Pump-probe techniques, molecular systems

The ground electronic state of 139La160 is X2S+ audits electronic spectrum involving the excited B2Y,1 has been studied by Doppler-free laser-induced fluorescence by Bacis, Collomb and Bessis [85] and by Bernard and Sibai [86]. Both states have therefore been well characterised and the system is ideal for radiofrequency/optical double resonance, as described by Childs, Goodman, Goodman and Young [87]. They used a collimated molecular beam, with the laser pump/probe technique described elsewhere in this chapter. 

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. 

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. 

This chapter focuses on the ultrafast ESIPT found in molecules containing an H-chelate ring (figure 4.1). We will introduce the most popular experimental techniques and discuss what kind of information can be extracted from the spectral signatures associated with the ESIPT and subsequent processes. In the remainder of the introduction, we introduce the investigated molecular systems. The subsequent experimental section describes different pump-probe techniques. Then the transient spectroscopic signatures and their interpretation and evaluation... 

Pump-probe diffraction techniques offer exciting new ways to probe transient structures in molecular, nanoscale, and biological systems. For dilute systems, or very small targets, electron diffraction is the preferred tool, because the cross sections for scattering of electrons from molecules are very large. In our research we show that pump-probe electron diffraction is an excellent technique to probe the dynamics of chemical reactions in the rarified environment of jet expansions, and for probing the diffraction signatures of individually excited vibronic states. 

The pump-and-probe technique has proved to be very well suited for studying shortlived transient states of molecular systems that had been excited by a short laser pulse before they dissociate ... 

Lasers are the precision tools of photochemistry and they have been used to both pump (initiate) and probe (analyse) chemical processes on time-scales that are short enough to allow the direct observation of intramolecular motion and fragmentation (i.e. on the femtosecond time-scale). Thus, laser-based techniques provide us with one of the most direct and effective methods for investigating the mechanisms and dynamics of fundamental processes, such as photodissociation, photoionization and unimolecu-lar reactions. Avery wide variety of molecular systems have now been studied using laser techniques, and only a few selected examples can be described here. 

There exists a large variety of spectroscopic techniques that employ ultra-short laser pulses. These methods may differ, for example, in the detection mechanism and in the number and properties of laser fields, and will in general monitor different aspects of the dynamics of the molecular system. Most of these experiments are of the pump-probe (PP) type, that is, the molecular system is prepared by a first laser pulse (the pump ) into a nonstationary state, the time evolution of which is interrogated by a time-delayed second laser pulse (the probe ). It is important to distinguish between resonant and nonresonant electronic excitation of the system. In the latter case, it is not possible to establish a population in the excited electronic state which survives the duration of the pump field. As a consequence, nonresonant excitation gives only rise to Raman-like emission, which is known... 

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