Electronic spectroscopies weakly-allowed transitions

Spectroscopy of rovibrational transitions in the electronic ground state can take advantage of these special conditions. The low translational temperature of the molecules increases the absorption rate significantly, and efficient excitation is possible even on weak overtone transitions [89]. This allows one to use simpler laser sources, which also simplifies experimentation. Due to the relatively long lifetime of vibrational levels of molecules ( msec to days), the potential line resolution can be huge. Spectroscopy of vibrational transitions on cold and localized samples of molecular ions was first achieved using HD+ ions [90]. 

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