Title: 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 highest-occupied 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.