Giant Enhancement of Air Lasing by Complete Population Inversion in N_{2}^{+}.

Area of Science:

• Quantum Chemistry
• Laser Physics
• Molecular Spectroscopy

Background:

• Precise control of population transfer in molecular quantum levels is crucial for managing molecular processes.
• High-intensity light fields (TW-PW cm⁻²) enable population transfer via nonlinear light-molecule interactions, but controlling the extent remains challenging.

Purpose of the Study:

• To achieve near-complete depletion of the X²Σg⁺(v=0) state in nitrogen ions (N2+).
• To significantly enhance N2+ lasing at 391 nm.
• To elucidate the mechanism of population transfer using intense, dual-color femtosecond laser pulses.

Main Methods:

• Focusing dual-color (800 nm and 1.6 µm) intense femtosecond laser pulses on nitrogen gas.
• Solving the time-dependent Schrödinger equation for population dynamics among the three lowest electronic states of N2+.

Main Results:

• Achieved almost complete depletion of the N2+ X²Σg⁺(v=0) state.
• Enhanced the intensity of N2+ lasing at 391 nm by 5-6 orders of magnitude.
• Identified the population depletion mechanism as vibrational Raman excitation followed by electronic excitation, leading to population inversion.

Conclusions:

• Demonstrated efficient population transfer in N2+ using a precisely designed intense laser field.
• The proposed method offers a promising route for controlling vibrational and electronic population transfer in molecules.
• This technique significantly enhances molecular lasing.