Hyperfine structure of pendular states and the sign of the dipole moment of ICl A state

Since a first study by Cummings and Klemperer [J. Chem. Phys. 60, 2035 (1974)] and further works by Friedrich and Herschbach, the dipole moments of the X and A states of ICl are considered to have opposite signs. We have performed a high resolution study of the Stark effect of the A–X transition near the 21-0 bandhead, and we have made a complete theoretical simulation of the spectrum, taking into account the hyperfine structure which is important. The comparison of the experimental spectrum with the simulation clearly prove that the dipole moments of the A and X state have the same sign. This result is in agreement with simple electronegativity arguments.