Picosecond lasers excited states

Picosecond laser excitation ( pump and probe experiments) allowed us to determine lifetimes of the singlet excited states of these dyes in some cases [88c, 88e],... 

In the numerical solution of these coupled equations, the shape of the incident laser pulse is assumed to be Gaussian, with FWHM pulse duration t. The range of tp covers picoseconds to nanoseconds. Note that because the intersystem crossing times are much greater than picoseconds, the excited state contribution usually becomes significant for nanosecond laser pulses. 

The most important common features in the photodecarboxylation of ketoprofen, suprofen, tiaprofenic acid, tohnetin, and benoxaprofen in neutral aqueous solutions are the following (1) All of them produce carbanion intermediates, which upon protonation give rise to the decarboxylated photoproducts (Scheme 4), and (2) the photodecarboxylation quantum yields are much higher from the carboxylate forms than from the non-dissociated acids. However, there are also some differences. Thus, while photodecarboxylation of benoxaprofen is produced from its singlet excited state, in the case of the benzophenone derivatives (ketoprofen, suprofen, tiaprofenic acid, tohnetin), the process seems to take place from the triplet excited states.Clear evidence for the participation of these states has been provided by laser flash photolysis studies. Thus, picosecond laser excitation of ketoprofen in aqueous... 

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]. 

With the advent of picosecond-pulse radiolysis and laser technologies, it has been possible to study geminate-ion recombination (Jonah et al, 1979 Sauer and Jonah, 1980 Tagawa et al 1982a, b) and subsequently electron-ion recombination (Katsumura et al, 1982 Tagawa et al, 1983 Jonah, 1983) in hydrocarbon liquids. Using cyclohexane solutions of 9,10-diphenylanthracene (DPA) and p-terphenyl (PT), Jonah et al. (1979) observed light emission from the first excited state of the solutes, interpreted in terms of solute cation-anion recombination. In the early work of Sauer and Jonah (1980), the kinetics of solute excited state formation was studied in cyclohexane solutions of DPA and PT, and some inconsistency with respect to the solution of the diffusion equation was noted.1... 

Laser flash photolysis experiments48,51 are based on the formation of an excited state by a laser pulse. Time resolutions as short as picoseconds have been achieved, but with respect to studies on the dynamics of supramolecular systems most studies used systems with nanosecond resolution. Laser irradiation is orthogonal to the monitoring beam used to measure the absorption of the sample before and after the laser pulse, leading to measurements of absorbance differences (AA) vs. time. Most laser flash photolysis systems are suitable to measure lifetimes up to hundreds of microseconds. Longer lifetimes are in general not accessible because of instabilities in the lamp of the monitoring beam and the fact that the detection system has been optimized for nanosecond experiments. 

The lifetime of the singlet excited state (the fluorescence lifetime TF) is of the order of picoseconds to 100 nanoseconds (10—12 - 10-7 seconds) and can now be measured accurately using pulsed laser excitation methods and other techniques. Since the radiative transition from the lowest triplet state to the ground state is formally forbidden by selection rules, the phosphorescence lifetimes can be longer, of the order of seconds. 

Recent developments in laser technology and fast detection methods now allow the kinetic behaviour of the excited state species arising from absorption of radiation by polymers to be studied on time-scales down to the picosecond region ( ). An example of a time-resolved fluorescence spectrometer which can be used to study such ultrafast phenomena is illustrated in Figure 5 Q). 

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. 

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-... 

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. 

Early picosecond studies were carried out by Schneider et al, [63] on the parent spiro-oxazine (NOSH in Scheme 8) and similar derivatives. In a back-to-back work, they also described a complimentary CARS (coherent anti-Stokes Raman spectroscopy) investigation [69], Simply put, these authors found that the closed spiro-oxazine ring opened in 2-12 psec after laser excitation. The reaction was slower in more viscous solvents. An intermediate state formed within the excitation pulse and preceded the formation of merocyanine forms. This transient was named X in deference to the X transient named by Heiligman-Rim et al. for the spiropyran primary photoproduct [8], (See also the previous section.) The name X has since been adopted by other workers for the spiro-oxazines [26,65],... 

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. 

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... 

Picosecond-resolved thermochemical information can be extracted from the evolution of a transient grating produced by the crossing of two laser pulses and interrogated with a third short pulse of light. Several groups have applied this method to thermodynamic questions about the decay of excited states and the evolution of excited states into reactive intermediates. 

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