The 2PPE spectrum measured with 420 nm light is shown on the left side of Fig. 4. The 2PPE intensity from the Si part is very weak, especially in the energy region of the bulk band gap [12]. On the other hand, the 2PPE spectrum of the Pd part has
Fig. 4. Right side shows a PEEM image of the Pd/Si sample measured with 210 nm light (fourth of Ti:Sa). Left side shows the TOF2PPE spectra measured with 420 nm light (a) at the bright Pd stripe and (b) at the dark Si base. 2PPE intensity in the shaded region was detected for the measurement in Fig. 5.
T. Munakata et al. / Surface Science 507–510 (2002) 434–440
439
Fig. 5. Dots show 2PPE intensity measured during line scans across the Pd/Si stripes. Thin line is the intensity profile of the PEEM image at the line in Fig. 4. The lateral resolution is better than 1 lm, confirming diffraction-limited focusing.
a sharp, intense peak due to the first image potential state [13]. The electron energy scale is not drawn in Fig. 4 because the work function of the sample was not calibrated. A 10 ns time gate of a boxcar integrator is fixed at the TOF indicated by the shade region in Fig. 4. During the measurement of the 2PPE signal in the time gate, the sample was line-scanned in 0.1 lm steps across the stripe. The result is shown in Fig. 5. The Pd parts of high 2PPE intensity are clearly discriminated from the low 2PPE intensity Si parts. An intensity profile of the PEEM image in Fig. 4 is also shown in Fig. 5. Although the measured sample positions are different for 2PPE and PEEM, the 2PPE intensity variation is satisfactorily similar to the PEEM profile. Even small intensity undulations at Si parts are similar for the 2PPE and PEEM traces. Judging from stepwise features of the 2PPE intensity variation, the lateral resolution is better than 1.0 lm. A shaded stripe of 1.0 lm width is drawn in Fig. 5 as a visual guide. Taking the diffraction-limited light spot diameter to be 0.61(k=NA) where k is the wavelength and
NA is the numerical aperture, the limit of the resolution is estimated to be 0.9 lm. This confirms that diffraction-limited focusing is achieved in the present system.
4. Outlook The present system has achieved photoemission spectroscopy in a <1 lm area with <40 meV energy resolution. Time resolution of 100 fs is an inherent characteristic of the light source. The system is characterized by energy/time resolutions that are close to the Fourier limit and by the lateral resolution that is close to the diffraction limit. By improvement of the mechanical stability of the apparatus and the sample stage, the resolutions will more closely approach the values limited by the physical principles. Recently, the sampling time was shortened to 0.4 s for the photoemission spectrum of a signal-to-noise ratio of five measured with the 140 nm light. The rapid sampling facilitates two-dimensional mapping of the
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electronic structure. When lateral resolution higher than the wavelength is required, the PEEM will be used. Though the PEEM has no energyresolution capability, complementary use with the micro-spot TOF system will be helpful for revealing the dynamics of electrons and atoms in mesoscopic regions. The performance of laboratory-based microbeam photoemission spectrometers has been limited by the incoherent nature of the conventional He I light source [1]. The high repetition rate VUV laser light source opens up new possibilities for laboratory-based photoemission spectroscopy with high performance in energy, lateral and temporal resolutions. The wavelength can be extended up to the X-ray region by employing highpeak-power lasers [7,10].
Acknowledgements This study was supported by the Special Coordination Funds of Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government.
