Picosecond Transient Absorption Measurements

Picosecond Transient Absorption Measurements.— Several picosecond excitation and probe experiments have been recently described. A 0.5 ps pulse from a passively mode-locked dye laser was utilized to excite and subsequently monitor the singlet absorption in tetraphenylethylene. A 5 ps relaxation time, ascribed to twisting around the ethylene double bond, and a long-lived twisted intermediate were observed. The involvement of an intermediate exciplex in the photoinduced hydrogen-atom transfer reaction of pyrene with diphenylamine was demonstrated 

Gardini, P. Bortolus, and E. Amouyal, Chem. Phys. Lett., 1981, 77, 115. 

Problems that may be encountered due to external birefringence effects in the anisotropic absorption method of Shank and Ippen have been discussed. In the event of these interferences being unavoidable, methods have been described to allow the rotational reorientation times of the dye molecules in fluid solution to be determined.  

The accumulated 3-pulse stimulated photon-echo method was used in order to monitor vibrational relaxation times of the first excited electronic state of pentacene. Two amplified dye lasers were used to perform ps photon-echo measurements on pentacene and naphthalene samples, which established that pseudo-local photon scattering was responsible for optical dephasing in vibronic transitions. A mode-locked cavity-dumped synchronously pumped dye laser system was used to demonstrate long coherence times for the delocalized optical excitation of dimer states, by ps photon-echo spectroscopy.  

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The proposed excited state fullerene-fullerene interactions are consistent with the results of picosecond transient absorption measurements, in which excimer-like transients were observed [181]. The transient absorption is probably responsible for the significant optical limiting effects of the CgQ-styrene copolymers [182],... 

Braun, C. L. and Scott, T W., Picosecond transient absorption measurements of geminate electron-cation recombination, J, Phys, Chem., 91, 4436,1987. 

Aramaki and Atkinson were also active in work on the spiro-oxazines [65]. They noted that for NOSH in many polar and nonpolar solvents the picosecond time-resolved resonance Raman spectra simply built up over 50 psec with no shape evolution. The same finding was concluded from transient absorption measurements over the same time scale. The spectra/absorbances were then constant for 1.5 nsec. These authors suggest that only two isomers can be expected to contribute to the merocyanine spectra because those trans about the y-methene bridge bond attached to the naphthalene ring are sterically crowded due to short interproton distances. There was no evidence for the X transient in their study however, the 50-psec convoluted pulse profile may be expected to mask this sortlifetime species even if it were present. 

We have obtained additional evidence supporting the electron transfer mechanism of fluorescence quenching in 2 and 2 from picosecond transient absorption and fluorescence measurements. The fluorescence lifetimes of 1-2 in butyronitrile are reported in Table II. These lifetimes are proportional to the observed fluorescence quantum yields of these compounds and therefore indicate that the observed fluorescence quenching is not due simply to a change in the radiative rate for emission. 

This technique utilizes a pulse pump-probe experiment and monitors the absorption of a weak probe beam in the presence of a strong pump beam. Fig. 8 depicts the experimental set-up for a two-beam pump-probe experiment, which includes homodyne and heterodyne Kerr gate measurements and polarization-controlled transient absorption measurement. Generally, the input beam is produced from an amplified pulse laser system with 1 KHz repetition rate, which can produce picosecond or femtosecond pulses. This pumping light beam is divided into two beams by a beam-splitter with an intensity ratio of 30 1 therefore, the one with the stronger intensity will act as the pump and the weaker one will be the probe. The position of the sample is where these two beams focus and overlap spatially. The time delay between the pulses from these two beams is controlled by a retroreflec-... 

Fig. 8. Transient absorbance change at 700 nm (left panel), 810 nm (center) and 690 nm (right) in a PS-I particle induced by 1.5-ps, 610-nm pulses (after changes due to excited antenna chlorophylls had been subtracted). Figure source Wasielewski, Fenton and Govindjee (1987) The rate of formation of [PTOCT Aq ] in photosystem I particles from spinach as measured by picosecond transient absorption spectroscopy. Photosynthesis Res 12 184-186.

