High harmonic spectra contributed by HOMO-1 orbital of aligned CO2 molecules

We observe the high harmonic generation (HHG) from anti-aligned CO2 molecules when the on-axis peak of HHG from HOMO-2 orbital disappears. The harmonic emission at anti-alignment can be attributed to the contribution of HOMO-1 orbital. Simulations reproduce these observations and reveal the angular distributions of tunneling ionization from HOMO and HOMO-1 respectively at different intensity. The determination of HOMO-1 orbital contributions in harmonic spectra is important for the tomography imaging of aligned molecules and analysis of the time evolved harmonic emission. (C) 2013 Optical Society of Americ