Two-photon Ionization Of He Research Articles

Electron vortices in two-photon ionization of He+ in two-color elliptically polarized laser field

We explored the photoelectron momentum distribution (PMD) generated by two-photon ionization of He+ in an elliptically polarized laser field, and co-rotation and counter-rotation elliptically polarized laser fields by solving the time-dependent Schrödinger equation (TDSE) in two dimensions. It is analyzed that as the ellipticity increases, the PMD localized along the y direction increases. We also investigated that the direction of rotation of the electron vortex structure in the counter-rotation laser field is determined by the larger wavelength of the laser pulse in the combination laser field.

Ionization of helium by an ultrashort extreme-ultraviolet laser pulse

We theoretically study photoelectron angular distributions and related anisotropy parameters for the competition of one- and two-photon ionization of He in an ultrashort extreme-ultraviolet laser pulse, using numerical results from the time-dependent Schrödinger equation. We explore the transition between the two processes for variation of the pulse duration, peak intensity, and central frequency or photoelectron energy. In the results obtained for fixed pulse parameters the transition and interference between the processes can be observed in the even (β2, β4) and odd (β1, β3) parameters, respectively, while the onset of the impact of three-photon ionization is observed via β5 at high peak intensities. Finally, we show that the transition can be observed via the even anisotropy parameters in pulses without carrier-to-envelope stabilization as well as in free-electron laser pulses with fluctuating pulse shape.

R -matrix-with-time-dependence theory for ultrafast atomic processes in arbitrary light fields

We describe an ab initio and non-perturbative $R$-matrix with time-dependence theory for ultrafast atomic processes in light fields of arbitrary polarization. The theory is applicable to complex, multielectron atoms and atomic ions subject to ultrashort (particularly few-femtosecond and attosecond) laser pulses with any given ellipticity, and generalizes previous time-dependent $R$-matrix techniques restricted to linearly polarized fields. We discuss both the fundamental equations, required to propagate the multielectron wavefunction in time, as well as the computational developments necessary for their efficient numerical solution. To verify the accuracy of our approach, we investigate the two-photon ionization of He, irradiated by a pair of time-delayed, circularly polarized, femtosecond laser pulses, and compare photoelectron momentum distributions, in the polarization plane, with those obtained from recent time-dependent close-coupling calculations. The predictive capabilities of our approach are further demonstrated through a study of single-photon detachment from F$^{-}$ in a circularly polarized, femtosecond laser pulse, where the relative contribution of the co- and counter-rotating $2p$ electrons is quantified.

Angular distributions in two-colour two-photon ionization of He

We present R-Matrix with time dependence (RMT) calculations for the photoionization of helium irradiated by an EUV laser pulse and an overlapping IR pulse with an emphasis on the anisotropy parameters of the sidebands generated by the dressing laser field. We investigate how these parameters depend on the amount of atomic structure included in the theoretical model for two-photon ionization. To verify the accuracy of the RMT approach, our theoretical results are compared with the experiment.

Nonresonant EUV-UV two-color two-photon ionization of He studied by single-shot photoelectron spectroscopy

Single-shot photoelectron spectroscopy of nonresonant two-color two-photon ionization of He has been carried out using an intense free-electron laser (FEL) at 59.7 nm and a femtosecond UV laser at 268 nm. By shot-by-shot analysis of photoelectron spectra, the cross section for nonresonant two-color ionization of He is determined to be ${\ensuremath{\sigma}}^{(2)}=4.1(6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}52}$ cm${}^{4}$ s. The cross-correlation trace derived from the nonresonant signals shows that the temporal resolution of the system is 0.36(3) ps, which is mainly governed by the timing jitter between the FEL and UV laser pulses.

