Vibrational state, infrared energy

Most infrared spectroscopy of complexes is carried out in tire mid-infrared, which is tire region in which tire monomers usually absorb infrared radiation. Van der Waals complexes can absorb mid-infrared radiation eitlier witli or without simultaneous excitation of intennolecular bending and stretching vibrations. The mid-infrared bands tliat contain tire most infonnation about intennolecular forces are combination bands, in which tire intennolecular vibrations are excited. Such spectra map out tire vibrational and rotational energy levels associated witli monomers in excited vibrational states and, tluis, provide infonnation on interaction potentials involving excited monomers, which may be slightly different from Arose for ground-state molecules. 

Tunable visible and ultraviolet lasers were available well before tunable infrared and far-infrared lasers. There are many complexes that contain monomers with visible and near-UV spectra. The earliest experiments to give detailed dynamical infonnation on complexes were in fact those of Smalley et al [22], who observed laser-induced fluorescence (LIF) spectra of He-l2 complexes. They excited the complex in the I2 B <—A band, and were able to produce excited-state complexes containing 5-state I2 in a wide range of vibrational states. From line w idths and dispersed fluorescence spectra, they were able to study the rates and pathways of dissociation. Such work was subsequently extended to many other systems, including the rare gas-Cl2 systems, and has given quite detailed infonnation on potential energy surfaces [231. 

The visible and near-infrared LID results for NO/Pt were discussed in terms of hot electrons combined with a charge transfer mechanism. For the 193 nm LID result considered here, the photon energy is above the substrate work function, thereby providing a direct source of electrons to bathe the adsorbed NO species. Comparison of translational energy and vibrational state distributions for NO/Pt(lll), NO/Pt(foil), and N0/Ni(100)-0 suggests that the mechanisms driving the desorption processes in these systems might be related. However, the details of the specific interaction potentials must be substantially different to account for the disparate spin-orbit and rotational population distributions. 

The techniques considered in this chapter are infrared spectroscopy (or vibrational spectroscopy), nuclear magnetic resonance spectroscopy, ultraviolet-visible spectroscopy (or electronic spectroscopy) and mass spectrometry. Absorption of infrared radiation is associated with the energy differences between vibrational states of molecules nuclear magnetic resonance absorption is associated with changes in the orientation of atomic nuclei in an applied magnetic field absorption of ultraviolet and visible radiation is associated with changes in the energy states of the valence electrons of molecules and mass spectrometry is concerned... 

Two vibronic states which happen to occur at nearly the same energy will perturb one another if certain conditions are fulfilled. The interacting states may be derived from different electronic states, but in polyatomic spectra are often merely different vibrational states of the same electronic states. The perturbations, which are either homogeneous (AK = 0) (e.g. Fermi resonance) or heterogeneous (AK = 1) (Coriolis resonance) (Mulliken, 1937) are then analogous to the perturbations observed in infrared and Raman spectra. Such perturbations are commonplace in electronic bands where the completely unperturbed band is the exception rather than the rule. 

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