- Email: [email protected]
Dissociation processes of core-excited CBr4 involving the Br(3d, 3p, 3s) and C(1s) inner-shells in the range 50–460 eV
Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 73–84 www.elsevier.com / locate / elspec
Dissociation processes of core-excited CBr 4 involving the Br(3d, 3p, 3s) and C(1s) inner-shells in the range 50–460 eV a, b b c Bong Hyun Boo *, Norio Saito , Isao H. Suzuki , Inosuke Koyano a
Department of Chemistry, Chungnam National University, Daejeon 305 -764, South Korea b Electrotechnical Laboratory, 1 -1 -4, Umezono, Tsukuba-shi, Ibaraki 305, Japan c Department of Material Science, Himeji Institute of Technology, 1479 -1 Kanaji, Kamigohri, Hyogo 678 -12, Japan Received 10 July 2001; received in revised form 5 December 2001; accepted 4 January 2002
Abstract The dissociative multiple photoionization of CBr 4 in the valence, and in the Br(3d, 3p, 3s) and C(1s) inner shell regions has been studied by using time-of-flight (TOF) mass spectrometry coupled to synchrotron radiation in the range 50–460 eV. Total and individual photoion yield curves have been recorded as a function of the photon energy. Several discrete resonances are observed below the Br(3d) threshold region. Also a giant shape resonance in the total ion yield curve is observed beyond the Br(3d) region, which can be identified as a shape resonance owing to the diffusion of the outgoing photoelectron by the angular momentum barrier in the 3d 9 ef outgoing channel. Various monocations of Br 1 and CBr n1 (n50–3) are detected along with dications of Br 21 and CBr 21 in the whole energy range examined. Dissociation processes 3 of multiply charged CBr 4 parent ions have also been investigated by photoelectron–photoion coincidence (PEPICO) and photoion–photoion coincidence (PIPICO) methods. The dominant dissociation pattern is found to involve C 1 –Br 1 and Br 1 –Br 1 in the whole energy range examined. Distinct energy dependence of dissociation processes is observed in the Br(3p) and Br(3s) regions. With the help of ab initio HF / 6-31111G(d) calculation, we estimated the photoabsorption positions and symmetries for the discrete core-excited states using the Z 1 1 equivalent ionic core model. The specific excitation and dissociation of molecules provides information on energy dissipation processes in conjunction with the relevant core hole states. 2002 Elsevier Science B.V. All rights reserved. Keywords: Carbon tetrabromide; Br core excitation; Multiple ionization; PEPICO; PIPICO
1. Introduction Inner-shell spectroscopy is an attractive tool to investigate atomic and electronic structures of molecules [1]. Owing to the localization of the core hole, site-selective photochemistry is observed to some *Corresponding author. Tel.: 182-42-821-6551; fax: 182-42823-1360. E-mail address: [email protected] (B.H. Boo).
extent, the degree of the selectivity relying on the competitive processes of electronic relaxation and bond cleavage. It is shown in the literature that Auger-electron–ion coincidence studies could reveal the individual decay channels for the various doubly charged ionic configurations populated in the Auger decay of the core hole [2,3]. These studies also enable us to correlate the ion fragmentation products, their kinetic energy, and their electronic configuration with the hole configuration in the doubly
0368-2048 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 02 )00005-1
74
B.H. Boo et al. / Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 73 – 84
charged molecule ion [2]. By employing the coincidence method, it is also possible to elucidate the effect of the core hole decay on the site-selectivity in the bond fragmentation. In addition, studies of the fragmentation pathways of dissociative multiple ionization involve a variety of coincidence methods such as photoelectron– photoion (PEPICO), photoion–photoion (PIPICO), photoelectron–photoelectron (PEPECO), and photoelectron–photoion–photoion (PEPIPICO, PE2PICO) coincidence [4]. Among these, the triple coincidence PEPIPICO technique was shown to be a powerful technique to elucidate the detailed dynamics in dissociative double ionization processes [5–7]. Recent development of position-sensitive ion detector in the PEPIPICO methods makes it possible to investigate the detailed dynamics in the dissociation of doubly charged ion [6,8]. Until now, however, the Auger-electron–ion coincidence and PEPIPICO methods are almost restricted to the determination of the three-body dissociation dynamics of relatively small dications. Hitchcock et al. [9] have investigated inner shell excitation in bromine-containing molecules such as methyl bromide and bromobenzene [10] via electron energy loss spectroscopy and then assigned inner shell excitation such as Br(3d)→s(a 1 )* and Br(3d)→5p(a,e) for methyl bromide, and Br(3d)→p(b 2 )* and Br(3d)→5p for bromobenzene. To our best knowledge, however, no photoabsorption data of CBr 4 in the Br(M) region have been reported. Among various molecules, CBr 4 is of special interest because of the following points: the molecule has four equivalent Br atoms in it, and thus we could investigate the effect of tetrahedrally oriented Br ligands on the giant resonance caused by continuum enhancement corresponding to 3d→ef; it has two different atoms, and we could compare dissociation patterns relying on the core excitation tuning the C and Br atoms. In this report, we present our recent experimental and theoretical results of the photoionization mass spectra measured both in the simple coincidence modes such as PEPICO and PIPICO modes, in the Br(M) and C(1s) region from 50 to 460 eV. The photoabsorption positions and symmetries for the core–hole states were roughly estimated by the equivalent ionic core valence orbital model (EIC-
VOM) calculation [11,12]. Various dissociation patterns of the multiply charged parent ions are proposed on the basis of the measurements of the ion TOF differences in the PIPICO mode. Comparison of the dissociation patterns was made in the Br(3d, 3p, 3s) and C(1s) core excitation regions. The specific excitation and dissociation of molecule provides information on energy dissipation processes of the core-excited states.
2. Theoretical and experimental Monochromatic vacuum ultraviolet and soft Xrays were obtained by dispersing synchrotron radiation from the TERAS storage ring at the Electrotechnical Laboratory using a Grasshopper monochromator. The principle and construction of the whole apparatus have been described in detail elsewhere [13,14] and thus only briefly described here. The monochromatized photons of 100–300 eV, with photon energy resolution of about 800–300, respectively, were obtained from a Grasshopper monochromator with a grating of 1200 l / mm. The monochromatized photons pass through two differential pumping stages and entered an ionization chamber equipped with a TOF spectrometer. Here an electric field of 175 V/ cm was applied to the ionization chamber for ion and electron extraction. The main chamber where the ionization cell, a photoelectron detector and the TOF spectrometer were equipped was evacuated down to ¯1310 28 Torr. When the CBr 4 gas molecules were introduced into the ionization cell, the pressure of the main chamber was maintained at ¯7310 26 Torr. The flight path length was 13 cm and the mass detection angle was perpendicular to the direction of incident photon beam and parallel to the photon beam polarization. The uncertainty in the photon beam energy lies within 60.1 eV. The total ion yield spectrum was obtained by recording the count rates of the total ions and the photon flux simultaneously while the photon wavelength is scanned. For characterization of individual ions and ion pairs, the time-correlated ion counting technique was used in the two modes of PEPICO and PIPICO, respectively. The PEPICO mode utilizes the start pulses for a time-to-amplitude converter (TAC)
B.H. Boo et al. / Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 73 – 84
provided by the photoelectron arrival signals at a microchannel plate (MCP), sampled from the collision chamber in the opposite direction to the ion flight direction, and the stop pulses generated by the photoion detection signals at another MCP located at the end of the TOF tube. However, the PIPICO mode uses the start pulses provided by the lighter ions, and the stop pulses generated by the heavier ions of the counter parts. Similarly the total PIPICO yield spectrum was obtained by measuring the count rates of total PIPICO simultaneously while the photon wavelength is scanned. For the measurements of the PIPICO count rates, the coincidence time range (gate width in TAC) was set to be 0–5 ms, because the TOF difference between any pairs of ions formed from CBr 4 is found to fall into this time range. The individual ion and PIPICO yields at a photon energy were directly obtained by measuring the integrated individual ion and PIPICO intensities, and by simultaneously measuring the photon flux while the photon wavelength is scanned. The corrected photon intensities were obtained by dividing the measured photoemission yields from a gold mesh photocathode placed between the exit slit of the monochromator and the experimental main chamber by the published photoemission efficiency of a gold photocathode [15]. Relative uncertainties in the normalized ion and PIPICO yields may mainly come from this normalization process and could be estimated as less than ¯10%. The sample CBr 4 with a nominal purity of 199% was purchased from Aldrich Chemical Co. and used without further purification. The geometry of CBr 4 was optimized by the HF / 6-3111 1G(d), B3LYP DFT / 6-3111 1G(d) [16–18], and MP2 / 6-3111 1G(d) [19–21] methods for comparison with the experimental geometry [22]. The HF / 6-3111 1G(d) calculation on the unoccupied orbital energies of the Br core-excited state was performed on CBr 3 Kr 1 at the experimental ˚ Simply geometry of CBr 4 [22], (r51.94260.002 A). the negative values of the resulting unoccupied orbital energies were taken in this study as the term values of CBr 4 . From these values, we roughly estimate the photoabsorption positions corresponding to the Br(3d 5 / 2 , 3p 3 / 2 , 3s) excitation by combining the computed term values, the experimental ionization limits for the Br core electrons, and the spin–
75
orbit splitting. The calculations were carried out using the Gaussian 94 program suite [23] installed at KISTI in Korea.
