Discriminating organic isomers by high harmonic generation: A time-dependent configuration interaction singles study

Area of Science:

• Quantum optics
• Molecular spectroscopy
• Computational chemistry

Background:

• High Harmonic Generation (HHG) is a nonlinear optical process producing high-energy photons and ultrashort laser pulses.
• HHG spectroscopy has shown potential for distinguishing between configurational isomers of polyatomic organic molecules, such as cis- and trans-1,2-dichloroethene (DCE).

Purpose of the Study:

• To propose an economical and accurate computational method for calculating HHG spectra of polyatomic species.
• To investigate the differences in HHG spectra between cis- and trans-DCE isomers using the proposed method.
• To explore the utility of time-frequency HHG spectra for isomer discrimination.

Main Methods:

• Utilizing Time-Dependent Configuration Interaction Singles (TD-CIS) theory with extended atomic orbital bases.
• Implementing models to account for ionization losses.
• Computing HHG spectra for both aligned and unaligned cis- and trans-DCE molecules, including coherent averaging for randomly oriented molecules.

Main Results:

• Distinct HHG spectra were computed for aligned and unaligned cis- and trans-DCE, correlating with their symmetries.
• The TD-CIS method successfully identified spectral differences between the isomers, consistent with experimental observations.
• Time-frequency HHG spectra revealed further distinctions between cis- and trans-DCE, highlighting their potential for isomer analysis.

Conclusions:

• The TD-CIS method offers an efficient and accurate approach for computing HHG spectra.
• HHG spectroscopy, particularly time-frequency analysis, shows promise as a tool for discriminating between molecular configurational isomers.
• This work provides a valuable computational framework for understanding and predicting HHG spectra in complex molecules.