L. Werner

Transition probabilities from the ground state of the states of H2

The position and the intensities of the Q lines of the absorption spectrum of H2 were studied in the 81–72 nm wavelength range, using synchrotron light. The transition probabilities of the transitions with 3 ≤ n ≤ 12 have been measured and compared to calculated values. A good agreement has been found. The main discrepancies observed may well be related to local non-adiabatic couplings not included in the calculations. Vibrational couplings between levels with large Δv values up to Δv = 5 were observed affecting the transition probabilities.

Investigation of photoionization and photodissociation of an oxygen molecule by the method of coupled differential equations

The photoionization and photodissociation of an oxygen molecule are investigated theoretically by the method of coupled differential equations. The molecular orbitals of the core are calculated in the MO LCAO approximation. The molecular orbitals of a photoelectron in the discrete and continuous spectra are determined by the single-center method. The wave functions of vibrational motion of the oxygen molecule are obtained within the diabatic approach. The described method is used for calculating the predissociation and autoionization widths of the 2σu−1(c4Σu−)n/σg, v Rydberg states of the oxygen molecule. The total cross sections of the resonant photoionization and neutral predissociation calculated for the oxygen molecule in the excitation energy range 20.6–24.8 eV are in good agreement with the available experimental data.

Observation of the oscillating absorption spectrum of a double-well state: the state of H2

The absorption spectrum of a double-well state, the state of H2, has been computed and compared with new experimental spectra in the energy range around its dissociation limit. Above the potential barrier, it is shown that the vibronic levels unexpectedly preserve a character linked to a definite well even if this character (the absorption probability) is partly shared by the neighbouring levels. The dissociation cross section presents beat-like oscillations at two frequencies as related to the classical oscillators of the two wells. The agreement with a measured high-resolution spectrum is excellent.The absorption spectrum of a double-well state, the state of H2, has been computed and compared with new experimental spectra in the energy range around its dissociation limit. Above the potential barrier, it is shown that the vibronic levels unexpectedly preserve a character linked to a definite well even if this character (the absorption probability) is partly shared by the neighbouring levels. The dissociation cross section presents beat-like oscillations at two frequencies as related to the classical oscillators of the two wells. The agreement with a measured high-resolution spectrum is excellent.

The predissociation of the 2σu−1(c4Σu−, v states of the oxygen molecular ion

The dynamics of predissociation of the 2σu−1(c4Σu−), v vibrational states of the O2+ ion was studied theoretically using the method of coupled differential equations. The main equations describing the vibrational motions of nuclei in the adiabatic and diabatic approximations are given. The applicability scope of approximate methods for solving these equations was studied. The predissociation widths for the v = 0 and 1 vibrational levels were found to be Γ0 = 0.054 meV and Γ1 = 9.71 meV. This substantiated the results of recent observations of neutral fragments formed after the dissociation of the O2 molecule. About 99% of the O2+ ions in the 2σu−1(c4Σu−), v states were found to decompose to the O(1D) + O+(4S) dissociation products.

Partial wave analysis of interfering Kr 3d95p resonant Raman Auger transitions based on measurements of alignment and orientation parameters within the natural line width

The alignment parameter A20 and the orientation parameter O10 for the 4p4(1D)5p2F7/2 and the 4p4(1D)5p2D5/2 states of Kr+ populated in resonant Auger transitions from 3d95p/6p photo-excited states of Kr were measured at BESSY II for photon energies between 91 eV and 92.8 eV. The method of fluorescence polarimetry was applied. A bandwidth of the exciting photons of 10 meV allowed to study A20 and O10 within the natural line width. Interference effects for resonances overlapping due to lifetime broadening were predicted and observed even when separated by 15 times the natural line width. The relative contribution of the three allowed electron partial waves was derived from A20 and O10 as a function of the exciting-photon energy and compared with the calculated contributions. The resulting good overall agreement between experiment and prediction is considered as a noticeable improvement of the understanding of excitation and decay of the prominent Kr 3d95p resonances.

