Population and deactivation of lowest lying barium levels by collisions with He, Ar, Xe and Ba ground state atoms

Abstract

Excitation transfer between the barium low lying excited states 6s6p ³ P ⁰₁ , 6s5d ¹ D ₂ and 6s5d ³ D _J by collisions with He,Ar,Xe and Ba has been investigated. The population densities in all levels involved were probed by absorption or by fluorescence usingcw lasers. The depopulation cross sections of the Ba³ P ⁰₁ state by collisions with noble gases were found to be σ_He(³ P ⁰₁ )=5.5·10⁻¹⁶ cm², σ_Ar(³ P ⁰₁ )=4.6·10⁻¹⁶ cm², and σ_Xe(³ P ⁰₁ )=1.7·10⁻¹⁶ cm². For Ar, the collisional depopulation of the³ P ⁰₁ level is exclusively due to the transition to the¹ D ₂ state. Under the assumption that the³ D _J metastable states are populated collisionally by¹ D ₂ →³ D _J transfer only, we have deduced the upper limit for the corresponding cross section σ ^Ar₁₃ =1.5·10⁻¹⁸ cm². From the Ba¹ D ₂ and Ba³ D _J steady-state diffusion distributions, collisional relaxation rates of the¹ D ₂ and³ D _J levels were evaluated. The collisional relaxation rates by Ar and Ba yielded total cross sections for the depopulation of metastable levels: σ_Ar(¹ D ₂)=1.5·10⁻¹⁷ cm², σ_Ba(¹ D ₂)℞1·10⁻¹³ cm², σ_Ar(³ D _J)=7·10⁻²¹ cm², and σ_Ba(³ D _J)=1·10⁻¹⁵ cm². Furthermore, it was found that the main contribution of the collisional depopulation of the¹ D ₂ state by Ar is related to back transfer to the³ P ⁰ _J state, whereas the deactivation of the³ D _J metastable state is due to back transfer to the¹ D ₂ state. Taking into account other cross sections reported in literature we can conclude that collisional deactivation of both metastable levels by Ba ground state atoms can be attributed to their mutual collisional mixing.