Born–Oppenheimer potential energy curves of the 1 , 1Π g , and 1Δ g states near the n = 2 dissociation threshold of H2 as obtained from Refs. 25–28. The dashed line represents the (2pσ u )2 diabatic potential causing strong dissociation in the singlet-g manifold.
The employed laser excitation scheme showing the possible channels contributing to the H+ and traces. In the figure the various steps (1)–(6) as discussed in the text are indicated.
Signal recording performed with multicolor laser excitation and time-of-flight mass separation. In the first step the Q(1) line in the C-X(2,0) band is excited. The upper panel shows the generic dissociation signal of the two-photon excited states of singlet-g symmetry in H+ detection. The middle panel shows the auxiliary signal channel recorded as . The background signal reflects the 1 XUV + 1 UV photoionization signal, which is independent of the wavelength of the additional dye laser tuned in the visible domain; the sharp resonances in the middle panel indicate bound levels of g symmetry. The lower panel is obtained by dividing the signal in the H+ channel by that in the signal, thus to a large extent eliminating the noise resulting from the XUV production and fluctuations in the molecular beam density. Before this signal division takes place the resonances are erased from the spectrum, so that only the noise in the trace is remained.
Identification of possible total angular momentum states of singlet-g symmetry based on selection rules in a 1 XUV + 1 VIS double-resonance excitation scheme, for the case of para- and ortho-hydrogen. (e) and (f) indicate electronic parities and (+) and (−) indicate total parities of all states. Dashed lines represent levels with positive total parity, while solid lines represent levels with negative total parity. All allowed electric-dipole transitions are indicated. The electric-dipole selection rules for the transitions depicted with dashed lines depend on the relative orientation of the polarizations of the two photons.
Dissociation spectra obtained by 1 XUV + 1 VIS two-photon excitation via the intermediate C1Π u , v = 2, J = 1 state, reached via an R(0) line in the first excitation step. In the upper trace the relative orientation of the polarizations of both excitation laser beams is perpendicular, while in the lower it is parallel. With parallel polarization J = 0 can be excited, while with perpendicular polarization J = 1 is observed. The J = 2 level can be excited via both polarization combinations.
Dissociation traces of ortho-hydrogen produced by 1 XUV + 1 VIS + 1 UV laser excitation. The horizontal axis represents the total term energy with respect to X1 reached in the two-step double-resonance excitation process.
Dissociation traces of para-hydrogen produced by 1 XUV + 1 VIS + 1 UV laser excitation. The horizontal axis represents the total term energy with respect to X1 reached in the two-step double-resonance excitation process.
Recorded spectra for transitions to rotational states belonging to I1 . The mechanism causing their predissociation involves tunneling through the potential barrier. For higher J states the effective barrier is smaller, leading to enhanced tunneling rates, which can be observed as increasing linewidths.
Rovibronic term values as a function of J(J + 1). Solid lines represent electronic states with negative rotationless parity and dashed lines with positive parity. The data points indicated with solid circles are based on emission spectroscopy (Ref. 10) and assigned by Yu and Dressler (Ref. 19). These transitions have also been observed in the present experiment detecting . The open circles represent the term values extracted from the dissociation spectra.
Predissociation widths Γ of I1Π g , v = 4, J states as obtained from the experiment and from a calculating of tunnelings widths via Eq. (8). In the last column the calculated values from Ross et al. (Ref. 18), as converted from lifetimes, are listed. All values are in cm−1.
Rovibronic term energy levels of the gerade states in the 118 500–120 500 cm−1 energy range and estimated linewidths. The accuracy of the term energy levels is 1 cm−1.
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