materials) ( 33 ). Three fitted structural parameters—
dihedral anglea, interior angleb,andaverage
bond length of the ringDr—are shown in Fig.
4A (circles with error bars), together with the
value extracted from the GAIMS simulation
(lines with shaded regions). Because the con-
trast between a nitrogen atom and a CH group
issmallinelectrondiffraction,itisdifficultto
experimentally distinguish which atom puckers
out of the plane. Therefore, the dihedral angle
aand interior anglebare both referring to
the heavy atom with the largest out-of-plane
torsion (Fig. 4A, inset). Both experiment and
simulation show a large(~30°) change in di-
hedral angle and a ~0.04-Å expansion of the
ring bond lengths, with a small change in
angleb. The most-accessed S 1 /S 0 CI geome-
try from the GAIMS simulation is shown in
Fig. 4B. It is described by ~60° out-of-plane
torsion on a carbon atom adjacent to the nit-
rogen atom. Because the molecule spent rela-
tively little time at the CI and the torsion angle
was smaller both before and after the CI (Fig.
1B), the averaged out-of-plane torsion at any
time appeared to be only ~30°.
Taking into account all of the analysis
discussed above, we were finally able to plot
electronic and nuclear dynamics together in
Fig. 4, C and D. The small-angle PD signal at
0.3 <s<0.7Å−^1 represents excited state pop-
ulation (blue curves), whereas the dihedral
angle represents the structural change along
the main reaction coordinate (red curves). The
torsion started ~100 fs after the S 0 →S 1 photo-
excitation, agreeing well with the model that
a small barrier is present in the out-of-plane
ring-puckering coordinate. The S 1 →S 0 IC
started only after the dihedral angle reached a
certain point (~150° for experiment and ~160°
for simulation), consistent with the prediction
that the S 1 →S 0 IC requires a relatively large
out-of-plane torsion. This plot, along with the
lack of structural changes in other degrees of
freedom, confirms that the out-of-plane tor-
sion was the major motion that drove the
S 1 →S 0 IC. Even though our data are consistent
with the UED data reported by Srinivasanet al.
( 21 ), we have a different interpretation of the
dynamics that does not include ring opening.
A detailed comparison between the two experi-
ments is given in the supplementary materials.
In summary, through the correlation of a
specific nuclear degree of freedom to electronic
structure change, we have demonstrated that
structural and electronic dynamics can be re-
trieved simultaneously and independently from
a single UED dataset. This method allows us
to identify the relaxation mechanism in the
np* state of pyridine. Owing to the universality
of the diffraction signature from correlation
effects ( 24 – 27 , 31 ), we expect that this method
will be widely applicable in ultrafast photo-
chemistry. Moreover, the inelastic electron
scattering is a Fourier transform of the change
of the two-electron density caused by elec-
tron correlation. In contrast to spectroscopic
probes, it measures spatial rather than ener-
getic aspects of electron correlation ( 23 ). Be-
cause electron correlation is at the heart of
modern quantum chemistry simulations, in-elastic electron scattering provides a bench-
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Fig. 4. Combined structural and electronic
dynamics.(A) Experimental structural evolution
(circles with error bars) retrieved from a geneticc^2
fitting algorithm. Vertical error bars represent
one SD of 86 individual fitting results; horizontal
error bars represent the time window used for each
fitting. Simulated values (lines with shaded regions)
were obtained from weighted averages of all
spawned trajectories from GAIMS simulation.
The two dashed lines show the position of 120°
and 180°. (B) Geometry of the most-accessed S 1 /S 0
CI from GAIMS simulation. Blue, nitrogen atom.
(CandD). Experimental (C) and simulated (D)
temporal evolution of small-angle PD signal (blue,
normalized to the maximum value) and dihedral
angle (red). Uncertainty is represented by shaded
regions, calculated by one SD of a bootstrapped
dataset (experiment) or one SEM of all trajectories
(simulation). Simulation results are convolved
with a 150-fs Gaussian kernel to account for
experimental IRF.RESEARCH | REPORT