Pump and probe method

Figure 1 Schematic describtion of the LP/LIF pump-and-probe method. 

Abstract The concepts at the basis of transient absorption measurements were illustrated with particular reference to nanosecond kinetic spectrophotometry and femtosecond pump and probe methods. The main features of the typical experimental setups for both techniques were illustrated as regards optical parts, geometrical layout of components and light detection systems. Examples of application of transient absorption spectroscopy were illustrated for the elucidation of photoinduced processes in supramolecular systems like a fullerenepyrrolidine-oligophenyleneethynylene hybrid derivative, a drug-protein complex and a tri-chromophoric system consisting of two porphyrins and one perylene bisimide. 

In the time-domain detection of the vibrational coherence, the high-wavenumber limit of the spectral range is determined by the time width of the pump and probe pulses. Actually, the highest-wavenumber band identified in the time-domain fourth-order coherent Raman spectrum is the phonon band of Ti02 at 826 cm. Direct observation of a frequency-domain spectrum is free from the high-wavenum-ber limit. On the other hand, the narrow-bandwidth, picosecond light pulse will be less intense than the femtosecond pulse that is used in the time-domain method and may cause a problem in detecting weak fourth-order responses. 

Since there are a large number of different experimental laser and detection systems that can be used for time-resolved resonance Raman experiments, we shall only focus our attention here on two common types of methods that are typically used to investigate chemical reactions. We shall first describe typical nanosecond TR spectroscopy instrumentation that can obtain spectra of intermediates from several nanoseconds to millisecond time scales by employing electronic control of the pnmp and probe laser systems to vary the time-delay between the pnmp and probe pnlses. We then describe typical ultrafast TR spectroscopy instrumentation that can be used to examine intermediates from the picosecond to several nanosecond time scales by controlling the optical path length difference between the pump and probe laser pulses. In some reaction systems, it is useful to utilize both types of laser systems to study the chemical reaction and intermediates of interest from the picosecond to the microsecond or millisecond time-scales. 

The characterization of the laser pulse widths can be done with commercial autocorrelators or by a variety of other methods that can be found in the ultrafast laser literature. " For example, we have found it convenient to find time zero delay between the pump and probe laser beams in picosecond TR experiments by using fluorescence depletion of trans-stilbene. In this method, the time zero was ascertained by varying the optical delay between the pump and probe beams to a position where the depletion of the stilbene fluorescence was halfway to the maximum fluorescence depletion by the probe laser. The accuracy of the time zero measurement was estimated to be +0.5ps for 1.5ps laser pulses. A typical cross correlation time between the pump and probe pulses can also be measured by the fluorescence depletion method. 

Another method is to measure the disappearance rate of the excited parent molecules, that is, the intensity changes of the disk-like images at various delay times (therefore, at various photolysis laser positions) along the molecular beam. This is very useful when the dissociation rate is slow and the method described above cannot be applied. This measurement requires a small molecular beam velocity distribution and a large variable distance between the crossing points of the pump and probe laser beams with the molecular beam. The small velocity distribution can be obtained through adiabatic expansion, and the available distances between the pump and probe laser beams depend on the design of the chamber. For variable distances from 0 to 10 cm in our system and AV/V = 10% molecular beam velocity distribution, dissociation rates as slow as 3 x 103 s 1 under collisionless condition can be measured. 

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... 

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. 

TR methods were originally developed in om laboratories to study excited-state structures and dynamics of transition metal complexes such as Ru + (bpy)s and metaUoproteins. TR measurements rely on a pump-probe approach in which two separate laser pulses are used, one to excite the system and the other to probe the transient Raman spectrum. The time resolution of the experiment is determined by the width of the laser pulses (typically 7 ns for a Q-switched laser or as short as 1 ps for a mode-locked laser). The pulses are variably delayed with respect to one another to achieve time resolution, either by optically dela)dng the probe pulse with respect to the pump pulse or by electronically delaying two independently tunable lasers. Thus, two different approaches are required depending on the time scale of interest. The fastest timescale (from 10 to 10 s) requires optical delay to achieve sufficiently short separation between the pump and probe pulses. In such a scheme, the probe pulse is sent through a fixed path, but the pump pulse is sent through a variable path that can be scanned. Since hght travels about 1 ft per ns, a difference in pathlength of a few feet is sufficient. The second approach typically uses two Q-switched Nd YAG lasers that are electronically delayed with respect to one another, to access... 

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. 

In this experiment type, the detector of the transmitted probe light behaves as a photon integrator only and the time resolution derives from the instrument s capability for measuring the difference in the time of arrival of the pump and probe pulses. This capability provides an intrinsic limitation to the pump-probe method, which depends critically on the width of the pulses being employed. As indicated earlier, exceedingly short duration pulses are commonplace these days. 

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