Rotationally resolved vacuum ultraviolet resonance-enhanced multiphoton ionization (VUV REMPI) of acetylene via the G̃ Rydberg state.

We present a 1 + 1′ resonance-enhanced multiphoton ionization (REMPI) scheme for acetylene via the linear G̃ 4sσ 1Πu Rydberg state, offering partial rotational resolution and the possibility to detect excitation in both the cis- and trans-bending modes. The resonant transition to the G̃ state is driven by a vacuum ultraviolet (VUV) photon, generated by resonant four-wave mixing (FWM) in krypton. Ionization from the short-lived G̃ state then occurs quickly, driven by the high intensity of the residual light from the FWM process. We have observed nine bands in the region between 79 200 cm–1 and 80 500 cm–1 in C2H2 and C2D2. We compare our results with published spectra in this region and suggest alternative assignments for some of the Renner–Teller split bands. Similar REMPI schemes should be applicable to other small molecules with picosecond lifetime Rydberg states