Communications: Development and characterization of a source of rotationally cold, enriched para- H₃⁺

In an effort to develop a source of H₃⁺that is almost entirely in a single quantum state (J=K=1), we have successfully generated a plasma that is enriched to ∼83% in para- H₃⁺ at a rotational temperature of 80 K. This enrichment is a result of the nuclear spin selection rules at work in hydrogenic plasmas, which dictate that only para- H₃⁺will form from para- H₂, and that para- H3+ can be converted to ortho- H3+ by subsequent reaction with H₂. This is the first experimental study in which the H₂ and H ₃⁺nuclear spin selection rules have been observed at cold temperatures. The ions were produced from a pulsed solenoid valve source, cooled by supersonic expansion, and interrogated via continuous-wave cavity ringdown spectroscopy.