Interference between resonant Raman Auger decay and direct excitation manifested in orientation and alignment of KrII 4p4(1D)5p 2P3/2 ions

The orientation parameter O10 for the 4p4(1D)5p2P3/2 state of KrII populated in resonant Auger transitions from 3d95p/6p photo-excited states was measured at BESSY II for photon energies between 90.8 eV and 92.9 eV. The (1D)5p2P3/2 satellite was populated in the Raman regime, i.e. with a bandwidth of the exciting photons (10 meV) smaller than the natural width of the 3d95p/6p states. Partial cross sections determined from the measurement of the orientation O10 and alignment A20 are compared with results of calculations where the non-resonant part of the photoionization cross section is computed within the sudden approximation. The electron partial wave analysis as the main intention of this investigation shows that this approach yields partial cross sections for the s1/2 (d5/2) waves that are larger (smaller) than the measured ones.

Interaction between doubly-excited 4p4nℓn′ℓ′ resonances in KrI

Photoionization cross sections for the 4p4(3P)5s4P5/2,3/2,1/2 satellites and 4s → el, 4p → el main lines of Kr II were measured using the photon-induced fluorescence spectroscopy in the exciting-photon energy range between 27.80 eV and 29.44 eV with extremely narrow bandwidth (1.7 meV at 28.55 eV) of the monochromatized synchrotron radiation. The observed resonances were assigned to the 4p45s(4P1/2)np and 4p45s(2P3/2)np Rydberg series on the basis of the calculations including core relaxation and interaction between many resonances and many continua. The calculation shows that the resonance structure in the photoionization channels exists due to the 4p4(1D)5s2D5/26p3/2 and 4p4(1D)5s2D3/2 6p3/2,1/2 promoter states which strongly perturb the above Rydberg series.

Cross sections for photoabsorption and 3p44s,3d satellite production at energies of the doubly excited Ar 3p4(3P)4s2P3/2,1/2 np resonances

The photoabsorption of Ar is studied experimentally and theoretically for energies between 32.5 eV and 33.0 eV. Resonant excitation and autoionization of doubly excited 3p4(3P)4s2P3/2,1/2 np Rydberg states are observed as dominant mechanism to populate doublet and quartet satellites of the 3p44s, 3d configurations. The perturbation of the Rydberg series by promoter states is reproduced by the calculations. Measured and calculated cross sections for the strong decay channels are in good agreement. A mirror behaviour of the 3s vacancy creation and the satellite production and the influence of the radiative decay of high-lying Rydberg states on the shape of a satellite threshold are manifested.

Absolute cross sections and branching ratios for the radiative decay of doubly excited helium determined by photon-induced fluorescence spectroscopy

The decay of doubly excited helium states below the N = 2 threshold, cascading radiatively over three steps, were investigated using the photon-induced fluorescence spectroscopy (PIFS) at BESSY II. Absolute cross sections as the product of the resonance excitation cross section of the doubly excited states and their fluorescence rate to decay into the singly excited 1sms(1S) and 1smd(1D) states were measured. The experiments showed that the (sp,2n+)(1P) states decay predominantly into the 1sns(1S) states, whereas the (pd,2n)(1P) states prefer to decay into the 1snd(1D) states. For the (sp,2n−)(1P) states with n = 4, 5 and 6 we observed a broad and complex decay pattern. In addition the angular distribution of the fluorescence radiation was measured. The results are in good agreement with theoretical predictions. Furthermore, the weakening of the LS coupling scheme and the mixing between singlet and triplet states in helium was confirmed by observation of the 1s6d(3D0,1,2) → 1s2p(3P0,1,2) transition on a doubly excited singlet state.

Investigation of N2+ (C2Σu+ → X2Σg+ fluorescence excited through auger decay of the 1s−1π* resonance

A theoretical investigation of N2+ (C2Σu+ → X2Σg+ molecular fluorescence excited through the Auger decay of the 1s−1π* resonance is carried out. The fluorescence cross sections are calculated with due regard for the dependence of the matrix element of the C → X dipole transition on the internuclear distance, the interference between channels of excitation via different vibrational levels vr of the 1s−1π* resonance, the rotational structure of the fluorescence band, and the predissociation of the N2+C2Σu+v′ ≥3) states. The calculated cross sections are in good agreement with the experimental results of recent measurements. The results of the calculations have demonstrated that the observed dependence of the cross section of the (C2Σu+ (v′) → X2Σg+ (v″) fluorescence on the excitation energy and the fluorescence wavelength for a group of bands with equal values of the difference Δv = v′ − v″ is associated with transitions between the vibrational levels of the electronic states involved in the excitation and subsequent cascade decay of the 1s−1π* resonance: N2 (v0 = 0) → N*2(1s−1π*(vr)) ⇒ N2+: (C2Σu+ (v′) → X2Σg+ (v″).