Pump-probe spectroscopy anisotropy

The methods discussed so far, fluorescence upconversion, the various pump-probe spectroscopies, and the polarized variations for the measurement of anisotropy, are essentially conventional spectroscopies adapted to the femtosecond regime. At the simplest level of interpretation, the information content of these conventional time-resolved methods pertains to populations in resonantly prepared or probed states. As applied to chemical kinetics, for most slow reactions (on the ten picosecond and longer time scales), populations adequately specify the position of the reaction coordinate intermediates and products show up as time-delayed spectral entities, and assignment of the transient spectra to chemical structures follows, in most cases, the same principles used in spectroscopic experiments performed with continuous wave or nanosecond pulsed lasers. 

Orientational dynamics inside a reverse micelle nanopool is explored using pump-probe spectroscopy. The initial experiments have had a lower time resolution and thus cannot measure rotational anisotropy for t < 200 Is. Thus mainly these experiments provide information about the dynamics in the longer timescale. The main results of these experiments are as follows (1) the rotational anisotropies for the larger reverse micelles (Wo > 20) are single exponential with a decay time the same as bulk water. (2) The rotational anisotropies for the smaller reverse micelles (Wo < 10) show an increasing slowing down of the dynamics as Wo decreases (Figure 17.5). These results have been understood in the fiamework of a two-component core-shell model [11]. 

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. 

The set-up is similar to the one used in normal saturation spectroscopy but the probe beam is now blocked by crossed polarizers that are placed at opposite sides of the absorption ceU. The polarizer in front of the cell is used to increase the linear polarization of the laser beam, which is frequently already well polarized. A matched pair of polarizers (frequently Gian-Thompson prisms. Fig. 6.46) can have a rejection ratio of 10 to 10 in the crossed position. The pump beam is circularly polarized and induces an anisotropy... 

Pulsed two-frequency (ultrafast, femtosecond) polarization-resolved mid-infrared spectroscopy was used in a series of papers by Bakker and coworkers to study the effect of ions on the structural dynamics of their aqueous solutions [44,120]. The solvent consisted of mixtures of Hp and D O (generally O.IM HDO in D O) and the first, the pump, pulse excited the 0-H or 0-D stretch vibration to the first excited state that then relaxed at a measurable rate. The second, the probe, pulse was red-shifted with respect to the first and probed the decay of this excited state. Generally, fairly concentrated electrolyte solutions were required for the application of this technique, in the range 0.5 to lOM. The rotational anisotropy is as follows ... 

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