Frequency-modulation spectroscopy with transform-limited nanosecond laser pulses

We demonstrate high-quality FM spectra with nanosecond laser pulses. Transform-limited pulses with FR sidebands are produced by pulsed amplification of a phase-modulated cw laser. The pulses can be shifted to the UV by nonlinear mixing. We report both initial experiments on It and what is to our knowledge the first observation of a far-UV transition by FM spectroscopy, at 214.5 nm. Major advantages of this method include (1) spectral resolution of the order of 0.001 cm(-1), (2) better-defined optical phase, and (3) a much smaller and more easily detected modulation frequency, similar to 500 MHz. The absorption sensitivity is similar to 10(-4), and considerable further improvement is expected. (C) 1996 Optical Society of America