Negative ions from liquid microdroplets irradiated with ultrashort and intense laser pulses

Highly energetic negative ions have been observed by irradiating water and heavy water micro droplets with ultrashort (35 fs) and highly intense ((0.8–1.2) × 1019 W cm−2) laser pulses. The creation of negative hydrogen (H−), deuterium (D−) as well as oxygen (O−) ions is suggested due to a process in which first their formation takes place in the hot plasma followed by an acceleration together with the positive ions. The negative ions are pulled by the energetic positive ions and propagate with them contributing to the quasineutrality of the beam. The observed phenomenon is discussed on the basis of possible formation and acceleration mechanisms of negative ions.