Why are Raman spectra of different substances unique

If you have heard of "light quanta" and know that one light corresponds to one energy (E=h ν), and molecules' (resonant) absorption and emission of light need to match this energy (Δ Ε=h ν), then the rest of the story is easy to explain. First of all, it's not just Raman. The Raman, infrared, ultraviolet visible, and microwave spectra of different molecules can basically be said to be unique. These different types of spectra only have different excitation forms, but the underlying reactions are the properties of the molecules themselves, which we call "energy levels". The meaning of "energy level" is that there are many energy states inside the molecule. Just like a person who can lie down, sit, stand, and run; Their corresponding energies are different. Molecules are the same, they can rotate, vibrate, and undergo "electron transitions" - that is, electrons in a molecule change their form. They also have different energies. Because energy is quantized, it becomes energy levels. Spectrum is a curve that records how much light a molecule absorbs at what frequency. The frequency of each absorbed light corresponds to the energy difference between two energy levels in the molecular energy level sequence. Therefore, different energy levels result in different spectra. So, why is the energy level of each molecule unique? It's easy to say. The energy level of a molecule is determined by its own size, mass, and the properties of its chemical bonds. Just like, why do different instruments produce different tones that you can distinguish with just one ear? Because instruments have different materials, sizes, shapes, and ways of producing sound. Even if the same instrument, such as every violin made by a legendary master, has its own unique "style", why? Because no two pianos have the same material and shape, there are always differences. Molecules are the same. As long as the molecules are different, the energy levels are different; Different energy levels result in different spectra. To be more advanced, the rotational energy level is determined by the moment of inertia of the molecule, the vibrational energy level is determined by the vibrational mode of the molecule - that is, the mass of the molecule and the strength of the chemical bond. The electronic energy level is of course determined by the structure of the electron arrangement. Microwave measurement of rotation, infrared measurement of vibration, and ultraviolet visible measurement of electrons.

Schematic diagram of molecular energy levels and corresponding spectra

However, it does not mean that any instrument can distinguish different molecules by measuring their spectra. The spectral resolution and measurement method of the instrument are important. It's like a layman and a performer listening to different violin sounds, of course, the performer can distinguish more subtle differences. Generally, it is necessary to use a "high-resolution spectrometer" with high spectral resolution, and in the case of molecules being rarefied gases, sufficient high-frequency spectral resolution can be obtained. That's also why in ordinary organic chemistry laboratories, infrared/Raman spectroscopy is usually only used as an auxiliary tool for molecular judgment and cannot make a definitive decision - because these ordinary spectrometers have too low resolution and measure liquid/solid samples, smoothing out many small differences and only allowing for qualitative guessing of how many functional groups the molecule carries.