Consider the following wave functions representing excited states of He: ψa = 1s(1)2s(2)α(1)α(2) ψb = [1s(1)2s(2) + 1s(2)2s(1)][α(1)β(2) − α(2)β(1)] ψc = [1s(1)2s(2) − 1s(2)2s(1)][α(1)β(2) − α(2)β(1)] ψd = [1s(1)2s(2) − 1s(2)2s(1)]α(1)α(2) (a) Determine which of the wave functions are eigenfunctions of the exchange operator, by acting the exchange operator. (b) Determine which are valid wave functions for excited state He, i. e., which have an eigenvalue of -1 and thus are antisymmetric with respect to exchange. (c) Which of the valid wavefunctions has/have the lowest energy