Dissociation dynamics of positive-ion and negative-ion fragments of gaseous and condensed Si(CH3)(2)Cl-2 via Si 2p, Cl 2p, and Cl 1s core-level excitations

Dissociation dynamics of positive-ion and negative-ion fragments of gaseous and

condensed Si ( C H 3 ) 2 Cl 2 via Si 2 p , Cl 2 p , and Cl 1 s core-level excitations

J. M. Chen, K. T. Lu, J. M. Lee, C. K. Chen, and S. C. Haw

Citation: The Journal of Chemical Physics 125, 214303 (2006); doi: 10.1063/1.2400229 View online: http://dx.doi.org/10.1063/1.2400229

View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/125/21?ver=pdfcov Published by the AIP Publishing

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Dissociation dynamics of positive-ion and negative-ion fragments

of gaseous and condensed Si

_„CH

₃

_…

₂

Cl

₂

via Si 2p, Cl 2p, and Cl 1s

core-level excitations

J. M. Chena兲,b兲 and K. T. Lua兲

National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan, Republic of China

J. M. Lee

National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan, Republic of China and Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan Republic of China

C. K. Chen and S. C. Haw

National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan, Republic of China

共Received 26 September 2006; accepted 26 October 2006; published online 1 December 2006兲 The state-selective positive-ion and negative-ion dissociation pathways of gaseous and condensed Si共CH3兲2Cl2following Cl 2p, Cl 1s, and Si 2p core-level excitations have been characterized. The

excitations to a specific antibonding state共15a₁*state兲 of gaseous Si共CH3兲2Cl2at the Cl 2p, Cl 1s,

and Si 2p edges produce significant enhancement of fragment ions. This ion enhancement at specific core-excited states correlates closely with the ion kinetic energy distribution. The results deduced from ion kinetic energy distribution are consistent with results of quantum-chemical calculations on Si共CH3兲2Cl2 using theADFpackage. The Cl−desorption yields for Si共CH3兲2Cl2/ Si共100兲 at ⬃90 K

are notably enhanced at the 15a₁* resonance at both Cl 2p and Si 2p edges. The resonant enhancement of Cl− _{yield occurs through the formation of highly excited states of the adsorbed}

molecules. These results provide insight into the state-selective ionic fragmentation of molecules via core-level excitation. © 2006 American Institute of Physics.关DOI:10.1063/1.2400229兴

I. INTRODUCTION

The photodissociation of core-excited molecules induced by x-ray photons has received much attention because a comprehensive knowledge of such fragmentation is not only of scientific importance but also of interest in other fields, such as chemical reactions induced by high-energy particles on interstellar dust and radiation damage of biomolecules and x-ray optics. By means of synchrotron radiation with energy tunable in the x-ray region, the site-selective photo-excitation and subsequent cleavage of chemical bonds of molecules on tuning the x-ray energy to a particular absorp-tion resonance have been a subject of extensive research. The site-specific fragmentation via core-level excitation was ob-served for several systems,1–9 but not for some molecules.10,11 A site-specific fragmentation via core-level excitation was observed for several systems,1–9 but not for some molecules.10,11Such photofragmentation of molecules might arise from a rapid dissociation of ions,12,13but no di-rect experimental evidence has been reported. The compli-cated reaction dynamics involved in site-selective fragmen-tation of core-excited molecules remains a topic of broad interest.14–17

Some highly excited states near or above the threshold energy for double ionization were observed for gaseous samples, such as SO2 and CO,18–20 but little research has

been devoted to these highly excited states and their role on

ion desorption processes for adsorbates on surfaces. The de-tection of negative ions has proved to be a powerful method of exposing highly excited states above the threshold energy for single ionization. However, the investigation of negative-ion fragments produced by inner-shell excitatnegative-ion of mol-ecules is still in its infancy.18,21–23The desorption dynamics of negative-ion fragments of absorbates on surfaces follow-ing core-level excitation are not yet fully understood.

