  We present a direct experimental evidence of photo-ionized CdTe quantum dot
core having luminescence "off" state at band-edge photo-excitation. We use the
autocorrelation of fluorescence fluctuations from dilute colloidal solution of
CdTe quantum dots in a diffraction limited detection volume to determine their
photo-ionization probability. The ionization probability is in the same order
of magnitude (\approx10-6) as the CdSe and CdSe/ZnS core/shell quantum dots.
Further, we measured the ionization/ neutralization rates (\approx104 s-1)
after addition of {\beta}-mercaptanol and found that ionization rate is
enhanced by two orders of magnitude after the addition. However, a comparable
neutralization rate enhances the light emission quantum yield. The ionization/
neutralization at microsecond time scales puts an upper limit on achieving
better quantum yield by thiol addition. The results also indicate that the
dominant ionization mechanism at band-edge excitation is tunneling.
