Cumulene carbenes in TMC-1: Astronomical discovery of <i>l</i>-H₂C₅

We report the first detection in space of cumulene carbon chain $l$-H$_2$C$_5$. A total eleven rotational transitions, with $J_{up}$ = 7-10 and $K_a$ 0 1, were detected TMC-1 31.0-50.4 GHz range using Yebes 40m radio telescope. derive a column density (1.8$\pm$0.5)$\times$10$^{10}$ cm$^{-2}$. In addition, we observations other carbenes previously TMC-1, to compare their abundances newly carbene chain. find that $l$-H$_2$C$_5$ is $\sim$4.0 times less abundant than larger $l$-H$_2$C$_6$, while it $\sim$300 $\sim$500 shorter chains $l$-H$_2$C$_3$ $l$-H$_2$C$_4$. discuss most likely gas-phase chemical routes these cumulenes stress kinetics studies able distinguish between different isomers are needed shed light on chemistry C$_n$H$_2$ $n$\,$>$\,3.