Picosecond absorption spectroscopy

Caldwell, K, Noe, L. J, Traylor, T. G. "Photodissociation of Carbon Monoxide Forms of Synthetic Heme Complexes Using Picosecond Absorption Spectroscopy", unpublished results, National ACS Meeting, Miami Beach, April 1985. 

The time and wavelength resolved fluorescence dynamics of bianthryl has been investigated by several groups [30, 82, 132, 133, 115, 116]. In addition, this molecule has been studied by picosecond absorption spectroscopy [115], electric field induced fluorescence anisotropy measurements [117] and optically induced dielectric absorption (microwave) measurements [118, 119]. The results are generally in accord with the theoretical model presented in Sections III.A and III.B. One of the challenges of studying the photodynamics of BA is that the LE and CT interconversion is so rapid (i.e., on the time scale of solvation) that it is necessary to employ ultraviolet subpicosecond and even femtosecond fluorescence spectroscopy which has only recently become available [30, 82, 132, 133]. 

Picosecond absorption spectroscopy was employed to study the dynamics of contact ion pairs produced upon the photolysis of substituted diphenylmethyl acetates in the solvents acetonitrile, dimethyl sulfoxide, and 2,2,2-trifluoroethanol.66 A review appeared of the equation developed by Mayr and co-workers log k = s(N + E), where k is the rate constant at 20 °C, s and N are nucleophile-dependent parameters, and is an electrophilicity parameter 67 This equation, originally developed for benzhydrylium ions and n-nucleophiles, has now been employed for a large number of different types of electrophiles and nucleophiles. The E, N, and s parameters now available can be used to predict the rates of a large number of polar organic reactions. Rate constants for the reactions of benzhydrylium ions with halide ions were obtained... 

Picosecond absorption spectroscopy studies of the contact ion pairs formed in the photo-initiated, S N 1 reaction of three substituted benzhydryl acetates (18) provided the rate constants for the k and k2 steps of the reaction (Scheme 10), in acetonitrile and DMSO.83 The activation parameters for the k and k2 steps were obtained from the temperature dependence of these steps and the transition state energies were calculated from the rate constants. This allowed the energy surfaces for three substituted substrates to be calculated in each solvent. The effect of solvent reorganization on the reactions of the unsubstituted and methyl-substituted benzhydryl contact ion pairs (CIP) was significant, causing a breakdown of transition state theory for these reactions. The results indicated that it will be very difficult to develop a simple theory of nucleophilicity in, S N1 reactions and that Marcus theory cannot be applied to SnI processes. 

In this system, the dynamics of pair decay (via intersystem crossing) and of pair separation by solvent molecules were determined by picosecond absorption spectroscopy [167, 168],... 

Concerning the analysis of different types of ion pairs and their discrimination using picosecond absorption spectroscopy, the work by Peters must be emphasized. He studied in great detail the photoreduction of benzophenone by aromatic amines... 

Figure 8. Ultrafast processes associated with bathorhodopsin and hypsorhodopsin monitored by picosecond absorption spectroscopy. (A) Absorbance changes (aA) as function of time monitored at 530 nm, showing the decay of a species (PBAT) identified as the precursor of BAT. (B) Arrhenius plot for the rate constant of this process in rhodopsin and in deuterated rhodopsin. (C) Decay of hypsorhodopsin. (D) Formation of bathorhodopsin. [(A) and (B) data from ref. 301 for bovine rhodopsin (in 0.1 M Ammonix LO, 66% ethylene glycol, at pH 7.0). (C) and (D) data from ref. 298 for squid rhodopsin in 2% digitonin (pH 10.5) at room temperature.]...

Shen Y, Cook AR. (2009) Optical fiber-based single-shot picosecond absorption spectroscopy. Rev Set Instrum 80 073106. 

Ono and Ware"" have measured the absorption, emission, and excitation spectra, the fluorescence decay times, and the quantum yields of a series of substituted diphenylmethylenes in rigid matrices at low temperatures. Acean-thrylene shows S2- So emission in hexane with a yield of 0.017 and lifetime of 4.3 ns. The low-temperature fluorescence spectra of bis-2-naphthyl-alkanes and their derivatives have been studied. Excimer formation is an activated process. The fluorescence and absorption spectra of 1,1-diphenyl-ethylenes have been analysed in some detail by Gustav and Bolke. " The S — Si transitions in trans isomers of phenylnaphthylethylenes have been assigned by picosecond absorption spectroscopy. Effects of solvent viscosity and the role of conformers in the mechanism of isomerization are elucidated. The production of non-equilibrium conformer concentrations in glassy solutions of diarylethylenes at 77 K due to restrictions imposed by the solid matrix has also been reported. Free jet excitation and emission spectra of diphenyl-butadiene show clearly the lowest excited Ag state and give a lifetime of 52.8 ns for 0-0 excitation.Electric field-induced charges in the optical... 

