Molecular excited-state relaxation dynamics at the colloidal microparticle interface monitored with pump-probe second...

Area of Science:

• Photochemistry
• Surface Science
• Spectroscopy

Background:

• Molecules adsorbed on surfaces exhibit altered photophysical properties.
• Understanding excited-state dynamics is crucial for molecular energy relaxation.
• Colloidal microparticles offer a tunable platform for surface studies.

Purpose of the Study:

• To investigate the excited-state relaxation dynamics of malachite green (MG) adsorbed on colloidal microparticles.
• To determine the influence of a negatively charged surface on MG's excited-state lifetime.
• To explore the role of local friction in molecular energy dissipation.

Main Methods:

• Time-resolved second harmonic generation (TR-SHG) spectroscopy.
• Photoexcitation of MG using a 615 nm pump pulse.
• Probing ground-state depletion via SHG of an 800 nm pulse.

Main Results:

• The excited-state lifetime of MG on polystyrene sulfate microparticles was determined to be 5.7 ± 0.4 ps.
• This lifetime is approximately three times longer than that observed at the air/water interface.
• The results indicate a significant impact of the negatively charged surface on local friction.

Conclusions:

• The negatively charged surface of colloidal microparticles enhances the excited-state lifetime of adsorbed malachite green.
• Local friction, influenced by the surface charge, plays a critical role in the energy relaxation pathways of photoexcited molecules.
• TR-SHG is a powerful technique for probing ultrafast dynamics at interfaces.