Ultrafast gas-phase x-ray scattering is a recently developed technique with immense potential for imaging chemical reactions in real time. In a pioneering study, we combined experimental pump-probe scattering data with calculated trajectories to image the ring-opening of 1,3-cyclohexadiene. In the present study, we demonstrate the wide-ranging applicability of this technique by disentangling the coherent vibrational motions from the anisotropic wavepacket that is initiated upon excitation of N-methylmorpholine to the 3p Rydberg state. Separate ultrafast photoelectron spectra show a coherent periodic motion with a period of 650 fs, attributed to the amine umbrella motion. Molecular dynamics simulations on the ground state molecular ion suggest that the amine umbrella motion has an oscillation period of 345 fs in that state. We recorded gas phase, time-resolved pump-probe x-ray scattering data of N-methylmorpholine where the molecule is excited to the 3p state using a 200 nm laser pulse, and the transient structure is recorded by x-ray scattering using the 9.5 keV photon beam from the LCLS free electron laser. The time-dependent scattering signals are in good agreement with the prior work, revealing clearly an oscillatory structural feature before dephasing washes out the structural motions. The data shows a significant anisotropy, which decays quickly to a persistent isotropic pattern. The experiment demonstrates that ultrafast gas-phase x-ray scattering can not only be applied to large-scale structural rearrangements such as a ring-opening reaction, but is also sensitive to the nuanced effects of wavepacket dephasing.

Deconvoluting the isotropic and anisotropic ultrafast x-ray scattering of gas-phase N-methylmorpholine following Rydberg excitation

256th National Meeting (2018)

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