Picosecond non-radiative relaxation via internal conversion in Ca10(PO4)F2:Cr

We use a femtosecond pump-probe technique to directly measure fast nonradiative relaxation following excitation of the 3A 23T1 transition in Ca10(PO4)6 F2(CFAP) doped with tetravalent chromium. We measure a 3T1 relaxation time of 137 ps at 10 K which decreases to 37 ps at room temperature through internal conversion directly to the 3A2 ground state. We find rates for quantum mechanical tunnelling and phonon assisted barrier hopping of 0 = 7.9 109 Hz and W1 = 3.4 1011 Hz respectively, with a barrier height of 630 cm-1.