Beam-probe techniques

Figure B2.5.8. Schematic representation of laser-flash photolysis using the pump-probe technique. The beam splitter BS splits the pulse coming from the laser into a pump and a probe pulse. The pump pulse initiates a reaction in the sample, while the probe beam is diverted by several mirrors M tluough a variable delay line.

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 surface sensitivity of most electron probe techniques is due to the fact that the penetration depth of electrons into metals falls to a minimum of 4 to 20 A when their kinetic energy is between 10 and 500 eV. It is also convenient that electrons at these energies have de Broglie wavelengths on the order of angstroms. With a monochromatic beam, it is possible to do LEED. 

Ion beam probes are used in a wide range of techniques, including Secondary Ion Mass Spectroscopy (SIMS), Rutherford backscattering spectroscopy (RBS) and proton-induced X-ray emission (PIXE). The applications of these and number of other uses of ion beam probes are discussed. 

High aspect ratio SFM probes can be also made by a focused-ion-beam (FIB) technique [209-211]. Microtips were reproducibly grown up to 1.0 pm in length and 0.1 pm in diameter. A tip radius as low as 5 nm could be achieved and was found to degrade only slightly after extensive SFM imaging. Conventional tips can be modified by etching techniques so that a sharper probe apex is provided [212]. 

The yield of dissociation products may be small, but sensitive methods of detection can be used. One of these is laser-induced fluorescence, shown schematically in Figure 9.31, in which a second, probe, laser is used to excite fluorescence in one of the products of dissociation. The C02 and probe laser beams are at 90° to each other and the fluorescence is detected by a photomultiplier at 90° to both beams. This technique has been used, for example, to monitor the production of NH2 from the dissociation of hydrazine (N2H4) or methylamine (CH3NH2). The probe laser was a tunable dye laser set at a wavelength of 598 nm corresponding to absorption in the 2g band, where v2 is the bending vibration, of the A2 A, —X2Bl electronic system of NH2, and total fluorescence from the 29 level was monitored. 

Fig. 6. Density of non-condensed fraction of the gas as the trap depth is reduced along the cooling path. The density is measured by the optical resonance shift, and the trap depth is set by the rf frequency. The lines (dash, solid, dot-dash) indicate the BEC phase transition line, assuming a sample temperature of (l/5th, l/6th, l/7th) the trap depth. The scatter of the data reflects the reproducibility of the laser probe technique and is dominated by alignment of the laser beam to the sample...

Another possibility to independently measure ionic fluxes is to combine the EQCM with real time beam probe deflection technique to identify the exchanged ions [29, 30], Ionic fluxes across the electrode-electrolyte interface result in changes of the solution refraction index adjacent to the electrode surface and a transient deflection of a laser beam is measured. 

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 resonance Raman spectrum of bathorhodopsin has recently been analyzed in detail by using a two laser beam pump-probe technique [140-142], On the basis of new experimental data as well as theoretical calculations [142] of the C=N stretching and C=N—H bend frequencies and the large shift seen upon deuteration (25 cm 1), it was concluded that the proton in both rhodopsin and bathorhodopsin must be... 

Pump-probe technique A flash photolysis technique in which the light beam (probe) used for spectral analysis is generated from a portion of the excitation (pump) beam. A time delay in the latter allows the obtention of kinetic data. 

The advent of supersonic molecular beam expansion techniques and high spectral-and time-resolved laser spectroscopic probing has transformed experimental... 

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