References [1] H. Ade (Ed.), J. Electron Spectrosc. Relat. Phenom. 84 (1997) 1. [2] T. Munakata, E. Ishikawa, I. Kinoshita, T. Kasuya, Rev. Sci. Instrum. 62 (1991) 2572. [3] S. Heun, T. Schmidt, B. Ressel, E. Bauer, K.C. Prince, Synchrotron Radiat. News 12 (1999) 25. [4] H. Spiecker, O. Schmidt, Ch. Ziethen, D. Menke, U. Kleineberg, R.C. Ahuja, M. Merkel, U. Heinzmann, G. Sch€ onhense, Nucl. Instr. and Meth. A 406 (1998) 499. [5] H. Yasufuku, M. Okumura, S. Kera, K.K. Okudaira, Y. Harada, N. Ueno, J. Electron Spectrsc. Relat. Phenom. 114–116 (2001) 1025. [6] H. Petek, H. Nagano, M.J. Weida, S. Ogawa, J. Phys. Chem. A 104 (2000) 10234. [7] R. Haight, Chem. Phys. 205 (1996) 231. [8] H. Petek, A.F. Heinz, Chem. Phys. 251 (1–3) (2000) (Special issue). [9] E. Knoesel, A. Hotzel, M. Wolf, Phys. Rev. B 57 (1998) 12812. [10] P. Siffalovic, M. Drescher, M. Spieweck, T. Wiesenthal, Y.C. Lim, R. Weidner, A. Elizarov, U. Heinzmann, Rev. Sci. Instrum. 72 (2001) 30. [11] T. Munakata, Surf. Sci. 454–456 (2000) 118. [12] K. Shudo, T. Munakata, Phys. Rev. B 63 (2001) 125324. [13] W. Steinmann, T. Fauster, in: H.L. Dai, W. Ho (Eds.), Advanced Series in Physical Chemistry, vol. 5, Laser Spectroscopy and Photochemistry on Metal Surfaces, World Scientific, Singapore, 1995, p. 184.
Time-resolved photoemission microspectroscopy based on fs-VUV laser light T. Munakata a
a,*
, T. Masuda b, N. Ueno b, A. Abdureyim a, Y. Sonoda
a
The Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako 351-0198, Japan Graduate School of Science and Technology, Chiba University, Inage-ku, Chiba 263-8522, Japan
b
Abstract A novel photoemission microspectrometer based on focused VUV coherent radiation is presented for simultaneous realization of <100 fs time resolution, <30 meV energy resolution, and sub-micrometer lateral resolution. VUV light at 140 nm wavelength (8.9 eV photon energy) is generated by frequency upconversion of a regeneratively amplified Ti:sapphire laser of 100 fs pulse duration. The VUV light is focused on a sample surface by a Schwarzschild objective of 0.29 numerical aperture. Photoelectrons are detected by a time-of-flight electron spectrometer. The width of the Fermi edge feature of cooled (30 K) polycrystalline copper was observed to be narrower than 40 meV. The energy resolution is close to the Fourier-transform-limited bandwidth of the laser light. Two-photon photoemission with 420 nm light revealed that the lateral resolution is very close to the diffraction-limited spot diameter of 0.9 lm. The microspectrometer is also equipped with a photoemission electron microscope for time-resolved sub-100 nm imaging. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Electron microscopy; Laser methods; Photoelectron spectroscopy; Surface electronic phenomena (work function, surface potential, surface states, etc.)
1. Introduction Recently, lateral resolution of photoemission spectroscopy has been rapidly improving by use of several types of energy filtered electron emission microscopes or focused light sources [1–5]. In addition, laser-based photoemission spectroscopy facilitates femtosecond time resolution, revealing dynamics of electrons and atoms on surfaces [6– 10]. Combination of lateral and temporal resolutions is the next step to be realized. Here, we * Corresponding author. Tel.: +81-48-467-4047; fax: +81-48467-4046. E-mail address: [email protected] (T. Munakata).
