A build-up cavity for Fourier transform emission experiments

Abstract: We have recorded electronic spectra of some diatomic species (I2, K2, and NaK), to illustrate the potential power of the combination of two high resolution techniques: intra-cavity laser induced fluorescence (ICLIF) and Fourier transform (FT) spectroscopy. Active and passive optical cavities have been used, working with visible continuous wave (cw) laser sources. The active cavity is a modified commercial ring dye laser, allowing for a sample up to 25cm in length. Dispersed fluorescence spectra recorded on a Bomem Fourier transform spectrometer showed a signal enhancement of about 10 when a molecular source was placed within the resonator. The system was tested with a heatpipe source, producing alkali metal vapour at about 300°C. These experiments illustrate enhanced cascade excitation mechanisms in K2; the highest vibrational levels of the electronic ground state of K2 can be observed with surprising ease. The increase in available power within the cavity has also led to the observation of fluorescence in NaK excited by a two-photon transition (Q (66) 61Σ+ ← X 1Σ+ transition). Spatial limitations have driven us to build a more versatile ring cavity able to accommodate larger sources. This broad-band (590–650nm) build-up cavity is locked by a Hänsch–Couillaud servo-loop to an input laser of (instantaneous) bandwidth ∼1MHz. Power enhancement factors of around 30 have been obtained with a 2.6% input coupler. The performance of the build-up cavity has been tested by recording FT spectra of intra-cavity laser induced fluorescence of iodine. The technique clearly has useful applications for weakly absorbing species, or for those whose electronic states are inaccessible to single-photon absorption techniques. This paper describes the arrangement we have used, highlighting some of the advantages and describing some of the particular difficulties we have encountered. [Copyright &y& Elsevier]