Knowledge Bank

La$3$C$2$O is produced for the first time by laser vaporization in a pulsed cluster source and identified by photoionization time-of-flight mass spectrometry. Vibrationally-resolved ion spectra are obtained with mass-analyzed threshold ionization (MATI) spectroscopy. The adiabatic ionization energy of La$3$C$2$O is measured to be 30891(5) cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. The spectra display several short vibrational progressions, and these progressions are associated mainly with La-La, La-C and La$3$C$2$O stretching excitations. The electron-spin multiplicities and molecular symmetries of La$3$C$2$O and La$3$C$2$O$+$ are determined by combining the experimental measurements with ab initio calculations at MP2 level. Preliminary data analysis shows that the $1$A$1$ $\leftarrow$ $2$A$1$ transition is responsible for the observed MATI spectra. The cluster has C symmetry with La$3$C$2$O in a bi-pyramid structure and oxygen being attached to the La$3$ plane.