A learning algorithm was used to manipulate optical pulse shapes and optimize retinal isomerization in bacteriorhodopsin, for excitation levels up to 1.8 ؋ 10 16 photons per square centimeter. Below 1/3 the maximum excitation level, the yield was not sensitive to pulse shape. Above this level the learning algorithm found that a Fourier-transform-limited (TL) pulse maximized the 13-cis population. For this optimal pulse the yield increases linearly with intensity well beyond the saturation of the first excited state. To understand these results we performed systematic searches varying the chirp and energy of the pump pulses while monitoring the isomerization yield. The results are interpreted including the influence of 1-photon and multiphoton transitions. The population dynamics in each intermediate conformation and the final branching ratio between the all-trans and 13-cis isomers are modified by changes in the pulse energy and duration.coherent control ͉ photoisomerization ͉ ultrafast science B acteriorhodopsin (bR) is a photosynthetic protein found in the purple membrane of Halobacterium salinarum and capable of conversion of solar energy into chemical energy. This energy conversion is efficient (1-3) and has several possible applications (4-12). A retinal chromophore is responsible for photon absorption. After photoexcitation retinal undergoes ultrafast isomerization from the all-trans to a 13-cis configuration, accompanied by additional changes in the conformation of bR (3,8). The initial steps of the bR photocycle (see Fig. 1) have been studied intensively (11, 13-33), but there are still unanswered questions regarding the electronic potential energy surfaces (PES) of retinal, the interaction with its surroundings in the protein, and related ultrafast vibrational coupling. A number of models have been proposed, each explaining parts of the large number of experiments (4,13,14,16,(34)(35)(36)(37)(38)(39)(40). Attempts have been made to reconcile the differences between these models (4).We aim to understand how the optical pulse shape and intensity affect the all-trans 3 13-cis yield and to explore potential pathways for producing high photoproduct yields on an ultrafast time scale. This is relevant for energy storage using bio-molecular machines (9, 32). Recently, Prokhorenko et al. showed that the isomerization yield of retinal in bR could be manipulated in a low intensity, biologically relevant regime through the use of phase and amplitude shaped optical fields (25). Modifications of as much as Ϯ20% were observed compared with unshaped pulses capable of exciting an equal number of molecules. Yet the ultimate yields remain small as photon flux was restricted to excite Ϸ0.3% of the chromophores in the excitation volume. In a different experiment, Vogt et al. used much higher intensity, shorter wavelength pump pulses to excite bR and a shaped 800-nm dump pulse to study the evolution of the molecule on the excited state PES (30). They found that the excited population is transferred most effectively back to t...

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.

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.