Correlated electronic and nuclear motion in molecules
Final Report Abstract
The aim of this project was to investigate time-resolved electronic and nuclear dynamics in molecules by bringing together a high repetition rate (400 kHz) laser system, a reaction microscope that allows simultaneous and coincident determination of the full 3D momentum vectors of several charged reaction fragments (electrons, ions). At the time of the proposal submission the laser system and the reaction microscope were under construction. After finishing the build-up of the laser system and the reaction microscope (ReMi) we were able to show that our ReMi design and the data acquisition electronics is well suited for high repetition rates. On the other hand it turned out that the few µJ pulse energy is too low for e.g. pump-probe experiments. On these grounds, we decided to move the ReMi to a different laser system with 10 kHz repetition rate delivering mJ pulses with 20 to 30 fs duration not only at 800 nm but also in the IR spectral range (1 to 2 µm). These laser parameters enabled the study of strong-field recollision in laser aligned 1,3-Butadiene, a molecule that has been studied extensively in the Canadian group of A. Stolow especially by J. Mikosch, who joined the Max-Born-Institute in 2014 and became part of this project. Strong-field-ionization of Butadiene leads not only to the population of the ionic ground state but also the first excited ionic state. The latter one undergoes fragmentation and by making use of the electron-ion coincidence detection of the ReMi the electron spectra of the two states can be separated. The investigation of the strong-field electron recollision of the two states is of special interest since it gives detailed insights into the process of self-imaging of molecules by laser induced electron diffraction (LIED). By measuring the angular dependence of the recollision yield in the molecular frame it was shown that the structure of the returning wave packet strongly depends on the initially ionized molecular state. This finding was confirmed by theoretical calculations performed in collaboration with T. Bredtmann and S. Patchkovskii at MBI. In collaboration with the group of A. Stolow strong-field ionization of 1-butene has been studied as a function of wavelength using photoion-photoelectron covariance and coincidence spectroscopy. A striking transition in the fragment-associated photoelectron spectra has been observed. While for photon energies less than the cation D0–D1 gap a single Above Threshold Ionization (ATI) progression is registered, two ATI progressions are seen for a photon energy greater than this gap. In the first case, electronically excited cations are created by sequential SFI population via the ground cationic state D0. In the second case, direct sub-cycle SFI to the D1 excited cation state contributes significantly. In order to demonstrate the multi coincidence capability of our ReMi we have studied the Coulomb explosion of CH2BrI3+. Although the three-fold ionization is a minor channel during strong-field ionization, the three final fragments could clearly be identified and their 3D momenta were determined with high precision. Five different break-up pathways have been identified: a non-sequential Coulomb explosion where all three ionic fragments are formed at the same time and four sequential channels where intermediate doubly-charged fragments are formed initially.
Publications
-
Molecular orbital imprint in laser-driven electron recollision. Sci. Adv. 4, 7 (2018)
F. Schell, T. Bredtmann, C. P. Schulz, S. Patchkovskii, M. J. J. Vrakking, and J. Mikosch
-
Sequential and direct ionic excitation in the strong-field ionization of 1-butene molecules. Phys. Chem. Chem. Phys. 20, 14708 (2018)
F. Schell, A. E. Boguslavskiy, C. P. Schulz, S. Patchkovskii, M. J. J. Vrakking, A. Stolow, and J. Mikosch
