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 H3+ 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.