Accurate calculations for the even-parity core-excitedP2states of neutral Li

Nonrelativistic configuration-interaction calculations have been carried out with orbital basis specifically adapted for each one of the first seven even-parity ${}_{}{}^2{}_{}{}^{}P$ bound states of neutral Li. The results show that the lowest four ${}_{}{}^2{}_{}{}^{}P$ states have recently been observed by absorption spectroscopy from the $1s^{2}2p$ state. The calculated transition wavelengths (20.7452, 19.7613, 19.4728, and 19.4065±0.0005 nm agree well with the experimental ones (20.744, 19.762, 19.474, and 19.405±0.002 nm). Electric dipole transition probabilities to the $1s^{2}2p$, $3p$ and $4p$ states have also been calculated. The configurations for the two lowest ${}_{}{}^2{}_{}{}^{}P\mathrm{}\left(1\right)\mathrm{}$ and ${}_{}{}^2{}_{}{}^{}P\mathrm{}\left(2\right)\mathrm{}$ states are $1s2p^2$ and $\mathrm{}\left(1s2p\right)\mathrm{}{}_{}{}^3{}_{}{}^{}P3p$, respectively. The $\mathrm{}\left(1s2p\right)\mathrm{}{}_{}{}^3{}_{}{}^{}P\mathrm{np}$ and $\mathrm{}\left(1s2p\right)\mathrm{}{}_{}{}^1{}_{}{}^{}P{n}^{\prime }p$ series almost coincide at $n=4\mathrm{}$, $n^{\prime }=3\mathrm{}$, and as a consequence, the ${}_{}{}^2{}_{}{}^{}P\mathrm{}\left(3\right)\mathrm{}$ and ${}_{}{}^2{}_{}{}^{}P\mathrm{}\left(4\right)\mathrm{}$ levels are approximately described by $\mathrm{}\left(1s2p\right)\mathrm{}{}_{}{}^3{}_{}{}^{}P4p\pm \mathrm{}\left(1s2p\right)\mathrm{}{}_{}{}^1{}_{}{}^{}P3p$. This is the simplest known example of two-level series which mutually perturb each other.