Vibrational normal modes of . The symmetric-stretch mode conserves the equilateral triangular symmetry and is infrared inactive, while the doubly degenerate bending mode has a transition dipole moment and is infrared active.
The barrier to linearity. A special representation of the potential energy surface, where two H–H bonds are forced to be equal, while their length is varied to give the minimum energy for a given angle between them. The plot shows that can sample linear geometries at energies (dashed line). Vibrational levels with energies are plotted and labeled. Those levels that have been observed in this work are emphasized in bold black color. The dash-dotted line marks the endothermicity of the formation reaction. (Potential surface taken from Ref. 8, zero point energy (level 0–0) set to with respect to the potential minimum, according to Ref. 37, level energies from Ref. 21).
Sketch of the trap setup and the laser system.
Timing scheme and typical particle numbers of the measurements. Upon loading of the trap up to 60% of the injected ions are highly excited and react to form immediately. After 200 ms of storage the average number of ions has decayed below 1 due to the inverse of the reaction shown in Eq. (8). At 200 ms the laser is activated for 100 ms and laser-induced ions are created. All ions are extracted after 300 ms and the ions are selected by a mass filter and counted. After each cycle consisting of 20–100 trap fillings at fixed frequency the laser power and wavelength are determined. For the next cycle the laser wavelength is increased until a full wavelength scan is completed. Typically 5–20 wavelength scans were taken for each transition and the number (after 300 ms of storage without laser interaction) and lifetime as well as the lifetime are recorded after each complete scan.
Four examples of spectra obtained by chemical probing spectroscopy. The translational temperature derived from the Doppler width is given in the insets.
List of observed transitions together with their vibrational and rotational assignments using Refs. 20 and 21. The transition frequencies are given for the present work as well as those published by Gottfried (Ref. 13) (where available) and the spectroscopically adjusted values of SAH (Ref. 21).
List of the observed transition frequencies and deviations to the unadjusted SAH calculation and corrected values of (Ref. 21) and the NMT calculation. The Einstein coefficients derived from NMT (Ref. 19) are also given. In the last line the parameters of the fundamental bending mode transition , are shown for comparison.
List of the brightest vibrational bands of up to , from a calculation of Le Sueur et al. (Ref. 34). The Einstein coefficients are those to the ground state. Two different potential energy surfaces have been employed: MBB (Ref. 6) and JS (Ref. 36), respectively.
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