Fluorescence action spectrum

The most widely employed optical method for the study of chemical reaction dynamics has been laser-induced fluorescence. This detection scheme is schematically illustrated in the left-hand side of figure B2.3.8. A tunable laser is scanned tlnough an electronic band system of the molecule, while the fluorescence emission is detected. This maps out an action spectrum that can be used to detemiine the relative concentrations of the various vibration-rotation levels of the molecule. 

Figure 13. Action spectrum of the linear He I Cl complex near the He + I Cl(By = 2) dissociation limit obtained by scanning the excitation laser through the ICl B—X, 2-0 region and monitoring the l Cl E—>X fluorescence induced by the temporally delayed probe laser, which was fixed on the l Cl E—B, 11-2 band head, (a). The transition energy is plotted relative to the I Cl B—X, 2-0 band origin, 17,664.08 cm . Panels (b), (c), and (d) are the rotational product state spectra obtained when fixing the excitation laser on the lines denoted with the corresponding panel letter. The probe laser was scanned through the ICl B—X, 11-2 region. Modified with permission from Ref. [51].

EMISSION SPECTRUM ACTION SPECTRUM EXCITATION SPECTRUM FLUORESCENCE Empirical rate equations,... 

Measurements of the intensity of fluorescence at any wavelength vs the wavelength of monochromatic light used to excite the fluorescence give a fluorescence excitation spectrum. The excitation spectrum is an example of an action spectrum, which is a measure of any response to absorbed light. At very low concentrations of pure substances, action spectra tend to be identical to absorption spectra. However, since the observed response (fluorescence in this case) is proportional to light absorbed, action spectra should be compared to plots of 1-T (where T = transmittance, Section B,l) vs wavelength rather than to plots of e vs X. The two plots are proportional at low concentrations. For a discussion of action spectra see Clayton.123... 

Figure 4-1. (a) Fluorescence excitation spectrum of the Hg-H2 complex, (b) Action spectrum of the Hg-H2 complex the pump laser is varied while the probe laser sits on the band head of the 0-0 transition in Hg H(2n1/2 <- 2E+). In the 2=1 domain extending into the continuum seen in (a), one sees vibrational bands—the reaction is then slow for the fl = 0 excitation (in the red), no structure appears—indicating a fast reaction. 

If a technique is to exploit differences in pigmentation, it needs prior measurements of the light-absorbing characteristics of the algae. Hence, the action spectrum for fluorescence of chlorophyll a is a tool for identifying the color types of algae (i). 

Hg-H2 fluorescence excitation spectra revealed only an absorption continuum having a sharp onset 40cm to the blue of the Hg line and decreasing in intensity toward shorter wavelengths, as shown in Figure 52. This emission was ascribed to free Hg( Pi), produced by exciting the complex to the B1 state at energies above the dissociation limit, since the Hg( Pi) action spectrum of the complex is identical to the fluorescence excitation spectra. Thus, in contrast to Ne, no Hg-H2 fluorescence was observed. This indicates that a nonradiative process occurs more efficiently than radiative decay, whose rate is lO s ... 

Figure 58. Ca-HCl action spectrum, monitoring CaCl - fluorescence ( 620nm), while scanning the pump laser in the vicinity of the Ca line. The bar graphs indicate the calculated areas of the corresponding absorption features (see text for details).

This effect is known variously as self-absorption, thickness effect or overabsorption. An analogous effect in photochemistry is known as saturation of the action spectrum. The first term is somewhat of a misnomer because it suggests that the problem has to do with re-absorption of the fluorescence radiation, by analogy with certain effects in optical spectroscopy. Actually, the absorption of the fluorescence is independent of the incident energy, hence does not contribute to any non-linearity. What is important is that the penetration depth for the incident radiation depends on the quantity one wants to measure. 

On extended irradiation, however, all wools will invariably yellow at about the same rate [67]. Leimox et al. reported an action spectrum for yellowing of wool with and without a fluorescent brightener (1% of a bistriazinyldiaminostilbene) [68]. From their reported data, an action spectrum of the following form can be derived ... 

However, this decay is J dependent, the total angular momentum increasing the predissociation rate, thus we would have to compare rotationally averaged values of the rate. The J dependence is not only seen in decay rates but also in the inspection of the action spectrum in figure 1 which shows much broader bands that the fluorescence excitation spectra, owing to the high J lines in the action spectrum. 

In fig. 2, we have represented in the top view the action spectrum to Pq and in the lower the fluorescence induction spectrum. The lower spectrum (LIF) reveals mainly a progression of stretching bands weakly appearing in the upper spectrum and assigned to an electronic state A. On the other hand the upper spectrum reveals another electronic state B and also transitions due to combination bands constructed on the A spectrum. 

The excimer XeBr(B,Q fluorescence was obtained by optical-optical double excitation of the complex Xe-Br2 (excitation of the B state of Br2 by the first harmonic of the laser and followed by the excitation of the valence state by the second harmonic [12]). The action spectrum, excitation of the complex and observation of the XeBr(B->X) fluorescence, is structureless and starts at the energetical threshold. The fluorescence spectrum ob ned with an excess of energy of 3800 cm is characteristic of a cold XeBr(B,Q, and presents a small emission assigned to the Br2(D - A0 transition. As the reaction occurs at the energetical threshold, then no barrier to the reaction was found. However, when the Rydberg states of the Br2 are excited no fluorescence is observed (direa two UV photon absorption). Probably... 

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