中文摘要：

在低于一个标准大气压的条件下对飞秒激光产生的N2等离子体光谱进行了实验研究．结果表明，各种样品气压下的激光N2等离子体光谱均表现为连续谱和线状谱的叠加．随着样品气压的降低，连续谱和原子谱线的强度经历了由缓慢增强发展为缓慢降低再到迅速降低的过程，而正一价离子谱线强度呈现逐步增强的特征．在气压低于0．3atm（1atm=101325Pa）时，出现了正二价态的离子谱线．给出了低压N2等离子体对于飞秒激光传输和能量吸收的物理特性，并初步讨论了低压等离子体通道特性．这些结果有助于加深了解飞秒激光等离子体的特性和机理，特别是给出了在实验上测量不同价态离子光谱的条件，为今后的研究提供了有益的实验依据．

英文摘要：

The spectra of N2 plasma induced by a femtosecond pulsed laser are studied experimentally at sub-atmospheric pressure. The results show that the spectra of laser-induced plasmas for all sample pressures are composed of continuous spectra and line spectra. As the sample pressure is reduced the intensities of continuous spectra undergo the transition from slow increase to rapid decrease; on the other hand, the intensities of N＋ spectra increase significantly with the decrease of pressure. The spectra of N＋＋ species are observed when the pressure is lower than 0.3 atm. The behaviors of the femtosecond laser propagation and energy absorption in N2 plasma at sub-atmospheric pressure are also given and the feature of the laser-induced plasma channel is briefly discussed. These results are helpful for better understanding the laser-induced plasma characteristics, especially provide the clue to the experimental measurement conditions for different charged species, which is useful for the future corresponding experimental research.