**1. The problem statement, all variables and given/known data**
Consider the formaldehyde molecule (A). The 2Pz orbitals of C and O are together a pi molecular orbital (MO, as drawned in (B). De pi* MO is marked in (C).

Show, with the help of symmetry arguments, that die transition dipole moment in formaldehyde of pi-pi* transition is polarised through the C=O axis. (Hint: expand the dipole operator mu in cartesian coordinates mu of x, mu of y and mu of z (MUx, MUy, MUz) and mirror the transition dipole moment in the xy, xz and yz plane).

**2. Relevant equations**
**3. The attempt at a solution**
How to start? (Not the answer, but a clue to the answer).

Thnx!