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State-specific enhanced production of positive and negative ions of gaseous SiCl4 and solid-state analogues following core-level excitation
Journal of Electron Spectroscopy and Related Phenomena 184 (2011) 140–143
Contents lists available at ScienceDirect
Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec
State-specific enhanced production of positive and negative ions of gaseous SiCl4 and solid-state analogues following core-level excitation K.T. Lu ∗ , J.M. Chen ∗ , J.M. Lee, S.C. Haw, S.A. Chen, Y.C. Liang National Synchrotron Radiation Research Center (NSRRC), 101 HsinAnn Road, Hsinchu Science Park, Hsinchu 30076, Taiwan, ROC
a r t i c l e
i n f o
Article history: Available online 21 December 2010 Keywords: Photon-induced ion dissociation X-ray absorption spectroscopy Dissociation dynamics Negative ions
a b s t r a c t We investigated dissociation dynamics of positive and negative ions of gaseous SiCl4 and SiCl4 adsorbed on Si(1 0 0) at ∼90 K following Cl 2p core-level excitations. The Cl 2p → 8a1 * excitation of SiCl4 /Si(1 0 0) leads to a significant enhancement of the yields of Cl+ and Cl− . Excitation of Cl 2p electrons to Rydberg orbitals near the Cl 2p ionization thresholds of gaseous SiCl4 enhances the production of anionic fragments Si− and Cl− . © 2011 Elsevier B.V. All rights reserved.
1. Introduction Synchrotron radiation with tunable energy in the soft X-ray region has stimulated extensive research on inner-shell spectra of gaseous molecules and molecular adsorbates on surfaces. Monochromatic synchrotron radiation can excite selectively a specific atom or a specific site in molecules. Inner-shell electrons of a particular element in a molecule can be promoted selectively to a specific valence state or to a Rydberg orbital below the core ionization threshold, or ejected into a shape resonance or a continuum above an ionization threshold. Relaxation of these core-excited or core-ionized molecules typically involves resonant Auger or normal Auger processes, producing multiply charged molecular ions that are unstable and subsequently undergo dissociation. Excitations of highly localized core electrons also conduct the siteselective fragmentation of molecules in several systems [1–5]. The photodissociation dynamics of core-excited molecules are not only of scientific significance but also of interest in fields such as chemical reactions induced by highly energetic particles on interstellar dust and radiation damage of biomolecules [6–8]. The intricate dissociation and relaxation dynamics of gaseous molecules and molecular adsorbates on surfaces via inner-shell excitation remains a topic of intense interest [9–12]. Most experiments have focused on measurements of positive ions, whereas anionic fragments have been overlooked. The detection of negative fragments has proved to be a powerful method to expose highly excited states of the relaxed core-excited states
and post-collision interaction (PCI) dynamics near the ionization threshold [13–17]. Such high-lying states might have only weak transitions in X-ray absorption spectra and positive-ion yield spectra, but appear as strong resonances in yield spectra of negative ions following core-level excitation [18,19]. The dissociation pathways of core-excited molecules in the solid phase are strongly modified relative to the gaseous phase because of electronic interaction with a substrate or neighboring molecules at the solid surface [1,3]. Accordingly, to elucidate the dissociation dynamics of core-excited molecules, coordinated studies of molecules in the gaseous phase and their analogues in the solid state with various techniques are indispensable. Few reports have appeared of coordinated investigations of ionic (positive and negative) fragments produced after inner-shell excitation of gaseous molecules and solid-state analogues. By combining measurements of the photon-induced ion dissociation (positive and negative ions) and X-ray absorption, we investigated the dynamics of dissociation of ionic fragments of gaseous SiCl4 and SiCl4 adsorbed on Si(1 0 0) at ∼90 K, following photoexcitation of Cl 2p electrons to various resonances. The most striking observation is that the Cl 2p → 8a1 * excitation enhances the Cl+ yield for condensed SiCl4 , but scarcely enhances the Cl+ yield for gaseous SiCl4 . The transitions of core electrons to high Rydberg orbitals near the Cl 2p ionization thresholds of gaseous SiCl4 enhance significantly the production of anionic fragments Si− and Cl− .