present the first report of our plan to develop a time-resolved photoemission microspectrometer based on focused fs-VUV laser radiation. We focus our interest on the energy region of the valence band. Upon adsorption of molecules on a surface, adsorption-induced occupied and unoccupied states are formed in the vicinity of the Fermi level (EF ) [11]. The occupied adsorption-induced states can be measured by one-photon photoemission with VUV light of about 10 eV photon energy. The unoccupied adsorption-induced states can be probed by two-photon photoemission (2PPE) with combination of a wavelength tunable pump light and the VUV probe light. The photoemission features from surface states or adsorption-induced states frequently appear as sharp
0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 2 ) 0 1 2 8 2 - 7
T. Munakata et al. / Surface Science 507–510 (2002) 434–440
peaks whose widths are <100 meV [11,12]. Thus an energy resolution better than 30 meV is required. This energy resolution can be achieved by employing a titanium sapphire (Ti:Sa) laser of 100 fs pulse duration, whose Fourier-transform-limited bandwidth is about 18 meV. Among many selections of laser pulse width, repetition rate and wavelength, the Ti:Sa laser of 100 fs pulse duration is advantageous in stability and in wavelength tunability. VUV light can be generated by frequency upconversion. Lateral resolution can be achieved either by an image magnification of photoemission or by microfocusing of the VUV light. While the imaging method is advantageous in lateral resolution, energy resolution of 30 meV is highly difficult to achieve in a magnification electron lens system [1,4]. Thus we fix the specifications of our system as follows: (a) VUV light generated by frequency upconversion of a 100 fs Ti:Sa laser is used as the light source, (b) the photoelectron energy resolution should be better than 30 meV, and (c) lateral resolution is determined by the light spot diameter which is close to the diffraction limit. The idea is similar to our previous ns-laser based photoemission microspectrometer [2], but the present plan aims the resolutions ap-
435
proaching the values of physical limitations. Lateral resolution higher than the diffraction-limited light spot diameter is also attempted for future improvement.
2. Experimental setup A schematic of the apparatus is shown in Fig. 1. The apparatus consists of a VUV light source and a measurement chamber containing a timeof-flight (TOF) electron spectrometer, a photoelectron emission microscope (PEEM) and a sample stage. The measurement chamber and a preparation chamber are evacuated, respectively, by turbo pumps and ion pumps to 10 8 Pa background pressure. The sample in the measurement chamber can be cooled to 30 K by thermal conduction from a low-vibration He refrigerator. The VUV light was generated by frequency tripling the second harmonic output of a regeneratively amplified Ti:Sa laser (Coherent, Mira 900F and RegA 900) operated at a 250 KHz repetition rate. The high repetition rate is essential in photoemission spectroscopy. Irradiation of laser
Fig. 1. Schematics of the apparatus for photoemission microspectroscopy.
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T. Munakata et al. / Surface Science 507–510 (2002) 434–440
pulses inevitably generates a large number of photoelectrons within the laser pulse width. The dense electron bunch frequently causes a spacecharge effect at the sample surface and within an electron energy analyzer [2]. The space-charge effect can be reduced by employing the high-repetition laser. The wavelength was tuned to 840 nm so that its sixth harmonics was at the short wavelength region of the Xe 6s level, increasing the VUV generation efficiency. By focusing the 840 nm light with a 70 cm focal length lens into a LBO crystal of 1.5 mm thickness, the second harmonic output at 420 nm was typically 300 mW. The 420 nm light was focused into a frequency conversion cell containing Xe gas. VUV light at 140 nm was generated as the third harmonics of the input light. The transmitted light was then dispersed through a LiF prism of 20° apex angle to eliminate the input visible light. The VUV light enters the measurement chamber through a LiF window, and is then focused onto a sample surface by a Schwarzschild objective of 0.29 numerical aperture. The objective consists of a pair of aluminum-coated mirrors mounted in a Cu–Be housing. The focus point is 40 mm from the front surface of the objective. The diffraction-limited spot diameter of the 140 nm wavelength light is estimated to be 0.3 lm. This determines the goal for the lateral resolution of the present system. Photoelectrons emitted from the sample are accelerated to a TOF tube which is typically biased by 0.5–1.0 V from the chamber ground. Passing through the TOF tube of 165 mm length, photoelectrons are detected by a microchannel plate (MCP) of 40 mm diameter. The entrance aperture of the TOF tube is 15 mm from the sample. The exit aperture of the TOF tube is covered with a sheet of electroformed Ni mesh of 90% transmission to suppress stray electric field from the MCP. The signal from the MCP is fed to a fast preamp of 1.3 GHz bandwidth and is detected by a constant fraction discriminator and a picosecond time analyzer (EG&G, 9308). The time analyzer is triggered by the output of a fast photodiode whose rise time is 1 ns. A commercial PEEM (Omicron) is installed at the opposite side of the TOF analyzer. By switching the light path, a PEEM image is gener-
ated by the femtosecond light. The PEEM is used for characterization of the sample as well as for time-resolved imaging with high spatial resolution. A standard UHV manipulator (VG, OMNIAX) was modified and used as the sample stage. In order to improve the resolution of sample translation, two sets of step motors and glass scales were mounted on the translation stage of the manipulator, outside of the UHV chamber. The motor actively controls the position of the stage through feedback from the glass scale. The nominal resolution of the closed loop stage control is 0.1 lm. The stage allows reproducible observation after translation of a sample as far as 100 mm.
