The electronic spectra of H 2 CO, D 2 CO, HDCO, and D2 \3CO have been recorded between 1250 and 2000 A and the vibrational fine structure associated with the n-> 3 s, 3 p" 3 py, 3 p" 3 d transitions assigned. The electronic origin bands in the Rydberg transitions show quite unusual isotope effects, with the v(D 2 CO)-v(H 2 CO) shift for the n-> 3 d transition being somewhat smaller than the limiting value of the ion while the lower energy n->3s and 3py transitions have anomalously large values. As these effects cannot be accommodated by the changes in the totally symmetric in-plane frequencies on excitation, they are attributed to large differences in Vs and V6 in the upper and lower electronic states. Direct information concerning the reduction in V6 on excitation comes from the identification of a series of low frequency bands in the n-> 3 Py system of D 2 CO which were assigned to quanta in V6' The potential function which was derived from this data was found to be very anharmonic and could contain a double minimum. The reduction in V( Q,;) at large Q,; was attributed to a coupling between the Rydberg IA1(n, 3py) state and a second IB2 state which lies at higher frequencies. An estimate of the coupling between these states lead to a value of 1480 em ~ I for the vibronic matrix element. While this value is entirely compatible with what would be expected for valence-valence coupling, it is somewhat larger than what would have been anticipated for vibronic coupling between Rydberg states. RESULTS Electronic structureThe electronic structure of formaldehyde in its ground and higher electronic states may be derived from molecular orbital theory as a linear combination

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