access inelastic scattering at small angles, we
intentionally set the main electron beam off-
center from the hole of the detector phosphor
screen (Fig. 1B). This setup provided access
down tos=0.3Å−^1 in one quadrant, wheres
is the momentum transfer of the scattered
electrons. Small-angle electron scattering is
dominated by the inelastic component. For
0.3 <s<1Å−^1 , previous gas electron diffraction
experiments have suggested that the inelastic
scattering intensity is typically 5 to 10 times
as large as the elastic scattering intensity ( 29 ).
The pump laser launches a wave packet on
the S 1 (np*) surface with ~3000-cm−^1 excess
energy, and the molecule relaxes through a
CI along a ring-puckering coordinate ( 14 , 15 ).
Figure 1C shows the S 0 and S 1 potential energy
surfaces at the floating occupation molecular
orbital–complete active space configuration in-
teraction (FOMO-CASCI) level of theory (sup-
plementary materials).
The experimental and simulated UED data
are given in Fig. 2. Figure 2A shows the ex-
perimental percentage difference (PD) signal,
PDexp,definedas
PDðs;tÞ¼Iðs;tÞIðs;t< 0 Þ
Iðs;t< 0 Þ 100 ð 1 ÞwhereI(s;t) is the radially averaged diffraction
intensity at any pump-probe delay timet,and
I(s;t< 0) is a reference pattern taken before
the arrival of the pump laser. In PDexp, the
signal ats>1.1Å−^1 appeared aroundt= 0 and
persisted throughout our observation window,
which reflects structural change of the mole-
cule and will be discussed later. The signal at
s< 1.1 Å−^1 , however, contained a sharp rise
(IRF-limited) and a slower decay (1.1 ± 0.2 ps);
see Fig. 2F. To understand this signal, we
performed dynamics, quantum chemistry and
diffraction simulations. The nonadiabatic
dynamics of the lowest three singlet and four
triplet states, including spin-orbit interactions,
was simulated using the generalized ab initio
multiple spawning (GAIMS) ( 30 )method(sup-
plementary materials). Diffraction patterns
and PD were first calculated using the IAM
(hereafter PDsimIAM); see Fig. 2B. PDsimIAMcap-
tured most of the large-angle features in
PDexpbut did not capture the strong increase
at small angles (s<1Å−^1 ;0<t<~1.5ps),
indicating that the small-angle signal origi-
nated from electron dynamics through ei-
ther binding (elastic) or correlation (inelastic)
effects.
To properly account for scattering signal
from both binding and correlation effects, we
performed ab initio electron diffraction (AIED)
simulations on GAIMS trajectories, similar to
the method developed by Breitensteinet al.
( 31 ) and implemented in TeraChem ( 32 )(sup-
plementary materials). The resulting total,
elastic, and inelastic PD (PDsimTotal,PDsimElastic,and
PDsimInelastic) are shown in Fig. 2, C to E. PDsimElastic
and PDsimInelasticare defined asPDsimElastic¼
IElasticðs;tÞIElasticðs;t< 0 Þ
Iðs;t< 0 Þ 100 ð 2 ÞPDsimInelastic¼
IInelasticðs;tÞIInelasticðs;t< 0 Þ
Iðs;t< 0 Þ 100 ð 3 ÞThe PDsimTotalnicely captured all the major fea-
tures in PDexp,withPDsimElasticand PDsimInelasticseparately located in thes>~1.1Å−^1 ands<
~1.1 Å−^1 regions. Lineout plots of the three stron-
gest features—the peak at 0.3 <s<0.7Å−^1 ,
the peak at 3.9 <s<4.9Å−^1 , and the trough at5.5 <s<6.5Å−^1 —are shown in Fig. 2, F to H.
Figure 2F shows that the IAM simulation
fails to reproduce the experimental data at
smallsand that the AIED simulation and
experiment agree reasonably well—both show
fast-rising (IRF-limited) and slow-decaying
(experiment: 1.1 ± 0.2 ps, simulation: 1.3 ±
0.1 ps) features. In addition, Fig. 2, D and E,
shows that this signal exclusively came from
the inelastic component.
We then inspected the connection between
the simulated small-angles scattering signal
and the electronic state population. The GAIMS
calculation predicted that the S 1 (np*) state was
exclusively populated and that >90% of the
population returned to the ground state S 0
within the 2.4-ps simulation window. The
small-angle signal PDsimTotalð 0 : 3 <s< 0 :7Å
1
;tÞ
correlates well with the S 1 population in the
GAIMS simulation, as illustrated in Fig. 3A.Yanget al.,Science 368 , 885–889 (2020) 22 May 2020 2of5
Fig. 1. Experiment overview.(A) Conceptual drawing of the experiment. The inelastic scattering
(red scattered wave) concentrates at small angles and encodes information about electron correlation
[represented graphically by the two electrons (yellow) in the lone-pair orbital (purple)]. The elastic
scattering (orange scattered wave) dominates athigh angles and encodes the molecular structure.
(B) Diffraction pattern with low-saccess. The two red rings near the center represent 0.25 Å−^1 (inner ring)
and 0.5 Å−^1 (outer ring), respectively. (C) Pyridine potential energy surfaces (S 1 and S 0 ) along the bond
length alternation and ring-puckering coordinates (fig. S6). Critical points (FC, Franck-Condon point)
and the minimum energy reaction pathway are marked. Time scales were obtained from simulation.RESEARCH | REPORT