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Dissociative multiple photoionization of Br2, IBr, and I2 in the VUV and X-ray regions: a comparative study of the inner-shell processes involving Br(3d,3p,3s) and I(4d,4p,4s,3d,3p)
Journal of Electron Spectroscopy and Related Phenomena 127 (2002) 139–152 www.elsevier.com / locate / elspec
Dissociative multiple photoionization of Br 2 , IBr, and I 2 in the VUV and X-ray regions: a comparative study of the inner-shell processes involving Br(3d,3p,3s) and I(4d,4p,4s,3d,3p) a, b Bong Hyun Boo *, Norio Saito a
Department of Chemistry, Chungnam National University, Daejeon 305 -764, South Korea b Electrotechnical Laboratory, 1 -1 -4, Umezono, Tsukuba-shi, Ibaraki 305, Japan Received 4 January 2002; received in revised form 6 June 2002; accepted 20 June 2002
Abstract Dissociative multiple photoionization of the bromine, the iodine monobromide, and the iodine molecules in the Br(3d,3p,3s) and I(4d,4p,4s,3d,3p) inner-shell regions has been studied by using time-of-flight (TOF) mass spectrometry coupled to synchrotron radiation in the ranges of 90|978 eV for Br 2 , 60|133 eV for IBr, and 86|998 eV for I 2 . Total photoion and photoion–photoion coincidence (PIPICO) yields have been recorded as functions of the photon energy. Here, giant shape resonances have been observed beyond the thresholds of the inner-shells owing to the Br(3d 10 )→Br(3d 9 e f), I(4d 10 )→I(4d 9 e f), and I(3d 10 )→I(3d 9 e f) transitions. The dissociation processes of the multiply charged parent ions have also been evaluated from variations of photoelectron–photoion coincidence (PEPICO) and PIPICO spectra with the photon energy. From each Br(3p 3 / 2 ) (189.9 eV) and I(4p 3 / 2 ) threshold (129.9 eV), quintuple ionization of the molecules begins to play important roles in the photoionization, subsequently yielding ion pairs of X 31 –X 21 (X5Br, I). From the I(3d 5 / 2 ) threshold (627.3 eV), loss of six electrons from iodine molecule additionally begins to play a minor role in the multiple photoionization, giving rise to the formation of ion pairs of either I 31 –I 31 or I 41 –I 21 . A direct comparison of the strengths and the ranges of the I(4d) and Br(3d) giant resonances was successfully made from dissociative photoionization of IBr. Over the entire energy range examined, 60 , E , 133 eV, biased charge spread relevant to the specific core-hole states of IBr is observed, presumably reflecting the fact that charge localizes mostly in the excited atoms, which can be accounted for mainly by a two step decay via a fast dissociation followed by autoionization upon the VUV absorption. 2002 Elsevier Science B.V. All rights reserved. Keywords: Bromine; Iodine monobromide; Iodine; Core excitation; Giant resonance; Multiple ionization; PEPICO; PIPICO PACS: 32.80.Fb
1. Introduction Inner-shell electronic excitation has proven to be a *Corresponding author. Fax: 182-42-823-1360. E-mail address: [email protected] (B.H. Boo).
most useful tool for investigating geometric and electronic structures of molecules [1]. Site-selective fragmentation has been one of the main concerns in the specific inner-shell transitions, which largely relies on how long it takes for the localization of the core-hole state to reside in a specific atom and its
0368-2048 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 02 )00162-7
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neighboring atoms for specific bond breakage, competing with energy and charge spread process in the molecule. Therefore, the elucidation concerning charge and energy redistribution following the core excitation / ionization provides an insight into the energy dissipation and the charge delocalization processes, mainly involving the Auger process and bond cleavage of core-excited molecules. Studies of the fragmentation pathways in dissociative multiple ionization of a molecule involve a variety of coincidence methods, such as photoelectron–photoion (PEPICO), photoion–photoion (PIPICO), and photoelectron–photoion–photoion (PEPIPICO, PE2PICO) coincidence [2]. Among the various coincidence methods, the triple coincidence PEPIPICO method has proven to be a powerful tool to elucidate the dissociation dynamics in double photoionization processes [3–5]. The recent addition of position-sensitive ion detection technology to the existing PEPIPICO spectroscopic tool has enabled us to investigate angle-resolved product distribution in the dissociation of doubly charged ions by using a two-dimensional imaging process [4,6]. Until now, however, the PEPIPICO method has been almost completely confined to the investigation of the threebody dissociation dynamics of relatively small dications. In addition, Auger electron–ion coincidence studies can reveal the individual decay channels for the various doubly charged ionic configurations populated in the Auger decay of the core hole [7,8]. These studies also enable correlation of the ion fragmentation products, their kinetic energies, and their electronic configuration with the hole configuration in the doubly charged molecule ion [7]. 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. However, these studies are also restricted to the elucidation of dissociative photoionization of small molecules following core excitation / ionization. Inner-shell excitation of molecular bromine has been elucidated by electron impact [9], but photoionization studies of the molecule in the VUV and X-ray regions are very few [10]. However, atomic and molecular iodines have been subject of photoionization [11–15] and electron impact ionization [16] studies in the VUV and X-ray regions. The
theoretical [13] and experimental [15] investigation on the photoionization of iodine atom and its ion shows that the photoionization cross sections, in the energy range above the I(4d) threshold, are dominated by powerful giant resonances, which are similar to each other in shape and strength, but show some shift in the energy dependence of the photoionization cross section for atomic iodine and its several multiply charged ions. In the present study, we have elucidated the dissociation processes following the valence and inner-shell photoexcitation / photoionization of bromine and iodine molecules involving Br(3d,3p,3s) and I(4d,4p,4s,3d,3p) in the ranges of 90|978 eV for Br 2 and 86|998 eV for I 2 . The study emphasizes the elucidation of the strengths and the ranges of the giant resonances involving Br(3d), I(4d) and I(3d), as well as the insight into the competitive processes of electronic relaxation and photochemical degradation of the core-hole states. Owing to the limitation of the PIPICO method for investigating the homonuclear diatomic molecules, we have further elucidated dissociative photoionization of IBr, which provides estimates of the relative strengths and the ranges of the I(4d) and Br(3d) giant resonances, and also is quite informative as to the characteristic dissociation processes following the Br(3d) and I(4d) core-excitation.
2. Experimental The present experiments have been performed using two different synchrotron radiation facilities located at Electrotechnical Laboratory (ETL) in Tsukuba for the Br 2 and I 2 samples, and at the Institute for Molecular Science (IMS) in Okazaki for the IBr sample. Monochromatic vacuum ultraviolet and soft X-rays were obtained by dispersing synchrotron radiation from the TERAS storage ring at ETL using a Grasshopper monochromator. The principle and the construction of the whole apparatus have been described in detail elsewhere [17,18], and thus are only briefly described here. The monochromatized photons of from 100 to 300 eV have photon energy resolutions of about from 800 to 300 respectively, with a grating of 1200 l / mm. The monochromatized photons passed through two dif-
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ferential pumping stages and entered an ionization chamber equipped with a TOF spectrometer. Here, an electric field of 142.9 V/ cm was applied to the ionization chamber for ion and electron extraction. The main chamber in which the ionization cell, a photoelectron detector and the TOF spectrometer were located was evacuated down to ¯ 2310 26 Torr. When the gas molecules were introduced into the ionization cell, the pressure of the main chamber was maintained at ¯ 8310 26 Torr for Br 2 and ¯ 5310 26 Torr for I 2 . The flight path length was 13 cm, and the ion detection angle was perpendicular to the direction of incident photon beam and 558 to the photon beam polarization for the isotropic collection of fragment ions. 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 was being scanned. For characterizing individual ions and ion pairs, the time-correlated ion counting technique was used in the PEPICO and PIPICO modes, respectively. The PEPICO mode utilizes a time-to-amplitude converter (TAC), for which start pulses are provided by the photoelectron arrival signals at a microchannel plate (MCP), and stop pulses are generated by the photoion detection signals at another MCP located at the end of the TOF tube. Note that the photoelectrons flying to the opposite direction to the ion flight direction were sampled from the collision chamber. However, the PIPICO mode uses start pulses provided by lighter ions, and stop pulses generated by heavier ions of their counterparts. 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 measurement of the PEPICO and PIPICO count rates, the coincidence time ranges (gate widths in the TAC) were set to be 0–10 and 0–5 ms, respectively because the TOF difference between any pairs of ions formed from Br 2 and I 2 was found to fall within 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, respectively, and by simultaneously measuring the photon flux while the photon wavelength was being scanned. The corrected photon intensities were ob-
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tained 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 [19]. The uncertainties in the normalized ion and PIPICO yields may mainly come from this normalization process and could be estimated to be less than ¯ 10%. The experimental conditions for measuring the PEPICO and PIPICO yields and their energy variations in IBr were basically the same as those used for the Br 2 and I 2 samples. The principle and construction for the PEPICO and PIPICO measurements have been detailed elsewhere [20,21], and thus are briefly described here. The experiment employing IBr was carried out using time-of-flight (TOF) mass spectrometry, coupled to a constant-deviation grazing incidence monochromator installed at the BL3A2 beam line of the ultraviolet synchrotron orbital radiation (UVSOR) facility at IMS. The slit width of the monochromator was 200 mm to give an optical resolution of 0.1 nm. The dispersed photon beam entered the ionization region through a hole of 3 mm f. The entrance aperture of the drift tube was 8 mm in diameter. The intensity of the monochromatized photon flux was monitored by using a gold-mesh photocathode. A relatively high electric field (2875 V/ cm) was applied to the ionization chamber which had a 6-mm-diameter hole for ion extraction. The flight path length was 20 cm, and the mass detection angle was perpendicular to the direction of the incident photon beam and was 558 with respect to the polarization direction of the synchrotron beam. The main chamber in which the ionization cell, a photoelectron detector, and the TOF spectrometer were located was evacuated down to ¯5310 29 Torr. When the IBr gas molecules were introduced into the ionization cell, the pressure of the main chamber was maintained at ¯6310 27 Torr. For the measurement of the PEPICO and PIPICO count rates, the coincidence time ranges (gate widths in the TAC) were set to be 0–5 ms. The individual ion yields at a given photon energy were obtained by measuring the ratio of the integrated individual PEPICO intensities, the total ion intensity, and the photon flux. The samples of Br 2 , IBr, and I 2 with nominal purities of 99.991%, 98%, and 99%, respectively,
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were purchased from Aldrich Chemical Co. (Br 2 and IBr) and Yakuri Pure Chemicals Co. (I 2 ), and were used without further purification except freeze– pump–thaw cycles.