MR Wasielewski, JM Fenton and Govindjee (1987) The rate of formation ofP700 -A in photosystem I par-tides from spinach as measured by picosecond transient absorption spectroscopy. Photosynthesis Res 12 181-190... 

Fig. 8. (A) Decay-associated difference spectra (DADS) for the 3.8- and 28-ps and the non-decaying components measured with the core compiex prepared from Synechocystis PS il-deieted mutant under neutrai redox conditions and room temperature in the biue region. (B) Absorbance difference spectrum of M[Ao -A,] obtained on the picosecond time scale by subtracting AA[P700" +P700] from AA [P700 -P700]+[Ao"-A,]. Figure source Mi, Lin and Blankenship (1999) Picosecond transient absorption spectroscopy in the blue spectral region of photosystem I. Biochemistry 38 15233,15234.

Picosecond transient absorption and emission measurements with no applied magnetic field show that the primary reaction, TAPD- ZP-NQ -> TAPD-ZP -NQ, occurs in r = 5 ps, while the secondary reaction, TAPD-ZP -NQ -> TAPD -ZP-NQ, occurs with r = 530 ps. Based on these kinetics the quantum yield of TAPD -ZP-NQ" formation is 67%. Charge recombination within the long-lived ion-pair, TAPD -ZP-NQ, occurs with a 4 ms time constant. The thermodynamic and kinetic parameters for TAPD-ZP-NQ following excitation are summarized in Figure 4. 

New results of calculations as well as picosecond (ps)- and femtosecond (fs) laser spectroscopy studies have extended the basis for mechanistic discussions. Nagele et al. performed fs transient absorption measurements of the photoisomerization in the (njr ) state. They found a very rapid formation of a... 

Using picosecond flash spectroscopy Gupta et al. 2k) reported for 2-hydroxyphenylbenzotriazole in ethanol a short-lived transient (6 ps) followed by a transient absorption whose lifetime is estimated to be 600 ps. The authors assigned the short-lived transient to the "vertical singlet" while the long-lived transient is presumably the "proton transferred species". These measurements of transient absorptions with the picosecond flash method confirm our results derived from the fluorescence emission using the phase fluorimetric method. 

The 8- and 4.5-nm particles are at most weakly emissive and the polarizations of spectral features assigned to these particles are determined by polarized bleach measurements. The bleach anisotropy was determined using femtosecond pulses, with the probe delayed a few picoseconds from the pump. This delay ensures electronic and vibrational relaxation as well as relaxation of optical Kerr effects induced in tire solvent. As a control, transient absorption experiments were performed with excitation at 475 nm and detection at 550 nm. This detection wavelengtli is to the red of the wavelengths at which a bleach would be observed and provides a measure of the transient absorption intensity in this general spectral... 

To study the excited state one may use transient absorption or time-resolved fluorescence techniques. In both cases, DNA poses many problems. Its steady-state spectra are situated in the near ultraviolet spectral region which is not easily accessible by standard spectroscopic methods. Moreover, DNA and its constituents are characterised by extremely low fluorescence quantum yields (<10 4) which renders fluorescence studies particularly difficult. Based on steady-state measurements, it was estimated that the excited state lifetimes of the monomeric constituents are very short, about a picosecond [1]. Indeed, such an ultrafast deactivation of their excited states may reduce their reactivity something which has been referred to as a "natural protection against photodamage. To what extent the situation is the same for the polymeric DNA molecule is not clear, but longer excited state lifetimes on the nanosecond time scale, possibly of excimer like origin, have been reported [2-4],... 

Precise measurements of the excited state lifetimes of the DNA constituents were not available till very recently, mainly due to the limited time resolution of conventional spectroscopic techniques. Studying the DNA nucleosides by transient absorption spectroscopy, Kohler and co-workers observed a very short-lived induced absorption in the visible which they assigned to the first excited state [5,6]. The lifetimes observed were all well below 1 picosecond. The first femtosecond fluorescence studies of DNA constituents were performed using the fluorescence upconversion technique. Peon and Zewail [7] reported that the excited state lifetimes of DNA/RNA nucleosides and nucleotides all fall in the subpicosecond time, thus corroborating the results obtained by transient absorption. 

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