Pump-probe techniques dynamics

In many respects the time-resolved pump-probe technique is similar to the CW counterpart. The use of pulsed laser light permits direct probing of both the magnitude of the PA and its dynamics. The experimental arrangement is practically the same as for the CW version, i.e., both pump and probe beams are focused and overlapped onto same spot on a sample. In addition, the pump and probe pulses are synchronized so that the lime interval t between them is constant and confined to a certain time range (in our case up to 3 ns). 

The power of these techniques in elucidating the detailed mechanisms of ion-molecule reactions is well demonstrated by this example. Undoubtedly, we can expect to see increasing use of the ultrafast pump-probe technique in the field of ion-molecule reaction dynamics. 

When metals have Raman active phonons, optical pump-probe techniques can be applied to study their coherent dynamics. Hase and coworkers observed a periodic oscillation in the reflectivity of Zn and Cd due to the coherent E2g phonons (Fig. 2.17) [56]. The amplitude of the coherent phonons of Zn decreased with raising temperature, in accordance with the photo-induced quasi-particle density n.p, which is proportional to the difference in the electronic temperature before and after the photoexcitation (Fig. 2.17). The result indicated the resonant nature of the ISRS generation of coherent phonons. Under intense (mJ/cm2) photoexcitation, the coherent Eg phonons of Zn exhibited a transient frequency shift similar to that of Bi (Fig. 2.9), which can be understood as the Fano interference [57], A transient frequency shift was aslo observed for the coherent transverse optical (TO) phonon in polycrystalline Zr film, in spite of much weaker photoexcitation [58],... 

Following another experimental approach, GWgoire et al [9] have tried to understand the influence of an increasing number of solvent molecules on the femtosecond dynamics of diatomic molecules, including the dimers Nal and Csl. Due to its relative simplicity, the isolated Nal molecule has been studied extensively with pump-probe techniques both experimentally [10], and theoretically [11,12], In this report, we investigate theoretically the femtosecond pump-probe ionization of Nal and Csl when aggregated with a molecule of acetonitrile CH3CN. 

In my talk I surveyed recent advances in the methodology and selected 2D-IR spectra of secondary structures. The results promise to provide structurally based kinetic probes for conformational dynamics, sharp tests of anharmonic potential surfaces and novel information regarding the transient and equilibrium vibrational dynamics of peptides. The heterodyned 2D-IR approach has proven useful in determining structures of peptides in solution and the anharmonic nature of the potential surfaces of peptides and secondary structures [1-10], as have polarized photon echo [2,6,10-12] or pump-probe techniques [4,13-16]. 

The optimum experiment would therefore proceed by preparing the molecules in a precise state using beam methods and laser excitation, followed by measurement of the radiative emission as well as other properties of the bound molecule. Such experiments are, in fact, underway on a variety of molecules in several laboratories around the world. In addition, information on the bound-state dynamics of molecules has emerged from pump-probe techniques in which two lasers are utilized, one to prepare the molecule in the desired state and the second to interrogate the dynamics. Along with these experimental developments, we note a need for reliable theories to understand the interrelationship between the observed features and the nature of the dynamics. Such developments are in progress. 

Since the first experiments on the I-CN [21] bond cleavage and the wavepacket oscillations between the ionic and covalent potentials in the photodissociation of Nal [22, 23], pump-probe techniques have been applied to a wide range of important photochemical processes. However, the data obtained Ifom such experiments are often difficult to interpret and theoretical modeling is needed to get further insight into the excited state dynamics of the systems of interest at the atomistic level. In this context, the development of efficient and accurate computational methods for the description of ground and excited electronic states of mid-size molecular systems in a balanced way [24, 25], has greatly facilitated the theoretical study of photochemical processes. 

To analyze the ultrafast dynamics of small molecules and clusters, the method of multiphoton ionization (MPI) spectroscopy combined with the pump probe technique was chosen. Adiabatic expansion or sputtering was used to produce the molecules and clusters of interest. Quadrupole mass spectrometers enabled a mass-selective detection of the ionized species. 

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