POPULATION AND DEACTIVATION OF LOWEST LYING BARIUM LEVELS BY COLLISIONS WITH HE, AR, XE AND PA GROUND-STATE ATOMS

Excitation transfer between the barium low lying excited states 6s6p ( 3)p(1)(0), 6s5d D-1(2) and 6s5D D-3(J) by collisions with He,Ar,Xe and Ba has been investigated. The population densities in all levels invo lved were probed by absorption or by fluorescence using cw lasers. The depopulation cross sections of the Ba P-3(1)0 State by collisions wit h noble gases were found to be sigma(He)(P-3(1)0) = 5.5 . 10(-16) cm(2 ), sigma(Ar)(3P(1)(0)) = 4.6 . 10(-16) cm(2), and sigma(Xe)(P-3(1)0) = 1.7 . 10(-16) cm(2). For Ar, the collisional depopulation of the P-3( 1)0 level is exclusively due to the transition to the D-1(2) state. Un der the assumption that the D-3(J) metastable states are populated col lisionally by D-1(2) --> D-3(J) transfer only, we have deduced the upp er limit for the corresponding cross section sigma(13)(Ar) = 1.5 . 10( -18) cm(2). From the Ba D-1(2) and Ba D-3(J) steady-state diffusion di stributions, collisional relaxation rates of the D-1(2) and D-3(J) lev els were evaluated. The collisional relaxation rates by Ar and Ba yiel ded total cross sections for the depopulation of metastable levels: si gma(Ar)(D-1(2)) = 1.5 . 10(-17) cm(2), sigma(Ba)(D-1(2)) congruent to 1 . 10(-13) cm(2), sigma(Ar)(D-3(J)) = 7 . 10(-21) cm(2), and sigma(Ba )(D-3(J)) = 1 . 10(-15) cm(2). Furthermore, it was found that the main contribution of the collisional depopulation of the D-1(2) state by A r is related to back transfer to the P-3(J)0 state, whereas the deacti vation of the D-3(J) metastable state is due to back transfer to the D -1(2) state. Taking into account other cross sections reported in lite rature we can conclude that collisional deactivation of both metastabl e levels by Ba ground state atoms can be attributed to their mutual co llisional mixing.