Spectroscopy and energetics of the acetylene molecule: Dynamical complexity alongside structural simplicity

This article reviews laser-spectroscopic studies of the structure, energetics, and dynamics of processes involving small polyatomic molecules, particularly acetylene (ethyne, C₂H₂). The linear, centrosymmetric structure of C₂H₂ is deceptively simple, given that aspects of its optical spectra and dynamics have proved to be unusually complicated. The article focuses on the ground electronic state of C₂H₂, where rovibrational eigenstates are only approximately described in normal-mode terms, because intramolecular processes (such as anharmonic mixing, ℓ-type resonances, and Coriolis coupling) introduce extensive global and local perturbations. These tend to spoil quantum numbers and symmetries that are well-defined in low-order basis states. Such effects within the rovibrational energy states of C₂H₂ are systematically characterized, together with dynamical descriptions in terms of polyad models and insight into photochemical or photophysical processes that may occur at high vibrational energies, without direct electronic excitation. Time-resolved optical double-resonance spectroscopy, probed by ultraviolet-laser-induced fluorescence and pumped by either infrared absorption or coherent Raman excitation, has proved particularly useful in exploring such effects in gas-phase C₂H₂; techniques of this type are discussed in detail, together with other laser-spectroscopic methods that provide complementary mechanistic information. A closely related topic concerns the area of optothermal molecular-beam spectroscopy, with particular emphasis on research by the late Roger E. Miller to whose memory this article is dedicated. Key publications by Miller and coworkers, in many of which C₂H ₂ and its isotopomers play a central role, are reviewed. These cover the following themes: structure of molecular complexes and clusters, infrared predissociation spectra, rotational and vibrational energy transfer, differential scattering, photofragmentation of oriented complexes, superfluid-helium nanodroplet spectroscopy, aerosols formed in low-temperature diffusion cells, surface scattering experiments, optically selected mass spectrometry, and characterization of biomolecules. A unifying issue that links the assorted topics of this article is the role that intramolecular perturbations can play to enhance (and sometimes suppress) the efficiency of rovibrational energy transfer in colliding molecules or in molecular complexes and clusters; C₂H₂ and its isotopomers have been a rich source of insight in this regard, although they continue to pose challenges to our understanding.