Rare earth ions doped inorganic materials have been extensively applied to many fields

such as display techniques

laser medium

and fiber communications. Trivalent thulium (Tm

3+

) in different hosts exhibits interesting optic and magnetic properties that are amenable to convenient experimental and theoretical studies. The fluoride compound doped with Tm

3+

has been identified as an excellent candidate of laser material

due to its high efficiency

narrow line widths

and unusually large transition cross sections. In addition

the crystal structure of the fluoride compound permits greater doping concentrations of larger rare earth ions than other widely used host materials such as yttrium aluminum garnet and yttrium scandium garnet. For many applications

it is important to know the detailed energy level scheme of RE

3+

in the appropriate host matrix. However

well-established investigations of the energy level scheme of Tm

3+

ion in various host matrices are rather scarce. Detailed characterization of the interactions effective in the formation of the energy level scheme of the Tm

3+

ion had been carried out mostly for the simple fluorides and oxide matrices. In addition

the maximum degeneracy of 91

which is rather low for an RE

3+

ion

can be treated theoretically and by computational means easily as a whole without any truncations. In this paper the experimental results of absorption and emission spectra of K

2

YF

5

:Tm

3+

at 12K were reported. The optical data were interpreted in terms of the

C

2v

point symmetry of the Tm

3+

site in the K

2

YF

5

matrix using the multipolar and group theoretical selection rules for electronic transitions. The Stark energy levels of ground state

3

H

6

and excited states

1

D

2

1

G

4

3

F

2

3

F

3

and

3

F

4

were assigned clearly. The experimental energy levels are compared with those of calculated by a phenomenological model

in which the Hamiltonian includes the electrostatic

spin-orbit interactions and crystal field interactions. The simulated treatment of the free ion and crystal field interactions within 4f

12

electron configuration yielded a satisfactory agreement with the experimental values of 32 Stark components

with a quite small root mean square deviation of 18cm

-1

. In older to acquire the information of crystal field effect

the parameters

S

and

S

k

were introduced.In addition

decay curves of

1

D

2

and

1

G

4

emissions in the samples with different Tm

3+

doping friction were discussed. Cross relaxation mechanisms have been used to explain the variations of the lifetimes.