3. Results and discussion The total ion and PIPICO yields as functions of energy are presented in Figs. 1 and 2, respectively. Some of the reported and estimated threshold energies for the inner-shell electrons of the molecules are marked with hatched lines on the yield curves. The reported ionization thresholds (IE) of the core electrons for Br 2 are: Br(3d 5 / 2 )577.40 eV [22]; those for atomic bromine are: Br(3d 5 / 2 )569.060.4 eV [23]; Br(3d 3 / 2 )570.160.4 eV [23]; Br(3p 3 / 2 )5 181.560.4 eV [23]; Br(3p 1 / 2 )5189.360.4 eV [23];
Fig. 2. Normalized total photoion (a) and PIPICO (b) yield curves of I 2 in the range of 89–998 eV.
Fig. 1. Normalized total photoion (a) and PIPICO (b) yield curves of Br 2 in the range of 90–978 eV.
Br(3s)5256.560.4 eV [23]; that for IBr: Br(3d 5 / 2 )5 76.00 eV [22]; those for I 2 are: I(4d 5 / 2 )557.2 eV [24] and 57.560.2 eV [25]; I(4p 3 / 2 )5129.960.5 eV [25]; I(4s)5194.860.5 eV [25]; I(3d 5 / 2 )5627.39 [22] and 627.260.2 eV [25]; those for atomic iodine are: I(3d 5 / 2 )5619.460.3 eV [23]; I(3p 3 / 2 )5 874.660.3 eV [23]; I(3p 1 / 2 )5930.560.3 eV [23]. The Br(3p 3 / 2 ) IE in Br 2 was estimated to be 189.9 eV, the value derived by adding the chemical shift of 8.460.4 eV (between Br(3d 5 / 2 ) IE in molecular bromine and that in atomic bromine) to the Br(3p 3 / 2 ) IE5181.560.4 eV in atomic bromine. Similarly, the Br(3s) threshold in Br 2 is estimated to be 264.960.6 eV. For I 2 , the core thresholds are estimated in the similar fashion as follows: I(3p 3 / 2 )5882.6 eV; I(3p 1 / 2 )5938.5 eV. Figs. 3 and 4 present the PEPICO (mass) spectra for Br 2 and I 2 taken at specific energies of 100, 150, 200, 270, 300, and 400 eV for Br 2 , and 88, 150, 200, 650, 900, and 950 eV for I 2 . Note that the photon energy selection for the
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Fig. 3. PEPICO (mass) spectra of Br 2 taken at specific photon energies of 100, 150, 200, 270, 300, and 400 eV.
Fig. 4. PEPICO (mass) spectra of I 2 taken at specific photon energies of 88, 150, 200, 650, 900, and 950 eV.
mass and the PIPICO measurements was made for a variety of the specific inner-shell excitation described in Figs. 3, 4, 7, and 8. The individual ion yields derived from the integrated ion counts and the photon intensities as functions of energy are given in Figs. 5 and 6. Figs. 7 and 8 present typical PIPICO spectra taken at the same photon energies as used for the PEPICO measurements. Also, their energy variations are presented in Figs. 9 and 10. Note that the PIPICO yields of the evenly charged ion pairs of X n1 –X n 1 (X5Br, I, n51–3) cannot be estimated accurately owing to an interference effect in the pulse-processing. However, we can determine accurately the PIPICO yields of the equally charged ion
pairs of Br n1 –I n1 (n 5 1, 2) by employing a heteronuclear diatomic molecule of IBr. Fig. 11 compares the individual photoion yield spectra for X n1 (X5Br, I, n51, 2) with the total ion yield spectra of Br 2 and I 2 measured in the present study.
3.1. Core threshold spectral behaviors 3.1.1. Br2 The bromine inner-shell photoionization spectra are dominated by a powerful giant resonance pertinent to the Br(3d) delayed onset phenomenon in the atomic photoabsorption and photoionization. The giant resonance is observed in the total ion and
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threshold as shown in Fig. 1, where the Br 1 and Br 21 ion intensities are prominent (Fig. 5).
Fig. 5. Individual photoion yields of Br 2 in the range of 100|527 eV. The yields (Iphotoion /Iphoton ) are presented on the same relative intensity scale.
PIPICO yield curves over the wide range of E , ¯ 540 eV, and can be identified as a shape resonance owing to the diffusion of the outgoing photoelectrons by the angular momentum barrier in the 3d 9 e f outgoing channel [26]. Therefore, the repulsive force is increasingly important with increasing orbital angular momentum quantum number , [26,27]. The initial core MO is almost pure 3d orbital which overlaps with continuous MO mainly in the region of the atomic core. The resultant orbitals are described by the e f atomic orbitals [11], the excitation to which is responsible for the enhancement of the photoionization cross section. Recently, this enormous spectral feature has also been observed spanning 50|450 eV in the photoionization of SiBr 4 [28,29] and CBr 4 [30]. In addition to the huge resonance in Br 2 , a discrete resonance is observed around the Br(3s)
3.1.2. IBr As seen in Fig. 11, the photoion yield spectra for I 1 and I 21 of iodine monobromide are also dominated by powerful giant resonance owing to the I(4d) delayed onset phenomenon. The individual photoionization spectra involving the I 1 and the I 21 ions were found to be very similar to the total ion yield spectrum of I 2 , indicating that the transition probability for the I(4d) core excitation to e f is much more probable than that for the Br(3d) one. The giant shape resonance observed in the Br 2 molecule [10] was not observed in the photoion yield spectra for the Br 1 and the Br 21 ions, in which a shape resonance occurs in a rather short range of 77.5|87 eV. The short-range shape resonance having a maximum at 77.5 eV may correspond to a shape resonance above the Br(3d) threshold coupled to the Rydberg and continuum, which arises from the excitation of the Br(3d) to a quasi-bound potential well which is created by a backscattering of the excited electrons by another bromine atom. This kind of shape resonance is commonly observed in molecules [28,29,31–33]. 3.1.3. I2 As seen in Fig. 2, two broad resonances are observed in the two regions spanning E , ¯ 167 eV and 635|850 eV, and the shape of the broad continuum with its maximum near 96.2 eV observed in the lower energy region looks very similar to that with a maximum near 92 eV observed previously in gaseous iodine molecules [11]. The spectral range for the occurrence of the I(4d) giant resonance in I 2 is found to be narrower than that of the Br(3d) one in Br 2 . Because the binding energy in I(4d 5 / 2 ) of I 2 is 57.2 eV [24], the broad resonance in the lower energy region definitely corresponds to the transition of I(4d 10 )→I(4d 9 e f) giving rise to the continuum enhancement. The giant resonance in the 4d inner-shell region in Xe is interpreted well in terms of a broad collective resonance in the 4d 10 shell in Xe [34], which is associated with dipolar and oscillatory motion of the entire 4d 10 inner-shell. This collective phenomenon can be thought of in terms of inhomogeneous
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Fig. 6. Individual photoion yields of I 2 in the range of 86|974 eV. The yields (Iphotoion /Iphoton ) are presented on the same relative intensity scale.
electron gas confined to a volume of atomic dimensions. The coupling gives rise to a collective interaction of the electrons with the incident radiation. It has been shown that the agreement between theory and experiment turns out to be excellent if a strong coupling among all the 4d electrons is taken into account [34]. Numerous studies show that the huge feature in the I(4d) threshold region is not a peculiarity of the iodine atom and molecule, but a characteristic behavior of the photoabsorption involving the 4d inner-shell electron, more generally inner electrons with large orbital angular momentum quantum number such as ,$2 [26]. Actually, photoionization cross sections for I 2 , I, and Xe are shown to be very similar in size and
energy dependence, and they both exhibit a prominent peak at ¯100 eV [11,14,35]. Going from Xe 1 to isoelectronic I, however, the shape of the (4d)21 multiplet is shifted about 15.5 eV toward lower ionization energy due to the lower nuclear charge by 1 amu [36]. Also the center of gravity of the (4d)21 multiplet is shifted toward to higher energy going from I 1 to I 31 [12,14]. Taking into account the facts, it seems to be appropriate to deduce relative partial photoionization cross sections for I(5p), I(5s), and I(4d) from the relatively better documented cross section data for the photoionization of Xe, where the partial cross section for Xe(4d) at 100 eV is observed to be predominant, being ¯17 times larger than that of
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Fig. 7. PIPICO spectra of Br 2 taken at the specific photon energies of 100, 150, 200, 270, 300, and 400 eV as used for the PEPICO spectra of Br 2 .
Xe(5p) and ¯64 times larger than that of Xe(5s) [35]. This provides a hint that the I(4d) partial photoionization almost accounts for the total photoionization around 96 eV. It is also shown that the partial cross section for the Xe(4d) photoionization at 100 eV is 8 times larger than that for Xe(3d) at 708 eV [35], being consistent with our observation that the total photoion yield in I 2 at 96.2 eV is about 5 times larger than that at 657 eV. It should be noted that the total ion yield ratio of the two maxima, 5 in I 2 , is lower than the cross section ratio, 8 in Xe, probably reflecting that at 656.5 eV, more ions are
Fig. 8. PIPICO spectra of I 2 taken at the specific photon energies of 88, 150, 200, 650, 900, and 950 eV as used for the PEPICO spectra of I 2 .
formed than at 96.2 eV, by dissociation of the multiply charged parent ions.
