Probing multiple molecular orbitals in an orthogonally polarized two-color laser field

High-harmonic radiation emitted from gaseous molecules is a coherent extreme ultraviolet (EUV) radiation that carries information on electronic structure and dynamics of the molecule. High-harmonics are generated when an electron, ionized and accelerated in a strong laser field, recombines with the parent ion. During the recombination, a dipole moment is induced by the returning electron and the parent ion, emitting the harmonic radiation after the periodic repetition of the process. Characteristics of the bound electron, thus, can be imprinted in the high-harmonic radiation. The highest occupied molecular orbital (HOMO) is mostly ionized in a strong laser field and reveals its characteristics dominantly. Energetically low-lying molecular orbitals referred to HOMO-1 and HOMO-2 also contribute to the radiation. Multi-orbital contributions to the radiation distort proper information on a specific orbital. Resolving the contribution of each orbital is, thus, crucial for understanding molecular dynamics. By applying an orthogonally polarized two-color laser field that consists of the fundamental and its second-harmonic field, we show that high-harmonic radiation emitted from the two highest-occupied molecular orbitals, HOMO and HOMO-1, of aligned molecules can be resolved. The characteristics attributed to the two orbitals are found to be separately imprinted in odd and even harmonics. Two-dimensional high-harmonic spectroscopy using orthogonal odd and even harmonics may enable us to observe multi-orbital dynamics during chemical reactions. © Springer International Publishing AG 2017011Nscopu