Role of nuclear dynamics in high harmonic generation redshift of H₂ ⁺

Abstract

The detailed spectral and temporal structures of high-order harmonic generation (HHG) have provided us with a wealth of information about the highly nonlinear response of atoms, molecules, and electronic structures to intense low-frequency laser fields. In this study, we aim to investigate the underlying physics behind the molecular high harmonic spectra redshift beyond the Born Oppenheimer approximation for H2+ and its isotopes with numerical simulations of the time-dependent Schrödinger equation. One widely recognized phenomenon is the occurrence of a spectral redshift at the trailing edge of a laser pulse. However, our findings reveal that even when driven by a sinusoidal laser pulse without a falling edge, the HHG redshift appears and highlights that the observed spectral redshift is not solely due to the declining intensity of the laser pulse. We show that it is a direct consequence of the decrease in ionization energy during the transition from recombination time to ionization time within wave packets at short internuclear distances. Interestingly, our results also demonstrate how a sinusoidal laser pulse without a rising part can disrupt the standard picture of redshift in literatures in the past. This finding suggests that there are additional factors influencing the redshift mechanism beyond just the trailing edge of the laser pulse. These phenomena are elucidated through the analysis of the dynamics of the nuclear wave packet. Our study provides valuable insights into the complex dynamics of HHG processes and sheds light on how different laser pulse characteristics can influence spectral redshifts and harmonic generation in molecular systems.

Control of quantum paths in harmonic generation through orthogonal fields specific frequency ratios

“…High-order harmonics serve as a primary source for generating attosecond pulses and soft x-rays, extending the research scale of ultrafast physics to the attosecond time regime [10][11][12]. With the intensive study of HHG, the effects of nuclear motion [13], the two-center 1 Present address: College of Physical Science and Technology, Bohai University, Jinzhou 121013, People's Republic of China. * Author to whom any correspondence should be addressed.…”