H/D isotopic exchange between water molecules at ice surfaces

Area of Science:

• Surface Science
• Physical Chemistry
• Materials Science

Background:

• Understanding isotopic exchange in water ice is crucial for astrochemistry and materials science.
• The kinetics and mechanisms of H/D exchange on ice surfaces are not fully understood.
• Proton mobility in ice plays a significant role in various chemical and physical processes.

Purpose of the Study:

• To investigate the kinetics of H/D isotopic exchange between H(2)O and D(2)O on ice films.
• To determine the influence of temperature and proton concentration on the exchange rate.
• To elucidate the directional nature of proton transfer in ice.

Main Methods:

• Cs(+) reactive ion scattering technique was employed to study H/D exchange on ice films.
• Ice films with varying H(2)O/D(2)O compositions and structures were deposited on a Ru(0001) substrate.
• Kinetic measurements were performed at temperatures ranging from 90 to 140 K.

Main Results:

• H/D exchange was very slow at 95-100 K but significant at 140 K.
• The rate coefficient at 140 K was determined as 1.6(±0.3) x 10(-19) cm(2) molecule(-1) s(-1).
• Addition of HCl dramatically accelerated surface exchange but restricted vertical propagation.

Conclusions:

• Proton transfer in ice films is highly directional, with faster lateral exchange at the surface.
• Surface disorder and protonic defect mobility facilitate lateral proton transfer.
• Vertical proton transfer is slow and likely assisted by water molecule self-diffusion.