Gaussian-Type Orbital Calculations for High Harmonic Generation in Vibrating Molecules: Benchmarks for H₂

Area of Science:

• Quantum dynamics simulation
• Strong-field physics
• Molecular ion spectroscopy

Background:

• Understanding the behavior of molecular ions like H₂⁺ under intense laser fields is crucial for attosecond science.
• Accurate theoretical modeling is needed to interpret experimental results and predict molecular responses.

Purpose of the Study:

• To simulate the response of H₂⁺ to few-cycle laser pulses.
• To benchmark Gaussian-type basis set approaches against all-grid solutions for electron-nuclear coupled motion.
• To investigate the influence of nuclear motion on high harmonic generation (HHG) and electronic coherences.

Main Methods:

• A mixed-basis approach combining Gaussian-type basis sets for electrons and grid representation for nuclei.
• Solving the time-dependent Schrödinger equation within and beyond the Born-Oppenheimer approximation.
• Comparison with all-grid-based solutions and fixed-nuclei calculations for benchmarking.

Main Results:

• Excellent agreement between Gaussian-type basis set methods and all-grid solutions for low intensities and small ionization probabilities.
• Reliable modeling of high harmonic generation (HHG) and high-frequency response using Gaussian basis sets for electrons up to moderate harmonic orders.
• Nuclear motion was found to disrupt electronic coherences on femtosecond timescales and influence HHG intensities.

Conclusions:

• Gaussian-type basis sets provide a reliable and computationally efficient method for modeling electron dynamics in H₂⁺ under laser fields, particularly for HHG.
• Nuclear motion plays a significant role in molecular ion dynamics, affecting electronic coherences and HHG spectra.
• The developed Gaussian-based quantum electron-nuclear dynamics approach is extendable to other quantum chemistry methods for systems lacking benchmarks.