Field-free alignment of molecules observed with high-order harmonic generation

Area of Science:

• Atomic, Molecular, and Optical Physics
• Quantum Dynamics
• Laser-Matter Interactions

Background:

• Molecular alignment dynamics are crucial for understanding chemical reactions and material properties.
• Strong laser fields can induce complex quantum phenomena in molecules, such as rotational wave packets.
• High-order harmonic generation (HHG) is a sensitive probe of electron dynamics in atoms and molecules.

Purpose of the Study:

• To investigate dynamic processes in field-free molecular alignment using high-order harmonic generation.
• To analyze the frequency components of time-dependent harmonic signals from aligned molecules.
• To understand the role of coherently populated rotational states in molecular wave packets.

Main Methods:

• Field-free alignment of N2, O2, and CO2 molecules using intense laser pulses.
• Time-resolved detection of high-order harmonic generation.
• Analysis of beat frequencies in the harmonic signal corresponding to molecular rotational states.

Main Results:

• Observed two distinct sets of beat frequencies in the harmonic signal from aligned molecules.
• Identified one frequency component related to field-free molecular alignment.
• Attributed the second frequency component to coherence within the molecular wave packet.

Conclusions:

• High-order harmonic generation is a sensitive technique for studying field-free molecular alignment dynamics.
• The observed beat frequencies provide detailed information about coherently populated rotational states.
• Understanding these frequency components is essential for interpreting revival signals in HHG.