Pump-probe absorption

Pump-probe absorption experiments on the femtosecond time scale generally fall into two effective types, depending on the duration and spectral width of the pump pulse. If tlie pump spectrum is significantly narrower in width than the electronic absorption line shape, transient hole-burning spectroscopy [101. 102. 103. 104. 105. 106. 107. 108. 109. 110. 111. 112 and 113] can be perfomied. The second type of experiment, dynamic absorption spectroscopy [57, 114. 115. 116. 117. 118. 119. 120. 121 and 122], can be perfomied if the pump and probe pulses are short compared to tlie period of the vibrational modes that are coupled to the electronic transition. 

Takeuchi T, Tahara T (2005) Coherent nuclear wavepacket motions in ultrafast excited-state intramolecular proton transfer sub-30-fs resolved pump-probe absorption spectroscopy of 10-hydroxybenzo[h]quinoline in solution. J Phys Chem A 109 10199-10207... 

We have investigated the picosecond photodissociation of the 2 and CO forms of a number of synthetic and natural heme complexes that differ in the geometry and strain associated with the imidazole-iron (porphyrin) interaction using the standard Nd 61ass laser pump-probe absorption method. Our results indicate that it takes some picoseconds for these complexes to dissociate and further suggest that a pseudo four coordinate complex may be present as a photointermediate in the synthetic compounds with the strained imidazole geometry. 

Analysis of pump-probe absorption data of Walther et al. (2005 not shown here) indicated two stretched exponentials characterizing the geminate recombination. In agreement with pump-probe results, their transient grating data (reproduced in Figure 1.12) have been fitted by an equation,... 

We investigated the ultrafast dynamics in a Na-NaBr melt at 1073 K by fs pump probe absorption spectroscopy. A simple model was used to simulate the dynamics of polaron-, bipolaron- and Drude-type electrons. The relaxation times for polarons and bipolarons are 210 fs and 3 ps, respectively. The existence of an isosbestic point at 1.35 eV indicates an inter-conversion between bipolarons and Drude-type electrons. 

Two-color pump-probe absorption spectroscopy is carried out with moderate pump energies producing small depletions of the vibrational ground state of only a few percent in order to avoid secondary excitation steps and minimize the temperature increase of the sample due to the deposited pump energy. 

The pump pulse in time-resolved pump-probe absorption spectroscopy is often linearly polarized, so photoexcitation generally creates an anisotropic distribution of excited molecules. In essence, the polarized light photoselects those molecules whose transition moments are nominally aligned with respect to the pump polarization vector (12,13). If the anisotropy generated by the pump pulse is probed on a time scale that is fast compared to the rotational motion of the probed transition, the measured anisotropy can be used to determine the angle between the pumped and probed transitions. Therefore, time-resolved polarized absorption spectroscopy can be used to acquire information related to molecular structure and structural dynamics. 

Transient intermediates are most commonly observed by their absorption (transient absorption spectroscopy see ref. 185 for a compilation of absorption spectra of transient species). Various other methods for creating detectable amounts of reactive intermediates such as stopped flow, pulse radiolysis, temperature or pressure jump have been invented and novel, more informative, techniques for the detection and identification of reactive intermediates have been added, in particular EPR, IR and Raman spectroscopy (Section 3.8), mass spectrometry, electron microscopy and X-ray diffraction. The technique used for detection need not be fast, provided that the time of signal creation can be determined accurately (see Section 3.7.3). For example, the separation of ions in a mass spectrometer (time of flight) or electrons in an electron microscope may require microseconds or longer. Nevertheless, femtosecond time resolution has been achieved,186 187 because the ions or electrons are formed by a pulse of femtosecond duration (1 fs = 10 15 s). Several reports with recommended procedures for nanosecond flash photolysis,137,188-191 ultrafast electron diffraction and microscopy,192 crystallography193 and pump probe absorption spectroscopy194,195 are available and a general treatise on ultrafast intense laser chemistry is in preparation by IUPAC. 

Pump-probe absorption experiments on the femtosecond time scale generally fall into two effective types, depending on the duration and spectral width of the pump pulse. If the pump spectrum is significantly narrower in width than the electronic absorption line shape, transient hole-burning spectroscopy [101. 102. 

Ultrafast TRCD has also been measured in chemical systems by incorporating a PEM into the probe beam optics of a picosecond laser pump-probe absorption apparatus [35]. The PEM resonant frequency is very low (1 kHz) in these experiments, compared with the characteristic frequencies of ultrafast processes and so does not interfere with the detection of ultrafast CD changes. 

The Si-photoisomerization of ris-stilbene was investigated by femtosecond pump-probe absorption spectroscopy in compressed solvents [79]. The viscosity dependence confirmed the existence of two pathways of the reaction. One showed an inverse viscosity dependence and led to trans-stilbene, the other one indicated no viscosity dependence and led to dihydrophenathrene. 

The femtosecond pump-probe absorption spectroscopy was used for the investigation of the SI-photoisomerization of cis-stilbene in compressed solvents [20]. The authors of the work [21] demonstrated a technique for femtosecond time-resolved optical pump-probe spectroscopy that allowed to scan over a nanosecond time delay at a kilohertz scan rate without mechanical delay line. Two mode-locked femtosecond lasers with 1 GHz repetition rate were linked at a fixed difference frequency of =11 kHz. One laser delivers the pump pulses, the other provides the probe pulses. The techniques enabled high-speed scanning over a 1-ns time delay with a time resolution of 230 fs. 

Fig. 5.10 Examples of ns pump-probe absorption spectroscopy for deuterated HK271 in 5CB. They show some indicators of our global best-fit procedure, (a, b) Typical probe transmittance signals (gray noisy line). The apparently constant isotropic signal reached after the pump passage is actually decaying in a time of several microseconds (data not shown), (c) Maximum ns dichroism, normalized to equilibrium transmittance T t = 0), vs. pump pulse energy density (circles). (d) Transmittance measured at 50 ns (triplet contribution) vs. pump pulse energy density. Best-fit curves are reported as black lines (see text for explanation). Reprinted with permission from [30]. Copyright 2003, American Physical Society...

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