Dephasing and Decoherence in Vibrational and Electronic Line Shapes

Area of Science:

• Physical Chemistry
• Chemical Physics
• Spectroscopy

Background:

• Spectral line shapes in liquids are broadened by interactions with the surrounding molecular environment, termed the 'bath'.
• Classical treatment of bath degrees of freedom leads to dephasing, while quantum mechanical treatment introduces decoherence.

Purpose of the Study:

• To investigate the relative importance of decoherence (quantum bath effects) versus dephasing in broadening spectral line shapes.
• To explore the impact of these broadening mechanisms on absorption and emission spectra, including the Stokes shift.

Main Methods:

• Developed general theoretical frameworks for analyzing absorption and emission line shapes considering both dephasing and decoherence.
• Derived new relationships connecting classical and quantum treatments of bath dynamics.
• Applied the theoretical models to a model system and a realistic system (vibrational OH stretch in HOD/D2O).

Main Results:

• Identified new connections between classical dephasing and quantum decoherence treatments of spectral broadening.
• For the vibrational OH stretch in HOD/D2O, quantum effects were found to be minor, except for their influence on the Stokes shift.
• The study suggests quantum bath effects are generally less significant for many vibrational and most electronic transitions in liquids at room temperature.

Conclusions:

• Quantum decoherence plays a limited role in spectral broadening for many common liquid-phase transitions at room temperature.
• Dephasing remains the dominant mechanism for spectral broadening in most cases.
• Quantum effects are more pronounced for the Stokes shift, highlighting its sensitivity to bath dynamics.