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Physical Review A

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”

Dimitrios Charalambidis, P. Tzallas, Nektarios A. Papadogiannis, Lampros A. A. Nikolopoulos, E. P. Benis, George D. Tsakiris

Published Sep 6, 2006• 8 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.74.037401

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We cannot agree with the Comment by Maquet et al. [Phys. Rev. A. 74, 027401 (2006)]. It seems that Maquet et al. are thinking in terms of classical fields. In our recent paper we have presented a study of the two-photon rate of He making use of quantized fields. This approach becomes a natural choice if we have in mind processes at high photon energy. In this description of the problem the squared vector potential $\mathbf{A}∙\mathbf{A}$ term is essential.

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The reported observation of two-photon ionization of He by 13eV13\phantom{\rule{0.3em}{0ex}}\mathrm{eV}13eV 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.
We cannot agree with the Comment by Maquet et al. [Phys. Rev. A. 74, 027401 (2006)]. It seems that Maquet et al. are thinking in terms of classical fields. In our recent paper we have presented a study of the two-photon rate of He making use of quantized fields. This approach becomes a natural choice if we have in mind processes at high photon energy. In this description of the problem the squared vector potential A∙A\mathbf{A}∙\mathbf{A}A∙A term is essential.
There have been relatively few investigations of multiphoton ionization from metastable helium. Of particular interest has been the work of Haberland et al. 1987 and Haberland and Oschwald 1988. In both the 1987 and 1988 papers they have described the two photon ionization of metastable helium. In each of these studies they have reported the occurrence of unexplained structure along the wings of their resonance profiles. Upon the performance of similar measurements by this study, the unexplained structure is not seen and the agreement of the experiment's measurements with the theoretical shape of the resonance curves has been good. To experimentally verify these resonance effects, we have used a tunable dye laser in conjunction with a time of flight mass spectrometer to create and detect ions from metastable helium by two-photon absorption. The use of a metastable state instead of the ground state is advantageous because of its proximity to the ionization continuum and its extended lifetime. Using a metastable state as a starting point for multiphoton absorption requires fewer photons to reach the ionization threshold. The extended lifetime of the state also makes it easy to access experimentally. For helium the singlet metastable state 2$\sp1$S lies at 20.61 eV above the ground level with a natural lifetime of close to a millisecond. Two photons of 501.7 nm and 504.35 nm are required for the ionization processes in resonance with the 3$\sp1$P and the 3$\sp1$D states. This thesis is the accounting of the experimental process involved in the measurement of the dipole and quadrupole resonances of two photon ionization from singlet metastable helium. The study includes the description of the laser, electron gun assembly for metastable helium creation, and the time of flight mass spectrometer. A discussion of the theory of multiphoton processes is included along with the discussion of the data, its reduction and analysis, and a comparison with theoretical prediction. This study has not only involved the acquisition of definitive atomic spectra data sets, but has also established a facility for resonant ionization spectroscopy at The American University.
Until recently, the nonlinear interaction between light and matter has been restricted to only low photon energies produced by optical lasers. However, about a decade ago, the rise of free-electron laser facilities revolutionized the field of nonlinear light-matter interaction by delivering intense high-energy light pulses. Today, such lasers are used for research in materials science, chemical technology, biophysical science, solid state physics as well as fundamental research. It is the new experimental possibilities provided by free-electron lasers that motivated the work presented in this thesis. Two-photon ionization process is one of the simplest nonlinear interactions in which absorption of two photons by an atom (or a molecule) leads to promoting one of its bound electrons to continuum. This work presents studies of two-photon ionization of neutral atoms. After a brief historical introduction to the topic of nonlinear light-matter interaction, the density matrix describing the state of an atom and a photoelectron following two-photon ionization is derived. In each successive chapter, part of this density matrix is used to obtain characteristic quantities such as total two-photon ionization cross section, photoelectron angular distributions, ion polarization or degree of polarization of fluorescence photon produced by subsequent decay of the photoion. Physical properties of these quantities are studied and intriguing phenomena, such as elliptical dichroism, polarization transfer as well as relativistic and screening effects are investigated.
We present a quantitative theory with detailed calculations pertaining to multiphoton ionization of helium under moderately strong uv radiation of photon energy 13eV13\phantom{\rule{0.3em}{0ex}}\mathrm{eV}13eV. Employing both, the nonperturbative solution of the time-dependent Schr\"odinger equation, as well as perturbation theory, we obtain detailed results for all processes expected to be of quantitative relevance within the range of intensities and pulse durations in this work, which have been chosen so as to match the conditions of a recent experiment on exactly this problem [T. Laarman et al., Phys. Rev. A 72, 023409 (2005)]. The conclusion, in connection with the experimental data, which our results lead to is that, without the presence of the second harmonic of the fundamental in the radiation, it is impossible to reconcile the reported observations with fundamental features of the processes involved. A more general conclusion seems to be that the presence of these harmonics, even at intensities less than 1% of that of the fundamental, are apt to produce profound side effects that will mask multiphoton processes by the fundamental.
Two-photon ionization of atomic helium has been measured by combining femtosecond extreme-ultraviolet pulses from the free-electron laser in Hamburg (FLASH at DESY) with intense light pulses from a synchronized neodymium-doped yttrium lithium fluoride laser. Sidebands appear in the photoelectron spectra when the two laser pulses overlap in both space and time. Their intensity exhibits a characteristic dependence on the relative time delay between the ionizing and the dressing pulses and provides an inherent time marker for time-resolved pump-probe experiments. The measurements of the sidebands are in good agreement with theoretical predictions and allow for a direct analysis of two-photon ionization, free from processes related to interference between multiple quantum paths.