Auger decay of the molecular field split S 2p core excited states in HS radical

Auger electron spectroscopy of fulminic acid, HCNO: an experimental and theoretical study

HCNO is a molecule of considerable astrochemical interest as a precursor to prebiotic molecules. It is synthesized by preparative pyrolysis and is unstable at room temperature. Here, we investigate its spectroscopy in the soft X-ray regime at the C 1s, N 1s and O 1s edges. All 1s ionization energies are reported and X-ray absorption spectra reveal the transitions from the 1s to the π* state. Resonant and normal Auger electron spectra for the decay of the core hole states are recorded in a hemispherical analyzer. An assignment of the experimental spectra is provided with the aid of theoretical counterparts. The latter are using a valence configuration interaction representation of the intermediate and final state energies and wavefunctions, the one-center approximation for transition rates and band shapes according to the moment theory. The computed spectra are in very good agreement with the experimental data and most of the relevant bands are assigned. Additionally, we present a simple approach to estimate relative Auger transition rates on the basis of a minimal basis representation of the molecular orbitals. We demonstrate that this provides a qualitatively good and reliable estimate for several signals in the normal and resonant Auger electron spectra which have significantly different intensities in the decay of the three core holes.

H2S ultrafast dissociation probed by energy-selected resonant Auger electron–ion coincidence measurements

We have studied the ultrafast dissociation of the H2S molecule upon S 2p3/2-->6a1 inner-shell excitation by combining high-resolution resonant Auger spectroscopy and energy-selected Auger electron-ion coincidence measurements. Auger final states have been correlated to the different fragmentation pathways (S+, HS+, and H2S+ ions). As an original result, we evidence a three-step mechanism to describe the resonant production of S+: the Auger recombination in the HS* fragment is followed for the A 3Pi and c 1Pi states by the S++H fragmentation mechanism.

Fluorescence emission from photo-fragments after resonant S 2p excitations in H2S

Visible-UV fluorescence emission of gas-phase hydrogen sulfide, H(2)S, has been studied at the S 2p edge with synchrotron radiation excitation. Dispersed fluorescence measurements in the wavelength range 300-900 nm were taken at several photon energies corresponding to the excitations of the S 2p electrons to the unoccupied molecular and Rydberg orbitals. The spectra reveal fluorescence from the H, S, S(+), HS and HS(+) photo-fragments. H is found to be the strongest emitter at Rydberg excitations, while the emission from S(+) is dominant at the molecular resonances and above the S 2p ionization thresholds. The intensities of hydrogen Lyman-alpha (122 nm), Balmer-alpha (656 nm), Balmer-beta (486 nm) transitions as well as the visible-UV total fluorescence yield (300-900 nm) and the total ion yield were measured by scanning the photon energy in small steps across the S 2p edge. The different Balmer lines show some sensitivity to the specific core excitations, which is, however, not so strong as that recently observed in the water molecule [E. Melero García, A. Kivimäki, L. G. M. Pettersson, J. Alvarez Ruiz, M. Coreno, M. de Simone, R. Richter and K. C. Prince, Phys. Rev. Lett., 2006, 96, 063003].

Radiative relaxation and fragmentation dynamics of S 2p-excited hydrogen sulfide

Radiative relaxation of S 2p-excited hydrogen sulfide (H(2)S) is investigated by dispersed ultraviolet and visible fluorescence spectroscopies. We observe distinct changes in the fluorescence spectra as a function of excitation energy. Excitation to Rydberg states below the S 2p ionization threshold yields intense fluorescence from neutral and ionic atomic fragments (H, S(+), and S(2+)). In addition to the atomic emission, fluorescence of the molecular fragment ion HS(+) is preferably found after excitation of the S 2p electron into the unoccupied 6a(1) and 3b(2) orbitals with sigma(*) character. This is interpreted as evidence for ultrafast dissociation of the core-excited molecule prior to electronic relaxation. The rotationally resolved fluorescence spectra of the A (3)Pi-->X (3)Sigma(-) transition are analyzed in terms of the fragmentation dynamics leading to the formation of the excited molecular fragment ion, where changes in bond angle are discussed in terms of the rotational population.

Constant-atomic-final-state filtering of dissociative states in the O1s→σ* core excitation in O2

The below-threshold region in core-excited O2 is very complex, consisting of a multitude of exchange-split states with mixed molecular orbital-Rydberg character. We have investigated the nature of these intermediate states by resonant Auger spectroscopy. In particular, we have obtained constant-atomic-final-state yield curves for several atomic peaks in the electron decay spectra which are stemming from ultrafast dissociation. The relative intensity of Auger decay leading to atomic final states is considered a signature of the relative weight of the sigma* character. This method allows one to "filter out" intermediate states with dissociative character. Extensive calculations have been performed by multi-reference configuration interaction at different interatomic distances in order to evaluate the potential curves of the core-excited states and propose a qualitative description of the dissociative molecular dynamics. The calculations show that the core-excited states have a relevant admixture of excitations to orbitals with Rydberg character and excitations to the sigma* orbital with different spin couplings. A diabatization of the adiabatic potential curves shows that the coupling between Rydberg and sigma* diabatic states is very different at the different crossing points and ultrafast dissociation occurs more easily on the lowest sigma* diabatic potential curve. As a consequence, the observation of atomic peaks only in the lower-energy region of the absorption curve is well justified.

Fragmentation dynamics of H2S following S 2p photoexcitation

The fragmentation dynamics of core-excited H(2)S has been studied by means of partial anion and cation yield measurements around the S L(2,3)-subshell ionization thresholds. All detectable ionic fragments are reported, and significant differences between partial ion yields are observed. Possible dissociation pathways are discussed by comparison to previous studies of electron spectra.