ENERGY-TRANSFER IN THE 3(1),2(1),4(1),5(1) FERMI-RESONANT STATES OF ACETYLENE .1. ROTATIONAL ENERGY-TRANSFER

An infrared-ultraviolet double resonance technique is used to probe the state-to-state rotational energy transfer dynamics of self-relaxation in acetylene. The output of an optical parametric oscillator at approximately 3 mum is used to excite C2H2 to a rotational level within one of its Fermi-resonant 3(1),2(1)4(1)5(1) states. By fixing this wavelength and scanning the frequency-doubled output of a tunable dye laser, laser induced fluorescence signals arising from collisional population of rotational levels within both dyads are observed and state-to-state rate constants for rotational relaxation are obtained. Rotational relaxation to J levels within the pumped (upper energy) Fermi-dyad accounts for 74% of the total rate of loss of the population of the J = 12 level, whereas relaxation to J levels in its partner accounts for only 16%. A further 7% of the absolute rotational relaxation rate is accounted for by vibrational relaxation out of the mixed levels, leaving only 3%-4% of the total relaxation to be accounted for.