Intramolecular collisional transfer in NO (a 4Π→B 2Π, b 4Σ−): Gateway-type, resonant versus direct, nonresonant mechanisms

A beam containing NO in the long-lived a 4Π state was allowed to interact with target particles in a collision cell. Intramolecular collision-induced transitions a 4Π→B 2Π and a 4Π→b 4Σ− were observed via the subsequent emissions in the β bands (B 2Π→X 2Π) and the Ogawa bands (b 4Σ−→a 4Π), respectively. In the ultraviolet part of the spectrum long β band progressions originating from the B-state vibrational levels v=0 and 3 were observed. In each band only a few lines appear, which were assigned to transitions from the rotational levels 2Π3/2(10.5) in v=0 and 2Π1/2(17.5) in v=3. These particular B 2Π levels are perturbed by specific levels of the a 4Π state, serving as so-called gateways to allow the otherwise spin-forbidden a 4Π→B 2Π collision-induced transition. An external magnetic field has a strong effect on the collision-induced emission from the NO(B, v=0) level. With paramagnetic target gases, direct spin-changing NO(a→B) collisional transfer was also observed in addition to the gateway transitions. The selectivity for the final state level is much weaker here. The collision-induced Ogawa band emission in the near-infrared showed smooth, partially resolved rotational contours with all target gases. This indicates that the collisional transfer NO(a→b) within the quartet system is also not of the highly restrictive gateway type, but results from direct collisional coupling.