(a) Propagation geometry: E

Figure 2. (a) Propagation geometry: EL, EXFEL and E mark the electric field polarization vectors of the optical laser used for impulsive alignment, XFEL and AXE radiation, respectively. All three pulses propagate along the z-axis direction, as is shown by the corresponding wave vectors kL, kXFEL and kAXE. (b) Degree of molecular alignment 〈cos 2ζ〉 as a function of the delay time ta computed using initial rotational temperature T = 100 K (\full) and 300 K (−− − −). The insets show the angular density distribution β(ζ) of CO molecules at times related to the maxima and minima of the 〈cos 2ζ〉 curves.

Abstract

We theoretically demonstrate the feasibility of x-ray lasing in the CO molecule by the core ionization of the C K- and O K-shell by x-ray free-electron laser sources. Our numerical simulations are based on the solution of generalized Maxwell–Bloch equations, accounting for the electronic and nuclear degrees of freedom. The amplified x-ray emission pulses have an extremely narrow linewidth of about 0.1 eV and a pulse duration shorter than 30 fs. We compare x-ray lasing transitions to the three lowest electronic states of singly ionized CO. The dependence of the lasing efficiency on the spectral width of the x-ray fluorescence band, value and orientation of the electronic transition dipole moment, lifetime of the core-excited state and the duration of the pump pulse is analysed. Using a pre-aligned molecular ensemble substantially increases the amplified emission. Moreover, by controlling the molecular alignment and thereby the alignment of the transition dipole moment polarization, the control of the emitted x-ray radiation is achievable. Preparing the initial vibrational quantum state, the x-ray emission frequency can be tuned within the fluorescence band. The present scheme is applicable to other diatomic systems, thereby extending the spectral range of coherent x-ray radiation sources based on stimulated x-ray emission on bound transitions.