Long-lasting molecular alignment: fact or fiction?

Area of Science:

• Quantum dynamics
• Molecular physics
• Laser-matter interactions

Background:

• Periodic laser pulses can theoretically maintain molecular alignment indefinitely.
• Aligned states are linked to cyclic eigenstates of truncated time propagators.
• Long-term localization in driven systems depends on the exact propagator's spectrum.

Purpose of the Study:

• Investigate the long-term stability of molecular alignment under realistic conditions.
• Determine if truncated cyclic states lose alignment under exact time evolution.
• Analyze the spectral properties of the time evolution operator for intense laser fields.

Main Methods:

• Analyzing the spectrum of the one-period time propagator U(T,0).
• Deriving error bounds for the exact time evolution of rotational wave packets.
• Comparing truncated propagator eigenstates with exact time evolution.

Main Results:

• For weak laser intensities, the evolution operator typically has a point spectrum.
• The spectral nature of U(T,0) for intense laser fields (needed for alignment) remains uncertain (point vs. singular continuous).
• Error bounds confirm that truncated aligned cyclic states maintain alignment for millions of rotational periods under exact evolution.

Conclusions:

• Truncated aligned states exhibit remarkable long-term stability, contrary to potential spectral ambiguities.
• The findings support the practical feasibility of using laser pulses for sustained molecular alignment.
• This work provides crucial insights into the robustness of quantum control in driven systems.