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For the experiment, an ultrahigh vacuum (UHV) with a base pressure of 10−8_mbar

or better is required in order to avoid fast surface contamination of the nanotip. Here a vacuum with a pressure of about 3×10−10mbar is used for laser-induced electron emission. We only give a brief overview of the vacuum setup and the detection systems. More details can be found in [118, 119].

The base pressure of about 3×10−10_{mbar is attained with the help of a rotary-vane}

roughing pump (intermediate pressure ∼10−3mbar) and a turbomolecular pump. The stainless-steel vacuum chamber is routinely baked out at a temperature of about 120◦C. In order to reach the final base pressure, an ion getter pump and a titanium sublimation pump assist the other pumps. The chamber can be backfilled with high-purity gases using a dosing valve for field ion microscopy.

Fig. 3.3 shows a sketch of the chamber and the detectors. The nanotip is held on a v-shaped wire and points towards one of the detectors. High-voltage biasing as well as resistive heating of the tip can be performed. The detection system should be able to provide high spatial resolution imaging and spectrally resolved measurements of electron emission from the tip. For this purpose, two different detectors can be used: A microchan- nel plate (MCP) detector and an electron spectrometer. Linear motion feedthroughs enablein-situexchange of the detectors. With the help of the MCP detector, tip charac- terization is possible with field emission and field ion microscopy (see subsection3.3.3).

OAP Vacuum chamber Tip MCP detector Spectrometer CCD camera E

Figure 3.3: Sketch of the vacuum chamber setup. The vacuum chamber, here in top view, is operated at a base pressure of 3×10−10mbar. The laser-illuminated tip is mounted on a v-shaped wire. Electrons emitted from the tip (trajectories indicated by blue curves) can be detected either with a microchannel plate (MCP) detector or a retarding field electron spectrometer. The former serves for high spatial resolution imaging of the electron emission pattern, whereas the latter is used to record electron energy spectra. More details can be found in [118,119].

The MCP detector is located at a distance of about 4 cm from the nanotip. Its active area has a diameter of about 4 cm. In a measurement with the MCP detector, the tip is biased with a negative voltageUtip and serves as a cathode. The front side of the MCP

is kept on ground potential and serves as an anode. The back side is biased with a high positive gain voltageUgain of up to 2 kV. The phosphor screen of the detector is biased

with +4 kV. In this configuration, the MCP serves as an electron multiplier with high spatial resolution. The gain factor can be as large as 107, enabling single electron detec- tion with a dark count rate of less than 5 electrons per second. The amplified current can be measured at the phosphor screen. The electron emission pattern is imaged with a CCD camera outside of the vacuum chamber.

Electron spectrometer

With the help of linear motion feedthroughs, the MCP detector can be exchanged with an electron spectrometer. The spectrometer that is used in the experiment6_{is a retarding}

field spectrometer, first implemented by E. W. M¨uller in 1936 [19]. The retarding field spectrometer features a fine mesh grid that is biased with voltage Ugrid, measured with

respect to the tip voltage. The grid serves as a high-pass energy filter for incoming electrons. Only electrons with a minimum kinetic energy of −|e|Ugrid pass. Electron

energies measured with this type of spectrometer are usually referenced to the Fermi energy EF. This thesis follows this convention common in surface science.

Recording the count rate transmitted through the filter grid as a function of the grid

voltageUgridyields an integrated electron spectrum. The actual spectrum is retrieved by

subsequent differentiation and smoothing of the recorded curve. In strong-field science, usually time-of-flight (TOF) spectrometers are used to measure photoelectron spectra. With a repetition rate of 80 MHz, however, arrival times of electrons from two subse- quent laser pulses easily overlap. Furthermore, the operation of such a spectrometer is only possible with pulsed (laser-triggered) electron emission. Continuous electron emis- sion, for example from field emission (see below), can not be resolved spectrally with a TOF spectrometer. A viable alternative to a retarding field spectrometer would be a hemispherical electron analyzer. This detector can also provide angular resolution in ad- dition to spectral resolution. Both spectrometer types are based on electrostatic energy filtering and are capable of measuring continuous electron currents.

In the experiment, the tip is kept on ground potential and electrons emitted from the tip are accelerated to the spectrometer’s entrance aperture that is biased with a positive extraction voltageUextr. Using a current preamplifier connected to the tip, it is

then possible to measure the total current drawn from the tip while recording electron spectra. This is particularly useful for monitoring the stability of the tip emission current over time. Only electrons emitted in forward direction are detected by the spectrometer. The acceptance angle of the spectrometer is not known, but estimated to be significantly smaller than 20◦. In the best case, a total detection efficiency of 10−2 _{was found from a}

comparison of the total current with the count rate detected at the spectrometer’s MCP detector. The resulting integrated electron spectrum is then numerically differentiated and smoothed with a Savitzky-Golay algorithm [123].

The electron spectrometer has a resolution of about 80 meV. This value has been estimated from a comparison of an experimental electron spectrum of field emission with its well-known theory counterpart [124]. Besides emitter instability, shot noise constitutes the main source of noise on spectral measurements (see [119] for a discussion).