Spectroscopy beyond molecular constants ROBERT W. FIELD, DAVID P. BALDWIN, ERNEST J. HILL, MINGGUANG LI* and MICHAEL C. MCCARTHY Department of Chemistry and the George R. Harrison Spectroscopy Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A. Abstract-The spectra of diatomic molecules containing an atom with a partially filled d or f shell are exceedingly complex. Nonetheless, simple bonding arguments and a simple, predictive zero-order electronic structure model can be used to organize and understand the unavoidably dense and confusing manifold of molecular states. The correspondence between molecular constants and a global periodicity-based structural model is only possible when large quantities of spectral information have been acquired and deciphered. This paper discusses how a variety of new and powerful, single and multiple resonance spectroscopies can be used to efficiently and rapidly generate the large amounts of high quality spectral information required to reveal what appears to be an elegant underlying electronic simplicity. INTRODUCTION FOR MANY spectroscopists, FATHER GATTERER'S photographic atlas, Molecular Spectra of Metallic Oxides [1], provided the first glimpse of the incredible congestion and complexity of diatomic electronic spectra. These spectra are so complex that, aside from some herioc studies by BARROW, LAGERQVIST and MERER, their successful analysis had to await the advent of tunable laser techniques and matrix, rather than algebraic, effective Hamiltonian models. The electronic structure of many of these seemingly simple diatomic molecules is so profoundly different from that of molecules from the second and third rows of the periodic table, that even the most powerful ab initio electronic structure calculations are unable to go beyond molecular constants to a simple, predictive, zero-order electronic structure model. Quantitative relationships between the electronic structures of isoelectronic as well as non-isoelectronic atoms, based on semi- empirical reduction of spectroscopic observables to one- and two-electron orbital integrals [e.g. spin-orbit Coulomb Fk(nl, n'l') and exchange Gk(nl, n'l')], which illustrate so beautifully the predictive and interpretive power of periodicity, are alto- gether unavailable for the electronic structures of gas phase diatomic molecules. In contrast, the power of periodicity-based ideas (e.g. crystal and ligand field theory) is universally acknowledged in the interpretation of spectra of ionic crystals and inorganic metal-ligand complexes. The purpose of this paper is to provide a summary of the powerful new experimental techniques that will be required to collect the huge quantity of high quality spectroscopic information from spectra of unprecedented complexity in order to begin to look beyond molecular constants for a periodicity-based, zero-order global electronic structure model for diatomic molecules with partially filled or low-lying d and f subshells. In their introduction to Molecular Spectra of Metallic Oxides, FATHER JUNKES and FATHER SALPETER discussed the difficulty of finding suitable sources for generating all of the molecules on FATHER GATTERER'S original list of target species. The required operating characteristics (path length, number density, temperature and temporal stability) of molecule sources differ enormously between classical, photographic grating spectrographs on the one hand and laser-based techniques on the other. One of * Dalian Institute of Chemical Physics, 129 Street, Dalian, Liaoning, China. This article was published in the Special Issue of Spectrochimica Acta dedicated to the 50th Anniversary of the Founding of the Journal. 75