Femtosecond charge and molecular dynamics of I-containing organic molecules induced by intense X-ray free-electron laser pulses

Abstract

We studied the electronic and nuclear dynamics of I-containing organic molecules induced by intense hard X-ray pulses at the XFEL facility SACLA in Japan. The interaction with the intense XFEL pulse causes absorption of multiple X-ray photons by the iodine atom, which results in the creation of many electronic vacancies (positive charges) via the sequential electronic relaxation in the iodine, followed by intramolecular charge redistribution. In a previous study we investigated the subsequent fragmentation by Coulomb explosion of the simplest I-substituted hydrocarbon, iodomethane (CHI). We carried out three-dimensional momentum correlation measurements of the atomic ions created via Coulomb explosion of the molecule and found that a classical Coulomb explosion model including charge evolution (CCE-CE model), which accounts for the concerted dynamics of nuclear motion and charge creation/charge redistribution, reproduces well the observed momentum correlation maps of fragment ions emitted after XFEL irradiation. Then we extended the study to 5-iodouracil (CHINO, 5-IU), which is a more complex molecule of biological relevance, and confirmed that, in both CHI and 5-IU, the charge build-up takes about 10 fs, while the charge is redistributed among atoms within only a few fs. We also adopted a self-consistent charge density-functional based tight-binding (SCC-DFTB) method to treat the fragmentations of highly charged 5-IU ions created by XFEL pulses. Our SCC-DFTB modeling reproduces well the experimental and CCE-CE results. We have also investigated the influence of the nuclear dynamics on the charge redistribution (charge transfer) using nonadiabatic quantum-mechanical molecular dynamics (NAQMD) simulation. The time scale of the charge transfer from the iodine atomic site to the uracil ring induced by nuclear motion turned out to be only ∼5 fs, indicating that, besides the molecular Auger decay in which molecular orbitals delocalized over the iodine site and the uracil ring are involved, the nuclear dynamics also play a role for ultrafast charge redistribution. The present study illustrates that the CCE-CE model as well as the SCC-DFTB method can be used for reconstructing the positions of atoms in motion, in combination with the momentum correlation measurement of the atomic ions created via XFEL-induced Coulomb explosion of molecules.

Molecular Dynamics of XFEL-Induced Photo-Dissociation, Revealed by Ion-Ion Coincidence Measurements

“…Acknowledgments: First and foremost, we are most grateful for all of the co-authors of [11,12,14,15] …”

“…Extracting high-quality coincidence datasets by MF is, on one hand, more difficult in this case because no clear axis of momentum distribution exists and it is shared by a number of ions. On the other hand, the ionization rate/pulse was relatively low in this experiment [14,15]. Here, again, preliminary Coulomb explosion simulations are valuable in deciding the strategy of applying MF.…”

“…The experiments were carried out at beam line 3 (BL3) of the SACLA X-ray free electron laser (XFEL) facility [35] and are described in detail in [12][13][14][15]. The XFEL beam is focused by the Kirkpatrick-Baez (KB) mirror system to a focal size of about 1 µm (FWHM) diameter.…”

“…Specifically, we present examples on how to extract detailed information about the nuclear, and also the related electron dynamics by using coincident ion spectroscopy and momentum filtering analysis methods. The full results of these experiments and their interpretation are published or submitted for publication elsewhere, in [12] for CH 3 I, in [13] for CH 2 I 2 , and in [14,15] for 5-iodouracil.…”

Coulomb implosion of tetrabromothiophene observed under multiphoton ionization by free-electron-laser soft-x-ray pulses

“…In our earlier works, we investigated molecules with one or two x-ray-absorption hot spots, i.e., iodine atoms in iodomethane [16], di-iodomethane [17], and iodouracil [18,19], and applied a Coulomb explosion model with a few empirical parameters representing the charge dynamics. The experimental technique used was multi-ion coincidence timeof-flight spectroscopy with three-dimensional momentum resolution [20][21][22].…”