
In molecular physics, the molecular term symbol is a shorthand expression of the group representation and angular momenta that characterize the state of a molecule, i.e. its electronic quantum state which is an eigenstate of the electronic molecular Hamiltonian. It is the equivalent of the term symbol for the atomic case. However, the following presentation is restricted to the case of homonuclear diatomic molecules, or symmetric molecules with an inversion centre. For heteronuclear diatomic molecules, the ''u/g'' symbol does not correspond to any exact symmetry of the electronic molecular Hamiltonian. In the case of less symmetric molecules the molecular term symbol contains the symbol of the group representation to which the molecular electronic state belongs. It has the general form: :$\_$ where * ''S'' is the total spin quantum number * Λ is the projection of the orbital angular momentum along the internuclear axis * Ω is the projection of the total angular momentum along the internuclear axis * ''u''/''g'' is the parity * +/− is the reflection symmetry along an arbitrary plane containing the internuclear axis ==Λ quantum number== For atoms, we use ''S'', ''L'', ''J'' and ''M_{J}'' to characterize a given state. In linear molecules, however, the lack of spherical symmetry destroys the relationship $($_{S}'', ''M_{L}'' and ''M_{J}''. The cylindrical symmetry of a linear molecule ensures that positive and negative values of a given ''m_{l}'' for an electron in a molecular orbital will be degenerate in the absence of spinorbit coupling. Different molecular orbitals are classified with a new quantum number, λ, defined as :λ = ''m_{l}'' Following the spectroscopic notation pattern, molecular orbitals are designated by a smallcase Greek letter: for λ = 0, 1, 2, 3,... orbitals are called σ, π, δ, φ... respectively. Now, the total ''z''projection of ''L'' can be defined as :$M\_L=\backslash sum\_i\; \_i.$ As states with positive and negative values of ''M_{L}'' are degenerate, we define :Λ = ''M_{L}'', and a capital Greek letter is used to refer to each value: Λ = 0, 1, 2, 3... are coded as Σ, Π, Δ, Φ... respectively. The molecular term symbol is then defined as :^{2''S''+1}Λ and the number of electron degenerate states (under the absence of spinorbit coupling) corresponding to this term symbol is given by: * (2''S''+1)×2 if Λ is not 0 * (2''S''+1) if Λ is 0. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Molecular term symbol」の詳細全文を読む スポンサード リンク