In this study, the dissociation dynamics for ionic 共posi-tive ion and nega共posi-tive ion兲 fragments from gaseous and con-densed Si共CH3兲2Cl2 following excitations of Cl 2p, Cl 1s,

and Si 2p electrons to various resonances have been charac-terized using synchrotron radiation. The most striking obser-vation is a strong enhancement of fragment ions following excitation of a core electron to a specific unoccupied anti-bonding valence state共15a₁* state兲. We provide a clear dem-onstration that this ion enhancement of specific core-relaxed states correlates closely with the ion kinetic energy distribu-tion. The Cl− _{desorption yields for Si共CH}

3兲2Cl2/ Si共100兲 at

⬃90 K are enhanced notably at the 15a1

* _{resonance at both}

Cl 2p and Si 2p edges. This resonant enhancement of Cl−

yield occurs through the formation of highly excited states of the adsorbed molecules. These results provide insight into the state-selective enhanced ionic fragments of gaseous and condensed molecules via core-level excitation.

II. EXPERIMENT

The experimental measurements were conducted at the high-energy spherical grating monochromator 共HSGM兲 a兲_{Authors to whom correspondence should be addressed.}

b兲_{Electronic mail: jmchen@nsrrc.org.tw}

0021-9606/2006/125共21兲/214303/8/$23.00 125, 214303-1 © 2006 American Institute of Physics

beamline and the double-crystal monochromator共DCM兲 ten-der x-ray beamline in the National Synchrotron Radiation Research Center共NSRRC兲 in Taiwan. For dissociation mea-surements in a condensed phase, an ultrahigh-vacuum 共UHV兲 chamber with a base pressure of ⬃1⫻10−10_{Torr was}

used. The Si共100兲 surface was cleaned by repeated resistive heating to ⬃1100 °C under vacuum before the measure-ments. The high purity Si共CH3兲2Cl2共Merck, 99.9%兲 was

de-gassed by several freeze-pump-thaw cycles before use. The vapor of Si共CH3兲2Cl2 was then condensed through a leak

valve the Si共100兲 surface at ⬃90 K. X-ray absorption spectra were recorded by the total-electron yield共TEY兲 mode using a microchannel plate detector. Negative ions were mass se-lected with a quadrupole mass spectrometer 共Balzers, QMA 410兲. The quadrupole detector was oriented perpendicular to the substrate surface, and photons were incident at an angle of 45° with respect to the substrate normal. The incident photon intensity 共I0兲 was monitored simultaneously by a Ni

mesh located after the exit slit of the monochromator. To measure photodissociation in the gaseous phase, an effusive molecular beam produced by expanding the gas through an orifice 共50␮m兲 into the experimental chamber was used. The pressure in this chamber was maintained at ⬃1 ⫻10−5_{Torr. Fragment ions were mass selected with a}

quad-rupole mass spectrometer 共Hiden, IDP兲. The ion kinetic en-ergy 共not calibrated兲 was measured by a quadrupole mass spectrometer with a 45° sector field analyzer 共Hiden, EQS兲. For ion kinetic energy distribution and photodissociation measurements, the HSGM beamline was operated with 100␮m slits corresponding to the energy resolution of ⬃0.2 eV at the Cl 2p edge and ⬃0.1 eV at the Si 2p edge. To obtain the high-resolution x-ray absorption spectrum, the HSGM beamline was set to a photon resolution of⬃0.05 eV at the Si 2p edge and⬃0.1 eV at the Cl 2p edge. The energy resolution of the DCM tender x-ray beamline was set to ⬃0.6 eV at the Cl 1s edge. All yield spectra of fragment ions and x-ray absorption spectra were normalized to the incident photon flux at the Si 2p, Cl 2p, and Cl 1s edges.

The surface coverage was determined by thermal de-sorption spectroscopy 共TDS兲. The Si共CH3兲2Cl2 TDS spectra

show a single molecular desorption peak with an exposure of 4 L or less共1 L=1⫻10−6_{Torr s兲. At a higher exposure,}

ad-ditional peak appears at a temperature of ⬃133 K and its intensity increases with Si共CH3兲2Cl2 exposures. 4 L

expo-sure of Si共CH3兲2Cl2 on Si共100兲 at ⬃90 K thus corresponds

to 1 ML.

III. RESULTS AND DISCUSSION

Figure1共a兲reproduces the yield spectra of fragment ions from gaseous Si共CH3兲2Cl2 following Cl 2p core-level

exci-tation, with the Cl L23-edge x-ray absorption spectrum at the

Cl L23-edge for comparison. The absorption features labeled

as 1, 1

⬘

and 2, 2

⬘

in Fig.1共a兲are ascribed to the transitions to the Cl 2p→15a₁* 共Si–Cl兲 antibonding orbital and Cl 2p

→10b1* 共Si–Cl兲 antibonding orbital, respectively.