A transient absorption band, with a profile similar to that of the dimethylani-line radical cation, was observed in polar solvents by time-resolved picosecond absorption spectroscopy. 

B Ke and VA Shuvaiov (1987) Picosecond absorption spectroscopy In photosynthesis and primary electron transfer processes, in J Barber (ed) Topics in Photosynthesis, Voi 8, The Light Reactions, pp 31-93. Eisevier... 

Figure 1. (A) Apparatus used for time-resolved picosecond absorption spectroscopy which utilizes the TPF technique. A set ofphotodiodes and oscilloscopes (PI, P2, P3, and P4)are used to measure the intensity of the laser pulse before and after each pass through the cell. (Reproduced with permission from Ref 15. Copyright 1969, North-Holland...

The Visual Transduction Process. Picosecond absorption spectroscopy which utilizes OMCDs also has provided important mechanistic information that previously was not available by means of other techniques. Detailed pathways of a number of reactions which are important from a physical, chemical, and/or biological viewpoint have been elucidated by means of this technique. A recent picosecond spectroscopic study by Spalink et. al. (30) has demonstrated that an experimental criterion, which has been used to support the hypothesis that cis-trans isomerization (31) is the primary event in the visual transduction process, is not true. This criterion is based on the commonly occurring statement that both the naturally occurring 11-cis-rhodopsin and the synthetic 9-cis-rhodopsin lead to the same primary photochemical product, bathorhodopsin. Of course, the existence of a common intermediate generated from either 11-cis- or 9-cis-rhodopsin would support the commonly proposed mechanism of cis-trans isomerization as the primary event in the visual transduction process. However, the data obtained by Spalink et. al. (30) indicate that a common intermediate is not generated from both rhodopsins. 

Intermolecular Photo-Induced Electron Transfer. Picosecond absorption spectroscopy has also been applied recently to studies of intermolecular electron transfer on the picosecond time scale.(34) As in the previously described study of rhodopsin, the photo-induced intermolecular electron transfer between chloranil (CHL) and the arenes, naphthalene (NAP), 9,10-dihydrophenanthrene (DHP), and indene (IN) was studied by means of picosecond absorption spectroscopy which utilizes an OMCD. Difference absorption spectra of samples of CHL and one of these particular arenes in acetonitrile were measured at selected delay times after excitation at 355-nm with 25-ps FWHM laser pulses. These picosecond spectroscopic studies revealed information about the mechanism of intermolecular electron transfer and subsequent radical ion formation that was not possible in previous spectroscopic studies performed on the nanosecond (35-37) and microsecond (38,39) time scales. 

Lifetime measurements were carried out on TPE in various solvents utilizing picosecond absorption spectroscopy [59,60]. Two absorption bands, 430 nm and 630 nm, were found for TPE excitation. The absorption band at 630 nm was found to be very weak and the decay was very rapid, within the experimental conditions. The absorption band around 430 nm was taken up for detail studies and its decay was studied in various solvents [61]. The solvent polarity is found to have a profound role in the decay of the 430 nm absorption band of excited TPE [61] and furthermore a linear relationship between log k (decay rate constant) and solvent polarity parameter was established. Based on these results, the authors mentioned that the 430-nm absorption band of singlet excited TPE has the twisted zwitterionic structure (as depicted in 1) and is nonfluorescent [61]. 

Picosecond absorption spectroscopy has been employed to follow the time-dependence of the optical absorption during the formation of e, while pulsed-laser saturation spectroscopy has been used to examine the... 

Temperature dependence of energy transfer fk-om the long wavelength antenna BChl-896 to the reaction center in Rhodospirillim Rubrum, Rhodobacter Sphaeroides (w.t. and M21 mutant) fk-om 77 to 177 K, studied by picosecond absorption spectroscopy. 

Most current models for ener transfer and trapping in the antenna of purple nonsulfur bacteria involve excitations visiting many reaction centers before trapping occurs. This so-called lake model has been supported by a variety of observations including singlet-singlet annihilation studies (1-3), low intensity picosecond absorption spectroscopy (4), and fluorescence depolarization studies (5). 

The photosynthetic apparatus of the purple bacterium Rhodobacter sphae-roides is composed of two light harvesting complexes, LHl and LH2, which surround and interconnect photochemical reaction centres. Picosecond absorption spectroscopy with weak picosecond laser pulses is a powerful technique to probe the excited state dynamics in antenna systems. For Rb. sphaeroides at 77K the measured picosecond absorption kinetics were interpreted to give the following sequence of energy transfer events and time constants (1-3). 

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