Lasing from molecular nitrogen ions at a wavelength of 391.4 nm in laser plasma

We report results of experimental studies on spectral, temporal, energy and spatial characteristics of coherent radiation at a wavelength of 391.4 nm [B²∑⁺_u(0) – X²∑⁺_g(0) transition of molecular nitrogen ion]. Lasing occurs in pure nitrogen under pumping by a radiation pulse with a duration of 70 fs at a wavelength of 950 nm. The lasing parameters are studied as functions of nitrogen pressure, pump beam energy, and its focusing conditions. Optimal values of pump energy (6 mJ) and nitrogen pressure (30 mbar) are determined. It is found that the divergence of radiation decreases with increasing focal length of the lens and correlates with the geometric dimensions of the laser plasma. It is also shown that with increasing specific pump power, the pulse duration decreases and can be close to the transform-limited one (1.83 ps).