Lifetime vibrational interference during the NO 1s^-1π^* resonant excitation studied by the NO⁺(A ¹Π → X ¹Σ⁺) fluorescence

Abstract.

Dispersed fluorescence from fragments formed after the de-excitation of the 1s^-1π^* resonances of N^*O and NO^* has been measured in the spectral range of 118–142 nm. This range is dominated by lines of atomic nitrogen and oxygen fragments and by the $A ~^{1}{\rm \Pi} \left(v^{\prime}\right) \rightarrow X ~^{1}{\rm \Sigma}^{+}\left(v^{\prime \prime}\right)$ bands in the NO⁺ ion which result from the participator Auger decay of the 1s^-1π^* resonances. Ab-initio calculations of the transition probabilities between vibrational levels during the reaction NO $X~^{2}{\rm \Pi}\left(v_{0}=0\right) \rightarrow$ N^*O$\left( {\rm NO}^{\ast}\right)$ $1s^{-1}\pi ^{\ast }\left( v_{r}\right)$ ⇒ NO$^{+}~A~^{1}{\rm \Pi} \left(v^{\prime}\right) \rightarrow X~^{1}{\rm \Sigma}^{+}\left(v^{\prime \prime}\right)$ were used to explain the observed intensity dependence for the $A\left(v^{\prime }\right) \rightarrow X\left(v^{\prime \prime}\right)$ fluorescence bands on the exciting-photon energy across the resonances and on both v^′ and v^′′ vibrational quantum numbers. The multiplet structure of the 1s^-1π^* resonance and lifetime vibrational interference explain the observed exciting-photon energy dependence of the $A\left( v^{\prime}\right) \rightarrow X\left(v^{\prime \prime}\right)$ fluorescence intensity. A strong spin-orbit coupling between singlet and triplet states of NO⁺ is proposed to reduce additional cascade population of the $A~^{1}{\rm \Pi}$ state via radiative transitions from the $W~^{1}{\rm \Delta}$ and $A^{\prime}~^{1}{\rm \Sigma}^{-}$ states and to explain remaining differences between measured and calculated integral fluorescence intensities.