Pulse Energy and Pulse Duration Effects in the Ionization and Fragmentation of Iodomethane by Ultraintense Hard X Rays

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 (CH 3 I) by ultraintense (∼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.

Simple model for sequential multiphoton ionization by ultra-intense x-rays

“…The functional dependence of the sequential n-photon ionization probability is then used to calculate the scaling of the ion yield with pulse energy, which is found to quantitatively describe the behaviour of the measured ion yields showing a "power-law" increase followed by saturation. The average time interval between ionizations for producing a given charge state is found to depend only on the pulse duration with a proportional relation, and not on any other pulse parameters, which is consistent with the conclusion drawn from a previous experiment studying ultra-intense hard x-ray ionization of CH 3 I molecules [21]. In that experiment, the kinetic energy of iodine ion fragments was found to be independent of pulse energy, and to increase with shorter pulse durations because of the decreased time interval between ionizations.…”

“…The dependence of the average time interval between ionizations on the x-ray pulse parameters can therefore be studied by measuring the kinetic energy of ions produced from ionization of molecules by x-rays with different pulse energies or durations. In a previous study [21], where we studied the pulse energy and pulse duration dependence of ultra-intense hard x-ray ionization of CH 3 I molecules, the iodine ion kinetic energy was found to be independent of pulse energy, but increases with shorter pulse duration. This experimental observation supports the conclusion reached from the sequential multiphoton ionization model that the average time interval between ionizations is proportional to the pulse duration τ and independent of all other pulse parameters.…”

“…This experimental observation supports the conclusion reached from the sequential multiphoton ionization model that the average time interval between ionizations is proportional to the pulse duration τ and independent of all other pulse parameters. An intuitive picture which explains such pulse parameter dependence can be found in [21].…”

Resonance-enhanced x-ray multiple ionization of a polyatomic molecule

“…In order to eliminate the effect of pulse energy fluctuations across the experimental runs and for a fair comparison between the molecular and atomic cases, all data are filtered on specific pulse energy windows and weighted according to the pulse energy distributions with a procedure previously discussed in the Supplemental Material of Ref. [15].…”

“…In molecules, x-ray ionization is often strongly localized at the atomic sites with the largest photoabsorption cross sections. It is typically accompanied by subsequent charge redistribution and molecular fragmentation [10][11][12][13][14][15].…”