Femtosecond photochemical ring opening of 1,3-cyclohexadiene studied by time-resolved intense-field ionization

Abstract

We found that hydrogen ion formation due to multielectron dissociative ionization by an intense-laser field is much less efficient with 1,3-cyclohexadiene than with its isomer 1,3Z,5-hexatriene (Z-HT). Moreover by suppressing the ionization barrier an intense-laser field ejects electrons most efficiently from molecular states of low ionization potential. After pumping 1,3-cyclohexadiene at 267 nm to its 1B2 state we probe the system by intense-laser field ionization with delayed 800 nm pulses. Monitoring of the parent ion C6H8+, of the main fragment C6H7+ and of H+ allows us to follow the motion from the 1B2 surface to the dark 2A1 state and from there towards the 2A1/1A1 conical intersection to the ground-state surface of the product. The measured 1B2 and 2A1 lifetimes are 43±3 and 77±7 fs, respectively, and the primary photoproduct cZc-HT is produced within 200 fs.

Femtosecond cluster studies of the solvated 7-azaindole excited state double-proton transfer

“…Experiment, as well as theory, indicates that the formation of ions in a femtosecond laser pulse (22) and the concomitant Coulomb explosion phenomenon (21) are completed on a very short time scale (ref. 23; unpublished calculations were performed to determine the separation time between two model 7-Aza monomers yielding times less than 25 fs), lending promise that the Coulomb explosion imaging method could be used in ''viewing'' intermediates at selected times during the course of a reaction (11,24). We have successfully used this method to follow the course of reactions in 7-Aza dimers and to detect the reaction intermediate as well.…”

Effects of probe energy and competing pathways on time-resolved photoelectron spectroscopy: the ring-opening of 1,3-cyclohexadiene

“…[14][15][16] The reaction has been studied extensively using a wide array of time-resolved techniques in solution and gas phase, including resonance Raman spectroscopy, [17][18][19] transient absorption, 12,20,21 photoelectron spectroscopy, [22][23][24] and dissociative intense-field ionisation. [25][26][27] Computational studies include electronic structure calculations 16,[28][29][30][31][32][33] and dynamics, [34][35][36][37][38][39][40][41][42] as well as joint theoretical and experimental work. 23,[43][44][45] It is notable that despite such extensive study, the understanding of the reaction is not complete.…”

Enhancement of strong-field multiple ionization in the vicinity of the conical intersection in 1,3-cyclohexadiene ring opening

“…In that case, enhanced double ionization would lead to an overall increase in fragmentation, manifested as an increase in lower--mass fragments and an increase in the fragment kinetic energy. Indeed, previous work by ourselves and others on CHD isomerization [4,13,15,20] also reports that proton and low mass fragment ejection goes through a delayed maximum, before reaching the value corresponding to the CHD and HT mixture at late times. The persistent high kinetic energy of lower--mass fragments suggests that the double ionization rate remains higher after the maximum compared to that of CHD.…”

“…The molecule has been studied extensively [3--13] (for a recent review see [14] and the references therein), and is suitable for testing new experimental techniques. Ion fragment time--of--flight (TOF) mass spectra of the parent molecule CHD and the product HT differ significantly when short intense 800 nm radiation is used for fragmentation [3,4,15]. The HT isomer is characterized by a more violent fragmentation that leads to an increase of smaller--mass fragments, particularly H + , compared to that of CHD.…”