Anomalous angular distribution of fragment ions from rare-gas diatomic molecules with intense, femtosecond, near-infrared laser pulses

Almost isotropic angular distributions are observed for fragment ions from Xe₂ produced with intense femtosecond laser pulses (8000.3em0exnm, 4×10¹⁴0.3em0exW∕cm²). The results observed for Xe₂ present a great contrast to those for I₂ molecules used as reference molecules, which show typical anisotropic angular distributions resulting from enhanced ionization. The kinetic energy spectra of the fragment ions from Xe₂ reveal that the internuclear distance remains near the equilibrium distance for various pulse widths from 45 to 7100.3em0exfs, indicating that in the rising edge of the pulses, the internuclear distance of Xe₂^0.4em0exn+ ($n=0$, 1, 2) does not reach the favorable region for enhanced ionization even though enhancement can potentially work for Xe₂^0.4em0exn+. The dynamic alignment of neutral Xe₂ during the pulses is also discussed on the basis of numerical simulations.