Number of AXE photons at the end of the medium (z = 5 mm) for the pre-aligned 〈cos 2ζ〉 = 0.64 (a) and isotropic (b) CO ensembles as a function of the XFEL pump pulse duration τ

Figure 8. Number of AXE photons at the end of the medium (z = 5 mm) for the pre-aligned 〈cos 2ζ〉 = 0.64 (a) and isotropic (b) CO ensembles as a function of the XFEL pump pulse duration τ. The peak pump intensity 4.5 × 1016 W cm−2 is the same for all calculations. The results are compared for the C 1s−1X (\full) and O 1s−1B (−− − −) transitions.

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.