Time- And intensity-dependent broadband cavity-enhanced absorption spectroscopy with pulsed intra-cavity laser-induced plasmas

A pulsed laser-induced plasma (LIP) was generated in ambient air inside a high-finesse (F ≈ 5200) near-concentric optical cavity. The optical plasma emission was successfully trapped and sustained by the cavity, manifested by ring-down times in excess of 4 µs indicating effective mirror reflectivities of ∼0.9994. The light leaking from the cavity was used to measure broadband absorption spectra of gaseous azulene under ambient air conditions between 580 and 645 nm, employing (i) intensity-dependent cavity-enhanced, and (ii) time-dependent cavity-ring down methodologies. Minimum detectable absorption coefficients of 4.7 × 10⁻⁸ cm⁻¹ and 7.4 × 10⁻⁸ cm⁻¹ were achieved for the respective approaches. The two approaches were compared and implications of pulsed excitation for gated intensity-dependent measurements were discussed.