Tracing charge transfer in argon dimers by XUV-pump IR-probe experiments at FLASH

Abstract

Charge transfer (CT) at avoided crossings of excited ionized states of argon dimers is observed using a two-color pump-probe experiment at the free-electron laser in Hamburg (FLASH). The process is initiated by the absorption of three 27-eV-photons from the pump pulse, which leads to the population of Ar 2+ *-Ar states. Due to nonadiabatic coupling between these one-site doubly ionized states and two-site doubly ionized states of the type Ar + *-Ar + , CT can take place leading to the population of the latter states. The onset of this process is probed by a delayed infrared (800 nm) laser pulse. The latter ionizes the dimers populating repulsive Ar 2+-Ar + states, which then undergo a Coulomb explosion. From the delay-dependent yields of the obtained Ar 2+ and Ar + ions, the lifetime of the charge-transfer process is extracted. The obtained experimental value of (531 ± 136) fs agrees well with the theoretical value computed from Landau-Zener probabilities.

A synchronized VUV light source based on high-order harmonic generation at FLASH

“…We choose here an in-line approach with a transmission grating spectrometer and a calibrated large area photodiode, which is described in detail below. Pump-probe parameter ranges of selected experiments in which an external optical laser was used in combination with a XUV/X-ray FEL [35][36][37][38][39][40][41] . The pulse duration of pump and probe pulses is displayed together with the corresponding photon energy.…”

Atomic, Molecular and Cluster Science with the Reaction Microscope Endstation at FLASH2

“…Momentum conservation in three-dimension is used to sort out all particles originating from the same fragmenting target particle, that is finding coincident particles. Angle-and energy-resolved measurements of charged particles in multi-coincidence are the key features of the REMI and enable to investigate atomic and molecular processes in detail [5,6,29,30].…”

“…The capabilities to observe molecular dynamics with this setup is demonstrated exemplarily in an experiment investigating charge transfer (CT) in argon dimers using an XUV-pump IR-probe scheme [5]. The experiment was performed at FLASH1, using the same reaction microscope apparatus, but a different IR pump laser [40].…”

“…In many cases a first "pump" pulse triggers certain dynamics, while a delayed "probe" pulse is used to interrogate the system at a later point in time. This way, the temporal evolution of any specific process under investigation like molecular break-up [4], charge migration [5,6] or isomerization [7] within a molecule, to just name a few, can be observed on their natural time scale by using lasers with correspondingly short pulse lengths in the range of a few femtoseconds. A small number of different technologies exists to produce such short laser-like or coherent light pulses.…”

“…Specific potential energies and corresponding internuclear distances are indicated by horizontal and vertical dashed lines, respectively. Reproduced from[5], with the permission of AIP (American Institute of Physics) Publishing.…”