Abstract
When exposed to intense infrared laser pulses of femtosecond duration, molecules such as N${}_{2}$, CO${}_{2}$, and H${}_{2}$O not only become ionized but can also produce lasers as a result of simultaneous population inversion. Scientists look deep into this ``air lasing'' phenomenon in N${}_{2}$ gas and uncover both quantum coherence in the rotational wave packets of the lasing molecular ions and its footprint in the air-laser spectrum.
Highlights
In the presence of strong laser fields, molecules can exhibit many intriguing behaviors, such as high-order harmonic generation [1], above-threshold ionization and dissociation [2], bond softening and hardening [3], and rotational excitation and molecular alignment [4]
Besides the population inversion between the B2Æuþ À X2Ægþ states, which is responsible for the observed simulated amplification of a seed pulse, a rotational wave packet in the ground vibrational state (v 1⁄4 0) of the excited electronic B2Æuþ state has been created in N2þ
We show that, surprisingly, the abovementioned externally seeded air laser [10,11,12] can be strongly affected by rotational wave packets of molecular ions in the ground vibrational state (v 1⁄4 0) of the excited electronic B2Æuþ state of N2þ produced in the femtosecond-laser-induced plasma spark
Summary
In the presence of strong laser fields, molecules can exhibit many intriguing behaviors, such as high-order harmonic generation [1], above-threshold ionization and dissociation [2], bond softening and hardening [3], and rotational excitation and molecular alignment [4]. We show that, surprisingly, the abovementioned externally seeded air laser [10,11,12] can be strongly affected by rotational wave packets of molecular ions in the ground vibrational state (v 1⁄4 0) of the excited electronic B2Æuþ state of N2þ produced in the femtosecond-laser-induced plasma spark. In our experiment, this effect has been observed in both the frequency domain and the time domain. The evidence obtained in such ‘‘indirect’’ observations contains entangled contributions from different unidentified rotational states and does not allow for straightforward decoding of the rotational states distribution for wave-packet reconstruction
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