In past years, in the molecular ions prepared by the femtosecond laser, scientists has observed laser-like radiations, coherence enhanced nonlinear effects, and many interesting quantum phenomena. All of these studies show that molecular ions are a versatile quantum platform to study quantum coherence in the strong-field regime.

Recently, researchers in Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences (CAS), observed rotational coherence in an excited state of CO⁺. Researchers demonstrated that the rotational coherence of the first excited state of CO⁺ can be recorded in the resonance enhanced vacuum ultraviolet (VUV) radiation, which is manifested as the periodic modulations of VUV radiation with the delay of two-color laser pulses. The results were published in Optics Letters on Aug. 06, 2021.

In this study, the researchers implemented 2+1 multiphoton resonant excitation in the strong-field-ionized CO molecules using near-infrared and ultraviolet pump pulses, which gives rise to VUV coherent radiation.

When they scanned the relative delay between two pulses, the rotational-resolved VUV radiations show periodic modulations lasting as long as 500 ps. Fourier analysis reveals that these modulations correspond to rotational beat frequencies of the first excited state of CO⁺, which is the result of multi-channel interference during the resonant excitation process.

They found that high resolution of Fourier transform spectra up to 0.067 cm^-1 allows them to obtain fine energy levels of the excited state of CO⁺. The theoretical calculation is in good agreement with the experimental observation.

This result opens up promising applications of rotational coherence spectroscopy in the accurate determination of molecular constants and ultrafast dynamics of molecular ions.

This work is supported by National Natural Science Foundation of China, Key Research Program of Frontier Sciences of Chinese Academy of Sciences, Shanghai Municipal Science and Technology Major Project, Program of Shanghai Academic Research Leader, and Youth Innovation Promotion Association of CAS.

Figure 1. (a) The rotational-resolved VUV emission spectra as a function of the time delay between two pump pulses. (b) Fourier transform spectra for the experimentally measured time-dependent VUV emission from different rotational levels and the theoretically calculated rotational beat frequencies of the first excited state of CO⁺. (Image by SIOM)

Article website:
https://doi.org/10.1364/OL.432315

Contact:
WU Xiufeng
General Administrative Office
Shanghai Institute of Optics and Fine Mechanics, CAS
Email: xfwu@siom.ac.cn
Web: http://english.siom.cas.cn/