High sensitivity broad-band mode-locked cavity-enhanced absorption spectroscopy: measurement of Ar*(3P2) atom and ion densities

Abstract

The recently developed broad-band absorption technique of ‘mode-locked cavity-enhanced absorption spectroscopy’ (ML-CEAS) is applied to the diagnostics of argon and nitrogen plasmas. Using a commercial tunable mode-locked Ti : Sa femtosecond laser, this combines the multipass advantage of the cavity-enhanced technique with the simultaneous acquisition over a broad spectral range of classic broad-band absorption spectroscopy. Absorption spectra in the 400 nm range are acquired after frequency doubling of the Ti : Sa femtosecond laser in a BBO crystal. The measurement of the metastable Ar*(3P2) atom density in a low pressure argon glow discharge through its weakly absorbing lines at 394.75 and 394.898 nm allows us to illustrate the importance of the apparatus' spectral resolution for determination of absolute number densities. Absorption spectra of the first negative band of the nitrogen ion, , around 391 nm are recorded in a nitrogen glow discharge and in the afterglow of a flowing μ-wave nitrogen plasma. The absolute ion density (and its rotational temperature) measured in the discharge zone and in the maximum of the short-lived afterglow (SLA) are 1.5 × 1015 ions m−3 (1300 K) and 1.0 × 1015 ions m−3 (800 K), respectively. Compared with the previously measured electron density at the maximum of the SLA, it is concluded that ions are not the dominant positive ions in this zone.