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Photoelectron Spectroscopy
2.14 Photoelectron Spectroscopy D.-S. YANG University of Kentucky, Lexington, KY, USA 2.14.1 BASIC CONCEPTS 2.14.2 SPECTRAL FEATURES 2.14.3 SPECTRAL ASSIGNMENTS 2.14.3.1 Electronic Structure Methods 2.14.3.2 Franck–Condon Principle 2.14.3.3 Photon Energy Dependence of Band Intensities 2.14.3.4 Ionization Energy Trends and Resolved Fine Structures 2.14.4 HeI, HeII, AND X-RAY PHOTOELECTRON SPECTROSCOPY 2.14.5 SYNCHROTRON RADIATION PHOTOELECTRON SPECTROSCOPY 2.14.6 LASER-BASED PULSED FIELD IONIZATION-ZERO ELECTRON KINETIC ENERGY PHOTOELECTRON SPECTROSCOPY 2.14.7 LASER-BASED NEGATIVE ION PHOTOELECTRON SPECTROSCOPY 2.14.8 OTHER PHOTOELECTRON TECHNIQUES 2.14.9 REFERENCES
2.14.1
187 188 189 189 189 189 190 190 190 191 194 194 195
BASIC CONCEPTS
Molecular photoelectron spectroscopy (PES) involves the application of the photoelectric effect to the study of electronic states and geometric structures.1–3 Electrons are ejected from molecules by irradiation of monochromatic photons with energy h, M þ h ! M þ þ e
ð1Þ
The energy difference E between a final ion state and an initial neutral state equals the incident photon energy minus the kinetic energy KE of the ejected electron, E ¼ h KE
ð2Þ
By defining adiabatic ionization energy (IE) as the minimum energy required to remove an electron from the ground electronic state of the neutral, E ¼ IE þ Ei En
ð3Þ
where Ei and En are the internal energies of the ion and neutral states, which include excited electronic, vibrational, and rotational energies. If the initial neutral molecules are in their ground states (En ¼ 0), the PES of neutral molecules provides information about the corresponding cations; if some of the initial neutral molecules are in their excited states, information about the neutral excited states may also be obtained. Photoelectron spectroscopy can also be used to study negative ions. In this case, the technique is termed negative ion (anion) PES or photodetachment PES. 187
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Photoelectron Spectroscopy
Mn þ h ! Mðn1Þ þ e
ð4Þ
E ¼ EA þ Eðn1Þ En
ð5Þ
and
where EA is the electron affinity of Mn that is the minimum energy to eject en electron from the ground state of Mn to the ground state of the M(n1) and En and E(n1) are the internal energies of Mn and M(n1).
2.14.2
SPECTRAL FEATURES
A photoelectron spectrum is a record of the electron signal as a function of the electron kinetic energies at fixed photon energy or as a function of photon wavelengths at specific electron kinetic energy. Because molecular energy states are quantized, a photoelectron spectrum will consist of a series of discrete bands at various energy positions. Because photoelectron transition probabilities are proportional to transition dipole moments, the intensity of each ionization band will reflect the difference of the potential energy surfaces in the initial and final states. Thus, the characteristic features of an ionization band involve its energy, intensity, shape, and resolved fine structures (vibrational, rotational, vibronic, spin-orbit, etc). Each of these features is sensitive to the electronic state and geometry of the molecule. Figure 1 shows a textbook example of the N2 photoelectron spectrum taken at the HeI photon energy (21.2 eV).4 The spectrum consists of three band systems, originating from transitions of the electronic ground state 1gþ of N2 to the 2gþ, 2 u, and 2uþ states of N2þ. The 2gþ 1gþ and 2uþ 1gþ transitions consist of two peaks, separated by 2,150 and 2,390 cm1, respectively, whereas the 2u 1gþ transition has six peaks with a spacing of 1,810 cm1. These energy intervals correspond to the vibrational frequencies in the three ionic states, which are significantly different from the frequency of 2,345 cm1 in the neutral ground state. The measured vibrational frequencies infer that the strength of the N2þ bonding follows the trend of 2uþ > 2gþ2 > u. The number of the observed vibrational quanta indicates a larger change of bond distance with the 2u 1gþ transition than the 2g,uþ 1gþ transitions. The bond
Πg2p
State of N2+
15
σu2p
2,150 cm–1
2
∑g
16 σg2p Πu2p σu2s
1,810 cm–1 17
2
Πu
18
σg2s
2,390 cm–1
2
∑u
19 σu1s σg1s N atom
J.P.(eV)
N2
Figure 1 Left: the MO diagram of N2; middle: the PE spectrum of N2; right: the ion states of N2þ (reproduced by permission of Bancroft and Hu, 1999;4 # 1999, Wiley).
Photoelectron Spectroscopy
189
lengths of the 2 g, 2u and 2u states of N2þ are 1.11642, 1.1749, and 1.074 A˚, respectively, compared to 1.09769 A˚ in the 1gþ ground state of N2. The energy of an ion state may be related to the energy of a molecular orbital (MO) from which an electron is ejected by photoionization. In N2, the 1gþ ground state has an electron configuration of 1g2(1s2)1u2(1s2)2g2(2s2)2u2(2s2)1u4(2p4)3g 2(2p2). The removal of an electron from the 2u2, 1u4, and 3g2 orbitals forms the electronic states 2uþ, 2u, and 2gþ of N2þ, respectively. In a zero-order approximation or Koopmans theorem, the experimental energy of an ionic state equals the negative value of the theoretical one-electron orbital energy. Though rarely accurate in practice (because electron relaxation, correlation, and relativistic effect are ignored), Koopmans theorem has proven molecular orbitals to be more than a product of the quantum chemist’s imagination and has contributed to the understanding of photoelectron spectra and thus, the bonding and structure of molecules. For example, the reduced vibrational frequencies and increased bond lengths of the 2gþ and 2u states of N2þ (with respect to those of N2) can be viewed as the result of removing the bonding electrons from the g and u orbitals in the 1gþ ground state of N2. The increased vibrational frequency and shortened bond distance of the 2uþ state are due to the ejection of an antibonding electron from the neutral u orbital (Figure 1). Moreover, the smaller vibrational frequency, longer vibrational progression, and larger bond distance of the 2u state (with respect to that of the 2gþ state) reflect a stronger electron binding in the u orbital than in the g orbital of the neutral 1gþ state. Thus, the spectral information about a molecular ion can be related to the MO picture of the corresponding neutral.
2.14.3 2.14.3.1
SPECTRAL ASSIGNMENTS Electronic Structure Methods
Because PES is an experimental version of quantum chemistry, quantum chemical calculations play a powerful role in the interpretation of PE spectra. Such calculations provide electronic transition energies, vibrational frequencies, and rotational constants to compare with PE band positions. Theoretical electron configurations and geometries of the initial and final states can be used to correlate with spectral intensity profiles. In the 1970s and 1980s, most computations were carried at semiempirical and non-correlation ab initio levels of theory. Since the 1990s, higher levels of computational methods are increasingly employed, which include density functional theory, configuration interaction, many-body perturbation theory, and coupled cluster methods.5 Density functional theory has been the most popular choice because of its efficiency and reasonable accuracy.
2.14.3.2
Franck–Condon Principle
For vibrationally resolved PE spectra, comparing Franck–Condon factors and PE peak intensities helps identify the geometric conformation and electronic state of the photoelectron carrier. According to the Franck–Condon principle, the intensity of a vibrational peak in an electronically allowed transition is proportional to the absolute square of the overlap integral of the vibrational wave functions of the initial and final states. This overlap integral is known as the Franck–Condon factor. To calculate such factors, potential functions must be known for both the initial and final states and can be obtained by ab initio calculations. In many cases, a harmonic approximation has yielded adequate descriptions for low vibrational levels of potential wells.6,7
2.14.3.3
Photon Energy Dependence of Band Intensities
For PE spectra of metal compounds where vibrational fine structures are not resolved, the identification of the nature of the ionized MOs is often aided by taking the advantage of the photon energy dependence of molecular ionization cross sections. It has been shown early that the intensities of the metal d orbital ionization bands increase dramatically from HeI to HeII (40.8 eV) radiation, compared to those of ligand-based s and p ionization bands.8–11 This is known as the HeI/HeII intensity rule. More recently, synchrotron radiation has been employed to measure relative band intensities at different photon wavelengths.4,12,13 The synchrotron radiation has
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Photoelectron Spectroscopy
the advantage of the extensive wavelength tunability over the HeI/HeII radiations, allowing for detailed studies of photon energy dependence of ionization band intensities.
