Large spectral blue shifts in intense laser irradiation of microdroplets

When methanol microdroplets of 15 mum size are irradiated by intense femtosecond laser pulses of moderate intensities (<2x10(16)W cm(-2)), we observe a 'red-light flash' from the microplasma. We report on the presence of a large 'blue shoulder' (that extends to about 200 nm from the incident laser wavelength) in the scattered spectra that corresponds to the red-light flash. A prepulse is found to be essential for producing the large blue shift, which is attributed to the rapid subsequent ionization of a near-critical density preplasma when the main pulse is incident.