UV multiphoton excitation: An access to the energy dependence of unimolecular reactions and collisional energy transfer

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Abstract
Repetitive absorption of UV (or visible) photons followed by fast internal conversions allows to deposit large amounts of vibrational energy in polyatomic molecules. Intra- and intermolecular processes with relatively weak energy dependences can be studied by this excitation over broad energy ranges. The technique is illustrated for the unimolecular isomerization azulene → naphthalene and collisional energy transfer of vibrationally excited azulene. With ns excimer laser pulses, up to three photons at λ=308 nm were deposited in the molecule, and collisional quenching competing with isomerization was studied. The results can be modeled conveniently by a multistep mechanism.