THEORETICAL CONFIGURATION-INTERACTION STUDY OF THE VERTICAL ELECTRONIC-SPECTRUM OF ETHANE

Ab initio multireference single- and double-excitation configuration i nteraction calculations are reported for the ground and 32 excited ele ctronic states of ethane, as well as its two lowest ionic states, 2E(g ) and 2A1g. The transition energy results indicate that the 3a1g molec ular orbital is 0.3 0.6 eV more stable than the 1e(g) lowest unoccupie d molecular orbital for the ethane D3d equilibrium conformation. The s trongest transition is computed to occur for the 3a1g --> 3psigma 1A2u - 1A1g excitation at 10.58 eV, with an optical f value of 0.1152. The n = 4 Rydberg transitions are also calculated and are found to occur with roughly 40% of their n = 3 counterparts. The observed broadness o f the ethane UV spectrum is believed to be caused primarily by the hig h density of Rydberg upper states, as well as significant relaxation e ffects which occur upon excitation from the ethane electronic ground s tate. The fact that the strongest ethane transition is computed to hav e a vertical energy which is 1.2-1.4 eV higher than the center of the observed discrete bands at 9.4 eV suggests that the previous interpret ation of this structure should be reconsidered.