Characterization of a simple gas expansion ion source for intense pulses of subthermal molecular ions

We describe a simple gas expansion ion source based on static discharge voltages and a commercially available pulsed valve. The discharge is initiated by the gas pulse itself between two high voltage electrodes, without the need for fast voltage switches or complex timing schemes. The ion source very reliably produces intense bursts of molecular ions (with currents exceeding 100 μA during the pulse-on phase) with only minor pulse-to-pulse variations in intensity and pulse shape. We have characterized the internal energy of H3+ and N2O+ ions produced by the ion source, using dissociative charge exchange and laser photodissociation, respectively. We find that collisions in the expanding gas reduce the internal energy of the molecular ions to well below room temperature.