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Observations of OH$^+$ are used to infer the interstellar cosmic rayionization rate in diffuse atomic clouds, thereby constraining the propagationof cosmic rays through and the shielding by interstellar clouds, as well as thelow energy cosmic ray spectrum. In regions where the H$_2$ to H number densityratio is low, dissociative recombination (DR) is the dominant destructionprocess for OH$^+$ and the DR rate coefficient is important for predicting theOH$^+$ abundance and inferring the cosmic ray ionization rate. We haveexperimentally studied DR of electronically and vibrationally relaxed OH$^+$ inits lowest rotational levels, using an electron--ion merged-beams setup at theCryogenic Storage Ring. From these measurements, we have derived a kinetictemperature rate coefficient applicable to diffuse cloud chemical models, i.e.,for OH$^+$ in its electronic, vibrational, and rotational ground level. Attypical diffuse cloud temperatures, our kinetic temperature rate coefficient isa factor of $\sim 5$ times larger than the previous experimentally derivedvalue and a factor of $\sim 33$ times larger than the value calculated bytheory. Our combined experimental and modelling results point to a significantincrease for the cosmic ray ionization rate inferred from observations ofOH$^+$ and H$_2$O$^+$, corresponding to a geometric mean of $(6.6 \pm 1.0)\times 10^{-16}\,\mathrm{s}^{-1}$, which is more than a factor of two largerthan the previously inferred values of the cosmic ray ionization rate indiffuse atomic clouds. Combined with observations of diffuse and densemolecular clouds, these findings indicate a greater degree of cosmic rayshielding in interstellar clouds than has been previously inferred.

Dissociative Recombination of Rotationally Cold OH$^+$ and Its Implications for the Cosmic Ray Ionization Rate in Diffuse Clouds