Extracting the phase of high-order harmonic emission from a molecule using transient alignment in mixed samples

We report measurements of both the phase and intensity of high-order harmonic emission from aligned CO2 molecules, using a mixture of a molecular and an atomic gas. The molecules are transiently aligned using an ultrashort pulse, and a subsequent stronger pulse is used to generate high-order harmonics from a mixed Kr/CO2 sample. The high-order harmonic emission from the molecules interferes with the reference emission from the atoms. By monitoring the change in harmonic emission as a function of gas mixture and molecular alignment, we can retrieve the orientational dipole that has general features consistent with a two-center emission model.