Revival structures of linear molecules in a field-free alignment condition as probed by high-order harmonic generation

Revival structures (rotational coherence) of three linear molecules (N ₂, O ₂, and CO ₂) in a field-free alignment condition have been investigated using high-order harmonic generation. The harmonic yields of these molecules were measured in a pump-probe manner by using a weak femtosecond (fs) laser pulse for field-free alignment of molecules and another intense fs laser pulse for harmonic generation. The harmonic intensities from 23rd to 29th order with respect to the time delay between the pump and the probe pulses showed revival structures in the condition of a field-free alignment of molecules. While the revival structure of a N ₂ molecule had one-fourth the period of the full revival time and different degrees of modulation among different fractional revival times, the revival structures of O ₂ and CO ₂ molecules showed one-eighth the periods of the full revival time and similar degrees of modulation among all fractional revival times. The revival structures could be interpreted in terms of the nature of the highest occupied molecular orbital and the total nuclear spin.