ECAMP 15

By combining a monochromatic electron source and high performance detectors, we build with ISMO and SPEC a new electronic microscope call HREELM. This microscope enables the imaging and analysis of vibrational interactions on surfaces. Applications include nanophysics, nanochemistry and photonics.
KEY-WORDS : surface microscopy; pulse electron source ; Rydberg atoms

1. INTRODUCTION: MOTIVATION FOR BUILDING HREELM

   For several decades now, we have been witnessing unprecedented development in the field of imaging and spectroscopy. Techniques are becoming increasingly sophisticated, pushing the limits of observation ever further. Nevertheless, each method has its advantages-and disadvantages.
   Two methods of analysis can be distinguished: imaging and spectroscopic techniques. Imaging techniques include LEEM (Low-Energy Electron Microscopy), a full-field electron microscopy technique using low-energy electrons to image the sample surface. It can achieve a spatial resolution of 20 nm and a spectral resolution of 300 meV. For spectroscopic techniques, we can cite HREELS (High-Resolution Electron Energy Loss Spectroscopy), which is a spectroscopic technique based on the detection of inelastically scattered low-energy electrons. HREELS has a spatial resolution of 1 mm and a spectral resolution of 1 meV.
   There is interest in the design of a new surface analysis instrument combining the features of LEEM and HREELS: HREELM (High-Resolution Electron Energy Loss Microscope). This is a full-field technique enabling simultaneous high-resolution imaging and spectroscopy. Resolution of phonons and surface plasmons requires an energy dispersion of less than 10 meV at a flux of 100 pA.Spatial resolution would be of the order of 10 nm[1].
   1. DESCRIPTION DU SET UP EXPERIMENTAL
   A) THE ELECTRONS SOURCE
   A key aspect of HREELM design is the electron source used.
   It must be capable of achieving a flux in excess of 100 pA for good brilliance, with an energy dispersion of the order of 5-10 meV.To meet these criteria, Rydberg atom ionization was the obvious choice[2].The entire source consists of a cesium jet, three lasers to excite the cesium atoms into a Rydberg state, and circular electrodes pierced at the center to create an electric field to which the Rydberg atoms will be exposed.
   In this situation, the energy dispersion of the source is given by the formula :
   ∆E=F∆z
   With ∆z the ionization zone imposed by the Rydberg resonance width and F the electric field imposed on the atoms.

B) ELECTRONS DETECTION SCHEME
Another important aspect of HREELM is the electron detection system. This is because the energy difference between electrons after their interaction with the sample will be small. A high-precision detection system is therefore required.
To achieve this, a time-of-flight measurement can be used. After interacting with the sample, the electrons are detected by a system of microchannel wafers coupled to a sensor which gives the position of the electrons and their arrival time on the sensor. In our case, we chose to use the Timepix 4, which is the only sensor capable of achieving the desired spatial and spectral resolution while supporting the flow of electrons imposed by the microchannel wafers.
However, to determine the time of flight, we need to know not only the arrival time of the particles, but also their departure time. So we use the electron/ion correlation to obtain the electron departure time.
3. RESULTS OBTAINED AND WORK IN PROGRESS
To be able to resolve phonons or molecular bonds, you need an electron source that meets all the above criteria. In order to test whether these criteria have been validated, it was necessary to build a first HREELM prototype.
The latter features an electron source based on ionization of Rydberg atoms and a detection system consisting of two microchannel wafers to amplify the electron flow, plus a phosphor screen to visualize the electron position. This device has proved that the three criteria can currently be achieved separately, but not simultaneously.
Current work consists in characterizing the electron source designed, in particular by studying the different Rydberg states, improving the detection system using Timepix, and working on the integration of all the elements of the experiment (time-of-flight tube, sensor, etc.).