Broad-band excitation

Time-domain waveforms generated from a constant-phase Initial spectrum by the SWIFT procedure just described have been successfully applied to broad-band excitation, windowed excitation, and multiple-ion monitoring (11), and to mult1ple-1on ejection for enhancement of FT/ICR dynamic range (21) as described below. 

From our consideration of the broad band excitation of ME spectra and its subsequent observation of the difference frequency as a quantum beat, the generalization to a more complex ME spectrum is obvious ... 

In the above diagram, the emission intensities of the three bands remain constant. It is the peak Intensity which changes as the band broadens. To this point, we have aceepted the fact that vibronic coupling leads to broad band excitation and emission in a phosphor. Take note that the above diagram is the result of experimental measurements which prove that as the temperature increases, the phonon spectrum becomes broadened, thereby leading to broadening of the bands. Thus, at - 200 °C., the number of phonon vibrations is restricted and a rather sharp emission band is seen. As the temperature rises, the number of separate phonon branches increases (the empty phonon levels become occupied) and the emission (excitation) band is further broadened. Note that at some temperature above 300 °C. (in this example), the phonon vibrations increase to the... 

Julian, Jr, R.K. Cooks, R.G. Broad-band excitation in the quadmpole ion trap mass spectrometer using shaped pulses created with the inverse Fourier transform. Anal. Chem. 1993, 58, 1827-1833 Soni, M.H. Cooks, R.G. Selective injection and isolation of ions in quadmpole ion trap mass spectrometry using notehed waveforms created using the inverse Fourier transform. Anal. Chem. 1994,66, 2488-2496. 

If now the period T of the pulses changes and becomes smaller or greater than the atomic period T] 2> it produces dephasing between the various atomic responses S (t) and blurring of their summation the total fluorescence intensity, in stationary regime, is modulated with a smaller amplitude. The amplitude of the fluorescence modulation passes through a maximum when the excitation frequency a> becomes equal to the atomic frequency 12 practice, it is not necessary to use a pulse excitation it is necessary to use a broad band excitation (Aa)exc 12 and to interrupt periodically the excitation light 13. ... 

The solution to this problem appears to lie in the use of narrow-band optical excitation using dye lasers (Szabo, 1970). Several workers have shown the power of optical-site selection spectroscopy (Eberly et al., 1974 Abram et al, 1974 Dinse et al, 1976,1978). The resulting emission spectrum is often dramatically sharper than that obtained with broad band excitation. Using this method, Dinse et al (1976,1978) have observed sharp ( 1 MHz) zf ODMR transitions of several triplets in different organic glasses. It was also indicated that insofar as ODMR linewidths are concerned, very narrow band selective detection ( 1 cm ) is equivalent to broadband detection combined with selection laser excitation. 

Fig. 12. Decay of the CO fluorescence from A jt p = o level (a) broad-band excitation, (b) and (c) selective excitation of rotational levels with essentially singlet and essentially triplet character (M. Lavollee and A. Tramer, unpublished).

When several metal ion sites are present in a compound, population analysis can be carried out in two main ways. For Eu , since the intensity of the MD transition Do —> Fi is independent of the metal-ion environment, a spectral decomposition of the transition recorded under broad band excitation into its components measured imder selective laser excitation, followed by integration yields the population Pi of each site [31]. More generally, one can rely on lifetime measurement, since the luminescence decay will be a multi-exponential function which may be analyzed, for instance, with Origin , using the following equations ... 

Transition Probabilities with Broad-Band Excitation... 

Fig. 7.2 (a) The emission spectra obtained at room temperature under broad band excitation at 274 nm (b) trichromatic diagram displaying the position of the studied samples in function of their composition and the excitation wavelength (gray, 274 mn violet, 285 nm blue, 334 nm green, 344 nm yellow, 353 nm orange, 361 nm red, 393 nm) (Reprinted with the permission from Ref [22], Copyright 2004 Amtaican Chemical Society)... 

The fluorescence spectrum emitted from selectively excited molecular levels of a diatomic molecule is therefore very simple compared with a spectrum obtained under broad-band excitation. Figure 6.34 illustrates this by two fluorescence spectra of the Naj-molecule, excited by two different argon laser lines. While the A = 488 nm line excites a positive A component in the (v" = 3, J = 43) level which emits only Q lines, the A = 476.5 nm line populates the negative A component in the (v =6, T=27) level of the state, resulting in P and R lines. 

Selective excitation with a laser tuned to a transition Eq - E. solves the cascade problem [11.19]. In this variant many excited levels of the atoms or ions are populated by broad-band excitation via collisions with target gas atoms in a differentialy pumped gas cell (Fig.11.18c). A few cm behind the exit aperture of the gas cell a laser crosses the ion beam. If the laser frequency is tuned to transition Ej - E. the populations of both levels are changed, due to optical pumping, by an amount AN which depends on the laser intensity, the transition probability A. j, and the initial population of the two levels. The laser intensity can be chopped and the fluorescence intensity Ip- (E -> Ej ) is observed as a function of x alternatively with (I ) or without (I2) laser excitation. The difference... 

Early studies lacked both the sensitivity and wavelength specificity required to fully characterize aquatic organic matter (AOM) fluorescence. Broad-band excitation and emission provided increased sensitivity to measure fluorescence intensity at low concentrations, but without wavelength resolution to determine peak positions. Smart et al. (1976) cited several literature values for excitation and emission maxima, however, results of early studies must be examined carefully for inaccuracies due to instrument biases, including errors in peak positions as well as in the number of peaks present in a sample. Many of these errors were not recognized in the 1970s and 1980s, but are the subject of detailed discussion in Chapter 5 of this volume (see also Holbrook et al., 2006). 

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