MASS-ANALYZED THRESHOLD IONIZATION OF LANTHANUM OXIDE CLUSTERS: La$_2$O$_2$ AND La$_3$O$_4$

Abstract

Lanthanum oxide clusters are produced by laser vaporization in a pulsed cluster source and identified by photoionization mass spectrometry. Vibrationally resolved ion spectra are obtained with mass-analyzed threshold ionization (MATI) spectroscopy. The MATI spectra of La$_2$O$_2$ and La$_3$O$_4$ exhibit a very strong 0-0 transition, indicating similar geometries for the neutral and ionized clusters and a very weakly bonding or non-bonding electron ejected from an outmost molecular orbital. The ionization energies of La$_2$O$_2$ and La$_3$O$_4$ are measured to be 36937(5) and 28028(5) cm$^{-1}$, respectively. In addition, the spectra of both clusters display a number of vibrational intervals that are associated with metal-metal, metal-oxygen, and oxygen-oxygen vibrations. Preliminary data analysis shows that the La$_2$O$_2$ cluster has a D$_{2h}$ planar structure and La$_3$O$_4$ has a C$_{3v}$ cage-like structure, both with alternating La-O-La bonds. The spectra may be assigned to the $^2$Ag$\leftarrow$$^1$Ag transition in the case of La$_2$O$_2$ and $^1$A$_1$$\leftarrow$$^2$A$_1$ in La$_3$O$_4$ .