Molecular emission from H2O/D2O[Ctilde]1B1and photodissociation dynamics on the[Btilde]1A1surface

Abstract

Bound to free emission centred near 420nm is reported from heavily predissociated levels of H₂O/D₂O [Ctilde](¹ B ₁)₀₀₀ (τ < 6ps) to the dissociative à ¹ B ₁ state. The quantum yield for fluorescence is small (∼ 10^-4) and provides an estimate for the radiative lifetime of the D₂O ([Ctilde] → Ã) emission. The ([Ctilde] → Ã) fluorescence excitation spectrum has been modelled to obtain refined heterogenous predissociation rates for D₂O ([Ctilde] ¹ B ₁ → [Btilde] ¹ A ₁) transfer. This has allowed the OD(A ²Σ⁺) photofragment fluorescence excitation spectrum to be modelled to obtain the J′_a dependent Renner-Teller crossing probability for [Btilde] ¹ A ₁ → à ¹ B ₁ transfer during dissociation, over a wide range of a-axis rotation. The results are in good, qualitative agreement with theory and provide insight into the dynamics of dissociation on the [Btilde] ¹ A ₁ surface. Resolved photofragment fluorescence excitation spectra for selected rotational levels N′, in OD (A ²Σ⁺)_v′ = 0 show a correlation between N′ and the [Btilde] → à crossing probabilities. This provides experimental evidence for the correlation between ‘direct’ trajectories and high product rotation and between ‘indirect’, bending trajectories and low product rotation, as suggested by previous 3-D scattering calculations. The effect of this correlation and of the intersections of [Btilde] ¹ A ₁ with à ¹ B ₁ and [Xtilde] ¹ A ₁, on the [Btilde] state dynamics and on the OD (A ²Σ⁺) quantum yield, rotational distributions and alignments are discussed.