3. Results and discussion We used a moderately low extraction field (175 V/ cm). The use of the low field enabled us to better evaluate the peak broadening owing to kinetic energy release in the dissociation event. However, a use of too low field can decrease the detection sensitivity owing to a fast departing flight via a strong repulsion between ionic fragments. The total ion yields as a function of energy are presented in Fig. 1. It is noticed that some of the literature values for the thresholds and estimated thresholds for the various core electrons are indicated with hatched lines over the yield curves, the reported thresholds of the core electrons for CBr 4 are: Br(3d 5 / 2 )576.7460.05 eV [24,25]; C(1s)5 294.6460.05 [24,25]. Those for atomic bromine are: Br(3d 5 / 2 )569.060.4 eV [26]; Br(3d 3 / 2 )570.160.4 eV [26]; Br(3p 3 / 2 )5181.560.4 eV [26]; Br(3p]h1 / 2j) 5 189.360.4 eV [26]; Br(3s)5256.560.4 eV [26]. Fig. 2 compares the total ion yield curve of CBr 4 with those of bromine-containing compounds. The total PIPICO yield spectrum is shown in Fig. 3. The energy dependence of the total PIPICO yield is similar to that of the total ion yield. This implies that dissociative multiple ionization is the main exit channel in the whole energy examined. It seems puzzling, however, that in the region from 145 to 260 eV, the total ion yield increases monotonously with increasing the energy, whereas the total PIPICO yield decreases. This may come from one possibility that in the Br(3p, 3s) threshold regions, locally ionized bromine may depart from the parent molecule with neutral counterpart. In this case, the total PIPICO intensity could decrease. In an attempt to elucidate the contributions of the total ion yields, we measured typical mass spectra in the PEPICO mode at some critical resonance and off resonance regions. Fig. 4 presents the PEPICO (mass) spectra recorded at 64.5, 70.0, 75.5, 84.5, 149.5, and 406.4 eV. It is noticed, however, that the photoelectrons in the PEPICO mode as the start
76
B.H. Boo et al. / Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 73 – 84
Fig. 1. Normalized total photoion yield curve of CBr 4 in the range 50–460 eV. The inset shows the scale-expanded spectrum in the Br(3d) excitation region.
pulse for TAC are not energy-selected. Below 149.5 eV, almost all the coincidence spectra exhibit a similar pattern. At 406.4 eV, however, the ion intensity of doubly charged species Br 21 is greatly enhanced. In Fig. 5, are presented the relative individual ion yields derived from the integrated ion counts shown in Fig. 4 and the photon intensities as a function of energy. It can be seen that energy variation of the total ion yield (sum of the individual ion yields, Fig. 5) is similar to that of total ion yield (Fig. 1) measured independently. Fig. 6 shows typical PIPICO spectra measured at 64.5, 70.0, 75.5, 84.5, 149.5, and 406.4 eV. Energy variation of the individual PIPICO yields is presented in Fig. 7. In the whole energy range, the most prominent PIPICO signal corresponds to Br 1 –Br 1 . At a higher energy (406.4 eV), the PIPICO peaks corresponding to Br 1 –CBr n1 (n51–3) are significantly diminished owing to near the total destruction of the four bonds. However, the relative PIPICO intensities for Br 21 –Br 1 and C 1 –Br 21 are greatly enhanced at this energy. It is interesting that any channel such as Br 21 –CBr n1 (n51,2,3) is not observed, which implies that in the core excitation
event, localization of the core hole is not preserved absolutely at the specific Br core, instead the rapid redistribution of excess internal energy and charge occurs yielding many singly charged atomic ions. Above the C(1s) threshold, the total ion and PIPICO yields decrease with increasing the photon energy, which might be a property of the oscillator strength of the molecule.
3.1. Core excitation spectra 3.1.1. Excitation of Br(3 d) electrons Several discrete resonances are observed over the broad structureless background resonance, which spans the whole energy range examined. The huge background resonance (giant resonance) can be identified as a shape resonance owing to the centrifugal barrier in the 3d 9 ef outgoing channel. This is responsible for the delayed onset and for the enhancement of the photionization cross section [27]. Therefore, the repulsive force is increasingly important with increasing the angular momentum quantum number , [28,29]. This spectral behavior has also been observed in the photoionization of Br-
B.H. Boo et al. / Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 73 – 84
Fig. 2. Comparison of normalized total photoion yield curve of MBr 4 (M5C, Si, Ge), Br 2 , and (CH 3 ) 3 SiBr.
containing compounds such as SiBr 4 [30], Br 2 [31], GeBr 4 [31], and (CH 3 ) 3 SiBr [31] as shown in Fig. 2. The energy range of the giant resonance corresponding to the Br(3d) core ionization is remarkably narrower in the Br 2 and (CH 3 ) 3 SiBr molecules than those in the other molecules (Fig. 2). Energy dependence of the giant spectral shape is quite similar to those observed in MBr 4 (M5Si, Ge) [31] as seen in Fig. 2. This affirms that the giant shape resonance arises solely from the bromine atoms. Apparently, several discrete excitation peaks below the Br(3d) edge (Fig. 1) consist of two doublet peaks at ¯70 and ¯75 eV, having a peak interval of ¯0.9 eV. Notice that the spin–orbit splitting between the Br(3d 5 / 2 ) and Br(3d 3 / 2 ) components was observed with an energy difference of
77
¯1.0 eV [9,10,30,32]. All the previous studies of bromine-containing molecules show that the transition of Br(3d) core electron to the first antibonding orbital occurs when the incident photon energy reached ¯70 eV, the value being more or less shifted with different ligands. It is accepted as a universal rule that the chemical shifts of photoelectron and Auger lines are linearly dependent on the average differences in Pauling’s electronegativities to the nearest neighbors, D x¯ P [33]. Based on the previous assignments of the Br(3d) core excitation in methyl bromide [9] and bromobenzene [10], the doublet peak at ¯70 eV in CBr 4 may correspond to Br(3d)→s(a 1 )* and the other doublet peak at ¯75 eV may be attributed to Br(3d)→5p(e). In order to help assign the transition observed in CBr 4 , we carried out ab initio calculation at the HF / 6-3111 1G(d) level on CBr 3 Kr 1 , which could be a counterpart of the Br(3d) core-excited CBr 4 in the view of equivalent ionic core virtual orbital model (EICVOM) [11,12]. The optimized geometries determined by the HF / 6-3111 1G(d) and MP2 / 6-3111 1G(d) calculations (Table 1) are quite consistent with the experimental geometry of CBr 4 [22], the term values were derived by a single point calculations on the experimental geometry at the HF / 6-3111 1G(d) level. The excitation energy corresponding to the Br(3d 5 / 2 )→s(a 1 )* derived using the calculated term values listed in Table 2 is 71.6 eV, in fair agreement with the experimental value of 69.6 eV as shown in Fig. 1. The discrepancy reflects that the simple HF / 6-3111 1G(d) calculation using the EICVOM model may not estimate the term values accurately or that the Br(3d) electrons cannot efficiently shield the nuclear charge owing to the extensive d character. Actually the term values should be estimated correctly by the separate calculations on one of the initial state and one of the final state. Therefore it is presumed that the HF calculation on CBr 3 Kr 1 could lead to only rough estimation of the photoabsorption position and the mode of the core hole state. The first two peaks at 69.6 and 70.5 eV in Fig. 1 may correspond to the transition from the Br(3d 5 / 2 ) and Br(3d 3 / 2 ) orbitals to the a 1 antibonding orbital, respectively, the arguments based on the following observations and calculation. (1) The peak shapes and splitting are almost the same as those in the
78
B.H. Boo et al. / Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 73 – 84
Fig. 3. Normalized photoion–photoion coincidence yield curve of CBr 4 in the range 50–450 eV. The inset shows the scale-expanded spectrum in the Br(3d) excitation region.
electron energy loss spectra of methyl bromide [9] and bromobenzene [10]. The spin–orbit splitting ¯1.0 eV owing to the Br(3d 3 / 2,5 / 2 ) core electrons was observed, being in excellent agreement with the reported splitting value of ¯1.0 eV [9,10,30,32]. (2) The ab initio calculation indicates that there is a transition with a 1 symmetry occurring first in the Br(3d) excitation region.