2. Experiments ∗ Corresponding authors. E-mail addresses: [email protected] (K.T. Lu), [email protected] (J.M. Chen). 0368-2048/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2010.12.010
The measurements were made at the 6-m high-energy spherical-grating monochromator (HSGM) beamline at the
K.T. Lu et al. / Journal of Electron Spectroscopy and Related Phenomena 184 (2011) 140–143
National Synchrotron Radiation Research Center (NSRRC) in Taiwan. For measurements of desorption of positive and negative ions in the condensed phase, an ultrahigh-vacuum (UHV) chamber with a base pressure ∼1 × 10−10 Torr was used. The Si(1 0 0) surface was cleaned on repeated resistive heating to ∼1100 ◦ C under vacuum before the measurements. Highly pure SiCl4 (Merck, 99.9%) was degassed with several freeze-pump-thaw cycles before use. The vapor of SiCl4 was then condensed through a leak valve onto the Si(1 0 0) surface at ∼90 K. The ion yields were detected with a quadrupole mass filter (Balzers model QMA 410 with off-axis secondary electron multiplier). The quadrupole detector was oriented perpendicular to the substrate surface; photons were incident at an angle 45◦ with respect to the substrate normal. Solidphase X-ray absorption spectra were recorded in a total-electron yield (TEY) mode with a microchannel plate detector [14]. The surface coverage was determined from thermal desorption spectra (TDS). All exposures are given in Langmuirs (1 L = 1 × 10−6 Torr s). Exposure (∼5 L) of SiCl4 on Si(1 0 0) corresponds to one monolayer. For measurements of photodissociation in the gaseous phase, an effusive molecular beam produced on expanding the gas through an orifice (50 m) into the experimental chamber was used. The pressure in this chamber was maintained at ∼2 × 10−5 Torr. Fragment ions were selected by mass with a quadrupole mass filter and detected with an off-axis channel electron multiplier (Hiden, EQS). The absorption spectrum of gaseous SiCl4 was measured with an ion chamber at pressure ∼1 × 10−4 Torr. For photodissociation measurements, the HSGM beamline was operated with 100-m slits corresponding to energy resolution ∼0.2 eV at the Cl 2p edge. To obtain the high-resolution X-ray absorption spectrum, the HSGM beamline was set to a photon resolution ∼0.1 eV at the Cl 2p edge. The photon energies were calibrated within an accuracy of 0.1 eV using the Si 2p absorption peaks at 104.1 eV and 104.2 eV in gaseous and solid-phase SiCl4 together with Cl 2p absorption peak at 201.7 eV in gaseous SiCl4 contributed from second-order light of synchrotron radiation at the Si 2p region [20,21]. 3. Results and discussion Fig. 1 shows the Cl L2,3 -edge X-ray-absorption total electron yield (TEY) spectrum from condensed SiCl4 with the gaseous X-ray absorption spectrum for comparison. The absorption features labeled 1 and 1 are assigned to transition Cl 2p → 8a1 * . The features labeled 2 and 2 correspond to excitation s Cl 2p → 9t2 * [21], but the diminished intensity of lines 2 and 2 in
Fig. 1. Cl L2,3 -edge X-ray-absorption spectra of molecular solid and gaseous SiCl4 .
141
Fig. 2. (a) PSID spectra of condensed SiCl4 following Cl 2p core-level excitation with the Cl L-edge TEY spectrum and (b) dependence on photon energy of the fragmented ions of gaseous SiCl4 at the Cl 2p edge with the Cl L-edge absorption spectrum.
the solid TEY spectrum shows that these two lines have some Rydberg character. Broad absorption features labeled 3 and 4 in the gaseous absorption spectrum clearly persist with decreased intensities, but sharper profiles, in the solid-phase Cl L2,3 -edge X-ray absorption spectrum of SiCl4 , indicating that these final states have a valence character with contributions from Rydberg orbitals [20]. According to a theoretical calculation, the valence-electron excitation energy for the highest occupied molecular orbital (HOMO, 8t2 ) to the lowest unoccupied molecular orbital (LUMO, 8a1 ) of SiCl4 is ∼7 eV [22]. These absorption features might include contributions from doubly excited states. The broad absorption features labeled 3 and 4 in Fig. 1 are accordingly explained as a mixing of Rydberg states with double excitations. As noted, on passing from the gaseous phase to the molecular solid, absorption line 1 shifts toward greater photon energy, whereas lines 2, 2 , 3, and 4 shift toward smaller energy. The broad band at ∼216 eV (label 5) is ascribed to the shape resonance [21]. Fig. 2a reproduces the photon-stimulated ion-desorption (PSID) spectra of SiCl+ , SiCl2 + , SiCl3 + , Si+ and Cl+ for SiCl4 adsorbed on Si(1 0 0) at ∼90 K with multilayer coverage (>25 monolayers) at the Cl 2p edge. Cl L2,3 -edge X-ray absorption spectrum for condensed SiCl4 measured by the TEY mode is also displayed for comparison. As noted from Fig. 2a, the PSID spectra of SiCl+ , SiCl2 + , SiCl3 + and Si+ show a close resemblance to the Cl L2,3 -edge X-ray absorption spectrum of solid SiCl4 , whereas the Cl+ PSID spectrum exhibits a clear dissimilarity with the Cl L2,3 -edge X-ray absorption spectrum. Transition Cl 2p → 8a1 * enhances significantly the Cl+ desorption yield relative to transitions Cl 2p → 9t2 * and Cl 2p → Rydberg orbitals. Even though these states have transitions assigned from the same atomic site, there is a significant difference in efficiency of producing various ions. This infers that the character of core-excited states hence plays a vital role in ion desorption. The Cl 2p → 9t2 * excitation enhances slightly the Si+ yield relative to the Cl 2p → 8a1 * and Cl 2p → Rydberg excitations.
