ToF MARIS

FOCUS has a long history of improving this technology, starting with the first lateral-resolved PEEMs in 1995 and publishing the first scientific angle and energy-resolved PEEM results in 2002 (nowadays often referred to as Momentum Microscope). Our first TOF-PEEM with lateral and momentum resolution was build in 2007. The latest revision of our microscope lens was launched in 2022 in combination with a double-hemispherical energy-filter, called NanoESCA MARIS. Besides its high lateral resolution of 35 nm we could improve the momentum resolution by a factor of 3 to reach an unsurpassed momentum resolution of < 0.005 Å-1.

The same lens in combination with a time-of-flight energy filter complements our microscopy product line as TOF MARIS.

The TOF MARIS utilizes two different detectors for an efficient work-flow. The Standard Imaging Unit (MCP + CCD Camera) is used for high count rates (> 30 Mcounts/s) and is perfectly suited to use continous lightsources with a high flux for the initial sample navigation in live view mode, e.g. to find interesting features on the sample under investigation.
Subsequently, the retractable delay-line-detector is moved into the optical axis and used for the time- / energy-resolved measurements with a pulsed light-source.
The microscope can be easily switched between real-space and momentum space projection with the projective lenses before the energy filter.

The benefit of a PEEM with time-of-flight energy filter is the parallel detection of the angular (or lateral) electron distribution and a large energy window. The data cube on the left was acquired in such a way. It shows the momentum vs energy measurement of a Ag (111) crystal, excited with a laser with a 3.1 eV photon energy.

With a 2 PPE process, electrons between the work function cut-off (E – EF = 4.4 eV) and the Fermi Level (E – E_F=6.2 eV) can be acquired. The cube was measured at room temperature.
The Fermi Edge on the right was acquired with the same set-up, but a sample temperature of 45 K. It shows an instrumental energy resolution of ∆E_inst = 23.5 meV.

The two raw images were measured on the same region of a structured test sample (Si grid covered with Ag) with two different detectors. The image quality and resolutions are comparable. In both cases, a mercury arc lamp was used for photo electron excitation.
The line scans along the marked edges of the structures were fitted with a convolution of a Gaussian and a Heaviside-Function.
The resolution value RES_16/84 is the width between 16% and 84% of the profile height. A resolution of 35 nm is achieved with both detectors.

A double-grid high-pass filter can be integrated in front of the DLD, to cut off unwanted secondary electrons. The different spectra below show, how the secondary background is systematically reduced by biasing the sample voltage relative to the grid potential.

Under typical angles of incidence for a PEEM (25°) a laser with 100 µm spot size has a time difference of up to 715 fs when reaching the surface. This can be an issue for studying effects on the fs timescale.

A light-reflecting mirror can be integrated into the objective lens, which allows to excite the sample under nearly normal incidence. This way, the light pulse reaches the sample surface homogeneously at the same time.

TOF MARIS (time-of-flight energy filter) and NanoESCA MARIS (double-hemispherical energy filter) follow a modular concept, in which the microscopy lens can be ordered or upgraded with different energy filters. A popular set-up is to combine the MARIS front lens with a double-hemispherical energy-filter (suitable for a wide range of light sources) with a time-of-flight energy-filter (often an efficient choice for pulsed sources).
In accordance with the main scientific application, it is possible to start with a TOF MARIS and upgrade the double-hemispherical energy-filter at a later time, when additional experiments with continuous lightsources and a high flux become of interest. Vice versa, a NanoESCA MARIS can be upgraded with a time-of-flight energy filter. The double-hemispherical energy-filter can be upgraded with an Imaging Spin-Filter.