24

Excita-tions to Rydberg orbitals are responsible for the absorption peaks labeled as 3, 3

⬘

. The broad band labeled as 4 is attrib-uted to the shape resonance. As shown in Fig. 1共a兲, the

photon-energy dependence of yields of various fragment ions, except Si共CH₃兲₂+, SiCH₃+, and Si+_{, of gaseous}

Si共CH₃兲₂Cl₂ resembles the Cl L₂₃-edge photoabsorption spectrum. Especially noteworthy is that excitations of Cl 2p to the 15a₁*共Si–Cl兲 antibonding state of gaseous Si共CH3兲2Cl2

produce significant enhancement of Si共CH3兲2+ and SiCH3+

yields.

In Fig.1共b兲, the ion yields of various ionic fragments for gaseous Si共CH3兲2Cl2as a function of photon energy near the

Si 2p edge are shown with the gaseous-phase Si L-edge x-ray absorption spectrum for comparison. The absorption peaks labeled as 1 and 1

⬘

in Fig.1共b兲are assigned to transi-tions from Si共2_P

3/2,1/2兲 initial states to the 15a1*共Si–Cl兲

anti-bonding orbital. The doublet structures labeled as 2 and 2

⬘

are assigned to excitations to the 17a₁* 共Si–Cl兲 antibonding orbital.25This result is consistent with the assignment in the

Si K-edge absorption spectrum of gaseous-phase

Si共CH3兲2Cl2 by Ferrer et al.26 The higher-energy peak

la-beled as 5 is due to excitation to Rydebrg orbitals. As noted from Fig. 1共b兲, the photon-energy dependence of yields of various fragment ions, except Si共CH3兲2+and SiCH3+, of

gas-eous Si共CH₃兲₂Cl₂ exhibits a close resemblance to the Si

L23-edge x-ray absorption spectrum. In contrast, the Si 2p

→15a1

*_{excitation of gaseous Si共CH}

3兲2Cl2induces a

substan-tially enhanced production of Si共CH3兲2 +

and SiCH₃+ yields, particularly of Si共CH3兲2

+

.

Figure 2 shows the ion yield spectra of various ionic fragments for gaseous Si共CH3兲2Cl2via Cl 1s core-level

ex-citation. The Cl K-edge x-ray absorption spectrum of gas-eous Si共CH3兲2Cl2is displayed also in Fig.2for comparison.

The assignment of the Cl K-edge x-ray absorption spectrum of Si共CH3兲2Cl2was discussed by Baba et al.27In contrast to

the Cl K-edge x-ray absorption spectrum of condensed phase, the double peaks labeled as 1 and 2 are clearly ob-served in the gaseous-phase Si共CH3兲2Cl2, as shown in Fig.2.

The absorption peaks labeled as 1 and 2 are virtually as-cribed to excitations from Cl 1s to the 15a₁*共Si–Cl兲 and 10b₁* 共Si–Cl兲 antibonding orbitals, respectively, similar to the Cl

L-edge absorption spectrum. The higher-energy peak labeled

as 3 is assigned to be a double excitation.27

As noted from Fig.2, the photon-energy dependence of yields of fragment ions H+, CH₃+, and Si共CH3兲Cl2

+

resembles the Cl K-edge x-ray absorption spectrum of gaseous Si共CH3兲2Cl2. In contrast, a significant dissimilarity of the

several ion yield spectra, particularly for Cl2+, and the Cl

K-edge x-ray absorption spectrum of gaseous Si共CH3兲2Cl2is

observed. As noted, the Cl 1s→15a₁* excitation of gaseous Si共CH3兲2Cl2 leads to significant enhancement of Cl2+ and

SiCH₃+yields, particularly for Cl2+_{. Besides, excitation of Cl}

1s to a 15a₁* state generates a moderate enhancement of Cl+

and Si+ yields, but a small enhancement of SiCl+ yield. Hence, yields of not only Cl+ and Cl2+ but also Si+ and SiCCH₃+were noticeably enhanced via the Cl 1s→15a₁* ex-citation of gaseous Si共CH3兲2Cl2.

