Transient absorption femtosecond measurements

Fig. 1. Top Reference spectra for femtosecond transient absorption measurements S-S abs. in solution (thin solid lines), oxidized dye (dye+) abs. in solution (thick solid line), fluorescence for solution (dotted line), steady-state absorption ofNKX-2311/ZnO (dotted-dashed line), and absorption of electrons in the conduction band (dashed line). Bottom Transient absorption spectra of NKX-23ll/ZnO in the spectral range between 600 and 1350 nm at the 2 (thick solid line), 10 (dotted line), 100 ps (thin solid line) delay times after excitation at 540 nm by the femtosecond pulse with the intensity of about 10 pJ.

Figure 13.7 Exciton diffusion to the heterojunction. Experimental (symbols) and modeled (lines) diffusion-limited exciton populations are compared. The experimental data show the exciton population recovered from femtosecond transient absorption measurements of charge generation in PFB F8BT polymer blends. The modeled data are a fit using a modified Fokker-Planck equation, with (dashed line) and without (solid line) the drift component close to the interface. The inset shows a cartoon of the diffusion (D ) in the bulk of the domain and the additional drift towards the heterojunction (DV in the interface region. (Reprinted with permission from Physical Review Letters, Probing the morphology and energy landscape of blends of conjugated polymers with sub-10 nm resolution by S. Westenhoff, I. A. Howard and R. H. Friend, Physical Review Letters, 101, art.no. 016102. Copyright (2008) American Physical Society)...

All systems were probed in steady-state fluorescence, absorption and time resolved emission lifetime studies at room temperature. Additionally, time resolved femtosecond transient absorption and nanosecond laser flash photolysis measurements were carried out. 

The femtosecond transient absorption studies were performed with 387 nm laser pulses (1 khz, 150 fs pulse width) from an amplified Ti Sapphire laser system (Model CPA 2101, Clark-MXR Inc). A NOPA optical parametric converter was used to generate ultrashort tunable visible pulses from the pump pulses. The apparatus is referred to as a two-beam setup, where the pump pulse is used as excitation source for transient species and the delay of the probe pulse is exactly controlled by an optical delay rail. As probe (white light continuum), a small fraction of pulses stemming from the CPA laser system was focused by a 50 mm lens into a 2-mm thick sapphire disc. A schematic representation of the setup is given below in Fig. 7.2. 2.0 mm quartz cuvettes were used for all measurements. 

Nanosecond laser Flash Photolysis experiments were performed with 355 and 532 nm laser pulses from a Brilland-Quantel Nd YAG system (5 ns pulse width) in a front face (VIS) and side face (NIR) geometry using a pulsed 450 W XBO lamp as white light source. Similarly to the femtosecond transient absorption setup, a two beam arrangement was used. However, the pump and probe pulses were generated separately, namely the pump pulse stemming from the Nd YAG laser and the probe from the XBO lamp. A schematic representation of the setup is given below in Fig. 7.3. 0.5 cm quartz cuvettes were used for all measurements. 

Time resolved hole burning spectra were measured by means of a femtosecond transient absorption spectrometer system. A second harmonics of a mode locked cw Nd + YAG laser (Quantronix, 82MHz) was used for a pumping source. A synchronously pumped rhodamine 6G dye laser with a saturable absorber dye jet (DODCl/DQOCI) and dispersion compensating prisms in the cavity was used. The output of the dye laser (lOOfs fwhm, 600pJ/pulse) was... 

The presence of a generation-dependent annihilation process and the influence of the substitution pattern have been validated by femtosecond time-resolved transient absorption measurements. 

This technique utilizes a pulse pump-probe experiment and monitors the absorption of a weak probe beam in the presence of a strong pump beam. Fig. 8 depicts the experimental set-up for a two-beam pump-probe experiment, which includes homodyne and heterodyne Kerr gate measurements and polarization-controlled transient absorption measurement. Generally, the input beam is produced from an amplified pulse laser system with 1 KHz repetition rate, which can produce picosecond or femtosecond pulses. This pumping light beam is divided into two beams by a beam-splitter with an intensity ratio of 30 1 therefore, the one with the stronger intensity will act as the pump and the weaker one will be the probe. The position of the sample is where these two beams focus and overlap spatially. The time delay between the pulses from these two beams is controlled by a retroreflec-... 

Fig. 8. Femtosecond two-beam pump-probe experimental set-up for Kerr gate and transient absorption measurement. P polarizers M mirrors PBS Pellicle beam splitter Ap aperture RR retroreflector... 

Direct observation of the CTI process in real time has been performed by use of femtosecond pulses. In 1991, two groups first reported femtosecond transient absorption spectroscopy of bovine rhodopsin [44,45], but their conclusions were remarkably divergent. One group excited bovine rhodopsin with a 35 fs pulse and probed with a 10 fs pulse, and concluded that product formation completed within 200 fs [44], In contrast, the other group measured transient absorption of bovine... 

In this chapter we mainly limit ourselves to the time region from several femtoseconds to milliseconds and to processes that are induced by illumination with light. One could roughly divide this time region into two parts, from femtoseconds to nanoseconds and from nanoseconds to milliseconds. The latter time scale can be investigated with electronic detection, and the described methods can also be used for processes that are initiated in other ways, for example, in stopped-flow experiments where reactants are rapidly mixed or pulse-radiolysis experiments where a short electron pulse induces a chemical reaction. After an introduction to the basic principles of transient absorption, we first treat slow transient absorption measurements (nanoseconds-milliseconds). 

Malone RJ, Miller AM, Kohler B (2003) Singlet excited-state lifetimes of cytosine derivatives measured by femtosecond transient absorption. Photochem Photobiol 77 158-164... 

Abstract The concepts at the basis of transient absorption measurements were illustrated with particular reference to nanosecond kinetic spectrophotometry and femtosecond pump and probe methods. The main features of the typical experimental setups for both techniques were illustrated as regards optical parts, geometrical layout of components and light detection systems. Examples of application of transient absorption spectroscopy were illustrated for the elucidation of photoinduced processes in supramolecular systems like a fullerenepyrrolidine-oligophenyleneethynylene hybrid derivative, a drug-protein complex and a tri-chromophoric system consisting of two porphyrins and one perylene bisimide. 

Several years ago, Youvan and coworkers produced an important mutant RC in the bacterium Rh.capsulatus in which one BChl of the SP was changed to bacterio-pheophytin (BPh) by the removal of the Mg atom. This heterodimer mutant can successfully carry out primary charge separation, albeit at a reduced rate and with a substantially reduced quantum yield.The femtosecond transient absorption experiments of Holten, Kirmaier and coworkers revealed that a charge transfer intermediate in this system is formed very rapidly after the initial flash the species created has been identified as the internal CT state of the heterodimer.Furthermore, Stark measurements on the heterodimer reveal two features, which are hypothesized to be admixtures of the exciton and CT states.Both spectroscopic and kinetic experiments were also performed on a similar heterodimer mutant for the Rb.sphaeroides system producing similar qualitative results. 

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