K.T. Lu et al. / Journal of Electron Spectroscopy and Related Phenomena 184 (2011) 140–143
(a) condensed SiCl4 Cl
-
(b) gaseous SiCl4 12 L -
Cl x 1 9L
5L
3L
-
Si x 1.5
Intensity (arb. units)
7L
-
Fig. 2b reproduces the fragment-ion yields of Cl+ , Si+ , SiCl+ , SiCl2 + and SiCl3 + for gaseous SiCl4 following Cl 2p core-level excitation, with the Cl L-edge X-ray absorption spectrum for comparison. The dependence of yields of various fragment ions, except Si+ , of gaseous SiCl4 on photon energy exhibits a close resemblance to the Cl L2,3 -edge X-ray absorption spectrum. Comparison of the Si+ yield spectrum and the Cl L2,3 -edge absorption spectrum in Fig. 2b shows that the transitions of Cl 2p electrons to 9t2 * orbitals (lines 2 and 2 ) and to Rydberg orbitals, lines labeled 3 and 4,enhance the Si+ yield, relative to transition Cl 2p → 8a1 * states. Based on resonant photoemission study, the spectator Auger and normal Auger transitions were the dominant decay channels for the excited Cl(2p) core hole of condensed and gaseous SiCl4 , producing excited states with multiple holes in the valence orbitals [23]. The spectator Auger decay produces a two-hole, one-electron (2h1e) final state in which the two holes in valence orbitals, and one electron is excited into an antibonding valence orbital or a Rydberg orbital. The normal Auger decay leads to a two-hole (2h) state in which the two holes in valence orbitals. A close similarity of the Cl L-edge TEY spectrum and ion-yield spectra for condensed and gaseous SiCl4 is accordingly attributed to the Auger decay of core-excited states and subsequent Coulomb repulsion of multivalence-hole final states, the mechanism socalled Auger-initiated desorption (AID) [24,25]. The state-specific enhancement of Cl+ yield via the Cl 2p → 8a1 * excitation implies, however, an additional process for Cl+ desorption from condensed SiCl4 . The 2h1e state following resonant core-level excitation is more effective for ionic dissociation than the 2h state produced by the normal Auger decay. If the spectator electron is localized in the antibonding orbital with a strongly repulsive ionic potential, the breaking of the chemical bond becomes enhanced [1,9,12,18,26]. Based on the Z + 1 core-equivalent model, after excitations of Cl 2p core electrons into the 8a1 * state in SiCl4 , the core equivalent molecule is Cl3 Si–Ar in its fundamental state. Thus, the coreexcited state with a spectator electron localized in 8a1 * states is expected to present a strongly repulsive potential-energy surface. The enhanced yield of Cl+ via the Cl 2p → 8a1 * excitation of condensed SiCl4 , as shown in Fig. 2, is therefore assisted by a strongly repulsive surface that is directly related to the spectator electrons localized in the antibonding orbitals. Unlike the desorption of positive ions, Cl− was the only ion predominantly observed in the desorption of negative ions from SiCl4 /Si(1 0 0) following the Cl 2p core-level excitation. To acquire additional insight into the mechanism of this desorption of negative ions, we measured the yield of Cl− as a function of SiCl4 exposure on Si(1 0 0) at ∼90 K, as presented in Fig. 3a. The Cl 2p → 8a1 * excitation enhances significantly the Cl− yield, particularly for small coverages. As Dujardin et al. proposed, formation of negative ions might originate from some highly excited states of the parent ion that are predissociated with an ion pair or that directly dissociate [19]. Based on this model, the intensities of the resonances in the negative-ion spectrum are demonstrated to be strongly correlated with the probability of spectator electrons being attached to the electronegative atom during fragmentation of the molecules. According to multiple-scattering X␣ calculations in SiCl4 , the spectator electron in the 8a1 * orbital is more localized at the Cl side, whereas the 9t2 * orbital is delocalized between Cl and Si atoms [21]. The Cl 2p → 8a1 * excitation is expected to enhance the Cl+ yield, relative to transitions Cl 2p → 9t2 * and Cl 2p → Rydberg orbitals. This expectation is verified in Fig. 3a. The dominant negative photo-ion fragments produced from gaseous SiCl4 via Cl 2p core-level excitation were Cl− and Si− . In Fig. 3b, the yields of anion fragments Cl− and Si− following Cl 2p core-level excitation of gaseous SiCl4 are reproduced with the Cl
Cl Yield (arb. units)
142
Cl 2p1/2 absorption Cl 2p3/2
1L
5
TEY
3 4 1 1' 2' 2
34
1 1' 2' 2
5
200 205 210 215 220 225
Photon Energy (eV)
200
205
210
215
220
Photon Energy (eV)
Fig. 3. (a) Cl− yield spectra for SiCl4 /Si(100) at ∼90 K with variable exposures at the Cl 2p edge. (b) Yields of anion fragments Cl− and Si− from gaseous SiCl4 following Cl 2p core-level excitation, and the Cl L-edge X-ray absorption spectrum [28]. The Cl 2p3/2 and Cl 2p1/2 ionization thresholds are marked by vertical dash lines [27].
