Femtosecond studies of the iodine–mesitylene charge‐transfer complex

Femtosecond laser studies have been performed to investigate the initial photodissociation reactions of I2–mesitylene charge transfer complexes. Photodissociation occurs along both the I2–mesitylene ‘‘bond’’ and the I–I bond with a branching ratio of 2:3 for the two reaction coordinates. Following excitation at 400 nm, geminate recombination occurs along both reaction coordinates. The reformed I2–mesitylene complexes are formed vibrationally hot and relax on a time scale of 13 ps. The I–mesitylene spectrum is fully developed within 500 fs of the pump pulse. Approximately 40% of the I–mesitylene complexes undergo geminate recombination on a time scale of 14 ps. Most of the remaining complexes recombine with their original partners on a time scale of 400 ps. The initial anisotropy of the photoproduct absorption is 0.09±0.02. This low anisotropy is a direct result of the geometry of the complex and nature of the electronic transition rather than indicative of ultrafast motion toward an asymmetric transition state preceding dissociation. © 1995 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70671/2/JCPSA6-103-18-7877-1.pd

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

One- and two-color kinetics have been combined with broadband ultraviolet transient absorption spectroscopy in the 265–300 nm region to elucidate the photophysics of cis-hexatriene in cyclohexane solvent. The lowest singlet excited state, the 2 1A121A1 state, is observed to have a lifetime of 200±50 fs. The ground-state hexatriene is produced vibrationally hot. The excess vibrational energy permits ultrafast isomerization around the C–C single bonds in hexatriene. This results in a dynamic equilibrium of the three cis-hexatriene rotamers, which then relaxes multiexponentially to the room-temperature distribution in which the di-s-trans-Z-hexatriene form predominates. The peak of the mono-s-trans (cZt-HT) population is estimated to be ∼50%. Vibrational cooling results in trapping of a small amount, ∼8%, of cZt-HT that relaxes on a much longer time scale as the barrier to isomerization becomes important. An estimate of the absorption spectrum of cZt-HT is deduced from analysis of the spectral data at 50 ps. © 1997 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70051/2/JCPSA6-107-13-4985-1.pd

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

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-hexatrienes-cis,Z,s-cis-1,3,5-hexatriene (cZc-HT) is formed on a ∼ 250 fs∼250fs 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 ps), vibrational cooling and thermal equilibration with the surroundings (10–20 ps), and activated isomerization of trapped cZt-HT to tZt-HT (≫100 ps).(≫100ps). © 1998 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70793/2/JCPSA6-108-2-556-1.pd

Extracting Information from Adaptive Control Experiments

Optical control of chemical reactivity is achieved through the use of photonic reagents, that is, “shaped” ultrafast optical pulses created using a pulse shaper. It has been demonstrated in a number of molecular systems that these pulses can effectively guide the system into a desired final state. Effective pulses are often found through an experimental search involving thousands of individual measurements. An examination of the pulses tested in these experiments can reveal the pulse features responsible for control and also the underlying molecular dynamics. In this article we review attempts to extract information from optical control experiments using adaptive learning algorithms to search the available parameter space, and we discuss how these kinds of experiments can be used to achieve and understand multiphoton optical control.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/91361/1/397_ftp.pd

Structure and function in the isolated reaction center complex of Photosystem II: energy and charge transfer dynamics and mechanism

The dynamics of energy and charge transfer in the Photosystem II reaction center complex is an area of great interest today. These processes occur on a time scale ranging from femtoseconds to tens of picoseconds or longer. Steady-state and ultrafast spectroscopy techniques have provided a great deal of quantitative and qualitative data that have led to varied interpretations and phenomenological models. More recently, microscopic models that identify specific charge separated states have been introduced, and offer more insight into the charge transfer mechanism. The structure and energetics of PS II reaction centers are reviewed, emphasizing the effects on the dynamics of the initial charge transfer.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43542/1/11120_2004_Article_406692.pd

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

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.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87876/2/114506_1.pd

Ultrafast electrocyclic ring opening of 7-dehydrocholesterol in solution: The influence of solvent on excited state dynamics

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98648/1/JChemPhys_134_104503.pd

Biophysics - Quantum path to photosynthesis

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62904/1/446740a.pd

Vibrational relaxation of I2I2 in complexing solvents: The role of solvent–solute attractive forces

Femtosecond transient absorption studies of I2–areneI2–arene complexes, with arene=hexamethylbenzenearene=hexamethylbenzene (HMB), mesitylene (MST), or m-xylene (mX), are used to investigate the effect of solvent–solute attractive forces upon the rate of vibrational relaxation in solution. Comparison of measurements on I2–MSTI2–MST complexes in neat mesitylene and I2–MSTI2–MST complexes diluted in carbontetrachloride demonstrate that binary solvent–solute attractive forces control the rate of vibrational relaxation in this prototypical model of diatomic vibrational relaxation. The data obtained for different arenes demonstrate that the rate of I2I2 relaxation increases with the magnitude of the I2–areneI2–arene attractive interaction. I2–HMBI2–HMB relaxes much faster than I2I2 in MST or mX. The results of these experiments are discussed in terms of both isolated binary collision and instantaneous normal mode models for vibrational relaxation. © 1998 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69406/2/JCPSA6-109-21-9494-1.pd