2.14.3.4
Ionization Energy Trends and Resolved Fine Structures
In some cases, ionization bands may be assigned by comparing the spectra of electronically and chemically related molecules. For example, ionization bands from highly localized metal orbitals are expected to show little energy shifts in a series of molecules consisting of a certain metal atom, whereas those from ligand-based orbitals should exhibit small energy changes for molecules containing the same ligand. In addition, resolved spin–orbit, Jahn–Teller, ligand-field, and vibrational splittings provide fingerprints about the nature of MO ionizations.4
2.14.4
HeI, HeII, AND X-RAY PHOTOELECTRON SPECTROSCOPY
HeI/HeII PES has greatly contributed to the understanding of electronic structures of coordinately saturated stable metal compounds. The technique has been widely used since the 1960s and is still being applied to new molecular systems.8–11,14–24 The HeI radiation, combined with a hemispheric electron analyzer, gives a total instrumental resolution of about 20 meV.21,22 This level of resolution is sufficient to resolve vibrational energy levels for simple and light molecules, but it is not enough to resolve low-frequency modes of metal compounds. Recently, a newly designed PE spectrometer consisting of a hemispherical analyzer and an electron cyclotron resonance He discharge lamp has improved the resolution to 3 meV,25 though the instrument has not been used on metal-containing molecules. HeII PES measurements are mainly used to aid spectral assignments according to the HeI/HeII intensity rule. Because the HeI/HeII radiation is in the UV region, the HeI/HeII PES is also known as UPS (ultraviolet photoelectron spectroscopy). X-ray PES or XPS has been used to study molecular core levels, and information from the core electron ionization can be used to evaluate orbital overlap and charge potential contributions to electron binding energies of valence orbitals.26 XPS resolution is lower than that of UPS due to broader X-ray photon line widths and shorter lifetimes of core electrons. The most commonly used X-ray sources involve MgK (1,253.6 eV ) and AlK (1,486.6 eV ). In XPS, a monochromator is used to remove the background continua and unwanted lines from the X-ray sources to increase the X-ray purity and reduce the X-ray line width. A line width of about 0.2 eV has been achieved for the MgK and AlK sources.27 The electron analyzer of an XPS spectrometer can be similar to that in a UPS spectrometer. Therefore, UPS and XPS spectra can be taken on the same instrument if both sources are available.
2.14.5
SYNCHROTRON RADIATION PHOTOELECTRON SPECTROSCOPY
Synchrotron radiation is produced by electrons circulating in a storage ring at nearly the speed of light and deflected in their trajectories by magnets.28 The major advantage of the synchrotron radiation is its wavelength tunability from UV to X-ray region. It can be used to study valence and core electrons and allows the investigation of photoionization cross sections as a function of photon energies. Since its application to metal carbonyls in 1987, synchrotron PES has been established as a powerful tool in elucidating molecular electronic structures.4,12,13,29 In studying core levels, synchrotron PE measurements have resolved ligand field splitting, vibrational splitting, and other fine structures of many molecular systems for the first time. For valence levels, it has provided unequivocal settlements for some controversies in PE spectral assignments. For example, TiCl4 was investigated by several HeI, HeII, and theoretical studies, however, the assignment of its PE spectrum has only been resolved by a combined synchrotron PE and theoretical study.30 The valence electron configuration of TiCl4 is predicted by theory to be either the form of 2a122t261e4 3t261t16 or 2t262a121e4 3t261t16, where the Cl 3s orbitals are not included. The 2a1 and 3t2 orbitals are predominantly Cl 3p, mixed with a small amount of Ti 4s in the 2a1 orbitals and Ti 3d in the 3t2 orbitals, the 2t2 and 1e are bonding orbitals between Cl 3p and Ti 3d, and the highest occupied orbital 1t1 is a non-bonding orbital of Cl 3p. The discrepancy between the two electron configurations lies in the energy ordering of the 2a1 and 2t2 orbitals. Figure 2 presents the PE spectra of the molecule recorded at photon energies (a) 24 and (b) 40 eV, which
191
Photoelectron Spectroscopy 1,200
A
(a) B
1,000
TiCl4 Band (C + D) (2t2 + 1e)
(C + D)
TiCl4 Band E (2a1)
0.8
0.3
E
400 200 0 11
12 13 14 Ionization energy (eV)
0.6
Branching ratio
600
Branching ratio
Counts
800
0.4 0.2
0.1
0
0
15
0.2
20
2,500
30 40 50 60 Photon energy/eV
70
20
TiCl4 Band (C + D) (2a1 + 1e)
(b)
70
TiCl4 Band E (2t2)
0.8
2,000
30 40 50 60 Photon energy/eV
0.6
1,000 500
0.6
Branching ratio
Branching ratio
Counts
0.5 1,500
0.4 0.2
11
12 13 14 Ionization energy (eV)
15
0.3 0.2 0.1 0
0
0
0.4
20
30
40
50
Photon energy/eV
60
70
20
30 40 50 60 Photon energy/eV
70
Figure 2 Left: PE spectra of TiCl4 recorded with photon energies at (a) 24 and (b) 40 eV; right: comparison of experimental (dashed line) and theoretical (solid line) branching ratios for the PE bands (C þ D) and E with two possible assignments (see text) (reproduced by permission of the American Chemical Society from Inorg. chem. 1994, 33, 5086–5093).
reproduce the band intensity profiles of HeI/HeII spectra. The spectra display four bands, labeled A, B, (C þ D), and E. The relative intensities of bands A and B decrease, whereas the intensity of band E increases from 20 to 40 eV. According to the HeI/HeII rule, bands A and B are expected to be the ionization of the ligand-based orbitals, which is consistent with the theoretical predictions of the Cl 3p-based 1t1 and 3t2 orbitals. Band (C þ D) is about twice as broad as any of the other bands and may be viewed as the ionization of the two closely lying 1e and 2t2 orbitals. However, the intensity increase for band E is not expected on the basis of the HeI/HeII intensity rule if the electron is ejected from the Cl 3p-based 2a1 orbital. Therefore, an alternative spectral assignment attributed band (C þ D) to the 1e and 2a1 orbitals and band E to the 2t2 orbitals. The disagreement between these two assignments is settled by synchrotron PE measurements and ionization cross section calculations in the photon energy range of 20–70 eV. Figure 2 presents theoretical (solid line) and experimental (dashed line) branching ratios (i.e., ratio of the intensity of a photoelectron band versus the sum of the intensities of the all bands in a spectrum) for the PE bands (C þ D) and E. The agreement between the experimental values for band (C þ D) and the theoretical values for the (2t2 þ 1e) orbitals is far superior to that from the alternative assignment. A branching ratio maximum is observed at 46 eV, in agreement with the calculated result if the band is from the ionization of the 2t2 and 1e orbitals, whereas the experimental and theoretical ratio profiles are completely different if the band is assigned to the 2a1 and 1e ionizations. For band E, the experimental branching ratio shows two peaks at 26 and 42 eV, which may be correlated with the calculated peaks at 26 and 48 eV from the 2a1 ionization and 22 and 46 eV from the 2t2 ionization. However, the experiment branching ratios exhibit no significant photon energy dependence of the band E intensity above 46 eV, whereas the calculation predicts a sharp decrease of the branching ratio for the 2t2 ionization. Therefore, band (C þ D) is assigned to the 2t2 þ 1e ionization and band E to the ionization of 2a1 orbital.
