in Physics Research A 393 (1997)548-551
INSTRUMENTS a METHODS IN PHVSICS RESEARCH SectIon A
Two color experiments combining Free Electron Laser and Synchrotron Radiation M. Marsiav*, M.E. Couprieb,“, L. Nahonb,c, D. Garzella”, R. Bakkerbsc, A. DelboulbZ, D. Nutarelli”, R. Roux”, B. Visentinb, C. Grupp”, G. Indlekofer”, G. Panaccione’, A. Taleb-Ibrahimi”, M. Billardoncyd “Sincrotrone Trieste, Padriciano 99, I-34012 Trieste, Itab! bC.E.A./D.R.E.C.A.M./S.P.A.M., Bdt. 522 Saclay. F-91191 Gif-sur-Yvette, France “L.U.R.E., Bdt. 209D, Centre Universitaire Paris-&d, F-91405 Orsay, France dE.S.P.C.I., 10 rue Vauquelin, F-75231 Paris, France
Abstract We present the results of the first two color experiment coupling FEL and Synchrotron Radiation, where the SuperACO FEL was used to photoexcite carriers at semiconductor surfaces and the consequently induced surface photovoltage was studied with soft X-ray photoemission spectroscopy. Thanks to the subnanosecond temporal resolution and high surface sensitivity of our experimental setup, we were able to measure for the first time the effect of surface states on band bending dynamics. Based on our direct experience during these experiments, and on the future developments related to other two color experiments proposed on SuperACO, we will discuss the advantages and the technical aspects of this mode of operating the storage ring and the FEL.
1. Introduction A new tool is available for two-photon spectroscopy: since 1994, the UV FEL on SuperACO has been successfully used in combination with Synchrotron Radiation (SR), in a series of experiments investigating the temporal dependence of the surface photovoltage (SPV) at semiconductor surfaces. The unique characteristics of this experimental setup - its high surface sensitivity and subnanosecond temporal resolution - made it possible to measure for the first time the effects of surface states on band bending dynamics in semiconductors. These results demonstrate that it is possible to use simultaneously X-rays from a standard SR undulator beamline and UV light from a storage ring FEL. From a spectroscopist point of view, the main advantages of using these two sources for two photon spectroscopy are their natural synchronization, independent tunability and high repetition rate; concerning the FEL operation, crucial points for the success of the experiments have been:
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(i) the possibility of operating the FEL at 800 MeV, the standard energy for SuperACO Cl], which allowed us to simultaneously use the SU3 high resolution photoemission beamline; (ii) the installation of a longitudinal feedback to increase the laser stability ; (iii) the long laser and positron beam lifetime .
2. Why Synchrotron Radiation combined with FEL? FELs, thanks to their pulsed nature and high peak power, have found extensive application in two-photon experiments, where both the pump and the probe come from the same photon beam. Although relatively simple, this approach is limited by possible interferences between pump and probe and by the fact that their wavelengths cannot be independently chosen. Although two color operation of FELs has been performed , the difference in photon energy attainable in this way is limited. Conventional lasers have been synchronized both with FELs and with SR in two photon experiments [5,7], but even with this approach the tunability is limited and the synchronization troublesome.
1997 Elsevier Science B.V. All rights reserved
M. Marsi et al. INucl. In&-. and Meth. in Ph~a Rex A 393 (1997) 548-551 Having the possibility of using two photon sources which are independently broadly tunable is the ideal situation for studying excited systems: in this way, excitation and spectroscopy can be separately optimized. If such two sources are both pulsed and synchronized, in addition one has the possibility of performing a time resolved study that can provide information on the lifetime of the excited state. SR X-rays and FELs represent one of this ideal comb;nations. Generally speaking, having an FEL synchronized with SR allows to fully extend the well established capabilities of SR to the study of excited systems, as long as the FEL can be tuned in the excitation region of interest. Many facilities, besides SuperACO, will offer in the near future the possibility of performing such experiments: synchronization tests have already been performed between the Mark III IR FEL and synchrotron radiation from a bending magnet of the Duke FEL Laboratory [S], FEL-SR experiments will be possible at UVSOR  and DELTA [lo], and projects for FELs coupled with SR sources are proposed in Sweden (MAX Lab), Italy (ELETTRA)  and France (SOLEIL) . We describe here the technical aspects which are peculiar for this kind of experiments, based on the direct experience we acquired on SuperACO during our studies of band bending dynamics at semiconductor surfaces.
