Chemical Physics ELSEVIER Chemical Physics 190 (1995) 191-205 Intramolecular vibrational dynamics of diacetylene and diacetylene-dl via eigenstate-resolved overtone spectroscopy Joan E. Gambogi, R. Zachary Pearson, Xueming Yang l, Kevin K. Lehmann, Giacinto Scoles Department of Chemistry, Princeton University, Princeton, NJ 08544, USA Received 31 May 1994 Abstract The high resolution spectra of several CH overtone bands in diacetylene and diacetylene-dl were measured using optothermally detected excitation of a collimated molecular beam. The first overtone of the acetylenic CH stretches in these two molecules were recorded in a single resonance scheme using a 1.5 Ixm color center laser. The second overtone spectra were taken using sequential infrared/infrared double resonance with a 3.0 and a 1.5 Ixm color center lasers. The perturbations in the spectra have been analyzed to obtain information about the nature and timescales of the underlying intramolecular vibrational redistribution processes. The uncovered dynamical features appear to be dominated by anharmonic couplings and exhibit regular, not chaotic, behavior. The first and second overtone spectra of diacetylene-d~ are consistent with a coupling model which involves coupling through a doorway state and then subsequent coupling to the bath. In diacetylene, a combination band was also recorded which, in the local mode picture, is equivalent to putting two quanta in one acetylenic CH stretch and one quanta at the other end of the molecule. Comparison of this spectrum with the spectrum obtained by putting three quanta in the same CH stretch, is consistent with earlier observations that delocalized combination bands are less perturbed than nearly isoenergetic pure overtone states. 1. Introduction During the past decade, several research groups have used high resolution, infrared spectroscopy of jet- cooled samples to study intramolecular vibrational energy redistribution (IVR) in a large number of mol- ecules (for a recent review see Ref. [ 1 ] ). In some cases several different vibrational states of the same molecule have been studied. Analysis of the spectra gives the coupling strengths of the initial (bright) state with the nearly isoenergetic vibrational states that have no direct absorption strength from the ground state (dark states), and allows the calculation of the population decay of Current address: Departmentof Chemistry,Universityof California at Berkeley, Berkeley, CA 94720, USA. 0301-0104/95 / $09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI0301-0104(94)00263-0 the bright state. These experiments, however, give only limited direct information about the characteristics of and the couplings between the dark states. In order to extract the maximum information from the experi- ments, extensive theoretical modeling of the bath lev- els, including important anharmonic interactions must be carried out. Such an analysis requires that the mol- ecule be well characterized spectroscopically. While many molecules have been thoroughly studied by vibrational spectroscopy, most have too small a den- sity of vibrational states to show extensive IVR in states that can be reached by excitation in the near-IR. A particularly interesting exception is diacetylene. The high symmetry of this molecule dramatically reduces the number of important low order anharmonic reso-