Why Are Detectors for Molecular Vibration Frequencies Less Common?

[kye]

We have common detectors for radio, microwaves, visible, ultraviolet, xray and gammy rays.. but how about the infrared frequencies from molecular vibrations. The frequencies of molecular vibrations range from less than 10^12 to approximately 10^14 Hz. These frequencies correspond to radiation in the infrared (IR) region of the electromagnetic spectrum. How come we don't commonly heard of detectors of it? I'm not talking about Raman Spectroscopy, but just plain detectors.. why can't we detect molecular vibrations frequencies yet we can easily detect radio, microwave, xray, etc.? anything I missed?

 

[Drakkith]

We can: http://en.wikipedia.org/wiki/Thermographic_camera

 

[kye]

We can: http://en.wikipedia.org/wiki/Thermographic_camera

Well. In fact I own thermal camera, but it can only detect the heat of object, it can't see molecular vibrations.. don't say all molecular vibrations are thermal... so what camera can detect non-thermal molecular vibrations?

 

[Drakkith]

I'm sorry I don't understand what you mean by asking if we can "see" molecular vibrations. The detection of IR radiation is similar to optical radiation. Both are, at this time, too high in frequency for current electronics to directly respond to the oscillating electric field, so we can't directly record it like we can for the radio and microwave bands. Above optical frequencies the radiation becomes hard to even detect, as with increasing energy it starts to simply pass through most materials and special techniques are used to focus and detect x-rays and gamma rays since normal lenses and mirrors don't work.

 

[kye]

I'm sorry I don't understand what you mean by asking if we can "see" molecular vibrations. The detection of IR radiation is similar to optical radiation. Both are, at this time, too high in frequency for current electronics to directly respond to the oscillating electric field, so we can't directly record it like we can for the radio and microwave bands. Above optical frequencies the radiation becomes hard to even detect, as with increasing energy it starts to simply pass through most materials and special techniques are used to focus and detect x-rays and gamma rays since normal lenses and mirrors don't work.

I mean my thermal imager can't detect the non-thermal frequencies of the IR which corresponds to molecular vibrations. So what kind of camera can detect them?

 

[Drakkith]

I mean my thermal imager can't detect the non-thermal frequencies of the IR which corresponds to molecular vibrations. So what kind of camera can detect them?

I don't know what "non-thermal frequencies" means in this context. A warm object sends out a broad range of frequencies across most of the spectrum. The camera will detect a small slice of the spectrum, specifically part of the IR band, regardless of its source. What non-thermal frequencies are you referring to? In your first post you listed 10¹² hz through 10¹⁴ hz as molecular vibration frequencies. If those are in the IR band, and your camera responds to those frequencies, then it should be able to detect them.

 

[sophiecentaur]

There is a bit of a sensitivity 'hole' in the measurement range at sub-mm wavelengths of EM radiation. The photon energy is low so they are more difficult to detect than light and much of the IR spectrum but they are more difficult to amplify coherently than microwaves and lower frequencies.
There is always the option of Bolometric measurement (heating effect) but, of course, that requires a significant amount of Power in the signal you're looking at/for.
Googling "sub millimeter microwave detectors" throws up a number of articles, including some which talk of Masers, mixing and sub-mm astronomy.

 

[f95toli]

There are lots of different detectors that work in that range.
The THz imagers that are used at certain airports is an obvious modern day applictation; but most such detectors were orginally developed to be used in radio astronomy.

There are TES, KIDs, bolometers, heterodyne Josephson mixers etc

 

[kye]

A thermal camera has range of 50 Celsius to 500 Celsius which corresponds to only small window of 9 - 12 um wavelength in the temperature blackbody curve. So a thermal camera can't detect the entire 1 - 1000 um IR frequencies.

Anyway. When I mentioned "non-thermal frequencies". I'm referring to the Raman frequencies.. which is none thermal (or do you think it has corresponding temperature from the blackbody curve corresponding to the wavelength or wave number detected since you consider all frequencies above zero as thermal)?

I have a separate question, in Wikipedia entry on Raman Spectroscopy. It is mentioned that:

"The Raman effect occurs when light impinges upon a molecule and interacts with the electron cloud and the bonds of that molecule. For the spontaneous Raman effect, which is a form of light scattering, a photon excites the molecule from the ground state to a virtual energy state. When the molecule relaxes it emits a photon and it returns to a different rotational or vibrational state."

