Precise Determination of Excited State Rotational Constants and Black-Body Thermometry in Coulomb Crystals of Ca+ and CaH+

We present high-resolution rovibronic spectroscopy of calcium monohydride molecular ions (CaH⁺) cotrapped in a Coulomb crystal with calcium ions (⁴⁰Ca⁺), focusing on rotational transitions in the $|X^1{\Sigma }^{+}$, ν″ = 0⟩ → |A¹Σ⁺, ν′ = 2⟩ manifold. By resolving individual P and R branch transitions with record precision and using Fortrat analysis, we extract key spectroscopic constants for the excited state |A¹Σ⁺, ν′ = 2⟩, specifically, the band origin, the rotational constant, and the centrifugal correction. Additionally, we demonstrate the application of high-resolution rotational spectroscopy of CaH⁺ presented here as an in situ probe of the local environmental temperature. We correlate the relative amplitudes of the observed transitions to the underlying thermalized ground-state rotational population distribution and extract the blackbody radiation (BBR) temperature.