3.1.2. Excitation of Br(3 p, 3 s) electrons The binding energies of the Br(3p, 3s) electron in CBr 4 are not available in the literature, but we can estimate the threshold energies of Br(3p 3 / 2 ) as 189.2 eV, the value determined by adding the chemical shift difference (7.72 eV) between the Br(3d 5 / 2 ) IP in Br free atom and that in CBr 4 to the Br(3p) IP of Br free atom. We also estimate the IP for the Br(3s) as 264.2 eV in the similar fashion. The estimated thresholds are marked as hatched lines over the various yield curves. As seen in Figs. 1 and 3, significant discrete features in the total ion and PIPICO yields are not observed in the Br(3p) and Br(3s) regions. However, in the partial ion yield spectra of C 1 and Br 21 (Fig. 5) and the PIPICO yield spectra of Br 21 –Br 1 and C 1 –Br 21 (Fig. 7),
two distinct features are clearly observed by clearly demonstrating the rises in the relative PIPICO yield spectra from the Br(3p) and Br(3s) edges. How can we justify the finding that Br 21 is formed more abundantly than C 21 even though the first (11.264 eV) [34] and 2nd (24.376 eV) [34] IPs of carbon atom in free atoms are close to those (first IP511.84 eV [34] and 2nd IP521.6 eV [34]) of bromine atom in free atoms? Why does the ion yield spectrum of Br 21 show no significant feature (the inset in Fig. 5) in the Br(3d) excitation / ionization region? Why does the local multiple ionization in the specific Br atom not occur in the Br(3d) region? This is accounted for by the MO concept in which the Br(3d) electrons are partly delocalized in the whole MO. Therefore even when the Br(3d) electrons are multiply ejected, charge redistribution in the whole molecule quickly occurs contributing to the decay of the doubly and triply charged parent ions into several singly charged product ions. However, the Br(3p, 3s) electrons are relatively more localized in the specific Br core atom, thus upon the ejection of the core electrons, charge and energy localization could partially remains to some extent in the specific core yielding the doubly charged bromine ions. However,
B.H. Boo et al. / Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 73 – 84
79
delayed excitation of Br(M), we could not isolate the C(1s) excitation spectrum. Also the abundance of the bromine atoms in a given molecule diminishes the contribution of the C(1s) core excitation spectrum. At 406.4 eV, the much higher energy than the Br(3p, 3s) and C(1s) thresholds, the relative intensities C 1 and Br 21 are significantly enhanced. However, C 21 is very slightly formed in the whole energy region examined. This indicates that the excess charge transfer from the Br atom to the central carbon atom is not feasible presumably because a fast dissociation could occur prior to charge redistribution in the multiply charged precursor ions, or multiple Auger processes may take place after the bond cleavage involved. In previous studies for core excitation of silane, de Souza et al. [35] proposed a two-step decay process for a valence resonance, namely a fast dissociation followed by the autoionization of the excited fragment, on the basis of the observation of Br(3d) core excitation of HBr [36].
3.2. Dissociation patterns
Fig. 4. Photoelectron–photoion coincidence (mass) spectra of CBr 4 measured at various photon energies.
it is not reasonable to justify the mechanism of a fast dissociation followed by local electronic relaxation by using only our simple PEPICO and PIPICO techniques because the electron kinetic energies are not specified for the photoelectron signals for this PEPICO experiment. For clarification of the mechanism, one needs elegant experiments such as an Auger electron–ion coincidence.
3.1.3. C(1 s) excitation Around the C(1s) threshold, significant feature in the ion and PIPICO yield spectra (Figs. 1 and 3) is not observed. However, because the resonance via the C(1s) core excitation is overlapped with the
A hole in the Br(M) shell resulting from excitation of an Br(M) electron may electronically relax by emitting Auger electrons before CBr 4 molecule begins to fragment. However, fast dissociation could also occur prior to the electronic relaxation, which explains the multiple ionization of a Br atom. Any dominant relaxation processes are expected to mainly occur via multiple-resonant Auger processes, which can involve the ejection of two electrons or sometimes, of more electrons [37,38]. When a Br(M) core electron is initially excited to a virtual orbital (one hole and one excited electron), CBr 4 will be left as a doubly charged ion owing to these double-resonant Auger processes. Various ion pairs are expected to arise from the bond destruction, of such doubly charged parent ion or fragment ions, because in polyatomic molecules in general, there are several possible combinations of valence MOs in which two holes are produced, thus giving several different fragmentation pathways. It was shown previously in the core excitation spectroscopy of N 2 O that the site specificity is not absolutely confined to breaking only the O–NN bond, even though the excitation of a core hole localized at the O site in the molecule in the event of
80
B.H. Boo et al. / Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 73 – 84
Fig. 5. Individual photoion yield spectra of CBr 4 in the range 65–460 eV. The spectral intensities (Iphotoion /Iphoton ) are presented on the same relative intensity scale. The inset shows the scale-expanded spectra in the Br(3d) excitation region.
initial absorption [2]. Even though this localization is to some extent preserved in the Auger decay, which favors the participation of valence electrons having wave functions with a large overlap at the site of the initial O core hole, some of these electrons are also crucial in forming the ON–N bond such that removal of these electrons in the Auger decay leads to the total destruction of the bonds in the molecule. But in our system, a total destruction followed by the local excitation tuning at the Br core excitation could occur to some extent. The localization of the core hole is more probable when the Br(3p, 3s) is excited than when the Br(3d) is. Processes leading to the ion pairs, Br 1 –Br 1 , Br 1 – 1 1 CBr 1 , Br 21 –Br 1 , C 1 –Br 21 , Br 1 –CBr 1 2 , C –Br , 1 1 and Br –CBr 3 are observed to occur in the whole range examined. Obviously, the ion pairs, C 1 –Br 1
and Br 1 –Br 1 are most abundant in the whole energy range examined as seen in Figs. 6 and 7. It should be noticed that the PIPICO yield of Br 1 –Br 1 cannot be estimated accurately owing to some interference effect in the pulse-processing. According to the seven ion pairs detected in the PIPICO spectra and their energy profiles, we can propose the dissociation channels of the core excited / ionized CBr 4 . In the Br(3d) giant excitation region, it is shown that the PIPICO yield for Br 1 –CBr n1 (n51–3) is significantly enhanced (Fig. 7) presumably due to process (1): CBr 4 1 hn → CBr 3 –Br * → f CBr 3 –Br g 21 1 1 2e → CBr 1 n (n 5 0–3) 1 Br 1 NP 1 2e
(1)
NP denotes neutral product(s). Why then do we not
B.H. Boo et al. / Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 73 – 84
81
whole energy range. It is noticed that the ion yield ratio for Br 1 is much larger than those for its possible ionic counterparts such as C 1 , CBr 1 , and so forth, in the dissociation of the multiply charged parent ion. Because for example, the intensity ratio IBr 1 /IC 1 is 4.960.8, the simple dissociation process, reaction (1) cannot explain the large intensity ratio. Thus we present processes (2)–(4) (nearly total destruction) as the most contributing processes to explain the ion yield ratios for the observed ionic species: CBr 4 1 hn → f CBr 431 g * 1 3e → CBr n1 (n 5 0–2) 1 1
2Br 1 (2 2 n)Br 1 3e
(2)
CBr 4 1 hn → f CBr 441 g * 1 4e → CBr n1 (n 5 0,1) 1 1
3Br 1 (1 2 n)Br 1 4e
(3)
CBr 4 1 hn → f CBr 451 g * 1 5e → C 1 1 4Br 1 1 5e (4) This argument is based on the slight ionization energy difference of C and Br atoms. It is presumed that carbon or carbon-containing ions are formed simultaneously together with the Br 1 –Br 1 ion pair. But we cannot rule out other possibility such as the occurrence of process (5) to account for the abundant formation of the Br 1 ion in comparison with CBr n1 (n51–3): Fig. 6. Photoion–photoion coincidence (PIPICO) spectra of CBr 4 measured at various photon energies.
