Pump-dump probe

A novel pump-damp-probe method (PDPM), which allows the characterization of solvation dynamics of a fluorescence probe not only in excited but also in the ground states has been recently developed (Changenet-Barret, 2000 and references therein). In PDPM, a pump produces a nonequilibrium population of the probe excited, which, after media relaxation, is simulated back to the ground states. The solvent relaxation of the nonequlibrium ground state is probed by monitoring with absorption technique. The inramolecular protein dynamics in a solvent-inaccessible region of calmodulin labeled with coumarin 343 peptide was examined by PDPM. In the pump-dump-probe experiments, part of a series of laser output pulses was frequency-doubled and softer beams were used as the probe. The delay of the probe with respect to the pump was fixed at 500 ps. 

An elegant way to study the dynamics from excited vibrational levels of the electronic ground state is to make use of the femtosecond pump dump probe scheme in transient absorption experiments. The molecular system is initially excited to the Si state and, using a second pulse of longer wavelength (the dump pulse), excited vibrational levels of the So state are populated via stimulated emission.197... 

Third-order nonlinear optical experiments make it, in principle, possible to study ground-state proton transfer reactions. In particular, a technique called pump-dump—probe [34], where the first pulse is used to excite the system, the second to bring population back to the ground state, and a third pulse to monitor the absorption of the ground state, which at least briefly after the pulse should still be in the tautomeric form T (Figure 9.4) would make it possible to monitor ground-state populations upon rapid creation of the ground-state tautomer. So far this technique has not been used on simple tautomeric compounds. It has been used in some photoactive proteins where proton transfer steps after excitation are important [34]... 

R. (2004) Incoherent manipulation of the photoactive yellow protein photocycle with dispersed pump-dump-probe spectroscopy. Biophysic. J., 87, 1858-1872. 

Figure 7-11. (a) Decay dynamics of FAt with and without the dump pulse. Inset time profiles of the pump, dump, and probe pulses, (b) Decay dynamics of PAi with and without the dump pulse. Shown for f = 0 and f i = 200 ps. 

The experimental setup has been described in detail elsewhere [10]. The current setup includes a peltier cooled GaAs photomuliplier to record weak signals. The intensity of the three beams was kept below 10 pJ for each beam in order to prevent distortion of the transients. At higher intensities a time independent signal can be observed and negative values of the exponent on Eq. 1 deplete the transient. The transients were recorded with a time resolution of 33 fs which is below the temporal width of the pump, dump and probe pulses (80fs from Grenouille measurement). 

An intuitive method for controlling the motion of a wave packet is to use a pair of pump-probe laser pulses, as shown in Fig. 13. This method is called the pump-dump control scenario, in which the probe is a controlling pulse that is used to create a desired product of a chemical reaction. The controlling pulse is applied to the system just at the time when the wave packet on the excited state potential energy surface has propagated to the position of the desired reaction product on the ground state surface. In this scenario the control parameter is the delay time r. This type of control scheme is sometimes referred to as the Tannor-Rice model. 

The modification of Eq. (7) allows one to use this formulation for the simulation of the fs pump-probe or pump-dump signals involving the ground and excited electronic states for the pump step and the cationic or the ground state for the probing of dynamics in the excited states. Moreover, the expression for the signals can be extended to treat, in addition to adiabatic dynamics, also nonadiabatic dynamics, which will be addressed in Section II.H.3. 

Based on ab initio classical trajectories and assuming Gaussian femtosecond envelopes for the laser fields, analytic expressions for the time-resolved pump-probe and pump-dump signals in the framework of the Wigner distribution approach are given by Eq. (6) (cf. Ref. 20). This ab initio Wigner distribution... 

Encouraged by the confirmation of the control concept, two-parameter control was considered in order to manipulate different processes in dimers and diatomic molecules. In addition to the pump-probe time delay, the second control parameter involved the pump [72, 73] or probe [66, 67] wavelength, the pump-dump delay [69, 74, 75], the laser power [121], the chirp [68, 76], or the temporal width [70] of the laser pulse. Optimal pump-dump control of K2 has been carried out theoretically in order to maximize the population of certain vibrational levels of the ground electronic state using one excited state as an intermediate pathway [71, 292-294]. The maximization of the ionization yield in mixed alkali dimers has been performed first experimentally using closed-loop learning control [77,78, 83] (CLL) and then theoretically in the framework of optimal control theory (OCT) [84]. 

To date, only IR photon energies in the H-stretching range have been used in IR-induced population transfer experiments, which for most systems results in excitation far above the isomerization barrier, impeding an experimental determination of the isomerization barriers directly. Zwier and coworkers presented an elegant alternative, where stimulated emission pumping (SEP) is combined with HF or PTS methods, see Fig. 2d. This combination allows one to probe the barrier to conformational isomerization [56]. The method consists of two steps a pump-dump SEP followed by a probe laser interrogation to determine the new cmifomiational distribution. In the early part of the molecular beam expansimi, SEP prepares the... 

The conceptual framework underlying the control of the selectivity of product formation in a chemical reaction using ultrashort pulses rests on the proper choice of the time duration and the delay between the pump and the probe (or dump) step or/and their phase, which is based on the exploitation of the coherence properties of the laser radiation due to quantum mechanical interference effects [56, 57, 59, 60, 271]. During the genesis of this field. 

Most double resonance experiments exploit one of the four energy level schemes shown in Fig. 1.17. The variety of detection schemes is enormous, but most schemes may be divided into those which result in a signal on an essentially dark background vs. those which result in a dip in an essentially constant background level. By definition, the frequency of the PROBE (or DUMP) laser is scanned and that of the PUMP or DETECT laser is held fixed while a double resonance spectrum is being recorded. 

The Ti-sapphire oscillator is extremely useful as a stand-alone source of femtosecond pulses in the near-IR region of the spectrum. Some ultrafast experiments, especially of the pump-probe variety (see below), can be conducted with pulses obtained directly from the oscillator or after pulse selection at a lower repetition rate. Far-IR (terahertz) radiation is usually generated using a semiconductor (usually GaAs) substrate and focused Ti-sapphire oscillator pulses [7]. If somewhat higher-energy pulses are required for an experiment, the Ti-sapphire oscillator can be cavity dumped by an intracavity acousto-optical device known as a Bragg cell. 

Figure 11.13 Time-resolved pump-probe vibrational relaxation of a C—H stretch vibration in liquid acetonitrile at room temperature detected by (Raman) emission from the excited state. In the isolated molecule such a v = 1 stretch mode will relax on the nanosecond time scale. The experiment maps out the energy transfer pathways in the liquid. The essential point is that the relaxation is by solvent-aided intramolecular transfer, as discussed in the text and indicated by arrows. Energy dumping into the liquid is far slower, requiring about 260ps (adapted from Deak etal. (1998) see also Iwaki and DIott (2001), Wang eta/. (2002)).

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