2.14.6
LASER-BASED PULSED FIELD IONIZATION-ZERO ELECTRON KINETIC ENERGY PHOTOELECTRON SPECTROSCOPY
Perhaps, the most important PES development over the past two decades is the introduction of pulsed field ionization-zero electron kinetic energy (PFI-ZEKE) technique.31–34 Other names in use for this technique include ZEKE-PFI, PFI-PE, or simply ZEKE. PFI-ZEKE involves the detection of electrons produced by delayed, pulsed, electric field ionization of very high-lying
192
Photoelectron Spectroscopy
Rydberg states with a principle quantum number n > 100. These Rydberg states are formed by laser excitation and are located a few cm1 (or a fraction of meV) below the ionization threshold. Because the electrons ejected from these Rydberg states carry near zero electron kinetic energy, these states are known as ZEKE states and the ejected electrons are named as ZEKEs. The measured electron peak position is lower than that without the presence of the field by the Stark shift ( ) ¼ CE 1=2 cm1
ð6Þ
where E is the magnitude of the pulsed electric field in the unit of V/cm1 and C is a proportional constant that can be determined experimentally. Experiments at various electric fields show that the value of C is 4 for metal-containing molecules.35 On the basis of the continuity of ionization oscillator strengths, the relative intensities of the photoelectron bands in a PFI-ZEKE spectrum are expected to be identical to those in a direct threshold photoionization PE spectrum, provided that perturbations by nearby autoionizing states are small. Indeed, PFI-ZEKE spectra of many metal compounds have shown Franck–Condon intensity profiles.6,7,36,37 The delay of pulsed field ionization from laser excitation is typically a few microseconds, which allows the ZEKE detection to be free from background electrons produced by direct photoionization and autoionization. Because of the pulsed nature of the experiment, a time-of-flight electron analyzer is used for the ZEKE detection. The longevity of the ZEKE states is currently contributed to the angular moment l mixing due to the presence of the weak d.c. field in the excitation volume and to the magnetic momentum ml mixing due to the inhomogeneous electric field formed by nearby prompt ions. The major advantage of PFI-ZEKE is its energy resolution, which allows the resolution of rotational and vibrational structures of metal compounds.6,7,34–51 For example, the ZEKE spectra of vanadium dimers (V2) have a line width of 1.5 cm1 (0.19 meV), from which the rotational
Figure 3 ZEKE spectra of MNH3 (M ¼ Al, Ga, In) and spectral simulations for indium adduct and insertion isomers (reproduced by permission of the American Chemical Society from J. Phys. Chem. A 2000, 104, 8178–8182).
Photoelectron Spectroscopy
193
constant and bond length of the corresponding cation have been accurately determined for the first time.51 Figure 3 shows the vibrationally resolved PFI-ZEKE spectra of MNH3 (M ¼ Al, Ga, In), with spectral line widths of 5.0 cm1.42–44 Each of the spectra displays a short vibrational progression and a small peak (a) at the left side of the strongest peak. Additionally, the AlNH3 spectrum exhibits a doublet in the first three strong bands. The doublet is separated by 58 cm1 and is due to the spin–orbit interaction in AlNH3. The splitting is not observed for the Ga and In species due to large energy separations between the two spin components of the heavy metal atoms, which prevent a significant population of the upper spin level. The vibrational progressions yield the MþN stretching frequencies of 339 (AlþN), 270 (GaþN), and 234 (InþN) cm1. The strongest peaks determine adiabatic IEs, 39,746, 40,135, and 39,689 cm1, for the Al, Ga, and In complexes, respectively. The small peaks (a) arise from the transitions of the first vibrational levels of MNH3 to the vibrational ground states of MNH3 and yield the M–N stretching frequencies of 227, 161, and 141 cm1 for the three neutral radicals. Converting the frequencies into stretching force constants shows the strength of the metal–nitrogen bonding in the order of Al > Ga > In, and the trend is explained by involving electrostatic and orbital interactions. To determine the geometric conformations of the complexes, density functional calculations were used to obtain the minimum energy structures and harmonic vibrational analyses for both the neutral and the ion; Franck–Condon factors were calculated using the theoretical geometries and vibrational analysis, and spectral simulations were performed with the experimental line widths and at various temperatures to obtain the best fit. A comparison of the 100 K simulations from the two indium isomers (Figures 4d and 4e) and the experimental spectrum of InNH3 (Figure 3(c)) clearly identifies that the electron carrier is a simple adduct rather than an insertion compound. An alternative to PFI-ZEKE would be mass-analyzed threshold ionization (MATI), where cations rather than ZEKE electrons are detected.52 In principle, MATI is attractive because of the inherent mass selection. However, MATI experiments are difficult to implement because of ion separation fields affecting the high Rydberg levels required for delayed pulsed field ionizations. The technique has so far only been used for the smallest metal molecules.53
Figure 4 Left: PE spectra of Re2Cl82 taken with 157, 193, 266, and 355 nm lasers; right: schematic drawing of potential energy curves showing the repulsive Coulomb barriers (RCB, eV) and vertical detachment energies (eV) of PE bands. The relative positions of the five laser wavelengths are also indicated (reproduced by permission of the American Chemical Society from J. Am. Chem. Soc. 2000, 122, 2096–2100).
194 2.14.7
Photoelectron Spectroscopy LASER-BASED NEGATIVE ION PHOTOELECTRON SPECTROSCOPY
Although the principle is the same for photoionization of neutrals and photodetachment of anions, their description differs in detail. First, negative ion PES has the advantage of species selectivity when combined with mass spectrometry. Second, all neutrals have positive IEs, whereas EAs of some anions may be negative. If a singly charged anion has a negative EA, the ion is energetically unstable. Third, an electron ejected from a neutral experiences an attractive Coulomb field of the positively charged core, an electron detached from a singly charged anion is subject to a angular momentum interaction with the residual neutral core, and photodetachment of a multiply charged anion results in Coulomb repulsion between two negatively charged particles (an anion and a free electron). Anions do not possess Rydberg-analogous states that converge to neutral states; thus, PFI technique does not apply to these species. For multiply charged anions, threshold photodetachment is not feasible because Coulomb repulsion barriers are often higher than the EAs of anions. Similar to neutral PES, negative ion PES can use a fixed-frequency light source and measure the energies of detached electrons, or scan photon wavelengths and detect ZEKE electrons. Most studies have been reported on singly charged anions and at fixed photon energies,54–64 although anion ZEKE experiments have also been reported.34,65–71 More recently, studies have also been carried out for multiply charged negative ions.72–76 An anion PES instrument consists of a photodetachment laser, an anion source, a mass selector, and an electron analyzer. Singly charged metal-containing anions are often prepared by pulsed laser vaporization in supersonic jets,54–61 or in electric discharge-flowing afterglow reactors,63,64 whereas multiply charged anions are produced by electrospray.73–76 Ions with a specific mass are selected by magnetic, electrostatic, or time-of-flight mass spectrometers. There are basically three widely used types of electron analyzers. The simplest one is a time-of-flight spectrometer. Its disadvantage is the small acceptance angle and thus, a small collection efficiency for photoelectrons. This problem is avoided in a magnetic bottle electron spectrometer that has high collection efficiency.57,61 The high efficiency is achieved by magnetic fields which guide the ejected electrons into the detector. However, the collection of electrons with relatively large velocity differences results in a reduced spectral resolution. For experiments with a continuous ion source (e.g., flowing afterglow), a hemispherical analyzer can be used to provide a better energy resolution, but a lower collection efficiency, than that of the magnetic bottle spectrometer.63,64 Combining an electrospray ion source with anion PES has opened up a new avenue for studying multiply charged anions in the gas phase. It provides information about the repulsive Coulomb barrier produced by the superposition of a long-range Coulomb repulsion between two negatively charged photoproducts and a short-range electron binding. Because of the presence of this barrier, electron detachment requires the incident photon energy to be greater than the Coulomb potential if electron tunneling is ignored. Figure 4 shows the PE spectra (left) and band positions (right) of Re2Cl82 at 157 nm (7.866 eV), 193 nm (6.424 eV), 266 nm (4.661 eV), and 355 nm (3.496 eV).75 The 157 nm spectrum exhibits eight bands labeled X—G. Bands X, A, and B are attributed to three ReRe bonding orbitals (, , and ), whereas features C—F are assigned to ReCl bonding and Cl non-bonding orbitals. The analysis of the PE spectrum confirms a (. . .242) electron configuration and a formal quadruple ReRe bond of the dianion. As photon wavelengths increase, the number of PE bands decrease. Because the Coulomb barrier should not be larger than the energy difference between the incident photon and the electron binding (if the binding energy is positive), the Coulomb barrier can be estimated to be <2.77 eV (hEG) from the 157 nm spectrum, <2.79 eV (hEC) from the 193 nm spectrum, <2.34 eV (hEA) from the 266 nm spectrum, and <2.34 eV (hEX) from the 355 nm spectrum. Moreover, a measurement at 532 nm (2.331 eV) shows no photoelectron signal, indicating that the Coulomb barrier is at least 2.33 eV. Therefore, the repulsion potential between Re2Cl82 and the free electron should be 2.3 eV at its ground state. If the two negative charges of Re2Cl82 are localized on the Cl atoms, the intramolecular Coulomb repulsion may be calculated according to Coulomb’s law, e2/4"0r. The calculated value is 2.5 eV, fairly close to the experimental value of 2.3 eV. If the Coulomb repulsion is stronger than the electron binding, the anion shall have a negative electron binding energy. For example, the binding energy of a quadruply charged anion, copper phthalocyanine, 3,40 ,400 ,4000 -tetrasulfonate [CuPc(SO3)4]4, is measured to be 0.9 eV.76
2.14.8
OTHER PHOTOELECTRON TECHNIQUES
Other PES methods have also been developed and may be of potential uses for metal compounds. These techniques include, among others, vacuum ultraviolet (VUV) laser-based PFI,77
Photoelectron Spectroscopy
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synchrotron-based PFI,78 photoelectron–photoion coincidence,79–83 two-dimensional imaging,84,85 and femtosecond time-resolved experiments.86,87 The VUV laser and synchrotron PFI may be used for high-resolution PES studies of metal compounds with higher IE (>6 eV). However, at the present time neither VUV lasers nor synchrotron radiations can provide photon intensities as high as UV lasers. The photoelectron–photoion coincidence technique correlates the electron signal to its carrier, but it often operates at relatively low particle counting rates. The two-dimensional PES, which measures photoelectron yields as a function of electron and photon energies, can provide new opportunities to study photoexcitation and photoionization processes. The femtosecond time-resolved technique is emerging as a new means for the study of ultrafast excited state dynamics.
