Abstract

Sub-Doppler infrared absorption spectra of jet- cooled 1-butyne and trans-ethanol exhibit contrasting patterns of molecular eigenstates. In the presence of intramolecular vibration energy redistribution, frequency resolved spectra reveal that transitions to zero order upper state, v, J, K_a, K_c levels are fragmented into clumps of molecular eigenstates. The number and frequencies of these discrete features contain information about the nature of the intramolecular coupling. In the methyl C-H stretch of ethanol at 2990 cm^-1 between J = 0 to 2, the number of coupled states increases sharply and is comparable to the rovibrational state density. In contrast, the actetylenic and methyl C-H stretches of 1-butyne at 3333 cm^-1, respectively, exhibit coupling that is approximately independent of J and coupled state densities comparable to the purely vibrational state density. It is therefore concluded that a rotationally mediated coupling mechanism such as Coriolis or centrifugal coupling is involved in ethanol whereas only anharmonic coupling is apparent in 1-butyne.

© 1991 Optical Society of America