Why molecules absorb light?

In summary, molecules absorb light through their electronic and vibrational transitions, which are governed by certain rules that give each molecule its unique "fingerprints." These transitions occur due to the coupling of the molecule's dipole moment with the electromagnetic field. In the case of non-polar molecules like CH4, only certain vibrational modes are active in infrared absorption, while rotations depend on the orientation of the molecule and the field. The absorption phenomenon can also be related to resonance, where the frequency of the applied external source matches the molecule's natural vibrational frequency. In condensed matter, the interaction between molecules affects the absorption process and can lead to rovibrational coupling. Stimulated emission can also occur, where a photon of the same
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We know that molecules absorb light based on their electronical, rotational, and vibrational transitions and the governed transition rules (so that each molecule has its own "fingerprints"). But I do not know why it happens?

Lets simplify it a little:

A methane molecule, for example, is charge-less (the net summation of p+e=0) while we know CH4 absorbs the photons with 3.3um wavelength. Now the question is why/how this happens? Why electromagnetic wave (photons) interacts with a charge-less "thing"?

Any help and/or advice is appreciated,
S
 
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  • #2
Molecules are not charge-less even though they are globally electrically neutral. They are still composed of positive nuclei and negative electrons. Changing the vibrational or electronic state will change the charge distribution, and therefore these can couple with the electromagnetic field.

The example of CH4 is a good one to explore some subtleties of this. Generally speaking, a vibrational transition will occur when it leads to a change in the dipole moment. If a molecule is polar (e.g., CO), then it can absorb IR light and be excited vibrationally, as this will necessarily change the dipole. However, a non-polar molecule (e.g., N2, O2) will not absorb in the infrared, as the charge symmetry is preserved even with vibrational excitation (the dipole moment is always 0). In the case of CH4, which is non-polar, certain modes will be active in the IR, such as an symmetric stretch, where some bonds contract at the same time others lengthen. But the purely symmetric stretch (breathing mode), where all bonds vibrate in phase, is not active (there will be no absorption of photons that can excite this mode).

Note that this is all true to first order only. There can be additional transition mechanisms (such as electric quadrupole, magnetic dipole, ...) or other processes (e.g., Raman) that can excite such vibrations. But dipole transitions are by far the strongest transitions in molecules, and the main reason why a molecule will absorb (or not) photons at a certain wavelength.

Rotations are a bit different, as they depend on the relative orientation of the molecule and the field. But it is again due to the coupling of the dipole (or induced dipole) of the molecule with the EM field.
 
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  • #3
DrClaude said:
Molecules are not charge-less even though they are globally electrically neutral. They are still composed of positive nuclei and negative electrons. Changing the vibrational or electronic state will change the charge distribution, and therefore these can couple with the electromagnetic field.

The example of CH4 is a good one to explore some subtleties of this. Generally speaking, a vibrational transition will occur when it leads to a change in the dipole moment. If a molecule is polar (e.g., CO), then it can absorb IR light and be excited vibrationally, as this will necessarily change the dipole. However, a non-polar molecule (e.g., N2, O2) will not absorb in the infrared, as the charge symmetry is preserved even with vibrational excitation (the dipole moment is always 0). In the case of CH4, which is non-polar, certain modes will be active in the IR, such as an symmetric stretch, where some bonds contract at the same time others lengthen. But the purely symmetric stretch (breathing mode), where all bonds vibrate in phase, is not active (there will be no absorption of photons that can excite this mode).

Note that this is all true to first order only. There can be additional transition mechanisms (such as electric quadrupole, magnetic dipole, ...) or other processes (e.g., Raman) that can excite such vibrations. But dipole transitions are by far the strongest transitions in molecules, and the main reason why a molecule will absorb (or not) photons at a certain wavelength.

Rotations are a bit different, as they depend on the relative orientation of the molecule and the field. But it is again due to the coupling of the dipole (or induced dipole) of the molecule with the EM field.
Thank you for all this helpful information.

Just one more question: How can we relate absorption phenomenon to the "resonance" which happens when the frequency of applied external source is very close to the natural frequency of the vibrating objects. In the case of CH4 , for example, can we say the CH4 molecule is vibrating with the frequency of c/3.3um (c is the speed of light) and when the light with this frequency hits the molecule, resonance occurs and so the energy of the molecule increases?
 
  • #4
Can anyone please give me an advice on the above/below question:

Can we relate the molecular vibration to resonance effect? i.e. Does it mean that if CH4 absorbs 3.3 um light, it is really vibrating with the frequency of c/3.3 um (c is the speed of light) and that is why higher or lower wavelengths are not absorbed?
 
  • #5
A substance that is in the gas phase absorbs EM radiation of frequencies that are equal to the vibration frequencies of its normal modes. In liquid or solid phase there aren't similar sharp spikes in the absorption spectrum, because the interaction between the molecules in condensed matter affects the absorption process. Sometimes there is also rovibrational coupling which means that molecules that are in different rotation states vibrate at slightly different frequencies, this can happen even in the gas phase.

A photon of a certain wavelength can also cause stimulated emission of a photon of same frequency from the molecule.
 
  • #6
Thank you for your explanation. With all due respect, I think this is not the answer of my question though.

Maybe I should rephrase it:

All I am asking is if CH4 molecule (for example) is vibrating with the frequency of light (in this case, c/3.3 um=9.1*10^13 Hz) that is absorbed or light frequency and vibration frequency are two different aspects of absorption process?
 
  • #7
Sotinam said:
All I am asking is if CH4 molecule (for example) is vibrating with the frequency of light (in this case, c/3.3 um=9.1*10^13 Hz) that is absorbed or light frequency and vibration frequency are two different aspects of absorption process?

Yes it is. That's what I said.
 
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  • #8
hilbert2 said:
Yes it is. That's what I said.
Thank you
 

1. Why do molecules absorb light?

Molecules absorb light because of the electrons present in their structure. When light energy hits a molecule, it can be absorbed by these electrons, causing them to jump to a higher energy level. This absorption of light energy is what gives molecules their color.

2. How does the structure of a molecule affect its ability to absorb light?

The structure of a molecule plays a crucial role in its ability to absorb light. Molecules with alternating single and double bonds, known as conjugated systems, have a higher chance of absorbing light due to the delocalization of electrons. Additionally, the size and shape of the molecule can also affect its absorption properties.

3. What determines the wavelength of light that a molecule can absorb?

The wavelength of light that a molecule can absorb is determined by the energy difference between its ground state and excited state. This energy difference is unique to each molecule and is influenced by factors such as its structure, electronic transitions, and the surrounding environment. As a result, different molecules can absorb different wavelengths of light.

4. Can all molecules absorb light?

No, not all molecules can absorb light. Only molecules with delocalized electrons or those with specific energy differences between their ground and excited states are capable of absorbing light. These molecules are often referred to as chromophores.

5. Why is the absorption of light important in biological systems?

The absorption of light is crucial in biological systems as it is responsible for photosynthesis, vision, and the production of vitamin D. Additionally, the absorption of light by molecules in the body can also have therapeutic effects, such as in photodynamic therapy for cancer treatment.

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