Seminars Archive

Attosecond measurements with ultrafast X-rays – the present and the future prospects

Abstract
We will motivate and describe recent progress on measuring non-adiabatic molecular dynamics associated with sudden ionization or excitation of molecules and polymers and unfolding on timescales from sub-femtosecond to few-femtosecond. First we will discuss the physical circumstances and nature of these processes, especially in terms of quantum coherence and decoherence, and then set-out what is required from incisive measurements to strengthen our understanding of these fundamental photo-physical events. Three methods have been used in our recent research based on ultrafast strong-field optical lasers and ultrafast X-ray lasers. In the first of these we have studied dynamics of strong field molecular ionization in some prototypical polyatomic organic molecules using an extension of the chirp-encoded high harmonic spectroscopy technique we introduced some years ago [2]. Using an 1800 nm OPA system to overcome the limits set by low ionization saturation intensity to HHG spectroscopy in organic molecules [3] we have developed a two-dimensional technique in which scans of HHG spectrum versus laser intensity can be used to extract electronic and nuclear dynamics in strong-field ionized benzene and substituted benzenes. By hollow fibre pulse compression of our OPA and optimization of HHG generation conditions in Ne and He gas we have demonstrated the generation of soft X-ray attosecond pulses with flux sufficient for XANES spectroscopy up to the O K edge at 540 eV [4]. This source was used to study the X-ray spectrum of an organic-optoelectronic polymer (P3HT) [5] and we are now developing time resolved measurements to study the dynamics of exciton formation and decay in these materials. The problem of measuring transient hole dynamics, arising from electron correlation driven processes such as charge migration, following sudden X-ray ionization of a molecule has long been an open question in attosecond science. We have developed a direct measurement concept of such transient hole dynamics [6] that we have recently demonstrated in isopropanol using an X-ray pump- X-ray probe method at LCLS employing the fresh slice two colour/two pulse mode. We report the first results on the evolution of an inner valence hole state in this molecule. Future prospects for sub-femtosecond resolved linear and non-linear X-ray measurements using X-ray FELs will be motivated by very recent results from our collaborations at LCLS. References [1] M.Vacher et al, Phys.Rev.Lett. 118, 083001 (2017) [2] S.Baker et al, Science, 312, 424 (2006) [3] F.McGrath et al, Rev.Sci,Inst., 88, 103108 (2017) [4] A.S.Johnson et al, Science Advances, 4, 3761 (2018) [5] A.S.Johnson et al, Structural Dynamics, 3, 062603 (2016) [6] B.Cooper et al, Faraday Discussion, 171, 93 (2014)