Third-harmonic generation and laser-induced continuum structure in sodium

Observation of laser-induced continuum structure (LICS) in sodium due to th e coupling of a bound state to the continuum by a strong laser field has be en reported elsewhere. Such experiments have mostly employed a single laser photon to probe the structured continuum from the ground state. Other expe riments have used multiphoton (third harmonic) probe schemes, but only one of these has successfully observed LICS, in agreement with theoretical pred ictions. We report here a very similar experiment generating third-harmonic radiation in sodium to probe the anticipated LICS produced by coupling the 5s state to the p continuum. However. under no conditions was any continuu m structure observed, in agreement with in a number of other experiments, b ut at variance with theoretical predictions.