Based on the resonant photoemission measurements of gaseous Si共CH3兲2Cl2, the spectator Auger transitions were

the dominant decay channels for resonant excitations of Cl 2p共and Si 2p兲 to the antibonding valence orbitals 共15a₁*and 10b₁*兲 and Rydberg orbitals of gaseous Si共CH3兲2Cl2.7,28The

214303-2 Chen et al. J. Chem. Phys. 125, 214303共2006兲

spectator Auger transition results in a two-hole, one-electron 共2h1e兲 final state, in which two holes are produced in valence orbitals and one electron is excited into an antibonding va-lence orbital or a Rydberg orbital. The spectator electron is localized at the respective valence orbital during the Auger decay. In contrast, the higher-energy shape resonance excita-tion is followed by the normal Auger decay, which leads to a two-hole共2h兲 state. Accordingly, a close resemblance of the fragment ion yield spectra and the corresponding Cl L-edge 共or Si L-edge兲 x-ray absorption spectra of gaseous Si共CH3兲2Cl2, as shown in Figs.1共a兲and1共b兲, is attributed to

the Auger decay of core-excited states and subsequent Cou-lomb repulsion of multi-valence-hole final states, which was called the Auger-initiated dissociation共AID兲 mechanism.29,30 As shown in Fig. 1共a兲, the Si共CH₃兲₂+ and SiCH₃+ yields show significant enhancement following the Cl 2p→15a₁* excitation when compared with the excitations of Cl 2p

→10b1 *

and Cl 2p→shape resonance. This infers that the spectator Auger decay and succeeding 2h1e final states with a spectator electron localized in a strong antibonding orbital 共15a1*兲 produce significant enhancement of specific ion

frag-ments. If this hypotheses is correct, it is reasonably expected that these specific 2h1e states populated by spectator Auger transitions of resonant Si共2p兲−1_共15a

1

*_兲1_{core-excited states of}

gaseous Si共CH3兲2Cl2 also lead to strong state-specific ion

enhancement. As shown in Fig.1共b兲, the yields of Si共CH3兲2+

and SiCH₃+ via the Si 2p→15a₁* excitation are clearly sig-nificantly enhanced, particularly for Si共CH3兲2+, providing

strong evidence to support this hypothesis.

The result deduced from Figs. 1 and 2 clearly reveals that, following the core-to-valence resonance excitation where the spectator Auger decay occurs, the effect of a spec-tator electron in an antibonding orbital must be taken into account in the ion dissociation processes. As demonstrated, if

FIG. 1. Photon-energy dependence of fragment ion yields of gaseous Si共CH3兲2Cl2following共a兲 Cl 2p and

共b兲 Si 2p core-level excitations to-gether with the corresponding photo-absorption spectrum. The ionization thresholds of Cl 2p3/2,1/2 and Si

2p3/2,1/2 of gaseous Si共CH3兲2Cl2 are

indicated in the corresponding absorp-tion spectrum.

the spectator electron is localized in the antibonding orbital with a strong repulsive ionic potential, the breaking of the chemical bond can be enhanced.31 Hence, the ion yield cor-relates closely with the slope of repulsive ionic potential. A possible reason for the significant difference in the Si共CH₃兲₂+ yields following Cl 2p 共or Si 2p兲 to 15a₁* and 10b₁* excita-tions might thus reflect the difference in the steepness of the repulsive potential between 15a₁*and 10b₁*. It is expected that the potential curve of 15a₁*is much steeper than that of 10b₁*. So, within the lifetime of 2h1e, Si共CH3兲2+ can gain more

kinetic energy from 15a₁* state, increasing the Si共CH₃兲₂+ yield.

The kinetic energy distribution of ionic fragments pro-duced by photoexcitation of molecules provides information about the dissociation dynamics such as the steepness of po-tential energy surfaces of electronically relaxed states and the energy partitioning among the internal degree of freedom of fragments.32To elucidate the mechanism of state-specific en-hancement of ionic fragments, we measure the kinetic energy distributions of various fragment ions from gaseous Si共CH3兲2Cl2following Cl 2p and Si 2p core-level excitations

to various resonances. In Figs.3共a兲–3共c兲, the average kinetic energies of fragment ions 关SiCl+, Si+, and Si共CH3兲2Cl+, as

representative examples兴 are shown as a function of the ex-citation energy near the Cl 2p edge. As noted, the average kinetic energies of fragment ions depend markedly on the excitation energy.

