Pumping K$_{\alpha}$ Resonance Fluorescence by Monochromatic X-Ray Sources

We demonstrate the correspondence between theoretically calculated K-shell resonances lying below the K-edge in multiple ionization states of an element\footnote{A. K. Pradhan, S. S. N. Nahar, et al, J. Phys. Chem. A \textbf{113}, 12356 (2009).} (Pradhan et al 2009), and recently observed K$_{\alpha}$ resonances in high-intensity X-ray free-electron laser (XFEL) plasmas\footnote{S. M. Vinko et al, Nature \textbf{482}, 59 (2012).} (Vinko et al 2012). Resonant absorptions due to K$_{\alpha}$ transitions in aluminum ions are computed and found to reproduce experimentally observed fluorescence features. Calculated fluorescence features for titanium are presented for possible observation of K$_{\alpha}$ resonances in the 4.5-5.0 keV energy range. A possibly sustainable excitation mechanism for K$_{\alpha}$ resonance fluorescence might be implemented using two monochromatic X-ray beams tuned to the K-edge and the K$_{\alpha}$ resonant energies simultaneously. This targeted ionization/excitation would create inner-shell vacancies via Auger decay, as well as pump K$_{\alpha}$ resonances. The required X-ray fluence to achieve resonance fluorescence would evidently be much less than in the XFEL experiments, and might enable novel biomedical applications.