Measuring 10 fs dynamics via resonant x-ray pump/x-ray probe spectroscopy*

We used two x-ray pulses to investigate the femtosecond scale molecular response to $K$-shell resonant excitation in O$_2$. Our results give three perspectives on this dynamic response: 1) sub-10 fs transient bleaching of resonant absorption, 2) a corresponding sub-10 fs evolution of the resonant Auger electron spectrum, and 3) a 10--15 fs evolution of electronic molecular symmetry. The x-ray pulses are tuned to the 531 eV $1s\rightarrow2p\pi^*$ resonance in O$_2$. Upon excitation by the first pulse, further absorption is suppressed until the dynamic molecular valence pulls a new valence state into resonance. The new resonance occurs only after about 5--10 fs and reveals opposite electronic symmetry to the $\pi$*. After 15 fs, this newly resonant state has lost molecular symmetry and undergoes atomic-like resonant absorption. We have thus used x-ray pump-probe spectroscopy to build a time-domain picture of the $\sim$10 fs molecular response to x-ray absorption.