The position of the electron energy bands at semiconductor surfaces is different from the bulk, due to charge accumulation on surface states. One way of determining their position is detecting the core level binding energy, since the latter shifts by the same amount as the valence and conduction bands when the charge distribution at the surface is changed; in particular, synchrotron radiation photoemission spectroscopy gives excellent accuracy in determining band bending, because the tunability of the photon source allows to maximize the surface sensitivity of the measurement . We performed a time-resolved study of the energy band dynamics, i.e. how the systems react to a perturbation of the equilibrium charge distribution. Such perturbation was produced by optically creating electron-hole pairs at the surface with the FEL pulse. Since electrons and holes move in opposite directions due to the preexisting band bending, the transient effect of this perturbation (called Surface PhotoVoltage, SPV) is to flatten the bands, until recombination processes bring the system back to equilibrium . We used the FEL to produce the electron-hole pairs and SR photoemission to probe the SPV (Fig. 1). Changing the delay between the FEL and SR pulses, a time dependence curve of the SPV was obtained for several
25.0 3. Time dependence of surface photovoltage at semiconductor surfaces
kinetic energy (eV) Si 2p core level spectra taken on Si(1 I 1)2 x 1 for different time delays between the FEL pump and the SR probe. The difference in binding energy between the peaks taken on the excited and non excited surface is a direct measurement of the FEL induced SPV. samples. In Fig. 2, we present the results obtained with different time delays for the Si( 1 1 1)2 x 1 surface: as one can see, the SPV presents fluctuations on the subnanosecond time scale, which are related to the inequivalent flow of electrons and holes to and from the surface states [lS], and which could be detected thanks to the high surface sensitivity and temporal resolution of our instrumental setup. In this paper we analyse in detail the experimental aspects of this study. 3.1. Super AC0 operation During the experiment, SuperACO was operated at the standard energy of 800 MeV Cl], in conditions which are essentially the same as the standard two-bunch mode. Thanks to the recent installation of the feedback for quadripolar oscillations, it is possible to lase at positron beam currents as high as 100 mA, even though the experiment was performed at 35-70 mA, to guarantee maximum stability of the FEL after the hexapolar oscillations
M. Marsi et al. /Nucl. In&-. and Meth. in Phys. Rex A 393 (1997) 548-551
(a) _*_._._.-.-.-.4 *-
140 120 -
0 I v ,I' 1.5
delay (ns) Fig. 2. Time dependence of the FEL induced SPV for Si surfaces ofdifferent quality: (a) was taken on the cleaved Si(1 1 1)2 x 1, (b) on the partially contaminated cleaved surface and (c) on a surface with a strong concentration of defects.
