The linear carbon chain
(HC5O) was detected by
McGuire and co-workers
TMC-1 using the
Green Bank Telescope, as reported in 2017.
While the detection is based on a single rotational transition, it includes the four lines that arise from the
hyperfine splitting associated with state of
the molecule. As a 2Π state, the
ground state of HC5O splits into two levels, which is known as fine structure.
The spectroscopic constants measured by
Mohammed and co-workers were used for the detection of HC5O.
They were able to measure the constants for the 2Π1/2 state without also generating the
spectrum of the 2Π3/2 state due to the very cold beam of molecules they were able
to form. The detection of HC5O toward TMC-1 was confirmed in 2021 by
Cernicharo et al., who observed a total
of 14 rotational transitions.