L-edge X-ray absorption spectrum. As discernible in Fig. 3b, the relative intensities of various features in anion-yield spectra differ significantly from those of the Cl L2,3 -edge absorption spectrum. As shown, excitations of Cl 2p electrons to Rydberg orbitals/doubly excited states (labels 3 and 4) near the Cl 2p ionization thresholds of gaseous SiCl4 enhance greatly the yields of anions Cl− and Si− , relative to the ratio of intensity of the corresponding transition to the excitation from core to valence (label 1) at ∼201.7 eV. The central position of the anion enhancement is about 207.0 eV, which is immediately above the Cl 2p3/2 ionization threshold (206.9 eV [27]) of gaseous SiCl4 . The enhanced atomic anionic – Cl− and Si− – via excitations from core level to Rydberg states near the Cl 2p ionization thresholds of gaseous SiCl4 , as shown in Fig. 3b, are explained in terms of the contribution from the shake-modified resonant Auger decay or/and post-collision interaction [28]. Similar observations have been reported for several small molecules following core-level excitation [29–31]. We compare the ionic dissociation between gaseous and condensed SiCl4 following Cl 2p core-level excitation. For ionic dissociation pathways, the Cl 2p → 8a1 * excitation enhances significantly the Cl+ yield in the condensed phase, but there is almost no enhancement of the Cl+ yield in the gaseous phase. The Cl 2p → 9t2 * excitation of condensed and gaseous SiCl4 enhances the Si+ yield, relative to the Cl 2p → 8a1 * excitation. For anionic dissociations, excitation of core electrons to Rydberg orbitals//doubly excited states near the Cl 2p ionization thresholds of gaseous SiCl4 significantly enhances the production of Cl− and Si− . In contrast to the gaseous phase, the core-to-valence (8a1 * ) excitation at the Cl 2p edge of SiCl4 /Si(1 0 0) leads to noteworthy enhancement of the Cl− yields. Through electronic interaction with a substrate or neighboring molecules at the solid surface, the potential-energy hypersurface of an intermediate excited ionized state of the parent molecule is modified in the condensed phase relative to gaseous molecules [32]. As evident in Fig. 3a, the Cl− desorption yield is clearly enhanced at a small coverage. On increasing the coverage, the enhancement via excitation from core to valence becomes less
K.T. Lu et al. / Journal of Electron Spectroscopy and Related Phenomena 184 (2011) 140–143
prominent. We propose accordingly that the surface or/and condensed effect might assist these high-lying excited ionic states to cross into ion-pair states and subsequently to produce a negative ion. The enhanced production of anionic fragments via core-level excitation near the ionization thresholds of Cl 2p edge of gaseous SiCl4 is attributed to the contribution from the shake-modified resonant Auger decay or/and post-collision interaction, whereas production of negative ions in the condensed phase is dominated by dissociative electron attachment and the high-lying excited states of the parent ions. As demonstrated, the channels for desorption of anions in the condensed phase differ notably from the pathways for dissociation of anions in the gaseous phase. Comparative studies are hence clearly imperative for an elucidation of the detailed dynamics of photofragmentation between gaseous molecules and molecular adsorbates on surfaces [1,3]. 4. Conclusion By combining measurements of photon-induced ion dissociation and X-ray absorption we investigated the dissociation dynamics of positive-ion and negative-ion fragments of gaseous SiCl4 and SiCl4 adsorbed on Si(1 0 0) ∼90 K following Cl 2p corelevel excitations. The Cl+ desorption yield is significantly enhanced following Cl 2p → 8a1 * excitation of condensed SiCl4 , whereas Cl 2p core-level excitation of gaseous SiCl4 produces little or no enhancement of the Cl+ yield. The Cl 2p → 9t2 * excitation of gaseous SiCl4 enhances appreciably the Si+ yield, relative to the Cl 2p → 8a1 * excitation. Excitations of the Cl 2p electrons to Rydberg orbitals near the Cl 2p ionization thresholds of gaseous SiCl4 greatly enhance the production of anionic fragments Si− and Cl− . The enhanced production of anionic fragments via core-level excitation near the ionization threshold of gaseous SiCl4 is explained in terms of contributions from the shake-modified resonant Auger decay or/and post-collisional interaction. The Cl 2p → 8a1 * excitation of SiCl4 /Si(1 0 0) leads to a notable enhancement in the Cl− yield, particularly for a small coverage. The resonant enhancement of Cl− yield occurs through the formation of high-lying excited states of the parent ions that are predissociated by the ion-pair channel. These complementary results provide new insight into the stateselective fragmentation of positive and negative ions of gaseous molecules and their solid-state analogues via core-level excitation. Acknowledgment
96-2113-M-213- 009-MY3 and NSC 99-2113-M-213- 006-MY3) supported this research. References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
[16] [17]
[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
[30] [31] [32]
We thank the NSRRC staff for their technical support. NSRRC and National Science Council of the Republic of China (Grant Nos. NSC
143
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Contents lists available at ScienceDirect
Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec
State-specific enhanced production of positive and negative ions of gaseous SiCl4 and solid-state analogues following core-level excitation K.T. Lu ∗ , J.M. Chen ∗ , J.M. Lee, S.C. Haw, S.A. Chen, Y.C. Liang National Synchrotron Radiation Research Center (NSRRC), 101 HsinAnn Road, Hsinchu Science Park, Hsinchu 30076, Taiwan, ROC
a r t i c l e
i n f o
Article history: Available online 21 December 2010 Keywords: Photon-induced ion dissociation X-ray absorption spectroscopy Dissociation dynamics Negative ions
a b s t r a c t We investigated dissociation dynamics of positive and negative ions of gaseous SiCl4 and SiCl4 adsorbed on Si(1 0 0) at ∼90 K following Cl 2p core-level excitations. The Cl 2p → 8a1 * excitation of SiCl4 /Si(1 0 0) leads to a significant enhancement of the yields of Cl+ and Cl− . Excitation of Cl 2p electrons to Rydberg orbitals near the Cl 2p ionization thresholds of gaseous SiCl4 enhances the production of anionic fragments Si− and Cl− . © 2011 Elsevier B.V. All rights reserved.
