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

Abstract

Almost isotropic angular distributions are observed for fragment ions from ${\mathrm{Xe}}_{2}$ produced with intense femtosecond laser pulses ($800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, $4\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$). The results observed for ${\mathrm{Xe}}_{2}$ present a great contrast to those for ${\mathrm{I}}_{2}$ molecules used as reference molecules, which show typical anisotropic angular distributions resulting from enhanced ionization. The kinetic energy spectra of the fragment ions from ${\mathrm{Xe}}_{2}$ reveal that the internuclear distance remains near the equilibrium distance for various pulse widths from 45 to $710\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$, indicating that in the rising edge of the pulses, the internuclear distance of ${\mathrm{Xe}}_{2}^{\phantom{\rule{0.4em}{0ex}}n+}$ ($n=0$, 1, 2) does not reach the favorable region for enhanced ionization even though enhancement can potentially work for ${\mathrm{Xe}}_{2}^{\phantom{\rule{0.4em}{0ex}}n+}$. The dynamic alignment of neutral ${\mathrm{Xe}}_{2}$ during the pulses is also discussed on the basis of numerical simulations.