I'd like to know if the so called "ground state" of the molecule is the blackbody radiation curve itself... say 10 micrometer wavelength that approx. corresponds to say 100 Celsius??

 

[Drakkith]

Anyway. When I mentioned "non-thermal frequencies". I'm referring to the Raman frequencies.. which is none thermal (or do you think it has corresponding temperature from the blackbody curve corresponding to the wavelength or wave number detected since you consider all frequencies above zero as thermal)?

I guarantee you the same frequencies are present in a blackbody radiation curve.

I'd like to know if the so called "ground state" of the molecule is the blackbody radiation curve itself... say 10 micrometer wavelength that approx. corresponds to say 100 Celsius??

No. Blackbody radiation is the result of the thermal motion of particles. This curve is based almost solely on temperature. Real objects will deviate slightly from the curve of an "ideal" blackbody in ways that depend on the makeup of the material, but they are still very similar. Since blackbody radiation is emitted in a broad range of frequencies, you cannot associate a single wavelength with a specific temperature.

 

[kye]

I guarantee you the same frequencies are present in a blackbody radiation curve.



No. Blackbody radiation is the result of the thermal motion of particles. This curve is based almost solely on temperature. Real objects will deviate slightly from the curve of an "ideal" blackbody in ways that depend on the makeup of the material, but they are still very similar. Since blackbody radiation is emitted in a broad range of frequencies, you cannot associate a single wavelength with a specific temperature.

how do you understand the ground state of molecules or where did the ground state get the energy if it is not relatated to temperatue? is ground state always at room temperature?

i read many wikipedia articles and about blackbody radiation and raman spectroscopy. anyway. it seems there are many categories of molecular vibration... vibration that produced temperature and vibrations explored by raman spectroscopy. so do you consider temperature caused not by vibrations of the ground state of the molecules but movements of the atoms? but molecules are made up of atoms, so how do you distinguish which can produce temperature and which produced molecule vibrations composed of the ground states probed by raman spectroscopy?

 

[Drakkith]

how do you understand the ground state of molecules or where did the ground state get the energy if it is not relatated to temperatue? is ground state always at room temperature?

The ground state is the minimum energy state for a system, such as electrons in molecular orbitals. (But there are many, many more) It's important to understand that all objects with a temperature above absolute zero have at least some of their composite particles NOT in the ground state.

so do you consider temperature caused not by vibrations of the ground state of the molecules but movements of the atoms? but molecules are made up of atoms, so how do you distinguish which can produce temperature and which produced molecule vibrations composed of the ground states probed by raman spectroscopy?

Raman spectroscopy uses an external laser to scatter light and measure the shift in wavelength from certain vibrational modes. These modes can also contribute to an objects temperature.

 

[kye]

I guarantee you the same frequencies are present in a blackbody radiation curve.



No. Blackbody radiation is the result of the thermal motion of particles. This curve is based almost solely on temperature. Real objects will deviate slightly from the curve of an "ideal" blackbody in ways that depend on the makeup of the material, but they are still very similar. Since blackbody radiation is emitted in a broad range of frequencies, you cannot associate a single wavelength with a specific temperature.

I know blackbody radiation is emitted a broad range of frequencies obeying the Planck distribution, but what would happen if an object emit only a particular wavelength say 10 um with zero value in other wavelengths? Can't such object exist, for example, bose condensates (or since this has zero temperature.. other objects with uniform atomic movements?)?

In a uniform object like wood, you are saying the temperature would have close to Planck distribution (ignoring emissivity) on the surface... but since wood has same uniform atoms.. why can't have one wavelength only. and again how do you force an object to emit in one wavelength only (except laser which is point beam only.. I'm talking of entire large object (say 1 foot across) with one wavelength only).. if there is such.. what would be its temperature?

 

[sophiecentaur]

There are plenty of non- thermal radiators. Examples are discharge tubes and radio transmitters. You cannot assign them a temperature on the basis of the power radiated at just one frequency (or even a number of frequencies). A TV display can produce a White that looks the same as a heated black body (filament) but this is achieved with three phosphors and the screen is not actually at 5500K ( obviously).
There are just different radiated spectra. Nothing more significant than that.