Collisional energy transfer in Na 2 between the excited states B1Π u and (2) 1Σ g+: Experimental results and theoretical interpretation

Selective laser excitation and fluorescence observation techniques (Fourier transform spectroscopy) have been used to study the collisional electronic excitation transfer process: N Na ₂( B¹Π _u, v', J') + Na(3 p²P) → Na ₂((2) ¹Σ _g⁺, v, J) + Na(3 p³P) + ∆ E for some selected rovibrational levels of Na ₂( B¹Π _u). An explicit formula is derived for the first-order, semiclassical, rotationally inelastic transition probability with an electric multipole long-range potential in the atom-dimer collision. The process is found to be resonant, with cross-sections strongly depending on ∆ E; being maximum when ∆ E ∼ 0 and negligible when ∆ E ≳ 300 cm ^-1 at T ∼ 770 K. At the same time the experimentally observed spectra indicate that both resonant and nonresonant channels contribute to the collisional energy transfer. The quantitative characteristics of these processes are presented, and they are compared with calculations based on a first-order Born approximation with dipole-quadrupole long-range interactions.