Lasers picosecond

With the advent of short pulsed lasers, investigators were able to perfonn time resolved coherent Raman scattering. In contrast to using femtosecond pulses whose spectral widtii provides the two colours needed to produce Raman coherences, discussed above, here we consider pulses having two distinct centre frequencies whose difference drives the coherence. Since the 1970s, picosecond lasers have been employed for this purpose [113. 114], and since the late 1980s femtosecond pulses have also been used [115]. Flere we shall briefly focus on the two-colour femtosecond pulsed experiments since they and the picosecond experiments are very similar in concept. 

Yu N-T, Nie S and Lipscomb L 1990 Surface-enhanced hyper-Raman spectrosocpy with a picosecond laser. New vibrational information for non-centrosymmetric carbocyanine molecules adsorbed on colloidal silver J. Raman Spectrosc. 21 797-802... 

In a typical time-resolved SHG (SFG) experiment using femtosecond to picosecond laser systems, two (tlnee) input laser beams are necessary. The pulse from one of the lasers, usually called the pump laser, induces the... 

Ultrafast TRCD has also been measured in chemical systems by incoriDorating a PEM into the probe beam optics of a picosecond laser pump-probe absorjDtion apparatus [35]. The PEM resonant frequency is very low (1 kHz) in these experiments, compared with the characteristic frequencies of ultrafast processes and so does not interfere with the detection of ultrafast CD changes. 

Major breakthroughs in early ultrafast VER measurements were made in 1972 by Laubereau et al [22], who used picosecond lasers in an SRS pump-incoherent anti-Stokes Raman probe configuration, to study VER of C-H... 

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... 

The LIF technique is extremely versatile. The determination of absolute intermediate species concentrations, however, needs either an independent calibration or knowledge of the fluorescence quantum yield, i.e., the ratio of radiative events (detectable fluorescence light) over the sum of all decay processes from the excited quantum state—including predissociation, col-lisional quenching, and energy transfer. This fraction may be quite small (some tenths of a percent, e.g., for the detection of the OH radical in a flame at ambient pressure) and will depend on the local flame composition, pressure, and temperature as well as on the excited electronic state and ro-vibronic level. Short-pulse techniques with picosecond lasers enable direct determination of the quantum yield [14] and permit study of the relevant energy transfer processes [17-20]. 

Noguchi, H., Okada, T, Onda, K., Kano, S. S., Wada, A. and Domen, K. (2003) Time-resolved SFG study of formate on a Ni(lll) surface under irradiation of picosecond laser pulses. Surf. Sci., 528, 183-188. 

Gratzel and Serpone and co-workers recently reported on a picosecond laser flash photolysis study of TiO. They observed the absorption spectrum immediately after the 30 ps flash and attributed it to electrons trapped on Ti" " ions at the surface of the colloidal particles. The absorption decayed within nanoseconds, the rate being faster as the number of photons absorbed per colloidal particle increased. This decay was attributed to the recombination of the trapped electrons with holes. 

A computer-controlled motorized translation stage mounted with a retro-reflector is used to vary the pump laser beam path relative to the probe laser beam path and this controls the relative timing between the pump and probe laser beams. Note that a one-foot difference in path length is about 1 ns time delay difference. The picosecond TR experiments are done essentially the same way as the nanosecond TR experiments except that the time-delay between the pump and probe beams are controlled by varying their relative path lengths by the computer-controlled motorized translation stage. Thus, one can refer to the last part of the description of the nanosecond TR experiments in the preceding section and use the pump and probe picosecond laser beams in place of the nanosecond laser beams to describe the picosecond TR experiments. 

Note added in typing In a very recent paper (81) Vaida and co-workers have used picosecond laser photolysis to show that, in cyclohexane solution, Cr(CO)5...cyclohexane (Amax 497 nm) is formed within 25 ps of the photolysis of Cr(C0)5 This suggests that, in solution, the primary photoproduct is Cr(C0)5 and that there is essentially no activation energy for the reaction of Cr(C0)5 with the solvent. Clearly, experiments with pulsed KrF lasers on carbonyls in solution and matrix may be very revealing. 

Diphenylmethylene was the first carbene to be studied using fast, time-resolved spectroscopic methods (Closs and Rabinow, 1976). Since then both nanosecond and picosecond laser techniques have been used to probe this intermediate (Eisenthal et al., 1980, 1984 Hadel et al., 1984a,b Griller et al., 1984b Langan et al., 1984 Sitzmann et al., 1984). The results of these experiments are essentially undisputed, but the interpretation of them still remains somewhat controversial. 

