! The allowed changes in the rotational quantum number J are DJ = ± l for parallel (S u +) transitions and DJ = 0, ± l for perpendicular (P u) transitions [3,5,7,8]. However, what these do not take into account is whether or not the state being transitioned from is actually populated, meaning that the molecule is in that energy state. These spectral lines are actually specific amounts of energy for when an electron transitions to a lower energy level. Imgur. In physics and chemistry, a selection rule, or transition rule, formally constrains the possible transitions of a system from one quantum state to another. In general the transition dipole moment is a complex vector quantity that includes the phase factors associated with the two states. Vibrational energy states. For a transition from the energy level denoted by J to that denoted by J + 1, the energy change is given by hν = E J + 1 − E J = 2(J + 1)(h 2 /8π 2 I) or ν = 2B(J + 1), where B = h/8π 2 I is the rotational constant of the molecule. The energy drop that an electron might have can be any where between fairly small (< 0.001 eV) for transitions between very high shells and the ionization energy, the highest to lowest orbitals, which is typically 10 to 20 eV. It can be approximated by the midpoint between the j=1,v=0->j=0,v=1 transition and the j=0,v=0->j=1,v=1 transition. The transition moment integral and the selection rule for rotational transitions tell if a transition from one rotational state to another is allowed. Ignoring electronic excitation, the total internal energy of a molecule is the sum of its vibrational and rotational energy. The desired transition frequency does not show up directly in the observed spectrum, because there is no j=0, v=0 to j=0, v=1 transition; the rotational quantum number must change by one unit. The transition dipole moment or transition moment, usually denoted for a transition between an initial state, , and a final state, , is the electric dipole moment associated with the transition between the two states. Since the energy level of the electron of a hydrogen atom is quantized instead of continuous, the spectrum of the lights emitted by the electron via transition is also quantized. E(v,J) = (v+½) e – (v+½)2xe e + Bv J(J+1) – DJ J2(J+1)2 Transitions between the E(v,J) levels in which v changes correspond to absorption of energy in the infrared region of the spectrum. Figure 1: Electronic Excitation Diagram. (Recall: E = hc/λ). Since vibrational energy states are on the order of 1000 cm -1 , the rotational energy states can be superimposed upon the vibrational energy states. Here we see that the absorption transitions by default involve a greater energy change than the emission transitions. This gives emission transitions of lower energy and consequently, longer wavelength than absorption. Radiation corresponding to this electron transition is absorbed, creating a peak at a corresponding energy (wavelength) in the absorbance spectrum. As you I just discussed in the Spectral Lines page, electrons fall to lower energy levels and give off light in the form of a spectrum. Due to vibrational relaxation in the excited state, the electron tends to relax only from the v'=0 ground state vibrational level. Electron transition from n ≥ 4 n\ge4 n ≥ 4 to n = 3 n=3 n = 3 gives infrared, and this is referred to as the Paschen series. an electron to move from its ground state (GS) to a much higher energy orbital (an electronic excitation state (EE)). A molecule’s rotation can be affected by its vibrational transition because there is a change in bond length, so these rotational transitions are expected to occur. Selection rules have been derived for electromagnetic transitions in molecules, in atoms, in atomic nuclei, and so on.The selection rules may differ according to the technique used to observe the transition.