Interaction of radiation with matter

[Jimmy87]

Hi, please could someone help as I have got myself desperately confused with how radiation interacts with matter largely because of conflicting sources on the internet. Apparently there are 2 main types of interaction of radiation with matter; atomic/molecular vibrations and electron transitions. There are a few things I am confused with:

1) Most sources I have read say a photon frequency of infrared or lower mainly interacts via atomic/molecular vibration although some sources say infrared can cause energy level transitions but most say infrared does not have enough energy to cause a discrete transition. Could someone clarify/confirm this?
2) If humans absorb visible light photons why can't they emit visible light (as we emit in the infrared). This seems stupid and clearly we don't but if we absorb visible light photons which cause discrete electron transitions then couldn't the electron jump straight back down to the ground state and emit a visible photon. What am I missing here?
3) I'm a bit confused with the vibrational absorption of radiation. It seems to suggests that you need a change in the dipole moment to absorb say, an infrared photon and that it depends on the natural frequency vibration of the molecule in question. But surely ALL molecules must interact not just ones that have a matching natural frequency because all objects emit infrared otherwise they would be at absolute zero?

Many thanks for any help anyone gives!

 

[mfb]

1) Most sources I have read say a photon frequency of infrared or lower mainly interacts via atomic/molecular vibration although some sources say infrared can cause energy level transitions but most say infrared does not have enough energy to cause a discrete transition. Could someone clarify/confirm this?

In metals, the energy is sufficient for transitions between electron energy levels. For other materials, that is rare.

2) If humans absorb visible light photons why can't they emit visible light (as we emit in the infrared).

We do! Otherwise you could not see humans. We don't do that in the dark (in relevant amounts), because that would need a temperature of ~1000K or chemical reactions that lead to such an emission.

3) I'm a bit confused with the vibrational absorption of radiation. It seems to suggests that you need a change in the dipole moment to absorb say, an infrared photon and that it depends on the natural frequency vibration of the molecule in question. But surely ALL molecules must interact not just ones that have a matching natural frequency because all objects emit infrared otherwise they would be at absolute zero?

All materials have many vibrations of various frequencies, some of them have transitions with a dipole moment.

because all objects emit infrared otherwise they would be at absolute zero?

No, they would have a low emissivity in the infrared.

 

[K^2]

1) IR is a broad range. There are electron transitions that correspond to near-visible IR. As you get to thermal ranges IR, the reason you don't get electron transitions that correspond to that is because any electrons that could pick up that much energy are going to be stripped off due to thermal excitations.

2) They can. It contributes to scattering. But typically, a visible frequency photon absorbed takes an electron to a level from which it has multiple decay options, which would typically result in light radiated back at several longer wavelengths. So if there is an absorption frequency matching incoming light, the net effect is still absorption.

3) There are lots of things going on that make it more complicated than that. Simplest example, imagine a lattice of some optically active atom. Suppose you are going to shine a light at it that doesn't quite match an absorption frequency. Well, due to vibrations in the lattice, the incoming light is going to be blue/red shifted depending on which way they move. That will allow absorption with a slight mis-match.

There are other things going on as well. Suffice it to say, when you have a lot of different molecules mixed together, you are going to have a fairly continuous absorption spectrum.

But yeah, if something can't absorb at specific frequency, it won't radiate at that frequency, either. So if you want to reduce thermal radiation of a body, you can coat it with silver paint, for example.

 

[Jimmy87]

In metals, the energy is sufficient for transitions between electron energy levels. For other materials, that is rare.

We do! Otherwise you could not see humans. We don't do that in the dark (in relevant amounts), because that would need a temperature of ~1000K or chemical reactions that lead to such an emission.


Thanks to both of you for your answers, very helpful indeed! I'm just a bit confused with the absorption and emission of visible photons. I see your point about not being able to see humans unless visible light photons are radited back. I always thought we absorb visible photons and re-radiate/emit them back in the infrared and the ones that aren't absorbed are reflected. So how does the process of reflection actually work in terms of qauntum transitions - does the photon actually get absorbed? Or is it just that some photons undergo a sharp transition back to ground state (what you said about seeing humans) whilst others are more gradual resulting in infrared emissions (the frequency humans mainly radiate in)?

 

[mfb]

Right - some photons get emitted back at the same frequency directly, the energy of other photons gets converted to heat and re-emitted as infrared radiation later.

 

[Imager]

Right - some photons get emitted back at the same frequency directly, the energy of other photons gets converted to heat and re-emitted as infrared radiation later.

Isn't infrared radiation just photons at a longer wavelength?

 

[sophiecentaur]

Reflection is not the same as emission. To emit light, humans would either need the chemicals that glow worms use or to be at at least 1000C. At 300K, we emit sufficient IR to be detected.

 

[mfb]

Isn't infrared radiation just photons at a longer wavelength?

Sure, so what?

 

[sophiecentaur]

Isn't infrared radiation just photons at a longer wavelength?

It's the energy of individual photons that counts! E =hf and all that

 

[mikeph]

Thanks to both of you for your answers, very helpful indeed! I'm just a bit confused with the absorption and emission of visible photons. I see your point about not being able to see humans unless visible light photons are radited back. I always thought we absorb visible photons and re-radiate/emit them back in the infrared and the ones that aren't absorbed are reflected. So how does the process of reflection actually work in terms of qauntum transitions - does the photon actually get absorbed? Or is it just that some photons undergo a sharp transition back to ground state (what you said about seeing humans) whilst others are more gradual resulting in infrared emissions (the frequency humans mainly radiate in)?

Going back to fundamentals, when light meets a medium, it can:

1. be absorbed: the light disappears and the medium gains energy
2. be transmitted: it goes through the medium and exits the other side
3. be scattered: it changes its direction

The energy gained by the medium from absorption can be re-radiated as:

4. emission: more light is generated, possibly at a different frequency as that which was absorbed.Whether the emitted light is the same frequency or not depends on the properties of the medium, because to emit at a different frequency requires some sort of internal energy transfer to occur (otherwise the outgoing photon would have the same energy and frequency as the incoming photon). There are too many different internal energy transfer mechanisms to cover here, or possibly even in a course on spectroscopy. It can all be broken down into quantum states of the absorber, but these states are far from simple. A solid can be viewed as one big quantum state: its lattice vibrations can correspond to many many many different energy levels so it can absorb/emit what looks like a continuous (blackbody like) spectrum, whereas an atom has very sharply defined electron levels so it will only radiate certain frequencies (e.g. the hydrogen balmer series or sodium's 580nm lines).