Probing cis-trans isomerization in the S₁ state of C₂H₂ via H-atom action and hot band-pumped IR-UV double resonance spectroscopies

We report novel experimental strategies that should prove instrumental in extending the vibrational and rotational assignments of the S₁ state of acetylene, C₂H₂, in the region of the cis-trans isomerization barrier. At present, the assignments are essentially complete up to ∼500 cm^-1 below the barrier. Two difficulties arise when the assignments are continued to higher energies. One is that predissociation into C₂H + H sets in roughly 1100 cm^-1 below the barrier; the resulting quenching of laser-induced fluorescence (LIF) reduces its value for recording spectra in this region. The other difficulty is that tunneling through the barrier causes a staggering in the K-rotational structure of isomerizing vibrational levels. The assignment of these levels requires data for K values up to at least 3. Given the rotational selection rule K′ - l″ = ± 1, such data must be obtained via excited vibrational levels of the ground state with l″ > 0. In this paper, high resolution H-atom resonance-enhanced multiphoton ionization spectra are demonstrated to contain predissociated bands which are almost invisible in LIF spectra, while preliminary data using a hyperthermal pulsed nozzle show that l″ = 2 states can be selectively populated in a jet, giving access to K′ = 3 states in IR-UV double resonance.