Experimental and theoretical studies of the np ? 1 ? u + and np ? 1 ? u + ( n ? 4 ,N' = 1 ?6) states of D2 : Energies, natural widths, absorption line intensities, and dynamics

Abstract : Over a thousand spectral lines in the photoexcitation spectrum of molecular deuterium (D2) to np 1 Σ + u and 1 Π + u Rydberg levels (n ≥ 4) were measured for rotational levels N = 1 − 6 in the 117 000 − 137 000 cm −1 spectral range by two different types of experiments at two synchrotron radiation sources: a vacuum ultraviolet (VUV) Fourier-transform (FT) spectrometer at SOLEIL, Paris and a 10m-normal-incidence monochromator (NIM) at BESSY II, Berlin. The experimental energies, the absorption cross sections, Einstein A-coefficients, and line widths are compared with ab initio multi-channel quantum defect (MQDT) calculations for these levels. More than 350 R(0) or P(2) lines were assigned, some 280 R(1) or P(3) lines, some 270 R(2) or P(4) lines, over 100 R(3) or P(5) lines, over 90 R(4) lines, and 24 R(5) lines to extract information on the N = 1 − 6 excited levels. Transition energies were determined up to excitation energies of 137 000 cm −1 above the ground state, thereby extending earlier work by various authors and considerably improving the spectral accuracy (< 0.1 cm −1), leading to several reassignments. The absorption and the dissociation, ionization and fluorescence excitation cross sections from the NIM experiment are measured on absolute scale and are used to calibrate intensities in the VUV-FT spectra. The overall agreement between experiment and first principles calculations, without adjustable parameters, is excellent in view of the multi-state interferences treated within the MQDT-framework: For the low N values the averaged deviations between those observed in the FT-SOLEIL spectra and those calculated with MQDT are ∼ 0.1 cm −1 with a spread of ∼ 0.5 cm −1. The line intensities in terms of Einstein coefficients are well represented in the MQDT-framework, as are the level widths representing the lifetimes associated with the sum of the three decay channels. These line intensities follow, in general, the 1/n 3 scaling behavior as characteristic in Rydberg series, but deviations occur and those are explained by MQDT. The decay dynamics of the excited N Rydberg levels is analyzed on the basis of the measured quantum yields for ionization, dissociation and fluorescence observed in the NIM experiment in terms of absolute cross sections for the distinctive channels. In particular in the n = 4 manifolds dissociation is found to play a major role, where in the n = 5 manifolds the behavior is most erratic due to strong competition between decay channels. At n = 6, ionization takes over as the dominant channel. Despite the excellent agreement between observations and the outcome of the MQDT calculations for both level energies and dynamics, some pronounced deviations are found as in the splitting of the 5pπ, v = 4 − 6, N = 1 levels. The shortcomings of the MQDT calculations are ascribed to the treatment of the excited states in terms of a 1snp single electron configuration, therewith neglecting possible interferences with 1snf or 2s core excited states. Some 27 lines remained unassigned; in view of their observation in fluorescence it is stipulated that these lines probe levels in the nf manifold.
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Journal of Molecular Spectroscopy, Elsevier, 2017, 338, pp.22 - 71. 〈10.1016/j.jms.2017.05.011〉
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M. Glass-Maujean, Ch. Jungen, A. Vasserot, H. Schmoranzer, A. Knie, et al.. Experimental and theoretical studies of the np ? 1 ? u + and np ? 1 ? u + ( n ? 4 ,N' = 1 ?6) states of D2 : Energies, natural widths, absorption line intensities, and dynamics. Journal of Molecular Spectroscopy, Elsevier, 2017, 338, pp.22 - 71. 〈10.1016/j.jms.2017.05.011〉. 〈hal-01537881〉

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