Abstract
High-order harmonic generation (HHG) is a nonlinear nonperturbative process in ultrashort intense laser-matter interaction. It is the main source of coherent attosecond (1 as = 10−18 s) laser pulses to investigate ultrafast electron dynamics. HHG has become an important table-top source covering a spectral range from infrared to extreme ultraviolet (XUV). One way to extend the cutoff energy of HHG is to increase the intensity of the laser pulses. A consequence of HHG in such intense short laser fields is the characteristic nonadiabatic red and blue shifts of the spectrum, which are reviewed in the present work. An example of this nonperturbative light-matter interaction is presented for the one-electron nonsymmetric molecular ion HeH2+, as molecular systems allow for the study of the laser-molecule orientation dependence of such new effects including a four-step model of MHOHG (Molecular High-order Harmonic Generation).
Highlights
Atomic high-order harmonic generation (HHG) was discovered in 1980s [1,2]
For long intense laser pulses, the sample is highly ionized on the rising laser envelope (RLE), little High-order harmonic generation (HHG) is emitted on the falling laser envelope (FLE)
We have reviewed in the presented work the nonadiabatic spectral shift of atomic HHG and molecular high-order harmonic generation (MHOHG) in intense laser fields
Summary
Atomic high-order harmonic generation (HHG) was discovered in 1980s [1,2]. It is a nonlinear nonperturbative process, which converts low-frequency fundamental femtosecond (1 fs = 10−15 s) laser fields to coherent high-energy attosecond (1 as = 10−18 s) pulse trains or an isolated attosecond laser pulse [3]. In MHOHG, the ionized electron can recombine with neighboring ions in the same molecule leading to a much larger cutoff energy depending on the internuclear distance R [8,9,10]. This larger cutoff energy comes from longer acceleration distance in the above process. We review the recent progress in these nonadiabatic spectral shifts of HHG and in particular MHOHG in this article
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