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Universität zu Köln
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Mathematisch-Naturwissenschaftliche Fakultät
Fachgruppe Physik

I. Physikalisches Institut

Laser Induced Reactions

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Laser induced reactions (LIR) belong to the family of "action spectroscopy" methods where the influence of the laser light on the investigated mass-selected trapped ions is monitored by detecting induced changes of the ion cloud composition in a high-efficiency ion counter. In the special case of LIR, changes of the rate coefficient of an endothermic ion-molecule reaction serve to detect the excitation of the parent ionic species. This offers not only the possibility of doing very high sensitivity spectroscopy on transient ions (a number of only 1000 ions per trapping period is enough), but LIR can yield information on state-selected reaction rate coefficients and lifetimes of excited states. Examples of this method include the IR spectrum of the highly fluxional CH5+ molecule, the laser induced charge transfer in the system N2+ + Ar, and the IR spectroscopy of the asymmetric stretching vibration ν3 as well as the asymmetric bending vibration of the acetylene cation. Recently, the method has enabled IR and even pure rotational spectroscopy of H2D+, other deuterated molecules of astrophysical interest will follow.

The adjacent figure explains LIR-spectroscopy of H2D+. In collisions with H2 it reacts to H3+. This reaction is endothermic by 232 K as indicated by the dashed line. As only the lowest rotational levels of H2D+ are populated at the low temperature of the experiment, the reaction is initially slow. Its speed can be substantially increased by exciting the ion prior to collision with the H2 molecule.