Competition of sequential and direct paths in two-photon ionization of He

We study the photoelectron angular distribution (PAD) from the two-photon ionization (TPI) of He by femtosecond EUV pulses. The calculation with the time-dependent Schrödinger equation (TDSE) and the measurement at the SPring-8 EUV-FEL show reasonable agreement. The extracted relative phase δ between the s and d wave packets are distinct from that between the corresponding eigenstates, due to the competition between sequential and direct ionization paths. When the pulse is resonant with an excited level, the sequential and direct TPI compete with each other, and δ and the PAD depend on the pulse width. On the other hand, when the Ry-dberg manifold is coherently excited, δ does not vary with the pulse width.

Suppression of ionization probability due to Rabi oscillations in the resonance two-photon ionization of He by EUV free-electron lasers

The two-photon resonance ionization probability of atoms in strong extreme-ultraviolet free-electron laser (EUV FEL) pulses has been investigated by the model of time-dependent wave packet propagation of a light-coupled multilevel atom. Under the simulation within the model assuming single-mode FEL pulses, the ionization probability ${P}_{\mathrm{ion}}$ has shown characteristic dependences on the scaled coupling parameter ${U}_{gi}$ between two levels of the ground ($g$) and intermediate ($i$) resonance states, namely, ${P}_{\mathrm{ion}}\ensuremath{\propto}{({U}_{gi})}^{n}$, with $n$ being equal to \ensuremath{\sim}2, less than 1, and \ensuremath{\sim}1 for the small, medium, and large ${U}_{gi}$ regimes, respectively. This power dependence of the ionization probability has been interpreted due to Rabi oscillations between $g$ and $i$ states. To compare with recent experimental results on the same condition, the multimode nature of self-amplitude spontaneous emission (SASE) FEL pulses has been managed in the simulation. Then, the recent experimental laser-power dependence of the two-photon resonance ionization of He [Sato et al., J. Phys. B 44, 161001 (2011)] has been well described by that for the large ${U}_{gi}$ regime of the simulation, i.e., $n$ \ensuremath{\sim} 1. Thus, the observed linear laser-power dependence has been rationalized as being caused by the strong Rabi oscillations between the (2$p$)--(1$s$) states.

EUV and soft X-ray photoelectron spectroscopy of isolated atoms and molecules using single-order laser high-harmonics at 42 eV and 91 eV

Photoelectron spectroscopy of isolated atoms and molecules using single-order high-harmonics of Ti:Sapphire laser pulses (800 nm, 12 fs/30 fs) is demonstrated. Dielectric multilayer mirrors, SiC/Mg and Mo/Si, are used to isolate the 27th (42 eV) and 59th (91 eV) order harmonics, respectively. The obtained harmonics are characterized by valence and inner-shell photoelectron spectroscopy of Xe. The applications to two-color two-photon ionization of He and pump-probe spectroscopy of ultrafast photodissociation of Br 2, Br 2(C 1Π u) → Br( 2P 3/2) + Br( 2P 3/2), are presented.

Comment on “Photoionization of helium atoms irradiated with intense vacuum ultraviolet free-electron laser light. Part I. Experimental study of multiphoton and single-photon processes”

The reported observation of two-photon ionization of He by $13\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ photons of the TESLA Facility in T. Laarmann, A. R. B. de Castro, P. G\urtler, W. Laasch, J. Schulz, H. Wabnitz, and T. M\oller, Phys. Rev. A72, 023409 (2005) is ambiguous, as it is not supported by the experimental data presented. We discuss the lack of evidence for the claimed two-photon ionization.

Frequency-resolved photoelectron spectra of two-photon ionization of He by an attosecond pulse train

We present measured and calculated energy-resolved photoelectron spectra obtained through two-photon ionization of He induced by a superposition from the 9th to the 15th harmonic of a Ti:Sapph laser forming an attosecond (asec) pulse train. The reported measured spectra are a decisive step towards frequency-resolved two-XUV-photon ionization-based second-order autocorrelation (AC) of asec pulse trains, and thus towards a complete reconstruction of asec pulses.

Two-Photon Ionization of He through a Superposition of Higher Harmonics.

