Multiphoton Control of the 1,3-Cyclohexadiene Ring-Opening Reaction in the Presence of Competing Solvent Reactions

Abstract

Although physical chemistry has often concentrated on the observation and understanding of chemical systems, the defining characteristic of chemistry remains the direction and control of chemical reactivity. Optical control of molecular dynamics, and thus of chemical reactivity provides a path to use photon energy as a smart reagent in a chemical system. In this paper, we discuss recent research in this field in the context of our studies of the multiphoton optical control of the photo-initiated ring-opening reaction of 1,3-cyclohexadiene (CHD) to form 1,3,5- cis-hexatriene (Z-HT). Closed-loop feedback and learning algorithms are able to identify pulses that increase the desired target state by as much as a factor of two. Mechanisms for control are discussed through the influence of the intensity dependence, the nonlinear power spectrum, and the projection of the pulses onto low orders of polynomial phase. Control measurements in neat solvents demonstrate that competing solvent fragmentation reactions must also be considered. In particular, multiphoton excitation of cyclohexane alone is capable of producing hexatriene. Statistical analyses of data sets obtained in learning algorithm searches in neat cyclohexane and for CHD in hexane and cyclohexane highlight the importance of linear and quadratic chirp, while demonstrating that the control features are not so easily defined. Higher order phase components are also important. On the basis of these results the involvement of low-frequency ground-state vibrational modes is proposed. When the population is transferred to the excited state, momentum along the torsional coordinate may keep the wave packet localized as it moves toward the conical intersections controlling the yield of Z-HT.

Closed-loop learning control of isomerization using shaped ultrafast laser pulses in the deep ultraviolet

“…This could be achieved via a pump-dump-pump scheme and leads to control over the momentum of the wave packet as it crosses the S 1 -S 0 CI, influencing the CHD/HT branching ratio at the CI. 11,35 A second possibility consists of strong-field dressing the PESs to change the wave packet evolution in the vicinity of the CI. While the time required for the wave packet to travel from the Frank-Condon region of the 1B 2 potential to the S 1 -S 0 CI ͑1B 2 / 1A 1 ͒ ͓ϳ130 fs ͑Ref.…”

Probing ultrafast electronic and molecular dynamics with free-electron lasers

“…The same motif appears in important biochemical processes such as vitamin D synthesis. The CHD/HT isomerization has received significant attention in experiments and calculations that seek to investigate the role that the nonBorn-Oppenheimer interactions play in the photorelaxation [24,75,[77][78][79][80][81][82][83][84][85][86][87][88][89][90][91][92][93][94][95][96]. It is now known that the ring opening is complete in less than 200 fs, as a result of nonradiative relaxation through a conical intersection between the S 1 and S 0 states [75,81,84].…”

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

“…46,47 Finally, and perhaps most importantly, CHD has been used extensively to demonstrate new techniques and concepts. This includes coherent control, both experiment 43,48,49 and theory, 7,50,51 and a range of new ultrafast imaging techniques 52 such as electron diffraction, [53][54][55][56] X-ray scattering, 44,45 Coulomb explosion, 57 and near-edge X-ray absorption fine structure (NEXAFS). 58 The variety of complementary observations generated by these techniques holds great promise for tying down a complete understanding of this important and complex reaction, for instance by combining X-ray scattering and photoelectron data such as in ref.…”