Fluorescent population

Let us find the magnitude of orientation and alignment of level J" at its fluorescent population J —+ J, when J is excited by the action of weak light causing a transition J" — J (see Fig. 3.14). The magnitude of moments j Pq created on level J may be found from (2.21), hence we know the rate of the generation of moments on level J". The... 

We will now consider the possibility of setting up an experiment to study the relaxation characteristics of level J" by applying fluorescent population (see Fig. 3.14). 

In deriving (3.29) we make use of the fact that ni oc ip . One of the important parameters affecting the magnitude of is the rate of decay Tjijii which is proportional to the Franck-Condon factor for the radiative transition J — J". As a matter of fact, this is where the term Franck-Condon pumping comes from (the term is used occasionally for denoting fluorescent population processes). 

It is simple to arrange the registration of the transient process in the case of fluorescent population considered in Section 3.4. For this purpose it is, for instance, sufficient that the first beam, which causes a transition J" — J (Fig. 3.14) should act in short pulses Tp = GS(t). Then in the time dependence of the fluorescence produced by the continuous probe beam (cycle J — J[ — J f dotted line in Fig. 3.14) we observe the relaxation of the polarization moments of level J" directly. 

Auzinsh, M.P. and Ferber, R.S. (1989). Optical orientation and alignment of high-lying vibrational-rotational levels of diatomic molecules under their fluorescence population, Opt. Spectrosc. (USSR), 66, 158-163. 

Figure Al.6,8 shows the experimental results of Scherer et al of excitation of I2 using pairs of phase locked pulses. By the use of heterodyne detection, those authors were able to measure just the mterference contribution to the total excited-state fluorescence (i.e. the difference in excited-state population from the two units of population which would be prepared if there were no interference). The basic qualitative dependence on time delay and phase is the same as that predicted by the hannonic model significant interference is observed only at multiples of the excited-state vibrational frequency, and the relative phase of the two pulses detennines whether that interference is constructive or destructive.

Greene C H and Zare R N 1983 Determination of product population and alignment using laser-induced fluorescence J. Chem. Rhys. 78 6741 -53... 

Altkorn R and Zare R N 1984 Effects of saturation on laser-induced fluorescence measurements of population and polarization Annual Review of Physical Chemistry ed B S Rabinovitch, J M Schurr and H L Strauss (Palo Alto, CA Annual Reviews)... 

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]. 

This book presents a detailed exposition of angular momentum theory in quantum mechanics, with numerous applications and problems in chemical physics. Of particular relevance to the present section is an elegant and clear discussion of molecular wavefiinctions and the detennination of populations and moments of the rotational state distributions from polarized laser fluorescence excitation experiments. 

A dye molecule has one or more absorption bands in the visible region of the electromagnetic spectrum (approximately 350-700 nm). After absorbing photons, the electronically excited molecules transfer to a more stable (triplet) state, which eventually emits photons (fluoresces) at a longer wavelength (composing three-level system.) The delay allows an inverted population to build up. Sometimes there are more than three levels. For example, the europium complex (Figure 18.15) has a four-level system. 

The population of may also be reduced by absorption of the fluorescence taking the molecule from into S2, if the wavelengths of the two processes correspond, as well as by... 

Measuring Protein Sta.bihty, Protein stabihty is usually measured quantitatively as the difference in free energy between the folded and unfolded states of the protein. These states are most commonly measured using spectroscopic techniques, such as circular dichroic spectroscopy, fluorescence (generally tryptophan fluorescence) spectroscopy, nmr spectroscopy, and absorbance spectroscopy (10). For most monomeric proteins, the two-state model of protein folding can be invoked. This model states that under equihbrium conditions, the vast majority of the protein molecules in a solution exist in either the folded (native) or unfolded (denatured) state. Any kinetic intermediates that might exist on the pathway between folded and unfolded states do not accumulate to any significant extent under equihbrium conditions (39). In other words, under any set of solution conditions, at equihbrium the entire population of protein molecules can be accounted for by the mole fraction of denatured protein, and the mole fraction of native protein,, ie. 

Hydrogen transfer in excited electronic states is being intensively studied with time-resolved spectroscopy. A typical scheme of electronic terms is shown in fig. 46. A vertical optical transition, induced by a picosecond laser pulse, populates the initial well of the excited Si state. The reverse optical transition, observed as the fluorescence band Fj, is accompanied by proton transfer to the second well with lower energy. This transfer is registered as the appearance of another fluorescence band, F2, with a large anti-Stokes shift. The rate constant is inferred from the time dependence of the relative intensities of these bands in dual fluorescence. The experimental data obtained by this method have been reviewed by Barbara et al. [1989]. We only quote the example of hydrogen transfer in the excited state of... 

Filthy conditions, due to rodent or insect population, may be observed in a grain warehouse. The inspector notes rat-chewed flour sacks and sacks contaminated with rat excrement. He removes a sample of sacking and flour from such contaminated areas and submits them to the analyst. Urine fluoresces under ultraviolet light. Where rodent urine is to be confirmed, the xanthydrol test is one of several that may be used. Excreta pellets may be moistened with water or an appropriate clearing solution and crushed for observation under the compound microscope. The presence of striated hair fragments indicates rodent excrement. 

VMAT1 is expressed in the adrenal medulla, by small intensely fluorescent cells in sympathetic ganglia, and by other nonneural cells that release monoamines. In contrast, VMAT2 is expressed by neuronal populations in the nervous system. The substrate specificity for the two isoforms is similar, but VMAT2 has a somewhat higher apparent affinity for all monoamines than VMAT1. In addition, only VMAT2 appears able to transport histamine, consistent with its expression by mast cells. 

Figure 3. Energy diagram for 1064 nm excitation of PuFg(g). The 5f electron states of PuF6 are shown at the left. The solid arrows Indicate photon absorption or emission processes. The wavy arrows indicate nonradiative processes by which excited states of PuF6 are lost. Comparison of observed fluorescence photon yields versus the fluorescence quantum yield expected for the 4550 cm" state indicate that the PuFg state initially populated following 1064 nm excitation may dissociate as shown.

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