High Harmonic Generation from Multiple Orbitals in N ₂

Abstract

Molecular electronic states energetically below the highest occupied molecular orbital (HOMO) should contribute to laser-driven high harmonic generation (HHG), but this behavior has not been observed previously. Our measurements of the HHG spectrum of N 2 molecules aligned perpendicular to the laser polarization show a maximum at the rotational half revival. This feature indicates the influence of electrons occupying the orbital just below the N 2 HOMO, referred to as the HOMO-1. Such observations of lower-lying orbitals are essential to understanding the sub-femtosecond/sub-angstrom electronic motion in laser excited molecules.Tomographic imaging of molecules using high harmonic generation (HHG) has attracted wide interest [1]. The method can be easily described in the framework of a strong-field three-step model [2,3].In this model, a portion of the electron wave function corresponding to the highest occupied molecular orbital (HOMO) tunnels into the continuum and is accelerated in a strong oscillating optical field. This continuum part of the wave function is treated as a free electron wave packet, which interferes coherently with the bound part of the HOMO when it returns to the molecule. Recombination dipole radiation is emitted on every half-cycle of the driving field and the coherent superposition of this radiation over multiple cycles forms a discrete spectrum of odd-order high harmonics. The spectrum contains information about the HOMO structure. Tomographic reconstruction achieves sub-angstrom spatial resolution de-1

Linked attosecond phase interferometry for molecular frame measurements

“…However, the dependence of that spectral feature on laser intensity [8] and wavelength [24] in CO 2 instead supports the participation of multiple orbitals [23]. 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.…”

High-Harmonic Spectroscopy: From Femtosecond to Attosecond Molecular Dynamics

“…The experiment is thus repeated in each half-cycle of the laser field and the temporal information is read on the photon energy axis, exploiting the one-to-one relationship between transit time and emitted photon energy. [17] Recent experiments show that strongfield ionization does not exclusively prepare the ground state of the cation [4][5][6]28] but that excited states can also be populated. The probability of accessing a certain ionic state is exponentially sensitive to I p [29] but also to the shape of orbitals, [30] which can enhance the contribution of a deeply bound orbital.…”

Two-center interference and ellipticity in high-order harmonic generation fromH2+

“…The correct explanation of the minimum in CO 2 , however, appears to require a multiorbital treatment [10]. Recent experiments on N 2 indicate multiorbital contributions as well [11]. Preparing molecules to control the harmonic emission is also possible: ring current states were used theoretically to generate circularly polarized high harmonics [12].…”