text: 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)