The dissociation dynamics of superexcited molecular hydrogen and its isotopes has been discussed from an experimental viewpoint. Recently, more attention has been paid to doubly excited states since new experiments on ionization and dissociation via doubly excited states have started due to the large progress in synchrotron radiation light source and electron spectroscopy. This paper reviews the experimental techniques and results obtained and indicates that the competition between neutral dissociation and autoionization is a key to investigating the molecular superexcited states.

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The absolute values of the fluorescence cross sections for CH(A, B → X) and H(n = 4– 7 → n′ = 2) in the photoexcitation of methane have been obtained as a function of incident photon energy in the range 12.65–41 eV, in which five superexcited states have been found and discussed quantitatively in detail. The lower two of them are single-hole one-electron superexcited states and the higher three around 25, 29 and 35 eV are doubly excited states, which have been investigated in detail for the first time in this experiment. It is remarkable that the oscillator strengths for the emission of the fluorescences originating from the doubly excited states are as large as or even larger than those from the single-hole one-electron superexcited (2a1)−1 (npt2) 1T2 states, which seems not to be amenable to the independent electron model.

The formation and decay of doubly excited methane in photon and electron interactions have been investigated through measuring (i) the cross sections for the emission of the Lyman-α fluorescence in the photoexcitation of CH4 as a function of incident photon energy in the range 18–51 eV and (ii) the electron-energy-loss spectrum of CH4 tagged with the Lyman-α photons at 80 eV incident electron energy and 10° electron scattering angle in the range of the energy loss 20–45 eV. Five superexcited states have been found, three of which are doubly excited states with the others being singly excited states. It has been found that the electron interaction with CH4 at 80 eV incident electron energy and 10° electron scattering angle accelerates the double excitation against the single excitation as compared with the photon interaction.

We have measured the coincidence time spectra of two Lyman-α photons emitted by a pair of H(2p) atoms in the photodissociation of H 2 at the incident photon energy of 33.66 eV and at the hydrogen gas pressures of 0.40 and 0.02 Pa. The decay time constant at 0.02 Pa is approximately half the lifetime of a single H(2p) atom, 1.60 ns, while the decay time constant at 0.40 Pa is in agreement with the lifetime of a single H(2p) atom. It turns out that the decay faster than the lifetime of a single H(2p) atom originates from the entanglement in the pair of H(2p) atoms. We have demonstrated an effect of entanglement on atomic decay.A large number of studies have been directed toward generating and manipulating entanglement in various qubits, which not only is at the heart of quantum theory but also plays an essential role in quantum information technologies [1][2][3][4]. A deeper understanding of entanglement has become increasingly important to overcome obstacles in the realization of quantum information technologies. One of them is that the entanglement in massive quantum particles is in general fragile against interaction with environments. From the perspective of this serious issue, theoretical studies were conducted on the dynamics of entanglement in two atoms located inside spatially separated cavities and a unique feature drastically different from single-particle dynamics was predicted, i.e., in some cases the entanglement disappears in a finite time due to spontaneous emission [5,6]. It was then experimentally demonstrated for an entangled photon pair, not for an entangled pair of massive quantum particles [7]. In this Rapid Communication, we reveal that entangled H(2p) atoms look as if they decay faster than a single H(2p) atom. This kind of investigation promotes the understanding of the transient properties of the entanglement emerging in a pair of massive quantum particles.Our group found an atom-pair formation process (1) followed by the emission of two photons and measured cross sections for the emission of two Lyman-α photons as a function of incident photon energy [8],(1)Recently, our group published two papers on the theoretical and experimental studies of the angular distribution of two Lyman-α photons in process (1). In the first paper, Miyagi et al. [9] pointed out that a pair of H(2p 0 ) and H(2p ±1 ) atoms produced from an H 2 molecule in the 1 u state, where the lower subscripts of 0 and ±1 denote the magnetic quantum * odagiri.t.aa@m.titech.ac.jp numbers m with respect to the internuclear axis, is entangled as follows:where two protons are labeled a and b and the two electrons are labeled 1 and 2. The internuclear distance r is infinite in the entangled state of two H(2p) atoms in Eq.(2). In fact, the value of r is 93 µm when the H(2p) atoms emit the Lyman-α photons, as calculated from the incident photon energy (33.66 eV as mentioned later), the dissociation limit of H(2p) + H(2p), and the lifetime of the H(2p) atom, i.e.,