As shown in Figs.3共a兲–3共c兲, the average kinetic energies of various fragment ions, such as SiCl+_{, at the shape}

reso-nance are much higher than those at Cl 2p core-to-valence and Cl 2p core-to-Rydberg excitations. This is due to the fact that the Coubomb repulsion of electronically relaxed two-hole 共2h兲 states at the shape resonance is much larger than that of 2h1e final states at resonant Cl 2p core-to-valence and Cl 2p core-to-Rydberg excitations. In Fig. 3共d兲, the kinetic energy distributions of Si共CH3兲2+ from gaseous Si共CH3兲2Cl2

following Cl 2p core-level excitation are reproduced. The resultant average kinetic energy of Si共CH₃兲₂+is shown in Fig.

3共e兲 as a function of the excitation energy near the Cl 2p edge. As noted from Figs. 3共d兲 and3共e兲, the ion kinetic en-ergy distribution of Si共CH3兲2+via the Cl 2p→15a1*excitation

is shifted to an energy of⬃0.2–0.3 eV greater than those via excitations of Cl 2p to 10b₁*, state, Rydberg orbital, and shape resonance. As mentioned, the kinetic-energy distribu-tion of ionic fragments from molecules via core-level exci-tation is related to the steepness of the potential energy curves of the electronically relaxed states.32Accordingly, the electronically relaxed 2h共15a1

*_兲1_{states with a spectator}

elec-tron localized at the 15a₁* states would have steeper repulsive-potential curves along the Si–Cl coordinates than for 2h共10b1*兲1and 2h states.

After spectator Auger decay of resonant core-excited molecules, the kinetic energy released to the dissociation processes is given the difference between the Coulomb re-pulsive energy of two holes and the dissociation energy of a specific bonding. We assumed that the Coulomb repulsive energy of two holes produced by the Cl共2p兲−1_15a

1

*_resonance

is approximately the same as that generated by the Cl共2p兲−1_10b

1

* _{resonance. As presented in Fig. 3, the kinetic}

energy for ionic fragment Si共CH3兲2+ via the Cl 2p→15a1*

excitation of gaseous Si共CH₃兲₂Cl₂ is ⬃0.2–0.3 eV greater than that via the Cl 2p→10b₁* excitation. Hence the disso-ciation energy of Si–Cl bonding of Si共CH₃兲₂Cl₂at the 2h1e states with a spectator electron in the 15a₁* orbital is smaller than that at the 2hle states with a spectator electron in the 10b₁*orbital.

Based on the molecular-orbital calculations in Si共CH3兲2Cl2 performed with the ADF package, the atomic

populations of the Si–Cl orbital, such as 11a₁state, are com-posed mainly of Si 3s orbital and Cl 3p orbital. It is therefore expected that the spectator electron in the Si 3s*_{orbital or Cl}

3p* _{orbital assists in breaking the Si–Cl bond. The 15a} 1 *

or-bital is composed of Si 3s共22%兲, Si 3p 共34%兲, Cl 3p 共26%兲, and other minor components. The 10b₁* orbital consists mainly of Si 3s共⬍1%兲, Si 3p 共61%兲, and Cl 3p 共10%兲. The contents of the Si 3s* _{and Cl 3p}*_{orbital components in the}

15a₁*orbital are much greater than those in the 10b₁* orbital. As a result, a spectator electron in the 15a₁* orbital is more effective in the cleavage of the Si–Cl bond than that in the 10b₁*orbital. This theoretical prediction is consistent with the present ion kinetic energy distribution measurements.

Figures4共a兲and4共b兲show the average kinetic energies of SiCl+_{and Cl}+ _{ions, respectively, as a function of the}

ex-citation energy in the vicinity of Si 2p edge. Similar to the Cl 2p edge, the average kinetic energies of SiCl+and Cl+ions above the Si 2p ionization threshold共⬃109.3 eV for Si 2p3/2

FIG. 2. Photon-energy dependence of fragment ion yields of gaseous Si共CH3兲2Cl2 near the Cl 1s edge with the Cl K-edge x-ray absorption

spectrum.

