The key difference between atomic spectroscopy and molecular spectroscopy is that the atomic spectroscopy refers to the study of the electromagnetic radiation absorbed and emitted by atoms whereas the molecular spectroscopy refers to the study of the electromagnetic radiation absorbed and emitted by molecules.
An electromagnetic wave is consisting of an electric field and magnetic field oscillating perpendicular to each other. Thus, the full range of electromagnetic radiation wavelengths is what we call the electromagnetic spectrum. In spectroscopy experiments, we use electromagnetic radiation of specific wavelengths to analyze a sample. There, we let the electromagnetic radiation to pass through our sample which contains the chemical species of interest.
