Ultrafast Measurement Techniques

Streak Cameras. For the recording of very fast phenomena a streak camera can be used [9.185]. The principle is Uhistrated in Fig. 9.33. The incoming transient light impinges on a photo-cathode. The released electrons are accelerated, focused and deflected by plates subjected to a rapidly rising voltage. The deflected electron beam hits a microchannel plate (Sect. 6.3) in which 

Optical parametric oscillators (OPOs) (Sect. 8.6) are very useful widely tunable secondary sources of ultrafast pulses that find a wide applicability in pump-probe experiments. Two independent OPOs, which are pumped by the same laser for automatic synchronization and minimal temporal jitter, are 

Molecules in the gas phase absorb on rotational or vibrational-rotational transitions. These features allow apphctions such as luggage inspection and spectroscopic gas analysis. THz techniques can also be implemented for object imaging. THz technology and applications are discussed in [9.190]. 

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The chapter is organized as follows in Section 8.2 a brief overview of ultrafast optical dynamics in polymers is given in Section 8.3 we present m-LPPP and give a summary of optical properties in Section 8.4 the laser source and the measuring techniques are described in Section 8.5 we discuss the fundamental photoexcitations of m-LPPP Section 8.6 is dedicated to radiative recombination under several excitation conditions and describes in some detail amplified spontaneous emission (ASE) Section 8.7 discusses the charge generation process and the photoexcitation dynamics in the presence of an external electric field conclusions are reported in the last section. 

Recently, Wright et al. (1994) used ultrafast photoionization techniques to detect vibrationally excited I2 following 220 nm photoexcitation of HBr-I2 complexes. They attributed this to quenching of Br by I2, pointing out that their measured lifetime of 51 + 5 ps is close to those reported by Sims et al. (1992). This raises exciting possibilities. It seems inevitable that Br I2 complexes will find their way eventually to the ground state PES since there is nowhere else to go. However, although the Br yield at 220 nm is unknown, it is expected to be modest. Specifically it is only 15% at 193 nm and is expected to diminish at... 

The various ultrafast reaction techniques have inevitably been used in a number of the reported investigations on dyes. A study of the saturation absorption dynamics of a cyanovinyldiethylamine dye has yielded a measured lifetime of 3 1 ps, a value which is determined by very rapid internal conversion of the Sj state. ... 

Nonetheless, these are only snapshots at selected points. To capture elemental and valence distribution in a representative area/volume a rapid measurement technique is essential. The authors have recently made use of the ultrafast scanning setup available at P06, DESY, Hamburg to investigate elemental distribution on the electrode level of nickel doped manganese spinel full cells after cycling [70]. 

Time-resolved X-ray absorption is a very different class of experiments [5-7]. Chemical reactions are triggered by an ultrafast laser pulse, but the laser-induced change in geometry is observed by absorption rather than diffraction. This technique permits one to monitor local rather than global changes in the system. What one measures in practice is the extended X-ray absorption fine structure (EXAFS), and the X-ray extended nearedge strucmre (XANES). 

Recently, Eisenthal and coworkers have developed time-resolved surface second harmonic techniques to probe dynamics of polar solvation and isomerization reactions occurring at liquid liquid, liquid air, and liquid solid interfaces [22]. As these experiments afford subpicosecond time resolution, they are analogous to ultrafast pump probe measurements. Specifically, they excite a dye molecule residing at the interface and follow its dynamics via the resonance enhance second harmonic signal. 

Ultrafast, time-resolved OKE measurements (which involve another type of four-wave mixing process) may also occasionally be used to determine x(3) and 7 values.5 7 In each of the THG, DFWM, Z-scan, and OKE techniques, 7 is derived from the measured x values by using the solute number density of the solution. In a few cases, x(3) values have also been determined by using Stark spectroscpy. It should be remembered that comparisons of 7 or x values obtained using different techniques with different experimental conditions are generally of little utility. 

Using this optical Keir cell the authors developed a technique to measure the lifetimes of atomic and molecular transitions on a picosecond time scale 138a), See also the reviews by Rentzepis 138b) about ultrafast processes and Merkelo 138c),... 

Today, ultrafast pulsed-laser techniques, high-speed computers, and other sophisticated instrumentation make it possible to measure the time evolutions of reactants, intermediates, transition structures, and products following an abrupt photoactivation of a starting material. Detailed theoretical calculations, experienced judgments based on the literature, and newly accessible femtosecond-domain experimental data providing observed intensities of chemical species versus time can provide insights on the atomic-scale events responsible for overall reaction outcomes. 

Ultrafast vibrational spectroscopy offers a variety of techniques for unraveling the microsopic dynamics of hydrogen bonds occurring in the femto- to picosecond time domain. In particular, different vibrational couplings can be separated in nonlinear experiments by measuring vibrational dynamics in real-time. Both coherent vibrational polarizations and processes of population and energy relaxation have been studied for a number of hydrogen bonded systems in liquids [1],... 

To study the excited state one may use transient absorption or time-resolved fluorescence techniques. In both cases, DNA poses many problems. Its steady-state spectra are situated in the near ultraviolet spectral region which is not easily accessible by standard spectroscopic methods. Moreover, DNA and its constituents are characterised by extremely low fluorescence quantum yields (<10 4) which renders fluorescence studies particularly difficult. Based on steady-state measurements, it was estimated that the excited state lifetimes of the monomeric constituents are very short, about a picosecond [1]. Indeed, such an ultrafast deactivation of their excited states may reduce their reactivity something which has been referred to as a "natural protection against photodamage. To what extent the situation is the same for the polymeric DNA molecule is not clear, but longer excited state lifetimes on the nanosecond time scale, possibly of excimer like origin, have been reported [2-4],... 

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