1. Introduction Synchrotron radiation with tunable energy in the soft X-ray region has stimulated extensive research on inner-shell spectra of gaseous molecules and molecular adsorbates on surfaces. Monochromatic synchrotron radiation can excite selectively a specific atom or a specific site in molecules. Inner-shell electrons of a particular element in a molecule can be promoted selectively to a specific valence state or to a Rydberg orbital below the core ionization threshold, or ejected into a shape resonance or a continuum above an ionization threshold. Relaxation of these core-excited or core-ionized molecules typically involves resonant Auger or normal Auger processes, producing multiply charged molecular ions that are unstable and subsequently undergo dissociation. Excitations of highly localized core electrons also conduct the siteselective fragmentation of molecules in several systems [1–5]. The photodissociation dynamics of core-excited molecules are not only of scientific significance but also of interest in fields such as chemical reactions induced by highly energetic particles on interstellar dust and radiation damage of biomolecules [6–8]. The intricate dissociation and relaxation dynamics of gaseous molecules and molecular adsorbates on surfaces via inner-shell excitation remains a topic of intense interest [9–12]. Most experiments have focused on measurements of positive ions, whereas anionic fragments have been overlooked. The detection of negative fragments has proved to be a powerful method to expose highly excited states of the relaxed core-excited states
and post-collision interaction (PCI) dynamics near the ionization threshold [13–17]. Such high-lying states might have only weak transitions in X-ray absorption spectra and positive-ion yield spectra, but appear as strong resonances in yield spectra of negative ions following core-level excitation [18,19]. The dissociation pathways of core-excited molecules in the solid phase are strongly modified relative to the gaseous phase because of electronic interaction with a substrate or neighboring molecules at the solid surface [1,3]. Accordingly, to elucidate the dissociation dynamics of core-excited molecules, coordinated studies of molecules in the gaseous phase and their analogues in the solid state with various techniques are indispensable. Few reports have appeared of coordinated investigations of ionic (positive and negative) fragments produced after inner-shell excitation of gaseous molecules and solid-state analogues. By combining measurements of the photon-induced ion dissociation (positive and negative ions) and X-ray absorption, we investigated the dynamics of dissociation of ionic fragments of gaseous SiCl4 and SiCl4 adsorbed on Si(1 0 0) at ∼90 K, following photoexcitation of Cl 2p electrons to various resonances. The most striking observation is that the Cl 2p → 8a1 * excitation enhances the Cl+ yield for condensed SiCl4 , but scarcely enhances the Cl+ yield for gaseous SiCl4 . The transitions of core electrons to high Rydberg orbitals near the Cl 2p ionization thresholds of gaseous SiCl4 enhance significantly the production of anionic fragments Si− and Cl− .
