Other dipole induction mechanisms also exist in molecular (as opposed to monatomic) gases and in mixtures of gases, when molecular gases are present. Molecules have centers of positive charge (the nuclei), which are surrounded by a cloud of electrons. Molecules thus may be thought of being surrounded by various electric multipolar fields which will polarize any collisional partner momentarily in a fly-by encounter, generating the so-called multipole-induced dipoles. In diatomic molecules such as H 2 and N 2, the lowest-order multipole moment is the quadrupole, followed by a hexadecapole, etc., hence the quadrupole-induced, hexadecapole-induced,... dipoles. Especially the former is often the strongest, most significant of the induced dipoles contributing to CIA and CIE. Other induced dipole mechanisms exist. In collisional systems involving molecules of three or more atoms (CO 2, CH 4...), collisional frame distortion may be an important induction mechanism. Collision-induced emission and absorption by simultaneous collisions of three or more particles generally do involve pairwise-additive dipole components, as well as important irreducible dipole contributions and their spectra.