Improved metastable de-excitation spectrometer using laser-cooling techniques

Details of a new approach for performing metastable de-excitation spectroscopy are given. A beam of metastable (2 S3) helium atoms, produced in a hollow cathode dc discharge, is collimated and subsequently focused using Doppler cooling of the 2 S13 -2 P23 transition at 1083 nm, forming an intense probe of up to 1× 1012 atoms s-1 cm-2. The large distance (2.5 m) between source and sample means that the beam is relatively free of UV photons and 2 S1 metastable atoms, removing the need for quench lamps and chopper wheels. As well as providing a clean high intensity source, the well defined nature of the beam is a necessary step towards using more sophisticated laser-cooling techniques with the ultimate aim of producing a metastable helium microscope. MDS and UPS spectra from Si(111) are shown.