# Rotationally Resolved Predissociation Spectrum of the 15Φ ← X5Δ Rovibronic Band of FeH+

## TL;DR

This paper reports the first detailed spectrum of FeH+ in the lab, revealing its structure and how it breaks apart when exposed to light.

## Contribution

The first rotationally resolved photodissociation spectrum of FeH+ is presented with theoretical and experimental analysis.

## Key findings

- The rotational structure of the 15Φ ← X5Δ transition in FeH+ is resolved and assigned.
- Predissociation broadens FeH+ spectral peaks due to curve crossings with repulsive states.
- The photodissociation cross-section of FeH+ does not exceed 5 × 10–20 cm2.

## Abstract

The elusive diatomic
molecule FeH+ has long
been hypothesized
to exist in cool interstellar environments, yet its spectral signature
has remained unidentified due to the lack of laboratory data. Its
neutral counterpart FeH, on the other hand, is a well-known feature
in the atmospheres of M-dwarfs, Sunspots, and hot Jupiter. Here we
present the first rotationally resolved photodissociation spectrum
of gas-phase FeH+, covering the energy range of 18550–18830
cm–1 (5390.8–5310.7 Å). The rotational
structure of the 15Φ ← X5Δ
electronic transition is resolved and conclusively assigned using
state-of-the-art multireference and coupled cluster calculations and
rovibrational spectra simulations. FeH+ spectral peaks
in this specific band are broadened by predissociation, which arises
from curve crossings of the bound 15Φ potential curve
with those of repulsive 7Π, 7Δ states.
The photodissociation cross-section does not exceed 5 × 10–20 cm2, and comparison of the laboratory
spectrum with observation data from HD 183143 does not reveal a match
for this relatively weak band. Due to the uncertainty of the spin–rotation
coupling constant, it is not possible to predict the precise positions
of the rotational line spectrum. However, the high spin and orbital
angular momentum quantum numbers of the electronic ground state place
the onset of the spectrum above 1300 GHz, a frequency region that
cannot be observed with ground-based telescopes.