  We report the detection of the propargyl radical (CH2CCH) in the cold dark
cloud TMC-1 in the lambda 3 mm wavelength band. We recently discovered this
species in space toward the same source at a wavelength of lambda 8 mm. In
those observations, various hyperfine components of the 2,0,2-1,0,1 rotational
transition, at 37.5 GHz, were detected using the Yebes 40m telescope. Here, we
used the IRAM 30m telescope to detect ten hyperfine components of the
5,0,5-4,0,4 rotational transition, lying at 93.6 GHz. The observed frequencies
differ by 0.2 MHz with respect to the predictions from available laboratory
data. This difference is significant for a radioastronomical search for CH2CCH
in interstellar sources with narrow lines. We thus included the measured
frequencies in a new spectroscopic analysis to provide accurate frequency
predictions for the interstellar search for propargyl at mm wavelengths.
Moreover, we recommend that future searches for CH2CCH in cold interstellar
clouds are carried out at lambda 3 mm, rather than at lambda 8 mm. The
5,0,5-4,0,4 transition is about five times more intense than the 2,0,2-1,0,1
one in TMC-1, which implies that detecting the former requires about seven
times less telescope time than detecting the latter. We constrain the
rotational temperature of CH2CCH in TMC-1 to 9.9 +/- 1.5 K, which indicates
that the rotational levels of this species are thermalized at the gas kinetic
temperature. The revised value of the column density of CH2CCH (including ortho
and para species) is (1.0 +/- 0.2)e14 cm-2, and thus the CH2CCH/CH3CCH
abundance ratio is revised from slightly below one to nearly one. This study
opens the door for future detections of CH2CCH in other cold interstellar
clouds, making possible to further investigate the role of this very abundant
hydrocarbon radical in the synthesis of large organic molecules such as
aromatic rings.
