  An x-ray pulse-shaping scheme is put forward for imprinting an optical
frequency comb onto the radiation emitted on a driven x-ray transition, thus
producing an x-ray frequency comb. A four-level system is used to describe the
level structure of N ions driven by narrow-bandwidth x rays, an optical
auxiliary laser, and an optical frequency comb. By including many-particle
enhancement of the emitted resonance fluorescence, a spectrum is predicted
consisting of equally spaced narrow lines which are centered on an x-ray
transition energy and separated by the same tooth spacing as the driving
optical frequency comb. Given a known x-ray reference frequency, our comb could
be employed to determine an unknown x-ray frequency. While relying on the
quality of the light fields used to drive the ensemble of ions, the model has
validity at energies from the 100 eV to the keV range.

Atomic Physics;Optics