Picosecond laser pulse excitation

Figure 5.13 illustrates the temporal evolution of photoinduced colour centres in colloidal TiOi following picosecond laser pulse excitation. These centres form as a result of carrier/exciton trapping by pre-existing defects (anion Va and cation Vc vacancies), and by some ions in irregular (defect) positions in the lattice, for example Ti rreg in titania (or Zr" i .eg in zirconia)—stages 16 to 21. ... 

Figure 5.13 Absorption spectra of photoinduced colour centres in colloidal TiOz formed after picosecond laser pulse excitation. Reprinted with permission from Sahyun and Serpone (1997). Copyright (1997) American Chemical Society.

The picosecond laser pulse excites coherently a linear combination of cluster eigenstates... 

The mechanism of spectral sensitisation has been shown to involve the direct transfer of an electron from the excited state of the dye into the AgX conduction band (see Fig. 11.8). In confirmation of this mechanism, excellent correlation has been demonstrated between the sensitisation capability of dyes and their electrochemical reduction potential. For the most efficient dyes, the quantum yield of the electron transfer step is close to 1.0. Using picosecond laser pulse excitation to measure the fluorescence decay rates of dyes adsorbed to AgX crystals, it has been shown that the electron transfer is very fast, with rate constants in the region of 10 s having been reported [24]. Once an electron has been transferred into the AgX conduction band, the latent image formation process ensues more or less as it does following direct absorption by AgX itself. The difference in the case of the dye-sensitised process is that the positive holes are trapped at the dye molecules. 

Hydrogen transfer in excited electronic states is being intensively studied with time-resolved spectroscopy. A typical scheme of electronic terms is shown in fig. 46. A vertical optical transition, induced by a picosecond laser pulse, populates the initial well of the excited Si state. The reverse optical transition, observed as the fluorescence band Fj, is accompanied by proton transfer to the second well with lower energy. This transfer is registered as the appearance of another fluorescence band, F2, with a large anti-Stokes shift. The rate constant is inferred from the time dependence of the relative intensities of these bands in dual fluorescence. The experimental data obtained by this method have been reviewed by Barbara et al. [1989]. We only quote the example of hydrogen transfer in the excited state of... 

How can such problems be counterbalanced Since a large capacitance of a semiconductor/electrolyte junction will not negatively affect the PMC transient measurement, a large area electrode (nanostructured materials) should be selected to decrease the effective excess charge carrier concentration (excess carriers per surface area) in the interface. PMC transient measurements have been performed at a sensitized nanostructured Ti02 liquidjunction solar cell.40 With a 10-ns laser pulse excitation, only the slow decay processes can be studied. The very fast rise time cannot be resolved, but this should be the aim of picosecond studies. Such experiments are being prepared in our laboratory, but using nanostructured... 

Let us mention the rotational beats which have recently been observed (with a trans-stilbene molecule in a gasodynamic jet) and interpreted by Felker and Zewail [54, 145]. The essence of this interesting phenomenon may be described in a simplified way as follows. Let a picosecond laser pulse be capable of coherently exciting from some rotational state J f of a... 

The detection efficiency of C6H5X (X = F, Cl, Br and I) was investigated with the laser multiphoton ionization method152. The laser-induced ion yield depends mainly on the cross sections of the transitions available to the molecule ground state and on the lifetime of the intermediate electronic state that is initially excited. If a species has a radiative lifetime that is very short compared to the pulse duration, it may relax after excitation and will not be ionized. Molecular ions will therefore be obtained when laser pulses that are at least as short as the excited-state lifetimes are employed. The S excited states of halobenzenes are estimated to have subnanosecond lifetimes, with the exception of fluorobenzene for which a lifetime of the order of 9-10 ns has been calculated at 2ex = 266 nm. Picosecond laser pulses are therefore found effective in producing ionization of halobenzenes with short lifetimes, whereas nanosecond pulses are not152. 

Therefore very narrow excitation pulse widths are necessary, for example, to measure sub-nanosecond relaxation times. A number of methods for generating picosecond laser pulses have been devised and several reviews of these techniques are available [10, 11]. 

A major breakthrough in the measurement of VER occurred in 1972. Laubereau et al. (32) used picosecond laser pulses to pump molecular vibrations via stimulated Raman scattering (SRS) and time-delayed incoherent anti-Stokes probing to study VER of C-H groups in ethanol and methanol ( " -3000 cm-1). Alfano and Shapiro (33) used the same technique to monitor both the decay of the initially excited (parent) C-H stretch excitation and the appearance and subsequent decay of a daughter vibration,... 

Figure 15-14. (a) Absorption and fluorescence spectra of propanodeoxyguanosine (PdG) and guanosine (G) in aqueous solution of pH 7 at 25°C. The steady-state fluorescence spectra of the two compounds were measured using solutions having the same absorbance (ca. 0.3) at the excitation wavelength (280 nm), (b) Observed dot) and fitted (blue) fluorescence decays of PdG in the room temperature aqueous solution, monitored at 365 nm after excitation with a picosecond laser pulse at 280 nm and 300 nm. Also shown is the temporal response function of the laser TCSPC system (FWHM 35 ps). (Reprinted with permission from Ref. [24].)... 

