January 19, 2026 | UR Gate
Introduction to Molecular Absorption Spectroscopy (UV–Vis)
Molecular absorption spectrum, UV–Vis spectroscopy, vibrational–rotational levels, electronic transitions, broadband bands, bonding and nonbonding electrons.
Introduction to Molecular Absorption Spectroscopy (UV–Vis): From Energy Levels to Broadband Bands
The atomic spectrum is a line spectrum, meaning it consists of discrete lines (either absorption or emission). This occurs because atoms possess quantized energy levels, and in practice these are electronic energy levels only. The energy gaps between these levels are highly specific; therefore, absorption or emission takes place only at well-defined wavelengths, producing a line spectrum.
In contrast, molecules (the subject of this discussion) contain vibrational and rotational energy levels (vibrational and rotational energy levels) in addition to the main electronic energy levels. This means that an electron’s transition is not limited to moving from one electronic level to another; it may also transition to many vibrational and rotational sublevels. As a result, the molecular spectrum typically appears as a broad band spectrum (broad band spectrum), rather than a set of sharp lines.
As illustrated in the figure, the broad, dark bands represent the main electronic energy levels. Within each electronic level, multiple vibrational levels exist (the upper dark lines), and between these vibrational levels there is a very large number of rotational levels. This greatly increases the number of possible transitions from a lower energy level to a higher one (or vice versa), which is why the molecular spectrum becomes broad (broad band), as shown.
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[Figure 1: Energy-level scheme showing electronic levels with vibrational/rotational sublevels, and the resulting broad absorption bands vs wavelength.]
It is possible for a transition to occur from the ground energy level to the first excited level (lower absorbed energy → longer wavelength), or from the ground level to the second or third excited level, depending on the amount of energy absorbed from the source (higher absorbed energy → shorter wavelength).
In practical UV–Vis molecular spectroscopy (ultraviolet and visible regions), the required absorbed (or emitted) energy typically corresponds to wavelengths in the range 200–780 nm, while many instruments can measure up to approximately 1100 nm. This energy is sufficient to promote valence electrons only. In other words, the present discussion of molecular spectra concerns the interaction of radiation incident on the sample (in the UV and visible regions) with valence electrons in the substances that compose the sample.
Valence electrons in molecular compounds are generally either:
- Bonding electrons (bonding electrons): electrons participating in the bonds that form the compound.
- Nonbonding electrons (non bonding electrons): electrons not shared in bonding (e.g., the lone-pair electrons on the nitrogen atom in ammonia).
Therefore, it is reasonable to begin by studying the types of chemical bonds in different compounds and their relationship to absorption, starting first with the molecular absorption spectrum.
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2- Hollas, J. M. (2004). Modern Spectroscopy (4th ed.). John Wiley & Sons.
3- Skoog, D. A., Holler, F. J., & Crouch, S. R. (2014). Principles of Instrumental Analysis (7th ed.). Cengage Learning.
4- Atkins, P., de Paula, J., & Keeler, J. (2018). Atkins’ Physical Chemistry (11th ed.). Oxford University Press.
