Picosecond calorimetry: time-resolved x-ray diffraction studies of liquid CH2Cl2

Area of Science:

• Physical Chemistry
• Materials Science
• Chemical Physics

Background:

• Liquid phase time-resolved X-ray diffraction (TRXRD) probes transient species.
• TRXRD signals include both solute and solvent contributions.
• Characterizing solvent signals is crucial for accurate analysis.

Purpose of the Study:

• To experimentally isolate and characterize the X-ray diffraction signal from a pure liquid solvent.
• To understand the structural dynamics of the solvent matrix during rapid heating.
• To investigate the energy relaxation pathways in multiphoton excited liquids.

Main Methods:

• Performed time-resolved X-ray diffraction on liquid dichloromethane (CH2Cl2).
• Utilized multiphoton excitation with 100 fs UV pulses.
• Studied the sample over a time range from -250 ps to 2.5 microseconds.

Main Results:

• Observed significant X-ray diffraction signals from a highly compressed, superheated liquid state before thermal expansion.
• Documented shock wave propagation and expansion dynamics in good agreement with theoretical models.
• Detected an unexpectedly slow initial energy release as heat, indicating a metastable state in multiphoton excited CH2Cl2.

Conclusions:

• Established a method to differentiate solvent signals in TRXRD studies.
• Provided experimental evidence for a metastable state in multiphoton excited CH2Cl2.
• Advanced the understanding of ultrafast structural dynamics and energy relaxation in liquids.