Rotational contour analysis of the (n, 3s) Rydberg transition in acetaldehyde

The rotational structure of the two-photon X̃B̃ transition in acetaldehyde is analyzed using the rigid rotor approximation. The transition is dominated by a zeroth rank tensor component S00 which explains the Q branch structure of the experimentally observed vibronic bands. Under the cold molecular beam conditions, the spectrum due to a minor second rank tensor component S22 becomes detectable. Its rotational structure confirms the rotational constants (A′ = 1.973 cm−1, B′ = 0.335 cm−1, C′ = 0.298 cm−1) derived from the analysis of the room temperature contours.