Photoionization and fragmentation of H3O+under XUV irradiation

Abstract

The photolysis of the hydronium cation H 3 O + has been studied at the extreme ultraviolet wavelengths of 35.56 ± 0.24 nm (34.87 ± 0.24 eV) and 21.85 ± 0.17 nm (56.74 ± 0.44 eV) using a crossed ion-photon beam setup at the free-electron laser FLASH. Coincidence photoelectron and photofragment spectroscopy was performed at 21.85 nm, where both inner and outer valence ionization are allowed, and revealed that the XUV photolysis of H 3 O + is by far dominated by ionization of outer valence electrons forming the 1 A 1 and 2 E states of the dication H 3 O 2+ . The dications were found to dissociate into the channels H 2 O + + H + (72 ± 4%), OH 0 + 2H + (18 ± 6%), and OH + + H + + H 0 (10 ± 1%). A kinematic analysis of the H 2 O + + H + channel after photoabsorption at 35.56 nm (where only outer valence ionization is possible) showed dissociation into excited states of the water radical ion, where the 1 A 1 state breaks up into the linearà 2 A 1 state of H 2 O + and the 2 E state decays into the strongly bentB 2 B 2 state. Finally, from the 2 E state of H 3 O 2+ , dissociation into OH 0 (X 2 ) + 2H + was identified to occur with a near linear dissociation geometry.

Lifetime of low vibrational levels of the metastableB̃2B2state ofH2O+<

“…The time of flight required in these experiments [54] by H 2 O + fragments to reach the detector amounted to a few μs, consistent with the present finding of a lifetime of 198 μs for the low vibrational levels of theB 2 B 2 PES.…”

“…5, panels (12) and (21), the H + fragments show coincidence with fragments on DET2, and we identify these fragments to originate from dissociation of H 2 O + ions into OH 0 + H + products as indicated. This assignment was furthermore confirmed by considering momentum conservation for the coincident particles [54].…”

“…The interaction zone is located inside a structure of electrodes that can be individually biased and allows detailed control of the electric potential both along and transverse to the ion direction of motion. The emerging photofragments from the dissociation of H 2 O + were analyzed using three fragment detectors (DET1-3); the additional detectors for analysis of photoelectrons (eDET1-2) [53,54] were not exploited in this experiment. As illustrated in Fig.…”

“…The significance of the excited states of the water radical ion was recently emphasized in studies on the xuv photoionization and fragmentation of H 3 O + [48,54], where it was established that under ionizing radiation, H 3 O + dissociates via dicationic states dominantly into excited H 2 O + , with ∼55% reaching thẽ B 2 B 2 state and ∼17% reaching theà 2 A 1 state. The time of flight required in these experiments [54] by H 2 O + fragments to reach the detector amounted to a few μs, consistent with the present finding of a lifetime of 198 μs for the low vibrational levels of theB 2 B 2 PES.…”

Photoelectron Spectroscopy of Ions: Study of the Auger Decay of the4d→nf(n=4,5) Resonances inBizau1, Cubaynes2, Guilbaud3 et al. 2016Phys. Rev. Lett.

“…The success of this experiment was favored by the huge cross section of the photoexcitation process (2000 Mb) and the existence of only one dominant decay channel.. Electron-ion coincidence technique was also performed at BESSY to improve the signal to noise ratio in the electron spectra, but with the draw-back of a very low counting rate [11] [12]. Since these pioneering experiments, a few electron spectra were obtained on ionic species at free electron laser facilities [13,14], taking advantage of the very high photon brilliance produced by these devices, several orders of magnitude higher than what is available at an undulator beam line on a storage ring light source. Nevertheless, even with such light sources photoelectron spectroscopy on ionic species is far to be a routine.…”