Science - USA (2020-05-22)

To further show this correlation, we simulated
the PDsimTotalsignal for S 0 and S 1 states over all
trajectories (Fig. 3B). We found that the S 1
signal was about three times as strong as
the S 0 signal at 0.3 <s<1Å−^1. This simulation
therefore confirms that the small-angle signal
couldbeusedtotracetheexcitedstatepopu-
lation, and the experimentally observed posi-
tive signal (s<1Å−^1 ;0<t<~1.5ps)showsboth
the S 0 →S 1 photoexcitation and the S 1 →S 0 IC.
Here we provide a simple physical picture
for the observed inelastic signal. The diffrac-
tion signature of the correlation effect is a

negative contribution to small-angle inelastic

scattering, and most (80 to 90%) of this ef-

fect comes from dynamic correlation due to

Coulomb repulsion ( 26 , 27 , 31 ). In the ground

state, the two electrons in the lone-pair orbital

arespatiallycloseandthedynamiccorrelation

is strong. In the photoexcited (np*) state, how-

ever, the two electrons no longer occupy the

same molecular orbital, leading to a marked

reduction of the dynamic correlation. According

to the above-mentioned arguments, small-

angle inelastic scattering is therefore sub-

stantially increased by photoexcitation and

decreased upon relaxation to the electronic

ground state. We have also simulated the

expected AIED signal for the S 2 ,T 1 ,andT 2

states (fig. S2), which confirms that all of these

open-shell excited states give rise to a strong

increase in small-angle inelastic scattering.

Although we believe this picture provides an

intuitive explanation of the observed inelastic

signal, the concrete details and generality are

subject to future studies.

To extract the nuclear structural dynamics,

we used 1.1 <s< 10.5 Å−^1 data to apply a genetic

c^2 structural fitting algorithm (supplementary

Yanget al.,Science 368 , 885–889 (2020) 22 May 2020 3of5

Fig. 2. Experimental and simulated UED signal for pyridine.(A) Experimental
PD signal, normalized by the 9% excitation ratio. (B) Simulated PD signal
from the IAM (elastic component only), using all 24 initial and 2040 spawned
trajectories. (C) Simulated PD total (elastic and inelastic) signal using AIED.
(D) Elastic part of simulated PD using AIED. (E) Inelastic part of simulated
PD using AIED. In (A) to (E), the red dotted line denotess=1.1Å−^1.

(FtoH) Lineout plots of the three strongest features from the experiment,

plotted together with IAM and AIED simulations. Uncertainties in (F) to (H)

are represented by shaded regions, calculated with 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 the

experimental IRF.

Fig. 3. Smallsscattering and excited
state population.(A) Simulated PD signal
using AIED (blue, leftyaxis) and S 1 population

(red, rightyaxis). (B) The PDsimTotalsignal on
S 0 and S 1 integrated over the time course of
the simulation (0 to 2.4 ps), calculated using all
accessed geometries by each state. Uncertainty
is represented by shaded regions, calculated
with one SEM of all trajectories (A) or one SD
of all visited geometries (B).

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