XSdosePES/Suricat

The ultrahigh vacuum set-up SurICat (Surface Investigation and Catalysis) is a photo emission/photo absorption station equipped with the high resolution electron energy analyzer Scienta SES100, a Bruker fluorescence detector, and a variety of sample characterization and preparation tools i.e. a LEED optic, a mass spectrometer, evaporators for organics and inorganics, and two quartz microbalances. One load lock and two preparation chambers, and the use of Omicron standard sample plates facilitate fast access. In order to provide excellent experimental conditions, the set up was designed as a four chamber system keeping the analyzer chamber free of contaminating materials such as dosed or evaporated adsorbates. The endstation SurICat is attached to the Optics beamline of the storage ring BESSY II, which is a dipole beamline. With three plane gratings it provides photon energies form 15eV to 1500eV with moderate flux. SurICat was designed in 2003, put into service in 2004, and became a dedicated user station in 2005.

Over the last 6 years more than 60 peer reviewed publications emerged from this chamber (see Table 1) mainly in the field of molecular interfaces, polymer research, and solar energy (38). The main reason for this success is (i) the wide energy range provided at this endstation spanning from Ultraviolet Photoemission (UPS) to X-ray Photoemission (XPS) and allowing Extended X-ray Absorption and Near Edge X-ray Absorption Spectroscopy (EXAFS and NEXAFS) and (ii) - even more importantly - the very ‘dilute’ photon beam, realized by the moderate flux, attenuator foils, and a large focal spot of the beam in measurement position. This low dose of the beam is of paramount importance to reproducibly investigate sensitive samples.

Vacuum photoemission spectroscopy is a very powerful tool for studying electronic properties of conjugated organic materials and their interfaces with inorganic materials. Many experimental studies have been performed using synchrotron radiation for generating photoelectrons. The advantage of high photon flux from synchrotron storage rings, however, may impose new experimental problems when sensitive samples are investigated, such as conjugated organic systems. These are prone to undergo chemical changes when exposed to high fluxes of photons and electrons. Occasionally it may be difficult to judge whether or not such sample degradation occurs especially when degradation happens on the same timescale as data acquisition. Positive charging near the surface of an organic specimen - due to the photoemission process - causes electric fields that distort or even suppress a meaningful photoemission spectrum. Photoelectrons and their secondaries with a kinetic energy of several eV can break intra-molecular bonds and result in chemical changes of the sample, e.g., via cross-linking and polymerization.
This effect is more pronounced with increasing thickness of the sample. When the sample gets less conducting this leads to enormous experimental problems when investigating polymer films or organic single crystals. In general these problems may be solved by considerably reducing the photon flux, which – on the other hand requires better high-performance detectors. Up to 2010 there was no report of angle-resolved photoemission spectroscopy to determine a full bandstructure of organic single crystals.

Angle-resolved ultraviolet photoelectron spectroscopy combined with synchrotron radiation is established as the most powerful tool for probing the energy band structure of solids. However, the band structure measurement of (single crystalline) organic semiconductors is difficult due to their low conductivity on one hand and the low tolerance of conjugated organic compounds towards beam damage on the other hand. Recent state-of-the-art systems are hemispherical analyzers with a 2-dimensional detector capable of running in angular mode. This allows measuring a selected cut of the Brillouin zone, while for the measurement of the full momentum plane next to the point of interest mechanical movement of the sample is required. Also the transmission of hemispherical analyzers is limited, resulting in the countrate-vs-resolution problem. In many cases increasing the intensity of the photon beam is beneficial; however, it cannot be applied for beam damage sensitive samples, like conjugated organic materials. In a Swedish-German collaboration we have recently performed very successful measurements on organic single crystals employing a novel type of photoemission detectors, which will help to overcome the fundamental problems mentioned above in the future- the Angle Resolved Time Of Flight electron energy analyzer (ARTOF) - which allows for ‘mild’ measurement conditions in terms of very short spectrum acquisition times combined with a very low photon flux.

This was achieved by:

  1. 300 times higher electron transmission than hemispherical analyzers
  2. angle-resolved measurements of a full 30°cone within one measurement
  3. data acquisition in the MHz range
  4. single bunch operation of the storage ring, i.e., low photon flux.

Therefore, the ARTOF design combines a number of properties that make it predestined for investigating organic compounds. Even more so, the measurement of a full 30° cone facilitates an experiment without change of any geometrical parameter of the sample during measurement. This can be of paramount advantage (in particular but not only for small samples) because it is no longer necessary to rotate the sample and thereby changing the investigated sample area, and the probed sample depth is also constant. Furthermore, the fixed geometry avoids parasitic effects on the measured band dispersion by a spatial variation of the incident photon energy and the correct orientation of a sample is no longer critical since the obtained energy dispersion can be rotated and shifted for correction after the measurement is finished. The continuous probing of the full band structure and the storage of the complete data set also makes it possible to check for any time dependent modifications of the sample.

While the drawback of an ARTOF is its limitation to a pulsed photon source, the benefits are capitalized by the combination of a time-of-flight tube with electron optics and a time- as well as a position-sensitive electron detector. Using electron optics theory it is possible to calculate trajectories of photoemitted electrons and from the measured transient time and position on the detector one can calculate its original energy and emission angle. Such design involves no slits and allows collecting most of the electrons emitted from the sample.

At the moment using single bunch beam is the method of choice, while implementing a chopper into the incident photon beam can eliminate above mentioned problem. As time of flight spectroscopy always requires temporal structure of the incident beam as a starting point of the measurement. We are currently working on a Mega Hertz shutter project. The beamline SurICat is attached to offers the precondition of realizing this project due to an internal beam focus. Therefore the system SurICat/Optics beamline is predestined to realize a state-of-the art very low dose photoemission set-up.