Spooktroscopy: a ghost imaging approach to ultrafast absorption spectroscopy*

With the advent of an X-ray free-electron laser, scientists can probe femtosecond ultrafast molecular dynamics with atomic-site specificity. One well-established technique is transient absorption spectroscopy, where transient sample absorption is determined by scanning the central photon energy and recording the resultant photoproducts. When using sub-femtosecond pulses, this method is limited in spectral resolution due to the large energy bandwidth inherent to the short pulses. Here, we present a novel technique based on the principle of ghost imaging, and demonstrate sub-bandwidth absorption spectroscopy measurements with sub-femtosecond pulses. We show both one-dimensional and two-dimensional results. Our method is applicable to any spectral measurement limited by the bandwidth or noise level of the probing source, provided the probe spectrum is known at each shot.