Prions in SrAl

12

O

19

occupy high

c

oordination sites

hence experience a weaker crystal field environment.The 4f

2

1

S

0

level lies below the 4f5d configuration

photon cascade emission has been reported.Photon cascade emission process also provides a unique method for directly measuring the quantum efficiency (QE)of

3

P

0

in this material.Since the separations between the Stark levels of

1

I

6

3

P

1

and subsequently

3

P

0

are small

it is reasonable to assume that the depopulation of

1

I

6

is dominated by multiphonon relaxation.All the electrons from

1

S

0

down to

1

I

6

would relax to

3

P

0

afterwards.The

1

S

0

→

1

I

6

fluorescence intensity can be taken as a measure of the initial

3

P

0

population.Thus the QE of

3

P

0

can be written as η(

3

P

0

)=photon number of photons emitted from

3

P

0

number of electrons arrived at

3

P

0

=intensity of all the emission lines from

3

P

0

intensity of the emission

1

S

0

→

1

I

6

=I(

3

P

0

- 3H

4

)βI(

1

S

0

-

1

I

6

)=Rβ

where β is the branching ratio of the

3

P

0

→3H

4

transition.The emission spectrum within 370～520nm of the 1% sample is shown in Fig 4.The excitation wavelength being used is 205nm.The ratio of the

3

P

0

→3H

4

and the

1

S

0

→

1

I

6

emission intensities

R

can be obtained from the spectrum.QE can then be derived from η=R/β.In Table 1 we listed the measured QE of some samples with different Pr concentrations or grown from different raw materials.For comparison

we also listed the result measured in CaAl

12

O

19

:Pr

Mg.As it is shown

QE decreases with increasing of Pr concentration.This reflects the effect of concentration quenching.It should also be noted that even in dilute sample

QE is still remarkably less than 100%.This might be the result of thermal excitation of the

1

I

6

and

3

P

1

states.In fact

in the spectrum measured at room temperature

we do observe

3

P

1

→3H

5

emission peaked at 525nm (B in Fig.1).Besides

there are also other lines (b

c in Fig.5) appear as the temperature rises from 77K to room temperature

which may be attributed to the emissions from

3

P

1

or

1

I

6

.According to the Stark levels given in Ref.

at room temperature

approximately 32% of the electrons would be at

1

I

6

while 7% at

3

P

1

.And it is estimated by Judd Ofelt analysis

comparably great part of the

1

I

6

emission intensities falls into infrared.