2.14.9 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46.
REFERENCES
Turner, D. W.; Baker, C.; Baker, A. D.; Brundle, C. R. Molecular Photoelectron Spectroscopy; Wiley: London, 1970. Eland, J. H. D. Photoelectron Spectroscopy; Butterworths: London, 1974. Rabalais, J. W. Principle of Ultraviolet Photoelectron Spectroscopy; Wiley: New York, 1977. Bancroft, G. M.; Hu, Y. F. In Inorganic Electronic Structure and Spectroscopy, Volume 1: Methodology. Solomon, E. I., Lever, A. B. P., Eds.; Wiley: New York, 1999, pp 443–512. Jensen, F. Introduction to Computational Chemistry; Wiley: Chichester, 1999. Yang, D. S. Coord. Chem. Rev. 2001, 214, 187–213. Be´rces, A.; Zgierski, M. Z.; Yang, D. S. In Computational Molecular Spectroscopy; Jensen, P., Bunker, P. R., Eds.; Wiley: New York, 2000, pp 109–134. Cowley, A. H. Prog. Inorg. Chem. 1979, 26, 45–160. Green, J. C. Struct. Bonding (Berlin) 1981, 43, 37–112. Solomon, E. I. Comm. Inorg. Chem. 1984, 3, 227–320. Lichtenberger, D. L.; Kellogg, G. E. Acc. Chem. Res. 1987, 20, 379–387. Green, J. C. Acc. Chem. Res. 1994, 27, 131–137. Li, X.; Bancroft, G. M.; Puddephatt, R. J. Acc. Chem. Res. 1997, 30, 213–218. Oskam, A. In Spectroscopy of Inorganic-based Materials; Clark, R. J. H., Hester, R. E., Eds.; Wiley: New York, 1987, pp 429–461. Yang, D. S.; Bancroft, G. M.; Puddephatt, R. J.; Bursten, B. E.; McKee, S. D. Inorg. Chem. 1989, 28, 872–877. Yang, D. S.; Bancroft, G. M.; Dignard-Bailey, L.; Puddephatt, R. J.; Tse, J. S. Inorg. Chem. 1990, 29, 2118–2122, 2496–2501. Yang, D. S.; Bancroft, G. M.; L. Puddephatt, R. J.; Tse, J. S. Inorg. Chem. 1990, 29, 2487–2495. Lichtenberger, D.; Rai-Chaudhuri, A.; Hogan, R. H. In Organometallic Chemistry; Fehlner, P., Ed.; Plenum: New York, 1992, pp 223–251. Lichtenberger, D. L.; Lynn, M. A.; Chisholm, M. H. J. Am. Chem. Soc. 1999, 121, 12167–12176. Westcott, B. L.; Gruhn, N. E.; Michelsen, L. J.; Lichtenberger, D. L. J. Am. Chem. Soc. 2000, 122, 8083–8084. Lichtenberger, D. L.; Gruhn, N.; Rai-Chaudhuri, A.; Renshaw, S. K.; Gladysz, J. A.; Jiao, H.; Seyler, J.; Igau, A. J. Am. Chem. Soc. 2002, 124, 1417–1423. Westcott, B. L.; Gruhn, N. E.; Enemark, J. H. J. Am. Chem. Soc. 1998, 120, 3382–3386. Downs, A. J.; Green, J. C.; McGrady, G. S.; Munkman, N.; Parkin, R. P. G. Dalton 2000, 21–27. Al-Ktaifani, M.; Green, J. C.; Hitchcock, P. B.; Nixon, J. F. J. Chem. Soc., Dalton Trans. 2001, 1726–1731. Baltzer, P.; Karlsson, L.; Lundqvist, M.; Wannberg, B. Rev. Sci. Instrum. 1993, 64, 2179–2189. Jolly, W. L. Acc. Chem. Res. 1983, 16, 370–376. Hollas, J. M. High Resolution Spectroscopy, Butterworths: London; 1982, pp 435–480. Margaritondo, G. Introduction to Synchrotron Radiation, Oxford University Press: New York, 1988. Hu, Y. F.; Bancroft, G. M.; Tan, K. H. J. Electron. Spectrosc. Relat. Phenom. 2001, 114–116, 147–151. Bursten, B. E.; Green, J. C.; Kaltsoyannis, N.; MacDonald, M. A.; Sze, K. H.; Tse, J. S. Inorg. Chem. 1994, 33, 5086–5093. Mu¨ller-Dethlefs, K.; Schlag, E. W. Ann. Rev. Phys. Chem. 1991, 42, 109–136. Mu¨ller-Dethlefs, K.; Schlag, E. W.; Grant, E. R.; Wang, K.; McKoy, B. V. In Advances in Chemistry and Physics, Volume XC.; Rice, S. A., Prigogine, I., Eds.; Wiley: New York, 1995, pp 1–103. Schlag, E. W. ZEKE Spectroscopy; Cambridge University Press: Cambridge, 1998. Mu¨ller-Dethlefs, K.; Schlag, E. W. Angew. Chem., Int. Ed. Engl. 1998, 37, 1346–1373. Yang, D. S.; James, A. M.; Rayner, D. M.; Hackett, P. A. Chem. Phys. Lett. 1994, 231, 177–182. Yang, D. S.; Hackett, P. A. J. Electron. Spectrosc. Relat. Phenom. 2000, 106, 153–169. Yang, D. S. In Advances in Metal and Semiconductor Clusters, Volume 5: Metal Ion Solvation and Metal–Ligand Interactions. Duncan, M. A., Ed.; Elsevier: Amsterdam, 2001, pp 187–226. Rodham, D. A.; Black, G. A. Chem. Phys. Lett. 1997, 264, 522–530. Panov, S. I.; Powers, D. E.; Miller, T. A. J. Chem. Phys. 1997, 108, 1335–1346. Barckholtz, T. A.; Powers, D. E.; Miller, T. A.; Bursten, B. E. J. Am. Chem. Soc. 1999, 121, 2576–2584. Agreiter, J. K.; Knight, A. M.; Duncan, M. A. Chem. Phys. Lett. 1999, 313, 162–170. Yang, D. S.; Miyawaki, J. Chem. Phys. Lett. 1999, 313, 514–518. Rothschopf, G. K.; Perkins, J. S.; Li, S.; Yang, D. S. J. Phys. Chem. A 2000, 104, 8178–8182. Li, S.; Rothschopf, G. K.; Pillai, D.; Sohnlein, B. R.; Wilson, B. M.; Yang, D. S. J. Chem. Phys. 2001, 115, 7968–7974. Rothschopf, G. K.; Li, S.; Perkins, J. S.; Yang, D. S. J. Chem. Phys. 2001, 115, 4565–4572. Pedersen, D. B.; Zgierski, M. Z.; Anderson, S.; Rayner, D. M.; Simard, B.; Li, S.; Yang, D. S. J. Phys. Chem. A 2001, 105, 11462–11469.
196 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87.
