Efficient K-α and He-α emission from Ti foils irradiated with 400 nm, 45 fs laser pulses

We have used the ASTRA laser facility to irradiate Ti foils with 45-fs 400 nm laser pulses at irradiances up to ∼1019Wcm-2. We have observed both thermal line radiation, in the form of the He-like 1s2-1s2p transitions (He-α), and the K-α emission line. We have noted that the efficiency of K-α production is in excess of 2×10-4 and thus compares well with using the fundamental 800 nm radiation. The behaviour of the K-α emission as a function of polarisation and angle of incidence suggests a vacuum heating mechanism as the principal source of the fast electrons necessary to create the inner-shell vacancies. Use of streak cameras in the X-ray regime has demonstrated that the emission of both K-α and He-α radiation lasts several picoseconds. In the former case, this would seem to imply that fast electrons generated by resonance absorption are circulated for some time before penetrating the foil.