#Whats absorption spectra series

Weaker absorption lines, called overtones, result from transitions in which ∆ ν is ☒ or ☓. Transitions in which ∆ ν is ☑ give rise to the fundamental absorption lines. A transition from the ground vibrational state to the first vibrational excited state ( ν = 1) requires absorption of a photon with an energy of h ν 0. For example, a carbon-carbon single bond (C–C) absorbs infrared radiation at a lower energy than a carbon-carbon double bond (C=C) because a single bond is weaker than a double bond.Īt room temperature most molecules are in their ground vibrational state ( ν = 0). The value of ν 0, which is determined by the bond’s strength and by the mass at each end of the bond, is a characteristic property of a bond. Where ν is the vibrational quantum number, which has values of 0, 1, 2, …, and ν 0 is the bond’s fundamental vibrational frequency. The energy for an allowed vibrational mode, E ν, is As shown in Figure 10.15, vibrational energy levels are quantized that is, a molecule may have only certain, discrete vibrational energies. The energy of infrared radiation produces a change in a molecule’s or a polyatomic ion’s vibrational energy, but is not sufficient to effect a change in its electronic energy. Infrared Spectra for Molecules and Polyatomic Ions A change in vibrational energy without a change in electronic energy levels occurs with the absorption of infrared radiation. Absorption of ultraviolet and visible radiation leads to a change in the analyte’s electronic energy levels and, possibly, a change in vibrational energy as well.

#Whats absorption spectra series

Superimposed on each electronic energy level is a series of lines representing vibrational energy levels.įigure 10.15 Diagram showing two electronic energy levels ( E 0 and E 1), each with five vibrational energy levels ( ν 0– ν 4). The lines labeled E 0 and E 1 represent the analyte’s ground (lowest) electronic state and its first electronic excited state. We can use the energy level diagram in Figure 10.15 to explain an absorbance spectrum. What is missing, however, is information about what types of energy states are involved, which transitions between energy states are likely to occur, and the appearance of the resulting spectrum. Figure 10.4 shows a simplified view of a photon’s absorption, which is useful because it emphasizes that the photon’s energy must match the difference in energy between a lower-energy state and a higher-energy state. The second requirement is that the photon’s energy, h ν, must exactly equal the difference in energy, ∆ E, between two of the analyte’s quantized energy states. ( Figure 10.3 provides a list of the types of atomic and molecular transitions associated with different types of electromagnetic radiation.) A bond’s vibrational energy is altered by the absorption of infrared radiation. For ultraviolet and visible radiation, absorption of a photon changes the energy of the analyte’s valence electrons.

First, there must be a mechanism by which the radiation’s electric field or magnetic field interacts with the analyte. There are two general requirements for an analyte’s absorption of electromagnetic radiation.