Resolving the ambiguity in the relation between Stokes shift and Huang-Rhys parameter

Area of Science:

• Solid-state physics
• Photochemistry
• Spectroscopy

Background:

• Electronic transitions in luminescent materials couple to vibrations, causing band broadening and Stokes shift.
• The Huang-Rhys parameter (S) quantifies vibrational coupling and equilibrium position offset (ΔQe).
• Existing textbook relations for EStokes and S are approximations and temperature-dependent.

Purpose of the Study:

• To investigate the precise relationship between Stokes shift (EStokes) and the Huang-Rhys parameter (S) under finite temperatures.
• To evaluate the validity of common approximations for EStokes and S.
• To identify a temperature-independent method for determining S.

Main Methods:

• Theoretical analysis of electronic-vibrational coupling in luminescent systems.
• Inclusion of finite temperature effects in the derivation of EStokes-S relationships.
• Comparison of band barycenter shifts with Stokes shift.

Main Results:

• The relationship between EStokes and S varies with temperature, necessitating different approximations in different temperature ranges.
• The difference between the barycenters of absorption and emission bands provides an unambiguous method for determining S.
• Band barycenter positions are unaffected by temperature, unlike the Stokes shift.

Conclusions:

• Commonly used approximations for the Huang-Rhys parameter are only valid under specific temperature conditions.
• Analyzing the difference between absorption and emission band barycenters offers a robust, temperature-independent method for quantifying vibrational coupling (S).
• This finding is crucial for accurate characterization of luminescent materials.