Enhancing High-Order Harmonic Generation Efficiency Through Molecular Size and Orientation Effects: A Pathway to Ultrafast Chemical Dynamics Studies.

High-order harmonic generation provides a powerful tool for probing ultrafast chemical dynamics, such as electron transfer, bond breaking, and molecular structural changes, with attosecond temporal resolution. The strong laser fields used in HHG can also directly influence chemical reaction pathways and rates, enabling coherent control of reaction selectivity. However, enhancing the efficiency of harmonic emission remains a critical challenge in ultrafast science. In this study, we investigate the effects of molecular size and orientation on HHG efficiency using time-dependent density functional theory simulations. By analyzing the linear molecules C18H2, C2H2, and CH under linearly polarized laser fields, we demonstrate that larger molecular sizes significantly enhance harmonic emission intensity. Our results reveal that C18H2, with its larger spatial dimensions, exhibits substantially higher harmonic intensity compared to smaller molecules like C2H2. This enhancement is further supported by examining charge redistribution and bond length changes during the HHG process. Additionally, we validate our findings with CH, a molecule of intermediate size, confirming the correlation between molecular size and harmonic efficiency.