Both techniques are based on photon-in -> photon-out in the X-ray regime. The photon-in part is identical for both.

X-ray reflectivity measures the specular reflection of a thin film heterostructure and is a very well established technique for structural analysis (thickness and roughnesses of of films). In most cases you have only one parameter to alter and this is the angle of the incoming (outgoing) light.  However, measuring complete reflectivity maps, in which also the energy is changed, can give you a much more detailed analysis of the structure. Take a look in the publication.

In this measurement symmetry the dominant part of the intensity is the elastic part, that means that the energy of the photons coming out are the same as the photons coming in.

However this is only one possible process. There is a chance for secondary transitions in the atom which induce photons of lower energy and different directions.

That means, if you move the detector out of the specular reflection geometry you might be able to see the fluorescence light after you have eliminated all the other effects which can play a role here, like diffuse scattering and stray light.

As the first approximation the fluorescence signal is similar to the X-ray absorption signal, but is usually direction dependent and suffer from self-absorption. The structural information is gone but the signal can give you valueable information about the electronic structure of the system.

Article Element Specific Monolayer Depth Profiling

extras.springer.com/2003/978-0-7354-0152-5/cdr_pdfs/.../646_1.pdf

Hi!

I am agree with Macke. Material suggested by dadkhahare useful. However, the common differences are :

1. XRF (source, sample and detector are fixed at an angle 2theta are static), XRR (source is fixed, sample is fixed or may be in oscillation,  but detector is in motion)

2. XRF (technique for elemental detection), but XRR (technique for structure, order, phase detection).