Impact of spatial chirp on high-harmonic extreme ultraviolet absorption spectroscopy of thin films

Extreme ultraviolet (XUV) absorption spectra of thin-film samples using high-harmonic light sources show periodic artifacts that align with the peaks and valleys in the spectrum of the incident XUV beam. We show that a major source of this error is the interaction of spatial chirp in the XUV beam with thickness variations in the film. This concept is demonstrated using a patterned silicon nitride film as well as semiconductor and transition metal complex films. Numerical simulations using the measured spatial chirp and sample morphology reproduce the artifacts in heterogeneous films nearly quantitatively. We use these simulations to place limits on the sample quality required for detailed studies of the electronic structure of transition metal complexes at the M_2,3 edge. Finally, we show that artifacts can be created in transient absorption spectroscopy for even flat samples due to the inhomogeneous distribution of excited states created by a Gaussian pump beam. The experiments are carried out in a semi-infinite neon gas cell at a phase-matching condition that gives a broad harmonic continuum.