CBr 4 1 hn → f CBr 421 g * 1 2e → 2Br 1 1 CBr n (n 5 0–2) 1 (2 2 n)Br 1 2e
observe the enhancement of the ion yield ratios for CBr 1 n (n51–3) in the Br(3p) and Br(3s) regions? In these regions, much higher energy could reside to some extent in the specific Br atom, which favors the participation of valence electrons having wave functions with a large overlap at the site of the initial Br core hole, some of these electrons play a decisive role in forming the C–Br bond such that loss of these electrons in the Auger decay leads to the total bond destruction in the molecule. It is elucidated that such simple decomposition as shown in process (1) cannot explain the formation of Br 1 ion with 7264% in the total intensity in the
(5)
Some minor channels such as process 6 should occur above the Br(3p) edges to explain the observation of the ion pair such as Br 21 –Br 1 and C 1 –Br 21 since any combination of the three ions in reaction (6) is observed in the PIPICO spectra (Fig. 6): CBr 4 1 hn → Br * –CBr 3 → Br * –CBr 41 3 1 4e → C 1 1 Br 21 1 Br 1 1 2Br
(6)
The occurrence of the channels (2)–(6) indicates that triple, quadruple and quintuple ionization could occur in the photoionization of CBr 4 in the range 50–460 eV.
B.H. Boo et al. / Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 73 – 84
82
Fig. 7. Individual photoion–photoion coincidence (PIPICO) yield spectra in the range 65–400 eV. The spectral intensities (IPIPICO /Iphoton ) are presented on the same relative intensity scale. The inset shows the scale-expanded spectra in the Br(3d) excitation region.
4. Conclusions The present coincidence spectroscopic study led to the detection of various monocations of Br 1 and 21 CBr 1 and n (n50–3) along with dications of Br Table 1 Optimized structures of CBr 4 determined by the ab initio and density functional calculations using a 6-3111 1G(d) biases set and their comparison with the experiment Parameter
C–Br a
˚ Value (A) HF
B3LYP DFT
MP2 a
Exp b
1.939
1.965
1.947
1.94260.002
Core electrons were excluded for electron correlation calculation. b From Ref. [22].
CBr 21 in the energy range 50–460 eV. Among the 3 ions, C 1 and Br 1 are predominantly observed in the whole energy range examined. In the total and
Table 2 Term values (eV) of CBr 3 Kr 1 determined by the HF calculation using a 6-3111 1G(d) basis set at the experimental geometry of CBr 4 [22] Symmetry
Term values (eV)
a1 a1 e a1 e a1 e a1
5.11 2.66 2.20 2.09 1.23 1.21 0.29 0.11
B.H. Boo et al. / Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 73 – 84
PIPICO yield curves, we have observed a prominently broad feature above the Br(3d) edge due to their enhanced oscillator strength, which originates from the diffusion of the outgoing photoelectron by the centrifugal barrier of the effective 3d potential. We have assigned the discrete resonance energies below the Br(3d) ionization edges by using the Z 1 1 equivalent ionic core approximation method and then by performing the HF / 6-3111 1G(d) calculation on the equivalent core CBr 3 Kr 1 at the experimental geometry of CBr 4 . The photoejection of the Br(3d) electrons followed by the Auger decay may end up with the observation of ion pairs involving singly charged ions because of the rapid charge spread along the molecule owing to the partial delocalization property of the 3d electron. However, the Br(3p, 3s) core excitation led to the formation of ion pairs such as Br 21 –Br 1 and C 1 –Br 21 in enhanced yields, implying that local multiple ionization of the specific excited bare Br atom or Br 1 ion occurs following a fast dissociation of the bond between the central carbon atom and the core-excited Br atom.
Acknowledgements This work was partially supported by the Korea Research Foundation through an non-directed research project (1998–1999). The staffs of the accelerator group of the Electrotechnical Laboratory in Tsukuba, Japan are greatly acknowledged for the use of the synchrotron radiation facility. The computations were carried out by using the CRAY C90 of the KISTI Supercomputer Center in South Korea, which is gratefully acknowledged.
References [1] A.P. Hitchcock, J. Electron Spectrosc. Relat. Phenom. 25 (1982) 245. [2] W. Eberhardt, E.W. Plummer, I.W. Lyo, R. Carr, W.K. Ford, Phys. Rev. Lett. 58 (1987) 207. [3] R. Murphy, W. Eberhardt, J. Chem. Phys. 89 (1988) 4054. [4] J.H.D. Eland, in: C.Y. Ng (Ed.), Vacuum Ultraviolet Photoionization and Photodissociation of Molecules and Clusters, World Scientific, Singapore, 1991, p. 297, and references therein. [5] J.H.D. Eland, Mol. Phys. 61 (1987) 725.
83
[6] S. Hsieh, J.H.D. Eland, J. Phys. B: At. Mol. Opt. Phys. 30 (1997) 4515. [7] I. Nenner, P. Morin, in: U. Becker, D.A. Shirley (Eds.), VUV and Soft X-Ray Photoionization, Plenum Press, New York, 1996, p. 291. [8] S. Hsieh, J.H.D. Eland, Rapid Commun. Mass Spectrom. 9 (1995) 1261. [9] A.P. Hitchcock, C.E. Brion, J. Electron Spectrosc. Relat. Phenom. 13 (1978) 193. [10] A.P. Hitchcock, M. Pocock, C.E. Brion, M.S. Banna, D.C. Frost, C.A. McDowell, B. Wallbank, J. Electron Spectrosc. Relat. Phenom. 13 (1978) 345. [11] H. Friedrich, B. Pittel, P. Rabe, W.H.E. Schwarz, B. Sonntag, J. Phys. B 13 (1980) 25. [12] W.H.E. Schwarz, R.J. Buenker, Chem. Phys. 13 (1976) 153. [13] N. Saito, I.H. Suzuki, Int. J. Mass Spectrom. Ion Process 82 (1988) 61. [14] N. Saito, I.H. Suzuki, Int. J. Mass Spectrom. Ion Process 115 (1992) 157. [15] B.L. Henke, Nucl. Instrum. Meth. 177 (1980) 161. [16] A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [17] A.D. Becke, Phys. Rev. A 38 (1988) 3098. [18] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [19] M. Head-Gordon, J.A. Pople, M.J. Frisch, Chem. Phys. Lett. 153 (1988) 503. [20] M.J. Frisch, M. Head-Gordon, J.A. Pople, Chem. Phys. Lett. 166 (1990) 281. [21] S. Saebo, J. Almlof, Chem. Phys. Lett. 154 (1989) 83. [22] H. Thomassen, K. Hedberg, J. Mol. Struct. 240 (1990) 151. [23] M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T. Keith, G.A. Petersson, J.A. Montgomery, K. Raghavachari, M.A. AlLaham, V.G. Zakrzewski, J.V. Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Replogle, R. Gomperts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. Defrees, J. Baker, J.P. Stewart, M. Head-Gordon, C. Gonzalez, J.A. Pople, Gaussian 94, Revision D.2, Gaussian, Inc, Pittsburgh, PA, 1995. [24] W.B. Perry, W.L. Jolly, Inorg. Chem. 13 (1974) 1211. [25] A.A. Bakke, H.-W. Chen, W.L. Jolly, J. Electron Spectrosc. Relat. Phenom. 20 (1980) 333. [26] J.A. Bearden, A.F. Burr, Rev. Mod. Phys. 39 (1967) 125. [27] J.P. Connerade, J.M. Esteva, R.C. Karnatak (Eds.), Giant Resonances in Atoms, Molecules, and Solids, Plenum Press, New York, 1986. [28] U. Fano, J.W. Cooper, Rev. Mod. Phys. 40 (1968) 441. [29] S.T. Manson, Adv. Electron. Electron Phys. 41 (1976) 73. [30] B.H. Boo, Z. Liu, S.Y. Lee, I. Koyano, J. Phys. Chem. 102 (1998) 8261. [31] B.H. Boo, I. Koyano, The photoionization studies are under way. [32] T.N. Olney, C.E. Brion, T. Ibuki, Chem. Phys. 201 (1995) 505. [33] P. Streubel, R. Franke, T. Chasse, R. Fellenberg, R. Szargan, J. Electron Spectrosc. Relat. Phenom. 57 (1991) 1. [34] C.E. Moore, Atomic Energy Levels, National Standards Reference Data Series, National Bureau of Standards, 1971.
84
B.H. Boo et al. / Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 73 – 84
[35] G.G.B. de Souza, P. Morin, I. Nenner, Phys. Rev. A: Gen. Phys. 34 (1986) 4770. [36] P. Morin, G.G.B. de Souza, I. Nenner, P. Lablanquie, Phys. Rev. Lett. 56 (1986) 131.
[37] B.H. Boo, S.Y. Lee, H. Kim, I. Koyano, J. Phys. Chem. 100 (1996) 523. [38] T. Imamura, C.E. Brion, I. Koyano, T. Ibuki, T. Masuoka, J. Chem. Phys. 94 (1991) 4936.
