Fluorescence pulse-method

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. 

The fluorescent lifetime of chlorophyll in vivo was first measured in 1957, independently by Brody and Rabinowitch (62) using pulse methods, and by Dmitrievskyand co-workers (63) using phase modulation methods. Because the measured quantum yield was lower than that predicted from the measured lifetime, it was concluded that much of the chlorophyll molecule was non-fluorescent, suggesting that energy transfer mechanisms were the means of moving absorbed energy to reactive parts of the molecule. 

Pulsed method. Using a pulsed or modulated excitation light source instead of constant illumination allows investigation of the time dependence of emission polarization. In the case of pulsed excitation, the measured quantity is the time decay of fluorescent emission polarized parallel and perpendicular to the excitation plane of polarization. Emitted light polarized parallel to the excitation plane decays faster than the excited state lifetime because the molecule is rotating its emission dipole away from the polarization plane of measurement. Emitted light polarized perpendicular to the excitation plane decays more slowly because the emission dipole moment is rotating towards the plane of measurement. 

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. 

The second method uses pulsed lasers and the laser-induced fluorescence is detected by telescope. If the telescope and the laser source have a definite base distance, the crossing of laser beam and the acceptance angle of the telescope define the height of the atmospheric layer at which fluorescence is detected. There is also the technique of delayed coincidence, where the time interval between laser pulse and detected fluorescence pulse determines the distance of the observed molecules from the observer (Lidar)... 

Applications making use of the nonlinear absorption of dyes are passive Q-switching in solid-state lasers, pulse shaping, pulse intensity measurements of high-power ultrashort pulses, optical isolation between amplifier stages of high power solid-state lasers, and pulse width measurements of ultrashort pulses by the two-photon-fluorescence (TPF) method. 

Fig. 8.8 Optical images (at an active zone I) of isolated silver clusters electrodeposited onto ITO by means of the double-pulse method ( i = —1,550 mV 2 = —VOO mV, 25 s) [37] (a) Scanning confocal microscopy image (topography mode), (b) Raman/fluorescence image of the same sample area, (c) SEM image corresponding to (a) and (b)...

The chemical composition of the catalysts was quantified by x-ray fluorescence analysis. B. E. T. surface area was measured by the B. E. T. one point method using nitrogen gas, and the amount of CO adsorption was measured by the pulse method. These analyses were carried out for the ground catalysts containing cordierite. The deviation of the results due to the uniformity of the thickness of the wash coat layer was around 5%, when 5g of fresh catalyst sample was used for each. The distribution of elements was measured by line analysis using EPMA (Shimazu EPMA 8705). 

U. Schreiber, T. Endo, H. Mi, K. Asada (1995). Quenching analysis of chlorophyll fluorescence by the saturation pulse method Particular aspects relating to the study of eukaryotic algae and cyanobacteria. Plant Cell Physiol, 36, 873-882. 

Cu/S102 catalysts were prepared by the ion-exchange technique [13,14], A 6.44 vt% Cu/Si02 catalyst was obtained from an aqueous solution of 0,2 mol Cu(N03)2/l at pH 11. Cu/MgO and Cu/-f-Al20a catalysts were prepared by Impregnation (incipient wetness technique). The catalysts were dried at 120 C for 12 hours, calcined at 400 °C for 12 hours and finally sieved (250 - 400 pm). The Cu loading was measured by X-ray fluorescence (XRF). The Cu surface area was measured by the decomposition of N20 at 90 °C (pulse method), assuming a stoichiometric factor (N20/Cu) of 0.5 and a specific atom density of 1.46 x 10 1 toma/mz 115). The total surface area (BET) was determined by N2 adsorption at - 195 G [16]. Table 1 shows a summary of the catalyst properties. 

The phenomena discussed above can be studied using various techniques, and not all methods are equally suitable in a particular case. Two principally different kinds of methods for measuring fluorescence lifetimes exist, namely, pulse methods and modulation or phase-shift methods. Phase-shift methods, despite the fact that they have been known for a longer time, have not found widespread use during the last decade. However, important technical advances have been made in phase-shift methods which in fact have inspired many researchers to apply them more frequently. Nevertheless, pulse methods are still the most widely used today, in particular for high time resolution. If carried out properly both types of methods must and will give the same result. Details of the measuring problem will determine which method is more appropriate in a particular case. 

In contrast to pulse methods described above, the phase-shift technique usually employs a continuous light source whose intensity is modulated by various means at some frequency /. The fluorescence response of the system is then also modulated at that frequency, albeit with some phase delay 0 and a reduced modulation depth m, as compared to the exciting light. "" From either of these quantities the fluorescence lifetime can be extracted. For a single-exponential decay the relationship between lifetime t, the modulation frequency /, phase shift 0, and the modulation depth m are given by tan(0) = /t and m = (1 -t-... 

The repetition rate / of the excitation pulses is chosen as high as possible since the measuring time for a given signal-to-noise-ratio is proportional to 1//. An upper limit for / is determined by the fact that the time T = /f between two successive laser pulses should be at least three times the lifetime r of the measured level A ). This technique is therefore ideally suited for excitation with mode-locked or cavity-dumped lasers. There is, however, an electronic bottleneck the input pulse rate of a TAC is limited by its dead time tq and should be smaller than 1 /td. It is therefore advantageous to invert the functions of the start and stop pulses. The fluorescence pulses (which have a much smaller rate than the excitation pulses) now act as start pulses and the next laser pulse stops the TAC. This means that the time (T — t) is measured instead of t. Since the time T between successive pulses of a mode-locked laser is very stable and can be accurately determined from the mode-locking frequency / = 1 / T, the time interval between successive pulses can be used for time calibration of the detection system [656]. In Fig. 6.91 the whole detection system is shown together with a decay curve of an excited level of the Na2 molecule, measured over a period of 10 min. More information about the delayed-coincidence method can be found in [798]. 

The widely used continuous instrumental method for determination of SO2 is based upon gas-phase fluorescence. Pulsed ultraviolet (UV) light (214 nm) is used to irradiate the air sample, which flows continuously through an optical cell. The SO2 reemits fluorescent radiation at 340 nm that is detected by a photomultiplier tube (PM), with the signal obtained being converted to concentration. This method is almost specific with a detection limit of 1 pgm in commercially available analyzers. 

Time-resolved fluorescence is perhaps the most direct experunent in the ultrafast spectroscopist s palette. Because only one laser pulse interacts with the sample, the mediod is essentially free of the problems with field-matter time orderings that arise in all of the subsequently discussed multipulse methods. The signal... 

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]. 

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