Photon Factory Activity Report 2017 #35 (2018)

BL-20A/2016G001

Comparative studies on the doubly excited states of H2O and H2S in thephotoexcitation
Kouichi Hosaka1,* Yutaro Torizuka1, Kai Minamizaki1, Philipp Schmidt2, Andre Knie2,
Arno Ehresmann2, Takeshi Odagiri3, Masashi Kitajima1 and Noriyuki Kouchi1
1Tokyo Institute of Technology, Meguro-ku, Tokyo, 152-8551, Japan
2University of Kassel, Kassel, D-34132, Germany
3Sophia University, Chiyoda-ku, Tokyo, 102-8554, Japan

1 Introduction
The atomic and molecular doubly excited states areembedded in the ionization continuum. Because of
the superposition of discrete and continuous
electronic states, the doubly excited states ofmolecules are not described by the BornOppenheimer products. The double photoexcitation
dynamics of molecules have thus been attractive
subjects of research. One of the means to observe thedoubly excited molecules is measuring cross sections
for the emission of the fluorescence from neutralfragments as a function of excitation energy [1-3]since the detection of such fluorescence photons
remove the superposition of discrete and continuous
electronic states. It turned out from our previous
investigations on CH 4 [1], NH 3 [2], and H 2O [3] thatthe superexcited states in the inner valence range are
not so amenable to the independent electron model as
those in the outer valence range. In other words theelectron correlation for the superexcited states in the
inner valence range is stronger than the electron
correlation for those in the outer valence range. In
the present experiment, the superexcited states of H 2S
by the absorption of a single photon are investigatedwith measuring the cross sections for the emission of
the Lyman-α fluorescence and the results arecompared with the corresponding ones of H 2O [4].
2 ExperimentLinearly polarized light was introduced into a gas cellfilled with H2S. The photons emitted at the 54.7◦ magic
angle with respect to the unit polarization vector of theincident light were detected by vacuum ultraviolet photondetectors. Two photon detectors, each of which consistsof an MgF2 window and a microchannel plate (MCP), arefitted to the gas cell and operated simultaneously: one ofthe detectors incorporates an MCP coated with CsI andthe other incorporates an MCP without coating andtherefore the detection efficiencies of them are different.
The energy of the incident light was scanned in the range
between 11 eV and 40 eV with 50 meV steps.
The dispersed fluorescence experiment was carried out
at Bessy II. The non-dispersive detection has theadvantage that the detection efficiency is much higher
than by the dispersive one, but has the disadvantage thatthe wavelength resolution is much poorer. Thus weskillfully combine the dispersive and non-dispersive
detections to determine the cross sections for Lyman−αfluorescence emission with small contamination of otherfluorescences and a small statistical uncertainty, which
would not be obtained by the dispersive or non-dispersive
detection alone.
3 Results and Discussion
The ground electronic states of H2O and H2S in C2v
symmetry are
H2O: (O1s)2(2a1) 2 (1b2) 2 (3a1) 2 (1b1) 2,22
6
H2S: (S1s) (S2s) (S2p) (4a1) 2 (2b2) 2 (5a1) 2 (2b1) 2.
Superexcited states of H2O and H2S are compared basedon the similarity and difference of the electronic structure.
In H2S, ten superexcited states have been found from theanalysis of the fluorescence cross sections against theincident photon energy, two of them are singly excited
2b2−1(mo) states with a single configuration and the othereight states are doubly excited states with multipleconfigurations. State-resolved dipole oscillator strengths
for H(2p) formation in the photoexcitation of H2S havebeen determined. It turns out that the energy-splitting of
the doubly excited ‘4a1−1 (mo′)’ states of H2S is enhanced
in comparison with that of the doubly excited ‘2a1−1(mo′)’states of H2O. This enhancement seems to be caused by
the stronger electron correlation in H2S than in H2O.
The similarity in shape is seen between the innervalence band and inner shell band in the fluorescencecross sections against the incident photon energy for H2S
and H2O. This similarity indicates that the electrons areexcited to similar orbitals from the valence orbitals or
inner shell orbitals.

References[1] H. Fukuzawa et al., J. Phys. B 38, 565 (2005).
[2] L. Ishikawa et al., J. Phys. B 41, 195204 (2008).
[3] M. Nakano et al., J. Phys. B 43, 215206 (2010).
[4] K. Hosaka et.al, to be submitted.
* hosakak@chem.titech.ac.jp