We resolve the structural dynamics of the ultrafast photoinduced ring opening reaction of 1,3-cyclohexadiene in space and time employing megaelectronvolt gas phase ultrafast electron diffraction. We, furthermore, observe coherent large amplitude motions of the photoproduct

Centurion, Martin

Coffee, Ryan

Cryan, James P.

Guhr, Markus

Hegazy, Kareem

Kirrander, Adam

Li, Renkai.

Martinez, Todd J.

Minitti, Michael P.

Nunes, J. Pedro F.

Parrish, Robert M.

Ruddock, Jennifer

Sanchez, David M.

Shen, Xiaozhe

Veccione, Theodore

Wang, Xijie

Weathersby, Stephen P.

Weber, Peter M.

Wilkin, Kyle

Wolf, Thomas J. A.

Yang, Jie

Yong, Hai-Wang

Zheng, Qiang

English

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University of Nebraska - Lincoln 
DigitalCommons@University of Nebraska - Lincoln 
Faculty Publications, Department of Physics 
and Astronomy Research Papers in Physics and Astronomy 
2018 
Imaging the ring opening reaction of 1,3-cyclohexadiene with MeV 
ultrafast electron diffraction 
Thomas J. A. Wolf 
Stanford PULSE Institute, thomas.wolf@stanford.edu 
Jie Yang 
SLAC National Accelerator Laboratory 
David M. Sanchez 
Stanford University 
J. Pedro F. Nunes 
University of York, joao-pedro.figueira-nunes@unl.edu 
Robert M. Parrish 
Stanford University 
See next page for additional authors 
Follow this and additional works at: https://digitalcommons.unl.edu/physicsfacpub 
Wolf, Thomas J. A.; Yang, Jie; Sanchez, David M.; Nunes, J. Pedro F.; Parrish, Robert M.; Shen, Xiaozhe; 
Centurion, Martin; Coffee, Ryan; Cryan, James P.; Guhr, Markus; Hegazy, Kareem; Kirrander, Adam; Li, 
Renkai.; Ruddock, Jennifer; Veccione, Theodore; Weathersby, Stephen P.; Weber, Peter M.; Wilkin, Kyle; 
Yong, Hai-Wang; Zheng, Qiang; Martinez, Todd J.; Wang, Xijie; and Minitti, Michael P., "Imaging the ring 
opening reaction of 1,3-cyclohexadiene with MeV ultrafast electron diffraction" (2018). Faculty 
Publications, Department of Physics and Astronomy. 288. 
https://digitalcommons.unl.edu/physicsfacpub/288 
This Article is brought to you for free and open access by the Research Papers in Physics and Astronomy at 
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Authors 
Thomas J. A. Wolf, Jie Yang, David M. Sanchez, J. Pedro F. Nunes, Robert M. Parrish, Xiaozhe Shen, 
Martin Centurion, Ryan Coffee, James P. Cryan, Markus Guhr, Kareem Hegazy, Adam Kirrander, Renkai. Li, 
Jennifer Ruddock, Theodore Veccione, Stephen P. Weathersby, Peter M. Weber, Kyle Wilkin, Hai-Wang 
Yong, Qiang Zheng, Todd J. Martinez, Xijie Wang, and Michael P. Minitti 
This article is available at DigitalCommons@University of Nebraska - Lincoln: https://digitalcommons.unl.edu/
