Liang Hao, Tsai Ming-Shian, Tseng Chun-Chia, Chen Ming-Chang, Thumm Uwe, Han Meng
Max Planck Institute for Physics of Complex Systems, Nöthnitzer Straße 38, Dresden 01187, Germany.
Institute of Photonics Technologies, National Tsing Hua University, Hsinchu 300044, Taiwan.
Sci Adv. 2025 Jun 6;11(23):eadu9200. doi: 10.1126/sciadv.adu9200. Epub 2025 Jun 4.
Laser-induced air fluorescence in the ultraviolet regime is primarily attributed to transitions between the C and B states in excited neutral [Formula: see text] molecules and between the B and X states in [Formula: see text] ions. However, the mechanism underlying the former remains contentious, as direct population to the C state by light fields is forbidden by electron spin constraints. In this work, we investigate the mechanism of air fluorescence from excited neutral [Formula: see text] molecules by carrier-envelope phase-stabilized sub-4 femtosecond pulses. Our results show that fluorescence from [Formula: see text] ions reaches a maximum with cosine-like pulses, while fluorescence from excited neutral [Formula: see text] molecules peaks with sine-like pulses. In addition, by scanning the chirp of the driving pulse, we find that ionic fluorescence is maximized with chirp-free pulses, whereas neutral fluorescence favors negatively chirped pulses. These observations, supported by classical trajectory Monte Carlo simulations, support the mechanism of intersystem crossing from excited spin-singlet states, which are populated via recollision-induced strong-field excitation.
紫外波段的激光诱导空气荧光主要归因于激发态中性[化学式：见原文]分子中C态与B态之间以及[化学式：见原文]离子中B态与X态之间的跃迁。然而，前者的潜在机制仍存在争议，因为光场直接将电子激发到C态受到电子自旋限制的禁止。在这项工作中，我们通过载波包络相位稳定的亚4飞秒脉冲研究了激发态中性[化学式：见原文]分子产生空气荧光的机制。我们的结果表明，[化学式：见原文]离子的荧光在类余弦脉冲时达到最大值，而激发态中性[化学式：见原文]分子的荧光在类正弦脉冲时达到峰值。此外，通过扫描驱动脉冲的啁啾，我们发现离子荧光在无啁啾脉冲时最大，而中性荧光则倾向于负啁啾脉冲。这些观察结果得到了经典轨迹蒙特卡罗模拟的支持，证实了通过再碰撞诱导的强场激发填充的激发自旋单重态之间的系间窜越机制。