Photoelectron Spectroscopy
Li, S.; Rothschipf, G. K.; Yang, D. S. J. Chem. Phys. 2002, 116, 6589–6594. Harrington, J.; Weisshaar, J. C. J. Chem. Phys. 1992, 97, 2809–2812. Ne´meth, G. I.; Ungar, H. Yeretzian, C.; Selzle, H. L.; Schlag, E. W. Chem. Phys. Lett. 1994, 228, 1–8. Linton, C.; Simard, B.; Peter-Look, H.; Wallin, S.; Rothschopf, G. K.; Gunion, R. F.; Morse, M. D. J. Chem. Phys. 1999, 111, 5017–5026. Yang, D. S.; James, A. M.; Rayner, D. M.; Hackett, P. A. J. Chem. Phys. 1995, 102, 3129–3134. Zhu, L.; Johnson, P.; J. Chem. Phys. 1991, 94, 5679–5681. Wiley, K. F.; Yeh, C. S.; Duncan, M. A. Chem. Phys. Lett. 1993, 211, 156–160. Kaya, K.; Nakajima, A. In Adv. Met. Semicond. Clusters, Volume 2; Duncan, M. A., Ed.; JIA: Greenwich, 1995, pp 87–114. Arnold, C. C.; Neumark, D. M. In Adv. Met. Semicond. Clusters; Duncan, M. A., Ed.; JIA: Greenwich, 1995, pp 113–148. Hendricks, J. H.; De Clercq, H. L.; Lyapustina, S. A.; Fancher, C. A.; Lippa, T. P.; Collins, J. M.; Arnold, S. T.; Lee, G. H.; Bowen, K. H. Front. Sci. Ser. 1996, 16, 321–328. Wang, L. S.; Wu, H. In Adv. Met. Semicond. Clusters, Volume 4; Duncan, M. A., Ed.; JIA: Greenwich, 1998, pp 299–343. Boesl, U.; Knott, W. J. Mass Spectrom. Rev. 1999, 17, 275–305. Fuke, K.; Hashimoto, K.; Takasu, R. In Adv. Met. Semicond. Clusters, Volume 4; Duncan, M. A., Ed.; Elsevier: Amsterdam, 2001, pp 299–343. Conceicao, J.; Laaksonen, R. T.; Wang, L. S.; Guo, T.; Nordlander, P.; Smalley, R. E. Phys. Rev. B 1995, 51, 4668–4671. Kietzmann, H.; Morenzin, J.; Bechthold, P. S.; Gantefor, G.; Eberhardt, W. J. Chem. Phys. 1998, 109, 2275–2278. Dessent, C. E. Johnson, M. A. In Fundamentals and Applications of Gas Phase Ion Chemistry; Jennings, K. R., Ed.; Kluwer: Netherlands, 1999, pp 287–306. Ervin, K. M.; Lineberger, W. C. In Adv. Gas Phase Ion Chem. Volume 1; Adams, N. G., Babcock, L. M., Eds.; JAI: Greenwich, 1992, pp 121–166. Green, S. M. E.; Alex, S.; Fleischer, N. L.; Millam, E. L.; March, T. P.; Leopold, D. G. J. Chem. Phys. 2001, 114, 2653–2668. Gantefor, G. F.; Cox, D. M.; Kaldor, A. Z. Phys. D Atoms Mol. Clusters 1991, 19, 59–61. Gantefor, G. F.; Cox, D. M.; Kaldor, A. Z. J. Chem. Phys. 1990, 93, 8395–8396. Gantefor, G. F.; Cox, D. M.; Kaldor, A. Z. J. Chem. Phys. 1992, 96, 4102–4105. Arnold, C. C.; Neumark, D. M. J. Chem. Phys. 1993, 99, 3353–3362. Arnold, C. C.; Neumark, D. M. J. Chem. Phys. 1993, 100, 1797–1804. Arnold, C. C.; Xu, C.; Burton, G. R.; Neumark, D. M. J. Chem. Phys. 1995, 102, 6982–6989. Drechsler, G.; Be¨ßmann, C.; Bosel, U.; Schlag, E. W. J. Mol. Struct. 1995, 348, 337–340. Compton, R. N. In Advances in Gas Phase Ion Chemistry; Adams, N. G., Babcock, L. M., Eds.; JAI: Greenwich, 1992, pp 257–305. Wang, L. S.; Wang, X. B. J. Phys. Chem. A 2000, 104, 1978–1990. Wang, L. S. Comm. Mod. Phys. 2001, 2, D207–221. Wang, X. B.; Wang, L. S. J. Am. Chem. Soc. 2000, 122, 2096–2100. Wang, X. B.; Wang, L. S. Nature 1999, 400, 2096–2100. Ng, C. Y. Int. J. Mass Spectrom 2000, 200, 357–386. Ng, C. Y. J. Electron. Spectrosc. Relat. Phenom. 2000, 108, 41–45. Baer, T. Int. J. Mass Spectrom. 2000, 200, 443–457. Continetti, R. E. Annu. Rev. Phys. Chem. 2001, 52, 165–192. Eland, J. H. D. J. Electron. Spectrosc. Relat. Phenom. 2000, 112, 1–8. Becker, U. J. Electron. Spectrosc. Relat. Phenom. 2000, 112, 47–65. Hall, R. I.; McConkey, A.; Ellis, K.; Dawber, G.; Avaldi, L.; MacDonald, M. A.; King, G. C. Meas. Sci. Technol. 1992, 3, 316–324. Sokell, E.; Wills, A. A.; Cubric, D.; Currell, F. J.; Comer, J.; Hammond, P. J. Electron. Spectrosc. Relat. Phenom. 1998, 94, 107–125. Berrah, N.; Langer, B.; Wills, A. A.; Kukk, E.; Bozek, J. D.; Farhat, A.; Gorczyca, T. W. J. Electron. Spectrosc. Relat. Phenom. 1999, 101–103, 1–11. Lochbrunner, S.; Larsen, J. J.; Shaffer, J. P.; Schmitt, M.; Schultz, T.; Underwood, J. G.; Stolow, A. J. Electron. Spectrosc. Relat. Phenom. 2000, 112, 183–198. Neumark, D. M. Annu. Rev. Phys. Chem. 2001, 52, 255–277.
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Comprehensive Coordination Chemistry II ISBN (set): 0-08-0437486 Volume 2, (ISBN 0-08-0443249); pp 187–196
2.14.1
187 188 189 189 189 189 190 190 190 191 194 194 195
BASIC CONCEPTS
Molecular photoelectron spectroscopy (PES) involves the application of the photoelectric effect to the study of electronic states and geometric structures.1–3 Electrons are ejected from molecules by irradiation of monochromatic photons with energy h, M þ h ! M þ þ e
ð1Þ
The energy difference E between a final ion state and an initial neutral state equals the incident photon energy minus the kinetic energy KE of the ejected electron, E ¼ h KE
ð2Þ
By defining adiabatic ionization energy (IE) as the minimum energy required to remove an electron from the ground electronic state of the neutral, E ¼ IE þ Ei En
ð3Þ
where Ei and En are the internal energies of the ion and neutral states, which include excited electronic, vibrational, and rotational energies. If the initial neutral molecules are in their ground states (En ¼ 0), the PES of neutral molecules provides information about the corresponding cations; if some of the initial neutral molecules are in their excited states, information about the neutral excited states may also be obtained. Photoelectron spectroscopy can also be used to study negative ions. In this case, the technique is termed negative ion (anion) PES or photodetachment PES. 187
188
Photoelectron Spectroscopy
Mn þ h ! Mðn1Þ þ e
ð4Þ
E ¼ EA þ Eðn1Þ En
ð5Þ
and
where EA is the electron affinity of Mn that is the minimum energy to eject en electron from the ground state of Mn to the ground state of the M(n1) and En and E(n1) are the internal energies of Mn and M(n1).