3. Results 3.1. TOF electron energy analyzer The photoemission spectrum of a clean polycrystalline copper sample is shown in Fig. 2. Here, the LiF prism and the Schwarzschild objective were not used. The stepwise feature due to the Fermi edge is observed at a energy of 4 eV, and the d-band peak, at 1.8 eV. The features below 1 eV are due to 2PPE by the input 420 nm light. A magnified view around EF is shown in the inset. The 10–90% width of the rising edge is 100 meV at room temperature and 40 meV for the sample cooled to 30 K. Taking into account the width of the Fermi–Dirac distribution at 30 K (10 meV) and the spectral width of 100 fs pulsed light (18 meV), the energy resolution of the present TOF analyzer is estimated to be 34 meV. In actuality, we believe the resolution to be better than 20 meV, because the tail of the unfocused VUV light irradiated a part of the sample holder, making the flight length slightly ambiguous. Overall time resolution of the TOF system is estimated by observing the reflected VUV light from the sample. The full width at half maximum of the reflected light signal was 0.2 ns. This width is mainly determined by the rise times of the MCP, preamp, and PIN photodiode. From the TOF at EF , 200 ns, the resolving power of the TOF system is estimated to be 1000, corresponding to 8 meV resolution at 4 eV energy. Thus we expect that the
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Fig. 2. TOF photoemission spectrum of polycrystalline copper. The magnified view around EF is shown in the inset.
bandwidth of the VUV light can be measured with the present system. By optimizing the Xe pressure, the intensity of the VUV light can be increased by orders of magnitude. Fig. 3 shows spectra measured at different VUV intensities. The intensity of the 2PPE component at 430 ns TOF is the same for all the spectra. The Fermi edge feature should have the appearance of a step function, but photoemission intensity below EF rapidly decreases as the VUV intensity increases. This deformation of the TOF spectrum is caused by the dead time of the time analyzer. Within the pulse pair resolution of 50 ns, the time analyzer fails to count a significant number of electron signals. The electron at a TOF of T1 can be counted when no electron is detected in the time span from (T1 Dpair ) to T1 , where Dpair is the pulse-pair resolution. Thus the spectrum is deformed to be the decreasing function of TOF. The counting error was simulated by randomly generating photoelectron pulses at TOF’s larger than EF . The thin line overlapping the topmost spectrum is the result of the simulation. The sim-
ulation reproduces the decay of the electron signal after the sharp rising edge as well as a small hump (marked by an arrow) at 50 ns delay from the EF feature. The hump is due to a partial recovery of the dead time. Thus a more efficient counting system is highly desirable. For a laser with a low repetition rate such as 10 Hz, analog measurement with an oscilloscope may be more advantageous than the fast counting system, although the linearity and dynamic range of analog measurement cannot be as high as those of the counting system. We examined the space charge effect caused by pulsed light irradiation. The space-charge effect results in the broadening and shift of the photoemission spectrum. The effect occurs both at the sample surface and inside of the energy analyzer. Since the width of the EF feature shown in Fig. 2 is not very sensitive to the VUV intensity, the spacecharge effect at the surface is not severe. On the other hand, a broadening of the spectrum was observed by exchanging the TOF analyzer with a hemispherical energy analyzer (VG, 100 AX) of 100 meV energy resolution. A count rate of
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trum measured with the hemispherical energy analyzer becomes broader as the counting rate exceeds 104 Hz. From a comparison with the result obtained with the TOF analyzer, the broadening is probably caused by the space-charge effect within the hemispherical energy analyzer. The TOF analyzer, in which no electron focusing is employed, is less sensitive to the space-charge effect than the hemispherical analyzer. The TOF energy analyzer is advantageous not only in simultaneous analysis of electrons with different kinetic energies but also in reducing the space-charge effect. 3.2. Lateral resolution
Fig. 3. Photoemission spectra measured with different VUV intensities. The VUV intensity increases from (a) to (d) by increasing the Xe pressure. The thin curve overlapping trace (d) is the result of simulation of the counting error.
