HIGH-RESOLUTION INFRARED MOLECULAR-BEAM SPECTROSCOPY OF CYANOACETYLENE CLUSTERS

High resolution infrared optothermal spectroscopy has been used to stu dy small clusters of cyanoacetylene, (HCCCN)(n), containing three or m ore monomer units. For the linear HCCCN trimer the fundamental of the free C-H stretch vibration has been rotationally resolved and analyzed , yielding a ground vibrational state rotational constant (B-0) of 94. 031(13) MHz. In addition, an inner C-H stretch fundamental band of the linear trimer (rotationally unresolved) has been identified by observ ing the effect of large electric fields on the band shape. This assign ment is also consistent with spectral intensity measurements as a func tion of molecular beam stagnation pressure. Predissociation lifetimes of the upper states of these two bands were determined from the observ ed homogeneous linewidths. Several other rotationally unresolved vibra tional bands have also been observed. The stagnation pressure and elec tric field dependence of these spectra are used to estimate the cluste r sizes and to make tentative structural assignments. (C) 1995 America n Institute of Physics.