Rotational energy levels are very close together. As a consequence, rotational energy transitions are initiated by relatively low-energy (long-wavelength) radiation. Typically, rotational changes are produced by radiation in the microwave region. To absorb radiation and thereby change its rotational energy, a molecule must have a permanent dipole moment. Also, heteronuclear diatomic molecules can change only one rotational level at a time—for example, from a level with J =1 to a level with J = 2, from J = 2 to J = 3, and so on. That is, for these molecules, DJ = +1.