Browsing by Author "Pen, UL"

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Direct momentum-space calculations for the resonant multiphoton processes of a hydrogen atom under intense laser pulses
(1996-01-01)
Pen, UL
;
Jiang, TF
;
物理研究所
;
Institute of Physics
We present a momentum-space solution for the time-dependent Schrodinger equation of a realistic hydrogen atom in a strong laser pulse. The method can integrate the nonperturbative system to thousands of optical cycles, previously thought not yet feasible. A hydrogen atom under a 285-nm (4.357-eV), 97.5-TW/cm(2) short pulse is found to produce two prominent photoelectron peaks above threshold, and significant intermediate bound-state resonance with the 4p state. With a 248-nm (5-eV) probe laser pulse that is ten times longer, the resonant structure is explored.
Friedmann equation and stability of inflationary higher derivative gravity
(2001-06-15)
Kao, WF
;
Pen, UL
;
Zhang, PJ
;
物理研究所
;
Institute of Physics
A stability analysis of the de Sitter universe in pure gravity theory is known to be useful in many aspects. We first show how to complete the proof of an earlier argument based on a redundant field equation. It is shown further that the stability condition applies to k not equal 0 Friedmann-Robertson-Walker spaces based on the nonredundant Friedmann equation derived from a simple effective Lagrangian. We show how to derive this expression for the Friedmann equation of pure gravity theory. This expression is also generalized to include scalar field interactions.