Fluorescence pumping

Let us consider the effect of an external magnetic field on the angular momenta distribution at a level populated in the fluorescence process see Section 3.4, Fig. 3.14. In the presence of an external magnetic field the following polarization moments are created on the lower level J via spontaneous transitions at weak excitation, x — 0  

What kind of information do these Hanle signals contain Fig. 4.15 might help to make this clear. There we see the expected shape of the Hanle signals at toj = u j and at j F = j/T. The curves 1 (Ei E) and 

4 (Ei L E) demonstrate the signal from the lower level J in scale on the assumption that tojn / 2 toj / j T, meaning that the upper 

Curves 2 (Ei E) and 5 (Ei L E) in Fig. 4.15 refer to the uoj Jj/T 1 scale, in which the excited state Hanle effect manifests itself (the ground state Hanle effect is already fully developed and does not manifest itself in this scale). The signal is of Lorentz shape  

Similarly to the above case, signals of level crossing may also be obtained for (P, i )-transitions, as well as for excitation by circularly polarized light [30]. 

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Fig. 4.15. Hanle signals produced by the probe beam at fluorescence pumping.

The resonance vector analysis has been used to explore all of the questions raised above on the fate of the polyad numbers in larger molecules, the most thoroughly investigated case so far probably being C2FI2- This molecule has been very extensively probed by absorption as well as stimulated emission pumping and dispersed fluorescence teclmiques [, 53, 70 and 71], the experimental spectra have been analysed in... 

The pump-probe concept can be extended, of course, to other methods for detection. Zewail and co-workers [16,18, 19 and 2Q, 93] have used the probe pulse to drive population from a reactive state to a state that emits fluorescence [94, 95, 96, 97 and 98] or photodissociates, the latter situation allowing the use of mass spectrometry as a sensitive and selective detection method [99, 100]. 

The main cost of this enlianced time resolution compared to fluorescence upconversion, however, is the aforementioned problem of time ordering of the photons that arrive from the pump and probe pulses. Wlien the probe pulse either precedes or trails the arrival of the pump pulse by a time interval that is significantly longer than the pulse duration, the action of the probe and pump pulses on the populations resident in the various resonant states is nnambiguous. When the pump and probe pulses temporally overlap in tlie sample, however, all possible time orderings of field-molecule interactions contribute to the response and complicate the interpretation. Double-sided Feymuan diagrams, which provide a pictorial view of the density matrix s time evolution under the action of the laser pulses, can be used to detenuine the various contributions to the sample response [125]. 

Figure C3.1.1. The basic elements of a time-resolved spectral measurement. A pump source perturbs tlie sample and initiates changes to be studied. Lasers, capacitive-discharge Joule heaters and rapid reagent mixers are some examples of pump sources. The probe and detector monitor spectroscopic changes associated with absorjDtion, fluorescence, Raman scattering or any otlier spectral approach tliat can distinguish the initial, intennediate and final...

Figure C3.1.8. Schematic diagram of a transient kinetic apparatus using iaser-induced fluorescence (LIF) as a probe and a CO2 iaser as a pump source. (From Steinfeid J I, Francisco J S and Fiase W L i989 Chemical Kinetics and. Dynamics (Engiewood Ciiffs, NJ Prentice-Fiaii).)...

In order to prevent this occurring a pulsed method of pumping is used with a repetition rate low enough to allow time for Tj — Sq relaxation. For CW operation either Tj must be sufflciently short or another dye has to be used for which T2 — Ti absorption does not overlap with the fluorescence. 

Figure 9.42 Intensity of sodium atom fluorescence as a function of time following excitation of Nal to the V potential with a pump wavelength of 307 nm (pulse duration ca 50 fs) and a probe wavelength of (a) 575 nm, (b) 580 nm, (c) 589 nm, and (d) 615 nm. (Reproduced, with permission, from Rose, T. S., Rosker, M. J. and Zewail, A. H., J. Chem. Phys., 91, 7415, 1989)...

Electrodriven separation techniques are destined to be included in many future multidimensional systems, as CE is increasingly accepted in the analytical laboratory. The combination of LC and CE should become easier as vendors work towards providing enhanced microscale pumps, injectors, and detectors (18). Detection is often a problem in capillary techniques due to the short path length that is inherent in the capillary. The work by Jorgenson s group mainly involved fluorescence detection to overcome this limit in the sensitivity of detection, although UV-VIS would be less restrictive in the types of analytes detected. Increasingly sensitive detectors of many types will make the use of all kinds of capillary electrophoretic techniques more popular. 

Pump laser photon absorption, fluorescence photon yield. 163-64... 

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