Unresolved sideband photon recoil spectroscopy of molecular ions

We reflect on the prospect of exploiting the recoil associated with absorption and emission of photons to perform spectroscopy of a single molecular ion. For this recoil to be detectable, the molecular ion is sympathetically cooled by a laser-cooled atomic ion to near their common quantum mechanical ground state within a trapping potential. More specifically, we present a general framework for simulating the expected photon recoil spectra in regimes where either the natural transition linewidth $\Gamma_t$ of the molecular ion or the spectral width $\Gamma_L$ of the exciting light source exceeds the motional frequencies of the two-ion system. To exemplify the framework, we present two complementary cases: spectroscopy of the broad 3s $^2$S$_{1/2}$ - 3p $^2$P$_{3/2}$ electronic transition ($\Gamma_t/2\pi = 41.8$ MHz) of a single $^{24}$Mg$^+$ ion at $\lambda=279.6$ nm by a narrow laser source ($\Gamma_L/2\pi \lesssim 1$ MHz) and mid-infrared vibrational spectroscopy of the very narrow $|v=0,J=1\rangle$ - $|v'=1,J'=0\rangle$ transition ($\Gamma_t/2\pi = 2.50$ Hz) at $\lambda=6.17$ $\mu$m in the $^1\Sigma^+$ electronic ground state of $^{24}$MgH$^+$ by a broadband laser source ($\Gamma_L/2\pi \gtrsim$ 50 MHz). The atomic ion $^{24}$Mg$^+$ has been picked to introduce a simple system to make comparisons with experimental results while still capturing most of the physics involved in electronic excitations of molecular ions.