physicsfacpub/288 
 Imaging the ring opening reaction of 1,3-
cyclohexadiene with MeV ultrafast electron 
diffraction 
Thomas J. A. Wolf1,*, Jie Yang2, David M. Sanchez1,3, J. Pedro. F. Nunes4, Robert M. 
Parrish1,3, Xiaozhe Shen2, Martin Centurion5, Ryan Coffee2, James P. Cryan1, Markus 
Gühr1,6, Kareem Hegazy1, Adam Kirrander7, Renkai. Li2, Jennifer Ruddock8, Theodore 
Veccione2, Stephen P. Weathersby2, Peter M. Weber8, Kyle Wilkin5, Hai-Wang Yong8, 
Qiang Zheng2, Todd J. Martinez1,3, Xijie Wang2, Michael P. Minitti2 
1Stanford PULSE Institute, SLAC National Accelerator Laboratory, Menlo Park, USA 
2SLAC National Accelerator Laboratory, Menlo Park, USA 
3Department of Chemistry, Stanford University, Stanford, USA 
4Department of Chemistry, University of York, Heslington, York, UK 
5Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, USA 
6Institut für Physik und Astronomie, Universität Potsdam, Potsdam, Germany 
7 School of Chemistry, University of Edinburgh, Edinburgh, UK 
8Department of Chemistry, Brown University, Providence, USA 
Abstract. We resolve the structural dynamics of the ultrafast photoinduced 
ring opening reaction of 1,3-cyclohexadiene in space and time employing 
megaelectronvolt gas phase ultrafast electron diffraction. We, furthermore, 
observe coherent large amplitude motions of the photoproduct.  
1 Introduction 
The ultrafast photoinduced ring opening reaction of 1,3-cyclohexadiene (CHD) is a model 
for a whole class of electrocyclic reactions with biological importance, for instance as a key 
step of vitamin D production in human skin [1,2]. Therefore, it has received a lot of research 
interest [2,3]. Except for a recent ultrafast x-ray scattering study [4], experimental knowledge 
about the reaction in the gas phase comes from valence electron spectroscopy, which is 
directly sensitive to transient changes in the electronic structure e.g. internal conversion 
through conical intersections [3,5,6]. It is, however, only indirectly sensitive to the nuclear 
wavepacket dynamics involved in this reaction. Direct access to the nuclear wavepacket 
dynamics can in principle be achieved by time-resolved x-ray or electron diffraction. 
However, temporal and/or spatial resolution has been insufficient so far to capture ultrafast 
reaction dynamics with spatial resolution down to atomic distances. Here, we present a study 
of the structural dynamics during the CHD ring opening reaction in the gas phase with an 
unprecedented combination of sub-Å spatial and fs temporal resolution using the MeV 
ultrafast electron diffraction facility at SLAC National Accelerator Laboratory [7].  
                                       
