Single-ionization of helium at Ti:Sapphire wavelengths: rates and scaling laws

• Published in: Journal of physics. B, Atomic, molecular, and optical physics Atomic, molecular, and optical physics, 2007-05, Vol.40 (10), p.1729-1743
• Main Authors: Parker, J S, Meharg, K J, McKenna, G A, Taylor, K T
• Format: Article
• Language: English
• Subjects: Atomic and molecular physics; Atomic properties and interactions with photons; Exact sciences and technology; Fundamental areas of phenomenology (including applications); Optics; Photoionization of atoms and ions; Photon interactions with atoms; Physics; Quantum optics; Strong-field excitation of optical transitions in quantum systems; multi-photon processes; dynamic stark shift
• ISSN: 0953-4075; 1361-6455
• DOI: 10.1088/0953-4075/40/10/008

We present a numerical and theoretical study of intense-field single-electron ionization of helium at 390 nm and 780 nm. Accurate ionization rates (over an intensity range of (0.175-34) X 1014 W cm-2, at 390 nm, and (0.275-14.4) X 1014 W cm-2 at 780 nm) are obtained from full-dimensionality integrations of the time-dependent helium-laser Schrodinger equation. We show that the power law of lowest order perturbation theory, modified with a ponderomotive-shifted ionization potential, is capable of modelling the ionization rates over an intensity range that extends up to two orders of magnitude higher than that applicable to perturbation theory alone. Writing the modified perturbation theory in terms of scaled wavelength and intensity variables, we obtain to first approximation a single ionization law for both the 390 nm and 780 nm cases. To model the data in the high intensity limit as well as in the low, a new function is introduced for the rate. This function has, in part, a resemblance to that derived from tunnelling theory but, importantly, retains the correct frequency-dependence and scaling behaviour derived from the perturbative-like models at lower intensities. Comparison with the predictions of classical ADK tunnelling theory confirms that ADK performs poorly in the frequency and intensity domain treated here.