Multiphoton ionization (MPI) is a fundamental first step in high-energy laser-matter interaction and is important for understanding the mechanism of plasma formation. With the discovery of MPI more than 50 years ago, there were numerous attempts to determine the basic physical constants of this process in direct experiments, namely photoionization rates and cross-sections of the MPI; however, no reliable data was available until now, and the spread in the literature values often reaches 2–3 orders of magnitude. This is due to the inability to conduct absolute measurements of plasma electron numbers generated by MPI, which leads to uncertainties and, sometimes, contradictions between MPI cross-section values utilized by different researchers across the field. Here, we report the first direct measurement of absolute plasma electron numbers generated at MPI of air, and subsequently we precisely determine the ionization rate and cross-section of eight-photon ionization of oxygen molecule by 800 nm photons σ8 = (3.3 ± 0.3)×10−130 W−8m16s−1. The method, based on the absolute measurement of the electron number created by MPI using elastic scattering of microwaves off the plasma volume in Rayleigh regime, establishes a general approach to directly measure and tabulate basic constants of the MPI process for various gases and photon energies.

In this work, a novel diagnostic technique for carbon monoxide (CO) number density measurements in a nitrogen buffer mixture at elevated pressures up to 5 bar was developed and tested. The technique utilizes 2 + 1 resonance enhanced multi-photon ionization (REMPI) of CO induced by a femtosecond laser pulse at 230.1 nm, followed by detection of the number of REMPI-induced electrons using the microwave scattering (MS) method (REMPI-MS technique). Dependences of the number of REMPI-generated electrons on CO number density and laser energy were measured and analyzed in conjunction with a four energy level model of the CO molecule. The number of REMPI-induced electrons scaled linearly with CO number density up to about 5 × 1018 cm−3 and was independent of the buffer gas pressure up to 5 bar. Higher CO number densities caused saturation onset associated with laser beam energy loss while travelling through the gaseous mixture due to two-photon absorption and photoionization. The number of REMPI-induced electrons was found to scale cubically with the laser pulse energy for the tested energy range of 8–20 μJ (intensity in the focal region about 7–18 GW/cm2), which is consistent with the operation regime where the number density of excited CO molecules increases throughout the laser pulse duration and does not saturate in time. The linear scaling region of the REMPI-MS signal can be used for a CO number density diagnostic after appropriate calibration of the system.

In this work, we present temporally resolved measurements of electron numbers created at photoionization of various gases by femtosecond laser pulse at 800 nm wavelength. The experiments were conducted in O 2 , Xe, Ar, N 2 , Kr and CO at room temperature and atmospheric pressure. Elastic microwave scattering was used to directly measure the electron numbers. Numbers of electrons in the range 310 8 to 310 12 electrons were produced by the laser pulse energies 100-700 J. After the laser pulse, plasma decayed on the time scale varied from 1 to 40 ns depending on the gas type and governed by two competing processes, namely, the creation of new electrons from ionization of the metastable atoms and loss of the electrons due to dissociative recombination and attachment to oxygen.

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