By Y Liu
World Scientific
The challenge of developing more intense, shorter-pulse lasers has already seen outstanding results and opened up completely new perspectives. In fact, the next generation of very-high-power laser facilities will provide the opportunity to explore even ultrarelativistic and vacuum nonlinearity at unprecedented levels, moving towards a QCD regime. At the same time, during the last few years, attosecond physics has provided a new, intriguing way to visualise both atoms and molecules, and the electromagnetic-field structure of the excitation wave packet itself, because this time domain is comparable with the classical periods of electrons orbiting around the nucleus. This growing research field is so recent that the literature on the subject is not yet adequate: in this sense, this book partially fills the gap. It contains contributions from several Chinese groups, both experimental and theoretical, and reports on recent studies of bound electron and molecular nonlinearities. The content is organised over eight chapters and spans a broad range of topics of this specialist subject.
Strong-field tunnelling is a possible key to the ionisation of neutrals. It offers a sophisticated method to image and probe atomic and molecular quantum processes. In fact, the study of direct and rescattered (by the nucleus) electrons in the ionisation process is able to resolve orbitals; in this context, it becomes important to go beyond strong-field approximation, and to evaluate the contribution of the long-range Coulomb field generated by the ion in the electron dynamical evolution (chapter 1).
Direct and rescattered electrons can be recorded together as a reference wave and a signal wave, respectively: the interferential patterns constitute the analogue of optical holography, reconstructing the illuminated objects. It is possible to integrate the influence of the Coulomb field, either in a numerical solution of the time-dependent Schrödinger equation (TDSE) or in a more intuitive quantum-trajectories Monte Carlo method describing the formation mechanisms of the photoelectron angular distribution of above-threshold ionisation (chapter 2).
Dissociation is a basic process of physical chemistry and, before the advent of new ultrafast tools, seemed completely out of scientists’ control, because the typical timescale is below the femtosecond range. For an easier comparison of theoretical predictions and experimental results for a molecule interacting with a strong ultrashort laser pulse, it is necessary to start with the simplest systems – the hydrogen molecular ion H+2. In chapter 3, on the basis of a numerical analysis of the related TDSE, the author suggests a pump–probe strategy to understand dissociation.
The theoretical discussion of double ionisation in a strong laser field is treated in chapters 4 and 5 for different kinds of atoms. In the case of high Z, the experiments show a different degree of correlation of the two expelled electrons, with respect to the low-Z case: this is due to the major importance of rescattering, as described by a semiclassical model. For the simpler systems H2 and He, TDSE is a powerful tool for calculating all of the main features of double ionisation (total and differential cross-sections, recoil-ion momentum spectra, two electron angular distributions, and two electron-interference phenomena).
A promising application of strong-field excitation on atoms and molecules is high-order harmonics generation (HHG), usually providing a XUV comb with different harmonics at the same intensities, both in a single attosecond pulse and in a train of attosecond pulses, by a conversion of the light frequency from IR to the X-ray regime. This technique provides a tomographic image of molecular orbitals as an alternative to scanning tunnelling microscopy or angle-resolved photoelectron spectroscopy, as well as a way to study ultrafast electronic structures, electron dynamics and multichannel dynamics (chapters 6 and 7).
Finally, chapter 8 presents an interesting review of the properties of free electron laser radiation, showing how nuclear motion in photo-induced reactions can be monitored in real time, the electronic dynamics in molecular co-ordinates can be extracted, and the site-specific information in the structural dynamics of chemical reactions can be provided. The experiments are based on EUV pump–probe and optical pump-X-ray probe excitation techniques, and are located at FLASH (Hamburg) and LCLS (SLAC), respectively.
As a summary, the book is a useful update for people who are interested in the specialised field of the interaction of atoms and molecules with femtosecond or sub-femtosecond high-intensity fields. The comprehensive bibliography allows the reader to gain a more exhaustive view of the subject.
