a State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China
b Key Laboratory for Laser Plasmas (Ministry of Education) and School of Physics and Astronomy, Collaborative Innovation Center of IFSA (CICIFSA), Shanghai Jiao Tong University, Shanghai 200240, China
c CAS Center for Excellence in Ultra-Intense Laser Science, Shanghai 201800, China
d Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi 030006, China
This work was supported by the National Key R&D Program of China (Grant Nos. 2018YFA0306303 and 2018YFA0404802), the National Natural Science Fund (Grant Nos. 11834004, 11621404, 11925405, and 91850203), the 111 Project of China (Grant No. B12024), Projects from Shanghai Science and Technology Commission (Grant No. 19JC1412200), and the Innovation Program of Shanghai Municipal Education Commission (Grant No. 2017-01-07-00-02-E00034). S. Pan acknowledges the support from the Academic Innovation Ability Enhancement Program for Excellent Doctoral Students of East China Normal University in 2021 (Grant No. 40600-30302-515100/141).
More than ten years ago, the observation of the low-energy structure in the photoelectron energy spectrum, regarded as an “ionization surprise,” has overthrown our understanding of strong-field physics. However, the similar low-energy nuclear fragment generation from dissociating molecules upon the photon energy absorption, one of the well-observed phenomena in light-molecule interaction, still lacks an unambiguous mechanism and remains mysterious. Here, we introduce a time-energy-resolved manner using a multicycle near-infrared femtosecond laser pulse to identify the physical origin of the light-induced ultrafast dynamics of molecules. By simultaneously measuring the bond-stretching times and photon numbers involved in the dissociation of H2+ driven by a polarization-skewed laser pulse, we reveal that the low-energy protons (below 0.7 eV) are produced via dipole-transitions at large bond lengths. The observed low-energy protons originate from strong-field dissociation of high vibrational states rather than the low ones of H2+ cation, which is distinct from the well-accepted bond-softening picture. Further numerical simulation of the time-dependent Schrödinger equation unveils that the electronic states are periodically distorted by the strong laser field, and the energy gap between the field-dressed transient electronic states may favor the one- or three-photon transitions at the internuclear distance larger than 5 a.u. The time-dependent scenario and our time-energy-resolved approach presented here can be extended to other molecules to understand the complex ultrafast dynamics.