Laser spectroscopy fluorescence decay

The vibrational energy levels of the B rio electronic state of I2 were studied by absorption spectroscopy in Exp. 39. In the present experiment, selected vibrational-rotational levels of this state will be populated using a pulsed laser. The fluorescence decay of these levels will be measured to determine the lifetime of excited iodine and to see the effect of fluorescence quenching caused by collisions with unexcited I2 molecules and with other molecules. In addition to giving experience with fast lifetime measurements, the experiment will illustrate a Stem-Volmer plot and the determination of quenching cross-sections for iodine. Student results for different quenching molecules will be pooled and the dependence of the cross sections on the molecular properties of the collision parmers will be compared with predictions of two simple models. 

This agrees quite well with the rate constants for intramolecular proton transfer in 2,4-bis(dimethyl-amino )-6-(2-hydroxy-5-methylphenyl)-5-triazine which had been measured by Shizuka et al. ( l6) using laser picosecond spectroscopy. The fluorescence decay constant t of (TIN) was found to be 60 20 ps. Because of the weak intensity all fluorescence lifetimes refer to the pure substance in crystalline form at room temperature. 

E. Gratton, B. Feddersen, M. vandeVen, Parallel acquisition of fluorescence decay using array detectors, in Time-Resolved Laser Spectroscopy in Biochemistry II (J. R. Lakowicz, ed.), Proc. SPIE 1204, 21-25(1990). 

The energy released as heat in the course of the nonradiative decay of P to the ground state and detected as a pressure wave by laser-induced optoacoustic spectroscopy (LIOAS) exhibits positive deviations (i.e., a> 1 cf. Eq. (1)) from the values which were calculated on the basis of the absorption spectrum of Pr alone (Figure 15) [90,115]. This indicates that already within the 15-ns duration of the excitation flash, one or several intermediates must have been formed. These in turn, within the same interval, may again absorb light from an intense laser flash and (at least in part) dissipate heat upon their return to the ground state of the same species (internal conversion) and/or to Pr (photochemical back reaction). The formation of primary photoproducts within the nanosecond flash duration was of course to be expected in view of the much shorter lifetimes of the photochromic fluorescence decay compo-... 

The resulting glass-ceramics obtained at various experimental conditions consist of a crystalline phase and a residual glassy phase. The nature of the crystalline phase corresponding to different heat treatment and precipitation conditions is determined by X-ray diffraction This together with a detailed spectroscopic study of the steady state fluorescence, absorption, decay dynamics (by means of selective laser spectroscopy) as well as electron paramagnetic resonance reveals the detailed nature of the crystalline phases. 

Direct measurement of the reaction of interest is sometimes possible using rapid reaction techniques. In laser flash photolysis, an intense, short-lived pulse of light irradiates the sample and the products are monitored by a variety of techniques, from basic UV/Vis spectroscopy to techniques - such as laser-excited fluorescence -which require a second, analytical pulse of radiation. In pulse radiolysis, a short (1-10 ns) pulse of high-energy (1-10 MeV) electrons irradiates the sample and the decay of the fragments can be analysed in the same way as the fragments from flash photolysis. The equipment for pulse radiolysis is even more complex and costly than that for flash photolysis, and tends to be concentrated in national facilities. 

Time-resolved fluorescence spectroscopy is also a valuable tool in biological and medical research [10.143-147]. Since the lifetimes involved are normally short, picosecond spectroscopy techniques are frequently employed (Sect. 9.4). Examples of fluorescence decay curves for tissue recorded with delayed coincidence techniques employing a frequency-doubled picosecond dye laser are depicted in Fig. 10.42. The decay characteristics allow the discrimination between tumour and normal tissue, and atherosclerotic plaque and normal vessel wall, respectively. General surveys of the use of LIF for medical diagnostics can be found in [10.148,149]. 

In the higher atmosphere the aerosol density decreases rapidly with altitude and other detection schemes may become more advantageous. Raman spectroscopy or detection of laser-induced fluorescence excited by frequency-doubled pulsed lasers has been utilized [14.22]. Both Raman and fluorescence intensities excited by the laser at a location x are proportional to the density n. (x) of scattering particles. However, because of the high pressure (p latm) the fluorescence is quenched if the collisional deactivation na v becomes faster than the spontaneous decay A. = 1/t. (see Sect. 12.2). Transition probabilities and quenching cross sections must therefore be known if quantitative results are to be obtained from measurements of the fluorescence intensity. 

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