Here, we report that intense ultra-short laser pulses produce a plasma of low energy electrons (LEEs) by the
inverse Bremsstrahlung effect and multiphoton ionization process. The phenomena show five striking characteristics.
First, the self-focusing of ultra-short laser pulses creates a plasma of LEEs (6.5 eV), which is concentrated in filaments
through an avalanche process. Second, kinetically hot 6.5 eV electrons interact with surrounding molecules resulting in
reactive radical species. Third, the dose rate reaches an enormous level of ~2.8 × 10<sup>11</sup> Gy/s as determined by a cericcerous
sulfate dosimetry and this leads to an ultra-high deposition of energy of between 4.6 × 10<sup>7</sup> to 8.16 × 10<sup>7</sup> keV/μm.
Fourth, filaments of variable length are produced by femtosecond pulses depending on the pulse duration as determined
by a tissue-equivalent radiation polymer gel dosimeter and imaged by magnetic resonance imaging (MRI). These results
reveal that one of the very interesting novelty of filamentation is the very low entrance dose, similar to proton irradiation.
Lastly, filamentary irradiation results in the decomposition of thymidine in the absence and the presence of oxygen
similar to the radiolysis of water.