Ionization of positive ions by relativistically-intense short laser pulses is analyzed in the framework of relativistic strong-field theory. We observe the appearance of broad interference-free patterns in the high-energy portion of the photoelectron spectra, which extend over hundreds of driving photon energies. These structures can be controlled by changing parameters of the driving laser field. As we also demonstrate, the electrons comprising these broad structures can form pulses of attosecond duration. While we present the fully numerical results for laser field intensities below 10<sup>20</sup>W=cm<sup>2</sup>, we also introduce the saddle point approximation to treat photoionization at larger intensities. In addition, the conditions enabling generation of ultrashort electron pulses are studied.
The optical frequency comb has become an indispensable tool for high precision spectroscopy. Also experiments in the field of ultrafast physics rely on the frequency comb technique to generate precisely controlled attosecond optical pulses by means of the high-order harmonic generation. However, in order to generate even shorter laser pulses or to apply this technique in investigations of nuclear structure, combs of frequencies of the order of MeV are necessary. It seems that it may not be possible to achieve such photon energies by high-order harmonic generation. In this context the possibility of the generation of Thomson and Compton-based frequency combs is presented. Diffraction of generated radiation by a sequence of laser pulses and its analogy to the diffraction grating is elucidated.