Pump-probe spectroscopy experiment

Pump-probe absorption experiments on the femtosecond time scale generally fall into two effective types, depending on the duration and spectral width of the pump pulse. If tlie pump spectrum is significantly narrower in width than the electronic absorption line shape, transient hole-burning spectroscopy [101. 102. 103. 104. 105. 106. 107. 108. 109. 110. 111. 112 and 113] can be perfomied. The second type of experiment, dynamic absorption spectroscopy [57, 114. 115. 116. 117. 118. 119. 120. 121 and 122], can be perfomied if the pump and probe pulses are short compared to tlie period of the vibrational modes that are coupled to the electronic transition. 

We now we summarize some of the procedures that are used in analyzing multidimensional IR data. Constants factors are often omitted from the formulas as are the transition dipole factors which are easily incorporated [74] when the modes are a collection of coupled harmonic oscillators. More generally the variations of transition dipole with nuclear displacement should be incorporated. It is often useful to compare the 2D-IR results with the results of other nonlinear experiments because it turns out that various manipulations of these multidimensional signals provide all of the common nonlinear results such as echoes, gratings, degenerate four wave effects, and pump-probe spectroscopy. 

Pump-probe absorption experiments on the femtosecond time scale generally fall into two effective types, depending on the duration and spectral width of the pump pulse. If the pump spectrum is significantly narrower in width than the electronic absorption line shape, transient hole-burning spectroscopy [101. 102. 

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. 

The application of pump-probe spectroscopy to electronic transitions of larger aggregates reveals the rapidly growing number of different dissociation channels. The result of two-color pump-probe femtosecond experiments performed on sodium clusters Na with 3 n 10 is shown in Fig. 8. For At > 0, the energy pump was 1.47 eV, whereas Sprobe> the energy of the... 

The systematically performed pump-probe spectroscopy on alkali clusters provided a good indication about suited candidates for a coherent control experiment. Among these, the fragmentation dynamics of the heteronu-clear trimer Na2K appeared to us the best. The corresponding pump-probe spectrum is shown in Fig. 14(a). It clearly exhibits — superimposed on an exponential decay with a time constant of 3.28 ps — an oscillatory behaviour with a period of roughly 500 fs. The Fourier-transform of this... 

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 spectroscopic tool to be considered here is femtosecond pump/probe spectroscopy. This experimental technique uses two ultrashort laser pulses which are time-delayed with respect to each other. They are sent into a molecular sample and a signal is recorded as a function of the delay-time between the pulses. To be more specific, we assume the molecule to be in an inital state 0o) O). Here o) denotes the wave function for the nuclear motion and 0) the wave function of the electrons (the adiabatic separation of nuclear and electronic motion is assumed throughout). The pump pulse induces a transition and the resulting wave function which describes the molecule after the interaction with the electric field may be assigned as 0i l). We treat electronic excitation so that the molecule is prepared in another electronic state 1). After the pump pulse passed the sample, the molecule evolves unperturbed until the probe pulse starts interacting. This interaction results in a second excitation to (in our case) a final electronic state 2) with the respective nuclear wave function 1 2) The scheme just described is depicted in Figure 1 and illustrates the idea of many pump/probe experiments. 

Figure 2 illustrates the basic concept of a typical pump-probe spectroscopy used in most ultrafast spectroscopy techniques. In its simplest form the output pulse train of an ultrafast laser is divided in two by a beam splitter. One pulse in train (called pump) first excites the sample under investigation. The second pulse train (called probe) will probe the sample with a suitable time delay with respect to the pump by introducing an optical delay in its path and some optical property (e.g., reflectivity, absorption, Raman scattering, luminescence, optical nonlinear responses) of the sample is then detected to investigate the changes produced by the pump. In most of the time-resolved pump-probe experiments, the time resolution is limited only by the pulse width of the laser or the jitter between the laser systems. 

Pump probe spectroscopy transient absorption spectroscopy is the most widely applied method of probing excited state dynamics. In these experiments, a laser pulse (pump, C0p ) places the nanostructure into the excited state a second... 

Although very detailed, fundamental information is available from ultrafast TRIR methods, significant expertise in femtosecond/picosecond spectroscopy is required to conduct such experiments. TRIR spectroscopy on the nanosecond or slower timescale is a more straightforward experiment. Here, mainly two alternatives exist step-scan FTIR spectroscopy and conventional pump-probe dispersive TRIR spectroscopy, each with their own strengths and weaknesses. Commercial instruments for each of these approaches are currently available. 

---