Metastable hydrogen in the 2s state as a probe
for dissociative doubly excited states of molecules
Yoshiaki Kumagai*1, Takeshi Odagiri*, Takehiko Tanabe*, Motoyoshi Nakano*,Isao H Suzuki†, ‡, and Noriyuki Kouchi**

Department of Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan†
Photon Factory, IMSS, KEK, Tsukuba, Ibaraki 305-0801, Japan‡
National Metrology Institute of Japan, AIST, Tsukuba, Ibaraki 305-8568, Japan1

E-mail: kumagai.y.ad@m.titech.ac.jp−1n''=4(2a 1) (n''pt 2)−2(2a1) (3a1)−1−1(2a1) (1t2) (2t2)−2(1t2) (3a1)−2(2a1)(1t2) (2t2)−1−1(1t2)(a)CH4n''=3(2a1)(1e)(b) NH3satellite−10−1

Cross sections / arb.units

The dynamics and spectroscopy of doubly
excited states of molecules are subjects of greatinterest in atomic and molecular physics since
the wave functions are not simply described as a
Born-Oppenheimer product due to the couplingwith ionization continua [1]. Such doubly excited states embedded in the electronic continua
are well observed in cross sections free fromionization such as dissociation cross sections [2].
In the present study, the cross sections for theproduction of metastable hydrogen in the 2sstate in photoexcitation of CH4 and NH3 weremeasured as a function of the incident photonenergy.
The measurements were carried out at BL20A of the Photon Factory, KEK. The experimental details were described in Odagiri et al.
[3].
Figure 1 shows the cross sections for the production of the H(2s) atom from photoexcited
CH4 (a) and NH3 (b) as a function of the incident photon energy. By a fitting based on asemi-classical analysis, superexcited states ofCH4 and NH3 were found in the cross sections
as shown by the dotted curves in the figure.
Among them, the three peaks at 29.7, 35.0 and
39.3eV in figure 1 (a) and the peak at 31.6eV in
figure 1 (b) were attributed to the doubly excited states. The doubly excited states of CH4seem to be the same that were observed in thecross sections to form H(2p) atoms in the photoexcitation of CH4 [4] .
For further investigations on the dynamics ofthe dissociative doubly excited states of molecules, we developed a new method based on atime-of-flight technique and the pulsed synchrotron light source for measuring the velocity ofthe metastable fragments in the photodissociation of molecules.

Cross sections / arb.units

Synopsis The cross sections for the production of the H(2s) atom in photoexcitation of CH4 and NH3 have beenmeasured as a function of the incident photon energy. Dissociative doubly excited states of CH4 and NH3 havebeen observed clearly in the cross section curves. A method to measure the velocity of the metastable fragmentsin photodissociation of molecules was newly developed.

0152025303540Incident photon energy / eV45Figure 1. Cross sections for the production of themetastable hydrogen atom in the 2s state for (a) CH4and (b) NH3 as a function of the incident photon energy. The vertical ionization potentials of CH4 [5]and NH3 [6] are displayed by thick vertical bars. Thecurves show the results of the fits by the semiclassical analysis.

References[1] Bardsley J N 1968 J. Phys. B 1 349[2] Odagiri T et al. 2009 J. Phys. B 42 055101[3] Odagiri T et al. 2010 Rev. Sci. Instrum. 81603108[4] Fukuzawa H et al. 2005 J. Phys. B 38 565[5] Potts A W et al. 1972 Proc. R. Soc. Lond. A 326
165[6] Piancastelli M N et al. 1987 J. Chem. Phys. 87
1982; Banna M S et al. 1975 J. Chem. Phys. 63 4759