Two kinds of complexes of molecules exist: the collisional complexes discussed 
above, which are short lived. Besides, bound (i.e. relatively stable) complexes
of two or more molecules exist, the so-called van der Waals molecules. These 
exist usually for much longer times than the collisional complexes and, under 
carefully chosen experimental conditions (low temperature, moderate gas density), 
their rotovibrational band spectra show "sharp" (or resolvable) lines (Heisenberg
uncertainty principle), much like ordinary molecules. If the parent molecules 
are nonpolar, the same induced dipole mechanisms, which are discussed above, 
are responsible for the observable spectra of van der Waals molecules.
Figure 1 (to be included)