Dissociation processes of core-excited CBr 4 involving the Br(3d, 3p, 3s) and C(1s) inner-shells in the range 50–460 eV a, b b c Bong Hyun Boo *, Norio Saito , Isao H. Suzuki , Inosuke Koyano a
Department of Chemistry, Chungnam National University, Daejeon 305 -764, South Korea b Electrotechnical Laboratory, 1 -1 -4, Umezono, Tsukuba-shi, Ibaraki 305, Japan c Department of Material Science, Himeji Institute of Technology, 1479 -1 Kanaji, Kamigohri, Hyogo 678 -12, Japan Received 10 July 2001; received in revised form 5 December 2001; accepted 4 January 2002
Abstract The dissociative multiple photoionization of CBr 4 in the valence, and in the Br(3d, 3p, 3s) and C(1s) inner shell regions has been studied by using time-of-flight (TOF) mass spectrometry coupled to synchrotron radiation in the range 50–460 eV. Total and individual photoion yield curves have been recorded as a function of the photon energy. Several discrete resonances are observed below the Br(3d) threshold region. Also a giant shape resonance in the total ion yield curve is observed beyond the Br(3d) region, which can be identified as a shape resonance owing to the diffusion of the outgoing photoelectron by the angular momentum barrier in the 3d 9 ef outgoing channel. Various monocations of Br 1 and CBr n1 (n50–3) are detected along with dications of Br 21 and CBr 21 in the whole energy range examined. Dissociation processes 3 of multiply charged CBr 4 parent ions have also been investigated by photoelectron–photoion coincidence (PEPICO) and photoion–photoion coincidence (PIPICO) methods. The dominant dissociation pattern is found to involve C 1 –Br 1 and Br 1 –Br 1 in the whole energy range examined. Distinct energy dependence of dissociation processes is observed in the Br(3p) and Br(3s) regions. With the help of ab initio HF / 6-31111G(d) calculation, we estimated the photoabsorption positions and symmetries for the discrete core-excited states using the Z 1 1 equivalent ionic core model. The specific excitation and dissociation of molecules provides information on energy dissipation processes in conjunction with the relevant core hole states. 2002 Elsevier Science B.V. All rights reserved. Keywords: Carbon tetrabromide; Br core excitation; Multiple ionization; PEPICO; PIPICO
1. Introduction Inner-shell spectroscopy is an attractive tool to investigate atomic and electronic structures of molecules [1]. Owing to the localization of the core hole, site-selective photochemistry is observed to some *Corresponding author. Tel.: 182-42-821-6551; fax: 182-42823-1360. E-mail address: [email protected] (B.H. Boo).
extent, the degree of the selectivity relying on the competitive processes of electronic relaxation and bond cleavage. It is shown in the literature that Auger-electron–ion coincidence studies could reveal the individual decay channels for the various doubly charged ionic configurations populated in the Auger decay of the core hole [2,3]. These studies also enable us to correlate the ion fragmentation products, their kinetic energy, and their electronic configuration with the hole configuration in the doubly
0368-2048 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 02 )00005-1
74
B.H. Boo et al. / Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 73 – 84
charged molecule ion [2]. By employing the coincidence method, it is also possible to elucidate the effect of the core hole decay on the site-selectivity in the bond fragmentation. In addition, studies of the fragmentation pathways of dissociative multiple ionization involve a variety of coincidence methods such as photoelectron– photoion (PEPICO), photoion–photoion (PIPICO), photoelectron–photoelectron (PEPECO), and photoelectron–photoion–photoion (PEPIPICO, PE2PICO) coincidence [4]. Among these, the triple coincidence PEPIPICO technique was shown to be a powerful technique to elucidate the detailed dynamics in dissociative double ionization processes [5–7]. Recent development of position-sensitive ion detector in the PEPIPICO methods makes it possible to investigate the detailed dynamics in the dissociation of doubly charged ion [6,8]. Until now, however, the Auger-electron–ion coincidence and PEPIPICO methods are almost restricted to the determination of the three-body dissociation dynamics of relatively small dications. Hitchcock et al. [9] have investigated inner shell excitation in bromine-containing molecules such as methyl bromide and bromobenzene [10] via electron energy loss spectroscopy and then assigned inner shell excitation such as Br(3d)→s(a 1 )* and Br(3d)→5p(a,e) for methyl bromide, and Br(3d)→p(b 2 )* and Br(3d)→5p for bromobenzene. To our best knowledge, however, no photoabsorption data of CBr 4 in the Br(M) region have been reported. Among various molecules, CBr 4 is of special interest because of the following points: the molecule has four equivalent Br atoms in it, and thus we could investigate the effect of tetrahedrally oriented Br ligands on the giant resonance caused by continuum enhancement corresponding to 3d→ef; it has two different atoms, and we could compare dissociation patterns relying on the core excitation tuning the C and Br atoms. In this report, we present our recent experimental and theoretical results of the photoionization mass spectra measured both in the simple coincidence modes such as PEPICO and PIPICO modes, in the Br(M) and C(1s) region from 50 to 460 eV. The photoabsorption positions and symmetries for the core–hole states were roughly estimated by the equivalent ionic core valence orbital model (EIC-
VOM) calculation [11,12]. Various dissociation patterns of the multiply charged parent ions are proposed on the basis of the measurements of the ion TOF differences in the PIPICO mode. Comparison of the dissociation patterns was made in the Br(3d, 3p, 3s) and C(1s) core excitation regions. The specific excitation and dissociation of molecule provides information on energy dissipation processes of the core-excited states.
2. Theoretical and experimental Monochromatic vacuum ultraviolet and soft Xrays were obtained by dispersing synchrotron radiation from the TERAS storage ring at the Electrotechnical Laboratory using a Grasshopper monochromator. The principle and construction of the whole apparatus have been described in detail elsewhere [13,14] and thus only briefly described here. The monochromatized photons of 100–300 eV, with photon energy resolution of about 800–300, respectively, were obtained from a Grasshopper monochromator with a grating of 1200 l / mm. The monochromatized photons pass through two differential pumping stages and entered an ionization chamber equipped with a TOF spectrometer. Here an electric field of 175 V/ cm was applied to the ionization chamber for ion and electron extraction. The main chamber where the ionization cell, a photoelectron detector and the TOF spectrometer were equipped was evacuated down to ¯1310 28 Torr. When the CBr 4 gas molecules were introduced into the ionization cell, the pressure of the main chamber was maintained at ¯7310 26 Torr. The flight path length was 13 cm and the mass detection angle was perpendicular to the direction of incident photon beam and parallel to the photon beam polarization. The uncertainty in the photon beam energy lies within 60.1 eV. The total ion yield spectrum was obtained by recording the count rates of the total ions and the photon flux simultaneously while the photon wavelength is scanned. For characterization of individual ions and ion pairs, the time-correlated ion counting technique was used in the two modes of PEPICO and PIPICO, respectively. The PEPICO mode utilizes the start pulses for a time-to-amplitude converter (TAC)
B.H. Boo et al. / Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 73 – 84
provided by the photoelectron arrival signals at a microchannel plate (MCP), sampled from the collision chamber in the opposite direction to the ion flight direction, and the stop pulses generated by the photoion detection signals at another MCP located at the end of the TOF tube. However, the PIPICO mode uses the start pulses provided by the lighter ions, and the stop pulses generated by the heavier ions of the counter parts. Similarly the total PIPICO yield spectrum was obtained by measuring the count rates of total PIPICO simultaneously while the photon wavelength is scanned. For the measurements of the PIPICO count rates, the coincidence time range (gate width in TAC) was set to be 0–5 ms, because the TOF difference between any pairs of ions formed from CBr 4 is found to fall into this time range. The individual ion and PIPICO yields at a photon energy were directly obtained by measuring the integrated individual ion and PIPICO intensities, and by simultaneously measuring the photon flux while the photon wavelength is scanned. The corrected photon intensities were obtained by dividing the measured photoemission yields from a gold mesh photocathode placed between the exit slit of the monochromator and the experimental main chamber by the published photoemission efficiency of a gold photocathode [15]. Relative uncertainties in the normalized ion and PIPICO yields may mainly come from this normalization process and could be estimated as less than ¯10%. The sample CBr 4 with a nominal purity of 199% was purchased from Aldrich Chemical Co. and used without further purification. The geometry of CBr 4 was optimized by the HF / 6-3111 1G(d), B3LYP DFT / 6-3111 1G(d) [16–18], and MP2 / 6-3111 1G(d) [19–21] methods for comparison with the experimental geometry [22]. The HF / 6-3111 1G(d) calculation on the unoccupied orbital energies of the Br core-excited state was performed on CBr 3 Kr 1 at the experimental ˚ Simply geometry of CBr 4 [22], (r51.94260.002 A). the negative values of the resulting unoccupied orbital energies were taken in this study as the term values of CBr 4 . From these values, we roughly estimate the photoabsorption positions corresponding to the Br(3d 5 / 2 , 3p 3 / 2 , 3s) excitation by combining the computed term values, the experimental ionization limits for the Br core electrons, and the spin–
75
orbit splitting. The calculations were carried out using the Gaussian 94 program suite [23] installed at KISTI in Korea.
