High-order harmonic phase in molecular nitrogen

Abstract

Electronic structure in atoms and molecules modulates the amplitude and phase of high harmonic generation (HHG). We report measurements of the high harmonic spectral amplitude and phase in N 2 . The phase is measured interferometrically by beating the N 2 harmonics with those of an Ar reference oscillator in a gas mixture. A rapid phase shift of 0.2π is observed in the vicinity of the HHG spectral minimum, where a shift of π had been presumed [J. Itatani et al., Nature 432, 867 (2004)]. We compare the phase measurements to a simulation of the HHG recombination step in N 2 that is based on a simple interference model. The results of the simulation suggest that modifications beyond the simple interference model are needed to explain HHG spectra in molecules.

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

“…The deep minimum is related to the σ orbital. This minimum has been observed in many experiments, either in unaligned or aligned N 2 [39][40][41][42]. The minimum shifts only slightly when the laser intensity is changed.…”

High-order-harmonic generation from field-distorted orbitals

“…Such a minimum may have different origin depending on the target and laser parameters, but it always carries important information about the target. For example, a Cooper minimum holds information about the electronic structure of the target [19][20][21][22][23]. Participation of multiple orbitals in the HHG process gives rise to a so-called dynamic minimum which allows elucidation of the interplay between the HOMO and lower-lying orbitals in the HHG process [24,25].…”

Linked attosecond phase interferometry for molecular frame measurements

“…(1) A two-source interferometer measures the phase versus molecular angle, Φ M ol (θ; q), and (2) aligned target molecule mixed with a reference atom is recorded. Each of these steps has been previously demonstrated [8,[17][18][19]. The difficulty until now was the lack of coupling between the two coordinates q and θ of the two-dimensional phase Φ M ol (q, θ).…”

“…Here, our calculations is not yet fully understood [8,9], ∆Φ I ≈ -0.9 π would explain the overall shift between the experiment and the model. As a result, comparing LAPIN measurements to our model allows to report an additional, previously unaccounted for, phase difference between the emission of two species in mixed gases experiments [18,19,22]. This needs to be further investigated by more detailed modeling which could include multielectron attosecond dynamics and subcycle laser-driven population evolution of the electron-hole first created by ionization [8,9].…”

“…We can add ∼4% by pressure of xenon to the helium, without affecting the gas expansion. However, we cannot directly measure the HHG spectrum from the reference atom alone S Ref (q), as in other experiments [18,19], because the pulsed valve operation and thereby the gas expansion are different without Br 2 vapors. Instead, we separately record the HHG spectra of the bromine molecules alone (S M ol (q, θ)) and of the gas mixture (S M ix (q, θ)).…”