Spectroscopic observation of bound ungerade ion-pair states in molecular hydrogen

Frequency-resolved observations of heavy Rydberg states in molecular hydrogen are reported in the ungerade manifold of states. Double-resonance spectroscopy via the $E,F$ ${}^1{\Sigma }_g^{+}$, $v^{\prime }=6$ state has been used to probe the energy region above the H($1s$) $+$ H(3$l$) dissociation threshold. Resonances are observed by ionizing H(3$l$) to produce H${}^{+}$, which is then detected by using a time-of-flight mass spectrometer. The kinetic energy of the H${}^{+}$ ion confirms that the observed signal is due to the photoionization of neutral H(3$l$) atoms, indicating that dissociation is a significant decay channel for the ion-pair states. The pattern of energies of the resonances agree well with the predictions of a mass-scaled Rydberg formula for bound quantum states of the H${}^{+}$H${}^{-}$ ion pair. Energies and quantum defects have been determined for principal quantum numbers in the range of $n=130$ to 207.

Figure 1

Potential energy curves of select ungerade double-well states in H${}_2$ [5, 6]. The H${}^{+}$H${}^{-}$ ion-pair Coulombic potential curve is shown as a dotted line. The gerade $E,F$ ${}^1{\Sigma }_g^{+}$ state is shown for comparison.

Figure 2

Schematic of the experimental arrangement. The laser setup depicted on the left is used to generate pump light pulses at 193 nm. The laser setup shown on the right is used to generate probe light pulses in the wavelength range of 300–330 nm.

Figure 3

H${}^{+}$ yield as a function of energy, above the H($1s$) $+$H(3$l$) dissociation limit, obtained by excitation from the $E,F$ ${}^1{\Sigma }_g^{+}$, $v^{\prime }=6$, $J^{\prime }=1$ state. The signal is magnified at high energy to show more detail. The insert provides an expanded view, showing an example of the regular pattern of resonances observed at high energy.

Figure 4

Excitation and dynamics that lead to the production of H${}^{+}$ ions from the excitation of the $E,F$ ${}^1{\Sigma }_g^{+}$, $v^{\prime }=6$, $J^{\prime }=1$ state. (a) Dissociation of the molecular ion. (b) Ionization of the H(3$l$) atomic fragment.

Figure 5

Observed quantum defects as a function of assigned ion-pair principal quantum number. The error bars are the standard deviation of a fit of the variation of the quantum defect to a linear dependence on energy.