PHOTOIONIZATION OF GROUP-V TRIMERS AND TETRAMERS

The photoionization of saturated antimony and bismuth vapors is invest igated. In antimony, the dominant vapor species is Sb-4. Its photoion yield curve is similar to those of P-4 and As-4, displaying three auto ionizing bands and an apparent adiabatic IP of 7.56 eV. The appearance potential of Sb-3(+)(Sb-4) occurs at 9.75(5-0.04)(+0.01) eV at 830 K, or 10.22(9-0.04)(+0.01) eV at 0 K. This value, together with Delta (H 10Sb3)-Sb-Theta), yields IP(Sb-3) less than or equal to 6.61 eV. Bismu th vapor contains approximate to 1% Bi-4 and even less Bi-3. The photo ion yield curve of Bi-4(+), with an apparent adiabatic IP of 6.81 eV, also displays three autoionizing bands. An analysis of these bands, an d comparison with the other Group V tetramers with T-d symmetry enable s one to estimate vertical IPs of 9.0 eV for (a(1))(-1), 7.5 and 8.9 e V for the spin-orbit split (t(2))(-1), 7.0 and 7.4 eV for the Jahn-Tel ler split (e)(-1). The photoion yield curve of Bi-3(+) has an adiabati c onset of less than or equal to 6.36 eV, corresponding to formation o f Bi-3(+), X(1)A(1)' in D-3h symmetry. An increase in slope at approxi mate to 7.4 eV is identified with the configuration... (la(2)'')(2e')( 4)(le''), which may be an E' state. At approximate to 8.8 eV, a pronou nced increase in slope may indicate a higher excited state, fragmentat ion of Bi-4, or a near coincidence of the two. The directly or indirec tly measured IPs of all Group V trimers are in fairly good agreement w ith ab initio calculations. The heats of formation of the neutral trim ers can be rationalized by a simple model involving transferability of sigma and pi bond energies from the corresponding dimers and tetramer s. The atomization energies of the trimer cations are significantly la rger than for the corresponding neutrals, which may be related to the closed shell structure of the cations.