3. Results and discussion We used a moderately low extraction field (175 V/ cm). The use of the low field enabled us to better evaluate the peak broadening owing to kinetic energy release in the dissociation event. However, a use of too low field can decrease the detection sensitivity owing to a fast departing flight via a strong repulsion between ionic fragments. The total ion yields as a function of energy are presented in Fig. 1. It is noticed that some of the literature values for the thresholds and estimated thresholds for the various core electrons are indicated with hatched lines over the yield curves, the reported thresholds of the core electrons for CBr 4 are: Br(3d 5 / 2 )576.7460.05 eV [24,25]; C(1s)5 294.6460.05 [24,25]. Those for atomic bromine are: Br(3d 5 / 2 )569.060.4 eV [26]; Br(3d 3 / 2 )570.160.4 eV [26]; Br(3p 3 / 2 )5181.560.4 eV [26]; Br(3p]h1 / 2j) 5 189.360.4 eV [26]; Br(3s)5256.560.4 eV [26]. Fig. 2 compares the total ion yield curve of CBr 4 with those of bromine-containing compounds. The total PIPICO yield spectrum is shown in Fig. 3. The energy dependence of the total PIPICO yield is similar to that of the total ion yield. This implies that dissociative multiple ionization is the main exit channel in the whole energy examined. It seems puzzling, however, that in the region from 145 to 260 eV, the total ion yield increases monotonously with increasing the energy, whereas the total PIPICO yield decreases. This may come from one possibility that in the Br(3p, 3s) threshold regions, locally ionized bromine may depart from the parent molecule with neutral counterpart. In this case, the total PIPICO intensity could decrease. In an attempt to elucidate the contributions of the total ion yields, we measured typical mass spectra in the PEPICO mode at some critical resonance and off resonance regions. Fig. 4 presents the PEPICO (mass) spectra recorded at 64.5, 70.0, 75.5, 84.5, 149.5, and 406.4 eV. It is noticed, however, that the photoelectrons in the PEPICO mode as the start
76
B.H. Boo et al. / Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 73 – 84
Fig. 1. Normalized total photoion yield curve of CBr 4 in the range 50–460 eV. The inset shows the scale-expanded spectrum in the Br(3d) excitation region.
pulse for TAC are not energy-selected. Below 149.5 eV, almost all the coincidence spectra exhibit a similar pattern. At 406.4 eV, however, the ion intensity of doubly charged species Br 21 is greatly enhanced. In Fig. 5, are presented the relative individual ion yields derived from the integrated ion counts shown in Fig. 4 and the photon intensities as a function of energy. It can be seen that energy variation of the total ion yield (sum of the individual ion yields, Fig. 5) is similar to that of total ion yield (Fig. 1) measured independently. Fig. 6 shows typical PIPICO spectra measured at 64.5, 70.0, 75.5, 84.5, 149.5, and 406.4 eV. Energy variation of the individual PIPICO yields is presented in Fig. 7. In the whole energy range, the most prominent PIPICO signal corresponds to Br 1 –Br 1 . At a higher energy (406.4 eV), the PIPICO peaks corresponding to Br 1 –CBr n1 (n51–3) are significantly diminished owing to near the total destruction of the four bonds. However, the relative PIPICO intensities for Br 21 –Br 1 and C 1 –Br 21 are greatly enhanced at this energy. It is interesting that any channel such as Br 21 –CBr n1 (n51,2,3) is not observed, which implies that in the core excitation
event, localization of the core hole is not preserved absolutely at the specific Br core, instead the rapid redistribution of excess internal energy and charge occurs yielding many singly charged atomic ions. Above the C(1s) threshold, the total ion and PIPICO yields decrease with increasing the photon energy, which might be a property of the oscillator strength of the molecule.
3.1. Core excitation spectra 3.1.1. Excitation of Br(3 d) electrons Several discrete resonances are observed over the broad structureless background resonance, which spans the whole energy range examined. The huge background resonance (giant resonance) can be identified as a shape resonance owing to the centrifugal barrier in the 3d 9 ef outgoing channel. This is responsible for the delayed onset and for the enhancement of the photionization cross section [27]. Therefore, the repulsive force is increasingly important with increasing the angular momentum quantum number , [28,29]. This spectral behavior has also been observed in the photoionization of Br-
B.H. Boo et al. / Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 73 – 84
Fig. 2. Comparison of normalized total photoion yield curve of MBr 4 (M5C, Si, Ge), Br 2 , and (CH 3 ) 3 SiBr.
containing compounds such as SiBr 4 [30], Br 2 [31], GeBr 4 [31], and (CH 3 ) 3 SiBr [31] as shown in Fig. 2. The energy range of the giant resonance corresponding to the Br(3d) core ionization is remarkably narrower in the Br 2 and (CH 3 ) 3 SiBr molecules than those in the other molecules (Fig. 2). Energy dependence of the giant spectral shape is quite similar to those observed in MBr 4 (M5Si, Ge) [31] as seen in Fig. 2. This affirms that the giant shape resonance arises solely from the bromine atoms. Apparently, several discrete excitation peaks below the Br(3d) edge (Fig. 1) consist of two doublet peaks at ¯70 and ¯75 eV, having a peak interval of ¯0.9 eV. Notice that the spin–orbit splitting between the Br(3d 5 / 2 ) and Br(3d 3 / 2 ) components was observed with an energy difference of
77
¯1.0 eV [9,10,30,32]. All the previous studies of bromine-containing molecules show that the transition of Br(3d) core electron to the first antibonding orbital occurs when the incident photon energy reached ¯70 eV, the value being more or less shifted with different ligands. It is accepted as a universal rule that the chemical shifts of photoelectron and Auger lines are linearly dependent on the average differences in Pauling’s electronegativities to the nearest neighbors, D x¯ P [33]. Based on the previous assignments of the Br(3d) core excitation in methyl bromide [9] and bromobenzene [10], the doublet peak at ¯70 eV in CBr 4 may correspond to Br(3d)→s(a 1 )* and the other doublet peak at ¯75 eV may be attributed to Br(3d)→5p(e). In order to help assign the transition observed in CBr 4 , we carried out ab initio calculation at the HF / 6-3111 1G(d) level on CBr 3 Kr 1 , which could be a counterpart of the Br(3d) core-excited CBr 4 in the view of equivalent ionic core virtual orbital model (EICVOM) [11,12]. The optimized geometries determined by the HF / 6-3111 1G(d) and MP2 / 6-3111 1G(d) calculations (Table 1) are quite consistent with the experimental geometry of CBr 4 [22], the term values were derived by a single point calculations on the experimental geometry at the HF / 6-3111 1G(d) level. The excitation energy corresponding to the Br(3d 5 / 2 )→s(a 1 )* derived using the calculated term values listed in Table 2 is 71.6 eV, in fair agreement with the experimental value of 69.6 eV as shown in Fig. 1. The discrepancy reflects that the simple HF / 6-3111 1G(d) calculation using the EICVOM model may not estimate the term values accurately or that the Br(3d) electrons cannot efficiently shield the nuclear charge owing to the extensive d character. Actually the term values should be estimated correctly by the separate calculations on one of the initial state and one of the final state. Therefore it is presumed that the HF calculation on CBr 3 Kr 1 could lead to only rough estimation of the photoabsorption position and the mode of the core hole state. The first two peaks at 69.6 and 70.5 eV in Fig. 1 may correspond to the transition from the Br(3d 5 / 2 ) and Br(3d 3 / 2 ) orbitals to the a 1 antibonding orbital, respectively, the arguments based on the following observations and calculation. (1) The peak shapes and splitting are almost the same as those in the
78
B.H. Boo et al. / Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 73 – 84
Fig. 3. Normalized photoion–photoion coincidence yield curve of CBr 4 in the range 50–450 eV. The inset shows the scale-expanded spectrum in the Br(3d) excitation region.
electron energy loss spectra of methyl bromide [9] and bromobenzene [10]. The spin–orbit splitting ¯1.0 eV owing to the Br(3d 3 / 2,5 / 2 ) core electrons was observed, being in excellent agreement with the reported splitting value of ¯1.0 eV [9,10,30,32]. (2) The ab initio calculation indicates that there is a transition with a 1 symmetry occurring first in the Br(3d) excitation region.