Figure 5. Schematic diagram of a time-resolved fluorescence spectrometer using a picosecond laser as an excitation source. Inset diagram intensity/time/ wavelength surface for poly (acenaphthalene) in benzene at 20°C. Excitation wavelength 295 nm. (Reproduced with permission from Ref. 21. Copyright 1987 Chemistry in Australia.)...

By the late 1960s the development of mode locking (Chapter 1) allowed the study of picosecond laser techniques. Excited-state processes carried out in the picosecond domain allow such processes as intersystem crossing, energy transfer, electron transfer and many pho-toinduced unimolecular reactions to be investigated. 

Soper SA, Mattingly QL (1994) Steady-state and picosecond laser fluorescence studies of nonradiative pathways in tricarbocyanine dyes implications to the design of near-IR fluor-ochromes with high fluorescence efficiencies. J Am Chem Soc 116 3744—3752... 

The ability of fluorescence to provide temporal information is of major importance. Great progress has been made since the first determination of an excited-state lifetime by Gaviola in 1926 using a phase fluorometer. A time resolution of a few tens of picosecond can easily be achieved in both pulse and phase fluorometries by using high repetition rate picosecond lasers and microchannel plate photo-... 

Intramolecular charge transfer in p-anthracene-(CH2)3-p-Ar,Af-dimethylaniline (61) has been observed174 in non-polar solvents. Measurements of fluorescence-decay (by the picosecond laser method) allow some conclusions about charge-transfer dynamics in solution internal rotation is required to reach a favourable geometry for the formation of intramolecular charge-transfer between the donor (aniline) and the acceptor (anthracene). 

The primary photochemical reaction for nitromethane in the gas phase is well supported by experiments to be the dissociation of the C—N bond (equation 98). The picosecond laser-induced fluorescence technique has shown that the ground state NO2 radical is formed in <5 ps with a quantum yield of 0.7 in 264-nm photolysis of nitromethane at low pressure120. The quantum yield of NO2 varies little with wavelength, but the small yields of the excited state NO2 radical increase significantly at 238 nm. In a crossed laser-molecular beam study of nitromethane, it was found that excitation of nitromethane at 266 nm did not yield dissociation products under collision-free conditions121. 

K. Berndt, H. Duerr and D. Palme, Picosecond laser spectroscopy with avalanche photodiodes, in Time Resolved Laser Spectroscopy in Biochemistry (J. R. Lakawicz, ed.), Proc. SPIE 909, 209-215... 

The direct detection of the S <- Sj absorption in organic compounds has so far been achieved by a nanosecond or picosecond laser flash photolysis method. The general features of transient absorption spectra of metalloporphyrins actually suggest the presence of strong absorption bands in visible or ultraviolet region (38-40). However, as the transient absorption of the state often overlaps with that of ground state depletion, it is usually difficult to evaluate the absolute absorption cross sections for the transition by... 

When molecules absorb a photon and produce an electronic excited state, the energy can be dissipated in several ways luminescence, radiationless decay to the ground state, and photochemistry. Luminescence dominated the older literature because it was easy to observe. A good review of luminescence is in Volume 3 of David Dolphin s seven-volume series The Porphyrins. Picosecond laser spectroscopy allowed for exploration of the radiationless decay pathways, particularly the initial steps that compete with luminescence and lead to photochemistry. Two principal forms of radiationless decay lead to long-term metastables ligand ejection and electron transfer. 

Nonlinear light-matter interactions have been successfully applied to create new visualization contrast mechanisms for optical microscopy. Nonlinear optical microscopy employs femtosecond and picosecond lasers to achieve a high photon flux density by focusing the beam onto a sample with a high numerical aperture (NA) microscope... 

Potma, E. O., Jones, D. J., Cheng, J.-X., Xie, X. S., and Ye, J. 2002. High-sensitivity coherent anti-Stokes Raman scattering microscopy with two tightly synchronized picosecond lasers. Opt. Lett. 27 1168-70. 

Petrov, G. L, Yakovlev, V. V., and Minkovski, N. 1.2004. Broadband nonhnear optical conversion of a high-energy diode-pumped picosecond laser. Opt. Commun. 229 441-45. 

More direct evidence for the intervention of excited states of triplet carbenes in reactions in solution is obtained by spectroscopic studies. Thus, picosecond lasers make it possible to study the quenching of carbene fluorescence by various substrates in solution at room temperature. Diphenylcarbene is generated upon laser photolysis of 30 and a second UV laser pulse is time delayed by 8 ns and is used to excite the carbene, thereby producing the excited triplet DPC (Scheme 9.32). The fluorescence of DPC is then monitored with a streak camera. The fluorescence... 

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