Ultrafast x-ray diffraction study of melt-front dynamics in polycrystalline thin films

Area of Science:

• Materials Science
• Condensed Matter Physics
• Physical Chemistry

Background:

• Melting is a fundamental phase transition, yet its microscopic dynamics remain incompletely understood.
• Investigating ultrafast melting provides critical insights into material behavior under extreme conditions.

Purpose of the Study:

• To examine the ultrafast melting process in polycrystalline gold thin films at the microscopic level.
• To identify and characterize any intermediate states formed during rapid melting.

Main Methods:

• Utilized time-resolved X-ray diffraction (TRXRD) with a laser pump and hard X-ray probe setup.
• Analyzed diffraction patterns to track structural changes over picosecond timescales.

Main Results:

• Observed the formation of a new diffraction peak, indicative of an intermediate solid-liquid state, appearing 50 picoseconds (ps) post-excitation.
• This intermediate state persisted beyond 500 ps, with its diffraction peak width evolving over time.
• The peak width initially increased rapidly within 50 ps, then distinctly narrowed at longer timescales, suggesting a melting band originating from grain boundaries.

Conclusions:

• The study identified a novel intermediate state during ultrafast melting of gold.
• This intermediate state, characterized by a melting band propagating from grain boundaries, has significant implications for understanding laser-matter interactions.
• Findings are relevant for optimizing pulsed laser deposition and laser ablation techniques.