3.2. Ionic dissociation behaviors The core-hole states resulting from excitation of the Br(M), I(M), and I(N) core electrons, respectively, may electronically relax by emitting Auger electrons before the molecules begin to fragment. Any dominant relaxation processes are expected to mainly occur via Auger processes, which can involve
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Fig. 9. Individual PIPICO yields of Br 2 in the range of 100|585 eV. The yields (IPIPICO /Iphoton ) are presented on the same relative intensity scale.
the ejection of two, or sometimes more, electrons. When a core electron is ejected, the molecules will be left as a multiply charged ion owing to the Auger processes. Various ion pairs are expected to arise from dissociation of such multiply charged parent ions because in molecules in general, there are several possible combinations of valence MO’s in which two or more holes are produced, thus giving several different fragmentation pathways. As seen in Figs. 3 and 4, the parent ion peaks look very sharp in comparison with the fragment ion peaks, strongly indicating that the fragment ions arise from decomposition of the multiply charged parent ions giving rise to the signal broadening in the TOF spectra via conversion of hot internal energy into kinetic energy. Because of the two possible
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Fig. 10. Individual PIPICO yields of I 2 in the range of 86|974 eV. The yields (IPIPICO /Iphoton ) are presented on the same relative intensity scale.
critical orientations of the departing ionic fragments with respect to the TOF axis just prior to the dissociation, the fragment ions fly to the MCP detector with a broad range of flight times. This explanation is also supported by the observation that each PIPICO channel in the PIPICO spectra except the X n1 –X n1 (X5Br, I, n51–3) has two humps (Figs. 7 and 8), also demonstrating the possibility of the two possible critical orientations of the departing ionic fragments with different charges with respect to the TOF axis. It seems puzzling that over the entire energy range examined, the intensities for the parent ions of I 1 2 increase with increasing energy. The most likely explanation can be found in scattered light contribution, which most likely occurs when we disperse
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the individual ion and PIPICO yields as functions of energy, we outline specific dissociation behaviors of the multiply charged parent ions relying on the excitation energy in Scheme 1.
3.2.2. IBr As seen in Fig. 11, below the Br(3d 5 / 2 ) threshold including the Br(3d) discrete excitation region, 60 , E , 75 eV, the I 1 and I 21 ions are exclusively formed with no trace amounts of the bromine ions. The biased charge localization in the I atoms seems surprising because the Br(3d 5 / 2 )→Br 2 (4ps 1 )* or Br(5p) excitation can lead to the detection of Br n1 (n51, 2), the extent of which is found to be negligibly small. The absence of the bromine ions with the concomitant formation of the iodine ions implies a prompt dissociation upon the I(4d) innershell excitation via process 1. IBr 1 hn → I*Br → I* 1 Br → I n1 1 Br 1 ne (n 5 1, 2)
Fig. 11. Individual photoion yields of IBr in the range of 60|133 eV. The yields (Iphotoion /Iphoton ) are presented on the same relative intensity scale. The total photoion yields of Br 2 and I 2 are scaled for better comparison.
high energy synchrotron radiation using some monochromators.
3.2.1. Br2 As seen in Fig. 3, the singly charged parent ion Br 1 2 is significantly formed only in the TOF mass spectrum at 100 eV, and then the intensity diminishes thereafter. This implies that valence ionization could occur at the low energy as a minor channel in the photoionization, where the molecule does not have a sufficient energy for bond cleavage, thus the parent ion seems to survive intact during the flight time. Over the entire energy range examined, double and triple ionization prevails, yielding ion pairs of Br 1 – Br 1 and Br 21 –Br 1 , respectively. From the Br(3p) threshold, loss of five electrons begins to occur, yielding ion pairs of Br 31 –Br 21 (Fig. 7). Based on
(1)
If we assume the intermediacy of IBr 21 species with two holes in the valence shells for the formation of I n1 (n51, 2), we could presumably observe both Br 1 and I 1 to some extent owing to the small ionization energy difference between IE(Br)511.84 eV and IE(I)510.454 eV [37]. The local phenomenon for the Auger process has previously been observed: In core excitation of silane, de Souza et al. [38] 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 [39]. Slightly beyond the Br(3d) threshold region, more Scheme 1 Br(3d) region Br 2 1hn Br 21 2 Br 31 2 Br(3p,3s) region Br 2 1hn Br 21 2 Br 31 2 Br 41 2
→Br n2 1 1ne (n51–3) →Br 1 1Br 1 →Br 21 1Br 1 →Br n2 1 1ne (n52–4) →Br 1 1Br 1 →Br 21 1Br 1 →Br 21 1Br 21 →Br 31 1Br 1
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specifically at 77.5 eV, the Br 1 and Br 21 ions are more intense than the I 1 and I 21 ions, respectively. Also, we have never observed photoion–photoion coincidence signals supporting the coincident formation of Br 1 and I 1 , etc. At relatively higher energies, 78 , E , 133 eV, however, the contribution of the exit channel leading to formation of Br 1 in the single ionization event turned out to be small again. Over the entire energy range examined, biased charge spread in the specific atomic products in the dissociative photoionization event is observed, presumably reflecting that charge localizes mostly in the excited product atoms relying on the specific inner-shell excitation, which can be accounted for mainly by a two step relaxation process via a fast dissociation followed by autoionization upon the VUV absorption. Based on the individual ion yield and the ion branching ratio as functions of energy, we outline the specific dissociation behaviors relevant to the inner-shell excitation in Scheme 2.
3.2.3. I2 At 88 eV, an energy near the lowest energy examined in this study, the I 1 and the I 21 ions are observed in significant yields along with a parent ion of I 1 2 with very weak intensity, where the PIPICO channels of I 1 –I 1 and I 21 –I 1 are uniquely formed through plausible intermediates of I 21 and I 231 , 2 Scheme 2 I(4d) excitation region(giant resonance) 60 , E , 75 eV IBr1hn →I*1Br→I 1 1Br1e IBr1hn →I*1Br→I 21 1Br12e Br(3d) ionization region around E 5 77.5 eV Single ionization event IBr1hn →Br*1I→Br 1 1I1e major IBr1hn →I*1Br→I 1 1Br1e minor Double ionization event IBr1hn →Br*1I→Br 21 1I12e major IBr1hn →I*1Br→I 21 1Br12e minor I(4d) and Br(3d) excitation region(giant resonance) 78 , E , 133 eV Single ionization event IBr1hn →I*1Br→I 1 1Br1e major IBr1hn →Br*1I→Br 1 1I1e minor Double ionization event IBr1hn →I*1Br→I 21 1Br12e major IBr1hn →Br*1I→Br 21 1I12e minor
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respectively. This indicates that the I 1 and I 21 ions observed here are related to dissociative double and triple photoionization, respectively. Previous studies show that in the case of atomic iodine, 4d direct single photoionization is most efficient and the shake contribution is only about 20610% [12], and in the photoionization of I 1 and I 21 , loss of two electrons is the main contributor to the total cross section in the I(4d) region [15]. Therefore, the direct I(4d) single ionization in I 2 may give rise to further loss of one or two more electrons via a single or double Auger process leading to double or triple ionization of I 2 , respectively. From 127 eV, the energy near the I(4p 3 / 2 ) threshold, the intensity of the PIPICO channel for I 31 – I 21 rises slowly and then at somewhere in the range from 593 to 974 eV, the PIPICO intensity would reach a maximum (Fig. 10). Even at high energies where multiply charged ions of I n1 (n53–5) begin to be additionally formed, the ion intensities for I 1 and I 21 do not seem to be significantly depleted (Fig. 6). This implies that even at the high energies near 635 eV, only the 3d inner electrons are not involved in the excitation and ionization, i.e., a variety of the other core electrons such as the 4d electron and valence electrons could rather be involved in the excitation leading to the formation of I n1 (n51–5). This explanation is supported by the observation that even above 900 eV (beyond the I(3p 3 / 2 ) threshold), a
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parent ion of I 1 2 is still formed in a significant yield (Fig. 4), strongly indicating that partial photoionization cross section for the I(3p) may not be high in comparison with those for the valence, the I(4d), and the I(3d). Based on the energy variation of the PEPICO and PIPICO yields, we outline the specific dissociation processes of the core excited / ionized I 2 pertinent to the excitation energy in Scheme 3. In view of the observation of the I 31 –I 21 channel (Fig. 10), why do we not observe a PIPICO channel, for example, such as the I 41 –I 1 ion pair. This biased charge distribution in the tentative parent ion of I 51 2 followed by ionic dissociation should be improbable because charge redistribution is more rapid than the bond cleavage upon the VUV photoabsorption. Expectedly, unequal charge distribution in the PIPICO channel of I 41 –I 21 is found to be inefficient. However, the observation of the ion-pair with the uneven charges strongly indicates that local Auger process, i.e., a prompt dissociation followed by local Auger process in the separate atoms or ions could still occur, because taking into account the sizable difference of IE(I 1 ) and IE(I 21 ), the possibility for the formation of the I 41 –I 21 ion pair from any longlived I 61 intermediate is presumed to be negligibly 2 small. Unfortunately, for example, comparison of the efficiencies due to I 31 –I 31 and I 41 –I 21 are not achieved because the PIPICO channels involving the
Scheme 3 I(4d) excitation I 2 1hn I 21 2 I 31 2 I(4p,4s) excitation I 2 1hn I 31 2 I 41 2 I 51 2 I(3d,3p) excitation I 2 1hn I 41 2 I 51 2 I 61 2
→I 2n 1 1ne (n52–3) →I 1 1I 1 →I 21 1I 1 →I 2n 1 1ne (n53–5) →I 21 1I 1 →I 21 1I 21 →I 31 1I 1 →I 31 1I 21 →I 2n 1 1ne (n54–6) →I 21 1I 21 →I 31 1I 1 →I 31 1I 21 →I 31 1I 31 →I 41 1I 21
equally charged ion pairs may be smeared in the PIPICO channel of I 21 –I 21 . Beyond the I(3d) threshold, the PIPICO channels of I 21 –I 21 and I 31 –I 21 are dominant, where also loss of six electrons from iodine molecule begins to play an important role in the photoionization, subsequently yielding ion pairs of either I 31 –I 31 or I 41 –I 21 . The assumption for the formation of I 21 – I 21 is based on the observation that at 650 eV, the I 21 ion intensity is prominently enhanced in comparison with the enhancement of the I 1 ion intensity (Fig. 4). Unexpectedly, the PEPICO and PIPICO spectra above the I(3p) threshold are not very different from the corresponding spectra around the I(3d) threshold (Figs. 4 and 8) except one observation that relative importance of the I 21 –I 1 channel is greatly visualized in the I(3p) region as seen in Fig. 8. This implies that even at these high energies near the I(3p) threshold, the I(3d) photoionization still plays a decisive role in the total cross section, and furthermore the I(4d) photoionization would contribute to the total photoionization yield. This argument is based on the previous observation by Becker at al. that the Xe(3d) partial photoionization cross section remains high in the range from the Xe(3d) threshold to 1000 eV, and furthermore, up to the high energy of 1000 eV, Xe(4d) partial cross section is kept not too low, i.e., the Xe(4d) partial cross section at 1000 eV is ¯0.21 Mb, being ¯1% of that at 100 eV, the energy giving the maximum Xe(4d) partial cross section [35].
