Soft X-ray Induced Production of Neutral Fragments in High-Rydberg States at the O 1s Ionization Threshold of the Water Molecule.

Antti Kivimäki,Tomasz J Wasowicz,Robert Richter

doi:10.1021/acs.jpca.0c06940

Abstract

Dissociation of water molecules after soft X-ray absorption can yield neutral fragments in high-Rydberg (HR) states. We have studied the production of such fragments by field ionization and ion time-of-flight (TOF) spectrometry. Neutral HR fragments are created at all resonances below the O 1s ionization potential (IP) and particularly within 1 eV above the O 1s IP. The latter effect is due to the recapture of the O 1s photoelectrons into HR orbitals of the molecular water ion after the emission of a fast Auger electron. H2O+(HR) fragments subsequently dissociate, yielding neutral H(HR) and O(HR) fragments, as were found by measuring the TOF spectra by pulsed field ionization. Such measurements were carried out at the O 1s → 4a1 and 2b2 resonances as well as just above the O 1s IP. The TOF spectra also reveal two series of oscillatory structures that are attributed to quantum beats involving Lyman emission in one of the series and field ionization of H(HR) fragments in the other series.

Highlights

Interaction of soft X-ray photons with water molecules produces several kinds of particles: electrons, positive ions, negative ions, neutral fragments, and photons of different wavelengths
Photodissociation of water molecules can occur in numerous ways when the incident photon energy approximately equals the ionization potential (IP) of the O 1s shell (539.79(2) eV1)
If the used photon energy is larger than the O 1s IP, an O 1s electron can be removed from the molecule; this process is known as O 1s photoionization

Summary

INTRODUCTION

Interaction of soft X-ray photons with water molecules produces several kinds of particles: electrons, positive ions, negative ions, neutral fragments, and photons of different wavelengths. Several different particle yields measurements of particle’s intensity as a function of photon energy have been reported in the 1s excitation region of the water molecule, e.g., a total ion yield,[3] partial ion yields (of ions such as O+, H+, O−, and H−),[4] a partial Auger electron yield,[5] the yield of metastable H atoms,[6] and Lyman-α and Balmer yields due to excited H atoms.[7,8] In the present paper, we will show a hitherto unpublished yield of neutral fragments in high-Rydberg (HR) states at the O 1s edge of H2O, where the term “high-Rydberg” refers to states with the principal quantum number n ≥ 20. Our TOF spectra obtained with pulsed field ionization show two different series of quantum beats in the same measurement: one of them is related to fluorescence emission from excited H atoms and the other to field ionization of H(HR) fragments

EXPERIMENTAL SECTION

RESULTS AND DISCUSSION

CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES