- Absorption and free-jet action spectroscopy of states in the region of four quanta of C–H stretching of acetylene (C2H2) around 12 676 cm−1 were studied. Rovibrational excitation combined with 243.135 nm promotion of C2H2 molecules to the upper electronic trans-bent states, 1Au/ 1Bu, and H photofragment ionization generated action spectra measuring the H yield as a function of the excitation wavelength. The findings show that the (1030000) IR bright state of the third C–H stretch overtone has a smaller photodissociation cross section than the (1214000) combination band containing trans-bend mode excitation and lying in its vicinity, due to a favorable Franck–Condon factor for the latter. Certain line pairs accessing similar J′ levels of the (1030000) state show anomalous intensities in the action spectrum, with P transitions excessively enhanced over R transitions. This implies enormous dissociation efficacy as a consequence of Coriolis-type local resonances with nearby states of another band and due to the overlap of the P transitions with another band. © 2002 American Institute of Physics.