Quantum State-Dependent Fragmentation Dynamics of DS Molecules Following Excitation at Wavelengths ∼ 129.1 and ∼ 139.1 nm.

The first high-resolution translational spectroscopy studies of D atom photoproducts following excitation to the Rydberg states of DS are reported. Excitation at wavelengths λ ∼ 139.1 nm reveals an unusual 'inverse' isotope effect; the B(3←2) Rydberg state of DS predissociates much faster than its counterpart in HS. This is attributed to accidental near resonance with a vibrationally excited level of a lower-lying, more heavily predissociated Rydberg state of DS that boosts the probability of nonadiabatic coupling to the dissociation continuum with A″ symmetry. Excitation at λ ∼ 129.1 nm populates the B(4←2) Rydberg state, which predissociates more slowly and allows the study of ways in which the branching into different quantum states of the SD products varies with the choice of parent excited () level. All excited parent levels yield both ground (X) and electronically excited (A) state SD fragments. The former are distributed over a wide range of rovibrational (″, ″) levels, while the population of levels with low ' and high ' is favored in the latter. These trends reflect the topographies of the dissociative A″ (A') potential energy surfaces that correlate with the respective dissociation limits. Rotational motion about the -inertial axis in the excited state molecule increases the relative yield of SD(A) products, consistent with dissociation by rotationally (Coriolis-) induced coupling from the photoexcited Rydberg level to the A' continuum. Molecules excited to the rotationless ( = 0) level also yield some SD(A) products, however, confirming the operation of a rival fragmentation pathway wherein photoexcited molecules decay by initial vibronic coupling to the A″ continuum, with subsequent nonadiabatic coupling between the A″ and A' continua enabling access to the D + SD(A) limit.