Simulation of X-ray laser by means of dissociative core-excited states

The recent advent of the X-ray free-electron laser (XFEL) based on self-amplified spontaneous emission (SASE) has brought new opportunities in X-ray physics and many scientific applications. On the other hand, the shot-noise start-up in the SASE mechanism lends an inherent stochastic character to X-ray pulses, leading to rather large variations both in wavelength and intensity. One strategy to solve the problem is to use an XFEL pulse to create a population inversion in a medium which then lases in the X-ray region (See, N. Rohringer et al., Nature, 481, 488 (2012)). Alternatively, resonant core excitation can be used as well. Recently, a theoretical chemistry group led by Professor F. Gel'mukhanov (Royal Institute of Technology, Sweden) has published a prediction of X-ray lasing based on resonant core excitation of a molecule to a state which is subject to ultrafast dissociation, i.e., a state in which dissociation precedes the femtosecond core hole decay. As an example, Cl 2p_1/2 → 6σ excitation of the HCl molecule by an XFEL pulse and the subsequent ultrafast dissociation were studied. For more information, see the paper, "Dissociative X-ray Lasing", Q. Miao et al., Phys. Rev. Lett., 109, 233905 (2012).