Molecular Physics seminar

Towards time-resolved electron diffraction from aligned molecules

by Per Johnsson (Atomic physics, Physics Department, Lund University)

FA31 ()


One of the main driving forces behind the development of high flux extreme ultraviolet (XUV) and X-ray free-electron lasers (FELs) is the possibility of diffractive imaging of small objects. One of the prospects of X-ray FELs is to image isolated bio-molecules by reconstruction from single-shot photon diffraction patterns and around the world enormous efforts are currently being invested in developing and building X-ray FELs. Complementary to diffraction of X-ray photons, one might employ diffraction of electrons, emitted as a result of photo-ionization, as a probe of molecular structure. In all molecular imaging experiments, it is an absolute requirement that the measurements can be done in the molecular frame, meaning in a coordinate system that is fixed with respect to the molecular axis. This can be done by aligning the molecules prior to doing the experiment, using for example laser-induced molecular alignment techniques which, in chemistry and physics research using table-top lasers, are today well-developed and have acquired a central role. I will present the results of initial experiments performed at FLASH, aiming towards time-resolved molecular structure determination using photoelectron diffraction. In a first proof-of-principle experiment, we have demonstrated laser-induced molecular alignment at FLASH, using the femtosecond infrared pulses from the in-house pump-probe laser system to create an aligned sample of CO2 molecules, and 46 eV photons from FLASH to probe the degree of alignment by recording the momentum distribution of O+ fragments resulting from Coulomb explosion of the CO22+ ions created through photo-ionization. For the detection we use a velocity map imaging spectrometer. Following this successful demonstration of alignment and probing of static molecules, we proceeded in April 2009 towards studying molecules undergoing dynamics. To that end, we have set up a three-pulse experiment at FLASH, where an 800 nm laser pulse from the in-house IR laser was used to align Bromine molecules, followed by a 400 nm pump pulse that induced dissociation of the neutral molecule, which was finally probed through XUV ionization by the FEL pulses. The main achievement of this campaign was that we managed to demonstrate FEL probing of dissociating aligned molecules, reaching pump-probe delay resolutions close to 100 fs, and thus observing a time-dependence of the fragment kinetic energy release at different points along the dissociation curve.