Stereochemical effects in the quenching of Na*(3 2P) by CO: Crossed beam experiment and ab initio CI potential energy surfaces

Electronic to vibration‐rotational‐translational energy transfer in the quenching of $\mathrm{Na}{\text{(3\hspace{0.17em}}}^2{P}_{\text{3/2}})$ by CO has been studied with state of the art crossed atomic, molecular, and laser beam techniques at 0.16 eV initial kinetic energy, and by ab initio CI calculations for the potential energy surfaces involved in the process. Double differential quenching cross sections are measured as a function of scattering angle and energy transferred to the molecule. A pronounced structure in the energy transfer spectra as well as a partial backward scattering is attributed to two different mechanisms, a ’’direct’’ one and one which proceeds through ’’complex’’ formation. The observations are explained by the calculated potential energy surfaces (PES) for the first excited states ${\mathrm{Ã\hspace{0.17em}}}^2{\mathrm{A\prime (Ã\prime \hspace{0.17em}}}^2\mathrm{A″)}$ and the ground state ${\mathrm{X̃\hspace{0.17em}}}^2\mathrm{A\prime }$ which exhibit two crossing seams below the 2.1 eV excitation energy: (i) one for colinear approach of ${\mathrm{Na}}^{*}$ on the carbon side of CO with its lowest energy 1.06 eV at $R_c(\mathrm{Na–CO}{\text{)= 5.5a}}_0{\mathrm{,\hspace{0.17em}r}}_c(\mathrm{C–O}{\text{) = 2.35a}}_0$ responsible for the direct process and (ii) one for colinear approach of ${\mathrm{Na}}^{*}$ on the oxygen end of CO with 1.28 eV at $R_c{\text{= 4.9a}}_0$ and $r_c{\text{= 2.47a}}_0,$ allowing the quenching after ’’complex’’ formation. The angularly integrated cross sections are maximal ${\text{(27\hspace{0.17em}Å}}^2/\mathrm{eV})$ at an energy transferred to the molecule equivalent to five vibrational quanta. Comparison with bulk data suggests strong rotational excitation (two vibrational quanta in the average) as can be rationalized from the anisotropy of the ${\mathrm{X̃\hspace{0.17em}}}^2\mathrm{A\prime }$ PES near the crossing region. Total quenching cross sections and their temperature dependence can be explained by the absorbing sphere model using the calculated location and energy of the crossing seams.