Atomic absorption with ultracold atoms

Recent advances in laser-atom cooling techniques and diode-laser technology now allow one to conduct an idealised atomic absorption experiment compris ing a sample of ultracold quasi-stationary absorbing atoms and a source of near-monochromatic resonant light. Under such conditions, the atomic absorp tion coefficient at line centre is independent of the oscillator strength o f the atomic resonance line. This offers the prospect of 'oscillator-streng th-free' atomic absorption spectroscopy in which the absorption signal is e qually large for both strong and weak (closed) transitions of the same wave length and in which absolute atomic absorption could be performed without k nowledge of the oscillator strength. Moreover, the resolution and sensitivi ty for a given atom density are greatly enhanced, typically by approximatel y three orders of magnitude (and even more for weak transitions), compared with conventional flame or graphite-furnace atomic absorption. We describe an atomic absorption experiment based on samples of ultracold, laser-cooled caesium atoms and a narrow-bandwidth diode laser source that approximates the idealised conditions for oscillator-strength-free atomic absorption. Th e absorption measurements are used to determine the number density and temp erature (approx. 6 mu K) of the sample of ultracold atoms. Some of the tech nical obstacles that would have to be overcome before samples of ultracold atoms and diode laser sources could be used in analytical atomic absorption spectroscopy are discussed. (C) 1999 Elsevier Science B.V. All rights rese rved.