Fast Track Communication

We have investigated dissociative ionization of N2 and O2 molecules by 52 nm extreme-ultraviolet light pulses at the free-electron laser facility in Japan. Distributions of kinetic energy release were measured at two different laser power densities below 3 × 1013 W cm−2 and intermediate and final states of the sequential two photon transitions were identified. A free-electron laser (FEL) based on self-amplified spontaneous emission (SASE) in the extreme ultraviolet (EUV) region below λ = 100 nm was first developed in Germany and has proven to be a very powerful tool to explore the interaction of strong EUV laser pulses with atoms [1–4], molecules [5–7] and clusters [8–11]. In May 2008, a new facility, the SPring-8 Compact SASE Source (SCSS) test accelerator in Japan started operation for users [12]. It provides linearly polarized EUV-FEL pulses (∼30 μJ per pulse, ∼100 fs pulse width, 10–20 Hz repetition rate) in the wavelength region 51–61 nm. In this energy regime, all atoms and molecules can be ionized by just a single photon with huge photoionization cross sections. We have demonstrated that this source is indeed ideal to investigate multiphoton multiple ionization of clusters [13, 14]. In the present work, we have investigated dissociative ionization of N2 and O2 molecules caused by two-photon absorption of the EUV-FEL pulses at 52 nm (photon energy of 24 eV) from this light source, using a dead-time-free multi-particle detection system [15]. The photon energy employed in the present experiment was set to be between single ionization thresholds and double ionization thresholds of N2 and O2, and twice the photon energy is above double ionization thresholds. Thus, N2 and O2 are ionized by single-photon absorption, and ionized N2 and O2 can be ionized or excited by absorption of another photon. Using tightly focused intense FEL pulses, first and second photoabsorption processes can occur with a single FEL pulse duration. Jiang et al also investigated sequential ionization of N2 by 44 eV FEL pulses at FLASH [7]. Because of difference in the photon energy, we found different intermediate and final states and decay pathways. The first evidence of multiphoton multiple ionization of N2 by the EUV-FEL pulses from the SCSS test accelerator was reported by Sato et al [16]. A new experimental setup employed here is similar to the previous one described and employed in [13, 14, 17]. We briefly describe the difference from the previous setup. To reduce background signals from residual gases, the base 0953-4075/09/181001+04$30.00 1 © 2009 IOP Publishing Ltd Printed in the UK J. Phys. B: At. Mol. Opt. Phys. 42 (2009) 181001 Fast Track Communication pressure at ion momentum spectrometer chamber has been improved by one order and kept below 10−9 Pa. The time-offlight axis of the spectrometer is vertical and thus perpendicular to the horizontal axes of the FEL beam and of its polarization. The supersonic molecular beam is horizontal and crosses the FEL beam at 45◦. The 52 nm FEL beam was steered by two upstream Au plane mirrors, skimmed by a 5 mm hole 1.2 m upstream the entrance of the experimental chamber, and then introduced to the chamber placed ∼26 m downstream from the radiation source point. To focus the FEL beam onto the molecular beam of ∼1 mm in diameter, we used a homemade concave mirror at normal incidence [13]. The FEL beam was partially blocked by a 1.5 mm wide horizontal beam stopper so that the non-focused beam did not irradiate directly the molecular beam. The intensity of the FEL beam was adjusted using a gas attenuator, i.e., an absorption cell filled with the Ar gas. Using electrostatic fields, ions are projected onto a 120 mm diameter microchannel plate (MCP) in front of a delay-line anode. The determination of the ion momentum is based on the measurements of the time-of-flight (TOF) and the detector hit position for each ion (see, e.g. [18]). Here, a three-layer type delay-line anode (Roentdek HEX120) is used to minimize the dead time. The design of the ion spectrometer with a two-stage acceleration section is similar to the one described in [19]. The lengths and fields are chosen to be 40 mm, 128 V mm−1 and 52 mm, 226 V mm−1, respectively. The acceleration is followed by a 308 mm long field-free drift tube with the ion detector mounted at its exit. All signals are fed to an eight-channel digitizer (Acqiris DC282 × 2) and software CFD is employed for extracting the timing signals [15]. Figure 1(a) shows kinetic energy release distributions (KERDs) from O2 (i.e. twice the kinetic energy of O+) irradiated by the FEL of 3 and 1 × 1013 W cm−2. The ratio of the FEL power was determined by the ion yields due to residual gases which are proportional to the FEL power. The O+ ions were also produced from the residual H2O. Thus, we recorded the signals with and without the molecular beam of O2. The spectra in figure 1 are the results after subtraction of the background contributions. For the spectrometer setting described above, we could collect all O+ ions emitted into 4π sr with kinetic energies up to ∼20 eV. The O2 in the B 2 − g state is known to emit O+ of 0.8 eV kinetic energy and ionization threshold of this state is 20.3 eV [20]. Thus, the sharp peak at 1.6 eV in figure 1(a) is assigned to single photoionization to the B 2 − g dissociative state. The KER in the 0–3 eV range does not exhibit significant FEL power dependence, indicating that one-photon processes are saturated in the present experimental conditions. The ion yields with KER above 3 eV decreases almost linearly with decreasing the FEL power. This power dependence indicates that the ion yields with KER above 3 eV is mostly due to two-photon processes. Close inspections reveal that the ion yields with KER above 13 eV decrease more rapidly with decreasing the FEL power, indicating contributions from the processes that involve more than two photons. The KERD depicted in figure 1(b) is extracted from 0.00 0.02 0.04 0.06 0.08 0.10 (a) O (non-coincidence) 3×10 W/cm 1×10 W/cm