Strong Fields

High Harmonic Generation and Molecules

A high harmonic generation (HHG)[1] by a strong laser field is a complicated process classically described by the three-step model[2] comprising of adiabatic ionization of a single electron in the strong field, propagation of this electron in the laser field, and subsequent radiative recombination with the core. This description works almost surprisingly well in atoms, but would need to be amended for molecules. Importantly, as already the first step (i.e. the strong field ionization) depends on molecule structure – the HHG may bear huge potential as a probe of molecular structure and, when applied in time-resolved fashion, - of molecular dynamics. This dynamics probe can eventually be complimentary to other existing ones such as transient absorption, time resolved ion yield, time-resolved photoelectron spectroscopy and photoelectron angular distribution, although it certainly is more complicated as it involves at least 2 processes (ionization and recombination) sensitive to intramolecular electronic and vibrational evolutions. On the way towards disentangling HHG we aim at exploring 1st and 3rd steps separately, by studying the ionization of polyatomic molecules in the strong field and by XUV photons.

Although the three-step model accounts only for a single electron, in reality, of course, all the electrons are involved so the multielectron dynamics may be of importance. The extent of the involvement varies from molecule to molecule and can be assessed experimentally via comparing percentage of ionization of HOMO vs. HOMO-1 etc (i.e. ionization into excited states of the ion). For example, in linear polyenes, where excited states of cation are not stable, monitoring the degree of the fragmentation that follows the strong field ionization can do this. However, since the ionized molecule can be excited within the same laser pulse and subsequently dissociate, a mass-spectrum alone is not a reliable observable. To identify different ionization routes one should record both the photoelectron and the photoion in coincidence (see PEPICO_Spectrometer). Then ionization into different ionic continua can be recognized from a difference in shapes of the photoelectron spectra correlated with a fragment and parent ion[3].

1: High Harmonic Generation (Wikipedia)
2: PRL: 71, 1994 (1993)
3: Science: 335, 1336 (2012)