Top: schematic of the NIST table-top x-ray probe. A pulsed laser induces photochemical reactions in the material under study. The same laser is focused onto a water jet, producing an x-ray generating plasma. The x-rays are used to interrogate the sample at controlled time delays after photoexcitation. Bottom: (left) measured time-resolved Kβ emission spectrum of iron-tris-bipyridine probed at a 3 ps time delay. The spectral shape changes in the excited (pumped) state due to a spin crossover in the system. (right) resultant time evolution of the high-spin fraction deduced from emission measurements at several time delays. Our data (see inset) show a fast rise of the high-spin fraction at time-zero followed by an exponential decay (see main figure) of the high-spin-state fraction with a time constant of 566 ± 100 ps.