The coherence and dephasing of vibrational motions of molecules constitute an integral part of chemical dynamics, influence material properties, and underpin schemes to control chemical reactions. In the present study, we measure coherent structural dynamics in optically excited N-methyl morpholine by scattering with ultrashort X-ray pulses from the Linac Coherent Light Source. The scattering signals are corrected for the different electron density in the excited electronic state of the molecule compared to the ground state. The experiment maps the evolution of the molecular geometry with femtosecond resolution, showing coherent motion that survives electronic relaxation and appears to persist for longer than previously seen using other methods.
We probe the dynamics of dissociating CS_{2} molecules across the entire reaction pathway upon excitation. Photoelectron spectroscopy measurements using laboratory-generated femtosecond extreme ultraviolet pulses monitor the competing dissociation, internal conversion, and intersystem crossing dynamics. Dissociation occurs either in the initially excited singlet manifold or, via intersystem crossing, in the triplet manifold. Both product channels are monitored and show that, despite being more rapid, the singlet dissociation is the minor product and that triplet state products dominate the final yield. We explain this by a consideration of accurate potential energy curves for both the singlet and triplet states. We propose that rapid internal conversion stabilizes the singlet population dynamically, allowing for singlet-triplet relaxation via intersystem crossing and the efficient formation of spin-forbidden dissociation products on longer timescales. The study demonstrates the importance of measuring the full reaction pathway for defining accurate reaction mechanisms.
When a molecule interacts with light, its electrons can absorb energy from the electromagnetic field by rapidly rearranging their positions. This constitutes the first step of photochemical and photophysical processes that include primary events in human vision and photosynthesis. Here, we report the direct measurement of the initial redistribution of electron density when the molecule 1,3-cyclohexadiene (CHD) is optically excited. Our experiments exploit the intense, ultrashort hard x-ray pulses of the Linac Coherent Light Source (LCLS) to map the change in electron density using ultrafast x-ray scattering. The nature of the excited electronic state is identified with excellent spatial resolution and in good agreement with theoretical predictions. The excited state electron density distributions are thus amenable to direct experimental observation.
Identification
of the initially prepared, optically active state
remains a challenging problem in many studies of ultrafast photoinduced
processes. We show that the initially excited electronic state can
be determined using the anisotropic component of ultrafast time-resolved
X-ray scattering signals. The concept is demonstrated using the time-dependent
X-ray scattering of N-methyl morpholine in the gas
phase upon excitation by a 200 nm linearly polarized optical pulse.
Analysis of the angular dependence of the scattering signal near time
zero renders the orientation of the transition dipole moment in the
molecular frame and identifies the initially excited state as the
3p
z
Rydberg state, thus bypassing the
need for further experimental studies to determine the starting point
of the photoinduced dynamics and clarifying inconsistent computational
results.
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