The stringent tolerances in the engineering of advanced optoelectronic integrated circuits with respect to control thickness and composition of ultra-thin layers require the development of monitoring and control techniques that follow the deposition process with sub-monolayer resolution. These demands led to the development of surface-sensitive real-time optical sensors that are able to move the control point close to the paint where the growth occurs, which in a chemical beam epitaxy process is the surface reaction layer, built up of physisorbed and chemisorbed precursor fragments between the ambient and film interface. In this contribution, we explore the application of p-polarized reflectance spectroscopy (PRS) in the context of real-time monitoring and control of pulsed chemical beam epitaxy (PCBE) Juring low temperature growth of epitaxial GaP heterostructures on Si(001) substrates by PCBE. The effect of periodic alterations in composition and thickness of a surface reaction lever (SRL) is monitored by PRS as a periodic modulated reflectance amplitude, denoted as fine structure. Using a reduced order kinetic model' we demonstrate the linkage of the PRS response towards surface reaction chemistry, film growth rate, and film properties. Mathematical control algorithms are introduced that link the PR signals to the growth process control parameters.