Uncovering multiple orbitals influence in high harmonic generation from aligned N 2

Recent measurements on high-order harmonic generation (HHG) from N 2 aligned perpendicular to the driving laser polarization (B K McFarland et al 2008 Science 322 1232) have shown a maximum at the rotational half-revival. This has been interpreted as the signature of the HHG contribution from the molecular orbital just below the highest occupied molecular orbital (HOMO). By using the recently developed quantitative rescattering theory combined with accurate photoionization transition dipoles, we show that the maximum at the rotational half-revival is indeed associated with the HOMO-1 contribution. Our results also show that the HOMO-1 contribution becomes increasingly more important near the HHG cutoff and therefore depends on the laser intensity. (Some figures in this article are in colour only in the electronic version) High-order harmonic generation (HHG) has been extensively investigated both experimentally and theoretically over the last two decades [1]. Until very recently, the HHG has been understood as being due to tunnelling ionization of an electron from the highest occupied molecular orbital (HOMO) and recombining back to the HOMO. The contribution from lower molecular orbitals is routinely neglected. That is not surprising since tunnelling is a highly nonlinear process, and therefore highly selective to the HOMO due to energy considerations. In general, the neglect of the contribution from lower molecular orbitals is not justified for systems where the HOMO and HOMO-1 are nearly degenerate, in other words, when the energy gap between the HOMO and lower molecular orbitals is much smaller than the ionization potential from the HOMO. Furthermore, for some molecular alignments, tunnelling ionization from the HOMO is suppressed due to symmetry of the wavefunction [2]. Clearly, in that case, the neglect of lower molecular orbitals is questionable. These two favourable conditions for observing a HOMO-1 contribution are present in N 2 , where the 1π u HOMO-1 has a binding energy of 16.93 eV, quite close to the binding energy of the 3σ g HOMO (15.58 eV). Early theoretical calculations based on the strong-field approximation (SFA) model [3, 4] have shown that harmonic yields from aligned N 2 are maximum if the molecules are aligned along the laser polarization direction. These results are in good agreements with the pump-probe delay time experimental data [5, 6] as well as the recent more direct measurements [7]. On the other hand, the contribution from the HOMO-1 is expected to peak near 90 •. Although the two molecular orbitals contribute …