2. Experiments ∗ Corresponding authors. E-mail addresses: [email protected] (K.T. Lu), [email protected] (J.M. Chen). 0368-2048/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2010.12.010
The measurements were made at the 6-m high-energy spherical-grating monochromator (HSGM) beamline at the
K.T. Lu et al. / Journal of Electron Spectroscopy and Related Phenomena 184 (2011) 140–143
National Synchrotron Radiation Research Center (NSRRC) in Taiwan. For measurements of desorption of positive and negative ions in the condensed phase, an ultrahigh-vacuum (UHV) chamber with a base pressure ∼1 × 10−10 Torr was used. The Si(1 0 0) surface was cleaned on repeated resistive heating to ∼1100 ◦ C under vacuum before the measurements. Highly pure SiCl4 (Merck, 99.9%) was degassed with several freeze-pump-thaw cycles before use. The vapor of SiCl4 was then condensed through a leak valve onto the Si(1 0 0) surface at ∼90 K. The ion yields were detected with a quadrupole mass filter (Balzers model QMA 410 with off-axis secondary electron multiplier). The quadrupole detector was oriented perpendicular to the substrate surface; photons were incident at an angle 45◦ with respect to the substrate normal. Solidphase X-ray absorption spectra were recorded in a total-electron yield (TEY) mode with a microchannel plate detector [14]. The surface coverage was determined from thermal desorption spectra (TDS). All exposures are given in Langmuirs (1 L = 1 × 10−6 Torr s). Exposure (∼5 L) of SiCl4 on Si(1 0 0) corresponds to one monolayer. For measurements of photodissociation in the gaseous phase, an effusive molecular beam produced on expanding the gas through an orifice (50 m) into the experimental chamber was used. The pressure in this chamber was maintained at ∼2 × 10−5 Torr. Fragment ions were selected by mass with a quadrupole mass filter and detected with an off-axis channel electron multiplier (Hiden, EQS). The absorption spectrum of gaseous SiCl4 was measured with an ion chamber at pressure ∼1 × 10−4 Torr. For photodissociation measurements, the HSGM beamline was operated with 100-m slits corresponding to energy resolution ∼0.2 eV at the Cl 2p edge. To obtain the high-resolution X-ray absorption spectrum, the HSGM beamline was set to a photon resolution ∼0.1 eV at the Cl 2p edge. The photon energies were calibrated within an accuracy of 0.1 eV using the Si 2p absorption peaks at 104.1 eV and 104.2 eV in gaseous and solid-phase SiCl4 together with Cl 2p absorption peak at 201.7 eV in gaseous SiCl4 contributed from second-order light of synchrotron radiation at the Si 2p region [20,21]. 3. Results and discussion Fig. 1 shows the Cl L2,3 -edge X-ray-absorption total electron yield (TEY) spectrum from condensed SiCl4 with the gaseous X-ray absorption spectrum for comparison. The absorption features labeled 1 and 1 are assigned to transition Cl 2p → 8a1 * . The features labeled 2 and 2 correspond to excitation s Cl 2p → 9t2 * [21], but the diminished intensity of lines 2 and 2 in
Fig. 1. Cl L2,3 -edge X-ray-absorption spectra of molecular solid and gaseous SiCl4 .
141
Fig. 2. (a) PSID spectra of condensed SiCl4 following Cl 2p core-level excitation with the Cl L-edge TEY spectrum and (b) dependence on photon energy of the fragmented ions of gaseous SiCl4 at the Cl 2p edge with the Cl L-edge absorption spectrum.
the solid TEY spectrum shows that these two lines have some Rydberg character. Broad absorption features labeled 3 and 4 in the gaseous absorption spectrum clearly persist with decreased intensities, but sharper profiles, in the solid-phase Cl L2,3 -edge X-ray absorption spectrum of SiCl4 , indicating that these final states have a valence character with contributions from Rydberg orbitals [20]. According to a theoretical calculation, the valence-electron excitation energy for the highest occupied molecular orbital (HOMO, 8t2 ) to the lowest unoccupied molecular orbital (LUMO, 8a1 ) of SiCl4 is ∼7 eV [22]. These absorption features might include contributions from doubly excited states. The broad absorption features labeled 3 and 4 in Fig. 1 are accordingly explained as a mixing of Rydberg states with double excitations. As noted, on passing from the gaseous phase to the molecular solid, absorption line 1 shifts toward greater photon energy, whereas lines 2, 2 , 3, and 4 shift toward smaller energy. The broad band at ∼216 eV (label 5) is ascribed to the shape resonance [21]. Fig. 2a reproduces the photon-stimulated ion-desorption (PSID) spectra of SiCl+ , SiCl2 + , SiCl3 + , Si+ and Cl+ for SiCl4 adsorbed on Si(1 0 0) at ∼90 K with multilayer coverage (>25 monolayers) at the Cl 2p edge. Cl L2,3 -edge X-ray absorption spectrum for condensed SiCl4 measured by the TEY mode is also displayed for comparison. As noted from Fig. 2a, the PSID spectra of SiCl+ , SiCl2 + , SiCl3 + and Si+ show a close resemblance to the Cl L2,3 -edge X-ray absorption spectrum of solid SiCl4 , whereas the Cl+ PSID spectrum exhibits a clear dissimilarity with the Cl L2,3 -edge X-ray absorption spectrum. Transition Cl 2p → 8a1 * enhances significantly the Cl+ desorption yield relative to transitions Cl 2p → 9t2 * and Cl 2p → Rydberg orbitals. Even though these states have transitions assigned from the same atomic site, there is a significant difference in efficiency of producing various ions. This infers that the character of core-excited states hence plays a vital role in ion desorption. The Cl 2p → 9t2 * excitation enhances slightly the Si+ yield relative to the Cl 2p → 8a1 * and Cl 2p → Rydberg excitations.
