Strong-field molecular ionization:: determination of ionization probabilities calibrated with field-free alignment

We report an original optical method providing the probability of molecular ionization induced by femtosecand laser pulses. The approach consists of exploiting molecular alignment to extract reliable information about ionization. The cross defocusing technique implemented for this purpose reveals a sensitivity with respect to postpulse alignment, as well as to the free electron density induced by the ultrashort laser pulse. The analysis of the resulting signal thus gives access to absolute single-ionization probabilities calibrated through the degree of alignment, provided that free electrons are produced mainly by single ionization. The relevance of the method is assessed in N-2. (c) 2006 Optical Society of America.