High-resolution Interferometric Diagnostics for Ultrashort Pulses

6. HIGH-HARMONIC GENERATIONcomponents of the bound state. Temporal information can be inferred because of the brevity andchirp of the returning wavepacket, and the resolution may be significantly below 1 fs.However, there are many challenges associated with extracting meaningful information fromhigh-harmonic radiation. The properties of the returning electron wavepacket affect the radiationand must be known or accounted for in some way. A strong laser field is inherent in the processand its influence upon the sample is also a consideration. Finally, the structural and dynamicalinformation in the measured harmonic spectra is often encoded in a subtle and complex manner.This section reviews various approaches.In molecules, the ground state has many centres of higher electron density. The recombinationdipole element is the coherent superposition of the dipole element resulting from each of thesecentres. Both the location of the centres [348–350] and the alignment of the molecule with respectto the laser polarisation [351–354] affect the interference of the emission from the centres.Instead of looking for specific correspondences between certain features in the harmonic spectrumand the molecular orbitals, tomographic reconstruction [355] was used to image the orbitalsby measuring a series of harmonic spectra at different alignment angles. The process is similarto computer-assisted tomography used in medicine. The interpretation of the molecular orbitalcreated controversy [356], chiefly because of the single-electron approximation used in the reconstruction.Theoretical analysis revealed that tomographic reconstructions represent a mixtureof molecular orbitals [357, 358] but for the nitrogen molecule used in the initial demonstration,the reconstructed orbital was a good qualitative picture of the highest-occupied molecular orbitalin the Hartree-Fock picture. Multi-electron behaviour is expected to become more important asmolecular size increases.Concerning dynamics, one strand of research has focused on the evolution of the core (comprisingthe other electrons and the nuclei) during the excursion of the ionized electron. The stateof the core is then sampled by the electron at the time of collision. This motivated examinationof HHG spectra for signs of sub-fs dynamics; extending previous studies [359–361] of the kineticenergy of fragments produced by double-ionization caused by the returning electron. Another exampleis the sensitivity of HHG to the vibrational motion of the H + 2ion created when the process128