Experimental Determination of the Dissociative Recombination Rate Coefficient for Rotationally Cold CH⁺ and Its Implications for Diffuse Cloud Chemistry

Abstract

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.

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

“…However, our experimentally derived value has a slope that varies significantly with temperature between T k 1 2 and T k 1 -. A variable slope with temperature has also been seen by Novotný et al (2019) for HeH + and by Paul et al (2022) for CH + . Together with those works, our findings demonstrate that the DR kinetic temperature rate coefficient can exhibit a temperature dependence that differs significantly from the theoretically derived T k 1 2 behavior for direct DR of diatomic ions (Guberman 1995).…”

“…The uncertainty in this rate coefficient represents a multiplicative The shaded area around the present results corresponds to the total systematic uncertainty of the measurement, mainly due to the absolute scaling of α mb and the uncertainty of T ⊥ . The particular relative contributions of the uncertainties have been discussed by Paul et al (2022). The single-pass merged-beams results of Mitchell (1990) have been incorporated into the UMIST database (McElroy et al 2013).…”

Simultaneous electrostatic trapping of merged cation & anion beams

“…In the first pulse, negative acceleration voltages are used to accelerate anions towards the HEIBT, while in the 2nd pulse, arriving after a B100 ms delay time, during which all the ion optics potentials are adjusted before the second pulse, positive acceleration potentials are applied to accelerate the cation ion bunch. To allow the ion beam to enter the trap, we lower the highest entrance mirror electrode potential from V M to 3 4 V M . Once the ions of interest enter the trap, the trap is rapidly closed by raising the potential back to V M , where the ion time of flight between the pulsed acceleration and the closing of the trap provides a rough selection of the velocity and charge over mass ratio of the ions of interest.…”

“…Alternatively cooling can be achieved by radiative thermalization of trapped ions. 3,37,51,52 Until this work, the double electrostatic ion ring experiment (DESIREE) was the only experimental setup that combines ion-trapping with a merged-beam section of velocity matched fast cation and anion beams. 19,53,54 In contrast to storage-ring devices, the electrostatic ion beam trap (EIBT) uses electrostatic mirrors to reflect and focus a fast ion beam in an analogous geometry to an optical resonator.…”

“…Electrostatic ion storage devices offer a powerful new technology to experimentally explore the interactions of atomic, molecular and cluster ion species with neutral targets, 1,2 electrons, 1,3,4 black-body radiation 5,6 and with various laser fields. [7][8][9][10][11][12][13][14][15] The detailed understanding of the interactions and time-evolution of ionic species is important for a fundamental understanding of the underlying quantum mechanical manyelectron and many-atom dynamics, 16,17 as well as for application to studies of the chemical evolution of a broad range of partly ionized interstellar-medium, [18][19][20][21][22] planetary, atmospheric and man-made environments.…”