Elucidating the interplay between nuclear and electronic degrees of freedom that govern the complex dielectric behavior of materials under intense photoexcitation is essential for tailoring optical properties on demand. However, conventional transient reflectivity experiments have been unable to differentiate between real and imaginary components of the dielectric response, omitting crucial electron-lattice interactions. Utilizing thin film interference we unambiguously determine the photoinduced change in the complex dielectric function in the Peierls semimetal bismuth and examine its dependence on the excitation density and nuclear motion of the $A_{1g}$ phonon. Our modeled transient reflectivity data reveal a progressive broadening and redshift of Lorentz oscillators with increasing excitation density and underscores the importance of both electronic and nuclear coordinates in the renormalization of interband transitions.

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• Dielectric properties
• Optical phonons
• Peierls transition
• Elemental materials
• Narrow band gap systems
• Semimetals
• Single crystal materials
• Ultrafast femtosecond pump probe
• Ultrafast pump-probe spectroscopy