Isotopic effects in low-energy lithium-hydrogen collisions

The cross sections and rate coefficients for the inelastic processes of mutual neutralization, ion-pair formation, and (de)-excitation in lithium-hydrogen collisions are calculated based on the ab initio electronic structure [J. Phys. B: At. Mol. Opt. Phys. 32, 81 (1999)] and the hopping probability current method [Phys. Rev. A 88, 052704 (2013)] for the nonadiabatic nuclear dynamics. For each given set of collisional parameters, calculations of the probability current evolution were repeated $2.62144\times {10}^9$ times, and this leads to a high accuracy of the inelastic state-to-state transition probabilities, the cross sections, and finally, the rate coefficients. The isotopic effects on the processes in collisions ${}^6\mathrm{Li}/{}^7\mathrm{Li}+\mathrm{H}/\mathrm{D}/\mathrm{T}$, where (H/D/T) represents hydrogen, deuterium, and tritium, for different isotopes are studied. In addition, the deexcitation resonance processes ${}^{6,7}\mathrm{Li}(2p\to 2s)+\mathrm{H}/\mathrm{D}/\mathrm{T}$ were treated by the branching probability current method. It is found that the isotopic effects are different for different collisional partners and for different electronic states involved into a process, varying from negligible effects for high-lying Li states to strong effects for low-lying Li states. The resonance transition process is one with the strongest isotopic effects: replacing H by T changes the rate coefficient by up to four orders of magnitude. Unfortunately, there is no experimental data for the processes with strong isotropic effects, only for the processes with moderate effects, up to 60%.

Figure 1

Branching ratio of the cross sections for the partial mutual neutralization processes ${}^7\mathrm{Li}^{+}+{\mathrm{D}}^{-}\to {}^7\mathrm{Li}\left(3s,3p,3d\right)+\mathrm{D}$ to the total cross section for the neutralization processes ${}^7\mathrm{Li}^{+}+{\mathrm{D}}^{-}\to {}^7\mathrm{Li}\left(3s,3p,3d\right)+\mathrm{D}$ as a function of the collision energy. Solid line corresponds to the present calculation, red circles correspond to the experimental data of Ref. [24], blue, magenta, and dark yellow circles correspond to the experimental data of Ref. [21].

Figure 2

Cross sections ${\sigma }_{if}$ (in the units of ${\mathring{\mathrm{A}}}^2$ multiplied by the collision energy $E_{\mathrm{col}}$ (in the units of eV) for the mutual neutralization processes in collisions of various isotopes of ${\mathrm{Li}}^{+}+{\mathrm{H}}^{-}$. Solid lines correspond to ${}^7{\mathrm{Li}}^{+}+{\mathrm{H}}^{-}/{\mathrm{D}}^{-}/{\mathrm{T}}^{-}$ collisions, and dashed lines correspond to to ${}^6{\mathrm{Li}}^{+}+{\mathrm{H}}^{-}/{\mathrm{D}}^{-}/{\mathrm{T}}^{-}$ collisions.

Figure 3

The same as in Fig. 2 but for low-lying final channels: $f=1,2$.

Figure 4

The partial cross sections corresponding to the deexcitation process $\mathrm{Li}\left(2p\right)+\mathrm{H}\to \mathrm{Li}\left(2s\right)+\mathrm{H}$. The black solid line corresponds to the process in ${}^7\mathrm{Li}+\mathrm{H}$ collisions, the black dashed line corresponds to the process in ${}^6\mathrm{Li}+\mathrm{H}$ collisions, the red solid line corresponds to the process in ${}^7\mathrm{Li}+\mathrm{D}$ collisions, the red dashed line corresponds to the process in ${}^6\mathrm{Li}+\mathrm{D}$, blue solid line corresponds to the process in ${}^7\mathrm{Li}+\mathrm{T}$, and the blue dashed line corresponds to the process in ${}^6\mathrm{Li}+\mathrm{T}$.

Figure 5

The rate coefficients for the processes ${}^{6,7}\mathrm{Li}\left(2p\right)+\mathrm{H}/\mathrm{D}/\mathrm{T}\to {}^{6,7}\mathrm{Li}\left(2s\right)+\mathrm{H}/\mathrm{D}/\mathrm{T}$ as a function of the temperature for the range of 1000–10 000 K. Black solid line corresponds to ${}^7\mathrm{Li}+\mathrm{H}$ collisions, red solid line corresponds to ${}^7\mathrm{Li}+\mathrm{D}$ collisions, blue solid line corresponds to ${}^7\mathrm{Li}+\mathrm{T}$ collisions, and dashed lines correspond to ${}^6\mathrm{Li}+\mathrm{H}/\mathrm{D}/\mathrm{T}$ collisions, respectively.

Figure 6

The rate coefficients for the mutual neutralization processes ${}^{6,7}{\mathrm{Li}}^{+}+{\mathrm{H}}^{-}/{\mathrm{D}}^{-}/{\mathrm{T}}^{-}\to {}^{6,7}\mathrm{Li}+\mathrm{H}/\mathrm{D}/\mathrm{T}$ as functions of the excitation energies of the final channels at temperatures of $T=6000\mathrm{K}$. The symbols correspond to different collisional partners, see the figure for the labels. The dashed line depicts the estimate according to the simplified model.

Figure 7

The rate coefficients for the excitation and deexcitation processes from the initial channels ${}^{6,7}\mathrm{Li}\left(3s\right)+\mathrm{H}/\mathrm{D}/\mathrm{T}$ as functions of the excitation energies of the final channels at the temperatures of $T=6000\mathrm{K}$. The symbols correspond to different collisional partners, see the figure for the labels.