We study the interplay of electronic-structure and multielectron effects in high-harmonic spectra of the two isoelectronic molecules CO${}_2$ and N${}_2$O as a function of the near-infrared driving wavelength and intensity. We show that the minima observed in the spectra of aligned molecules ($\langle {\cos }^2\theta \rangle =0.54–0.64$) are intensity dependent at the shortest investigated wavelength (1.17 $\mu$m) but become gradually intensity independent at longer wavelengths (1.36 and 1.46 $\mu$m). These results combined with detailed theoretical modeling show that the contributions of several ionization channels are important in the spectra recorded with short-wavelength driving pulses because the minima are located in the cutoff region where the contributions from different channels are comparable. In spectra recorded with longer driving wavelengths, the minima are located in the plateau region of the spectrum and are caused by the electronic structure of the molecules. Our results also rationalize and reconcile previous results and provide general guidelines for future studies.

• Received 27 September 2012

DOI:https://doi.org/10.1103/PhysRevA.87.033409