Spectroscopy picosecond

Optoelectronic detection systems such as fast photodiodes and sampling oscilloscopes have reached a time resolution of lO s. However, this is still not sufficient to resolve many fast transient events on a picosecond time scale. In picosecond spectroscopy, therefore, new techniques had to be invented to measure durations and profiles of picosecond pulses and to probe ultrafast relaxation processes. 

For many applications in the picosecond range the streak camera [11.21] can be used which may reach time resolutions of a few picoseconds. It consists essentially of a fast image intensifier (see Sect.4.5.5) where the 

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]. 

Most of the methods used to measure picosecond phenomena are based on optical delay lines where the picosecond laser pulse is divided by a beam splitter and the two replica pulses travel different path lengths before they are recombined. The measurement of a time interval At is thus transferred to that of a path difference ax = cAt where c is the velocity of light. 

Picosecond pulse width measurement using two-photon induced f1uorescence 

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Although very detailed, fundamental information is available from ultrafast TRIR methods, significant expertise in femtosecond/picosecond spectroscopy is required to conduct such experiments. TRIR spectroscopy on the nanosecond or slower timescale is a more straightforward experiment. Here, mainly two alternatives exist step-scan FTIR spectroscopy and conventional pump-probe dispersive TRIR spectroscopy, each with their own strengths and weaknesses. Commercial instruments for each of these approaches are currently available. 

This agrees quite well with the rate constants for intramolecular proton transfer in 2,4-bis(dimethyl-amino )-6-(2-hydroxy-5-methylphenyl)-5-triazine which had been measured by Shizuka et al. ( l6) using laser picosecond spectroscopy. The fluorescence decay constant t of (TIN) was found to be 60 20 ps. Because of the weak intensity all fluorescence lifetimes refer to the pure substance in crystalline form at room temperature. 

All these features were observed experimentally for solutions of 3-amino-/V-methylphthalimide, 4-amino-/V-methylphthalimide, and for nonsubstituted rhoda-mine. The results were observed for cooled, polar solutions of phthalimides, in which the orientational relaxation is delayed. Exactly the same spectral behavior was observed [50] by picosecond spectroscopy for low viscosity liquid solutions at room temperature, in which the orientational relaxation rate is much higher. All experimental data indicate that correlation functions of spectral shifts Av-l(t), which are used frequently for describing the Time Dependent Stokes Shift, are essentially the functions of excitation frequency. 

Rubinov AN, Bushuk BA, Stupak AP (1983) Picosecond spectroscopy of intermolecular interactions in dye solutions. Appl Phys B 30 99-104... 

To conclude our description of techniques, the use of nanosecond and picosecond spectroscopy which has been applied to excited state intramolecular proton transfer (ESIPT) will be mentioned briefly (Beens et al., 1965 Huppert et al., 1981 Hilinski and Rentzepis, 1983). A large number of inter-and intramolecular proton transfers have been studied using these methods (Ireland and Wyatt, 1976) but in the case of processes which are thought to involve simple proton transfer along an intramolecular hydrogen bond it is usually only possible to estimate a lower limit for the rate coefficient. 

Spontaneous ionization from the charge-transfer state of 2-anilinonaphthalene (62) in water/methanol mixtures175 shows (using picosecond spectroscopy) that the hydration of the electron limits the rate in the overall kinetics. For 8-(phenylamino)-l-naphthalenesulphonate, a water cluster (of 4 members) is the charge acceptor in the same way as observed for proton hydration175. 

Sitzman, E. V. Eisental, K. H. Application of Picosecond Spectroscopy to Chemistry, Reidel 1984. 

Picosecond spectroscopy was performed using an apparatus described by Reynolds and Rentzepis (9 ). The duration of the excitation pulse was 25 psec at 530 nm. Pulse intensity was about 1.5 m joules. Dependence of relaxation time on power from 0.3 up to 2.0 m joules revealed no effects of pulse intensity on decay times. 

Dissociation of axial ligands has been followed by picosecond spectroscopy for a number of metalloporphyrins. For the well-known photodissociation of O2 and CO from hemoglobin and myoglobin the photoproducts appear very early < 10 psec. Dissociation of basic axial ligands such as pyridine and piperadine occurs within the lifetime of the excited state for Ni(II), Co(III) as well as for Fe(II) porphyrins. Whether the ejected species is "hot" with energy from the electronic deactivation of the porphyrin is not known, but the dissociation process does not appear to be dependent upon the wavelength of the excitation pulse (30,32). 

Example 1— Excitation of stretching vibration of C—H bond in C2H5OH takes place at 2900 cm-k With the help of picosecond spectroscopy it has been found that during relaxation this vibration gets transformed into two bending vibrations of the C—bond at 1450 cm-k... 

Example 2—The first stage in this process of vision has been the excitation of rhodopsin. Rhodopsin partially gets deactivated forming an intermediate, prelumirhodopsin or bathorhodopsin. Picosecond spectroscopy reveals that prelumirhodopsin gets formed because of an intramolecular proton transfer—a jump of a proton from one position to another. 

E. V. Sitzman and K. B. Eisenthal, in Applications of Picosecond Spectroscopy to Chemistry Reidel Publishing, Dordrecht, The Netherlands, 1983, pp. 41-63. 

AOT heptane H20 reversed micelles CdS CdS duster growth by picosecond spectroscopy is examined 99... 

Determination of Excited-State Rotational Constants and Structures by Doppler-Free Picosecond Spectroscopy, J. S. Baskin and A. H. Zewail, J. Phys. Chem. 93, 5701 (1989). 

With the photographic flash lamp the light pulse has a duration of several microseconds at best. The Q-switched pulsed laser provides pulses some thousand times faster, and the kinetic detection technique remains similar since photomultiplier tubes and oscilloscopes operate adequately on this time-scale. The situation is different with the spectrographic technique electronic delay units must be replaced by optical delay lines, a technique used mostly in picosecond spectroscopy. This is discussed in Chapter 8. 

Cyanoantracene (and indene) form CT complexes with TCNE these complexes were studied27 using time-resolved picosecond spectroscopy. Irradiation of the CT complex produced an ion radical pair as shown in Scheme 2. 

Letokhov VS (ed) (1987) Laser picosecond spectroscopy and photochemistry of biomolecules, Adam Hilger, Bristol... 

The results for 6T1 (dodecylsexithiophene) after one-photon excitation are very similar to that of 5T. At first, the transient absorption A0 was observed. The maximum of the excited-state absorption A was found by picosecond spectroscopy at 900 nm with a decay time t = HOOps. With excitation by two photons (7,exc = 616 nm, 80 fs), a broad Ao band appears first followed by the A and fluorescence bands. A0 is seen for about 500 fs later, the fluorescence predominates. 

The conventional flash photolysis setup to study photochemical reactions was drastically improved with the introduction of the pulsed laser in 1970 [17], Soon, nanosecond time resolution was achieved [13], However, the possibility to study processes faster than diffusion, happening in less than 10 10 s, was only attainable with picosecond spectroscopy. This technique has been applied since the 1980s as a routine method. There are reviews covering the special aspects of interest of their authors on this topic by Rentzepis [14a], Mataga [14b], Scaiano [18], and Peters [14c],... 

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