Schematic picture of the dynamics after excitation to the V state of ethylene. Two conical intersections are depicted, at twisted/pyramidalized and ethylidene geometries. Several instances of the XUV probe are also indicated, to representative cation states. When the geometrical configuration of the molecule is CH2CH2, one can expect that many of these cation states will lead to symmetric fragmentation of the ion (CH2 + CH2 +). On the other hand, when the geometrical configuration of the molecule is more nearly ethylidene-like, one can expect that XUV excitation will lead to asymmetric fragmentation of the ion (CH + CH3 +). As depicted on the right, XUV absorption after H2 elimination can also lead to ionization of H2 (and analogously to ionization of H atoms, not shown). Thus, monitoring the dependence of ion fragments on pump-probe time delay provides a means to map out the dynamics of the excited state and potentially to determine whether and when each of the two types of intersections is involved in nonradiative decay. Note that this schematic is not meant to imply that the twisted/pyramidalized intersection must be accessed prior to arrival at the ethylidene intersection geometry (the true potential is multidimensional and the molecule can proceed from the Franck-Condon point to the ethylidene intersection without going through the twisted/pyramidalized intersection geometry).
Pump/Probe apparatus. High order harmonics are recombined with variable delay at the common focus of two spherical concave mirrors. Photo-ions are extracted from the focal region through a 500 μm pinhole with a strong electric field in the y-direction.
Absolute change in photo-ion yields in ions/shot for fragments CH2 +, CH3 +, H+, and H2 +, and corresponding relative change in percent for C2H4 +. Positive delay corresponds to the 7.7 eV probe pulse arriving first. Note the different y-scale for each fragment. The dashed black lines indicate the ground state bleach components present in the CH3 + and CH2 +, with their associated systematic uncertainties present in the shaded grey regions.
CH2 + and CH3 + panes (upper two panels) show simulated CH3 + and CH2 + ion fragment yields as a function of pump/probe time delay for excited ethylene molecules, reported as a fraction of the total population initially launched on the V state surface. The solid line corresponds to the total yield. The filled areas show the contributions from photoionization of population on the excited V state surface (dark gray) and the ground N state surface (light gray). The lower two panels show the cumulative fraction of the initial population that has undergone H or H2 elimination. The shaded regions represent 1σ statistical uncertainty. Positive delay corresponds to the 7.7 eV pump pulse arriving first.
Comparison between theory and experiment for CH2 + (upper panel) and CH3 + (lower panel) fragments. Data Points: Excited molecule photoionization signals extracted from the raw data as described in the text. Solid Lines: CH2 and CH3 theory curves scaled by a common scale factor. Shaded regions represent the 1σ statistical uncertainty in the theoretical results.
Density plot (created using volmap tool of VMD (Ref. 53)) for trajectory basis functions that undergo direct (within 200 fs) dissociation after XUV photo-ionization to give CH2 + (upper panel) or CH3 + (lower panel) fragments. The density plots are overlaid on the twisted/pyramidalized (upper panel) and ethylidene (lower panel) MECIs, showing that the experimental CH2 +/CH3 + signals are strongly correlated with the two different MECIs that are operative in excited state quenching for ethylene.
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