Transmission change

We employed a comparative Z-scan procedure, wherein we perform a reduced-aperture Z-scan on CS2 immediately followed by reduced and open-aperture Z-scans on the polymer solution at a particular wavelength. The peak on-axis intensity, /0, is then calculated from the CS2 peak to valley transmission change using previously measured values for its non-linear axis of refraction. Based on the work of Sheik-Bahae et al the equation for /0is given by 37]... 

A suitable method for a detailed investigation of stimulated emission and competing excited state absorption processes is the technique of transient absorption spectroscopy. Figure 10-2 shows a scheme of this technique. A strong femtosecond laser pulse (pump) is focused onto the sample. A second ultrashort laser pulse (probe) then interrogates the transmission changes due to the photoexcita-lions created by the pump pulse. The signal is recorded as a function of time delay between the two pulses. Therefore the dynamics of excited state absorption as... 

Figure 10-3. Tinic-rcsolved pholoinduccil transmission changes in PPV probed at 560 nm lor dillcrenl photon eneigies (460 nm open circles, 480 nm solid circles, 500 nm open squares, 510 nm solid squares) (after Ref. [23]).

Direct evidence for the competition of two counteracting contributions to the transient absorption changes stems from the temporal evolution of the transmission change at 560 nm. From Figure 10-3 it can be seen that the positive transmission change due to the stimulated emission decays very fast, on a time scale of picoseconds. On the other hand the typical lifetime of excitations in the 5, slate is in the order of several hundred picoseconds. Therefore, one has to conclude that the stimulated emission decay is not due to the decay of the. Sj-population (as is typically the case in dye solutions). The decay is instead attributed to the transiei.i build up of spatially separated charged excitations that absorb at this wavelength. 

Figure 10-4. Temporal behavior of the pholoinduccd transmission changes in LPPP alter excitation with a femtosecond pump pulse at 400 nnt. The two curves correspond to probe photon eneigies of 2.48 eV (dotted line) and 1.91 eV (solid line). At 2.48 eV the transmission change is positive due to stimulated emission (SE) while a photoin-dueed absorption (PIA) is observed at 1.91 eV (according to Ref.(24J).

Figure 10-5. Transient transmission changes AV/Po in PPV for different lime delays between the pump and probe pulse. The pump pulse is a 100 fs laser pulse at 325 nm obtained by frequency doubling ol amplified dye laser pulses, (a) and (b) correspond to different sides of a PPV-film. The spectra in (a) were obtained lor the unoxidized side of the sample while the set of spectra in (b) was measured for the oxidized side of the same sample. The main differences observed are a much lower stimulated emission effect for the oxidized side. The two bottom spectra depict the PL-spectra for comparison. The dashed line indicates the optical absorption (according to Kef. (281).

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To study the orientational relaxation of the excited HDO molecules, we rotated the polarization of the probe pulse by 45 degrees with respect to the pump polarization using a zero-order A/2 plate. After the sample the probe is split in two beams, and using two polarizers and two detectors, the transmission changes of the probe parallel to the pump (ln(T/7o) ) and perpendicular to the pump (ln(T/7o)j ) are measured simultaneously. 

Fig. 1. a) Absorption and fluorescence spectra of indole solvated in cyclohexane and ethanol, b) Time resolved transmission change of indole probed at 740 nm. 

Fig. 2. a) Transient spectra of indole solvated in water 500 fs (solid thick line) and 1 ps (broken line) after the excitation. The dotted trace shows the spectrum of the solvated electron, b) Time resolved transmission change of the indole solution (triangles) and pure water (circles) at 470 nm and 650 nm. 

Fig. 2. Transmission change of CN-DHA induced at 350 nm and probed at 545 nm exponential dynamics and oscillatory behavior of the coherently excited vibronic wavepacket. Ring opening (1.2 ps) and internal conversion (13 ps) to CN-VHF-cis take place on different time scales.

Figure 8.13 Left Structures and energies of a hydroxybenzotriazole in the course of intramolecular proton transfer. Right Kinetics of transmission changes at 325 nm (a), 355 nm (b), 385 nm (c) and 400 nm (d) horizontal axis, time in ns...

This expression can be analysed in two ways depending on the nature of the sample. In the case of a thin film [in comparison with the absorption depth of the pump light (D transmission changes (aNoD [Pg.18]

The noncollinear pump-probe experiment is depicted schematically in Fig. 13. The linearly polarized (P3) pump pulse is focused (LI) into the sample producing induced transmission changes. The polarization of the probe beam is adjusted to 45° relative to the pump with a half-wave plate (A./2) and a Gian polarizer (PI). By the help of an analyzer (P2) simultaneous detection of the parallel ( ) and perpendicular ( L) components of the energy transmission T(v, to) of the probe through the sample is installed. For blocked excitation (chopper, Ch) the sample transmission... 

T0(v) is measured. The resulting relative transmission changes ln(T/T0)n,i for variable probe frequency v and probe delay time to (VD) are used in the following as the relevant signal quantities, from which the following quantities are derived, as demonstrated by Graener et al. (11) ... 

Transient hole burning is clearly indicated by the data for the anisotropic component of the probe transmission change (a, b) with a halfwidth (true FWHM) of 45 5 cm-1 of the Lorentzian shaped hole at early delay times, tD < 0 (a). For the isotropic component of the probing signal, the narrow hole (dotted lines in d, e) is superimposed on a broader... 

The parallel component of the probe transmission change is plotted in Fig. 17 in the range 2850-3650 cm 1 for different delay times (left-hand ordinate scales, experimental points, calculated solid curves). The transient spectrum during the excitation process, to = 0 ps, is depicted in Fig. 17a. A bleaching at the frequency position of 2974 cm-1 is shown because of the excitation of the CH vibration. The excess population of the upper level v = 1 can be directly monitored from the induced absorption around 2952 cm-1 and is attributed to excited-state absorption (width of 17 2 cm-1). The bleaching signal at lower frequencies indicates... 

Relaxation Kinetics. The details of the experimental procedure have been described earlier (14). 0.1 M phosphate buffer, pH 7.0, containing 2 X 10"5 M EDTA was used in all relaxation experiments. These were performed with solutions of different initial reagent composition— either ferrocyanide was added to oxidized azurin or ferricyanide to reduced azurin. Temperature jumps of 2.9° or 4.7° were applied to the reaction solution. The subsequent transmission changes were monitored at 625 nm (absorption of oxidized azurin) or 420 nm (absorption of ferricyanide ). Each plotted value of the relaxation time or amplitude represents the average of at least four measurements. 

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