Ultrafast intermolecular energy transfer in heavy water

Area of Science:

• Physical Chemistry
• Spectroscopy
• Molecular Dynamics

Background:

• Understanding vibrational energy transfer is crucial for chemical reaction dynamics.
• Deuterium oxide (D2O) and its mixtures with water (H2O) are fundamental systems in chemistry and biology.
• Femtosecond mid-infrared spectroscopy provides high temporal resolution for studying ultrafast molecular processes.

Purpose of the Study:

• To investigate the vibrational energy relaxation dynamics of OD vibrations in D2O.
• To quantify resonant vibrational (Förster) energy transfer between OD vibrations in D2O/H2O mixtures.
• To determine the Förster radius for OD vibrations in HDO/H2O solutions.

Main Methods:

• Utilized femtosecond mid-infrared spectroscopy to probe OD vibrations.
• Measured vibrational lifetimes and anisotropy decay dynamics.
• Analyzed concentration-dependent changes in anisotropy to assess energy transfer rates.

Main Results:

• Observed a lifetime of 400 +/- 30 fs for OD vibrations in bulk D2O.
• Found negligible resonant energy transfer at 0.5% D2O in H2O, with a molecular reorientation time constant of 2.6 +/- 0.1 ps.
• Demonstrated increased and non-exponential anisotropy decay with higher D2O concentrations, indicating significant Förster energy transfer.
• Determined the Förster radius for OD vibration of HDO in H2O to be r0 = 2.3 +/- 0.2 Å.

Conclusions:

• Vibrational energy relaxation of OD in D2O occurs on a sub-picosecond timescale.
• Resonant Förster energy transfer becomes significant at higher concentrations of D2O in H2O.
• The Förster radius provides a quantitative measure of the interaction range for OD vibrational energy transfer.