Role of resonance-enhanced multiphoton excitation in high-harmonic generation of N2: A time-dependent density-functional-theory study

Abstract

A minimum at $\ensuremath{\sim}$39 eV is observed in the high-harmonic-generation spectra of N${}_{2}$ for several laser intensities and frequencies. This minimum appears to be invariant for different molecular orientations. We reproduce this minimum for a set of laser parameters and orientations in time-dependent density-functional-theory calculations, which also render orientation-dependent maxima at 23--26 eV. Photon energies of these maxima overlap with ionization potentials of excited states observed in photoelectron spectra. Time profile analysis shows that these maxima are caused by resonance-enhanced multiphoton excitation. We propose a four-step mechanism, in which an additional excitation step is added to the well-accepted three-step model. Excitation to a linear combination of Rydberg states ${c}_{4}^{\ensuremath{'}}{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Sigma}}_{u}^{+}$ and ${c}_{3}\phantom{\rule{0.16em}{0ex}}{{}^{1}\ensuremath{\Pi}}_{u}$ gives rise to an orientation-invariant minimum analogous to the ``Cooper minimum'' in argon. When the molecular axis is parallel to the polarization direction of the field, a radial node goes through the atomic centers, and hence the Cooper-like minimum coincides with the minimum predicted by a modified two-center interference model that considers the de-excitation of the ion and symmetry of the Rydberg orbital.