New S1 state vibrational and T3, 2, 1 spin–rotational assignments in the vicinity of the acetylene [formula omitted] band

Abstract

Pulsed supersonic jet spectra of acetylene have been recorded in the vicinity of the A ˜ 1 A u – X ˜ 1 Σ g + V 0 3 K 0 1 band in two mutually exclusive detection channels: ultraviolet laser-induced fluorescence (UV-LIF) and surface electron ejection by laser excited metastables (SEELEM). No eigenstate of the upper state(s) can appear via transitions in both UV-LIF and SEELEM channels. The UV-LIF channel is blind to transitions into eigenstates that have lifetimes longer than a few microseconds, while the SEELEM channel is exclusively sensitive to transitions into electronically excited eigenstates that have lifetimes longer than ∼30 μs. It is observed that intersystem crossing from the à ( S 1) bright state to the dark triplet states is mediated through a single “doorway state” vibrational level of the T 3 electronic state. We have characterized the bright state, the doorway state, and the dark states (singlet as well as triplet) illuminated by the bright state. Rotational transitions that belong to several previously unassigned vibrational-symmetry-allowed but nominally Franck–Condon forbidden S 1 ← S 0 electronic-vibrational bands are detected and assigned in the UV-LIF spectrum. The UV-LIF spectrum reveals that the electronic symmetry of the T 3 state is 3 B u and the K′ value of the observed T 3 ← S 0 vibrational band segment is 1. From the SEELEM spectrum, nine vibrational levels of dark triplet states are observed, spread over only ∼2.42 cm −1. Unique N, K, and e/ f quantum numbers are assigned for all of the observed triplet spin-rovibronic levels. Electronic symmetries for seven of the nine triplet states are determined. Most of the triplet state vibrational levels observed in the SEELEM spectrum belong to the T 1 surface ( 3 B u and 3 B 2) at energy well above the barrier to linearity. Despite the high vibrational state density, the upper levels of all of the assigned spin-rovibronic transitions fall onto surprisingly regular rotational term value plots [ E′ vs. N′ ( N′ + 1)] and the intensities fall onto surprisingly smooth Boltzmann plots. The most remarkable thing about the SEELEM spectrum is its apparent regularity, combined with a vibrational density of states so high that anharmonic and a-type Coriolis coupling among the T 1 and T 2 vibrational levels is essentially complete. The spectroscopically observed density of K-assigned vibrational states is comparable to the calculated symmetry-sorted state density.