15min:

UGO JACOVELLA, BÉRENGER GANS AND FRÉDÉRIC MERKT, ETH Zürich, Laboratorium für Physikalische Chemie, Wolfgang-Pauli-Strasse 10, 8093 Zürich, Switzerland.

Using a coherent narrow-band vacuum-ultraviolet (VUV) laser source (bandwitdh of 0.008~cm^-1) coupled to a photoionization and pulse-field-ionization zero-kinetic-energy photoelectron (PFI-ZEKE) spectrometer, the threshold photoionization of polyatomic molecules can be studied at high resolution.
We present a new measurement of the PFI-ZEKE photoelectron spectrum of the origin band of the X⁺~²E_2(d) ~X~¹\mathrmA_1(s) ionizing transition of 2-butyne at a resolution of 0.15~cm^-1. Despite this high resolution, the spectral congestion originating from the combined effects of the internal rotation, the spin-orbit coupling and the Jahn-Teller effect prevented the full resolution of the rotational structure of the photoelectron spectrum. Combined with the known structure of the X ¹A_1(s) ground state of 2-butyne, including the free internal rotation$, the spectrum was used to derive information on the X⁺~²E_2(d) ground state of the 2-butyne radical cation. The rotational branch structure of the spectrum points at a complex energy-level structure of the cation and at the importance of a shape resonance enhancing g photoelectron partial waves.