| Much of our knowledge about molecular structure and reactivity is based on interpreting how molecules interact with light. In femtochemistry experiments, information about an evolving molecular structure is inferred by relying on available knowledge about the way that molecular absorption spectra depend on the instantaneous molecular structure. In small molecules this is possible, but it soon becomes problematic when the size of a molecule increases. In larger molecules it becomes more attractive to rely on diffraction as a means to obtain structural information. In a diffraction experiment structural information is encoded in interference patterns that result from the way that an electron or light wave scatters. In the present proposal we will perform experiments making use of 4th generation light sources that generate intense, femtosecond XUV and x-ray pulses, such as the FLASH and XFEL lasers in Hamburg and the LCLS laser at Stanford. We will study structural rearrangement in molecules by means of electron diffraction, making use of electrons that are generated within the molecule by photo-ionization. Using electrons formed in photo-ionization as a means to ?illuminate the molecule from within?, time-resolved molecular dynamics can be studied in a manner that is radically different from the methods that have so far been used in femtochemistry experiments, and that connects in a much more direct way to the structural transformations that are occurring. On the one hand, (inner-shell) electrons that are emitted from a single location within the molecule will diffract off other atoms in the molecule, leading to interferences in the molecular frame photoelectron angular distribution (MF-PAD) that encode structural information. On the other hand, the production of equivalent electrons at different locations within a molecule will lead to multi-centre interference in the MF-PAD due to the indistinguishibility of the emitted electrons (thereby again encoding structural information into the angular distributions). The proposed experiments will exploit the recent development (in our laboratory) of highly pre-aligned and oriented molecular samples. The workplan that we aim to carry out consists of pump-probe experiments at FLASH where two-center interference and intra-molecular electron diffraction will be used to monitor photodissociation of aligned Br2 molecules and oriented NO molecules, as well as Br2 bond-formation in photofragmentation of CHBr3. In addition, we will take part in user experiments at LCLS (as part of a large international consortium) measuring x-ray diffraction and electron diffraction in aligned 1,4-diiodo-benzene molecules. In our laboratory at AMOLF we will work on further improvement of our molecular sample preparation techniques and on the theory of measuring photoelectron angular distributions in the molecular frame. Also, we will perform experiments at AMOLF measuring MF-PADs at XUV wavelengths that can be produced by means of high-harmonic generation (HHG). |