2.14.2
SPECTRAL FEATURES
A photoelectron spectrum is a record of the electron signal as a function of the electron kinetic energies at fixed photon energy or as a function of photon wavelengths at specific electron kinetic energy. Because molecular energy states are quantized, a photoelectron spectrum will consist of a series of discrete bands at various energy positions. Because photoelectron transition probabilities are proportional to transition dipole moments, the intensity of each ionization band will reflect the difference of the potential energy surfaces in the initial and final states. Thus, the characteristic features of an ionization band involve its energy, intensity, shape, and resolved fine structures (vibrational, rotational, vibronic, spin-orbit, etc). Each of these features is sensitive to the electronic state and geometry of the molecule. Figure 1 shows a textbook example of the N2 photoelectron spectrum taken at the HeI photon energy (21.2 eV).4 The spectrum consists of three band systems, originating from transitions of the electronic ground state 1gþ of N2 to the 2gþ, 2 u, and 2uþ states of N2þ. The 2gþ 1gþ and 2uþ 1gþ transitions consist of two peaks, separated by 2,150 and 2,390 cm1, respectively, whereas the 2u 1gþ transition has six peaks with a spacing of 1,810 cm1. These energy intervals correspond to the vibrational frequencies in the three ionic states, which are significantly different from the frequency of 2,345 cm1 in the neutral ground state. The measured vibrational frequencies infer that the strength of the N2þ bonding follows the trend of 2uþ > 2gþ2 > u. The number of the observed vibrational quanta indicates a larger change of bond distance with the 2u 1gþ transition than the 2g,uþ 1gþ transitions. The bond
Πg2p
State of N2+
15
σu2p
2,150 cm–1
2
∑g
16 σg2p Πu2p σu2s
1,810 cm–1 17
2
Πu
18
σg2s
2,390 cm–1
2
∑u
19 σu1s σg1s N atom
J.P.(eV)
N2
Figure 1 Left: the MO diagram of N2; middle: the PE spectrum of N2; right: the ion states of N2þ (reproduced by permission of Bancroft and Hu, 1999;4 # 1999, Wiley).
Photoelectron Spectroscopy
189
lengths of the 2 g, 2u and 2u states of N2þ are 1.11642, 1.1749, and 1.074 A˚, respectively, compared to 1.09769 A˚ in the 1gþ ground state of N2. The energy of an ion state may be related to the energy of a molecular orbital (MO) from which an electron is ejected by photoionization. In N2, the 1gþ ground state has an electron configuration of 1g2(1s2)1u2(1s2)2g2(2s2)2u2(2s2)1u4(2p4)3g 2(2p2). The removal of an electron from the 2u2, 1u4, and 3g2 orbitals forms the electronic states 2uþ, 2u, and 2gþ of N2þ, respectively. In a zero-order approximation or Koopmans theorem, the experimental energy of an ionic state equals the negative value of the theoretical one-electron orbital energy. Though rarely accurate in practice (because electron relaxation, correlation, and relativistic effect are ignored), Koopmans theorem has proven molecular orbitals to be more than a product of the quantum chemist’s imagination and has contributed to the understanding of photoelectron spectra and thus, the bonding and structure of molecules. For example, the reduced vibrational frequencies and increased bond lengths of the 2gþ and 2u states of N2þ (with respect to those of N2) can be viewed as the result of removing the bonding electrons from the g and u orbitals in the 1gþ ground state of N2. The increased vibrational frequency and shortened bond distance of the 2uþ state are due to the ejection of an antibonding electron from the neutral u orbital (Figure 1). Moreover, the smaller vibrational frequency, longer vibrational progression, and larger bond distance of the 2u state (with respect to that of the 2gþ state) reflect a stronger electron binding in the u orbital than in the g orbital of the neutral 1gþ state. Thus, the spectral information about a molecular ion can be related to the MO picture of the corresponding neutral.
2.14.3 2.14.3.1
SPECTRAL ASSIGNMENTS Electronic Structure Methods
Because PES is an experimental version of quantum chemistry, quantum chemical calculations play a powerful role in the interpretation of PE spectra. Such calculations provide electronic transition energies, vibrational frequencies, and rotational constants to compare with PE band positions. Theoretical electron configurations and geometries of the initial and final states can be used to correlate with spectral intensity profiles. In the 1970s and 1980s, most computations were carried at semiempirical and non-correlation ab initio levels of theory. Since the 1990s, higher levels of computational methods are increasingly employed, which include density functional theory, configuration interaction, many-body perturbation theory, and coupled cluster methods.5 Density functional theory has been the most popular choice because of its efficiency and reasonable accuracy.
2.14.3.2
Franck–Condon Principle
For vibrationally resolved PE spectra, comparing Franck–Condon factors and PE peak intensities helps identify the geometric conformation and electronic state of the photoelectron carrier. According to the Franck–Condon principle, the intensity of a vibrational peak in an electronically allowed transition is proportional to the absolute square of the overlap integral of the vibrational wave functions of the initial and final states. This overlap integral is known as the Franck–Condon factor. To calculate such factors, potential functions must be known for both the initial and final states and can be obtained by ab initio calculations. In many cases, a harmonic approximation has yielded adequate descriptions for low vibrational levels of potential wells.6,7
2.14.3.3
Photon Energy Dependence of Band Intensities
For PE spectra of metal compounds where vibrational fine structures are not resolved, the identification of the nature of the ionized MOs is often aided by taking the advantage of the photon energy dependence of molecular ionization cross sections. It has been shown early that the intensities of the metal d orbital ionization bands increase dramatically from HeI to HeII (40.8 eV) radiation, compared to those of ligand-based s and p ionization bands.8–11 This is known as the HeI/HeII intensity rule. More recently, synchrotron radiation has been employed to measure relative band intensities at different photon wavelengths.4,12,13 The synchrotron radiation has
190
Photoelectron Spectroscopy
the advantage of the extensive wavelength tunability over the HeI/HeII radiations, allowing for detailed studies of photon energy dependence of ionization band intensities.
2.14.3.4
Ionization Energy Trends and Resolved Fine Structures
In some cases, ionization bands may be assigned by comparing the spectra of electronically and chemically related molecules. For example, ionization bands from highly localized metal orbitals are expected to show little energy shifts in a series of molecules consisting of a certain metal atom, whereas those from ligand-based orbitals should exhibit small energy changes for molecules containing the same ligand. In addition, resolved spin–orbit, Jahn–Teller, ligand-field, and vibrational splittings provide fingerprints about the nature of MO ionizations.4
2.14.4
HeI, HeII, AND X-RAY PHOTOELECTRON SPECTROSCOPY
HeI/HeII PES has greatly contributed to the understanding of electronic structures of coordinately saturated stable metal compounds. The technique has been widely used since the 1960s and is still being applied to new molecular systems.8–11,14–24 The HeI radiation, combined with a hemispheric electron analyzer, gives a total instrumental resolution of about 20 meV.21,22 This level of resolution is sufficient to resolve vibrational energy levels for simple and light molecules, but it is not enough to resolve low-frequency modes of metal compounds. Recently, a newly designed PE spectrometer consisting of a hemispherical analyzer and an electron cyclotron resonance He discharge lamp has improved the resolution to 3 meV,25 though the instrument has not been used on metal-containing molecules. HeII PES measurements are mainly used to aid spectral assignments according to the HeI/HeII intensity rule. Because the HeI/HeII radiation is in the UV region, the HeI/HeII PES is also known as UPS (ultraviolet photoelectron spectroscopy). X-ray PES or XPS has been used to study molecular core levels, and information from the core electron ionization can be used to evaluate orbital overlap and charge potential contributions to electron binding energies of valence orbitals.26 XPS resolution is lower than that of UPS due to broader X-ray photon line widths and shorter lifetimes of core electrons. The most commonly used X-ray sources involve MgK (1,253.6 eV ) and AlK (1,486.6 eV ). In XPS, a monochromator is used to remove the background continua and unwanted lines from the X-ray sources to increase the X-ray purity and reduce the X-ray line width. A line width of about 0.2 eV has been achieved for the MgK and AlK sources.27 The electron analyzer of an XPS spectrometer can be similar to that in a UPS spectrometer. Therefore, UPS and XPS spectra can be taken on the same instrument if both sources are available.
