Geometry- and diffraction-independent ionization probabilities in intense laser fields: Probing atomic ionization mechanisms with effective intensity matching

Abstract

We report a novel experimental technique for the comparison of ionization processes in ultrafast laser pulses irrespective of pulse ellipticity. Multiple ionization of xenon by 50 fs 790 nm, linearly and circularly polarized laser pulses is observed over the intensity range 10 TW/cm 2 to 10 PW/cm 2 using Effective Intensity Matching (EIM), which is coupled with Intensity Selective Scanning (ISS) to recover the geometry-independent probability of ionization. Such measurements, made possible by quantifying diffraction effects in the laser focus, are compared directly to theoretical predictions of multiphoton, tunnel and field ionization, and a remarkable agreement demonstrated. EIM-ISS allows the straightforward quantification of the probability of recollision ionization in a linearly polarized laser pulse. Furthermore, probability of ionization is discussed in terms of the Keldysh adiabaticity parameter γ, and the influence of the precursor ionic states present in recollision ionization is observed for the first time.

Extreme-ultraviolet light generation in plasmonic nanostructures

“…The presence of low-order harmonics and substantial gas ionization already implies rather high intensities within the nanostructures [72,73,74]. (Note that the ionization energies of the gases used (argon: 15.8 eV, xenon: 12.1 eV) exceed the energy of ten and seven laser photons, respectively.)…”

Atomic excitation during recollision-free ultrafast multi-electron tunnel ionization

“…Here we report a unique combination of experimental techniques 5 -8 that enables us to accurately measure the tunnel ionization probability for argon exposed to 50 femtosecond laser pulses. Most significantly for the current study, this measurement is independent of the optical focal geometry 7,8 , equivalent to a homogenous electric field. Furthermore, circularly-polarized radiation negates recollision.…”

“…Recollision is the key mechanism for attosecond XUV pulse generation, as the kinetic energy of the electron is dissipated photonically if the electron is recaptured by the parent ion 19 . However, in the present work, we make recollision events negligible by employing circularly polarized light: the absorption of a large number of photons transfers considerable angular momentum to the liberated electron, preventing it from returning to the ionic core 7,20 , thus the masking effect of recollisional excitation and ionization are negated.…”

“…As circularly polarized radiation is employed, this data is free from the influence of recollision effects. The partial probability is recovered from the ISS data in figure 2 using the method described in reference[7].The uncertainty is estimated by calculating the statistical deviation in the recorded average time-of-flight signal, and the upper and lower confidence interval corresponding to 1 s.d. was propagated through the deconvolution procedure.…”

Extreme-ultraviolet light generation in plasmonic nanostructures

“…It is important to resolve whether the lack of any HHG signatures in these measurements is caused by insufficient local intensities or has other physical origins. The presence of low-order harmonics and substantial gas ionization already implies rather high intensities within the nanostructures [72,73,74]. (Note that the ionization energies of the gases used (argon: 15.8 eV, xenon: 12.1 eV) exceed the energy of ten and seven laser photons, respectively.)…”