Two-photon Resonant Excitation of a Doubly Excited State in He atoms by High-harmonic Pulses

Abstract

We experimentally demonstrate a two-color two-photon resonant excitation of the doubly excited 2p 2 1 S state in helium atoms by the combination of 19th and 21st harmonic photons of a Ti:sapphire laser. Production of the 2p 2 1 S 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.

Characterization of attosecond XUV pulses utilizing a broadband UV~VUV pumping

“…The major drawback of our scheme is that the photoelectron signal will not be as strong as that of FROG-CRAB due to the use of one-photon resonant twophoton ionization by the UV∼VUV pump and XUV probe pulses in the perturbation regime. But we believe it will not a big problem in these days, since even nonresonant two-photon above-threshold ionization of He by XUV (30 eV) photons has been experimentally observed in recent experiments [17] with a good agreement with a theory. After the derivation of all necessary equations we perform some analysis from the few different aspects before presenting numerical results for the H atom as an example.…”

“…where ω xuv 0 is the central frequency of the probe pulse. Specifically we choosehω xuv 0 = 30 eV based on the recent experimental work at this photon energy [17], and additionally assume a 20 eV spectral bandwidth. Of course our scheme is rather general, and can be applied to any other XUV photon energies.…”

“…If we assume that the low-order HHG is used for the pump pulse, the peak intensity would be rather modest, say, 10 11 W/cm 2 [17,21,22], for which |C 1,2 (t)| 1 and C 0 (t) ∼ 1 for all t. Under this weak pumping condition, we can simplify the expressions for the excited states as…”

Double-configuration grating monochromator for extreme-ultraviolet ultrafast pulses

“…Spectroscopic applications of HHs often require the spectral selection of a single harmonic in a narrow XUV band, especially for experiments aimed to gain insight into electronic structures of materials, where multiple orders of HHs have to be filtered out. Although multilayer mirrors are the simplest optical elements for the spectral selection of one harmonic order and are successfully employed for HHs and synchrotron beam lines, they may give a poor contrast ratio between adjacent harmonics [7].…”

Nonperturbative generation of above-threshold harmonics from pre-excited argon atoms in intense mid-infrared laser fields

“…On the other hand, around one decade ago, Paul et al experimentally demonstrated HHG completely from the excited states of alkali-metal atoms (i.e., rubidium), which was realized by cascade excitation in the presence of a weak cw laser field [18]. It should be noted that HHG from pure excited states of noble gases has not been reported, as a similar near-resonant optical excitation requires strong sources of extreme ultraviolet light [14,19,20]. Another difficulty is that for atoms and molecules on the excited states, their ionization potentials are too low to maintain a sufficiently low Keldysh parameter (i.e., , where Ip represents ionization potential and Up represents ponderomotive energy) at the wavelengths near 800 nm [21].…”