2.14.5
SYNCHROTRON RADIATION PHOTOELECTRON SPECTROSCOPY
Synchrotron radiation is produced by electrons circulating in a storage ring at nearly the speed of light and deflected in their trajectories by magnets.28 The major advantage of the synchrotron radiation is its wavelength tunability from UV to X-ray region. It can be used to study valence and core electrons and allows the investigation of photoionization cross sections as a function of photon energies. Since its application to metal carbonyls in 1987, synchrotron PES has been established as a powerful tool in elucidating molecular electronic structures.4,12,13,29 In studying core levels, synchrotron PE measurements have resolved ligand field splitting, vibrational splitting, and other fine structures of many molecular systems for the first time. For valence levels, it has provided unequivocal settlements for some controversies in PE spectral assignments. For example, TiCl4 was investigated by several HeI, HeII, and theoretical studies, however, the assignment of its PE spectrum has only been resolved by a combined synchrotron PE and theoretical study.30 The valence electron configuration of TiCl4 is predicted by theory to be either the form of 2a122t261e4 3t261t16 or 2t262a121e4 3t261t16, where the Cl 3s orbitals are not included. The 2a1 and 3t2 orbitals are predominantly Cl 3p, mixed with a small amount of Ti 4s in the 2a1 orbitals and Ti 3d in the 3t2 orbitals, the 2t2 and 1e are bonding orbitals between Cl 3p and Ti 3d, and the highest occupied orbital 1t1 is a non-bonding orbital of Cl 3p. The discrepancy between the two electron configurations lies in the energy ordering of the 2a1 and 2t2 orbitals. Figure 2 presents the PE spectra of the molecule recorded at photon energies (a) 24 and (b) 40 eV, which
191
Photoelectron Spectroscopy 1,200
A
(a) B
1,000
TiCl4 Band (C + D) (2t2 + 1e)
(C + D)
TiCl4 Band E (2a1)
0.8
0.3
E
400 200 0 11
12 13 14 Ionization energy (eV)
0.6
Branching ratio
600
Branching ratio
Counts
800
0.4 0.2
0.1
0
0
15
0.2
20
2,500
30 40 50 60 Photon energy/eV
70
20
TiCl4 Band (C + D) (2a1 + 1e)
(b)
70
TiCl4 Band E (2t2)
0.8
2,000
30 40 50 60 Photon energy/eV
0.6
1,000 500
0.6
Branching ratio
Branching ratio
Counts
0.5 1,500
0.4 0.2
11
12 13 14 Ionization energy (eV)
15
0.3 0.2 0.1 0
0
0
0.4
20
30
40
50
Photon energy/eV
60
70
20
30 40 50 60 Photon energy/eV
70
Figure 2 Left: PE spectra of TiCl4 recorded with photon energies at (a) 24 and (b) 40 eV; right: comparison of experimental (dashed line) and theoretical (solid line) branching ratios for the PE bands (C þ D) and E with two possible assignments (see text) (reproduced by permission of the American Chemical Society from Inorg. chem. 1994, 33, 5086–5093).
reproduce the band intensity profiles of HeI/HeII spectra. The spectra display four bands, labeled A, B, (C þ D), and E. The relative intensities of bands A and B decrease, whereas the intensity of band E increases from 20 to 40 eV. According to the HeI/HeII rule, bands A and B are expected to be the ionization of the ligand-based orbitals, which is consistent with the theoretical predictions of the Cl 3p-based 1t1 and 3t2 orbitals. Band (C þ D) is about twice as broad as any of the other bands and may be viewed as the ionization of the two closely lying 1e and 2t2 orbitals. However, the intensity increase for band E is not expected on the basis of the HeI/HeII intensity rule if the electron is ejected from the Cl 3p-based 2a1 orbital. Therefore, an alternative spectral assignment attributed band (C þ D) to the 1e and 2a1 orbitals and band E to the 2t2 orbitals. The disagreement between these two assignments is settled by synchrotron PE measurements and ionization cross section calculations in the photon energy range of 20–70 eV. Figure 2 presents theoretical (solid line) and experimental (dashed line) branching ratios (i.e., ratio of the intensity of a photoelectron band versus the sum of the intensities of the all bands in a spectrum) for the PE bands (C þ D) and E. The agreement between the experimental values for band (C þ D) and the theoretical values for the (2t2 þ 1e) orbitals is far superior to that from the alternative assignment. A branching ratio maximum is observed at 46 eV, in agreement with the calculated result if the band is from the ionization of the 2t2 and 1e orbitals, whereas the experimental and theoretical ratio profiles are completely different if the band is assigned to the 2a1 and 1e ionizations. For band E, the experimental branching ratio shows two peaks at 26 and 42 eV, which may be correlated with the calculated peaks at 26 and 48 eV from the 2a1 ionization and 22 and 46 eV from the 2t2 ionization. However, the experiment branching ratios exhibit no significant photon energy dependence of the band E intensity above 46 eV, whereas the calculation predicts a sharp decrease of the branching ratio for the 2t2 ionization. Therefore, band (C þ D) is assigned to the 2t2 þ 1e ionization and band E to the ionization of 2a1 orbital.
2.14.6
LASER-BASED PULSED FIELD IONIZATION-ZERO ELECTRON KINETIC ENERGY PHOTOELECTRON SPECTROSCOPY
Perhaps, the most important PES development over the past two decades is the introduction of pulsed field ionization-zero electron kinetic energy (PFI-ZEKE) technique.31–34 Other names in use for this technique include ZEKE-PFI, PFI-PE, or simply ZEKE. PFI-ZEKE involves the detection of electrons produced by delayed, pulsed, electric field ionization of very high-lying
192
Photoelectron Spectroscopy
Rydberg states with a principle quantum number n > 100. These Rydberg states are formed by laser excitation and are located a few cm1 (or a fraction of meV) below the ionization threshold. Because the electrons ejected from these Rydberg states carry near zero electron kinetic energy, these states are known as ZEKE states and the ejected electrons are named as ZEKEs. The measured electron peak position is lower than that without the presence of the field by the Stark shift ( ) ¼ CE 1=2 cm1
ð6Þ
where E is the magnitude of the pulsed electric field in the unit of V/cm1 and C is a proportional constant that can be determined experimentally. Experiments at various electric fields show that the value of C is 4 for metal-containing molecules.35 On the basis of the continuity of ionization oscillator strengths, the relative intensities of the photoelectron bands in a PFI-ZEKE spectrum are expected to be identical to those in a direct threshold photoionization PE spectrum, provided that perturbations by nearby autoionizing states are small. Indeed, PFI-ZEKE spectra of many metal compounds have shown Franck–Condon intensity profiles.6,7,36,37 The delay of pulsed field ionization from laser excitation is typically a few microseconds, which allows the ZEKE detection to be free from background electrons produced by direct photoionization and autoionization. Because of the pulsed nature of the experiment, a time-of-flight electron analyzer is used for the ZEKE detection. The longevity of the ZEKE states is currently contributed to the angular moment l mixing due to the presence of the weak d.c. field in the excitation volume and to the magnetic momentum ml mixing due to the inhomogeneous electric field formed by nearby prompt ions. The major advantage of PFI-ZEKE is its energy resolution, which allows the resolution of rotational and vibrational structures of metal compounds.6,7,34–51 For example, the ZEKE spectra of vanadium dimers (V2) have a line width of 1.5 cm1 (0.19 meV), from which the rotational
Figure 3 ZEKE spectra of MNH3 (M ¼ Al, Ga, In) and spectral simulations for indium adduct and insertion isomers (reproduced by permission of the American Chemical Society from J. Phys. Chem. A 2000, 104, 8178–8182).
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193
constant and bond length of the corresponding cation have been accurately determined for the first time.51 Figure 3 shows the vibrationally resolved PFI-ZEKE spectra of MNH3 (M ¼ Al, Ga, In), with spectral line widths of 5.0 cm1.42–44 Each of the spectra displays a short vibrational progression and a small peak (a) at the left side of the strongest peak. Additionally, the AlNH3 spectrum exhibits a doublet in the first three strong bands. The doublet is separated by 58 cm1 and is due to the spin–orbit interaction in AlNH3. The splitting is not observed for the Ga and In species due to large energy separations between the two spin components of the heavy metal atoms, which prevent a significant population of the upper spin level. The vibrational progressions yield the MþN stretching frequencies of 339 (AlþN), 270 (GaþN), and 234 (InþN) cm1. The strongest peaks determine adiabatic IEs, 39,746, 40,135, and 39,689 cm1, for the Al, Ga, and In complexes, respectively. The small peaks (a) arise from the transitions of the first vibrational levels of MNH3 to the vibrational ground states of MNH3 and yield the M–N stretching frequencies of 227, 161, and 141 cm1 for the three neutral radicals. Converting the frequencies into stretching force constants shows the strength of the metal–nitrogen bonding in the order of Al > Ga > In, and the trend is explained by involving electrostatic and orbital interactions. To determine the geometric conformations of the complexes, density functional calculations were used to obtain the minimum energy structures and harmonic vibrational analyses for both the neutral and the ion; Franck–Condon factors were calculated using the theoretical geometries and vibrational analysis, and spectral simulations were performed with the experimental line widths and at various temperatures to obtain the best fit. A comparison of the 100 K simulations from the two indium isomers (Figures 4d and 4e) and the experimental spectrum of InNH3 (Figure 3(c)) clearly identifies that the electron carrier is a simple adduct rather than an insertion compound. An alternative to PFI-ZEKE would be mass-analyzed threshold ionization (MATI), where cations rather than ZEKE electrons are detected.52 In principle, MATI is attractive because of the inherent mass selection. However, MATI experiments are difficult to implement because of ion separation fields affecting the high Rydberg levels required for delayed pulsed field ionizations. The technique has so far only been used for the smallest metal molecules.53
Figure 4 Left: PE spectra of Re2Cl82 taken with 157, 193, 266, and 355 nm lasers; right: schematic drawing of potential energy curves showing the repulsive Coulomb barriers (RCB, eV) and vertical detachment energies (eV) of PE bands. The relative positions of the five laser wavelengths are also indicated (reproduced by permission of the American Chemical Society from J. Am. Chem. Soc. 2000, 122, 2096–2100).