4. Conclusions The present coincidence spectroscopic study led to the detection of various multiply charged ions of Br n1 (n51–3) and I n1 (n51–5) in the ranges of 90|978 for Br 2 , 60|133 eV for IBr, and 86|998 eV for I 2 . In the total photoion and PIPICO yield curves for Br 2 , IBr, and I 2 , the prominent features have been observed as the giant shape resonances owing to the the Br(3d 10 )→Br(3d 9 e f), I(4d 10 )→I(3d 9 e f), and I(3d 10 )→I(4d 9 e f) transitions. The similar pattern for the giant resonance as seen in I 2 is observed in the individual photoion yield spectra in IBr. In the range of 86|126 eV in the Br 2 and I 2 systems, the
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energy range specifically for the Br(3d) and I(4d) excitation / ionizations, only singly and doubly charged atomic ions in the Br 2 and I 2 systems are predominantly observed, as the results of dissociation of the doubly and triply charged parent ions. Also at this energy, a parent ion of Br 1 2 is observed, revealing the possibility of the occurrence of valence ionization. Below the Br(3d) threshold including the Br(3d) discrete excitation region in IBr, 60 , E , 75 eV, I 1 and I 21 ions are exclusively formed with no trace amounts of the Br 1 and Br 21 ions. Slightly above the Br(3d) threshold, more specifically at 77.5 eV, however, the photoionization events leading to the formations of Br 1 and Br 21 are dominant. At higher energies beyond the Br(3d) threshold, 78 , E , 133 eV, the I 1 and I 21 ion intensities turn out again to exceed those for the Br 1 and Br 21 ones, respectively. Over the entire energy range examined, biased charge localization in the specific atomic products in the dissociative photoionization event is observed, presumably reflecting that charge localizes mostly in the excited product atoms relying on the specific inner-shell excitation, which can be accounted for mainly by a two step relaxation process via a fast dissociation followed by autoionization upon the VUV absorption. For the I 2 system, the I 1 ions are formed in the two I(4d) and I(3p) critical regions, and the intensity of the I 21 –I 1 channel is shown to rise beyond the I(3p) threshold, providing a hint that beyond the I(3p) threshold, valence and 4d ionization could still remain as minor exit channels in the photoionization, and partial photoionization cross section for the I(3p) may not be high in comparison with that for the I(3d). From the Br(3p 3 / 2 ) (189.9 eV) and I(4p 3 / 2 ) thresholds (129.9 eV), losses of five electrons from the molecules begin to play important roles in the photoionization, subsequently yielding ion pairs of X 31 –X 21 (X5Br, I). From the I(3d 5 / 2 ) threshold (627.3 eV), loss of six electrons from the iodine molecule additionally begin to take part in the photoionization, yielding either I 31 –I 31 or I 41 –I 21 . Contrary to the heteronuclear system, the unequal charge distribution in the dissociation processes of plausible Br 41 and I 61 intermediate ions is ineffi2 2 cient because charge spreading processes are more rapid than the bond cleavages in the multiply charged parent ions.
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Acknowledgements The authors wish to acknowledge the financial support of the Korea Research Foundation through the Non-Directed Research Program of 1998–1999. The staffs of the accelerator group of the Electrotechnical Laboratory in Tsukuba, Japan and Ultraviolet Synchrotron Orbital Radiation in Okazaki, Japan are greatly acknowledged for the use of the synchrotron radiation facility.
References [1] A.P. Hitchcock, J. Electron Spectrosc. Relat. Phenom. 25 (1982) 245. [2] 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. [3] J.H.D. Eland, Mol. Phys. 61 (1987) 725. [4] S. Hsieh, J.H.D. Eland, J. Phys. B: At. Mol. Opt. Phys. 30 (1997) 4515. [5] I. Nenner, P. Morin, in: U. Becker, D.A. Shirley (Eds.), VUV and Soft X-Ray Photoionization, Plenum, New York, 1996, p. 291. [6] S. Hsieh, J.H.D. Eland, Rapid Commun. Mass Spectrom. 9 (1995) 1261. [7] W. Eberhardt, E.W. Plummer, I.W. Lyo, R. Carr, W.K. Ford, Phys. Rev. Lett. 58 (1987) 207. [8] R. Murphy, W. Eberhardt, J. Chem. Phys. 89 (1988) 4054. ´ G.C. King, F.H. Read, J. Phys. [9] D.A. Shaw, D. Cvejanovic, B: at. Mol. Phys. 17 (1984) 1173. [10] B.H. Boo, I. Koyano, J. Korean Phys. Soc. 40 (2002) 826. [11] F.J. Comes, U. Nielsen, W.H.E. Schwarz, J. Chem. Phys. 58 (1973) 2230. [12] L. Nahon, A. Svensson, P. Morin, Phys. Rev. A 43 (1991) 2328. [13] M.Y. Amusia, N.A. Cherepkov, L.V. Chernysheva, S.T. Manson, Phys. Rev. A 61 (2000) 020701. [14] G. O’Sullivan, C. McGuinness, J.T. Costello, E.T. Kennedy, B. Weinmann, Phys. Rev. A 53 (1996) 3211. [15] H. Kjeldsen, P. Andersen, F. Folkmann, H. Knudsen, B. Kristensen, J.B. West, T. Andersen, Phys. Rev. A 62 (2000) 020702. ¨ [16] Ch. Achenbach, A. Muller, E. Salzborn, R. Becker, Phys. Rev. Lett. 50 (1983) 2070. [17] N. Saito, I.H. Suzuki, Int. J. Mass Spectrom. Ion Process. 82 (1988) 61. [18] N. Saito, I.H. Suzuki, Int. J. Mass Spectrom. Ion Process. 115 (1992) 157. [19] B.L. Henke, Nucl. Instrum. Meth. 177 (1980) 161. [20] T. Masuoka, T. Horigome, I. Koyano, Rev. Sci. Instrum. 60 (1989) 2179.
152
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[21] E. Ishiguro, M. Suzui, J. Yamazaki, E. Nakamura, K. Sakai, O. Matsudo, N. Mizutani, K. Fukui, M. Watanabe, Rev. Sci. Instrum. 60 (1989) 2105. [22] A.A. Bakke, H.-W. Chen, W.L. Jolly, J. Electron Spectrosc. Relat. Phenom. 20 (1980) 333. [23] J.A. Bearden, A.F. Burr, Rev. Mod. Phys. 39 (1967) 125. [24] B. Wallbank, J.S.H.Q. Perera, D.C. Frost, C.A. McDowell, J. Chem. Phys. 69 (1978) 5405. [25] S. Aksela, H. Aksela, T.D. Thomas, Phys. Rev. A 19 (1979) 721. [26] J.P. Connerade, J.M. Esteva, R.C. Karnatak (Eds.), Giant Resonances in Atoms, Molecules, and Solids, Plenum, New York, 1987. [27] U. Fano, J.W. Cooper, Rev. Mod. Phys. 40 (1968) 441. [28] B.H. Boo, Z. Liu, S.Y. Lee, I. Koyano, J. Phys. Chem. 102 (1998) 8261. [29] B.H. Boo, N. Saito, J. Electron SpecFrosc. Related Phen. in Press. [30] B.H. Boo, N. Saito, I.H. Suzuki, I. Koyano, J. Electron Spectrosc. Relat. Phenom. 123 (2002) 73.
[31] H. Friedrich, B. Pittel, P. Rabe, W.H.E. Schwarz, B. Sonntag, J. Phys. B: At. Molec. Phys. 13 (1980) 25. [32] T. Imamura, C.E. Brion, I. Koyano, T. Ibuki, T. Masuoka, J. Chem. Phys. 94 (1991) 4936. [33] B.H. Boo, S.M. Park, I. Koyano, J. Phys. Chem. 99 (1995) 13362. [34] G. Wendin, J. Phys. B: At. Mol. Phys. 6 (1973) 42. [35] U. Becker, D. Szostak, H.G. Kerkhoff, M. Kupsch, B. Langer, R. Wehlitz, A. Yagishita, T. Hayaishi, Phys. Rev. A 39 (1989) 3902. [36] A. Gottwald, Ch. Gerth, M. Richter, Phys. Rev. Lett. 82 (1999) 2068. [37] C.E. Moore, Atomic Energy Levels, National Standards Reference Data Series, National Bureau of Standards, 1971. [38] G.G.B. de Souza, P. Morin, I. Nenner, Phys. Rev. A 34 (1986) 4770. [39] P. Morin, I. Nenner, Phys. Rev. Lett. 56 (1986) 1913.
