The Metastable De-excitation Spectroscopy (MDS) apparatus consists of two main sections; a high-intensity source of He* atoms, and a sample analysis chamber for investigating surfaces. To produce the He* beam, established techniques of laser cooling are used to first collimate, and then focus the He* atoms to a point on the sample under study. Opposite is a picture of the He* source, collimation section, and flight chamber. A Micro-Channel Plate and phosphor screen assembly is mounted on the end of the system in order to characterise the He* beam. A further vacuum chamber for focussing will soon be added.
Manipulation of the He* beam is performed using the output from a 1W 1083nm single mode CW ytterbium fibre amplifier (IPG Laser YAM-1-1083) specifically designed for spectroscopic applications. The seed for this amplifier is a grating stabilised DBR diode laser whose 40mW 1083nm output has a linewidth of ~1 MHz. Stabilisation of the laser is performed using saturated absorption spectroscopy in which an absorption signal is fed into a lock-in circuit to generate a phase-locked feedback loop.
Surfaces are studied using a 180 degree hemispherical energy analyser (Omicron EA 125) with up to seven channeltron detectors. In addition to performing MDS this analyser also has Auger Electron Spectroscopy (AES), Ultra-violet Photoelectron Spectroscopy (UPS), and Low Energy Electron Diffraction (LEED) capabilities allowing thorough characterisation of surfaces.
The
MDS apparatus. The He* beamline shown here consists of the source,
colimation, and flight chambers, all of which are suspended from a frame
to prevent vibrations disturbing the operation of the frequency-locked
laser.
