We report measurements and theoretical simulations of high-order harmonic generation (HHG) in aligned N₂ 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 N₂ 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.



Keywords and Phrases

High Order Harmonic Generation; Intense Laser Field; Intensity Dependence; Laser Intensities; Orbital Contribution; Orbitals; Photoionization Spectra; Propagation Effect; Shape Resonance; Theoretical Simulation, Harmonic Generation, Electron Resonance

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Document Type

Article - Journal

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Final Version

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© 2012 American Physical Society (APS), All rights reserved.

Publication Date

01 Jan 2012

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Physics Commons