Knowledge Bank

The rotational spectrum of a pure silicon cluster, the $Si3$ trimer, has been observed for the first time. From the rotational constants of the normal and the $\mathrm{<mrow\; data-mjx-texclass="ORD">29</mrow>}Si$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">30</mrow>}Si$ isotopic species, a precise geometrical structure has been derived: the trimer is an isosceles triangle with a bond to the apex Si of length 2.165(7) {\AA} and an apex angle of $78.1(4)\circ$. The substantial inertial defect and fairly large centrifugal distortion suggest that the molecule possesses a shallow bending potential. $Si3$ is a good candidate for astronomical detection because radio lines of comparably massive silicon molecules (e.g., $SiC2,SiC4$, and SiS) are readily observed in at least one astronomical source: the circumstellar shell of $IRC+10216$. In excess of $1013$ silicon trimers are produced per gas pulse in our molecular beam, implying that the rotational spectra of $Si6,Si9$, and even larger polar silicon clusters may be detectable with the present technique, as well as similar germanium clusters.