CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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2.3. Fluorescence Spectroscopy
Fluorescence spectroscopy is a commonly used technique in characterizing different
samples. This technique exploits the emission from a species on optical excitation. The
emission peak position, width contains valuable information on the sample [2.4, 2.5]. For the
present thesis we have used a Hitachi fluorimeter F-4010 model which employed 150W high
pressure xenon lamp as excitation source, different monochromators for excitation and
emission wavelengths and photomultiplier tube (PMT) as detector. For spectral range 280-
650nm, IP-28 (Hamamatsu) and for range 400-800nm, R-928F (Hamamatsu) PMT was used.
In fluorescence spectroscopy, molecule of interest is excited into a higher electronic level
with a particular wavelength of light and radiative relaxation of the molecule is recorded in a
wavelength range. The emission generally occurs from a vibrationally relaxed configuration
of excited electronic state, therefore is generally red shifted with respect to absorption spectra
of that particular transition. The shift between absorption and emission maxima is referred to
as Stoke shift. Stoke shift can be due to vibrational relaxation, intramolecular charge transfer
etc. In the case of semiconductors radiative relaxation can take place from band edge also
called band edge emission or it takes place from a trap which is called a trap state emission.
Generally trap state emission band is broad due to energetic heterogeneity arising and
distribution of bonding configurations. Apart from fluorescence one can also monitor what is
called excitation spectra. In excitation spectra, emission wavelength is held constant and
excitation wavelength (till position of emission wavelength) is varied. This gives rise to a
plot of emission intensity versus excitation wavelength.
One of the most important applications of fluorescence spectroscopy is in biolabeling,
therefore a high quantum yield is a desirable trait for such applications. Therefore