Symmetry reduction induced by argon tagging gives access to low-lying excited states of FeH+ in the overtone region of the Fe–H stretching mode

Abstract

Iron is the most abundant transition metal in the interstellar medium (ISM), and is thought to be involved in a variety of astrochemical processes. Here, we present the infrared multiple photon dissociation (IRMPD) spectra of Ar_1,2FeH⁺ and their deuterated isotopologues in the region of 2240–14 000 cm⁻¹. The Fe–H overtone stretching mode in ArFeH⁺ and Ar₂FeH⁺ is observed at 3636 ± 28 cm⁻¹ and 3659 ± 13 cm⁻¹, respectively. Deuteration shifts these bands to 2618 ± 31 cm⁻¹ and 2650 ± 14 cm⁻¹ in ArFeD⁺ and Ar₂FeD⁺, respectively. Additionally, the spectra of Ar₂FeH⁺ and Ar₂FeD⁺ feature broad transitions at ∼2200–4000 cm⁻¹ and ∼4500–6500 cm⁻¹. We assign these bands to electronic transitions from the thermally populated X⁵A₂/X′⁵A₁ ground state manifold into the A′⁵B₂ and B⁵A₁ states, which we model with multi-reference quantum chemical calculations including spin–orbit coupling. The calculations show that these transitions are symmetry forbidden in FeH⁺ and in the equilibrium geometry of ArFeH⁺/ArFeD⁺, while the zero-point oscillation of the bending mode of the triatomic molecule leads to some oscillator strength. Upon addition of the second argon atom, the transitions become weakly allowed in the equilibrium geometry of Ar₂FeH⁺/Ar₂FeD⁺ due to symmetry reduction from C_∞v to C_2v.