Hindered alignment in ultrashort, intense laser-induced fragmentation of O₂

Molecules ionized by intense (10-100 TW/cm²) and ultrashort (tens of femtoseconds) laser fields undergo rotation and alignment mediated through their polarizability. The expected alignment is indeed observed in the case of O₂ molecules ionized by intense laser pulses of 800 nm wavelength and 25 fs duration, as observed through velocity imaging of the fragments. Strikingly, when 35 fs pulses of 400 nm wavelength of comparable intensity are employed, an anomalous hindering of this alignment is observed. In both cases, we propose dissociation pathways for the energetic ions consistent with the recorded kinetic energy distributions. Using a semiclassical model of induced rotation of the molecular ion that involves polarizabilities of the participating excited states, both behaviors are reproduced. The model suggests that the difference in the observations can be attributed to a transient negative polarizability in an intermediate state of the proposed pathway.