Limits of laser spectroscopy for determining size of a molecule or particle Say you have a number of particles respectively molecules suspended in air. The diameter of the particles $d_{particle}$ are of the order 1 to 100nm. Can you actually, i.e. in practice, measure the size (i.e. the diameter) of those particles using laser spectroscopy? The suspension in air may be a knock-out criterion for this measurement (I'm not sure). If so, would it change anything if the particles/molecules were placed in a vacuum chamber? What are the current experimental limits on doing such measurements with laser spectroscopy?  
 A: Short answer - it will not work because of several reasons:


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*Elastic scattering of light from particles much smaller than the wavelength of the light is known as Rayleigh scattering. The scattering efficiency is proportional to $d^6$, where $d$ is the particle diameter, thus it is gets very small for molecules. You would have to use maximal laser power, but then you cannot tell if the particle scattered 1 or 2 or 50 photons.

*The momentum of a single photon is too small to shift 1 nm particle. Let's take even smaller particle - hydrogen atom, which has a diameter of about 0.1 nm. If we consider a photon of green light (500 nm), elastically bouncing of the hydrogen atom, then from the conservation of momentum I calculate the additional velocity for the hydrogen atom is 1.6 m/s. This looks large enough, but keep in mind that the average velocity of a hydrogen atom at room temperature is 2200 m/s, but it can be anything between 0 and 5000 m/s, according to Maxwell-Boltzmann distribution.

*There are $10^{19}$ molecules per $cm^3$ of air in normal conditions, so even in highest vacuum ($10^{-12} $bar) you cannot isolate a single molecule. Aerosol particles in vacuum will just drop down.
But even if this doesn't work, such technique can be used for other interesting measurements. While researching this question, I found that Rayleigh Doppler Technique can be used to measure temperature and wind speed high in the atmosphere. The idea is to distinguish between the light scattered from molecules and aerosol particles. 
Molecules move fast in random directions, so there will be some Doppler broadening of the scattered light that increases with the speed of molecules, that is proportional to temperature. Aerosol particles, on the other hand, are too heavy for random motion and just follow the air flow. The light that they scatter back will not be broadened, but shifted according to the speed of particles relative to the laser. Of course, this shift is very small, as air moves much slower than the speed of light, but it can still be observed with an interferometer.
