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

M. Fushitani, Y. Hikosaka, A. Matsuda, T. Endo, E. Shigemasa, M. Nagasono, T. Sato, T. Togashi, M. Yabashi, T. Ishikawa, and A. Hishikawa

Phys. Rev. A 88, 063422 – Published 31 December 2013

Abstract

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 $σ^{(2)} = 4.1 (6) \times 10^{- 52}$ 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.

Received 26 September 2013

DOI:https://doi.org/10.1103/PhysRevA.88.063422

Authors & Affiliations

M. Fushitani^1,2, Y. Hikosaka^2,3, A. Matsuda¹, T. Endo¹, E. Shigemasa^2,4, M. Nagasono², T. Sato², T. Togashi⁵, M. Yabashi^2,5, T. Ishikawa², and A. Hishikawa^1,2,*

¹Department of Chemistry, Graduate School of Science, Nagoya University, Nagoya, Aichi 464-8602, Japan
²RIKEN, SPring-8 Center, Sayo, Hyogo 679-5148, Japan
³Department of Environmental Science, Faculty of Science, Niigata University, Niigata, Niigata 950-2181, Japan
⁴Institute for Molecular Science, National Institutes of Natural Sciences, Myodaiji, Okazaki, Aichi 444-8585, Japan
⁵Japan Synchrotron Radiation Research Institute, Sayo, Hyogo 679-5198, Japan

^*hishi@chem.nagoya-u.ac.jp

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Vol. 88, Iss. 6 — December 2013

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Figure 1
(a) Experimental setup. The EUV-FEL beamline includes an Ar gas attenuator for the FEL intensity monitor, a mirror system consisting of elliptical (EM) and cylindrical (CM) mirrors for focusing, and a magnetic bottle-type photoelectron spectrometer. The magnetic mirror formed by a permanent magnet (MG) and a solenoid (S) directs all the photoelectrons to a microchannel plate (MCP) with a phosphor screen. An ultrashort laser pulse from the optical laser line is introduced with a certain time delay $Δ t$ into the spectrometer and coupled with FEL pulses with an angle of $\sim$ 1 $^{\circ}$ by a steering mirror (SM) inside the FEL beamline. The time delay between the FEL and optical laser pulses is controlled by an optical delay-line stage. (b) One-color two-photon ionization via the He 2 $^{1} P$ state by EUV FEL at 58.4 nm (21.2 eV). (c) Two-color two-photon ionization with EUV FEL at 59.7 nm (20.8 eV) and UV pulses at 268 nm (4.63 eV).
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Figure 2
(a) Photoelectron spectrum of He-Ar gas mixture at $h ν = 21.2$ eV, obtained by averaging over $2 \times 10^{3}$ single-shot photoelectron spectra. The FEL field intensity is $\sim 3 \times 10^{12}$ W/cm $^{2}$ . The number density ratio between He and Ar is $n_{Ar}^{0} / n_{He}^{0} = 3.3 \times 10^{- 3}$ . (b) Ar $^{+}$ (upper) and He $^{+}$ (lower) photoelectron intensities plotted against the FEL field intensity. The Ar $^{+}$ signals are multiplied by a factor of 1.5 for clarity. The result of the least-squares fitting of He $^{+}$ signals is also shown (thin solid line). The deviation from a linear dependence (broken line) observed for Ar $^{+}$ at high FEL intensities is well explained by a simulation incorporating the volume effect and the depletion of the ground state (thick solid line).
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Figure 3
(a) Averaged photoelectron spectra of He-Ar gas mixture using EUV-FEL (20.8 eV, $1.6 \times 10^{13}$ W/cm $^{2}$ ) and UV laser (4.63 eV, $1.4 \times 10^{13}$ W/cm $^{2}$ ) pulses at $Δ t = 0$ ps. The number density ratio between He and Ar is $n_{Ar}^{0} / n_{He}^{0} = 1.2 \times 10^{- 4}$ . Two peaks seen at 0.6 and 1.3 eV are assigned to nonresonant and resonant two-color two-photon ionization of He, respectively. (b) Temporal evolution of photoelectron spectra of resonant and nonresonant two-color two-photon ionization of He. (c) Photoelectron signal intensities of He $^{+}$ for nonresonant (filled circles) and resonant (open squares) components as a function of the time delay between the FEL (20.8 eV) and UV laser (4.63 eV) pulses. The FWHM of the cross-correlation trace of the nonresonant trace is obtained to be 0.36(3) ps by fitting the data to a Gaussian function.
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Figure 4
(a) Distribution of nonresonant two-color two-photon ionization of He. The nonzero signal by the temporal overlap of EUV and UV pulses is well explained by a linear function of the EUV-FEL pulse intensity. (b) Distribution of nonresonant signal intensity obtained by the shot-by-shot analysis of the photoelectron spectra at $Δ t = 0$ ps in Fig. 3. (c) Distribution of the normalized $T^{(2)}$ at $Δ t = 0$ ps simulated for jitter-free FEL pulses. (d) Same as (c), but including the timing jitter of 0.33 ps (FWHM).
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