Observations of CH⁺ are used to trace the physical properties of diffuse clouds, but this requires an accurate understanding of the underlying CH⁺ chemistry. Until this work, the most uncertain reaction in that chemistry was dissociative recombination (DR) of CH⁺. Using an electron–ion merged-beams experiment at the Cryogenic Storage Ring, we have determined the DR rate coefficient of the CH⁺ electronic, vibrational, and rotational ground state applicable for different diffuse cloud conditions. Our results reduce the previously unrecognized order-of-magnitude uncertainty in the CH⁺ DR rate coefficient to ∼20% and are applicable at all temperatures relevant to diffuse clouds, ranging from quiescent gas to gas locally heated by processes such as shocks and turbulence. Based on a simple chemical network, we find that DR can be an important destruction mechanism at temperatures relevant to quiescent gas. As the temperature increases locally, DR can continue to be important up to temperatures of ∼600 K, if there is also a corresponding increase in the electron fraction of the gas. Our new CH⁺ DR rate-coefficient data will increase the reliability of future studies of diffuse cloud physical properties via CH⁺ abundance observations.

The electron-ion recombination rate coefficient for $\mathrm{Si}\mathrm{IV}$ forming $\mathrm{Si}\mathrm{III}$ was measured at the heavy-ion storage-ring TSR. The experimental electron-ion collision energy range of $0$–$186\mathrm{eV}$ encompassed the $2p^{6}nl{n}^{\prime }{l}^{\prime }$ dielectronic recombination (DR) resonances associated with $3s\to nl$ core excitations, $2s2p^{6}3snl{n}^{\prime }{l}^{\prime }$ resonances associated with $2s\to nl$ $(n=3,4)$ core excitations, and $2p^{5}3snl{n}^{\prime }{l}^{\prime }$ resonances associated with $2p\to nl$ $(n=3,\dots ,\infty )$ core excitations. The experimental DR results are compared with theoretical calculations using the multiconfiguration Dirac-Fock (MCDF) method for DR via the $3s\to 3p{n}^{\prime }{l}^{\prime }$ and $3s\to 3d{n}^{\prime }{l}^{\prime }$(both $n^{\prime }=3,\dots ,6$) and $2p^{5}3s3l{n}^{\prime }{l}^{\prime }$ $(n^{\prime }=3,4)$ capture channels. Finally, the experimental and theoretical plasma DR rate coefficients for $\mathrm{Si}\mathrm{IV}$ forming $\mathrm{Si}\mathrm{III}$ are derived and compared with previously available results.

We present state-selected measurements of rotational cooling and excitation rates of ${\mathrm{CH}}^{+}$ molecular ions by inelastic electron collisions. The experiments are carried out at a cryogenic storage ring, making use of a monoenergetic electron beam at matched velocity in combination with state-sensitive laser dissociation of the ${\mathrm{CH}}^{+}$ ions for simultaneous monitoring of the rotational level populations. Employing storage times of up to 600 s, we create conditions where electron-induced cooling to the $J=0$ ground state dominates over radiative relaxation, allowing for the experimental determination of inelastic electron collision rates to benchmark state-of-the-art theoretical calculations. On a broader scale, our experiments pave the way to probe inelastic electron collisions for a variety of molecular ions relevant in various plasma environments.