Two-photon resonant excitation of a doubly excited state in He atoms by high-harmonic pulses

Taro Sekikawa,Eisuke Haraguchi,Tatsuya Okamoto,Takashi Nakajima,Mikio Yamashita

doi:10.1364/oe.16.021922

Abstract

We experimentally demonstrate a two-color two-photon resonant excitation of the doubly excited 2p2 1S state in helium atoms by the combination of 19th and 21st harmonic photons of a Ti:sapphire laser. Production of the 2p2 1S state is confirmed by the experimental observation that the electron emission from this state does not depend on the direction of harmonic polarization. Our ab-initio theoretical results through the solution of time-dependent Schrödinger equation are consistent with the experimental results and confirm the first successful production of a doubly excited state by high-harmonic pulses.

Highlights

Development of ultrashort pulse lasers has led to the generation of coherent extremeultraviolet (XUV) and soft x-ray (SXR) light [1, 2]
We experimentally demonstrate a two-color two-photon resonant excitation of the doubly excited 2p2 1S state in helium atoms by the combination of 19th and 21st harmonic photons of a Ti:sapphire laser
Our ab-initio theoretical results through the solution of time-dependent Schrödinger equation are consistent with the experimental results and confirm the first successful production of a doubly excited state by high-harmonic pulses

Summary

Introduction

Development of ultrashort pulse lasers has led to the generation of coherent extremeultraviolet (XUV) and soft x-ray (SXR) light [1, 2]. Use of the negatively and positively chirped laser pulses enables us to almost continuously shift (tune) the center photon energy upwards and downwards, respectively [15] Because of this practical tunability of the photon energy, high harmonics are very useful for XUV and SXR nonlinear spectroscopy of atoms and molecules, etc. Toward this goal, experimental observation of twophoton resonant excitation of a doubly excited state by high-harmonic pulses would serve as a cornerstone for ultrafast XUV nonlinear spectroscopy. We report the first experimental observation of two-photon resonant excitation to the doubly excited state of helium atoms by XUV harmonic photons together with our ab-initio theoretical results. In order to confirm the successful detection of autoionizing εs electrons, we have measured the electron signals with two different polarization geometries, the detail of which will be explained later in this paper

Experimental

Observation of the doubly excited state

Comparison with ab-initio theory

Summary