K.T. Lu et al. / Journal of Electron Spectroscopy and Related Phenomena 184 (2011) 140–143
(a) condensed SiCl4 Cl
-
(b) gaseous SiCl4 12 L -
Cl x 1 9L
5L
3L
-
Si x 1.5
Intensity (arb. units)
7L
-
Fig. 2b reproduces the fragment-ion yields of Cl+ , Si+ , SiCl+ , SiCl2 + and SiCl3 + for gaseous SiCl4 following Cl 2p core-level excitation, with the Cl L-edge X-ray absorption spectrum for comparison. The dependence of yields of various fragment ions, except Si+ , of gaseous SiCl4 on photon energy exhibits a close resemblance to the Cl L2,3 -edge X-ray absorption spectrum. Comparison of the Si+ yield spectrum and the Cl L2,3 -edge absorption spectrum in Fig. 2b shows that the transitions of Cl 2p electrons to 9t2 * orbitals (lines 2 and 2 ) and to Rydberg orbitals, lines labeled 3 and 4,enhance the Si+ yield, relative to transition Cl 2p → 8a1 * states. Based on resonant photoemission study, the spectator Auger and normal Auger transitions were the dominant decay channels for the excited Cl(2p) core hole of condensed and gaseous SiCl4 , producing excited states with multiple holes in the valence orbitals [23]. The spectator Auger decay produces a two-hole, one-electron (2h1e) final state in which the two holes in valence orbitals, and one electron is excited into an antibonding valence orbital or a Rydberg orbital. The normal Auger decay leads to a two-hole (2h) state in which the two holes in valence orbitals. A close similarity of the Cl L-edge TEY spectrum and ion-yield spectra for condensed and gaseous SiCl4 is accordingly attributed to the Auger decay of core-excited states and subsequent Coulomb repulsion of multivalence-hole final states, the mechanism socalled Auger-initiated desorption (AID) [24,25]. The state-specific enhancement of Cl+ yield via the Cl 2p → 8a1 * excitation implies, however, an additional process for Cl+ desorption from condensed SiCl4 . The 2h1e state following resonant core-level excitation is more effective for ionic dissociation than the 2h state produced by the normal Auger decay. If the spectator electron is localized in the antibonding orbital with a strongly repulsive ionic potential, the breaking of the chemical bond becomes enhanced [1,9,12,18,26]. Based on the Z + 1 core-equivalent model, after excitations of Cl 2p core electrons into the 8a1 * state in SiCl4 , the core equivalent molecule is Cl3 Si–Ar in its fundamental state. Thus, the coreexcited state with a spectator electron localized in 8a1 * states is expected to present a strongly repulsive potential-energy surface. The enhanced yield of Cl+ via the Cl 2p → 8a1 * excitation of condensed SiCl4 , as shown in Fig. 2, is therefore assisted by a strongly repulsive surface that is directly related to the spectator electrons localized in the antibonding orbitals. Unlike the desorption of positive ions, Cl− was the only ion predominantly observed in the desorption of negative ions from SiCl4 /Si(1 0 0) following the Cl 2p core-level excitation. To acquire additional insight into the mechanism of this desorption of negative ions, we measured the yield of Cl− as a function of SiCl4 exposure on Si(1 0 0) at ∼90 K, as presented in Fig. 3a. The Cl 2p → 8a1 * excitation enhances significantly the Cl− yield, particularly for small coverages. As Dujardin et al. proposed, formation of negative ions might originate from some highly excited states of the parent ion that are predissociated with an ion pair or that directly dissociate [19]. Based on this model, the intensities of the resonances in the negative-ion spectrum are demonstrated to be strongly correlated with the probability of spectator electrons being attached to the electronegative atom during fragmentation of the molecules. According to multiple-scattering X␣ calculations in SiCl4 , the spectator electron in the 8a1 * orbital is more localized at the Cl side, whereas the 9t2 * orbital is delocalized between Cl and Si atoms [21]. The Cl 2p → 8a1 * excitation is expected to enhance the Cl+ yield, relative to transitions Cl 2p → 9t2 * and Cl 2p → Rydberg orbitals. This expectation is verified in Fig. 3a. The dominant negative photo-ion fragments produced from gaseous SiCl4 via Cl 2p core-level excitation were Cl− and Si− . In Fig. 3b, the yields of anion fragments Cl− and Si− following Cl 2p core-level excitation of gaseous SiCl4 are reproduced with the Cl
Cl Yield (arb. units)
142
Cl 2p1/2 absorption Cl 2p3/2
1L
5
TEY
3 4 1 1' 2' 2
34
1 1' 2' 2
5
200 205 210 215 220 225
Photon Energy (eV)
200
205
210
215
220
Photon Energy (eV)
Fig. 3. (a) Cl− yield spectra for SiCl4 /Si(100) at ∼90 K with variable exposures at the Cl 2p edge. (b) Yields of anion fragments Cl− and Si− from gaseous SiCl4 following Cl 2p core-level excitation, and the Cl L-edge X-ray absorption spectrum [28]. The Cl 2p3/2 and Cl 2p1/2 ionization thresholds are marked by vertical dash lines [27].