Dissociative multiple photoionization of Br 2 , IBr, and I 2 in the VUV and X-ray regions: a comparative study of the inner-shell processes involving Br(3d,3p,3s) and I(4d,4p,4s,3d,3p) a, b Bong Hyun Boo *, Norio Saito a
Department of Chemistry, Chungnam National University, Daejeon 305 -764, South Korea b Electrotechnical Laboratory, 1 -1 -4, Umezono, Tsukuba-shi, Ibaraki 305, Japan Received 4 January 2002; received in revised form 6 June 2002; accepted 20 June 2002
Abstract Dissociative multiple photoionization of the bromine, the iodine monobromide, and the iodine molecules in the Br(3d,3p,3s) and I(4d,4p,4s,3d,3p) inner-shell regions has been studied by using time-of-flight (TOF) mass spectrometry coupled to synchrotron radiation in the ranges of 90|978 eV for Br 2 , 60|133 eV for IBr, and 86|998 eV for I 2 . Total photoion and photoion–photoion coincidence (PIPICO) yields have been recorded as functions of the photon energy. Here, giant shape resonances have been observed beyond the thresholds of the inner-shells owing to the Br(3d 10 )→Br(3d 9 e f), I(4d 10 )→I(4d 9 e f), and I(3d 10 )→I(3d 9 e f) transitions. The dissociation processes of the multiply charged parent ions have also been evaluated from variations of photoelectron–photoion coincidence (PEPICO) and PIPICO spectra with the photon energy. From each Br(3p 3 / 2 ) (189.9 eV) and I(4p 3 / 2 ) threshold (129.9 eV), quintuple ionization of the molecules begins to play important roles in the photoionization, subsequently yielding ion pairs of X 31 –X 21 (X5Br, I). From the I(3d 5 / 2 ) threshold (627.3 eV), loss of six electrons from iodine molecule additionally begins to play a minor role in the multiple photoionization, giving rise to the formation of ion pairs of either I 31 –I 31 or I 41 –I 21 . A direct comparison of the strengths and the ranges of the I(4d) and Br(3d) giant resonances was successfully made from dissociative photoionization of IBr. Over the entire energy range examined, 60 , E , 133 eV, biased charge spread relevant to the specific core-hole states of IBr is observed, presumably reflecting the fact that charge localizes mostly in the excited atoms, which can be accounted for mainly by a two step decay via a fast dissociation followed by autoionization upon the VUV absorption. 2002 Elsevier Science B.V. All rights reserved. Keywords: Bromine; Iodine monobromide; Iodine; Core excitation; Giant resonance; Multiple ionization; PEPICO; PIPICO PACS: 32.80.Fb
1. Introduction Inner-shell electronic excitation has proven to be a *Corresponding author. Fax: 182-42-823-1360. E-mail address: [email protected] (B.H. Boo).
most useful tool for investigating geometric and electronic structures of molecules [1]. Site-selective fragmentation has been one of the main concerns in the specific inner-shell transitions, which largely relies on how long it takes for the localization of the core-hole state to reside in a specific atom and its
0368-2048 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 02 )00162-7
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neighboring atoms for specific bond breakage, competing with energy and charge spread process in the molecule. Therefore, the elucidation concerning charge and energy redistribution following the core excitation / ionization provides an insight into the energy dissipation and the charge delocalization processes, mainly involving the Auger process and bond cleavage of core-excited molecules. Studies of the fragmentation pathways in dissociative multiple ionization of a molecule involve a variety of coincidence methods, such as photoelectron–photoion (PEPICO), photoion–photoion (PIPICO), and photoelectron–photoion–photoion (PEPIPICO, PE2PICO) coincidence [2]. Among the various coincidence methods, the triple coincidence PEPIPICO method has proven to be a powerful tool to elucidate the dissociation dynamics in double photoionization processes [3–5]. The recent addition of position-sensitive ion detection technology to the existing PEPIPICO spectroscopic tool has enabled us to investigate angle-resolved product distribution in the dissociation of doubly charged ions by using a two-dimensional imaging process [4,6]. Until now, however, the PEPIPICO method has been almost completely confined to the investigation of the threebody dissociation dynamics of relatively small dications. In addition, Auger electron–ion coincidence studies can reveal the individual decay channels for the various doubly charged ionic configurations populated in the Auger decay of the core hole [7,8]. These studies also enable correlation of the ion fragmentation products, their kinetic energies, and their electronic configuration with the hole configuration in the doubly charged molecule ion [7]. 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. However, these studies are also restricted to the elucidation of dissociative photoionization of small molecules following core excitation / ionization. Inner-shell excitation of molecular bromine has been elucidated by electron impact [9], but photoionization studies of the molecule in the VUV and X-ray regions are very few [10]. However, atomic and molecular iodines have been subject of photoionization [11–15] and electron impact ionization [16] studies in the VUV and X-ray regions. The
theoretical [13] and experimental [15] investigation on the photoionization of iodine atom and its ion shows that the photoionization cross sections, in the energy range above the I(4d) threshold, are dominated by powerful giant resonances, which are similar to each other in shape and strength, but show some shift in the energy dependence of the photoionization cross section for atomic iodine and its several multiply charged ions. In the present study, we have elucidated the dissociation processes following the valence and inner-shell photoexcitation / photoionization of bromine and iodine molecules involving Br(3d,3p,3s) and I(4d,4p,4s,3d,3p) in the ranges of 90|978 eV for Br 2 and 86|998 eV for I 2 . The study emphasizes the elucidation of the strengths and the ranges of the giant resonances involving Br(3d), I(4d) and I(3d), as well as the insight into the competitive processes of electronic relaxation and photochemical degradation of the core-hole states. Owing to the limitation of the PIPICO method for investigating the homonuclear diatomic molecules, we have further elucidated dissociative photoionization of IBr, which provides estimates of the relative strengths and the ranges of the I(4d) and Br(3d) giant resonances, and also is quite informative as to the characteristic dissociation processes following the Br(3d) and I(4d) core-excitation.
2. Experimental The present experiments have been performed using two different synchrotron radiation facilities located at Electrotechnical Laboratory (ETL) in Tsukuba for the Br 2 and I 2 samples, and at the Institute for Molecular Science (IMS) in Okazaki for the IBr sample. Monochromatic vacuum ultraviolet and soft X-rays were obtained by dispersing synchrotron radiation from the TERAS storage ring at ETL using a Grasshopper monochromator. The principle and the construction of the whole apparatus have been described in detail elsewhere [17,18], and thus are only briefly described here. The monochromatized photons of from 100 to 300 eV have photon energy resolutions of about from 800 to 300 respectively, with a grating of 1200 l / mm. The monochromatized photons passed through two dif-
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ferential pumping stages and entered an ionization chamber equipped with a TOF spectrometer. Here, an electric field of 142.9 V/ cm was applied to the ionization chamber for ion and electron extraction. The main chamber in which the ionization cell, a photoelectron detector and the TOF spectrometer were located was evacuated down to ¯ 2310 26 Torr. When the gas molecules were introduced into the ionization cell, the pressure of the main chamber was maintained at ¯ 8310 26 Torr for Br 2 and ¯ 5310 26 Torr for I 2 . The flight path length was 13 cm, and the ion detection angle was perpendicular to the direction of incident photon beam and 558 to the photon beam polarization for the isotropic collection of fragment ions. 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 was being scanned. For characterizing individual ions and ion pairs, the time-correlated ion counting technique was used in the PEPICO and PIPICO modes, respectively. The PEPICO mode utilizes a time-to-amplitude converter (TAC), for which start pulses are provided by the photoelectron arrival signals at a microchannel plate (MCP), and stop pulses are generated by the photoion detection signals at another MCP located at the end of the TOF tube. Note that the photoelectrons flying to the opposite direction to the ion flight direction were sampled from the collision chamber. However, the PIPICO mode uses start pulses provided by lighter ions, and stop pulses generated by heavier ions of their counterparts. 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 measurement of the PEPICO and PIPICO count rates, the coincidence time ranges (gate widths in the TAC) were set to be 0–10 and 0–5 ms, respectively because the TOF difference between any pairs of ions formed from Br 2 and I 2 was found to fall within 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, respectively, and by simultaneously measuring the photon flux while the photon wavelength was being scanned. The corrected photon intensities were ob-
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tained 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 [19]. The uncertainties in the normalized ion and PIPICO yields may mainly come from this normalization process and could be estimated to be less than ¯ 10%. The experimental conditions for measuring the PEPICO and PIPICO yields and their energy variations in IBr were basically the same as those used for the Br 2 and I 2 samples. The principle and construction for the PEPICO and PIPICO measurements have been detailed elsewhere [20,21], and thus are briefly described here. The experiment employing IBr was carried out using time-of-flight (TOF) mass spectrometry, coupled to a constant-deviation grazing incidence monochromator installed at the BL3A2 beam line of the ultraviolet synchrotron orbital radiation (UVSOR) facility at IMS. The slit width of the monochromator was 200 mm to give an optical resolution of 0.1 nm. The dispersed photon beam entered the ionization region through a hole of 3 mm f. The entrance aperture of the drift tube was 8 mm in diameter. The intensity of the monochromatized photon flux was monitored by using a gold-mesh photocathode. A relatively high electric field (2875 V/ cm) was applied to the ionization chamber which had a 6-mm-diameter hole for ion extraction. The flight path length was 20 cm, and the mass detection angle was perpendicular to the direction of the incident photon beam and was 558 with respect to the polarization direction of the synchrotron beam. The main chamber in which the ionization cell, a photoelectron detector, and the TOF spectrometer were located was evacuated down to ¯5310 29 Torr. When the IBr gas molecules were introduced into the ionization cell, the pressure of the main chamber was maintained at ¯6310 27 Torr. For the measurement of the PEPICO and PIPICO count rates, the coincidence time ranges (gate widths in the TAC) were set to be 0–5 ms. The individual ion yields at a given photon energy were obtained by measuring the ratio of the integrated individual PEPICO intensities, the total ion intensity, and the photon flux. The samples of Br 2 , IBr, and I 2 with nominal purities of 99.991%, 98%, and 99%, respectively,
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were purchased from Aldrich Chemical Co. (Br 2 and IBr) and Yakuri Pure Chemicals Co. (I 2 ), and were used without further purification except freeze– pump–thaw cycles.
