Temporal and spatial distribution profiles of lithium atoms in low-pressure discharges by concentration-modulated absorption spectroscopy

The application of the new pump–probe spectroscopic technique, COMAS (concentration-modulated absorption spectroscopy), to the study of sputtered lithium atoms in two glow-discharge devices is discussed. Information is presented on the absolute concentration of lithium atoms in a hollow cathode lamp (1.62 × 1011 cm–3 in the centre at 14 mA) and a plane-parallel dc discharge (1.24 × 109 cm–3 close to the cathode at 1.05 mA). In addition an experimentally derived absorption cross-section for the (2s)2, S →(2p)2, P1/2, 3/2 resonance transition in lithium (σ≈ 2.95 × 10–12 cm2) is presented for the first time. In producing concentration profiles for both a commercial hollow cathode lamp and a glow-discharge system built in-house, use is made of the high spatial resolution attainable with COMAS. Results obtained in the dc plane-parallel system are compared to a theoretical model describing the transport of sputtered atoms through a low-pressure glow discharge. By varying the delay between the pump and probe laser pulses, the lifetime of the (2p)2, P1/2, 3/2 state has been measured as 25 ± 1 ns showing good agreement with values obtained by other methods found in the literature (ca. 24–27 ns).