Excitation spectra correction factors

Montalti M, Credl A, Prod L, Gandolfi MT (2006), Handbook of photochemistry, 3rd edn. CRC Press, Boca Raton. An essential reference book containing data tables for a wide range of compounds, and a variety of reference materials including quantum yields, lifetimes, quenching rate constants, electrochemical potentials and solvent properties as well as information on standard procedures used in chemical actinometry, determination of emission and excitation spectra correction factors, and quantum yield measurements and also information on equipment such as lamps and filters. 

In order to minimize the effects of possible inaccuracy of the correction factors for the emission spectrum, the standard is preferably chosen to be excitable at the same wavelength as the compound, and with a fluorescence spectrum covering a similar wavelength range. 

The absolute excitation spectrum is then identical to the absorption spectrum. The correction factor can again be stored in a computer memory and used to multiply other instrumental excitation spectra (so long as the lamp is not changed). 

The fluorescence spectrum (emission spectrum) of a sample is obtained by scanning M2 while keeping the excitation wavelength constant. Since the efficiency of M2 is wavelength dependent, and the detector (see below) also has a sensitivity that varies with wavelength, such a fluorescence spectrum is normally uncorrected , i.e., it will depend upon the instrument as well as the sample. The correction of emission spectra is less easy to achieve than for excitation spectra, and is less often performed, so published emission spectra often vary from instrument to instrument. At least three methods are available for emission correction. A sound but tedious method is to calibrate the emission system (i.e., M2 plus the detector) with a standard light source of known emission profile. Such devices are available from NIST and other standards bodies. Comparison of the output of the fluorescence spectrometer with the certified output of the lamp then provides a correction factor at each wavelength, which can be applied to subsequent sample spectra. A related technique is to... 

The fluorescent components are denoted by I (intensity) followed by a capitalized subscript (D, A or s, for respectively Donors, Acceptors, or Donor/ Acceptor FRET pairs) to indicate the particular population of molecules responsible for emission of/and a lower-case superscript (d or, s) that indicates the detection channel (or filter cube). For example, / denotes the intensity of the donors as detected in the donor channel and reads as Intensity of donors in the donor channel, etc. Similarly, properties of molecules (number of molecules, N quantum yield, Q) are specified with capitalized subscript and properties of channels (laser intensity, gain, g) are specified with lowercase superscript. Factors that depend on both molecular species and on detection channel (excitation efficiency, s fraction of the emission spectrum detected in a channel, F) are indexed with both. Note that for all factorized symbols it is assumed that we work in the linear (excitation-fluorescence) regime with negligible donor or acceptor saturation or triplet states. In case such conditions are not met, the FRET estimation will not be correct. See Chap. 12 (FRET calculator) for more details. 

Fig. 22. 10 K absorption spectrum of 5% Mo + Cs2NaYbCl6. The absorption features are labeled according to the chromophore, Yb + or Mo +. 10 K luminescence of this sample with 10,826 cm Yb + excitation. This spectrum has been corrected for the instrument response. Note that the Yb + luminescence is magnified hy a factor of 1000, showing nearly quantitative Yb3+ Mo + ET...

A plot of the photocurrent quantum yield versus excitation wavelength is termed the photoaction or photocurrent action spectrum. These spectra are obtained at short-circuit in a two-electrode arrangement or with an external bias in a three-electrode configuration. The photocurrent quantum yield is defined as the number of electrons measured in the external circuit divided by the number of absorbed photons. It is experimentally difficult to calculate the number of absorbed photons and corrections for scattered or transmitted light often appear to be fudge factors that increase the uncertainty of the absolute photocurrent yield. Therefore, the incident photocurrent yield is often reported which represents a lower limit of the true photocurrent quantum yield. 

Principally, the same phenomenon is observed with the somewhat larger particles (8.0 nm, solution e). In this case, the nonactivated colloid shows a detectable fluorescence (spectrum e. Figure 15). This fluorescence is enhanced by a factor of 5 by the addition of 1 x 10 M Cd and the raising of the pH to 11.5 (spectrum ea. Figure 15). The intensities of the fluorescence spectra shown in Figure 15 are comparable with each other since they have been corrected for the absorbance of the respective solutions at the excitation wavelength. In all cases a single fluorescence band is observed which is almost symmetrical and fairly narrow. Its maximum is located close to the onset of absorption. 

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