Absolute Measurement of Laser Ionization Yield in Atmospheric Pressure Range Gases over 14 Decades

Abstract

Strong-field ionization is central to intense laser-matter interactions. However, standard ionization measurements have been limited to extremely low density gas samples, ignoring potential high density effects. Here, we measure strong-field ionization in atmospheric pressure range air, N2 and Ar over 14 decades of absolute yield, using mid-IR picosecond avalanche multiplication of single electrons. Our results are consistent with theoretical rates for isolated atoms and molecules and quantify the ubiquitous presence of ultra-low concentration gas contaminants that can significantly affect laser-gas interactions.The unification of tunneling ionization and multiphoton ionization (MPI) of atoms in intense laser fields by Keldysh in 1965 [1] provided an analytic foundation for strong field laser physics [2-6], but measurements of the transition from MPI to tunneling had to await later advances in short pulse lasers [7][8][9][10]. This transition is characterized in atomic units by the dimensionless Keldysh parameter = 2 ⁄ / , where is the atom's ionization potential, is the peak laser field, and is the laser frequency. At moderate intensity ( ≫ 1, MPI regime), the yield is proportional to (∝ ), while at higher intensities or longer wavelengths ( < 1), the transition to tunneling and barrier suppression ionization [7,9] is characterized by ∝ , where is the integer number of photons needed to exceed . These early measurements were conducted in extremely low density gases (typically ~10 8 −10 12 cm −3 ) in order to prevent ionization products interacting with background gas or experiencing space charge effects in transit to high voltage detectors [7,9,11]. However, many applications of strong-field ionization, such as high harmonic

Direct-write laser-induced self-organization and metallization beyond the focal volume in tellurite glass

“…This oxidation process in the nanostructured zone support the following phase transformation sequence: t-Te → a-Te → glassy-TeO 2 [46]. Localized ionization of the ambient air due to the beam high-peak power (∼10-30 TW/cm 2 ) [47] in the vicinity of the material surface is likely to further enhance this effect by feeding ionized oxygen atoms to the oxidation process. In parallel, the migration of tungsten elements contributes to preserving the glassy-TeO 2 along the nanoplanes.…”

Dynamics of a buffer-gas-loaded, deep optical trap for molecules

“…As a starting point, we use Popruzhenko's formula [99] for hydrogen-like atoms, which is a Perelomov-Popov-Terent'ev (PPT) ionization formula [100]. The formula matches (within two orders of magnitude) experimental results for ionization rates of noble gases [101,102] and air [103]. PPT formulae also match ab initio estimates of ionization rates for polyatomic molecules [104].…”

“…This can lead to exponential growth in the electron number. The process is well understood in high-intensity, pulsed lasers and at high gas densities [103,[175][176][177][178][179]. However, the treatment of breakdown in continuous-wave (CW) laser beams is often left as an afterthought, because CW intensities are rarely high enough to induce breakdown.…”

Powerful terahertz waves from long-wavelength infrared laser filaments

“…The apparent discrepancy between the experiments of Lai et al 48 , showing the validity of the PPT ionization rate formula, and the EID calculations can be explained by the fact that the experiments by Lai et al were conducted at low gas pressures, where the many-body interactions lose their importance. To clarify this situation, Woodbury et al 56 experimentally measured the ionization yield in air, nitrogen, and argon at atmospheric (0.5-3 bar) pressures for 1.024 and 3.9 μm picosecond laser pulses. The ionization yield was measured over a dynamic range of intensities covering 14 orders of magnitude (the sensitivity of the experiment allowed to observe individual ionization events at low intensities).…”

“…In addition to the ionization yield measurements, Woodbury et al 56 estimated the effective collisional frequency ν c , which determines the avalanche ionization rate. ν c = 0.55 ps −1 was found for both the 1.024 and 3.9 μm wavelengths.…”