Thermodynamic investigation of the Si7 and Si8 clusters by Knudsen cell mass spectrometry

The Knudsen cell mass spectrometric method has been employed to measure the partial pressures of the Si7 and Si8 clusters under equilibrium conditions above liquid silicon, contained in a boron nitride liner inside a graphite Knudsen cell. Gaussian 2 (G2) theory and B3LYP density functional method were employed to determine the geometry, the vibrational frequencies, and the binding energy of the Si8 cluster. From the all-gas analyzed equilibria the following atomization enthalpies, ΔaH0o(Sin), and enthalpies of formation, ΔfH298.15o(Sin), in kJ mol−1, have been obtained: Si7, 2381±36 and 743±36; Si8, 2735±65 and 837±65. Experimental literature values for the electron affinities of Sin(n=3–8) have been combined with present and previous results to obtain the bonding energies for the Sin−(n=3–8) cluster anions. The experimental atomization energies are compared with available theoretical values.