3.1.2. Excitation of Br(3 p, 3 s) electrons The binding energies of the Br(3p, 3s) electron in CBr 4 are not available in the literature, but we can estimate the threshold energies of Br(3p 3 / 2 ) as 189.2 eV, the value determined by adding the chemical shift difference (7.72 eV) between the Br(3d 5 / 2 ) IP in Br free atom and that in CBr 4 to the Br(3p) IP of Br free atom. We also estimate the IP for the Br(3s) as 264.2 eV in the similar fashion. The estimated thresholds are marked as hatched lines over the various yield curves. As seen in Figs. 1 and 3, significant discrete features in the total ion and PIPICO yields are not observed in the Br(3p) and Br(3s) regions. However, in the partial ion yield spectra of C 1 and Br 21 (Fig. 5) and the PIPICO yield spectra of Br 21 –Br 1 and C 1 –Br 21 (Fig. 7),
two distinct features are clearly observed by clearly demonstrating the rises in the relative PIPICO yield spectra from the Br(3p) and Br(3s) edges. How can we justify the finding that Br 21 is formed more abundantly than C 21 even though the first (11.264 eV) [34] and 2nd (24.376 eV) [34] IPs of carbon atom in free atoms are close to those (first IP511.84 eV [34] and 2nd IP521.6 eV [34]) of bromine atom in free atoms? Why does the ion yield spectrum of Br 21 show no significant feature (the inset in Fig. 5) in the Br(3d) excitation / ionization region? Why does the local multiple ionization in the specific Br atom not occur in the Br(3d) region? This is accounted for by the MO concept in which the Br(3d) electrons are partly delocalized in the whole MO. Therefore even when the Br(3d) electrons are multiply ejected, charge redistribution in the whole molecule quickly occurs contributing to the decay of the doubly and triply charged parent ions into several singly charged product ions. However, the Br(3p, 3s) electrons are relatively more localized in the specific Br core atom, thus upon the ejection of the core electrons, charge and energy localization could partially remains to some extent in the specific core yielding the doubly charged bromine ions. However,
B.H. Boo et al. / Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 73 – 84
79
delayed excitation of Br(M), we could not isolate the C(1s) excitation spectrum. Also the abundance of the bromine atoms in a given molecule diminishes the contribution of the C(1s) core excitation spectrum. At 406.4 eV, the much higher energy than the Br(3p, 3s) and C(1s) thresholds, the relative intensities C 1 and Br 21 are significantly enhanced. However, C 21 is very slightly formed in the whole energy region examined. This indicates that the excess charge transfer from the Br atom to the central carbon atom is not feasible presumably because a fast dissociation could occur prior to charge redistribution in the multiply charged precursor ions, or multiple Auger processes may take place after the bond cleavage involved. In previous studies for core excitation of silane, de Souza et al. [35] proposed a two-step decay process for a valence resonance, namely a fast dissociation followed by the autoionization of the excited fragment, on the basis of the observation of Br(3d) core excitation of HBr [36].
3.2. Dissociation patterns
Fig. 4. Photoelectron–photoion coincidence (mass) spectra of CBr 4 measured at various photon energies.
it is not reasonable to justify the mechanism of a fast dissociation followed by local electronic relaxation by using only our simple PEPICO and PIPICO techniques because the electron kinetic energies are not specified for the photoelectron signals for this PEPICO experiment. For clarification of the mechanism, one needs elegant experiments such as an Auger electron–ion coincidence.
3.1.3. C(1 s) excitation Around the C(1s) threshold, significant feature in the ion and PIPICO yield spectra (Figs. 1 and 3) is not observed. However, because the resonance via the C(1s) core excitation is overlapped with the
A hole in the Br(M) shell resulting from excitation of an Br(M) electron may electronically relax by emitting Auger electrons before CBr 4 molecule begins to fragment. However, fast dissociation could also occur prior to the electronic relaxation, which explains the multiple ionization of a Br atom. Any dominant relaxation processes are expected to mainly occur via multiple-resonant Auger processes, which can involve the ejection of two electrons or sometimes, of more electrons [37,38]. When a Br(M) core electron is initially excited to a virtual orbital (one hole and one excited electron), CBr 4 will be left as a doubly charged ion owing to these double-resonant Auger processes. Various ion pairs are expected to arise from the bond destruction, of such doubly charged parent ion or fragment ions, because in polyatomic molecules in general, there are several possible combinations of valence MOs in which two holes are produced, thus giving several different fragmentation pathways. It was shown previously in the core excitation spectroscopy of N 2 O that the site specificity is not absolutely confined to breaking only the O–NN bond, even though the excitation of a core hole localized at the O site in the molecule in the event of
80
B.H. Boo et al. / Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 73 – 84
Fig. 5. Individual photoion yield spectra of CBr 4 in the range 65–460 eV. The spectral intensities (Iphotoion /Iphoton ) are presented on the same relative intensity scale. The inset shows the scale-expanded spectra in the Br(3d) excitation region.
initial absorption [2]. Even though this localization is to some extent preserved in the Auger decay, which favors the participation of valence electrons having wave functions with a large overlap at the site of the initial O core hole, some of these electrons are also crucial in forming the ON–N bond such that removal of these electrons in the Auger decay leads to the total destruction of the bonds in the molecule. But in our system, a total destruction followed by the local excitation tuning at the Br core excitation could occur to some extent. The localization of the core hole is more probable when the Br(3p, 3s) is excited than when the Br(3d) is. Processes leading to the ion pairs, Br 1 –Br 1 , Br 1 – 1 1 CBr 1 , Br 21 –Br 1 , C 1 –Br 21 , Br 1 –CBr 1 2 , C –Br , 1 1 and Br –CBr 3 are observed to occur in the whole range examined. Obviously, the ion pairs, C 1 –Br 1
and Br 1 –Br 1 are most abundant in the whole energy range examined as seen in Figs. 6 and 7. It should be noticed that the PIPICO yield of Br 1 –Br 1 cannot be estimated accurately owing to some interference effect in the pulse-processing. According to the seven ion pairs detected in the PIPICO spectra and their energy profiles, we can propose the dissociation channels of the core excited / ionized CBr 4 . In the Br(3d) giant excitation region, it is shown that the PIPICO yield for Br 1 –CBr n1 (n51–3) is significantly enhanced (Fig. 7) presumably due to process (1): CBr 4 1 hn → CBr 3 –Br * → f CBr 3 –Br g 21 1 1 2e → CBr 1 n (n 5 0–3) 1 Br 1 NP 1 2e
(1)
NP denotes neutral product(s). Why then do we not
B.H. Boo et al. / Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 73 – 84
81
whole energy range. It is noticed that the ion yield ratio for Br 1 is much larger than those for its possible ionic counterparts such as C 1 , CBr 1 , and so forth, in the dissociation of the multiply charged parent ion. Because for example, the intensity ratio IBr 1 /IC 1 is 4.960.8, the simple dissociation process, reaction (1) cannot explain the large intensity ratio. Thus we present processes (2)–(4) (nearly total destruction) as the most contributing processes to explain the ion yield ratios for the observed ionic species: CBr 4 1 hn → f CBr 431 g * 1 3e → CBr n1 (n 5 0–2) 1 1
2Br 1 (2 2 n)Br 1 3e
(2)
CBr 4 1 hn → f CBr 441 g * 1 4e → CBr n1 (n 5 0,1) 1 1
3Br 1 (1 2 n)Br 1 4e
(3)
CBr 4 1 hn → f CBr 451 g * 1 5e → C 1 1 4Br 1 1 5e (4) This argument is based on the slight ionization energy difference of C and Br atoms. It is presumed that carbon or carbon-containing ions are formed simultaneously together with the Br 1 –Br 1 ion pair. But we cannot rule out other possibility such as the occurrence of process (5) to account for the abundant formation of the Br 1 ion in comparison with CBr n1 (n51–3): Fig. 6. Photoion–photoion coincidence (PIPICO) spectra of CBr 4 measured at various photon energies.
