Pump probe techniques decay

Table 1. Parameters Used in Fitting the Signal Decay of 10-CPT in methanol-water using the pump-probe technique [4],...

The validity of the physics that adopts the point of view of decaying states depends on the characteristics of the process of excitation-preparation. Specifically, one must assume that the duration of the pulse of excitation energy is much shorter than the lifetime of the unstable state. This implies that indeed the system is prepared in a nonstationary state at f = 0, i.e., in the localized state (T o/ Eo)/ while losing memory of the excitation step. For long-lived unstable states, this is expected to be achievable easily. For shortlived unstable atomic or molecular states, say of the order of 10 s, this is also achievable, in principle, via modern pump-probe techniques with time-delays in the range of a few femtoseconds or of a couple of hundreds of attoseconds. 

The solvation shells of ions may not be spherically symmetrical, a phenomenon that occurs at the surfaces of solutions. The solvent molecules may then polarize the ions, even if they are monatomic, and thus affect their properties. However, in the bulk of the solutions, monatomic ions reside in a spherically symmetrical field and are themselves little affected by the solvent. This is not necessarily the case for polyatomic ions, even for those that have a globular shape. This effect is most noted in the vibrational and rotational relaxation times of polyatomic ions. The relaxation is measurable by the pump/probe technique, where the laser beam at the wave-number of the ground state is followed after a short interval by a probe beam at the level of the excited state, following its exponential decay. 

Oscillations of fluorescence, stimulated emission and excited-state absorption have been studied by pump-probe techniques and fluorescence upconversion, and have been seen in numerous small molecules in solution (Fig. 11.7A [120, 122-124]), and also in photosynthetic bacterial reaction centers [27, 125, 126]. They typically damp out over the course of several picoseconds as a result of vibrational relaxations and dephasing. Vibrational coherences generally decay more slowly than electronic coherences because the energies of vibrational states are not coupled as strongly to fluctuating interactions with the surroundings. Vibrational dephasing also tends to be less dependent on the temperature. 

It is clear that a core-hole represents a very interesting example of an unstable state in the continuum. It is, however, also rather complicated [150]. A simpler system with similar characteristics is a doubly excited state in few-body systems, as helium. Here, it is possible [151-153] to simulate the whole sequence of events that take place when the interaction with a short light pulse first creates a wave packet in the continuum, including doubly excited states, and the metastable components subsequently decay on a timescale that is comparable to the characteristic time evolution of the electronic wave packet itself. On the experimental side, techniques for such studies are emerging. Mauritsson et al. [154] studied recently the time evolution of a bound wave packet in He, created by an ultra-short (350 as) pulse and monitored by an IR probe pulse, and Gilbertson et al. [155] demonstrated that they could monitor and control helium autoionization. Below, we describe how a simulation of a possible pump-probe experiment, targeting resonance states in helium, can be made. 

Principally, the pump and probe technique depicted in Fig. 1.21 is apphed in time-resolved transient absorption experiments. A pump beam, directed onto the sample, generates excited species or reactive intermediates such as free radicals. The formation and decay of these species can be monitored with the aid of an analyzing (probe) light beam that passes through the sample perpendicular to the direction of the pump beam. In principle, a set-up of this kind is also suitable for recording luminescence, if it is operated without the probe beam. 

Ultrafast (femtosecond) pulsed two-color mid-infrared spectroscopy was used by Bakker et al. in a series of papers to study the effect of ions on the structural dynamics of their aqueous solutions as recently reviewed (Bakker 2008). The first intense pulse (pump pulse) excites the O-H (or O-D) stretch vibration to the first excited state and the second pulse (probe pulse), red-shifted with respect to the first, probes the decay of this state. This technique has been applied to aqueous (0.1-0.5 M HDO in D2O) solutions of LiX, NaX, and MgX2 (X = Cl, Br, I), KF, NaC104, and Mg(C104)2 over wide concentration ranges, 0.5-10 mol dm. ... 

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 ... 

In general, a sample will contain molecules that interact with their surroundings in a variety ways, for example because some of the fluorescing molecules are buried in the interior of a protein while others are exposed to the solvent. The fluorescence then decays with multiphasic kinetics that can be fit by a sum of exponential terms (Eq. 1.4). Fluorescence lifetimes can be measured by time-correlated photon counting, by fluorescence upconversion, or by modulating the amplitude of the excitation beam and measuring the modulation and phase shift of the fluorescence (Chap. 1). Pump-probe measurements of stimulated emission become the method of choice for sub-picosecond lifetimes (Chap. 11). For further information on these techniques and ways of analyzing the data see [30-34]. 

Fig.11.22. Pump and probe technique to measure decay times of short-lived excited states...

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