Excited state bandpass

Figure 1 A schematic representation of a fairly advanced single-molecule setup. BS, beam splitter (nonpolatizing, polarizing, or a dichroic mirror) PD, fast photodiode POL, polarizer BE, beam expander BP, bandpass filter WP, quarter waveplate or Berek compensator EF, emission filter. If a dichroic mirror is used to split the emission between the APDs, then additional filters are usually placed in front of each APD to prevent leakage of the emission into the incorrect channel. The photodiode is only used in combination with pulsed excitation in the determination of the excited-state lifetime.

In practice, the steady-state fluorescence intensity If(7f) measured at wavelength AE (selected by a monochromator with a certain wavelength bandpass AAF) is proportional to F (/.f) and to the number of photons absorbed at the excitation wavelength AE (selected by a monochromator). It is convenient to replace this number of photons by the absorbed intensity 1a( e), defined as the difference between the intensity of the incident light 10( e) and the intensity of the transmitted light Jt( e) ... 

Figure 7. Collection of emission-wavelength-dependent steady-state excitation scans for 100 pM pyrene in sub- and supercritical C02. For the monomer and excimer scans, the emission wavelength (16 nm bandpass) is adjusted to 380 and 460 nm, respectively.

From Fig. 4 it can be seen that, for finite bandpass detection, one will obtain different fluorescent intensities per emitting molecule depending on the level pumped. This can produce systematic errors in both the determination of absolute concentrations and the use of excitation scans to obtain ground state rotational temperatures (21) Also, the lack of a thermal distribution imposes restrictions on models of and data analysis in optical saturation techniques. 

Flame photometric detector (FPD) is a representative of optical detectors for gas chromatography. In FPD, column effluent is introduced to a hydrogen flame, which breaks analyte molecules into atoms. The temperature of the flame is sufficient to excite some atoms, especially sulphur and phosphorus. These excited atoms emit characteristic lights on return to the ground state. The light emitted by the element of interest is selected by a suitable bandpass filter and measured... 

The experimental problems associated with SVL fluorescence are presently severe, and obtaining exciting radiation that is simultaneously tunable, monochromatic, and intense provides the principal challenge. Short-arc xenon arcs coupled with a monochromator have been used with benzene where an exciting bandpass of 15-45 cm has been sufficiently monochromatic to yield SVL fluorescence from a number of states. Narrower bandpasses are needed to extend the list of available levels in the Ba state and to extend this type of experiment to other systems where spectral congestion in absorption is more severe. It is certain that this will be among the important applications of tunable lasers in the near future. 

Absorption measurements were carried out using Hitachi 150-20 or Varian Cary Bio50 absorption spectrometers. Steady-state fluorescence measurements were recorded using Hitachi 4010,Varian Eclipse or Spex Fluorolog-2 spectrofluorimeters. Steady-state fluorescence polarization spectra were recorded using a Hitachi 4500 spectrofluorimeter equipped with a fluorescence polarization accessory, and using excitation and emission bandpass values of 3 nm. 

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