Electron Rearrangement Dynamics in DissociatingI2n+Molecules Accessed by Extreme Ultraviolet Pump-Probe Experiments

Abstract

The charge rearrangement in dissociating I n+ 2 molecules is measured as a function of the internuclear distance R using XUV pulses delivered by the free-electron laser (FEL) in Hamburg. Within an XUV pump-probe scheme the first pulse initiates dissociation by multiply ionizing I 2 , and the delayed probe pulse further ionizes one of the two fragments at a given time, thus triggering charge rearrangement at a well-defined R. The electron transfer between the fragments is monitored by analyzing the delay-dependent ion kinetic energies and charge states. The experimental results are in very good agreement with predictions of the classical over-the-barrier model demonstrating its validity in a thus far unexplored quasimolecular regime relevant for FEL, plasma and chemistry applications.The dynamics of charge relaxation, rearrangement and equilibration at the transition between chemically bound and unbound systems is essential for understanding many chemical [1, 2] and plasma reactions [3,4]. It is also crucial for the imaging of biomolecules with atomic resolution, a key application of Free-Electron Lasers (FELs). Here, the idea is to take a snapshot of the molecule by irradiating the system with intense ultra-short (< 100 fs) X-ray pulses before it is damaged [5]. Recent experiments on nano-crystalized lysozyme, however, have shown that the photon-induced damage leads to a scattering pattern that is different from what is expected for the intact molecule even for X-ray pulses as short as 70 fs [6]. Photon absorption is strongly localized at constituents with high atomic numbers, often having more than an order of magnitude larger cross sections as compared to H or even C atoms [7]. Therefore, it is crucial to understand the underlying ultrafast electronic and nuclear rearrangement dynamics in order to develop improved damage models that take into account the spatio-temporal spread of the locally induced charge.Localized photon absorption efficiently triggers atomic movement and electron rearrangement across the entire molecular ion, mainly leading to its fragmentation. In a recent study methylselenol [8] and ethylselenol [9] molecules, containing one heavy selenium atom as photoabsorption center, were irradiated with single intense 5 fs X-ray pulses. From the observed charge-state distributions and the fragments' kinetic energies it could be concluded that even for such short pulses ultrafast charge rearrangement takes place, accompanied by considerable atomic displacements which is relevant for imaging with atomic resolution. However, the central question concerning the underlying time and length scales remained open. In this work we present experimental results on the electron transfer between two iodine ions at freely chosen internuclear distances R. A dissociation of the I 2 molecule is triggered by multiple ionization with a femtosecond pump pulse [10]. The evolving system is then further ionized by the delayed probe pulse. Depending on the time delay, or the corresponding internuclear distance, and the charg...

X-ray multiphoton ionization dynamics of a water molecule irradiated by an x-ray free-electron laser pulse

“…Specifically, a core ionization that is purely located on one atomic site may give rise to charging up at another atomic site of the molecule, so the charges are redistributed among neighboring atoms within the molecule. This charge rearrangement might yield different ion fragments than one would expect from the x-ray ionization of the individual constituent atoms [20,22,23]. Theoretical descriptions for these observations have been developed so far only based on phenomenological models [19,24,25].…”

“…In this context electronic transition rates, i.e., photoionization cross sections and decay rates, were derived from the values for individual atoms. However, it has been found that relevant aspects of the radiation damage in molecules must be understood rather from the molecular electronic structure [7,[19][20][21][22][23]. Specifically, a core ionization that is purely located on one atomic site may give rise to charging up at another atomic site of the molecule, so the charges are redistributed among neighboring atoms within the molecule.…”

Nonlinear resonant Auger spectroscopy in CO using an x-ray pump-control scheme

“…These versatile x-ray sources will therefore soon open the pathway for implementing all x-ray pump-probe schemes. One very successful scheme turns out to be splitting of the FEL pulse into two replicas and using them as time-delayed pump and probe pulses; this scheme has been successfully applied to study interatomic Coulombic decay in neon dimers [11], charge transfer in iodine molecules [12] and iodomethane [13], photoexcited molecular dynamics [14] and so on; an even more elaborated scheme, which makes use of a pair of x-ray pulses with slightly different wavelengths [15][16][17][18][19], was used to probe ultrafast electronic and molecular dynamics [19][20][21][22][23]. Here, we propose a different x-ray pump-probe scheme based on two time-delayed x-ray pulses of different frequencies [15][16][17][18] to study vibrational wave-packet dynamics in core-excited electronic states.…”

Ultrafast x-ray-induced nuclear dynamics in diatomic molecules using femtosecond x-ray-pump–x-ray-probe spectroscopy

“…However, extracting information from the experimental schemes involving strong-field interactions requires theoretical modeling to account for the induced strong-field effects [14]. With the advent of FELs, a new generation of pump-probe schemes was conceived that uses two XUV pulses [15][16][17][18]. The high flux of photons in a femtosecond pulse generated by FELs is crucial to obtain the needed signal in the pump-probe scheme, in which absorption of at least one photon from each pulse by the same molecule is required.…”