L-edge X-ray absorption spectrum. As discernible in Fig. 3b, the relative intensities of various features in anion-yield spectra differ significantly from those of the Cl L2,3 -edge absorption spectrum. As shown, excitations of Cl 2p electrons to Rydberg orbitals/doubly excited states (labels 3 and 4) near the Cl 2p ionization thresholds of gaseous SiCl4 enhance greatly the yields of anions Cl− and Si− , relative to the ratio of intensity of the corresponding transition to the excitation from core to valence (label 1) at ∼201.7 eV. The central position of the anion enhancement is about 207.0 eV, which is immediately above the Cl 2p3/2 ionization threshold (206.9 eV [27]) of gaseous SiCl4 . The enhanced atomic anionic – Cl− and Si− – via excitations from core level to Rydberg states near the Cl 2p ionization thresholds of gaseous SiCl4 , as shown in Fig. 3b, are explained in terms of the contribution from the shake-modified resonant Auger decay or/and post-collision interaction [28]. Similar observations have been reported for several small molecules following core-level excitation [29–31]. We compare the ionic dissociation between gaseous and condensed SiCl4 following Cl 2p core-level excitation. For ionic dissociation pathways, the Cl 2p → 8a1 * excitation enhances significantly the Cl+ yield in the condensed phase, but there is almost no enhancement of the Cl+ yield in the gaseous phase. The Cl 2p → 9t2 * excitation of condensed and gaseous SiCl4 enhances the Si+ yield, relative to the Cl 2p → 8a1 * excitation. For anionic dissociations, excitation of core electrons to Rydberg orbitals//doubly excited states near the Cl 2p ionization thresholds of gaseous SiCl4 significantly enhances the production of Cl− and Si− . In contrast to the gaseous phase, the core-to-valence (8a1 * ) excitation at the Cl 2p edge of SiCl4 /Si(1 0 0) leads to noteworthy enhancement of the Cl− yields. Through electronic interaction with a substrate or neighboring molecules at the solid surface, the potential-energy hypersurface of an intermediate excited ionized state of the parent molecule is modified in the condensed phase relative to gaseous molecules [32]. As evident in Fig. 3a, the Cl− desorption yield is clearly enhanced at a small coverage. On increasing the coverage, the enhancement via excitation from core to valence becomes less
K.T. Lu et al. / Journal of Electron Spectroscopy and Related Phenomena 184 (2011) 140–143
prominent. We propose accordingly that the surface or/and condensed effect might assist these high-lying excited ionic states to cross into ion-pair states and subsequently to produce a negative ion. The enhanced production of anionic fragments via core-level excitation near the ionization thresholds of Cl 2p edge of gaseous SiCl4 is attributed to the contribution from the shake-modified resonant Auger decay or/and post-collision interaction, whereas production of negative ions in the condensed phase is dominated by dissociative electron attachment and the high-lying excited states of the parent ions. As demonstrated, the channels for desorption of anions in the condensed phase differ notably from the pathways for dissociation of anions in the gaseous phase. Comparative studies are hence clearly imperative for an elucidation of the detailed dynamics of photofragmentation between gaseous molecules and molecular adsorbates on surfaces [1,3]. 4. Conclusion By combining measurements of photon-induced ion dissociation and X-ray absorption we investigated the dissociation dynamics of positive-ion and negative-ion fragments of gaseous SiCl4 and SiCl4 adsorbed on Si(1 0 0) ∼90 K following Cl 2p corelevel excitations. The Cl+ desorption yield is significantly enhanced following Cl 2p → 8a1 * excitation of condensed SiCl4 , whereas Cl 2p core-level excitation of gaseous SiCl4 produces little or no enhancement of the Cl+ yield. The Cl 2p → 9t2 * excitation of gaseous SiCl4 enhances appreciably the Si+ yield, relative to the Cl 2p → 8a1 * excitation. Excitations of the Cl 2p electrons to Rydberg orbitals near the Cl 2p ionization thresholds of gaseous SiCl4 greatly enhance the production of anionic fragments Si− and Cl− . The enhanced production of anionic fragments via core-level excitation near the ionization threshold of gaseous SiCl4 is explained in terms of contributions from the shake-modified resonant Auger decay or/and post-collisional interaction. The Cl 2p → 8a1 * excitation of SiCl4 /Si(1 0 0) leads to a notable enhancement in the Cl− yield, particularly for a small coverage. The resonant enhancement of Cl− yield occurs through the formation of high-lying excited states of the parent ions that are predissociated by the ion-pair channel. These complementary results provide new insight into the stateselective fragmentation of positive and negative ions of gaseous molecules and their solid-state analogues via core-level excitation. Acknowledgment
96-2113-M-213- 009-MY3 and NSC 99-2113-M-213- 006-MY3) supported this research. References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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[30] [31] [32]
We thank the NSRRC staff for their technical support. NSRRC and National Science Council of the Republic of China (Grant Nos. NSC
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