3 104 Hz was obtained at the d-band peak under conditions similar to those in Fig. 3(d). The spec-
The lateral resolution of the present system is measured by focusing 420 nm laser light on a striped sample. For comparison, a PEEM image of the sample measured with 210 nm (fourth harmonics of Ti:Sa) light is shown on the right side of Fig. 4. Stripes of Pd were formed on a silicon base with a 5.9 lm period. The Pd stripes are visible as bright lines, and the Si base, as dark lines. The 2PPE spectrum measured with 420 nm light is shown on the left side of Fig. 4. The 2PPE intensity from the Si part is very weak, especially in the energy region of the bulk band gap [12]. On the other hand, the 2PPE spectrum of the Pd part has
Fig. 4. Right side shows a PEEM image of the Pd/Si sample measured with 210 nm light (fourth of Ti:Sa). Left side shows the TOF2PPE spectra measured with 420 nm light (a) at the bright Pd stripe and (b) at the dark Si base. 2PPE intensity in the shaded region was detected for the measurement in Fig. 5.
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Fig. 5. Dots show 2PPE intensity measured during line scans across the Pd/Si stripes. Thin line is the intensity profile of the PEEM image at the line in Fig. 4. The lateral resolution is better than 1 lm, confirming diffraction-limited focusing.
a sharp, intense peak due to the first image potential state [13]. The electron energy scale is not drawn in Fig. 4 because the work function of the sample was not calibrated. A 10 ns time gate of a boxcar integrator is fixed at the TOF indicated by the shade region in Fig. 4. During the measurement of the 2PPE signal in the time gate, the sample was line-scanned in 0.1 lm steps across the stripe. The result is shown in Fig. 5. The Pd parts of high 2PPE intensity are clearly discriminated from the low 2PPE intensity Si parts. An intensity profile of the PEEM image in Fig. 4 is also shown in Fig. 5. Although the measured sample positions are different for 2PPE and PEEM, the 2PPE intensity variation is satisfactorily similar to the PEEM profile. Even small intensity undulations at Si parts are similar for the 2PPE and PEEM traces. Judging from stepwise features of the 2PPE intensity variation, the lateral resolution is better than 1.0 lm. A shaded stripe of 1.0 lm width is drawn in Fig. 5 as a visual guide. Taking the diffraction-limited light spot diameter to be 0.61(k=NA) where k is the wavelength and
NA is the numerical aperture, the limit of the resolution is estimated to be 0.9 lm. This confirms that diffraction-limited focusing is achieved in the present system.
4. Outlook The present system has achieved photoemission spectroscopy in a <1 lm area with <40 meV energy resolution. Time resolution of 100 fs is an inherent characteristic of the light source. The system is characterized by energy/time resolutions that are close to the Fourier limit and by the lateral resolution that is close to the diffraction limit. By improvement of the mechanical stability of the apparatus and the sample stage, the resolutions will more closely approach the values limited by the physical principles. Recently, the sampling time was shortened to 0.4 s for the photoemission spectrum of a signal-to-noise ratio of five measured with the 140 nm light. The rapid sampling facilitates two-dimensional mapping of the
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electronic structure. When lateral resolution higher than the wavelength is required, the PEEM will be used. Though the PEEM has no energyresolution capability, complementary use with the micro-spot TOF system will be helpful for revealing the dynamics of electrons and atoms in mesoscopic regions. The performance of laboratory-based microbeam photoemission spectrometers has been limited by the incoherent nature of the conventional He I light source [1]. The high repetition rate VUV laser light source opens up new possibilities for laboratory-based photoemission spectroscopy with high performance in energy, lateral and temporal resolutions. The wavelength can be extended up to the X-ray region by employing highpeak-power lasers [7,10].
Acknowledgements This study was supported by the Special Coordination Funds of Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government.
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