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Abstract:
The interaction of intense femtosecond x-ray pulses with molecules
sensitively depends on the interplay between multiple photoabsorptions,
Auger decay, charge rearrangement, and nuclear motion. Here, we report
on a combined experimental and theoretical study of the ionization and
fragmentation of iodomethane (CH3I) by ultraintense (similar to 10(19)
W=cm(2)) x-ray pulses at 8.3 keV, demonstrating how these dynamics
depend on the x-ray pulse energy and duration. We show that the timing
of multiple ionization steps leading to a particular reaction product
and, thus, the product's final kinetic energy, is determined by the
pulse duration rather than the pulse energy or intensity. While the
overall degree of ionization is mainly defined by the pulse energy, our
measurement reveals that the yield of the fragments with the highest
charge states is enhanced for short pulse durations, in contrast to
earlier observations for atoms and small molecules in the soft x-ray
domain. We attribute this effect to a decreased charge transfer
efficiency at larger internuclear separations, which are reached during
longer pulses.