* Corresponding author: thomas.wolf@stanford.edu 
, (201E Web of Conferences https://doi.org/10.1051/e onf /201920PJ pjc9)205 0 5070
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  © The Authors,  published  by EDP Sciences.  This  is  an  open  access  article  distributed  under  the  terms  of the Creative
Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). 
2 Experiment 
The details of the experimental setup are described elsewhere [7]. In short, 3.7 MeV 
electron pulses with 180 Hz repetition rate were created by directing the third harmonic of a 
Ti:Sapph laser system onto the photocathode of an electron gun and subsequent acceleration 
of the ejected electrons in an RF cavity. The electron pulses were temporally and spatially 
overlapped with another portion of the Ti:Sapph third harmonic in the experimental chamber. 
The sample was introduced into the interaction region using a pulsed nozzle. Diffracted 
electrons were detected at 4 m distance from the interaction region using a combination of a 
phosphor screen and a camera. The instrument response function is estimated to be 160 fs. 
3 Results and Discussion 
Figure 1 shows an atomic pair distribution function (PDF) from real-space 
transformation of radially averaged static diffraction signals (blue). It exhibits two maxima 
associated with C-C bond distances and additional carbon-carbon distances across the CHD 
ring, respectively (see inset of Fig. 1).  
 
Photoabsorption transfers part of the CHD population to an excited state. The difference PDF 
(ΔPDF, PDF after time zero minus PDF before time zero) in Fig. 1 (green) 0.7 ps after 
photoexcitation shows negative amplitudes at the positions of the static PDF due to the loss 
of ground state population. In addition, it exhibits positive amplitudes between 3 Å and 6 Å, 
which stem from the photoexcited population. The range is outside the C-C distances of 
CHD. Comparison with C-C distances of two isomers of the photoproduct 1,3,5-hexatriene 
(HT) shows good agreement. The broad amplitude distribution suggests a high level of 
vibrational excitation due to redistribution of large amounts of the absorbed photon energy.   
The ΔPDFs around time zero show, apart from an overall amplitude increase due to the finite 
instrument response function, two sets of time-dependent changes in their shape. These must 
be due to the underlying ring opening dynamics. The two negative peaks from ground state 
population loss initially exhibit similar intensity. At the latest delay, the peak at 2.4 Å shows 
approximately twice the amplitude of the peak at 1.4 Å. Furthermore, the positive signature 
first peaks at 3.4 Å. At later delays, the signature develops a second peak at 4.5 Å. 
Both effects can be associated with the nuclear wavepacket dynamics during the ring 
opening: Directly after photoexcitation, the positive contribution from the photoexcited 
Fig. 1. Static pair distribution function  
(PDF) of 1,3-cyclohexadiene (CHD, blue).  
The two maxima in the PDF can be  
associated with carbon-carbon distances as  
color-coded in the inset and marked by  
arrows. A representative difference PDF  
(ΔPDF) 0.7 ps after photoexcitation  
(green) shows negative contributions at the  
peak positions of the static PDF due to  
ground state population loss. It exhibits  
positive amplitudes from the photoexcited  
population, in a distance range, where  
CHD has negligible contributions.  
Comparison with carbon-carbon distances  
in two isomers of the photoproduct 1,3,5- 
hexatriene (inset) shows good agreement. 
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2
population to the ΔPDF and the negative contribution from the ground state population loss 
cancel each other out, since the nuclei have not moved yet.  
During the ring opening, the positive PDF amplitudes associated with the breaking C-C bond 
must move from 1.4 Å into the range beyond 3 Å. Likewise, amplitudes connected to part of 
the C-C distances across the CHD ring (2.4 Å) move to larger distances. The negative PDF 
contribution from ground state population loss, however, remains static. Thus, the negative 
PDF amplitudes from ground state population loss at 1.4 Å become exposed earlier than the 
negative amplitudes at 2.4 Å. This is, what we observe in the changes of the relative negative 
amplitude. After ring opening, the molecules arrive on the HT ground state potential energy 
surface, with substantial amounts of vibrational energy. The nuclear wavepacket broadens 
considerably, which is visible in the time-dependent broadening of the positive contributions 
in Fig.1. In addition to the immediate ring opening dynamics, we find evidence for coherent 
large amplitude motion of the HT carbon chains in the investigated time window of 0.7 ps.  
We compare our experimental findings with multi-reference excited state trajectory 
simulations, which we extend by simulations in the ground state on density functional theory 
level to cover the experimentally investigated 0.7 ps time window. Except for slight 
disagreements in time scales, Simulated ΔPDFs are in excellent agreement with our 
experimental data. They support both our interpretation of the experimental data about both 
the ring opening dynamics and the following large amplitude motion on the sub-ps timescale. 
 
This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, 
Chemical Sciences, Geosciences, and Biosciences Division. The experimental part of this research was 
performed at the SLAC megaelectronvolt ultrafast electron diffraction facility, which is supported in 
part by the DOE BES SUF Division Accelerator & Detector R&D program, the Linac Coherent Light 
Source (LCLS) Facility, and SLAC under contract Nos. DE-AC02-05-CH11231 and DE-AC02-
76SF00515. MG is funded via a Lichtenberg Professorship of the Volkswagen Foundation. DMS. is 
grateful to the NSF for a graduate fellowship. JPFN acknowledges the support by the Wild Overseas 
Scholars Fund of Department of Chemistry, University of York. KW and M. are supported by the U.S. 
Department of Energy Office of Science, Basic Energy Sciences under Award No. DE-SC0014170. 
PMW is supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under 
Award No. DE-SC0017995. AK is supported by the Carnegie Trust for the Universities of Scotland 
(grant ref. CRG050414) and an RSE/Scottish Government Sabbatical Research Grant (ref. 58507). 
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Imaging the ring opening reaction of 1,3-cyclohexadiene with MeV ultrafast electron diffraction

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Imaging the ring opening reaction of 1,3-cyclohexadiene with MeV ultrafast electron diffraction

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