Nitrogen molecules in ambient air exposed to an intense near-infrared femtosecond laser pulse give rise to cavity-free superradiant emission at 391.4 and 427.8 nm. An unexpected pulse duration-dependent cyclic variation of the superradiance intensity is observed when the central wavelength of the femtosecond pump laser pulse is finely tuned between 780 and 820 nm, and no signal occurs at the resonant wavelength of 782.8 nm ($2{\omega }_{782.8\mathrm{nm}}={\omega }_{391.4\mathrm{nm}}$). On the basis of a semiclassical recollision model, we show that an interference of dipolar moments of excited ions created by electron recollisions explains this behavior.

中文翻译：

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氮离子超辐射发射对泵浦激光波长和持续时间的意外敏感性

暴露于强烈的近红外飞秒激光脉冲的环境空气中的氮分子会在391.4和427.8 nm处产生无腔的超辐射发射。当飞秒泵浦激光脉冲的中心波长在780 nm至820 nm之间进行微调时，观察到超辐射强度的与脉冲持续时间有关的意外循环变化，并且在共振波长782.8 nm（$\mathrm{2个}{\omega }_{782.8\mathrm{纳米}}={\omega }_{391.4\mathrm{纳米}}$）。在一个半经典的碰撞模型的基础上，我们表明由电子碰撞产生的激发离子的偶极矩的干扰可以解释这种现象。