Ultrafast AMO physics at the LCLS x-ray FEL

Abstract

The Linac Coherent Light Source at the SLAC National Accelerator Laboratory, Stanford University, began operation in 2009 as the world's first hard x-ray free electron laser. Early experiments have concentrated on atomic physics, and have demonstrated several key features of the ultrafast high field x-ray-atom interaction. This paper reviews some of these early results. 1. INTRODUCTION: WORLD'S BRIGHTEST ULTRAFAST X-RAY SOURCE, LCLS The new class of so-called fourth generation synchrotron light sources are a revolutionary departure from storage ring-based electron synchrotrons. They are true x-ray lasers, which use relativistic electrons as a gain medium. The Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory in Menlo Park California is the world's first hard x-ray free electron laser (XFEL) (1). It produces coherent x-ray fields that exceed the brightness of any previous source (measured in energy per unit time per unit solid angle per unit frequency interval) by a factor of one billion or more. The increase of peak brightness is due to a combination of increased pulses energy and decreased pulse duration. These novel lasers allow us to observe the fastest timescales in atoms and molecules, which are due to the interactions among electrons. The timescales involved can be characterized by electronic energy separations, which can be many electron volts. Any ultrafast laser source is limited to pulse durations no shorter than a single cycle of the electromagnetic field. For the conventional Ti : Sapphire laser ( = 800nm), this is about 2fs. But for x-rays, the pulse duration can be much shorter. Simulations show that the LCLS is capable of sub-femtosecond pulses, which could capture electronic coherences in molecules. X-rays can reveal multielectron processes inside atoms and molecules. Furthermore, these sources have not yet reached their intrinsic limits in pulse duration, where sub-femtosecond pulses are possible (2). Accelerator and x-ray laser scientists and engineers at SLAC have also begun to explore techniques that will one day provide routine operation with pulses whose time-bandwidth product is near the Fourier transform limit (3). Such control over x-ray fields could be exploited just as in the field of quantum optics with visible and ultraviolet laser pulses. This short review will highlight some of the first results, and the opportunities for further work in strong-field x-ray physics of atoms and molecules. The work described here was all done at the LCLS x-ray free electron laser by the Stanford PULSE Institute AMO group in collaboration with other AMO research groups around the world (4-9). Some of the experiments described form the thesis research of two Stanford students, Mike Glownia and James Cryan (10, 11).