Picosecond measurements

Picosecond Measurements of Electron Transfer in Bacterial Photosynthetic Reaction Centers... 

By strobing the time intervals such that their number equals the number of k values, we can try to invert the e"1 matrix of Eq. (7) to obtain the unknown Xm vector. However, it follows from Eq. (8) that the em matrix cannot be inverted, as it contains a number of columns, explicitly all the s = s columns, composed of a single number. This is due to the fact that for s = s the Eg - Es> terms vanish, leaving the ys decay rates as the only source of time-dependence. Since for spontaneous radiative decay (and many other processes), the decay times, l/ys, are orders of magnitude longer than the duration of the sub-picosecond measurement, the e s s matrix elements are essentially time-independent and hence identical to one another at different times. As a result, the e matrix, which becomes nearly singular, cannot be inverted. 

Figure 4-15. Picosecond measurements for decay of reactant clusters ROH M (as detected by two photon ionization of the cluster ROH-M + ) and formation of product RO -H+M (as detected by the fragment M H+) (from Steadman and Syage 1990).

O Neil, M. Marohn, J. McLendon, G. Picosecond measurements of exciton trapping in semiconductor clusters, Chem. Phys. Lett. 1990, 168, 208. 

Detection systems for nanosecond and microsecond pulse radiolysis are mostly the same, but that for picosecond measurement is somewhat different. The description is confined here to nanosecond and microsecond systems and a brief explanation to the picosecond system will be given in the following subsection. 

A more complete account of these studies is cuirendy in preparation. (M. L. Homg, J. Gaidecki, and M. Maroncelli, "Sub-Picosecond Measurements of the Solvation Dynamics of Coumain 153, to be submitted to J. Chem. Phys.)... 

Binding and release of ligands in solvent-complexed nickel(n) porphyrins occurs during excitation by picosecond measurements. These processes appear to occur mainly from the lowest (diol) excited state of the species being pumped. 

Azulene is one of the most interesting molecules from the photophysical point of view. A picosecond measurement of the vibrational energy decay in matrix isolated polyatomic molecules shows at 4K that molecular modes in a polyatomic matrix does not affect decay in the 2 vibrational manifold . The S2 Sq fluorescence of pseudoazulenes has been studies in Shpolski... 

Germer TA, Stephenson JC, Heilweil EJ, Cavanagh RR (1993) Picosecond measurement of substrate-to-adsorbate energy transfer The frustrated translation of CO/Pt(l 11). J Chem Phys 98 9986... 

Note the data points marked by x in the near infrared region between 1000 and 1300 nm, as reported by Dutton, Kaufmann, Chance and Rentzepis". This combination of extinction coefficients is almost certainly due to the oxidation of the primary electron donor, P870, i.e., the bacteriochlorophyll special-pair [BChl]2, since it is not found for species such as BChF, BChl", or B (generated electrochemi-cally). Picosecond measurements have revealed the maximum absorbance increase at 1250 nm in Rb. sphaeraides and C. vinosum and at 1300 nm mRp. viridis. Picosecond measurements in this wavelength region can thus provide direct information on the reaction course of P870 photooxidation. 

However, data reported by other workers (see R1-R3 for further discussion) gave rise to some controversy and for this reason Shuvalov and Duysens " extended their picosecond measurements to a modified reaction-center complex ofRb. sphaeroides R-26. It is known that the bacterial reaction center contains two monomeric BChl molecules, and that only cofactors in the L- (i.e., A-) branch, including BChl, participate in electron transfer. To eliminate any possible band shift of the absorbance bands of the BChl molecule, Bb, during the formation of the [P" -BA ]-state, Shuvalov and Duysens used the procedure of Ditson, Davis and Pearlstein to selectively remove the Bg molecules by a well-documented NaBH4 treatment. Furthermore, it is known that the conversion ofP in the Be-deleted reaction centers to P"" is nearly 100% and the interaction of B with BO is unaltered. 

II.A. Early Picosecond Measurements Urbana, Yellow Springs-Pushchinoy ... 

II.C. Nano- and Picosecond Measurements with P700-Enriched Complexes Sac/ay, Sagamiko-Okazaki) ... 

Picosecond measurements were performed at room temperature at 750 nm and 800 nm, giving preferential excitation of BChl c and a respectively. Anisotropy measurements were made with the exciting light polarized parallel or perpendicular to the measuring light. Isotropic measurements were made with the polarization set to 54.7. The excitation Intensity was varied with neutral density filters. Fluorescence spectra and were measured on a Spex Fluorolog spectrometer. The anisotropy was calculated from Iu-Ij /(Ii i+2Ii), where I i and Ii are the intensity of the absorbance or fluorescence signal. 

OUT recent theoretical work and for inviting us to join in the study of cyanine photophysics and photochemistry. We wish also thank Prof E.Castelluci for his generous help in the picosecond measurements carried out using the facilities provided by the LENS laboratory of Florence. The sample of BMPC perchlorate used in this work was donated by Prof Sheves, Rehovot, to whom we are very grateful. This research was supported by the Ministero della cerea Scientifica e Tecnologia (Rome), the Consiglio Nazionale delle Ricerche (Rome) and the Centro Interdipartimentale di Calculo Automatico e Informatica Applicata (University ofModena). 

The solvation process in alcoholic solutions of molecular species has been examined by picosecond measurements on the time-dependence of the Stokes shift , i.e., of the frequency difference between the maxima of the absorption and fluorescence spectra. The time-constant for this quantity for solutions of 1-naphthylamine in propanol is 52 ps, which agrees as regards order of magnitude with the result of an electrostatic calculation from a model in which the solvent is represented by a dielectric continuum and the solute by a spherical cavity of dielectric constant c- This model predicts the relaxation time of the reactive field to be not x but the longitudinal relaxation time tl which equals Ti ( c + 2 oo)/( c -f 2 q), and can be much smaller than td for highly-polar solvents (cf. Section 4.3.4). 

Braun, C. L. and Scott, T. W., Picosecond measurements of time-resolved geminate charge recombination, /. Phys, Chem, 87, 4776,1983. 

Contributions to the third-order susceptibility (-G) CO-O), co) from third-order hyperpolarizability have received considerable attention. Picosecond measurements have been carried out on liquid crystals in the isotropic phase [69, 70] from the criti-... 

In order to test the expectation of a largely mono-exponential decay of the radical pair P+H at low temperatures where the formation of this state is clearly non-exponential, we confront the above picosecond measurements with the decay characteristics of the radical pair P+H . 

Using powerful polarized light pulses instead of electrical pulses to induce birefringence in traditional Kerr cell liquids, an optical Kerr shutter has been developed with wide ranging applications to picosecond measurements [11.22]. 

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