in the ring disappear. This is to be compared to the 400 mA, normal injection current in 24 positron bunch mode, and 200 mA, normal injection current in 2 bunch mode on SuperACO. Due to the high brightness of its undulator, the working conditions on SU3 were perfectly acceptable for performing a surface science experiment, thanks also to the long lifetime of the positron beam (about 10 h). 3.2. Coherence Since the FEL operates on undulator SU7, which is on the opposite side of the ring with respect to SU3, the light from the FEL hutch was transferred to the SU3 experimental station via a series of multilayer mirrors to cover a distance of about 50 m. In this way, the sample was illuminated with a laser power of l-5 mW, corresponding to about 2.8 x 1014 ph~-‘rnrn-~ (0.5 nJ per pulse). The high degree of spatial coherence of the FEL was essential for its effective transport to the focal point of the SR beamline. 3.3. Tunability of the two sources Although the photon energy for the two beams was not changed during the experiment, it was crucial to be able to exploit the tunability of the FEL and of SR to get maximum surface sensitivity for both pump and probe. Consequently, the laser was operated at 350 nm (which gives the maximum absorption and minimum penetration depth in Si ), and a 130eV energy was used for probing the binding energy of the Si2p core
level with maximum surface sensitivity: since the photoelectron escape depth at this energy is just a few A, the position of the energy bands at the surface can be determined [ 131. 3.4. Non-interference
between pump and probe
The SR photon flux used during the experiment, which can be estimated to be about 10’ photons s-l mm-‘. negligibly small with respect to the FEL intensity, is known not to produce any detectable SPV at room temperature . Consequently, the SR beam does not affect the FEL induced SPV. Conversely, the FEL photon energy is less than the work function of the Si surface, so that the laser beam does not contribute to the photoemission yield. Therefore, the probe did not interfere with the pumping process and viceversa. 3.5. Synchronicity The natural synchronization between the FEL and SR pulses is the most important feature of this experimental setup. To the best of our knowledge, it is not possible to have an intense source of UV light, which can be tuned to 350 nm (minimum penetration depth in Si) and satisfactorily synchronized with the SR pulses other than a storage ring FEL. Nevertheless, the temporal stability of the SuperACO FEL was further optimized thanks to the installation of a longitudinal feedback, to keep a high degree of stability while remaining in the perfect tuning zone, with no jitter . The possibility of operating in this way was vital to attain the necessary subnanosecond
M. Marsi et al. /Nucl. Instr. and Meth. in Phvs. Res. A 393 (1997) 548-551
temporal resolution (of course, if the jitter is larger than the FEL FWHM, the effective laser pulse duration and consequently the temporal resolution of the experiment becomes worse). The separation between consecutive micropulses, with SuperACO operating in the two positron bunch mode, is 120 ns, the FWHM of the FEL and of the SR pulses are 50 and 500 ps, respectively. A computer controlled optical delay line was used to change the delay between the FEL and the SR pulse on the sample. The zero delay time was determined in situ with a fast response photodiode introduced in the UHV experimental chamber to detect at the beamline focal point the FEL pulse and the zero order light from SU3, measured with wide open slits to give a signal strong enough to be detectable by the diode. The precision in the determination of the zero delay was about _+ 0.2 ns, where as the precision for rrlntiw time delays is better than 1 ps.
4. Future perspectives Two color experiments coupling SR with FEL are planned to be further developed at SuperACO. A new RF cavity, to be installed in 1997, should markedly increase the performances of the FEL  and open new possibilities in that respect. Apart from the continuation of SPV experiments on semiconductor interfaces (Schottky barriers. heterojunctions). new applications are planned for the near future: in atomic physics, the study of photoionization of excited atoms (He, Xe); - in semiconductor physics, an experiment is planned that will use the IR light from a SuperACO bending magnet to probe the FEL photoexcited carriers in heterostructures; _ in biology. transient absorption SR studies of FEL excited tumor cells.
5. Conclusions The successful experiments on the temporal dependence of the SPV at semiconductor surfaces have proven that the combination of SR and FEL represents a valuable tool for two photon spectroscopy. The main requirements for the operation of the FEL and of the storage ring have been pointed out, as well as the advantages of the simultaneous use of these two advanced photon sources. The first experiments at SuperACO show that for this kind of user applications the main emphasis should be
put on FEL stability, long laser and positron beam lifetime (several hours), and compatibility with the SR beamlines on the storage ring. If these requirements will be satisfied, the increasing number of FEL facilities coupled with SR storage rings worldwide is likely to provide many interesting opportunities for the study of the excited states of matter.
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