Does Dynamic Light Scattering Follow the Same Frequency as the Incident Signal?

[Ras9]

Hi guys, I was studying dynamic light scattering experiment and I had a problem! Here's the thing:
I read that in case of weak coupling between x-rays and matter the whole system behaves as a linear system. I knew that linear systems respond to an input signal with an output that has the same frequency. So I would expect the scattered field to have the same frequency of the incident one, but obviously this doesn't happen (otherwise we would have no inelastic scattering). So? What am I missing?
Thanks

 

[M Quack]

X-ray DLS is called XPCS (x-ray photon correlation spectroscopy). The change in frequency is negligible, you can interpret the speckle pattern as elastic snapshot of the system's configuration at that point in time. You take another snapshot later and compare.

http://sector7.xray.aps.anl.gov/~dufresne/UofM/xpcs.html

 

[Ras9]

By doing what you are saying I can get the field's correlation function, am I right? And what about the spectral density? If I use a filter in my experiment and I can measure the intensity of the scattered field at different frequencies. So if the change in frequency is negligible i would get something that looks like a delta, is that so?

 

[DrDu]

I don't see immediately why linearity should exclude frequency change. After all, linearity only means that the intensity of the exiting light is proportional to the intensity of the incoming light.

 

[Ras9]

I don't know exactly if there is a particular proof for that, I just remember it from my classes. But giving a quick look on wikipedia it seems to be truth:
"In simplest terms, if a sine wave is injected into a system at a given frequency, a linear system will respond at that same frequency with a certain magnitude and a certain phase angle relative to the input" (http://en.wikipedia.org/wiki/Frequency_response)
I will look for other sources!