3. Results and discussion The total ion and PIPICO yields as functions of energy are presented in Figs. 1 and 2, respectively. Some of the reported and estimated threshold energies for the inner-shell electrons of the molecules are marked with hatched lines on the yield curves. The reported ionization thresholds (IE) of the core electrons for Br 2 are: Br(3d 5 / 2 )577.40 eV [22]; those for atomic bromine are: Br(3d 5 / 2 )569.060.4 eV [23]; Br(3d 3 / 2 )570.160.4 eV [23]; Br(3p 3 / 2 )5 181.560.4 eV [23]; Br(3p 1 / 2 )5189.360.4 eV [23];
Fig. 2. Normalized total photoion (a) and PIPICO (b) yield curves of I 2 in the range of 89–998 eV.
Fig. 1. Normalized total photoion (a) and PIPICO (b) yield curves of Br 2 in the range of 90–978 eV.
Br(3s)5256.560.4 eV [23]; that for IBr: Br(3d 5 / 2 )5 76.00 eV [22]; those for I 2 are: I(4d 5 / 2 )557.2 eV [24] and 57.560.2 eV [25]; I(4p 3 / 2 )5129.960.5 eV [25]; I(4s)5194.860.5 eV [25]; I(3d 5 / 2 )5627.39 [22] and 627.260.2 eV [25]; those for atomic iodine are: I(3d 5 / 2 )5619.460.3 eV [23]; I(3p 3 / 2 )5 874.660.3 eV [23]; I(3p 1 / 2 )5930.560.3 eV [23]. The Br(3p 3 / 2 ) IE in Br 2 was estimated to be 189.9 eV, the value derived by adding the chemical shift of 8.460.4 eV (between Br(3d 5 / 2 ) IE in molecular bromine and that in atomic bromine) to the Br(3p 3 / 2 ) IE5181.560.4 eV in atomic bromine. Similarly, the Br(3s) threshold in Br 2 is estimated to be 264.960.6 eV. For I 2 , the core thresholds are estimated in the similar fashion as follows: I(3p 3 / 2 )5882.6 eV; I(3p 1 / 2 )5938.5 eV. Figs. 3 and 4 present the PEPICO (mass) spectra for Br 2 and I 2 taken at specific energies of 100, 150, 200, 270, 300, and 400 eV for Br 2 , and 88, 150, 200, 650, 900, and 950 eV for I 2 . Note that the photon energy selection for the
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Fig. 3. PEPICO (mass) spectra of Br 2 taken at specific photon energies of 100, 150, 200, 270, 300, and 400 eV.
Fig. 4. PEPICO (mass) spectra of I 2 taken at specific photon energies of 88, 150, 200, 650, 900, and 950 eV.
mass and the PIPICO measurements was made for a variety of the specific inner-shell excitation described in Figs. 3, 4, 7, and 8. The individual ion yields derived from the integrated ion counts and the photon intensities as functions of energy are given in Figs. 5 and 6. Figs. 7 and 8 present typical PIPICO spectra taken at the same photon energies as used for the PEPICO measurements. Also, their energy variations are presented in Figs. 9 and 10. Note that the PIPICO yields of the evenly charged ion pairs of X n1 –X n 1 (X5Br, I, n51–3) cannot be estimated accurately owing to an interference effect in the pulse-processing. However, we can determine accurately the PIPICO yields of the equally charged ion
pairs of Br n1 –I n1 (n 5 1, 2) by employing a heteronuclear diatomic molecule of IBr. Fig. 11 compares the individual photoion yield spectra for X n1 (X5Br, I, n51, 2) with the total ion yield spectra of Br 2 and I 2 measured in the present study.
3.1. Core threshold spectral behaviors 3.1.1. Br2 The bromine inner-shell photoionization spectra are dominated by a powerful giant resonance pertinent to the Br(3d) delayed onset phenomenon in the atomic photoabsorption and photoionization. The giant resonance is observed in the total ion and
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threshold as shown in Fig. 1, where the Br 1 and Br 21 ion intensities are prominent (Fig. 5).
Fig. 5. Individual photoion yields of Br 2 in the range of 100|527 eV. The yields (Iphotoion /Iphoton ) are presented on the same relative intensity scale.
PIPICO yield curves over the wide range of E , ¯ 540 eV, and can be identified as a shape resonance owing to the diffusion of the outgoing photoelectrons by the angular momentum barrier in the 3d 9 e f outgoing channel [26]. Therefore, the repulsive force is increasingly important with increasing orbital angular momentum quantum number , [26,27]. The initial core MO is almost pure 3d orbital which overlaps with continuous MO mainly in the region of the atomic core. The resultant orbitals are described by the e f atomic orbitals [11], the excitation to which is responsible for the enhancement of the photoionization cross section. Recently, this enormous spectral feature has also been observed spanning 50|450 eV in the photoionization of SiBr 4 [28,29] and CBr 4 [30]. In addition to the huge resonance in Br 2 , a discrete resonance is observed around the Br(3s)
3.1.2. IBr As seen in Fig. 11, the photoion yield spectra for I 1 and I 21 of iodine monobromide are also dominated by powerful giant resonance owing to the I(4d) delayed onset phenomenon. The individual photoionization spectra involving the I 1 and the I 21 ions were found to be very similar to the total ion yield spectrum of I 2 , indicating that the transition probability for the I(4d) core excitation to e f is much more probable than that for the Br(3d) one. The giant shape resonance observed in the Br 2 molecule [10] was not observed in the photoion yield spectra for the Br 1 and the Br 21 ions, in which a shape resonance occurs in a rather short range of 77.5|87 eV. The short-range shape resonance having a maximum at 77.5 eV may correspond to a shape resonance above the Br(3d) threshold coupled to the Rydberg and continuum, which arises from the excitation of the Br(3d) to a quasi-bound potential well which is created by a backscattering of the excited electrons by another bromine atom. This kind of shape resonance is commonly observed in molecules [28,29,31–33]. 3.1.3. I2 As seen in Fig. 2, two broad resonances are observed in the two regions spanning E , ¯ 167 eV and 635|850 eV, and the shape of the broad continuum with its maximum near 96.2 eV observed in the lower energy region looks very similar to that with a maximum near 92 eV observed previously in gaseous iodine molecules [11]. The spectral range for the occurrence of the I(4d) giant resonance in I 2 is found to be narrower than that of the Br(3d) one in Br 2 . Because the binding energy in I(4d 5 / 2 ) of I 2 is 57.2 eV [24], the broad resonance in the lower energy region definitely corresponds to the transition of I(4d 10 )→I(4d 9 e f) giving rise to the continuum enhancement. The giant resonance in the 4d inner-shell region in Xe is interpreted well in terms of a broad collective resonance in the 4d 10 shell in Xe [34], which is associated with dipolar and oscillatory motion of the entire 4d 10 inner-shell. This collective phenomenon can be thought of in terms of inhomogeneous
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Fig. 6. Individual photoion yields of I 2 in the range of 86|974 eV. The yields (Iphotoion /Iphoton ) are presented on the same relative intensity scale.
electron gas confined to a volume of atomic dimensions. The coupling gives rise to a collective interaction of the electrons with the incident radiation. It has been shown that the agreement between theory and experiment turns out to be excellent if a strong coupling among all the 4d electrons is taken into account [34]. Numerous studies show that the huge feature in the I(4d) threshold region is not a peculiarity of the iodine atom and molecule, but a characteristic behavior of the photoabsorption involving the 4d inner-shell electron, more generally inner electrons with large orbital angular momentum quantum number such as ,$2 [26]. Actually, photoionization cross sections for I 2 , I, and Xe are shown to be very similar in size and
energy dependence, and they both exhibit a prominent peak at ¯100 eV [11,14,35]. Going from Xe 1 to isoelectronic I, however, the shape of the (4d)21 multiplet is shifted about 15.5 eV toward lower ionization energy due to the lower nuclear charge by 1 amu [36]. Also the center of gravity of the (4d)21 multiplet is shifted toward to higher energy going from I 1 to I 31 [12,14]. Taking into account the facts, it seems to be appropriate to deduce relative partial photoionization cross sections for I(5p), I(5s), and I(4d) from the relatively better documented cross section data for the photoionization of Xe, where the partial cross section for Xe(4d) at 100 eV is observed to be predominant, being ¯17 times larger than that of
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Fig. 7. PIPICO spectra of Br 2 taken at the specific photon energies of 100, 150, 200, 270, 300, and 400 eV as used for the PEPICO spectra of Br 2 .
Xe(5p) and ¯64 times larger than that of Xe(5s) [35]. This provides a hint that the I(4d) partial photoionization almost accounts for the total photoionization around 96 eV. It is also shown that the partial cross section for the Xe(4d) photoionization at 100 eV is 8 times larger than that for Xe(3d) at 708 eV [35], being consistent with our observation that the total photoion yield in I 2 at 96.2 eV is about 5 times larger than that at 657 eV. It should be noted that the total ion yield ratio of the two maxima, 5 in I 2 , is lower than the cross section ratio, 8 in Xe, probably reflecting that at 656.5 eV, more ions are
Fig. 8. PIPICO spectra of I 2 taken at the specific photon energies of 88, 150, 200, 650, 900, and 950 eV as used for the PEPICO spectra of I 2 .
formed than at 96.2 eV, by dissociation of the multiply charged parent ions.
