Toward Detection of FeH + in the Interstellar Medium: Infrared Multiple Photon Dissociation Spectroscopy of Ar 2 FeH .

The iron hydride molecular cation FeH + is expected to be present in the interstellar medium. Because of the lack of laboratory data, it is yet to be identified in spectrally resolved astronomic observations. As a benchmark for computational predictions and to guide an experimental search for the ro-vibrational lines of FeH + , we performed infrared multiple photon dissociation (IRMPD) spectroscopy of FeH + tagged with two argon atoms. The Fe-H stretching mode in Ar 2 FeH + is observed at 1860 cm -1 . Combination bands of the Fe-H stretch with the two Fe-H bending and the asymmetric Fe-Ar stretching modes are observed at 2012 cm -1 , 2054 cm -1 , and 2078 cm -1 . Quantum chemical calculations show that the molecule has C 2v symmetry. The Ar-Fe-Ar bending mode at 46 cm -1 is significantly populated at the temperature of the experiment, causing thermal broadening of the Fe-H stretch and its redshift with increasing internal energy.