The ultrafast ground and excited state dynamics of cis -hexatriene in cyclohexane

Pump-probe gas phase X-ray scattering experiments, enabled by the development of Xray Free Electron Lasers, have advanced to reveal scattering patterns of molecules far from their equilibrium geometry. For polyatomic molecular systems, large amplitude vibrational motions are associated with anharmonicity and shifts of interatomic distances (known as the 'shrinkage effect' in linear molecules), making analytical solutions using traditional harmonic approximations inapplicable. More generally, the interatomic distances in a polyatomic molecule are not independent and the traditional equations commonly used to interpret the data may give unphysical results. Here we introduce a novel method based on molecular dynamic trajectories and illustrate it on two examples of hot, vibrating molecules at thermal equilibrium. When excited at 200 nm, 1,3cyclohexadiene (CHD) relaxes on a sub-picosecond time scale back to the reactant molecule, the dominant pathway, and to various forms of 1,3,5-hexatriene (HT). With internal energies of about 6 eV, the energy thermalizes quickly, leading to structure distributions that deviate significantly from their vibrationless equilibrium. The experimental and theoretical results are in excellent agreement and reveal that a significant contribution to the scattering signal arises from transition state structures near the inversion barrier of CHD. In HT, the analysis clarifies that previous inconsistent structural parameters determined from electron diffraction were artifacts resulting from the use of inapplicable analytical equations.

supporting

confidence: 89%

Scattering off molecules far from equilibrium

Yong

Ruddock

Stankus

et al. 2019

“…We have recently published an ultrafast transient absorption study of the internal conversion and conformational relaxation of Z-HT in cyclohexane following excitation into the strongly allowed 1 1 B 2 state. 17 These measurements demonstrate the recovery of vibrationally hot ground state hexatriene on a time scale of ϳ200 fs. The subsequent multiexponential relaxation is interpreted in terms of an evolving equilibrium of Z-HT rotamers and energy transfer to the surroundings ͑thermalization͒.…”

Section: Introductionmentioning

confidence: 84%

The ultrafast photochemical ring-opening reaction of 1,3-cyclohexadiene in cyclohexane

Pullen

Anderson

Walker

et al. 1998

Self Cite

102

The ring-opening reaction of 1,3-cyclohexadiene in cyclohexane solution and the subsequent photoproduct cooling dynamics have been investigated by using two-color transient absorption kinetic measurements and novel time-resolved absorption spectroscopy in the 260-300 nm spectral region. The initial photoproduct in this reaction, s-cis,Z,s-cis-1,3,5-hexatriene ͑cZc-HT͒ is formed on a ϳ250 fs time scale. Spectra deduced for time delays very close to zero, as well as calculated Rice-Ramsperger-Kassel-Marcus unimolecular reaction rates, provide strong evidence that the quantum yield for the reaction is determined before any relaxation occurs on the ground state. Upon formation, the vibrationally excited hexatriene photoproduct is able to isomerize around C-C single bonds freely. As a result, the evolution observed in the transient absorption measurements represents a combination of rotamer population dynamics and thermalization due to energy transfer to the solvent. Three distinct time scales for relaxation are observed. These time scales correspond approximately to the development of an evolving equilibrium of Z-HT rotamers ͑1-5 ps͒, vibrational cooling and thermal equilibration with the surroundings ͑10-20 ps͒, and activated isomerization of trapped cZt-HT to tZt-HT (ӷ100 ps).

“…around both of the single bonds, while a few percent is trapped as cZt-HT, relaxing to the tZt-HT ground-state configuration within a nanosecond. 17 Recently CHD has been recognized as a candidate for coherent control and for potential use as a molecular logic switch. 18 Direct one-photon 1A → 2A transitions are effectively forbidden.…”

Section: Introductionmentioning

confidence: 99%

Spectral phase effects on nonlinear resonant photochemistry of 1,3-cyclohexadiene in solution

Carroll

Pearson

Florean

et al. 2006

Self Cite

We have investigated the ring opening of 1,3-cyclohexadiene to form 1,3,5-cis-hexatriene (Z-HT) using optical pulse shaping to enhance multiphoton excitation. A closed-loop learning algorithm was used to search for an optimal spectral phase function, with the effectiveness or fitness of each optical pulse assessed using the UV absorption spectrum. The learning algorithm was able to identify pulses that increased the formation of Z-HT by as much as a factor of 2 and to identify pulse shapes that decreased solvent fragmentation while leaving the formation of Z-HT essentially unaffected. The highest yields of Z-HT did not occur for the highest peak intensity laser pulses. Rather, negative quadratic phase was identified as an important control parameter in the formation of Z-HT.