3.2. Ionic dissociation behaviors The core-hole states resulting from excitation of the Br(M), I(M), and I(N) core electrons, respectively, may electronically relax by emitting Auger electrons before the molecules begin to fragment. Any dominant relaxation processes are expected to mainly occur via Auger processes, which can involve
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Fig. 9. Individual PIPICO yields of Br 2 in the range of 100|585 eV. The yields (IPIPICO /Iphoton ) are presented on the same relative intensity scale.
the ejection of two, or sometimes more, electrons. When a core electron is ejected, the molecules will be left as a multiply charged ion owing to the Auger processes. Various ion pairs are expected to arise from dissociation of such multiply charged parent ions because in molecules in general, there are several possible combinations of valence MO’s in which two or more holes are produced, thus giving several different fragmentation pathways. As seen in Figs. 3 and 4, the parent ion peaks look very sharp in comparison with the fragment ion peaks, strongly indicating that the fragment ions arise from decomposition of the multiply charged parent ions giving rise to the signal broadening in the TOF spectra via conversion of hot internal energy into kinetic energy. Because of the two possible
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Fig. 10. Individual PIPICO yields of I 2 in the range of 86|974 eV. The yields (IPIPICO /Iphoton ) are presented on the same relative intensity scale.
critical orientations of the departing ionic fragments with respect to the TOF axis just prior to the dissociation, the fragment ions fly to the MCP detector with a broad range of flight times. This explanation is also supported by the observation that each PIPICO channel in the PIPICO spectra except the X n1 –X n1 (X5Br, I, n51–3) has two humps (Figs. 7 and 8), also demonstrating the possibility of the two possible critical orientations of the departing ionic fragments with different charges with respect to the TOF axis. It seems puzzling that over the entire energy range examined, the intensities for the parent ions of I 1 2 increase with increasing energy. The most likely explanation can be found in scattered light contribution, which most likely occurs when we disperse
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the individual ion and PIPICO yields as functions of energy, we outline specific dissociation behaviors of the multiply charged parent ions relying on the excitation energy in Scheme 1.
3.2.2. IBr As seen in Fig. 11, below the Br(3d 5 / 2 ) threshold including the Br(3d) discrete excitation region, 60 , E , 75 eV, the I 1 and I 21 ions are exclusively formed with no trace amounts of the bromine ions. The biased charge localization in the I atoms seems surprising because the Br(3d 5 / 2 )→Br 2 (4ps 1 )* or Br(5p) excitation can lead to the detection of Br n1 (n51, 2), the extent of which is found to be negligibly small. The absence of the bromine ions with the concomitant formation of the iodine ions implies a prompt dissociation upon the I(4d) innershell excitation via process 1. IBr 1 hn → I*Br → I* 1 Br → I n1 1 Br 1 ne (n 5 1, 2)
Fig. 11. Individual photoion yields of IBr in the range of 60|133 eV. The yields (Iphotoion /Iphoton ) are presented on the same relative intensity scale. The total photoion yields of Br 2 and I 2 are scaled for better comparison.
high energy synchrotron radiation using some monochromators.
3.2.1. Br2 As seen in Fig. 3, the singly charged parent ion Br 1 2 is significantly formed only in the TOF mass spectrum at 100 eV, and then the intensity diminishes thereafter. This implies that valence ionization could occur at the low energy as a minor channel in the photoionization, where the molecule does not have a sufficient energy for bond cleavage, thus the parent ion seems to survive intact during the flight time. Over the entire energy range examined, double and triple ionization prevails, yielding ion pairs of Br 1 – Br 1 and Br 21 –Br 1 , respectively. From the Br(3p) threshold, loss of five electrons begins to occur, yielding ion pairs of Br 31 –Br 21 (Fig. 7). Based on
(1)
If we assume the intermediacy of IBr 21 species with two holes in the valence shells for the formation of I n1 (n51, 2), we could presumably observe both Br 1 and I 1 to some extent owing to the small ionization energy difference between IE(Br)511.84 eV and IE(I)510.454 eV [37]. The local phenomenon for the Auger process has previously been observed: In core excitation of silane, de Souza et al. [38] 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 [39]. Slightly beyond the Br(3d) threshold region, more Scheme 1 Br(3d) region Br 2 1hn Br 21 2 Br 31 2 Br(3p,3s) region Br 2 1hn Br 21 2 Br 31 2 Br 41 2
→Br n2 1 1ne (n51–3) →Br 1 1Br 1 →Br 21 1Br 1 →Br n2 1 1ne (n52–4) →Br 1 1Br 1 →Br 21 1Br 1 →Br 21 1Br 21 →Br 31 1Br 1
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specifically at 77.5 eV, the Br 1 and Br 21 ions are more intense than the I 1 and I 21 ions, respectively. Also, we have never observed photoion–photoion coincidence signals supporting the coincident formation of Br 1 and I 1 , etc. At relatively higher energies, 78 , E , 133 eV, however, the contribution of the exit channel leading to formation of Br 1 in the single ionization event turned out to be small again. Over the entire energy range examined, biased charge spread in the specific atomic products in the dissociative photoionization event is observed, presumably reflecting that charge localizes mostly in the excited product atoms relying on the specific inner-shell excitation, which can be accounted for mainly by a two step relaxation process via a fast dissociation followed by autoionization upon the VUV absorption. Based on the individual ion yield and the ion branching ratio as functions of energy, we outline the specific dissociation behaviors relevant to the inner-shell excitation in Scheme 2.
3.2.3. I2 At 88 eV, an energy near the lowest energy examined in this study, the I 1 and the I 21 ions are observed in significant yields along with a parent ion of I 1 2 with very weak intensity, where the PIPICO channels of I 1 –I 1 and I 21 –I 1 are uniquely formed through plausible intermediates of I 21 and I 231 , 2 Scheme 2 I(4d) excitation region(giant resonance) 60 , E , 75 eV IBr1hn →I*1Br→I 1 1Br1e IBr1hn →I*1Br→I 21 1Br12e Br(3d) ionization region around E 5 77.5 eV Single ionization event IBr1hn →Br*1I→Br 1 1I1e major IBr1hn →I*1Br→I 1 1Br1e minor Double ionization event IBr1hn →Br*1I→Br 21 1I12e major IBr1hn →I*1Br→I 21 1Br12e minor I(4d) and Br(3d) excitation region(giant resonance) 78 , E , 133 eV Single ionization event IBr1hn →I*1Br→I 1 1Br1e major IBr1hn →Br*1I→Br 1 1I1e minor Double ionization event IBr1hn →I*1Br→I 21 1Br12e major IBr1hn →Br*1I→Br 21 1I12e minor
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respectively. This indicates that the I 1 and I 21 ions observed here are related to dissociative double and triple photoionization, respectively. Previous studies show that in the case of atomic iodine, 4d direct single photoionization is most efficient and the shake contribution is only about 20610% [12], and in the photoionization of I 1 and I 21 , loss of two electrons is the main contributor to the total cross section in the I(4d) region [15]. Therefore, the direct I(4d) single ionization in I 2 may give rise to further loss of one or two more electrons via a single or double Auger process leading to double or triple ionization of I 2 , respectively. From 127 eV, the energy near the I(4p 3 / 2 ) threshold, the intensity of the PIPICO channel for I 31 – I 21 rises slowly and then at somewhere in the range from 593 to 974 eV, the PIPICO intensity would reach a maximum (Fig. 10). Even at high energies where multiply charged ions of I n1 (n53–5) begin to be additionally formed, the ion intensities for I 1 and I 21 do not seem to be significantly depleted (Fig. 6). This implies that even at the high energies near 635 eV, only the 3d inner electrons are not involved in the excitation and ionization, i.e., a variety of the other core electrons such as the 4d electron and valence electrons could rather be involved in the excitation leading to the formation of I n1 (n51–5). This explanation is supported by the observation that even above 900 eV (beyond the I(3p 3 / 2 ) threshold), a
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parent ion of I 1 2 is still formed in a significant yield (Fig. 4), strongly indicating that partial photoionization cross section for the I(3p) may not be high in comparison with those for the valence, the I(4d), and the I(3d). Based on the energy variation of the PEPICO and PIPICO yields, we outline the specific dissociation processes of the core excited / ionized I 2 pertinent to the excitation energy in Scheme 3. In view of the observation of the I 31 –I 21 channel (Fig. 10), why do we not observe a PIPICO channel, for example, such as the I 41 –I 1 ion pair. This biased charge distribution in the tentative parent ion of I 51 2 followed by ionic dissociation should be improbable because charge redistribution is more rapid than the bond cleavage upon the VUV photoabsorption. Expectedly, unequal charge distribution in the PIPICO channel of I 41 –I 21 is found to be inefficient. However, the observation of the ion-pair with the uneven charges strongly indicates that local Auger process, i.e., a prompt dissociation followed by local Auger process in the separate atoms or ions could still occur, because taking into account the sizable difference of IE(I 1 ) and IE(I 21 ), the possibility for the formation of the I 41 –I 21 ion pair from any longlived I 61 intermediate is presumed to be negligibly 2 small. Unfortunately, for example, comparison of the efficiencies due to I 31 –I 31 and I 41 –I 21 are not achieved because the PIPICO channels involving the
Scheme 3 I(4d) excitation I 2 1hn I 21 2 I 31 2 I(4p,4s) excitation I 2 1hn I 31 2 I 41 2 I 51 2 I(3d,3p) excitation I 2 1hn I 41 2 I 51 2 I 61 2
→I 2n 1 1ne (n52–3) →I 1 1I 1 →I 21 1I 1 →I 2n 1 1ne (n53–5) →I 21 1I 1 →I 21 1I 21 →I 31 1I 1 →I 31 1I 21 →I 2n 1 1ne (n54–6) →I 21 1I 21 →I 31 1I 1 →I 31 1I 21 →I 31 1I 31 →I 41 1I 21
equally charged ion pairs may be smeared in the PIPICO channel of I 21 –I 21 . Beyond the I(3d) threshold, the PIPICO channels of I 21 –I 21 and I 31 –I 21 are dominant, where also loss of six electrons from iodine molecule begins to play an important role in the photoionization, subsequently yielding ion pairs of either I 31 –I 31 or I 41 –I 21 . The assumption for the formation of I 21 – I 21 is based on the observation that at 650 eV, the I 21 ion intensity is prominently enhanced in comparison with the enhancement of the I 1 ion intensity (Fig. 4). Unexpectedly, the PEPICO and PIPICO spectra above the I(3p) threshold are not very different from the corresponding spectra around the I(3d) threshold (Figs. 4 and 8) except one observation that relative importance of the I 21 –I 1 channel is greatly visualized in the I(3p) region as seen in Fig. 8. This implies that even at these high energies near the I(3p) threshold, the I(3d) photoionization still plays a decisive role in the total cross section, and furthermore the I(4d) photoionization would contribute to the total photoionization yield. This argument is based on the previous observation by Becker at al. that the Xe(3d) partial photoionization cross section remains high in the range from the Xe(3d) threshold to 1000 eV, and furthermore, up to the high energy of 1000 eV, Xe(4d) partial cross section is kept not too low, i.e., the Xe(4d) partial cross section at 1000 eV is ¯0.21 Mb, being ¯1% of that at 100 eV, the energy giving the maximum Xe(4d) partial cross section [35].
