High-order harmonic generation from highly excited states in acetylene

Abstract

High-order harmonic generation (HHG) from aligned acetylene molecules interacting with mid infra-red (IR), linearly polarized laser pulses is studied theoretically using a mixed quantum-classical approach in which the electrons are described using time-dependent density functional theory while the ions are treated classically. We find that for molecules aligned perpendicular to the laser polarization axis, HHG arises from the highestoccupied molecular orbital (HOMO) while for molecules aligned along the laser polarization axis, HHG is dominated by the HOMO-1. In the parallel orientation we observe a double plateau with an inner plateau that is produced by ionization from and recombination back to an autoionizing state. Two pieces of evidence support this idea. Firstly, by choosing a suitably tuned vacuum ultraviolet pump pulse that directly excites the autoionizing state we observe a dramatic enhancement of all harmonics in the inner plateau. Secondly, in certain circumstances, the position of the inner plateau cut-off does not agree with the classical three-step model. We show that this discrepancy can be understood in terms of a minimum in the dipole recombination matrix element from the continuum to the autoionizing state. As far as we are aware, this represents the first observation of harmonic enhancement over a wide range of frequencies arising from autoionizing states in molecules.

High-Order harmonic generation of aligned acetylene in elliptically polarized strong laser fields

“…The contribution of different molecular orbitals on the HHG of C 2 H 2 was theoretically investigated very recently, in which a longer wavelength of 1450 nm was employed as the driving laser [24]. Our experimental results are qualitatively consistent with this theoretical prediction.…”

“…Our experimental results are qualitatively consistent with this theoretical prediction. We should also mention that the difference in the measured HHG signal between the parallel and the perpendicular alignments is not so significant, comparing to the theoretical results [24]. A possible reason is that in experiments, one can never achieve completely pure alignment of the molecule axis parallel or perpendicular to the laser, indicating that both HOMO and DOI:10.1063/1674-0068/31/cjcp1805117 c ⃝2018 Chinese Physical Society HOMO−1 could be attributed to the observed HHG for either θ=0 • or θ=90 • thus may smear out the difference.…”

“…Later, Negro et al reported the molecular orbital tomography of nitrous oxide (N 2 O) as well as C 2 H 2 using high-order harmonic spectroscopy [23]. Very recently, Mulholland and Dundas performed a theoretical study on HHG of C 2 H 2 using a mixed quantumclassical approach, the results of which imply the significant effect of different structure of the molecular orbitals on molecular HHG [24]. Nevertheless, further experimental and theoretical studies on the HHG of C 2 H 2 are still in demand, especially for the aligned molecules in elliptically polarized strong laser fields which has been sparsely investigated in the literature.…”

Probing the role of excited states in ionization of acetylene

“…In addition, by resonantly tuning the laser wavelength to these excited states, ionization and harmonic generation are significantly modified. In a recent paper we showed that HHG from acetylene can be enhanced whenever the molecule is excited to an autoionizing state [66]. Here, we show that this behaviour is due to resonant excitation by considering a range of VUV wavelengths.…”

“…The inner plateau is due to ionization from and recombination back to the LUMO+1. However, the cut-off energy for this inner plateau is lower than that predicted by the classical three-step model: we attribute this to suppression of ionization from the LUMO+1 due to transitions back to the HOMO-1 [66]. These transitions from the LUMO+1 to the HOMO-1 give rise to a window of enhanced harmonics (harmonics [11][12][13][14][15][16][17][18][19][20][21].…”