Figure 11 Absorption and emission spectra of 2-(4-styrylphenyl)benzoxazole. Absorption spectrum (solid green line) was registered with Hitachi U-2900 Spectrophotometer, and emission spectrum (red dashed line) by Hitachi Fluorescence Spectrophotometer F 7000, the plot was corrected by the detector sensitivity. The energy absorbed and emitted by particular dyes is associated with electron
transitions to different vibrational states. According to Fig. 4, the energy of excited
electrons is lost in the transitions from S
1
vibrational states to S
1
basic state and from S
1
ground level to S
0
. The energy emitted in these processes is less than the energy which was
absorbed during electron excitation from S
0
to S
1
. As a result, spectra of absorption and
emission are shifted relatively to each other what reduces re-absorption. This phenomenon
is based on Stokes law, which implies that the wavelength of emission was greater than the
wavelength of absorption. The difference between maxima of absorption and emission
peak is identified as Stokes shift. The larger the Stokes shift is, the smaller probability of
re-absorption of scintillating light in the material. Re-absorption is undesired because it
shortens the attenuation length. That is why substances with large Stokes shift are valuable
scintillator dopants [33] [38].
In Fig. 12 absorption and emission spectra of POPOP (1,4-bis(5-phenyloxazol-2-
yl)benzene) which is widely used wavelength shifter in plastic scintillators are shown. The
value of the Stokes shift which is calculated as the difference between maxima of
absorption and emission, is equal to about 50 nm.