4. Conclusions The present coincidence spectroscopic study led to the detection of various multiply charged ions of Br n1 (n51–3) and I n1 (n51–5) in the ranges of 90|978 for Br 2 , 60|133 eV for IBr, and 86|998 eV for I 2 . In the total photoion and PIPICO yield curves for Br 2 , IBr, and I 2 , the prominent features have been observed as the giant shape resonances owing to the the Br(3d 10 )→Br(3d 9 e f), I(4d 10 )→I(3d 9 e f), and I(3d 10 )→I(4d 9 e f) transitions. The similar pattern for the giant resonance as seen in I 2 is observed in the individual photoion yield spectra in IBr. In the range of 86|126 eV in the Br 2 and I 2 systems, the
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energy range specifically for the Br(3d) and I(4d) excitation / ionizations, only singly and doubly charged atomic ions in the Br 2 and I 2 systems are predominantly observed, as the results of dissociation of the doubly and triply charged parent ions. Also at this energy, a parent ion of Br 1 2 is observed, revealing the possibility of the occurrence of valence ionization. Below the Br(3d) threshold including the Br(3d) discrete excitation region in IBr, 60 , E , 75 eV, I 1 and I 21 ions are exclusively formed with no trace amounts of the Br 1 and Br 21 ions. Slightly above the Br(3d) threshold, more specifically at 77.5 eV, however, the photoionization events leading to the formations of Br 1 and Br 21 are dominant. At higher energies beyond the Br(3d) threshold, 78 , E , 133 eV, the I 1 and I 21 ion intensities turn out again to exceed those for the Br 1 and Br 21 ones, respectively. Over the entire energy range examined, biased charge localization in the specific atomic products in the dissociative photoionization event is observed, presumably reflecting that charge localizes mostly in the excited product atoms relying on the specific inner-shell excitation, which can be accounted for mainly by a two step relaxation process via a fast dissociation followed by autoionization upon the VUV absorption. For the I 2 system, the I 1 ions are formed in the two I(4d) and I(3p) critical regions, and the intensity of the I 21 –I 1 channel is shown to rise beyond the I(3p) threshold, providing a hint that beyond the I(3p) threshold, valence and 4d ionization could still remain as minor exit channels in the photoionization, and partial photoionization cross section for the I(3p) may not be high in comparison with that for the I(3d). From the Br(3p 3 / 2 ) (189.9 eV) and I(4p 3 / 2 ) thresholds (129.9 eV), losses of five electrons from the molecules begin to play important roles in the photoionization, subsequently yielding ion pairs of X 31 –X 21 (X5Br, I). From the I(3d 5 / 2 ) threshold (627.3 eV), loss of six electrons from the iodine molecule additionally begin to take part in the photoionization, yielding either I 31 –I 31 or I 41 –I 21 . Contrary to the heteronuclear system, the unequal charge distribution in the dissociation processes of plausible Br 41 and I 61 intermediate ions is ineffi2 2 cient because charge spreading processes are more rapid than the bond cleavages in the multiply charged parent ions.
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Acknowledgements The authors wish to acknowledge the financial support of the Korea Research Foundation through the Non-Directed Research Program of 1998–1999. The staffs of the accelerator group of the Electrotechnical Laboratory in Tsukuba, Japan and Ultraviolet Synchrotron Orbital Radiation in Okazaki, Japan are greatly acknowledged for the use of the synchrotron radiation facility.
References [1] A.P. Hitchcock, J. Electron Spectrosc. Relat. Phenom. 25 (1982) 245. [2] 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. [3] J.H.D. Eland, Mol. Phys. 61 (1987) 725. [4] S. Hsieh, J.H.D. Eland, J. Phys. B: At. Mol. Opt. Phys. 30 (1997) 4515. [5] I. Nenner, P. Morin, in: U. Becker, D.A. Shirley (Eds.), VUV and Soft X-Ray Photoionization, Plenum, New York, 1996, p. 291. [6] S. Hsieh, J.H.D. Eland, Rapid Commun. Mass Spectrom. 9 (1995) 1261. [7] W. Eberhardt, E.W. Plummer, I.W. Lyo, R. Carr, W.K. Ford, Phys. Rev. Lett. 58 (1987) 207. [8] R. Murphy, W. Eberhardt, J. Chem. Phys. 89 (1988) 4054. ´ G.C. King, F.H. Read, J. Phys. [9] D.A. Shaw, D. Cvejanovic, B: at. Mol. Phys. 17 (1984) 1173. [10] B.H. Boo, I. Koyano, J. Korean Phys. Soc. 40 (2002) 826. [11] F.J. Comes, U. Nielsen, W.H.E. Schwarz, J. Chem. Phys. 58 (1973) 2230. [12] L. Nahon, A. Svensson, P. Morin, Phys. Rev. A 43 (1991) 2328. [13] M.Y. Amusia, N.A. Cherepkov, L.V. Chernysheva, S.T. Manson, Phys. Rev. A 61 (2000) 020701. [14] G. O’Sullivan, C. McGuinness, J.T. Costello, E.T. Kennedy, B. Weinmann, Phys. Rev. A 53 (1996) 3211. [15] H. Kjeldsen, P. Andersen, F. Folkmann, H. Knudsen, B. Kristensen, J.B. West, T. Andersen, Phys. Rev. A 62 (2000) 020702. ¨ [16] Ch. Achenbach, A. Muller, E. Salzborn, R. Becker, Phys. Rev. Lett. 50 (1983) 2070. [17] N. Saito, I.H. Suzuki, Int. J. Mass Spectrom. Ion Process. 82 (1988) 61. [18] N. Saito, I.H. Suzuki, Int. J. Mass Spectrom. Ion Process. 115 (1992) 157. [19] B.L. Henke, Nucl. Instrum. Meth. 177 (1980) 161. [20] T. Masuoka, T. Horigome, I. Koyano, Rev. Sci. Instrum. 60 (1989) 2179.
152
B.H. Boo, N. Saito / Journal of Electron Spectroscopy and Related Phenomena 127 (2002) 139–152
[21] E. Ishiguro, M. Suzui, J. Yamazaki, E. Nakamura, K. Sakai, O. Matsudo, N. Mizutani, K. Fukui, M. Watanabe, Rev. Sci. Instrum. 60 (1989) 2105. [22] A.A. Bakke, H.-W. Chen, W.L. Jolly, J. Electron Spectrosc. Relat. Phenom. 20 (1980) 333. [23] J.A. Bearden, A.F. Burr, Rev. Mod. Phys. 39 (1967) 125. [24] B. Wallbank, J.S.H.Q. Perera, D.C. Frost, C.A. McDowell, J. Chem. Phys. 69 (1978) 5405. [25] S. Aksela, H. Aksela, T.D. Thomas, Phys. Rev. A 19 (1979) 721. [26] J.P. Connerade, J.M. Esteva, R.C. Karnatak (Eds.), Giant Resonances in Atoms, Molecules, and Solids, Plenum, New York, 1987. [27] U. Fano, J.W. Cooper, Rev. Mod. Phys. 40 (1968) 441. [28] B.H. Boo, Z. Liu, S.Y. Lee, I. Koyano, J. Phys. Chem. 102 (1998) 8261. [29] B.H. Boo, N. Saito, J. Electron SpecFrosc. Related Phen. in Press. [30] B.H. Boo, N. Saito, I.H. Suzuki, I. Koyano, J. Electron Spectrosc. Relat. Phenom. 123 (2002) 73.
[31] H. Friedrich, B. Pittel, P. Rabe, W.H.E. Schwarz, B. Sonntag, J. Phys. B: At. Molec. Phys. 13 (1980) 25. [32] T. Imamura, C.E. Brion, I. Koyano, T. Ibuki, T. Masuoka, J. Chem. Phys. 94 (1991) 4936. [33] B.H. Boo, S.M. Park, I. Koyano, J. Phys. Chem. 99 (1995) 13362. [34] G. Wendin, J. Phys. B: At. Mol. Phys. 6 (1973) 42. [35] U. Becker, D. Szostak, H.G. Kerkhoff, M. Kupsch, B. Langer, R. Wehlitz, A. Yagishita, T. Hayaishi, Phys. Rev. A 39 (1989) 3902. [36] A. Gottwald, Ch. Gerth, M. Richter, Phys. Rev. Lett. 82 (1999) 2068. [37] C.E. Moore, Atomic Energy Levels, National Standards Reference Data Series, National Bureau of Standards, 1971. [38] G.G.B. de Souza, P. Morin, I. Nenner, Phys. Rev. A 34 (1986) 4770. [39] P. Morin, I. Nenner, Phys. Rev. Lett. 56 (1986) 1913.