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Laser vaporisation in association with a helium droplet source has been used to embed a range of metal atoms and dimers within the droplets. Using tuneable UV laser radiation, photoionisation of the metal atoms has revealed the presence of long-lived metastable excited states. These states undergo 1-photon excitation to Rydberg states which then autoionises. In the case of aluminium atoms, the metastable state excitation spectrum consists of a series of narrow lines with very small shifts from their gas-phase values; however, ab-initio calculations suggest that these states should be as strongly bound to the helium as the ground state of aluminium. For copper atoms, metastable states within a few thousand wavenumbers of the ground state ionisation limit have been detected. Transitions out from these states (possibly members of the series 3d10 nd, n>10) have line widths of 20-50 cm-1 and are blue-shifted by several hundred wavenumbers from their gas phase values. None of these metastable states can be detected in the absence of the helium droplet beam, and current experiments are concentrating on the mechanism(s) responsible for their generation. |
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