Energy transfer mechanisms in backward scattering of ${\mathrm{Cs}}^{+}$ ions from $N_2$ molecules

The energy transfer processes in ${\mathrm{Cs}}^{+}-N_2$ collisions were experimentally studied at laboratory (lab) collision energies $E_{\mathrm{lab}}=50-250\mathrm{eV}$ and lab scattering angles $\theta >2^{\circ }$ via differential scattering spectroscopy. A classical trajectory calculation method was used to analyze the experimental results. We observed specific electronic excitations to the $C{s}^{+}+N_2\left(A{}^3\Sigma \right)$ and $C{s}^{+}+N_2\left(a{}^1\Pi \right)$ states in the energy-transfer spectra with significant excitation probabilities in backward scatterings at lower center-of-mass (CM) energies $E=13.6-18.3\mathrm{eV}$ $(E_{\mathrm{lab}}=78-105\mathrm{eV})$ and a CM angle $\Theta \sim {165}^{\circ }$ $(\theta =5^{\circ })$. The results of the simulations of energy transfer spectra through classical trajectory calculations indicate that electronic transitions occur around the molecular orientation $\gamma =\pi /2$ and at vibrotational excitation energies $\Delta E/E<0.12$. The potential crossings for the $N_2\left(A{}^3\Sigma \right)$ and $N_2\left(a{}^1\Pi \right)$ excitations are located at the intermolecular potentials of $V_{\mathrm{IM}}\left(R,\gamma =\pi /2\right)=13.4$ and 16.4 eV, respectively. $N_2\left(A{}^3\Sigma \right)$ excitation is the dominant electronic excitation process at lower energies of $E<17.5\mathrm{eV}$ $(E_{\mathrm{lab}}<100\mathrm{eV})$. Evaluations using an empirical charge-overlap model reasonably reproduce intermolecular potential crossings. Moreover the proposed model can provide effective information on the spin-changing $N_2\left(A{}^3\Sigma \right)$ excitation.