Knowledge Bank

Sub-Doppler infrared absorption spectra of l-butyne and trans ethanol cooled in a slit jet expansion under the same conditions are reported. Spectra of the c-type component of the asymmetric methyl C-H stretch near $2990cm-1$ exhibit a substantially different appearance in these molecules. Mixing of the zero-order methyl C-H stretch vibration with bath vibrational (or rotational-vibrational) states gives rise to multiplets of molecular eigenstates. This behavior is the spectroscopic signature of the kinetic phenomena of intramolecular vibrational redistribution (IVR). Such eigenstates are assigned unambiguously to zero-order transitions by comparison of ground state combination differences to microwave data for $J\prime =0-4$ and $Ka\prime =0-2$. Both molecules reveal fragmentation of each upper state rotational level into multiplet of molecular eigenstates. However, in ethanol a dramatic increase in the number of coupled levels as a function of $J\prime$ indicates the presence of a Coriolis or centrifugal coupling mechanism for IVR. This is in sharp contrast to 1-butyne where no such strong rotational effect is observed suggesting a primarily anharmonic mechanism. The data provide a test of the keyhole model which was previously proposed to explain IVR in these molecules. According to the model the bright state (C-H stretch) and the primary anharmonic coupling to the bath is similar in each case. However, the couplings among the zero-order bath states is different for the two molecules. The asymmetric internal O-H rotor in the ethanol molecule allows for rotationally induced coupling mechanisms among vibrational bath states and for exploration of much of the available rotational-vibrational phase space. In contrast, no rotational effects are found in l-butyne and the anharmonic bath couplings allow for exploration of vibrational phase space only.