Knowledge Bank

The history of the determination of several multipole moments of $H2$ and its isotopes will be reviewed. The interplay between theory and experiment will be illustrated using some recent experimental results in which new features become apparent at higher resolution or at higher densities. In particular, the importance of collision-induced effects for the determination of the dipole moment matrix elements of HD and the quadrupole moment matrix elements of $H2$ will be discussed. Finally, some complications involved in determining higher moments, e.g. the hexadecapole moment of $H2$, will also be considered.