AMO Seminar

Title: Ionization of He by an Intense, Few-cycle Attosecond XUV Pulse

Speaker:J. M. Ngoko Djiokap, Department of Physics and Astronomy, University of Nebraska-Lincoln

Abstract:

The experimental production of isolated attosecond pulses with stable and tunable carrier-envelope-phase (CEP) is a key step toward a major goal of attosecond science: controlling electronic motion on its natural timescale. The crucial next step is to increase the intensity of such pulses such that one can investigate nonlinear XUV attosecond processes. We have recently developed a perturbation theory for nonlinear attosecond pulse photoionization [1, 2] and have developed numerical codes to solve the one- and two-electron, time-dependent Schrödinger equation (TDSE) [1, 2, 3, 4, 5, 6]. These capabilities have allowed us to investigate both analytically and numerically few-cycle pulse CEP-induced asymmetries in ionization of H to H⁺ [1, 3], of He to He⁺(1s) [1, 3, 4], of He to He⁺(n = 2) [5], and of He to He⁺⁺ [2, 6].

In this talk, I will focus on our latest results: double ionization of He by an intense, elliptically-polarized, few-cycle attosecond XUV pulse [6]. A key result is the prediction of the existence of a new type of CEP-sensitive polarization asymmetry that is normally absent in single photon double ionization of He, and which only occurs for an elliptically-polarized, few-cycle attosecond XUV pulse. This new effect that we call nonlinear dichroism (ND) is sensitive not only to the ellipticity, peak intensity I, and temporal duration of the pulse, but also to the energy sharing. This dichroic effect (i.e., the difference of the two-electron angular distributions for opposite helicities of the ionizing XUV pulse) is proportional to I^3/2, indicating from a perturbation theory analysis [6] that it originates from interference of first- and second-order amplitudes owing to the broad pulse bandwidth, allowing one to probe and control S- and D-wave channels of the two-electron continuum. ND probes electron correlation on its natural timescale since it vanishes for long pulses.

This work is supported in part by DOE, Office of Science, Division of Chemical Sciences, Geosciences, and Biosciences, Grant No. DE-FG03-96ER14646.

References
[1] E.A. Pronin, A.F. Starace, M.V. Frolov and N.L. Manakov, Phys. Rev. A 80, 063403 (2009).
[2] J.M. Ngoko Djiokap, N.L. Manakov, A.V. Meremianin, and A.F. Starace, Phys. Rev. A 88, 053411 (2013).
[3] L.-Y. Peng, E.A. Pronin and A.F. Starace, New J. Phys. 10, 025030 (2008).
[4] J.M. Ngoko Djiokap, S.X. Hu, W.-C. Jiang, L.-Y. Peng, and A.F. Starace, New J. Phys. 14, 095010 (2012).
[5] J.M. Ngoko Djiokap, S.X. Hu, W.-C. Jiang, L.-Y. Peng, and A.F. Starace, Phys. Rev. A 88, 011401(R) (2013).
[6] J.M. Ngoko Djiokap, N.L. Manakov, A.V. Meremianin, S.X. Hu, L.B. Madsen and Anthony F. Starace, submitted
to Phys. Rev. Lett. (2014).

Coffee/tea and cake will be served at 15:00