Trapping centers and transfer process of electrons and holes in infrared stimulable phosphors

X

S:

Ra

Sm (

X

=Sr

Ca;

Ra

=Ce

Eu) were investigated in the photo-stimulated luminescence. The difference of infrared absorption spectra before and after excitation exhibits the same shape as the infrared stimulation spectra

with several notches cut by the absorption lines of Sm

3+

4f-4f transition on the broad band. This suggests that each of the electron-trapping centers is adjacent to a Sm

3+

ion

and thus the fact that Sm ions keep in +3 valence without being changed into Sm

2+

in the trapped state. Further more

electron paramagnetic resonance measurements show that the EPR absorption from Eu

2+

in SrS:Eu

Sm remains unchanged after excitation in comparison with the spectrum before excitation. It can be speculated that even the Eu

2+

ions are not changed into Eu

3+

during the trapping process. Different from the covalent Ⅲ-Ⅴ semiconductors

in the ionic crystals such as alkali-earth sulfides

the carriers are trapped by the defects instead of by the doped impurity ions. In the case of several kinds of luminescent center

the stimulated luminescence is dependent on the preceding excitation wavelength that determined which centers being excited and thereby binding the holes left by the excitation of electrons into the conduction band.