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The processes leading to the N2+ lasing are rather complex and even the population distribution after the pump laser excitation is unknown. In this paper, we study the population distribution at electronic and vibrational levels in N2+ driven by ultra-short laser pulse at the wavelengths of 800 nm and 400 nm by using the quantum-mechanical time-domain incoherent superposition model based on the time-dependent Schrödinger equation and the quasi-classical model assuming instantaneous ionization injection described by density matrix. It is shown that while both models provide qualitatively similar results, the quasi-classical instantaneous ionization injection model underestimates the population inversions corresponding to the optical transitions at 391 nm, 423 nm and 428 nm due to the assumption of quantum mixed states at the ionization time. A fast and accurate correction to this error is proposed. This work solidifies the theoretical models for population at vibrational states in N2+ and paves the way to uncover the mechanism of the N2+ lasing.

International audience; We present a model describing ionization and excitation of nitrogen molecules by a strong and short laser pulse. In difference to previous publications, both processes are considered within the same formalism of density matrix. We account for the dependence of the dipole moment on the vibrational quantum number and for a large number of excited levels. Populations of the excited levels depend significantly on the laser intensity, wavelength, polarization and pulse duration.

Extremely high pressure (∼10 Tpa) and temperature (5×105 K) have been produced using a single laser pulse (∼100 nJ, 800 nm, 200 fs) focused inside transparent dielectrics. The laser pulse of intensity over 0.1 PW/cm2 converts a material within the absorption volume of ∼0.15 μm3 into plasma in a few femtoseconds. A pressure of ∼10 Tpa, far exceeding the strength of any material builds up to the end of the pulse generating strong shock and rarefaction waves. This results in the formation of a nano‐void surrounded by a shell of shock‐compressed material. In sapphire, the compressed shell revealed that it has a density 1.14 times of the initial one and increased chemical reactivity. The unique conditions: extreme pressure and temperature at record high heating and cooling rates become available in a well‐controlled laboratory environment.

Linearly polarized laser radiation accelerates electrons to very high velocities and these electron form a sheath layer on the rear side of thin targets where preferentially protons are accelerated. When mass‐limited targets are used, the lateral transport of the absorbed laser energy is reduced and the accelerating field is enhanced. For targets consisting of two ion species, heavier ions facilitate formation of quasi‐monoenergetic bunch of lighter ions. For circularly polarized light, fast electron production is suppressed by the absence of the oscillatory component of the ponderomotive force. Ions are accelerated on the front side by the separation field and very thin foil can be accelerated as one massive quasi‐neutral block. As all ion species acquire the same velocity, this acceleration mechanism is preferred for heavier ions.