Cavity ringdown spectroscopy, CRDS [1], is used to measure absorption spectra of dense lithium vapour generated in the heat-pipe oven. Absorption spectra obtained from fitting procedure in time windows of different lengths and placed at different positions across the ringdown decaying curve are presented. Non-linear effects are detected, such as decreasing of absorption with increasing laser pulse energy. Pronounced dips in the line center of strong transitions occur depending on molecular densities, injected laser pulse energies and chosen time window [2]. CRD spectra are compared with integrated cavity output spectra, ICOS [3].
 
CRDS absorption method is applied on measurements of small absorption coefficients in the spectral region of two-photon transitions. In this case differential equation for the intracavity field intensity becomes non-linear. Absorption spectra around two-photon Li(2S-3D), Li(2S-5D) and Li(2S-6D) transitions and quasiresonant Li(2P-4S) transition are measured. In addition, vibrational-rotational transitions of Li2(X ¹S⁺_g -A ¹S⁺_u ) and Li2(X ¹S⁺_g -B ¹P_u) bands in this spectral regions are identified.

Using CRDS, absorption spectra of lithium and cadmium vapour mixture is measured, in order to observe bound-bound transition of LiCd excimer molecule [4]. Comparing this spectra with absorption spectra of pure lithium vapour we identify narrow intervals of wavelengths in which absorption of Li2(X ¹S⁺_g -B ¹P_u) band transition is small or zero. The
application of selective wavelength excitation spectroscopy in this spectral windows allows us to discriminate between overlapping Li2(X ¹S⁺_g -B ¹P_u) and LiCd bands.
 

1. M. D. Wheeler, S. M. Newman, A. J. Orr-Ewing, M. N. R Ashfold, J. Chem. Soc.,
    Faraday Trans. 94 (1998) 337-351
2. I. Labazan, S. Rudic, S. Milosevic, Chem. Phys. Lett. 320 (2000) 613-623
3. A. O' Keefe, Chem. Phys. Lett. 293 (1998) 331-336
4. S. Milosevic,  'Spectral Line Shapes' (A. D. May, J. R. Drummond, and E. Oks, Eds), Vol. 8, Am. Inst.of
    Phys. Conf. Proc. 328, p.391-405. Am. Inst. of Phys., College Park, MD, 1995