Scanning drift tube measurements and kinetic computations of electron swarm parameters in CO*

We present scanning drift tube measurements and modelling results of electron swarm transport coefficients in CO as a function of the reduced electric field E/N at room temperature. The measurements are performed under time-of-flight (TOF) conditions: a cloud of electrons is initiated by short UV laser pulses and its evolution is traced by detecting the electrons beyond a drift region whose length is varied. This way, so-called swarm maps are recorded, which are used to extract the transport coefficients of the electrons by fitting the current signals. Using the most recent set of cross-sections for electron scattering in CO from Magboltz (v11.11), modelling results are obtained by Monte Carlo (MC) simulations and by solving the Boltzmann equation using a multi-term approach and the density gradient expansion procedure. The measured and computed values of electron swarm transport coefficients agree very well, except for E/N<5 Td. For such low values of E/N the more advanced model for the strongly forward-peaked nature of rotational excitation is required. Measurements and calculations under TOF conditions are augmented by MC simulations of electron transport for idealized steady-state Townsend (SST) conditions. We found a ‘window’ of E/N where the SST transport properties of the electrons exhibit oscillatory behaviour as they relax towards the equilibrium state far downstream from the electron emitting boundary.