Hirata Y, Mataga N. (1990) Solvation dynamics of electrons ejected by picosecond dye laser pulse excitation of p-phenylenediamine in several alcoholic solutions. J Phys Chem 94 8503-8505. 

More recently time-resolved techniques have been applied for studying photocarrier dynamics at the semiconductor-liquid interface. One of the main motivations is that such studies can lead to an estimation of the rate at which photo-induced charge carriers can be transferred from the semiconductor to a redox acceptor in the solution. This method is of great interest because rate constants for the transfer of photocarriers cannot be obtained from current-potential curves as in the case of majority carrier transfer (Section 7.3.5). The main aim is a detailed understanding of the carrier dynamics in the presence of surface states. The different recombination and transfer processes can be quantitatively analyzed by time-resolved photoluminescence emitted from the semiconductor following excitation by picosecond laser pulse. Two examples are shown in Fig. 7.60 [82, 83]. 

Irradiation performed with racemic substrate at room temperature, unless noted otherwise. Anisotropy (g) factor at or around irradiation wavelength, if reported or estimated. Extent of destruction. Maximum observed rotation a of irradiated solution, or specific rotation [a] of isolated sample or of residue obtained upon evaporation. Maximum observed ellipticity of irradiated solution or molar ellipticity of isolated sample. Enantiomeric excess of isolated sample. Not reported. Compound (mp 113 C) of unknown structure, obtained in a reaction of humulene with sodium nitrite, according to the reported procedure Chapman, AC. J. Chem. Soc. 1895 67 780. A mixed case of asymmetric destruction and photoderacemization irradiation performed at 0 C. Enantiomerically enriched sample used. Estimated g factor enhanced by two-quantum excitation with high intensity picosecond laser pulse. High-inten-sity laser of indicated pulse duration used. "Irradiation performed at 77 K in a hydrocarbon glass matrix. Optically pure sample photolyzed only to evaluate the enhanced g factor. Estimated g factor enhanced by two—quantum excitation with high-intensity femtosecond laser pulse. 

Fluorescence Lifetimes and Picosecond Dynamics. The fluorescence lifetime (ts) of fran.s -stilbenes at room temperature is rather short. A value of approximately lOOps may be estimated from radiative rate constant ( r) using ts = f//rr [295]. Direct lifetime measurements with picosecond laser pulses confirm this finding (Table 14). Important progress in understanding of the excited state dynamics was achieved in several laboratories [314-375], especially by Yoshihara [314 319], Hochstrasser [262, 320 335], Fleming [343-348], Troe [352 355], Zewail [361-364] and their co-workers. Saltiel and Sun [28] extensively discussed the literature con-... 

Here, /lfexp —(E(/RT) and k0 are rate constants for activated and nonac-tivated decay steps of t, respectively. For stilbene, halogenated, and some other stilbenes (4-R CN, CH3, OCH3) the activated step has been assigned to twisting in the excited singlet state [27, 105, 114, 357, 358], whereas for nitrostilbenes intersystem crossing is involved in the activated step [31]. At low temperatures lifetimes of 1-2 ns have been reported for several trans-stilbenes [314, 337, 342], Below — 150°C an enhanced frans-stilbene was excited by a 532-nm picosecond laser pulse, in addition to a 266-nm pulse [317]. 

However, as we noted earlier (in Section II), it is most important to have a full temporal and spatial profile of the picosecond laser pulses in order to correctly interpret the one or two photon excited molecular response functions (absorption, emission, Raman scattering) of the system under study. While autocorrelation techniques such as second harmonic generation (SHG) or two photon fluorescence have always been the route to such short pulse measurements, a significant advance has recently been made in bringing these conventional autocorrelation measurements into the real-time domain. ... 

Figure 11,22 Two-photon STED. The sample is excited with a ca. 100 fs laser pulse (PUMP). A time-delayed picosecond laser pulse (DUMP) is used to partially deplete the exdted state population. This causes an abrupt change in fluorescence polarization due to the preferential removal of molecules oriented parallel to the polarization direction of the PUMP. Analysis of the anisotropy change shows that it is sensitive to both the initial (pre-dumped) fluorescence anisotropy (/ u) and the pre-dumped excited state value of (a4o)- This order parameter is unobservable in conventional fluorescence experiments...

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). 

In TCSPC, the sample is excited with a picoseconds laser pulse and each single emitted photon is recorded with picoseconds accuracy. The sample is exited at MHz... 

A novel pump-probe degenerate four-wave mixing technique was introduced to study the excited state enhanced nonlinear optical response in polymers [7]. In this technique (see Fig, 3) a picosecond laser pulse at w, is divided into three beams... 

Generally speaking, excitation of a medium by short laser pulses can be used to study dynamic properties of the medium over a very wide time range. Here, we have shown that nanosecond-pulse excitation can yield information about the dynamics of molecular reorientation on the -10-sec time scale, and thermal effect on the 10—l(X)-msec time scale. The power of this technique lies in the fact that a single 6-function-like laser pulse may induce a number of fundamental excitation modes of vastly different time constants. Consider, for example, molecular reorientation coupled with flow induct by a picosecond laser pulse in a liquid crystal. It can be shown that, aside from the thermal effect, the transient behavior will manifest itself with three characteristic time constants ... 

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