CBr 4 1 hn → f CBr 421 g * 1 2e → 2Br 1 1 CBr n (n 5 0–2) 1 (2 2 n)Br 1 2e
observe the enhancement of the ion yield ratios for CBr 1 n (n51–3) in the Br(3p) and Br(3s) regions? In these regions, much higher energy could reside to some extent in the specific Br atom, which favors the participation of valence electrons having wave functions with a large overlap at the site of the initial Br core hole, some of these electrons play a decisive role in forming the C–Br bond such that loss of these electrons in the Auger decay leads to the total bond destruction in the molecule. It is elucidated that such simple decomposition as shown in process (1) cannot explain the formation of Br 1 ion with 7264% in the total intensity in the
(5)
Some minor channels such as process 6 should occur above the Br(3p) edges to explain the observation of the ion pair such as Br 21 –Br 1 and C 1 –Br 21 since any combination of the three ions in reaction (6) is observed in the PIPICO spectra (Fig. 6): CBr 4 1 hn → Br * –CBr 3 → Br * –CBr 41 3 1 4e → C 1 1 Br 21 1 Br 1 1 2Br
(6)
The occurrence of the channels (2)–(6) indicates that triple, quadruple and quintuple ionization could occur in the photoionization of CBr 4 in the range 50–460 eV.
B.H. Boo et al. / Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 73 – 84
82
Fig. 7. Individual photoion–photoion coincidence (PIPICO) yield spectra in the range 65–400 eV. The spectral intensities (IPIPICO /Iphoton ) are presented on the same relative intensity scale. The inset shows the scale-expanded spectra in the Br(3d) excitation region.
4. Conclusions The present coincidence spectroscopic study led to the detection of various monocations of Br 1 and 21 CBr 1 and n (n50–3) along with dications of Br Table 1 Optimized structures of CBr 4 determined by the ab initio and density functional calculations using a 6-3111 1G(d) biases set and their comparison with the experiment Parameter
C–Br a
˚ Value (A) HF
B3LYP DFT
MP2 a
Exp b
1.939
1.965
1.947
1.94260.002
Core electrons were excluded for electron correlation calculation. b From Ref. [22].
CBr 21 in the energy range 50–460 eV. Among the 3 ions, C 1 and Br 1 are predominantly observed in the whole energy range examined. In the total and
Table 2 Term values (eV) of CBr 3 Kr 1 determined by the HF calculation using a 6-3111 1G(d) basis set at the experimental geometry of CBr 4 [22] Symmetry
Term values (eV)
a1 a1 e a1 e a1 e a1
5.11 2.66 2.20 2.09 1.23 1.21 0.29 0.11
B.H. Boo et al. / Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 73 – 84
PIPICO yield curves, we have observed a prominently broad feature above the Br(3d) edge due to their enhanced oscillator strength, which originates from the diffusion of the outgoing photoelectron by the centrifugal barrier of the effective 3d potential. We have assigned the discrete resonance energies below the Br(3d) ionization edges by using the Z 1 1 equivalent ionic core approximation method and then by performing the HF / 6-3111 1G(d) calculation on the equivalent core CBr 3 Kr 1 at the experimental geometry of CBr 4 . The photoejection of the Br(3d) electrons followed by the Auger decay may end up with the observation of ion pairs involving singly charged ions because of the rapid charge spread along the molecule owing to the partial delocalization property of the 3d electron. However, the Br(3p, 3s) core excitation led to the formation of ion pairs such as Br 21 –Br 1 and C 1 –Br 21 in enhanced yields, implying that local multiple ionization of the specific excited bare Br atom or Br 1 ion occurs following a fast dissociation of the bond between the central carbon atom and the core-excited Br atom.
Acknowledgements This work was partially supported by the Korea Research Foundation through an non-directed research project (1998–1999). The staffs of the accelerator group of the Electrotechnical Laboratory in Tsukuba, Japan are greatly acknowledged for the use of the synchrotron radiation facility. The computations were carried out by using the CRAY C90 of the KISTI Supercomputer Center in South Korea, which is gratefully acknowledged.
References [1] A.P. Hitchcock, J. Electron Spectrosc. Relat. Phenom. 25 (1982) 245. [2] W. Eberhardt, E.W. Plummer, I.W. Lyo, R. Carr, W.K. Ford, Phys. Rev. Lett. 58 (1987) 207. [3] R. Murphy, W. Eberhardt, J. Chem. Phys. 89 (1988) 4054. [4] J.H.D. Eland, in: C.Y. Ng (Ed.), Vacuum Ultraviolet Photoionization and Photodissociation of Molecules and Clusters, World Scientific, Singapore, 1991, p. 297, and references therein. [5] J.H.D. Eland, Mol. Phys. 61 (1987) 725.
83
[6] S. Hsieh, J.H.D. Eland, J. Phys. B: At. Mol. Opt. Phys. 30 (1997) 4515. [7] I. Nenner, P. Morin, in: U. Becker, D.A. Shirley (Eds.), VUV and Soft X-Ray Photoionization, Plenum Press, New York, 1996, p. 291. [8] S. Hsieh, J.H.D. Eland, Rapid Commun. Mass Spectrom. 9 (1995) 1261. [9] A.P. Hitchcock, C.E. Brion, J. Electron Spectrosc. Relat. Phenom. 13 (1978) 193. [10] A.P. Hitchcock, M. Pocock, C.E. Brion, M.S. Banna, D.C. Frost, C.A. McDowell, B. Wallbank, J. Electron Spectrosc. Relat. Phenom. 13 (1978) 345. [11] H. Friedrich, B. Pittel, P. Rabe, W.H.E. Schwarz, B. Sonntag, J. Phys. B 13 (1980) 25. [12] W.H.E. Schwarz, R.J. Buenker, Chem. Phys. 13 (1976) 153. [13] N. Saito, I.H. Suzuki, Int. J. Mass Spectrom. Ion Process 82 (1988) 61. [14] N. Saito, I.H. Suzuki, Int. J. Mass Spectrom. Ion Process 115 (1992) 157. [15] B.L. Henke, Nucl. Instrum. Meth. 177 (1980) 161. [16] A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [17] A.D. Becke, Phys. Rev. A 38 (1988) 3098. [18] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [19] M. Head-Gordon, J.A. Pople, M.J. Frisch, Chem. Phys. Lett. 153 (1988) 503. [20] M.J. Frisch, M. Head-Gordon, J.A. Pople, Chem. Phys. Lett. 166 (1990) 281. [21] S. Saebo, J. Almlof, Chem. Phys. Lett. 154 (1989) 83. [22] H. Thomassen, K. Hedberg, J. Mol. Struct. 240 (1990) 151. [23] M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T. Keith, G.A. Petersson, J.A. Montgomery, K. Raghavachari, M.A. AlLaham, V.G. Zakrzewski, J.V. Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Replogle, R. Gomperts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. Defrees, J. Baker, J.P. Stewart, M. Head-Gordon, C. Gonzalez, J.A. Pople, Gaussian 94, Revision D.2, Gaussian, Inc, Pittsburgh, PA, 1995. [24] W.B. Perry, W.L. Jolly, Inorg. Chem. 13 (1974) 1211. [25] A.A. Bakke, H.-W. Chen, W.L. Jolly, J. Electron Spectrosc. Relat. Phenom. 20 (1980) 333. [26] J.A. Bearden, A.F. Burr, Rev. Mod. Phys. 39 (1967) 125. [27] J.P. Connerade, J.M. Esteva, R.C. Karnatak (Eds.), Giant Resonances in Atoms, Molecules, and Solids, Plenum Press, New York, 1986. [28] U. Fano, J.W. Cooper, Rev. Mod. Phys. 40 (1968) 441. [29] S.T. Manson, Adv. Electron. Electron Phys. 41 (1976) 73. [30] B.H. Boo, Z. Liu, S.Y. Lee, I. Koyano, J. Phys. Chem. 102 (1998) 8261. [31] B.H. Boo, I. Koyano, The photoionization studies are under way. [32] T.N. Olney, C.E. Brion, T. Ibuki, Chem. Phys. 201 (1995) 505. [33] P. Streubel, R. Franke, T. Chasse, R. Fellenberg, R. Szargan, J. Electron Spectrosc. Relat. Phenom. 57 (1991) 1. [34] C.E. Moore, Atomic Energy Levels, National Standards Reference Data Series, National Bureau of Standards, 1971.
84
B.H. Boo et al. / Journal of Electron Spectroscopy and Related Phenomena 123 (2002) 73 – 84
[35] G.G.B. de Souza, P. Morin, I. Nenner, Phys. Rev. A: Gen. Phys. 34 (1986) 4770. [36] P. Morin, G.G.B. de Souza, I. Nenner, P. Lablanquie, Phys. Rev. Lett. 56 (1986) 131.
[37] B.H. Boo, S.Y. Lee, H. Kim, I. Koyano, J. Phys. Chem. 100 (1996) 523. [38] T. Imamura, C.E. Brion, I. Koyano, T. Ibuki, T. Masuoka, J. Chem. Phys. 94 (1991) 4936.