Single rotational levels of ungerade vibrational levels, 2ν3′+ν6′ and 3ν3′+ν6′ (both with bu symmetry), in the à 1Au electronically excited state of acetylene were excited by an IR-UV double resonance scheme via the ν3″ fundamental level in the X̃ 1Σg+ state, and the rotationally resolved dispersed fluorescence (DF) spectra were recorded at 3.2–4.5 cm−1 resolution. The term values of the new ungerade levels were determined within an accuracy of 0.56 cm−1(1σ) through careful calibration achieved by frequency standard atomic Fe and Hg lines. A total of 111 new ungerade vibrational levels with Σu+, Σu−, and Δu symmetry below 10 000 cm−1 was identified in the high-resolution IR-UV-DF spectra, which provide access to new classes of X̃ 1Σg+ bright states: (i) (0,v2″,0,v4″1,1−1)Σu+, (0,v2″,0,v4″1,11)Δu, and (0,v2″,0,v4″3,1−1)Δu, which are the Franck–Condon (FC) bright levels from the nν3′+ν6′ (n=2,3) levels in the à 1Au state; (ii) (0,v2″,0,v4″−1,11)Σu− levels which appear through the a-axis Corioris interaction between nν3′+ν6′ and nν3′+ν4′ (n=2,3) in the à 1Au state; and (iii) (0,v2″,1,v4″0,0)Σu+ and (0,v2″,1,v4″2,0)Δu levels which gain transition intensity from the Duschinsky effect associated with the bent-linear ÖX̃ transition. All observed ungerade term values and previously determined gerade and ungerade term values below 10 000 cm−1 were fitted by two effective model Hamiltonians, i.e., a pure-bend effective Hamiltonian and a stretch–bend effective Hamiltonian. The stretch–bend effective Hamiltonian is expressed in terms of 31 Dunham expansion parameters and 11 anharmonic resonance parameters associated with (i) five stretch–bend anharmonic resonances; (ii) one stretch–stretch and two bend–bend Darling–Dennison resonances; and (iii) one vibrational l resonance. The parameters in this Hamiltonian were determined from a least-squares fit of 287 vibrational term values (111 new ungerade levels, 128 levels from absorption, 1 level from stimulated Raman, 13 levels from stimulated emission pumping (SEP), and 34 levels from UV-DF spectroscopy) below 10 000 cm−1 with a standard deviation of σ=1.21 cm−1. The FC patterns for the v4″=odd ungerade levels, (0,v2″,0,v4″,1), in the IR-UV-DF spectra were derived, and the nodes along the v4″ trans-bend mode were found at v4″=11 via the 2ν3′+ν6′ upper state, and at v4″=9 and 15 via the 3ν3′+ν6′ upper state, which is consistent with the ν3′ dependence of the FC patterns observed in previous UV-DF studies.
