Intensity dependence of multiple orbital contributions and shape resonance in high-order harmonic generation of aligned N2molecules

Abstract

We report measurements and theoretical simulations of high-order harmonic generation (HHG) in aligned N 2 molecules using a 1200-nm intense laser field when the generating pulse is perpendicular to the aligning one. With increasing laser intensity, the minimum in the HHG spectra first shifts its position and then disappears. Theoretical simulations including the macroscopic propagation effects in the medium reproduce these observations and the disappearance of the minimum is attributed to the additional contribution of HHG from inner orbitals. We also predict that the well-known shape resonance in the photoionization spectra of N 2 should exist in the HHG spectra. It is most clearly seen when the generating laser is parallel to the aligning one and disappears gradually as the angle between the two lasers increases. No clear evidence of this shape resonance has been reported so far when using lasers with different wavelengths. Further experimentation is needed to draw conclusions.

N2 HOMO-1 orbital cross section revealed through high-order-harmonic generation

“…This represents a large enhancement for harmonics in the cutoff. Figure 5 compares the theoretical PICS for HOMO and HOMO-1 [43] with the experimental HHG signal as a function of angle for harmonics 35 to 41. In the figure we can observe that the HOMO PICS (dotted line) for harmonics 35 and 37 have a nonzero contribution at 90…”

“…A comparison between the experimental angle scans (solid red line) at the full revival of 17.04 ps and theoretical calculations of the photoionization cross sections of HOMO (dotted line) and HOMO-1 (dashed line) from Ref. [43]. The cross section is normalized to 1, while the experimental data are normalized to the isotropic harmonic yield.…”

“…We thank Cheng Jin and Anh-Thu Le for the fruitful discussions and access to the data first presented in Ref. [43].…”

Optical Parametric Amplification Techniques for the Generation of High-Energy Few-Optical-Cycles IR Pulses for Strong Field Applications

“…The role of ionization from multiple molecular orbitals has been studied in CO 2 and N 2 by performing HHG measurements at different pulse intensities and IR wavelengths [73,74]. Here, intensity-dependent spectral features are attributed to the increasing contribution of deeper orbitals, and wavelength-dependent spectral features are recognized as due to the interference among different ionization-recombination channels involving multiple orbitals.…”

Theory of High Harmonic Generation for Probing Time-Resolved Large-Amplitude Molecular Vibrations with Ultrashort Intense Lasers

“…Developed initially for the case of fixed nuclei, the QRS is computationally efficient and has been well tested [22,23]. Furthermore, with the inclusion of macroscopic propagation effect [24][25][26], the predicted HHG spectra based on QRS have been shown to agree well with experiments for different molecules.We consider a homonuclear diatomic molecule in the electronic ground state under a few-cycle intense laser pulse. We will show below that the induced dipole DðtÞ can be calculated as DðtÞ ¼ Z dRjðR; tÞj 2 Dðt; RÞ;…”