214303-4 Chen et al. J. Chem. Phys. 125, 214303共2006兲

ionization threshold兲 exceed those at Si 2p core-to-valence excitations. In Fig. 4共c兲, the ion kinetic energy distributions of Si共CH3兲2

+ _{of gaseous Si共CH}

3兲2Cl2 following Si 2p

core-level excitation are reproduced. The resultant average kinetic energy of Si共CH3兲2

+

is shown Fig. 4共d兲 as a function of the

excitation energy in the vicinity of the Si 2p edge. As noted from Figs.4共c兲and4共d兲, the average kinetic energy for frag-ment Si共CH3兲2

+ _{at the Si共2p兲}−1_共15a 1

*_兲1 _{resonance is greater}

than those at the Si共2p兲−1_共17a 1

*_兲1_{resonance and above the Si}

2p ionization threshold. Hence the slope of the potential

en-FIG. 3. Average kinetic energies of共a兲 SiCl+_{,共b兲 Si}+_{, and共b兲 Si共CH}

3兲2Cl+as a function of excitation energy near the Cl 2p edge. 共d兲Kinetic energy

distributions of Si共CH3兲2+of gaseous Si共CH3兲2Cl2following Cl 2p core-level excitation. The photon energy used for excitation is indicated in each spectrum.

The number indicated in each spectrum corresponds to an absorption peak marked in the absorption spectrum in Fig.1共a兲.共e兲Average kinetic energy of Si共CH3兲2+as a function of the photon energy in the vicinity of the Cl 2p edge.

ergy curves along the Si–Cl coordinates of the electronically relaxed 2h共15a1

*_兲1 _{states is steeper than those of the}

2h共17a₁*兲1_{and 2h states, as confirmed by Cl 2p excitation in}

Figs.3共d兲 and3共e兲. The enhancement of the Si共CH3兲2 +

yield at the specific resonance therefore correlates strongly with the ion kinetic energy distribution which is related to the steepness of potential surface of core-relaxed states. Accord-ingly, after spectator Auger decay of resonant Si 2p, Cl 2p, and Cl 1s core-excited states of gaseous Si共CH3兲2Cl2, the

subsequent electronically relaxed 2h1e or multihole, one-electron 共mh1e兲 final states with a steeper potential energy surface along the dissociation coordinates lead to significant enhancement of specific ion fragments. The present results clearly demonstrate that the bond breaking is assisted by a specific antibonding state, not just any antibonding state. Per-haps for this reason site selectivity was not found in some molecules.11

Unlike positive-ion desorption, Cl−_{and H}−_{were the ions}

predominantly observed in the negative-ion desorption of

Si共CH₃兲₂Cl₂/ Si共100兲 following Cl 2p and Si 2p core-level excitations. Figures5共a兲and5共b兲show the Cl−yield and H− yield spectra from Si共CH₃兲₂Cl₂/ Si共100兲 following Cl 2p and Si 2p core-level excitations, respectively, with the corre-sponding Cl L-edge and Si L-edge TEY spectra of condensed Si共CH3兲2Cl2 for comparison. As noted from Figs. 5共a兲 and 5共b兲, the H−yield curves nearly follow the corresponding Cl

L-edge and Si L-edge total electron yield curves. The

pos-sible desorption mechanism of H− _from

Si共CH₃兲₂Cl₂/ Si共100兲 was likely due to dissociative attach-ment on molecules of secondary electrons produced by pho-toabsorption of molecular adsorbates. This desorption mechanism was called dissociative electron attachment 共DEA兲. Also, the H−_{and Cl}−_{yields at⬃101 eV, indicated by}

arrows in Fig. 5共b兲, are induced by the secondary electrons produced by the Si 2p core-level excitation of the Si共100兲 substrate, likewise providing evidence of the existence of DEA processes.22,33

Especially noteworthy is that the Cl 2p→15a₁*excitation

FIG. 4. Average kinetic energies of共a兲 SiCl+_{and共b兲 Cl}+_{as a function of excitation energy near the Si 2p edge.共d兲 Kinetic energy distributions of Si共CH} 3兲2

+

of gaseous Si共CH3兲2Cl2following Si 2p core-level excitation. The photon energy used for excitation is indicated in each spectrum. The number indicated in

each spectrum corresponds to an absorption peak marked in the absorption spectrum in Fig.1共b兲.共e兲 Average kinetic energy of Si共CH₃兲₂+_{as a function of the}

photon energy in the vicinity of the Si 2p edge.

