The passage of molecules through conical intersections can be probed with unprecedented temporal and spectral resolutions by X-ray pulses. We propose spectroscopic techniques that use molecular coherences as novel windows into the non-adiabatic passage. The first application is TRUECARS, where a hybrid broadband/narrowband probing scheme records the coherence signature free from the usually dominating population contributions. We present the sensitivity of the TRUECARS signal to the underlying photophysics for the RNA-nucleobase uracil. Through its Wigner representation, the signal directly maps the topology of the potential energy surface around the conical intersection explored by the wavepacket coherence and gives the vibronic energy distribution of the latter. In the second application, time-resolved X-ray diffraction is simulated for the photoisomerization of azobenzene. Mixed elastic/inelastic scattering from coherences emerges as a weak contribution in the total signal. This can be amplified by employing hard X-rays, with high momentum transfer amplitudes, where the coherence contribution is stronger. The transient real-space molecular charge density provides a movie of the conical intersection passage. X-ray free-electron lasers (FELs) relying on the self-amplified spontaneous emission (SASE) mechanism generate stochastic x-ray pulses lacking phase control. This presents a major bottleneck, since time-resolved multidimensional nonlinear x-ray spectroscopy schemes are typically based on sequences of coherent, phase-controlled pulses. We show that suitable correlation covariance signals averaged over independent realizations of stochastic FEL pulses can retrieve the same joint temporal and spectral resolutions of signals with phase-controlled pulses. This is demonstrated for both Raman spectroscopy and imaging signals. We employ optimal control theory to directly amplify desired spectral features and potentially bring them above the detection threshold.

Probing vibronic coherences at conical intersections by ultrafast X-ray stimulated Raman and diffraction

ACS Spring 2022

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