A deep UV trigger for ground-state ring-opening dynamics of 1,3-cyclohexadiene

Abstract

We explore the photo-induced kinetics of 1,3-cyclohexadiene upon excitation at 200 nm to the 3p state by ultrafast time-resolved, gas-phase x-ray scattering using the Linac Coherent Light Source. Analysis of the scattering anisotropy reveals that the excitation leads to the 3px and 3py Rydberg electronic states, which relax to the ground state with a time constant of 208 ± 11 fs. In contrast to the well-studied 266 nm excitation, at 200 nm the majority of the molecules (76 ± 3%) relax to vibrationally hot cyclohexadiene in the ground electronic state. A subsequent reaction on the ground electronic state surface leads from the hot cyclohexadiene to 1,3,5-hexatriene, with rates for the forward and backward reactions of 174 ± 13 and 355 ± 45 ps, respectively. The scattering pattern of the final hexatriene product reveals a thermal distribution of rotamers about the carbon-carbon single bonds.

Advances in ultrafast gas-phase x-ray scattering

“…Shortly afterward, analysis of the time-dependent scattering signals of NMM following photoexcitation revealed coherent vibrations that dephase on a picosecond time scale 7 . Recently, similar experiments also revealed multiple competing pathways in the UV-induced photodissociation of trimethylamine 42 and an array of insights into the initial electronic redistribution 14 , structural dynamics 45 , and product species formed 43 A c c e p t e d M a n u s c r i p t result, rather than the overall accuracy. As the calculation of structures in highly excited electronic states of polyatomic molecules is unreliable, theoretical results cannot be used as absolute benchmarks.…”

Tracking the ultraviolet-induced photochemistry of thiophenone during and after ultrafast ring opening

“…Recent advances in time-resolved experimental techniques and in computational methods for treating (coupled) electronic and nuclear dynamics are revolutionizing the field of ultrafast photochemistry, enabling direct probing of evolving molecular structures with unprecedented structural and temporal resolution [1][2][3][4][5][6][7][8] . Such studies provide the ultimate test of our knowledge and understanding of light-initiated chemistry.…”

“…The photo-induced ring opening of the polyene 1,3-cyclohexadiene 13 is widely employed as a model system for benchmarking and validating ultrafast methods, e.g. by ultrafast X-ray 1,7,8 and electron 6 diffraction, by femtosecond transient X-ray absorption 4 and fragmentation 14 , and by time-resolved photoelectron spectroscopy (TRPES) [15][16] . However, few other photo-induced ringopening reactions have been probed so thoroughly and, of these, even fewer have provided a comprehensive picture of the reaction dynamics on both the excited and ground (S 0 ) state potential energy surfaces (PESs).…”

An Assessment of Different Electronic Structure Approaches for Modeling Time-Resolved X-ray Absorption Spectroscopy

“…X-ray free electron lasers (XFELs) and instruments based on high-harmonic generation (HHG) enable spectroscopic measurements on the femtosecond [2][3][4] and attosecond [5][6][7][8] timescales. Methods for investigating femtosecond dynamics can be classified into two categories: (i) methods that track the electronic structure as parametrically dependent on the nuclear dynamics, such as time-resolved photoelectron spectroscopy (TR-PES), [9][10][11][12] and (ii) methods that directly visualize the nuclear dynamics, such as ultrafast X-ray scattering [13][14][15][16] and ultrafast electron diffraction. 12,17 Time-resolved X-ray absorption spectroscopy (TR-XAS) belongs to the former category.…”