Excited-state lifetime determination pulses

Excited State Lifetime Determination with I tosecond Laser Pulses... 

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-... 

Dr can be determined by time-resolved fluorescence polarization measurements, either by pulse fluorometry from the recorded decays of the polarized components I l and 11, or by phase fluorometry from the variations in the phase shift between J and I as a function of frequency (see Chapter 6). If the excited-state lifetime is unique and determined separately, steady-state anisotropy measurements allow us to determine Dr from the following equation, which results from Eqs (5.10) and (5.41) ... 

At still shorter time scales other techniques can be used to determine excited-state lifetimes, but perhaps not as precisely. Streak cameras can be used to measure faster changes in light intensity. Probably the most useful techniques are pump-probe methods where one intense laser pulse is used to excite a sample and a weaker pulse, delayed by a known amount of time, is used to probe changes in absorption or other properties caused by the excitation. At short time scales the delay is readily adjusted by varying the path length travelled by the beams, letting the speed of light set the delay. 

The redistribution of free voliunes also influences the sub-glass transition temperatures Tp and T observed for photoisomerization reactions in polymer solids. T, Tp and T are frequency-dependent, and the response of any process to the transitions at these temperatures depends on the time scale. The time scale of photoprocesses may not be equal to those of DSC or dynamic mechanical methods, which are of the order of 10 to 1( Hz. However, for photodecoloration of the merocyanine form of spiro-bepzopyran in polycarbonate film under steady-state irradiation of 560 nm light after laser-single-pulse induced coloration, it was found that the Arrhenius plot of the apparent rate coefficient broke at T (150 °C), Tp (20 C), and T (—120 °Q of the matrix polycarbonate these temperatures are the ones determined by dynamic mechanical measurements. The excited state lifetime of the merocyanine form in polycarbonate was 1.8 ns . Hence, the decolorating isomerization during the lifetime proceeded only in a small fraction of the molecules surrounded by a sufficient amount of free volume. Thus, it is likely that the temperature dependence of the apparent rate coefficient reflecting the relative quantum yield is controlled by the frequency of redistribution of free volumes, which may be comparable with the frequency determined by dynamic mechanical measurements. 

Hence, a plot of In [ A] versus time (t) should give a straight line with a slope of 1/Xf. The value of [ A] is determined from the fluorescence intensity. Experimentally, lifetime measurements are obtained using a pulsed laser source. Pulsing leads to the population of the excited state of A, followed by emission of light by A with a time profile according to Equation [4]. Figure 5 shows a schematic description of a luminescence decay curve (A) and the plot used for the determination of the excited state lifetime (B). 

Figure 5 Experimental determination of excited state lifetimes (A) a plot of the intensity (I) versus time after the laser pulse. The lifetime corresponds to the time at which the intensity decays to 1/e of its maximum value (B) a plot of In (/) versus time after laser pulse. The lifetime here can be calculated directly from the slope of the linear equation.

Figure 1 A schematic representation of a fairly advanced single-molecule setup. BS, beam splitter (nonpolatizing, polarizing, or a dichroic mirror) PD, fast photodiode POL, polarizer BE, beam expander BP, bandpass filter WP, quarter waveplate or Berek compensator EF, emission filter. If a dichroic mirror is used to split the emission between the APDs, then additional filters are usually placed in front of each APD to prevent leakage of the emission into the incorrect channel. The photodiode is only used in combination with pulsed excitation in the determination of the excited-state lifetime.

Figure 4 (a) A schematic overview of the recording of a photon arrivai stream fora measurement involving a pulsed laser, (b) An example intensity time trace constructed for a muitichromophoric moiecuie using binned macro times, (c) The excited-state lifetime can be determined by histogramming the micro times (shown here on a iogarithmic y-axis). 

A Gd + doped crystal is illuminated with a pulsed light source, so that the l7/2 excited state of this ion is populated by absorbing 1 mJ of energy per incident pulse. Determine the heat delivered to the crystal per excitation pulse if the nonradiative rate from this state is 10 s The fluorescence lifetime of the l7/2 state is 30 /xs. 

A series of chlorophyll-like donor (a chlorin) linked having C60 (chlorin-C60) or porphyrin-C60 dyads with the same short spacer have been synthesized as shown in Schemes 13.1 and 13.2 [39, 40]. The photoinduced electron-transfer dynamics have been reported [39, 40]. A deoxygenated PhCN solution containing ZnCh-C60 gives rise upon a 388-nm laser pulse to a transient absorption maximum at 460 nm due to the singlet excited state of ZnCh [39]. The decay rate constant was determined as 1.0 X 10u s-1, which agrees with the value determined from fluorescence lifetime measurements [39]. This indicates that electron transfer from 1ZnCh to C60 occurs rapidly to form the CS state, ZnCh +-C60 . The CS state has absorption maxima at 790 and 1000 nm due ZnCh+ and C60, ... 

Ethylene-propylene copolymer films gave a very broad absorption in the visible region upon electron-pulse irradiation (Fig. 16) [93], It was comprised of at least three species, electrons, excited states, and alkane radical cations. At about 700 nm and 800 nm the contributions from excited states and radical cations, respectively, were largest. The lifetime of the radical cation determined... 

Triplet excited states of four derivatives of ubiquinone-6 (256), in which various ring substituents are progressively altered, have been studied by laser flash photolysis (265 nm) and pulse radiolysis (9—12MeV electrons). The triplet absorption spectra, extinction coefficients, lifetimes, energy levels, and quantum efficiencies of formation were determined.117... 

An electronic or vibrational excited state has a finite global lifetime and its de-excitation, when it is not metastable, is very fast compared to the standard measurement time conditions. Dedicated lifetime measurements are a part of spectroscopy known as time domain spectroscopy. One of the methods is based on the existence of pulsed lasers that can deliver radiation beams of very short duration and adjustable repetition rates. The frequency of the radiation pulse of these lasers, tuned to the frequency of a discrete transition, as in a free-electron laser (FEL), can be used to determine the lifetime of the excited state of the transition in a pump-probe experiment. In this method, a pump energy pulse produces a transient transmission dip of the sample at the transition frequency due to saturation. The evolution of this dip with time is probed by a low-intensity pulse at the same frequency, as a function of the delay between the pump and probe pulses.1 When the decay is exponential, the slope of the decay of the transmission dip as a function of the delay, plotted in a log-linear scale, provides a value of the lifetime of the excited state. 

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