Inner-valence excited and multiply excited states of molecular oxygen around the double-ionization potential as probed by a pair of fluorescence photons

Abstract

The cross sections for the generation of a pair of fluorescence photons in the photoexcitation of O 2 differential with respect to solid angles for the emission of the photon pair have been measured as a function of incident photon energy in the range 23-47 eV using the photon-photon coincidence method, the (γ , 2γ ) method, to investigate the inner-valence excited states and multiply excited states of O 2 as superexcited states. Four superexcited states of O 2 with the 3 − u or 3 u symmetry have been newly found around 29, 36, 38 and 44 eV in the measured cross section curve free from ionization. It turns out that they are inner-valence excited states and multiply excited states. Two remarkable points are shown: (1) there exist superexcited states of O 2 dissociating into neutral fragments even in the energy range above the double-ionization potential of O 2 at 36.13 eV and (2) the values of the doubly differential cross sections decrease to a large extent around the double-ionization potential with increasing incident photon energy, both of which have been compared with the results of N 2 by our previous (γ , 2γ ) experiment. The origin of (2) is discussed in detail.

Three-body neutral dissociations of a multiply excited water molecule around the double ionization potential

“…The experiment was conducted at beamline 3B of the Photon Factory in KEK (KEK-PF) by using a similar apparatus to the one used in our previous studies [16][17][18][19][20]. Synchrotron radiation from the 24 m spherical grating monochromator [21,22] was introduced into a gas cell filled with water vapour.…”

“…Recently we have developed a more powerful method, the (γ , 2γ ) method: a pair of fluorescence photons emitted by a pair of neutral fragments produced in photoexcitation of a molecule are detected in coincidence for eliminating the contribution of ionization [16][17][18][19][20]. By this method, for diatomic molecules, we were able to measure the ionizationfree cross sections in photoexcitation even in the range where the dissociative ionizations accompanied by the emission of fluorescences take place.…”

“…By this method, for diatomic molecules, we were able to measure the ionizationfree cross sections in photoexcitation even in the range where the dissociative ionizations accompanied by the emission of fluorescences take place. It has been found through the (γ , 2γ ) experiments on the diatomic molecules that in the energy range above the double ionization potential of the molecules there still exist multiply excited neutral states dissociating into neutral fragments [17][18][19][20]. Measuring curves of the cross sections due purely to the multiply excited states with the decay channels being specified enabled more detailed discussions on formation and decay.…”

Neutral Dissociation of Superexcited Oxygen Molecules in Intense Laser Fields

“…They roughly assigned several excited O atoms observed in their excitation spectra to dissociation products in Rydberg states converging to the B ( 2 Σ g − ), b ( 4 Σ g − ), or A ( 2 Π u ) states of the O 2 + ion, respectively, but did not supply enough theoretical explanation of the dissociation mechanism. While Odagiri et al focused on experimentally indentifying the inner-valence excited states and multiply excited states of O 2 as SESs in the higher energy range of 23−47 eV using the photon−photon coincidence method …”

High Resolution Multiphoton Spectroscopy by a Tunable Free-Electron-Laser Light

“…In atoms and molecules there is a class of short-lived resonances, the so-called superexcited states with excitation energies above the first ionization threshold and spectral widths in the meV range [8][9][10][11][12][13][14]. These correlated, doubly excited electronic states are coupled to the continuum, and accurate description of resonance excitation and decay dynamics requires state-of-the-art modeling.…”