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

The interaction of He atoms with intense vacuum-ultraviolet light of a free-electron laser is investigated using time-of-flight mass spectroscopy and photoelectron spectroscopy. The atoms were irradiated with $100\mathrm{fs}$ pulses at $95\mathrm{nm}$ wavelength, which corresponds to $\sim 13\mathrm{eV}$ photon energy. The ionization of He atoms is observed at a peak intensity of ${10}^{10}–{10}^{13}\mathrm{W}∕{\mathrm{cm}}^2$, which is due both to nonlinear multiphoton ionization with the fundamental wavelength and single-photon ionization with third harmonic radiation of the free-electron laser. The observation of two sharp photoelectron peaks in the kinetic energy spectra, that are separated by the photon energy, is in agreement with the numerical solution of the time-dependent Schrödinger equation. The calculation was done using the fully quantized field and a limited but representative set of basis states. The ionization rate dependence on the laser peak intensity indicates that: (a) The low-energy peak in the photoelectron spectra is mainly due to two-photon absorption of the fundamental, but (b) the high-energy peak at $15.4\mathrm{eV}$ is probably due to third harmonic FEL radiation. The theoretically predicted contribution from three-photon absorption of the fundamental is of about the same order of magnitude and could not be separated from the third harmonic background signal. Particularly, the photoelectron spectra and ${\mathrm{He}}^{+}$ time-of-flight data give evidence that the intensity of third harmonic light is high enough to perform single-shot spectroscopy on gas phase samples.

Figure 1

Longitudinal section of the mirror chamber. A small tube (diameter $2\mathrm{mm}$) separates the mirror chamber from the main chamber. The adjustment of both mirrors was remote controlled.

Figure 2

Transmission curve of photoelectrons as a function of their kinetic energy.

Figure 3

Ionic products after irradiation of an atomic beam of helium. The FEL parameters are given in the figure. ${\mathrm{H}}_2{\mathrm{O}}^{+}$ ions result from single-photon ionization of residual gas molecules.

Figure 4

$n$-photon ionization rate ${\Gamma }_n$ of ${\mathrm{He}}^{+}$ ions as a function of the FEL peak intensity $I$ in a double-logarithmic scale. The slope of the linear fit is approximately $n=1$. The generalized cross section of the ionization process is given by ${\xi }_n$. Data points obtained in the focus (triangles) and at reduced peak intensities $8\mathrm{mm}$ outside the focus (circles) are indicated.

Figure 5

Electron kinetic energy distribution from He atoms irradiated with VUV FEL light at similar peak intensity of (a) $5.4\bullet {10}^{12}\mathrm{W}∕{\mathrm{cm}}^2$ (without band pass filter), respectively (b) $\sim {10}^{12}\mathrm{W}∕{\mathrm{cm}}^2$ (with band pass filter). The calculated energy positions of He photoelectrons are indicated in the figure by dotted lines.

Figure 6

(a) Dependence of the He photoelectron signal at $1.2\mathrm{eV}$ kinetic energy (peak A) on the FEL peak intensity in log-log scale. Experimental data (squares) is compared with theoretical prediction for the two-photon absorption (solid line). (b) Probability of two-photon absorption calculated at the indicated FEL intensities after irradiation with a single laser pulse (open squares). The He atom goes from the ground state into a state with one bound electron and one quasi-free electron. A polynomial fit to the theoretically derived values is indicated by the solid line, for details see the text and the companion paper.

Figure 7

$n$-photon ionization rate ${\Gamma }_n$ related to peak C in Fig. 5a as a function of the FEL peak intensity $I$ in a double-logarithmic scale. The slope of the linear fit $(n=1)$ reflects, that the ionization of He atoms leading to $15\mathrm{eV}$-electrons, is governed by third harmonic FEL radiation. The generalized cross section of the ionization process is given by ${\xi }_n$. Data points obtained in the focus and at reduced peak intensities 3, 8, and $15\mathrm{mm}$ outside the focus are indicated in the figure.

Figure 8

Comparison between the measured (a) and simulated (b) time-of-flight photoelectron distribution that corresponds to the energy spectrum shown in Fig. 5a. Electrons ejected into the half-sphere facing towards (forward) and away (backward) from the detector are indicated (for details see the text).

Figure 9

Peak intensity dependence of the resonant absorption probability for two- and three-photon transition from the He $1s1s$ singlet ground state into quasifree ${\mathrm{He}}^{+}+{\mathbf{k}}_{\mathbf{1}}$, respectively, ${\mathrm{He}}^{+}+{\mathbf{k}}_{\mathbf{2}}$ states. VUV-FEL parameters are given in the figure.