214303-6 Chen et al. J. Chem. Phys. 125, 214303共2006兲

significantly enhanced the Cl−desorption yield. To elucidate the origin of the Cl−_{enhancement at the 15a}

1

*_{resonance, we}

measured the Cl− _{yield spectra from Si共CH}

3兲2Cl2/ Si共100兲

with variable coverage following Cl 2p core-level excitation, as presented in Fig. 5共a兲. As noted, the Cl− _{yield at shape}

resonance increases with Si共CH3兲2Cl2 exposures and thus

show a linear behavior with electron yield. Hence the desorp-tion mechanism of Cl− at shape resonance excitation for Si共CH3兲2Cl2/ Si共100兲 is likely due to DEA. If the

enhance-ment of Cl− _{yield at the 15a} 1

* _{resonances is due to a DEA}

process, the Cl− yield at the 15a₁* resonance should show a trend like that at the shape resonance, because the energy distribution of the secondary electrons becomes indistin-guishable for photoexcitation at shape resonance and at other photon energies. Hence, the Cl−_{yield at the 15a}

1

*_resonance

should monotonically increase with Si共CH3兲2Cl2 exposure.

However, as noted from Fig.5共a兲, the Cl−yield at the 15a₁* resonance increased up to 10 L exposure and then slowly decreased after 10 L exposure of Si共CH3兲2Cl2 on Si共100兲.

Moreover the observed 15a₁*resonances have the same ener-gies and widths between Cl−_{yield and TEY spectra. It allows}

us to give a reasonable explanation that the enhancement of Cl−yield at the 15a₁*resonance is due not to DEA processes but to unimolecular processes.

It is speculated that the enhancement of Cl−yield at the Cl共2p兲−1_15a

1

* _{resonance might originate from some highly}

excited states of the parent ions that are predissociated by an ion-pair or direct dissociation.18–20 It was proposed that the core-excited states of Si共CH₃兲₂Cl₂ molecules via the Cl 2p core-level excitation can decay by an Auger transition to these highly excited states. If such highly excited states exist, it is thus expected that these states can also be populated by

an Auger transition of Si 2p core-excited states of Si共CH3兲2Cl2. As shown in Fig. 5共b兲, the Cl− yield at

⬃103.8 eV via the Si 2p→15a1* excitation shows a notable

enhancement relative to that at ⬃105.5 eV on excitation to Si–C antibonding states. This gives evidence in support of the existence of some highly excited states. A similar phe-nomenon has been found for OPCl3共P 2p and Cl 2p edges兲,

Si共CH3兲4−nCln 共n=1–4兲 共Si 2p and Cl 2p edges兲, etc.21The

formation of negative ions through highly excited states is thus not specific to Si共CH3兲2Cl2 molecules. It is therefore

believed that the present experimental finding is of a general nature.

IV. CONCLUSION

In conclusion, the state-selective positive-ion and negative-ion dissociation pathways of gaseous and con-densed Si共CH3兲2Cl2following Cl 2p, Cl 1s, and Si 2p

core-level excitations have been characterized using photon-induced dissociation, x-ray absorption spectroscopy, and ion kinetic energy distribution. The excitations to a specific an-tibonding state共15a₁*state兲 of gaseous Si共CH3兲2Cl2at the Cl

2p, Cl 1s, and Si 2p edges lead to significant enhancement of fragment ions. This ion enhancement at specific core-excited states correlates strongly with the ion kinetic energy which is related to the steepness of a potential surface along the dis-sociation coordinates of core-relaxed states. The results de-duced from ion kinetic energy distribution are consistent with results of molecular orbital calculations on Si共CH₃兲₂Cl₂ using the ADF package. The Cl− desorption yields from

Si共CH₃兲₂Cl₂/ Si共100兲 at ⬃90 K are notably enhanced at the 15a₁*resonance at both Cl 2p and Si 2p edges. The resonant enhancement of Cl− _{yield occurs through the formation of}

highly excited states of the adsorbed molecules. These highly excited states hence play an important role in ion desorption of adsorbed molecules via core-level excitation. Our experi-mental results provide important insight into the roles and relative importance of dissociative electron attachment, highly excited states, and Auger initiated desorption. These results contribute to a comprehensive understanding of the state-selective ionic 共positive ions and negative ions兲 frag-mentation of gaseous and condensed molecules via core-level excitation.

ACKNOWLEDGMENTS

We thank the NSRRC staff for their technical support. This research is supported by the NSRRC and the National Science Council of the Republic of China under Grant No. NSC 94-2113-M-213-001 and NSC 93-2113-M-213-009.

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