Threshold behavior of laser ionization spectra for small potassium clusters

Production and ionization energies of K(n)F (n = 2-6) clusters by thermal ionization mass spectrometry

RATIONALE

The very small clusters of the type K(n)F are of particular importance since their first ionization energies (IEs) are lower than those of the alkali metal atoms. Theoretical calculation has demonstrated that this kind of cluster represents a potential 'building block' for cluster-assembly materials with unique structural, electronic, optical, magnetic, and thermodynamic properties. To date, however, there have been no experimental results on the IEs of K(n)F (n >2) clusters.

METHOD

K(n)F (n = 2-6) clusters were produced by the evaporation of a solid potassium fluoride salt using a modified thermal ionization source of modified design, and mass selected by a magnetic sector mass spectrometer where their IEs were determined.

RESULTS

Clusters K(n)F (n = 3-6) were detected for the first time. The order of the ion intensities was K(2)F(+)> > K(4)F(+)> > K(3)F(+)K(6)F(+)> K(5)F(+). The determined IEs were 3.99 ± 0.20 eV for K(2)F, 4.16 ± 0.20 eV for K(3)F, 4.27 ± 0.20 eV for K(4)F, 4.22 ± 0.20 eV for K(5)F, and 4.31 ± 0.20 eV for K(6)F. The IEs of K(n)F increase slightly with the increase in potassium atom number from 2 to 6. We also observed that the presence of a fluorine atom leads to increasing ionization energy of bare metal potassium clusters.

CONCLUSIONS

The modified thermal ionization source provides an efficient way of obtaining the fluorine-doped potassium clusters. These results also present experimental proof that K(n)F (n = 2-6) clusters belong to the group of 'superalkali' species.