Pump and probe technique

Pulse ultrasonic relaxation method, 32 18 Pump-and-probe techniques, 46 137 Purification, of actinide metals, see Actinide, metals, purification XjPj Purified protein, 36 94 Purple acid phosphatases, 40 371, 376, 43 362, 395-398, 44 243-245 biological function, 43 395 homology, 43 397... 

As demonstrated for CS2, the pump-and-probe technique with CMI is a promising tool for investigating how a nuclear wavepacket evolves in real time. Considering that few-cycle laser pulses are now becoming available, we are now entering into a new era when it will be possible to see molecules in intense laser fields in real time with highest temporal resolution [34,35]. 

Since all these excitations possess clear spectroscopic fingerprints, a very powerful tool for studying their photophysics is the so-called pump-and-probe technique, which can be related to the optical nonlinearities of the material. As far as optical properties are concerned, the change in refractive index upon excitation (An or Ak), either optical or otherwise, is the nonlinear part of the optical response. Traditionally two approaches have been developed to measure nonlinearities, based on different techniques ... 

This discourse tries to give an overview of the current state-of-the-art instrumentation in real-time pulse radiolysis experiments utilizing optical, conductometric and other methods. Pump-and-probe techniques for the sub-nanosecond time domain are believed to be beyond the scope of this discussion. 

The two-photon photochemistry of C6H5Br has been studied in an ion cyclotron resonance (ICR) spectrometer. A pump-and-probe technique... 

The pump-and-probe technique using powerful ultra short (7 ps) laser pulses has been applied to investigate the process of intraband relaxation of the excited carriers in CdSe/ZnS quantum dots. The slowing down of intraband relaxation through the energy levels of holes have been observed at powerful excitation in the case of resonant excitation of the first electron excited state lP(e). 

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. 

During the last 15 years, several technical developments in the light source and detector made time-resolved Raman spectroscopy an important branch of time-resolved surface spectroscopy [60]. It should be emphasized that there are two kinds of time-resolved studies. The first kind of measurement is triggered by a certain surface process (reaction) named as a single shot experiment and the second one is a pump and probe technique. 

Today two important techniques are used to avoid secondary collisions. The first technique is the molecular beam method, in which the density i kept so low that no collisions occur before the products reach the detector (for example a mass spectrometer). The second technique, that became more popular in the recent years, is based on time resolution. In this "pump and probe" technique, two short laser pulses... 

For measurements of very fast relaxation processes that demand a time resolution below 10 s most detectors (except the streak camera) are not fast enough. Here the pump-and-probe technique is the best choice. It is based on the following principle shown in Fig. 6.96. 

Fig. 6.97 Pump-and-probe technique for the measurements of ultrafast relaxation processes...

These developments widen the range of applications considerably. We will now give some examples of applications of the pump-and-probe technique. 

A very interesting problem is concerned with the physical limitations of the ultimate speed of electronic computers. Since any bit corresponds to a transition from a nonconducting to a conducting state of a semiconductor, or vice versa, the relaxation time of electrons in the conduction band and the recombination time certainly impose a lower limit for the minimum switching time. This electronic relaxation can be measured with the pump-and-probe technique. The electrons are excited by a femtosecond laser pulse from the upper edge of the valence band into levels with... 

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 ... 

Using such short attosecond XUV pulses, the temporal evolution of the Auger process after inner shell excitation can be followed with the pump-and-probe technique. 

Quantum beats can be observed not only in emission but also in the transmitted intensity of a laser beam passing through a coherently prepared absorbing sample. This has first been demonstrated by Lange et al. [872, 873]. The method is based on time-resolved polarization spectroscopy (Sect. 2.4) and uses the pump-and-probe technique discussed in Sect. 6.4. A polarized pump pulse orientates atoms in a cell placed between two crossed polarizers (Fig. 7.12) and generates a coherent superposition of levels involved in the pump transition. This results in an oscillatory time dependence of the transition dipole moment with an oscillation period AF = 1/Av... 

One example of this pump-and-probe technique is the investigation of collision-induced vibrational-rotational transitions in the different isotopes HDCO and D2CO of formaldehyde by an infrared-UV double resonance [1041]. A CO2 laser pumpes the V6 vibration of the molecule (Fig. 8.18). The collisional transfer into other vibrational modes is monitored by the fluorescence intensity induced by a tunable UV dye laser with variable time delay. 

The time resolution of the pump-and-probe technique is not limited by the rise time of the detectors. It can therefore be used in the pico- and femto-second range (Sect. 6.4) and is particularly advantageous for the investigation of ultrafast relaxation phenomena, such as collisional relaxation in liquids and solids [1042, 1043]. It is also useful for the detailed real-time study of the formation and dissociation of molecules where the collision partners are observed during the short time interval when forming or breaking a chemical bond [1044]. 

Fig. 8.19 Pump- and probe-technique for the measurement of ultrafast processes...

The detailed knowledge of the different steps of biological processes on a molecular level is one of the ambitious goals of molecular biology. The importance of this field was underlined by the award of the Nobel Prize in chemistry in 1988 to J. Deisenhofer, R. Huber, and H. Michel for the elucidation of the primary steps in photosynthesis and the visual process [1511]. This subsection illustrates the importance of time-resolved Raman spectroscopy in combination with pump-and-probe techniques (Sect. 6.4) for the investigation of fast biological processes. 

PA) measurements in the pump and probe technique is described elsewhere (8). The carriers were photogenerated by an Ar laser at 2.7eV chopped at 140 Hz thus optimizing the measurement to carrier lifetime of order Imsec. The changes AT of the transmission T were measured via a probe beam that was provided by broad band CW lamps followed by a monochromator. 

A more direct manner to determine the lifetime of these excited complexes is to use real-time picosecond pump and probe techniques. For example, the complex I2 Ne is excited to a given initial vibrational state v[ and the nascent I2 is then detected in a given final vibrational state Vf by using the laser... 

A comparison between measured lifetimes for the l2 - Ne complex, using line broadening and transients experiments based on the aforementioned picosecond pump and probe technique, is shown in Figure 24.25. Good agreement in the qualitative v dependence is observed as v increases, the predissociation rate increases and, correspondingly, the lifetime decreases. 

Figure 24.25 Lifetimes of the I2---Ne complex in the range v = 12-25. Top comparison between the complex lifetimes measured by the pump and probe technique and line width measurements. Reproduced from Willberg et al, J. Chem. Phys., 1992, 96 198, with permission of the American Institute of Physics. Bottom comparison between the experimental l2 - Ne lifetime values obtained by the pump and probe technique and theoretically calculated values using ab initio methods. Reproduced from Delgado-Barrio, in Dynamicai Processes in Molecular Physics, 1991, with permission of lOP Publishing Ltd...

The pump and probe technique using picosecond lasers also allows us to investigate the I2 (C02) cluster ion photofragmentation dynamics in real time. An example of such an investigation is shown in Figure 25.5 for cluster size n= 12-17, where the absorption recovery of I2 is displayed as a function of the pump-probe delay. 

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