Stereochemical Control of NO(2Π) Λ-Doublet Fine Electronic Structure from tert -Butylnitrite UV Photodissociation

The photodissociation stereodynamics of tert-butylnitrite at 355 nm have been investigated using pulsed slice imaging combined with polarization-resolved detection of NO(2Π, v = 1, J) fragments. By selectively probing different rotational branches and analyzing both translational energy distributions and Dixon’s bipolar moments, we establish a direct connection between the dissociation pathway, fragment stereodynamics, and NO(2Π) Λ-doublet population. The measured translational energy distributions, although apparently structureless, exhibit a clear bimodal character. This bimodality is accompanied by distinct anisotropy parameters and polarization signatures, demonstrating the coexistence of two competing dissociation mechanisms. A theoretical analysis of the potential energy surfaces along the O–NO dissociation coordinate shows that these pathways originate from excitation at different regions of the NO vibrational phase, leading either to prompt dissociation on a steeply repulsive surface or to delayed dissociation following transient trapping in flatter regions of the excited-state potential. Crucially, the two dynamical regimes produce different Λ-doublet populations of the NO fragment. The selective sensitivity of the P11, Q11 + P21, and Q22 + R12 rotational branches reveals that each pathway maps onto distinct parity outcomes, reflecting different projections of the NO π orbital at the instant of O–NO bond rupture. The analysis of the bipolar moments further demonstrates that fragment polarization and vector correlations are strongly pathway-dependent and evolve with rotational excitation. These results demonstrate that Λ-doublet population constitutes a direct and robust fingerprint of the dissociation pathway. Translational energy bimodality, anisotropy, fragment polarization, and Λ-doublet selectivity are shown to arise from the same underlying stereochemical mechanisms, providing a unified description of the photodissociation process in this polyatomic system.