Shape resonances and multielectron effects in the core-level photoionization ofCO2

Abstract

The partial photoionization cross sections and angular distributions of the C 1s ' and 0 1s ' singlehole states of CO2 have been measured with high precision near threshold. The 4o. "shape resonance is well reproduced in the 1s single-hole cross sections, but strong site-specific effects are observed with respect to both its energy and intensity. A striking feature of the present data is the presence of fine structure in the 1s single-hole cross sections o. » due to multielectron excitations: virtually all spectral details in the total 1s cross sections o.&, are also present in o. sH implying that multielectron excitations couple strongly to the underlying 1s continuum. The decay dynamics of the double-excitation feature at 303.5 eV in the C 1s cross section of CO2 have been characterized by Auger spectroscopy. Partial cross sections and angular distributions have also been measured for the first group of strong shake-up satellites in the C 1s photoelectron spectrum. Similar to the C 1s ionization of CO, a strong enhancement of intensity near threshold is observed for some satellite channels as well as the occurrence of features that are not present in the sudden limit. The latter are attributed to conjugate shake-up.PACS number(s): 33.80.Eh, 33.60.Fy I. INTRQDUCTIGNA considerable body of experimental and theoretical work on photoabsorption and photoionization cross sections has shown that resonances and multielectron effects are commonly encountered in core-level excitation of small molecules. In some studies these features have been carefully characterized and provide models for the interpretation of similar efFects in larger molecules [1,2]. The current upsurge of interest in such K-shell properties is due to recently improved instrumentation, in particular high-resolution monochromator s in combination with undulator sources, for absorption and photoelectron spectroscopy experiments, e.g. , [3 -6]. These allow detailed examination of the energetics and dynamics of core-level photoionization.The excitation of a core electron into an unoccupied bound molecular orbital or the continuum is generally accomplished by a strong relaxation of the remaining system of charges. Provided that sufhcient energy is available from the absorbed photon, there is a relatively high probability that a valence electron and vibrations are excited simultaneously with the core electron, giving rise to shake-up satellites in the photoelectron spectra [7 -10] and to vibrational structure in both photoabsorption [3 -5] and photoemission spectra [6,11]. Transitions to unoccupied states accompanied by shake-up often produce characteristic features in the continuum just below the onset of the shake-up satellites seen in the photoelectron spectra and have been referred to as double excitations. In the same energy region, i.e. , within the erst tens 'Present address: Max-Planck-Arbeitsgruppe "Rontgenbeugung, " Hausvogteiplatz 5-7, 10117Berlin, Germany. Present address: Hamburger Synchrotronstrahlungslabor at DESY, Notkestralje 85, 22603 Hamb...

Triple ionization of CO2 by valence and inner shell photoionization

“…1 from measurements at 303 and 546 eV photon energy, where there are no prominent resonant structures in the excitation spectra. 15,16 The structures are in excellent agreement with the better resolved spectra of Hatamoto et al, 1 but the energies of the peaks agree rather with those of Püttner et al 2 It is notable that in the spectrum at 546 eV the molecular symmetry is reduced from D ∞ν to C ∞ν by localisation of the O1s hole on one atom on the time scale of photoelectron emission, with the consequence that the vibration excited by O1s ionization is the antisymmetric stretch ν 3 . At 303 eV, ionization from the central C1s orbital excites only the symmetric stretch ν 1 .…”

“…14 Other less abundant routes to triple ionization of the molecule include direct photoionization, a channel which is open below the inner shells, and single Auger decay from core-valence doubly ionized states; these routes have not been explored hitherto. At distinct photon energies above the inner shell edges, shape resonances and shake-up satellites are also formed 15,16 and may complicate the observation of simple hole states.…”

Vibrationally resolved partial cross sections and asymmetry parameters for carbon K-shell photoionization of the CO2molecule

“…This is also the case for molecular photoionization where photoemission is accompanied by vibrational excitation of the residual molecular ion. Recent high-resolution soft x-ray photoelectron spectroscopic studies [1][2][3][4][5][6][7][8][9][10][11] show that the spectral and angular distributions for inner-shell photoemission from molecules in shape resonance regions depend on the final vibrational state, as predicted theoretically some decades ago [12,13].…”

O 1s excitation and ionization processes in the CO2molecule studied via detection of low-energy fluorescence emission

“…The energies of these features (547-549 eV) agree very well with those estimated for the core valence double excited states, obtained by adding the energy of the O 1s → π * excitation (535.4 eV) and the shake-up energies of 11.5-13.8 eV for the O 1s photoelectron satellites S 1 -S 3 , as labeled in [37] and in figure 3. In other words, the same valence-to-virtual orbital excitation is suggested to accompany both the O 1s → π * transition in core valence double excitation and O 1s photoionization in a shake-up process; the 1π u → 2π u monopole excitation is likely involved here, similarly to the case of the C 1s shake-up transitions [22]. The better-known double excitation feature around 554 eV also appears with high intensity in the excitation functions of the 845-nm oxygen line.…”

“…The O 1s edge of the CO 2 molecule is a good choice for such a study because it shows a rich collection of various core excitation phenomena. The pre-edge O 1s photoabsorption spectrum contains excitations to both valence and Rydberg orbitals, while the above-edge part reveals double excitations and shape resonances, see e.g., [21,22,23,24,25,26]. Furthermore, fragmentation of the CO 2 molecule after core excitation and core ionization has been studied extensively using different electron -ion and ion -ion coincidence spectroscopies, see e.g., [27,28,29,30,31], which can provide complementary information to the present investigation.…”