Photon Factory Activity Report 2005 #23 Part B (2006)
Atomic and Molecular Science

20A/2003G006(γ, 2γ) studies on N2 revealing multiply excited states embedded in the double
ionization continuum
Takeshi Odagiri*, Makoto Murata, Noriyuki Kouchi
Department of Chemistry, Tokyo Institute of Technology,O-okayama 2-12-1, Meguro-ku, Tokyo 152-8551, Japan

ResultsFigure 1 shows the doubly differential cross sections
for the emission of two photons of N fluorescence as afunction of the incident photon energy. The short verticallines in the range 30.42-37.15eV in the figure indicate thedissociation limits of N*+N*' which give twofluorescence photons detectable by the present photon
detectors.
The doubly differential cross section as a function ofthe incident photon energy rises above approximately 33
eV and shows peak structures at 36, 39, and 45 eV and abroad structure around 40-44 eV. Photoelectron spectra in

2Users' Report+

1 +N2 (X Σ g )
–+ 2e

N22+

N* + N *′References[1] T. Odagiri et al., J. Phys. B 37 (2004) 3909
[2] M. Murata et al., J. Electron Spectrosc. Relat. Phenom.
144-147 (2005) 147[3] M. Murata et al., J. Phys. B 39 (2006) 1285
[4] S. Svensson et al., Physica Scripta 44 (1991) 184[5] Z. F. Liu et al., Chem. Phys. Lett. 203 (1993) 337[6] G. Dawber et al., J. Phys. B 27 (1994) 2191

2Experimental
The measurements were carried out at BL-20A. The
linearly polarized synchrotron radiation from the 3-m
normal incidence monochromator was introduced in a gas
cell filled with N2 with the pressure of approximately 0.13
Pa. Two VUV-photons emitted parallel to the electricvector of the incident light and opposite to each otherwere detected by two photon detectors, each of which iscomposed of an MgF2 window and a microchannel plate
providing a filter range of 115-150 nm. The coincidence
counts were normalized for gas pressure, flux of theincident photons, detection efficiencies and geometricalfactor to obtain cross sections for the emission of two
photons (process (1)) in N2 differential with respect to2
solid angles for the photons, d σ/dΩidΩj [3].

-1

this energy range reveal the (2σg) N2 states and thecorrelation states associated with them [4,5]. Hence the36, 39 and 40-44 eV peaks can be attributed to multiplyexcited states of N2 built on these ionic states [3]. It isremarkable that the 45 eV peak is above the double
ionization potential of N2 (43.0 eV [6]). The precursor
excited state of N2 for the 45 eV peak thus turns out to beembedded in the single and double ionization continua. It
is interesting that such a highly excited state of N2
dissociates into neutral fragments escaping from theelectronic autoionization and makes a contribution
comparable to those below the double ionization potential.

d σ/dΩi dΩj / arb. units

Introduction
Molecular multiply excited resonances in the extreme
ultraviolet regime are highly correlated systems in which
the electronic and nuclear motions strongly couple due tointeractions with many electronic continua. Their
formation and decay is hence of great importance incollision physics. However, such resonances in innervalence range have been hardly observed by ordinary
photoabsorption or photoionization spectroscopies
because direct ionization processes dominate the spectra.
Recently we have developed (γ, 2γ) method forinvestigating the formation and decay of multiply excitedresonances: two fluorescence photons from fragment
atoms, hν’ and hν” in photodissociation process (1), aredetected in coincidence to eliminate direct ionization andobserve spectral features due to multiply excited states [13].
AB+hν → AB** → A*+B* → A+hν’+B+hν” (1)In the present study the (γ, 2γ) method has been applied tomolecular nitrogen.

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Incident photon energy / eV50Figure 1. Doubly differential cross sections for theemission of two photons of N fluorescence as afunction of the incident photon energy. The verticalline with crossed hatch indicates the double ionization
potential of N2 [6]. The short vertical lines indicate thedissociation limits of N*+N*' (see text).

*joe@chem.titech.ac.jp