Vibrational parameters, molecular photonics

As was already suggested in the introduction, once the mechanism as well as the experimental parameters of laser induced multiple photon ionization have been investigated these ions may be used in spectroscopic and ion-molecule interaction studies. For two cases such an application has now been demonstrated in this laboratory by Proch, Trickl and Sha /II,12/. For practical experimental reasons the two diatomic molecules N and CO were chosen to address questions of cold- eam formation and E to E energy transfer. Nitrogen molecular ions N X were made in selected vibrational states by going through the folfowii sequence of excitation steps. 

In sharp contrast to conventional spectroscopic methods based on direct mie-photon absorption, IRMPD spectroscopy relies on the sequential absorption of a large number of IR photons. This excitation mechanism leaves an imprint on the observed IR spectrum in the sense that vibrational bands are typically broadened, red-shifted and affected in relative intensity to some extent. While the intramolecular processes underlying these spectral modifications have been addressed and qualitatively modelled in a large number of studies [166-172], it is often hard to predict quantitatively an IRMPD spectrum because the required molecular parameters, in particular the anharmonic couplings between vibrational normal modes at high internal energies, are usually unknown and cannot be calculated accurately using current quantum-chemical methods, fri practice, most experimental IRMPD spectra are therefore analysed oti the basis of computed linear absorption spectra, which usually provide a reasonable approximation to the IRMPD spectrum. 

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