Quantum State-Dependent\nFragmentation Dynamics of\nD<sub>2</sub>S Molecules Following Excitation at Wavelengths ∼\n129.1 and ∼ 139.1 nm

The first high-resolution translational spectroscopy\nstudies of\nD atom photoproducts following excitation to the Rydberg states of\nD<sub>2</sub>S are reported. Excitation at wavelengths λ ∼\n139.1 nm reveals an unusual ‘inverse’ isotope effect;\nthe <sup>1</sup>B<sub>1</sub>(3<i>da</i><sub>1</sub>←2<i>b</i><sub>1</sub>) Rydberg state of D<sub>2</sub>S predissociates\nmuch faster than its counterpart in H<sub>2</sub>S. This is attributed\nto accidental near resonance with a vibrationally excited level of\na lower-lying, more heavily predissociated Rydberg state of D<sub>2</sub>S that boosts the probability of nonadiabatic coupling to\nthe dissociation continuum with <sup>1</sup>A″ symmetry. Excitation\nat λ ∼ 129.1 nm populates the <sup>1</sup>B<sub>1</sub>(4<i>da</i><sub>1</sub>←2<i>b</i><sub>1</sub>) Rydberg state, which predissociates more slowly and allows\nthe study of ways in which the branching into different quantum states\nof the SD products varies with the choice of parent excited (<i>J</i><sub><i>KaKc</i></sub>) level. All excited parent\nlevels yield both ground (X) and electronically excited (A) state\nSD fragments. The former are distributed over a wide range of rovibrational\n(<i>v</i>″, <i>N</i>″) levels, while\nthe population of levels with low <i>v</i>′ and high <i>N</i>′ is favored in the latter. These trends reflect\nthe topographies of the dissociative <sup>1</sup>A″ (<sup>1</sup>A′) potential energy surfaces that correlate with the respective\ndissociation limits. Rotational motion about the <i>b</i>-inertial axis in the excited state molecule increases the relative\nyield of SD(A) products, consistent with dissociation by rotationally\n(Coriolis-) induced coupling from the photoexcited Rydberg level to\nthe <sup>1</sup>A′ continuum. Molecules excited to the rotationless\n(<i>J</i><sub><i>KaKc</i></sub> = 0<sub>00</sub>) level also yield some SD(A) products, however, confirming the operation\nof a rival fragmentation pathway wherein photoexcited molecules decay\nby initial vibronic coupling to the <sup>1</sup>A″ continuum,\nwith subsequent nonadiabatic coupling between the <sup>1</sup>A″\nand <sup>1</sup>A′ continua enabling access to the D + SD(A)\nlimit.