We present experimental results and theoretical analysis of two-photon ionization of He by a superposition of the 7th to the 13th harmonic of a Ti:sapphire laser. Solving the time-dependent Schrödinger equation for He in a coherent polychromatic field, the He+ yield is calculated. From this yield the number of He+ ions produced has been estimated and found in reasonable agreement with its measured value. The present results establish the feasibility of a second-order autocorrelation measurement of superposition of harmonics, and thus they represent the precursor towards the direct temporal characterization of attosecond pulse trains.

Two-color pump-probe experiments in helium using high-order harmonics

A pump-probe technique has been applied for measuring the lifetimes and absolute photoionization cross-sections of excited He states. The 1s2p P-1 and 1s3p P-1 states of He are excited by using the 13th and the 14th harmonic, respectively, of a tunable 70 ps dye laser generated in a Kr gas jet. The states are ionized after a varying time delay, by absorption of probe photons with energies between 1.6 and 4.5 eV. Lifetimes of tau(1s2p) = 0.57 ns and tau(1s3p) = 1.76 ns are determined with a precision of about 15%. A significant enhancement of the number of ions present in the lifetime curves at zero time delay for pressures above 6 x 10(-5) mbar is attributed to direct two-photon ionization of He in combination with AC Stark broadening of the excited state and absorption of the XUV light in the medium. Absolute photoionization cross-sections from the He 1s2p P-1 and He 1s3p P-1 states in the threshold region are determined by measuring the saturation of the ionization process with a precision of similar to 25%. In addition, the variation of the relative orientation between the polarization vectors of the pump and probe beams enables the determination of partial photoionization cross-sections. (Less)

Resonance-enhanced two-photon ionization of He and Xe using a mode-locked Ar + laser synchronized to VUV synchrotron radiation pulses

The light pulses of mode-locked Ar + laser have been synchronized to the synchrotron radiation pulses of the SuperACO storage ring in Orsay. In order to test the feasibility and to demonstrate the capability for the investigation of free atoms we have studied the two-photon ionization of atomic Helium and Xenon via the He ∗ 1s3p and Xe ∗ 5p 55d resonant state, respectively. The experimental results show clearly an ionization signal which strongly depends on the temporal delay between the two light pulses. The spectra can be interpreted as the convolution of an exponential function representing the lifetime of the intermediate resonant state and a Gaussian function representing the cross correlation function of the synchrotron and laser radiation. The overall time resolution of the present set-up has been estimated to about 1 ns.

A theoretical study of the two-photon ionization of metastable He with linearly and circularly polarized laser light

Generalized cross sections <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\hat{\sigma}_{L}</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\hat{\sigma}_{C}</tex> for the two-photon ionization of He from the metastable <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2^{1}S</tex> state are calculated for linearly and circularly polarized incident laser light, respectively, in the wavelength range 3725-5100 Å. Significant enhancement of cross sections occurs through real <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3^{1}P</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4^{1}P</tex> p intermediate states. The ratio <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\hat{\sigma}_{C}</tex> / <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\hat{\sigma}_{L}</tex> shows a minima at the wavelengths close to 3743 and 4450 Å.

Calculation of cross sections for multiphoton ionization of complex atoms

A method is proposed for solving the equations of perturbation theory appropriate for multiphoton excitation or ionization of complex atoms, and detailed calculations are carried out for two-photon ionization of He. The method consists of writing the wave function for order $N$ as a product of $N\mathrm{th}$- and zeroth-order wave functions for the electron absorbing the photon ("optical" electron) and the remaining electrons ("passive" electrons), respectively, with symmetrization of the spatial wave function appropriate for an eigenfunction of spin. Projection onto the space of the passive electrons then generates an integro-differential equation for the optical electron. The Coulomb Green's function is used to convert this equation into an integral equation, which is solved numerically using a modified version of the noniterative method of solution used extensively in scattering theory. Results obtained with the Hartree-Fock zeroth-order model show good agreement with the results of coupled Hartree-Fock perturbation theory. Results are also obtained ignoring exchange, and the error is observed to be about a factor of 2 over a wide range of frequencies.