Counting the electrons in a multiphoton ionization by elastic scattering of microwaves

Abstract

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.

Investigation of a Collisional Radiative Model for Laser-Produced Plasmas

“…These authors found rate coefficents for photoionization between 10 −9 to 10 −15 cm 3 s −1 depending on the electron temperature and density [10]. Other photoionization rates have been reported [11], but these rates describe multi-photon ionization during the formation of LPPs and thus are not applicable to the CR model considered here. However, it is noteworthy that these high photoionization rates during the laser pulse duration play a significant role in creating LPPs when the laser beam is of sufficient intensity.…”

“…Comparing the order of magnitude of the photoionization rate coefficients determined by Su et al with the multi-photon rate coefficients, clearly shows the difference between the two. For the photoionization of O (Z A = 8) with laser intensities on the order of 10 13 W/cm −2 , Sharma et al report the multi-photon rates being between 10 8 to 10 10 s −1 , where n e is on the order of 10 14 to 10 15 cm −3 [11], corresponding to rate coefficients between 10 −6 to 10 −5 cm 3 s −1 . The resonant process of dielectronic recombination (DR) has recently been shown to have a significant effect upon the charge state distribution of Si (Z A = 14) LPPs [12].…”

Intensity-dependent self-induced dual-color laser phase modulation and its effect on terahertz generation

“…( 5 ), the electron density N e ( ξ ) is dominated by the ionization rate associated with the incident laser intensity I ( ξ ) 33 , which gradually increases with I ( ξ ) and reaches a maximum increment at the peak. However, because the electron–ion recombination time is usually much longer than the femtosecond scale of the pulse width 34 , 35 , N e ( ξ ) should be considered static after the peak. Therefore, the first term on the r.h.s.…”

Thomson and collisional regimes of in-phase coherent microwave scattering off gaseous microplasmas

“…Notable advantages of the technique include a high sensitivity, good temporal resolution, low shot noise 11 , non-intrusive probing, and the capability of time gating due to continuous scanning. The advantages of CMS over other available diagnostic techniques (microwave interferometry, laser interferometry, time-of-flight mass spectrometry, laser Thomson scattering, Langmuir probes, hairpin resonators) refer to limited sensitivities, lack of absolute calibration, necessity of signal accumulation, and plasma perturbations of the latter as previously reported elsewhere 5 , 17 .…”

“…A careful understanding of electromagnetic-plasma and scattering interactions is essential for the proper extraction of information on total electron numbers and subsequent derivable quantities (e.g., the electron number density ) from CMS. Vast historical art exists for weakly-ionized, strongly collisional microplasmas in which electron motion is restricted by electron-neutral collisions 4 , 5 , 9 , 10 . However, application of the diagnostic to collisionless plasmas has remained relatively unexplored (no N e correspondence is given by 11 , 13 , 18 )—despite garnering some attention in recent years 19 due to a prevalence in studies on electric propulsion devices 20 , photoionization in low-pressure conditions 21 , 22 , etc.…”