M. Glass-Maujean

H2 and D2 photon impact predissociation

Atomic Lyman α and Balmer α, β, and γ emission have been observed by irradiating H2 and D2 in the 680–860 A region with synchrotron radiation from the ACO storage ring at Orsay. Comparison of the observed excitation spectra with the absorption spectrum shows clear evidence for the predissociation of molecular Rydberg states above the dissociation thresholds which are observed for the dissociation reactions: H2+hν→H(1S) +H* (n=2, 3, 4, 5). Undispersed visible fluorescence is also observed on excitation in the 750–860 A region which is attributed to molecular emission from excited Rydberg states. The results of these photon impact experiments are compared with those from electron impact processes.

Photodissociation of doubly excited states of H2: Emission of Balmer lines

The cross section for the formation of H(n=3) or H(n=4) by photodissociation of H2 has been measured for the first time, using synchrotron radiation between 500 and 300 A. The comparison with computed cross section curves allows the identification of the doubly excited molecular dissociating states. The importance of the diabatic behavior of these states is discussed.

Synchrotron vacuum ultraviolet radiation studies of the D(1)Pi(u) state of H(2)

The 3pπD (1)Π(u) state of the H(2) molecule was reinvestigated with different techniques at two synchrotron installations. The Fourier transform spectrometer in the vacuum ultraviolet wavelength range of the DESIRS beamline at the SOLEIL synchrotron was used for recording absorption spectra of the D (1)Π(u) state at high resolution and high absolute accuracy, limited only by the Doppler contribution at 100 K. From these measurements, line positions were extracted, in particular, for the narrow resonances involving (1)Π(u) (-) states, with an accuracy estimated at 0.06 cm(-1). The new data also closely match multichannel quantum defect calculations performed for the Π(-) components observed via the narrow Q-lines. The Λ-doubling in the D (1)Π(u) state was determined up to v=17. The 10 m normal incidence scanning monochromator at the beamline U125/2 of the BESSY II synchrotron, combined with a home-built target chamber and equipped with a variety of detectors, was used to unravel information on ionization, dissociation, and intramolecular fluorescence decay for the D (1)Π(u) vibrational series. The combined results yield accurate information on the characteristic Beutler-Fano profiles associated with the strongly predissociated Π(u) (+) parity components of the D (1)Π(u) levels. Values for the parameters describing the predissociation width as well as the Fano-q line shape parameters for the J=1 and J=2 rotational states were determined for the sequence of vibrational quantum numbers up to v=17.

VUV spectroscopic study of the D1Piu state of molecular deuterium

The D 1Π u – absorption system of molecular deuterium has been re-investigated using the VUV Fourier-Transform (FT) spectrometer at the DESIRS beamline of the synchrotron SOLEIL and photon-induced fluorescence spectrometry (PIFS) using the 10 m normal incidence monochromator at the synchrotron BESSY II. Using the FT spectrometer absorption spectra in the range 72–82 nm were recorded in quasi static gas at 100 K and in a free flowing jet at a spectroscopic resolution of 0.50 and 0.20 cm−1 respectively. The narrow Q-branch transitions, probing states of Π− symmetry, were observed up to vibrational level v = 22. The states of Π+ symmetry, known to be broadened due to predissociation and giving rise to asymmetric Beutler–Fano resonances, were studied up to v = 18. The 10 m normal incidence beamline setup at BESSY II was used to simultaneously record absorption, dissociation, ionization and fluorescence decay channels from which information on the line intensities, predissociated widths, and Fano q-parameters were extracted. R-branch transitions were observed up to v = 23 for J = 1–3 as well as several transitions for J = 4 and 5 up to v = 22 and 18 respectively. The Q-branch transitions are found to weakly predissociate and were observed from v = 8 to the final vibrational level of the state v = 23. The spectroscopic study is supported by two theoretical frameworks. Results on the Π− symmetry states are compared to ab initio multi-channel-quantum defect theory (MQDT) calculations, demonstrating that these calculations are accurate to within 0.5 cm−1. Furthermore, the calculated line intensities of Q-lines agree well with measured values. For the states of Π+ symmetry a perturbative model based on a single bound state interacting with a predissociation continuum was explored, yielding good agreement for predissociation widths, Fano q-parameters and line intensities.

Photoabsorption probabilities of H2 levels lying near the H(1S) + H(n=2) dissociation limit

The transition probabilities for Hydrogen B 2pσ1Σ+u (v′=32–35)←X 1Σ+g (v″=0), C 2pπ1Πu (v′=11–13)←X (v″=0), and B′ 3pσ1Σ+u (v′=5–8)←X (v″=0) were measured using synchrotron radiation in order to investigate the variation of the transition probabilities in the region of the dissociation threshold. The experimental data which agree with calculated probabilities indicate that B′ and C state continua may be of comparable intensities.