XDL2011 Workshop 3

Ultra-fast Science with "Tickle and Probe"
Monday, June 20th - Tuesday, June 21st, 2011

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Organizers: Robert Schoenlein (Lawrence Berkeley National Laboratory), Brian Stephenson (Argonne National Laboratory), Eric Dufresne (APS), & Joel Brock (Cornell University)

Workshop Agenda (html)

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Purpose: The purpose of the workshop is to assess the potential use of ultra-fast (50 fs) x-ray pulses at high repetition rates (MHz to GHz) for time-resolved studies of weakly pumped (tickled) condensed matter systems. We are especially interested in exploring what might be feasible with the (nearly) diffraction-limited hard x-ray beams produced by Energy Recovery Linac (ERL) and Ultimate Storage Ring (USR) sources.

Description: The characteristic time for the structural rearrangement of atoms in condensed matter is set by the characteristic vibrational frequency of an atom, T_vib=2π(k/m)^½≈1ps (k~eV/a², a=3A, and m = 10^-25kg). Thus, the ultra-short (ps to < 50 fs) x-ray pulses produced by an ERL, or the ps pulses produced by a USR with crab cavities, will enable the opportunity to study the rearrangement of atoms (structural phase transitions, chemical reactions) on their natural time- and length-scales. Similarly, the natural time-scale for electronic structural dynamics (bond dynamics, valence charge flow, electronic phase transitions, correlated electron systems) mediated by the electron-phonon interaction is on the order of 1 ps.

The nature of the x-ray pulse train from an ERL or USR is complimentary to that of an X-FEL. An X-FEL produces very intense pulses at low repetition rates (100Hz @ LCLS); the ERL or USR with a crab cavity arrangement produces small pulses at very high repetition rates (MHz to GHz) but the time-average flux of these two sources is comparable. High repetition rate pump-probe experiments on atomic length- and time-scales take place in a largely unexplored regime of experimental phase space. Clearly, due to the low signal/x-ray pulse, ERL/USR experiments will require signal averaging over many pulses. On the other hand, ERL/USR pulses will not damage many samples. Similarly, to take advantage of the high repetition rate, the sample must be able to relax between driving pulses. Hence the driving pulse must only "tickle" rather than "pump" the sample. Weak optical pumps (e.g., Ti:Al₂O₃ laser running at >800MHz), nanofabricated (e.g. stripline) excitation/sample cells, and ultrafast THz pulses are possible pumps. Micro-focusing minimizes pump power required and speeds up relaxation via (thermal) diffusion. Time-resolved microscopy is also possible.

Examples of Potential Science:

1. AMO (EXAFS, SAXS, XRD)
   1. Molecular rotational dynamics
   2. Cluster dynamic
   3. Spin dynamics
   4. Dipole-dipole interactions
2. Chemistry (Laser-Induced Time-Resolved: XAS, SAXS, and XRD)
   1. Photo-electrochemistry
      1. Photo-induced solvation
      2. Charge transfer in ionic liquids
   2. Coherent control
      1. Pump-dump vs pump-pump
      2. Shaping of single or multi-photon spectrum
      3. Control of excited state wave packets directing reactivity
3. Condensed Matter (XMCD, XRD, XAS, XANES, ARPES)
   1. Beyond Born-Oppenheimer approximation
   2. Complex correlated materials
      1. Strongly correlated systems (high-Tc cuprates, CMR manganites)
      2. Magnetic materials (ferromagnetics, heterostructures)
      3. Multiferroics (ferromagnetism/ferroelectricity, interfaces)
      4. Organics - molecular crystals
      5. Optical quenching of CDWs and SDWs
   3. Molecular Dynamics
      1. Solute dynamics
      2. Solvent dynamics
      3. Surface chemistry/catalysis
   4. Magnetism
      1. TR Microscopy

Workshop contributions: Presentations will focus on the issues of ultra-fast, x-ray (2-15keV) studies, and on the advantages and disadvantages of ERL and USR hard x-ray sources, for such experiments.

Method: The workshop will occur at Cornell's Robert Purcell Center on June 20-21, 2011 and is open to anyone who may be interested. The workshop will start with a short overview of ERL and USR specifications and capabilities for frontier science with nanometer hard x-ray beams (see accompanying attachment for some ERL/USR background). The program, outlined below, will then continue with a core group of invited world-leaders to give short talks and lead the discussion in the relevant science areas. In order to allow time for discussion after each talk, speakers are asked to rigorously adhere to the 20 minute time limit. There will be ample additional time for open discussion and poster sessions. Workshop participants are especially encouraged to be inventive and explore unorthodox ideas.

Poster Session: There will be space to put up poster of up to 4'(height) x 6'(width) [1.2 x 1.8 meters]. If you plan to have a poster, please submit a ½ page abstract to Laura Houghton at lab49@cornell.edu and she will try to compile them into the program. There is a specific poster session time shown in the program, but they can remain up for the entire duration of the workshop.

Workshop Results: Our goal is to communicate the possibilities for science with an ERL or USR, as well as with other sources, and engage the community in developing ideas for the science case. Imaginative thinking will be required -- that's why we are assembling this workshop group! Documenting the results of the workshop is essential. We hope to explore compelling science that is uniquely enabled by an ERL or USR. Accordingly, each invited participant will be asked to present at least one novel experiment of interest (described by a 1-3 paragraphs of text, an appropriate graphic, and relevant references) that would be very difficult to perform without the capabilities of an ERL or USR. Time will be provided towards the end of the workshop to summarize results and conclusions.

Of particular importance is the group discussion at the end of the meeting. It is therefore very important that all invited workshop attendees stay through to the end of the workshop. It is in this period that our discussion leaders will try to pull all the good ideas generated into a summary and we need your very active participation to do this well. As a motivator to stay through to the end and to further the development of "community" for this brief several day period, we have arranged a series of excursions. Depending on the workshop these include, for example, Cayuga Lake Boat dinner cruises, local winery tours, visits to Ithaca's local spectacular gorge parks, etc.