E.M. Reinhold

Observation of the I ' (1)Pi(g) outer well state in H-2 and D-2

We observed bound levels of the I′ state in H2 and D2, confined in the outer well of the lowest 1Πg adiabatic potential close to its (1s+2p) dissociation limit, with an equilibrium internuclear distance of ≈8 a.u. Rovibronic levels (v=0–2, J=1–5 for H2 and v=0–5, J=1–6 for D2) are populated with pulsed lasers in resonance enhanced XUV+IR (extreme ultraviolet+infrared) excitation, and probed by a third laser pulse. Level energies are measured with an accuracy of ≈0.03 cm−1, and are in reasonable agreement with predictions from ab initio calculations in adiabatic approximation; the smallness of Λ-doublet splitting indicating that nonadiabatic interactions with 1Σg+ states are generally weak. Additional resonances are observed close to the n=2 dissociation limit, some of which can be assigned as high vibrational levels of the EF 1Σg+ state.

Phenomena of g-u symmetry-breakdown in HD

Phenomena associated with the breakdown of inversion symmetry in the HD molecule are reviewed and discussed. A distinction is made between three kinds of physical effects observed in HD spectra. The existence of a small electric dipole moment in the ground state gives rise to vibrational and pure rotational transitions following selection rules of electric dipole transitions. Coupling between electronic states of g and u symmetry occurs, which is associated with the appearance of forbidden lines in the electronic spectrum. This effect occurs predominantly at near coincidences between levels of opposite inversion symmetry and a recently observed example of strongly interacting states (H¯ 1 + g and B¯ 1 + u ) is highlighted. Electronic coupling between states of g and u symmetry always takes place near dissociation threshold; as a result of the mass difference and the electronic isotope shift the behaviour at long range cannot be described in an adiabatic picture. A procedure is developed to construct long-range potentials near the n = 2 dissociation limit in which the breakdown of g-u symmetry is incorporated.

Optical observation of the 3sσgFΠu3 Rydberg state of N2

Using ultrahigh-resolution 1XUV+1UV two-photon ionization laser spectroscopy, the FΠu3←XΣg+1(0,0) transition of N2 has been optically observed for the first time, and the 3sσgFΠu3(υ=0) Rydberg level fully characterized with rotational resolution. The experimental spectroscopic parameters and predissociation level widths suggest strong interactions between the F state and the 3pπuGΠu3 Rydberg and C′Πu3 valence states, analogous to those well known in the case of the isoconfigurational Πu1 states.

Tunneling and decay dynamics of (H)over-bar(1)Sigma(+)(g) outer well states in hydrogen

We present an experimental study of the decay dynamics of rovibrational states confined in the outer minimum of the HH 1Σg+ potential of the hydrogen molecule. Specific (v,J) quantum states are populated using a two-step excitation process involving a pulsed extreme ultraviolet laser source. Decay products are selectively probed with additional laser pulses, using the timing of these pulses to determine lifetimes. The competition between dissociation and ionization, occurring at short internuclear distance after tunneling through the potential barrier, is addressed. Observed lifetimes and decay channels of H levels are consistent with a semiclassical description of tunneling of the nuclear motion through the potential barrier towards small internuclear distance. For the HD isotopomer breaking of the g−u symmetry is found to strongly affect the decay dynamics.

Spectroscopic observation and characterization of H(+)H(-) heavy Rydberg states.

A series of discrete resonances was observed in the spectrum of H2, which can be unambiguously assigned to bound quantum states in the 1/R Coulombic potential of the H+H- ion-pair system. Two-step laser excitation was performed, using tunable extreme ultraviolet radiation at lambda = 94-96 nm in the first step, and tunable ultraviolet radiation in the range lambda = 310-350 nm in the second step. The resonances, detected via H+ and H2+ ions produced in the decay process, follow a sequence of principal quantum numbers (n = 140-230) associated with a Rydberg formula in which the Rydberg constant is mass scaled. The series converges upon the ionic H+H- dissociation threshold. This limit can be calculated without further assumptions from known ionization and dissociation energies in the hydrogen system and the electronegativity of the hydrogen atom. A possible excitation mechanism is discussed in terms of a complex resonance. Detailed measurements are performed to unravel and quantify the decay of the heavy Rydberg states into molecular H2+ ions, as well as into atomic fragments, both H(n = 2) and H(n = 3). Lifetimes are found to scale as n3.

High precision frequency calibration of N I lines in the XUV domain

With the use of a narrowband and tunable extreme ultraviolet laser source transition frequencies in the spectrum of neutral atomic nitrogen were measured for transitions originating from the (2p3)4S3/2 ground state to 12 levels in the 2p23d and 2p24s configurations in the wavelength range 95.1–96.5 nm. The present laboratory calibrations, performed at an absolute accuracy of 0.005 cm−1 or 5 × 10−8, should be useful for comparison with nitrogen absorption features observed in quasars for assessment of possible temporal variation of the fine structure constant. Simultaneous recordings of spectra for both 14N and 15N yield accurate values for transition isotope shifts, which reveal large specific-mass shifts almost to the extent of cancelling the isotope shifts.

Optical observation of the 3s sigma F-g (3)Pi(u) Rydberg state of N-2

Using ultrahigh-resolution 1XUV+1UV two-photon ionization laser spectroscopy, the FΠu3←XΣg+1(0,0) transition of N2 has been optically observed for the first time, and the 3sσgFΠu3(υ=0) Rydberg level fully characterized with rotational resolution. The experimental spectroscopic parameters and predissociation level widths suggest strong interactions between the F state and the 3pπuGΠu3 Rydberg and C′Πu3 valence states, analogous to those well known in the case of the isoconfigurational Πu1 states.

Optical observation of the 3sσ gF 3Π u Rydberg state of N 2

Using ultrahigh-resolution 1XUV+1UV two-photon ionization laser spectroscopy, the FΠu3←XΣg+1(0,0) transition of N2 has been optically observed for the first time, and the 3sσgFΠu3(υ=0) Rydberg level fully characterized with rotational resolution. The experimental spectroscopic parameters and predissociation level widths suggest strong interactions between the F state and the 3pπuGΠu3 Rydberg and C′Πu3 valence states, analogous to those well known in the case of the isoconfigurational Πu1 states.