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

Abstract

We report novel experimental strategies that should prove instrumental in extending the vibrational and rotational assignments of the S1 state of acetylene, C[subscript 2]H[subscript 2], in the region of the cis-trans isomerization barrier. At present, the assignments are essentially complete up to ∼500 cm[superscript −1] below the barrier. Two difficulties arise when the assignments are continued to higher energies. One is that predissociation into C[subscript 2]H + H sets in roughly 1100 cm[superscript −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.