Generation of attosecond pulses and attometrology

Progress in the development of compact sources of coherent X-ray radiation using high harmonics of laser radiation and the generation of attosecond pulses underlie the emergence and rapid advance of attosecond physics and attometrology. Attometrology allows probing internal processes in matter related to the motion of electrons and light nuclei.
A number of important results in the field of attosecond physics and attometrology were obtained by the IAP RAS team headed by A. M. Sergeev and M. Yu. Ryabikin. The generation of high harmonics of laser radiation was investigated theoretically taking into consideration the structure of electron wave packets detached from atoms and rescattered at parent ions and features of cross sections of free-bound transitions in different atoms. In particular, it was proposed to increase the efficiency of high-harmonic generation by using frequency anomalies in cross sections of free-bound electron transitions. The ellipticity of high harmonics was explained and the conditions for efficient generation of high harmonics with elliptical and circular polarization were found. The effect of a sharp increase in the efficiency of generation of attosecond pulses at over-barrier ionization of atoms by powerful optical pulses using initially excited electronic states was revealed. It was shown theoretically that the efficiency of generation of high harmonics and attosecond pulses at laser ionization of molecules can significantly exceed the efficiency of analogous processes in atomic systems. The capabilities of controlling energy, spectral and temporal characteristics of attosecond pulses using molecular vibrational, dissociative and rotational wave packets were demonstrated.

Generation of broadband radiation with spectrum cor-responding to "water window" (λ = 2.3—4.4 nm) at over-barrier ionization of stretched molecular ions aligned along the electric field of laser pulse (wavelength 400 nm, intensity 1.4 · 1017 W/cm2)

Spectrogram of integral signal of high harmonics generated by D2+ ions. Instantaneous values of intensities of different frequency components of the signal are shown by color. Revival orders of molecular vibrational wave packet for the corresponding delays between exciting and probing pulses are shown on top

Methods of imaging ultrafast internal dynamics of molecules by probing them by electrons detached from the molecules in a strong laser field and rescattered at the parent ions were developed. In particular, a method of probing long-term evolution of molecular vibrational wave packets based on measuring the signal of high harmonics of the probe laser pulse using the effect of enhanced ionization of stretched molecules was proposed. Techniques of retrieving information about molecule structure from the spectra of rescattered electrons and generated photons that enable attaining subfemtosecond temporal and subangstrom spatial resolution were developed.