KEY2CHEM

Spectroscopy: Interaction of Light with Matter

Spectroscopy is the interaction of electromagnetic radiation (or light) with matter. This interaction produces some kind of change in the matter that often provides useful information regarding the structure of the atoms in the matter. Different photon energies produce different changes in the molecules that are excited. For example, visible light allows electrons to change between different energy shells in an atom. This is called an electronic transition since the electrons are moving. Infrared (IR) light causes transitions in vibrational motions of atoms or molecules.

Example 1.

A student uses a green laser pointer (wavelength = $$540\text{ nm}$$) to excite a transition in a molecule. What type of transition is likely observed?

A. Vibrational

B. Rotational

C. Electronic

Solution

C. Electronic

A wavelength of $$540\text{ nm}$$ is in the visible range of the electromagnetic spectrum. This likely induces an electronic transition (movement of electrons between energy shells) in a molecule.

Example 2.

Why is visible light useful for exciting electronic transitions and infrared radiation is useful for exciting molecular vibrations?

A. Molecules cannot undergo electronic transitions.

B. Vibrational motion is lower energy, so a lower energy photon can be used.

C. Electronic transitions change color depending on what color visible light they absorb.

Solution

B. Vibrational motion is lower energy, so a lower energy photon can be used.

The photon ($$E = h\nu$$) used to induce a transition is equal to the energy gap of the transition. Since infrared radiation is lower in energy than visible light, the energy gaps between vibrational energy levels are smaller than those for electronic transitions.

Example 3.

To determine the concentration of Red Food Dye #40 in a sports drink, what type of spectroscopy would be most useful?

A. UV-vis spectroscopy

B. IR spectroscopy

C. Mass spectrometry

Solution

A. UV-vis spectroscopy

UV-vis (electronic) spectroscopy is good for determining the concentration of highly-pigmented substances. The Beer-Lambert law relates the amount of radiation absorbed to the concentration of absorbing molecules.