光 丝 诱 导 量 子 拍 频 测 量 氮 原 子 超 精 细 结 构 研 究

Objective Accurate measurement of hyperfine structure (HFS) constants in atomic systems plays a fundamental role in precision spectroscopy, quantum metrology, and atomic structure theory. Traditional high-resolution spectroscopic techniques often rely on vacuum conditions and interferometric configurations, limiting their adaptability in complex or open environments. To address these limitations, this research proposes and experimentally validates a novel approach based on filament-induced quantum beat spectroscopy. By utilizing the coherent fluorescence signals generated from nitrogen atoms excited by femtosecond laser-induced filaments in ambient air, we aim to extract quantum beat frequencies arising from hyperfine level interference. This approach allows for high temporal and frequency resolution without requiring complex vacuum or interference systems, thus offering a compact and efficient pathway for HFS constant retrieval in atmospheric conditions. Methods A femtosecond laser pulse (central wavelength of 800 nm, pulse duration of ~40 fs) is focused on ambient air to generate stable filament, which excites neutral nitrogen atoms and induces spontaneous fluorescence. The filament-induced fluorescence is collected in the lateral direction and directed into a spectrometer. Time-resolved fluorescence signals correspond to three characteristic spectral lines in the 3p(⁴S^°_{3 2})→3s(⁴P_J) transitions of neutral nitrogen (NI)—742.364, 744.229, and 746.831 nm. To isolate quantum beat components embedded in the fluorescence decay, the raw temporal signals are subjected to nonlinear exponential fitting and the background decay is subtracted to obtain the residual oscillatory signals. These residuals are analyzed by Fourier transform to extract beat frequencies arising from hyperfine level interference. Theoretical model of hyperfine energy levels is employed to construct a multi-frequency fitting framework, enabling the retrieval of the magnetic dipole constant A and electric quadrupole constant B of the excited state. Results and Discussions First, three characteristic spectral lines of NI at 742.364, 744.229, and 746.831 nm are identified from the filament-induced time-resolved fluorescence spectrum, corresponding to transitions from the 3p(⁴S^°_{3 2}) state to different 3s(⁴P_J) levels (Fig. 2 and Table 1). The associated hyperfine energy levels and the allowed transition branches are illustrated to guide the construction of beat frequency models. Fluorescence intensity signals for the three characteristic spectral lines exhibit exponential decay modulated by weak oscillations, attributed to quantum beats between hyperfine components (Fig. 3). Residual signals obtained by subtracting the exponential background are used for frequency-domain analysis. The Fourier spectra of the residuals reveal clear beat frequency components for the fluorescence signals of the 744.229 nm and 746.831 nm channels, while the fluorescence signal of the 742.364 nm channel shows only weak modulations (Fig. 4). This variation is primarily attributed to the differences in the number and strength of allowed hyperfine transitions as well as the disparities in transition dipole moments and initial sublevel population distributions. By matching the experimental beat frequencies with theoretical models, the HFS constants for the 3p(⁴S^°_{3 2}) state are determined as A=60.99 MHz and B=16.88 MHz. These results demonstrate the effectiveness of filament-induced quantum beat spectroscopy for precise hyperfine measurements in air. Conclusions This paper demonstrates a novel approach for measuring hyperfine structure constants of neutral nitrogen atoms using filament-induced quantum beat spectroscopy. By generating stable filaments in ambient air and recording the time-resolved fluorescence of characteristic spectral lines, we extract beat frequencies through Fourier analysis and determine the hyperfine structure constants of the 3p(⁴S^°_{3 2}) state as A=60.99 MHz and B=16.88 MHz. Compared to traditional vacuum-based or magnetically shielded spectroscopic methods, this approach achieves high temporal resolution and frequency sensitivity in open environment without complex interferometric setups. Its applicability to nitrogen atoms suggests its potential for broader use in atomic diagnostics in complex gas environments, remote sensing, and ultrafast spectroscopy.