Knowledge Bank

Laser Induced Reaction (LIR) is a novel technique for the spectroscopy of molecular ions as well as the study of state-specific rate coefficients and lifetimes of excited states. It is based on trapping mass-selected ions in the cold environment of a reactant gas and probing the laser-induced excitation of the ion by the outcome of the corresponding endothermic ion-molecule reaction. The feasibility of this method has been demonstrated in the past for the % %\begin{wrapfigure}{c}{70mm} %\includegraphics[width=70mm]{Fig1} % aption{Measured and calculated spectra of CH$\mathrm{5+}$} %\end{wrapfigure} % % electronic spectroscopy of N$\mathrm{2+}$ in the charge transfer reaction with Ar, and the spectroscopy of the IR-active modes of the acetylene cation in the abstraction reaction of C$2$H$\mathrm{2+}$ with hydrogen}, 2068--2075 (2002)}}, 073001, (2005)}.\ % Recently, LIR has been applied to record the first far infrared spectra of bare CH$\mathrm{5+}$ and some of its deuterated versions combining a low temperature 22--pole ion trap with the large tuning range of the free electron laser \mbox{FELIX}. % }, 134--141 (2005)}. To obtain the LIR spectrum, the product ions of the endothermic proton transfer reaction CH$\mathrm{5+}$ + CO$2$ $⟶$ CH$4$ + OCOH$+$ have been counted as a function of the \mbox{FELIX} wavelength. In addition to the earlier observed C--H-stretching feature}, 135--137 (1999)} at 3000 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, low-frequency extensions down to 2200 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ become discernible, as well as a H--C--H bending feature at 1250 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. Comparison of the measured data to computed ones shows that CH$\mathrm{5+}$ is fluxional at the temperature of the experiment, $T$=110K.