194 2.14.7
Photoelectron Spectroscopy LASER-BASED NEGATIVE ION PHOTOELECTRON SPECTROSCOPY
Although the principle is the same for photoionization of neutrals and photodetachment of anions, their description differs in detail. First, negative ion PES has the advantage of species selectivity when combined with mass spectrometry. Second, all neutrals have positive IEs, whereas EAs of some anions may be negative. If a singly charged anion has a negative EA, the ion is energetically unstable. Third, an electron ejected from a neutral experiences an attractive Coulomb field of the positively charged core, an electron detached from a singly charged anion is subject to a angular momentum interaction with the residual neutral core, and photodetachment of a multiply charged anion results in Coulomb repulsion between two negatively charged particles (an anion and a free electron). Anions do not possess Rydberg-analogous states that converge to neutral states; thus, PFI technique does not apply to these species. For multiply charged anions, threshold photodetachment is not feasible because Coulomb repulsion barriers are often higher than the EAs of anions. Similar to neutral PES, negative ion PES can use a fixed-frequency light source and measure the energies of detached electrons, or scan photon wavelengths and detect ZEKE electrons. Most studies have been reported on singly charged anions and at fixed photon energies,54–64 although anion ZEKE experiments have also been reported.34,65–71 More recently, studies have also been carried out for multiply charged negative ions.72–76 An anion PES instrument consists of a photodetachment laser, an anion source, a mass selector, and an electron analyzer. Singly charged metal-containing anions are often prepared by pulsed laser vaporization in supersonic jets,54–61 or in electric discharge-flowing afterglow reactors,63,64 whereas multiply charged anions are produced by electrospray.73–76 Ions with a specific mass are selected by magnetic, electrostatic, or time-of-flight mass spectrometers. There are basically three widely used types of electron analyzers. The simplest one is a time-of-flight spectrometer. Its disadvantage is the small acceptance angle and thus, a small collection efficiency for photoelectrons. This problem is avoided in a magnetic bottle electron spectrometer that has high collection efficiency.57,61 The high efficiency is achieved by magnetic fields which guide the ejected electrons into the detector. However, the collection of electrons with relatively large velocity differences results in a reduced spectral resolution. For experiments with a continuous ion source (e.g., flowing afterglow), a hemispherical analyzer can be used to provide a better energy resolution, but a lower collection efficiency, than that of the magnetic bottle spectrometer.63,64 Combining an electrospray ion source with anion PES has opened up a new avenue for studying multiply charged anions in the gas phase. It provides information about the repulsive Coulomb barrier produced by the superposition of a long-range Coulomb repulsion between two negatively charged photoproducts and a short-range electron binding. Because of the presence of this barrier, electron detachment requires the incident photon energy to be greater than the Coulomb potential if electron tunneling is ignored. Figure 4 shows the PE spectra (left) and band positions (right) of Re2Cl82 at 157 nm (7.866 eV), 193 nm (6.424 eV), 266 nm (4.661 eV), and 355 nm (3.496 eV).75 The 157 nm spectrum exhibits eight bands labeled X—G. Bands X, A, and B are attributed to three ReRe bonding orbitals (, , and ), whereas features C—F are assigned to ReCl bonding and Cl non-bonding orbitals. The analysis of the PE spectrum confirms a (. . .242) electron configuration and a formal quadruple ReRe bond of the dianion. As photon wavelengths increase, the number of PE bands decrease. Because the Coulomb barrier should not be larger than the energy difference between the incident photon and the electron binding (if the binding energy is positive), the Coulomb barrier can be estimated to be <2.77 eV (hEG) from the 157 nm spectrum, <2.79 eV (hEC) from the 193 nm spectrum, <2.34 eV (hEA) from the 266 nm spectrum, and <2.34 eV (hEX) from the 355 nm spectrum. Moreover, a measurement at 532 nm (2.331 eV) shows no photoelectron signal, indicating that the Coulomb barrier is at least 2.33 eV. Therefore, the repulsion potential between Re2Cl82 and the free electron should be 2.3 eV at its ground state. If the two negative charges of Re2Cl82 are localized on the Cl atoms, the intramolecular Coulomb repulsion may be calculated according to Coulomb’s law, e2/4"0r. The calculated value is 2.5 eV, fairly close to the experimental value of 2.3 eV. If the Coulomb repulsion is stronger than the electron binding, the anion shall have a negative electron binding energy. For example, the binding energy of a quadruply charged anion, copper phthalocyanine, 3,40 ,400 ,4000 -tetrasulfonate [CuPc(SO3)4]4, is measured to be 0.9 eV.76
2.14.8
OTHER PHOTOELECTRON TECHNIQUES
Other PES methods have also been developed and may be of potential uses for metal compounds. These techniques include, among others, vacuum ultraviolet (VUV) laser-based PFI,77
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synchrotron-based PFI,78 photoelectron–photoion coincidence,79–83 two-dimensional imaging,84,85 and femtosecond time-resolved experiments.86,87 The VUV laser and synchrotron PFI may be used for high-resolution PES studies of metal compounds with higher IE (>6 eV). However, at the present time neither VUV lasers nor synchrotron radiations can provide photon intensities as high as UV lasers. The photoelectron–photoion coincidence technique correlates the electron signal to its carrier, but it often operates at relatively low particle counting rates. The two-dimensional PES, which measures photoelectron yields as a function of electron and photon energies, can provide new opportunities to study photoexcitation and photoionization processes. The femtosecond time-resolved technique is emerging as a new means for the study of ultrafast excited state dynamics.
2.14.9 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46.
REFERENCES
Turner, D. W.; Baker, C.; Baker, A. D.; Brundle, C. R. Molecular Photoelectron Spectroscopy; Wiley: London, 1970. Eland, J. H. D. Photoelectron Spectroscopy; Butterworths: London, 1974. Rabalais, J. W. Principle of Ultraviolet Photoelectron Spectroscopy; Wiley: New York, 1977. Bancroft, G. M.; Hu, Y. F. In Inorganic Electronic Structure and Spectroscopy, Volume 1: Methodology. Solomon, E. I., Lever, A. B. P., Eds.; Wiley: New York, 1999, pp 443–512. Jensen, F. Introduction to Computational Chemistry; Wiley: Chichester, 1999. Yang, D. S. Coord. Chem. Rev. 2001, 214, 187–213. Be´rces, A.; Zgierski, M. Z.; Yang, D. S. In Computational Molecular Spectroscopy; Jensen, P., Bunker, P. R., Eds.; Wiley: New York, 2000, pp 109–134. Cowley, A. H. Prog. Inorg. Chem. 1979, 26, 45–160. Green, J. C. Struct. Bonding (Berlin) 1981, 43, 37–112. Solomon, E. I. Comm. Inorg. Chem. 1984, 3, 227–320. Lichtenberger, D. L.; Kellogg, G. E. Acc. Chem. Res. 1987, 20, 379–387. Green, J. C. Acc. Chem. Res. 1994, 27, 131–137. Li, X.; Bancroft, G. M.; Puddephatt, R. J. Acc. Chem. Res. 1997, 30, 213–218. Oskam, A. In Spectroscopy of Inorganic-based Materials; Clark, R. J. H., Hester, R. E., Eds.; Wiley: New York, 1987, pp 429–461. Yang, D. S.; Bancroft, G. M.; Puddephatt, R. J.; Bursten, B. E.; McKee, S. D. Inorg. Chem. 1989, 28, 872–877. Yang, D. S.; Bancroft, G. M.; Dignard-Bailey, L.; Puddephatt, R. J.; Tse, J. S. Inorg. Chem. 1990, 29, 2118–2122, 2496–2501. Yang, D. S.; Bancroft, G. M.; L. Puddephatt, R. J.; Tse, J. S. Inorg. Chem. 1990, 29, 2487–2495. Lichtenberger, D.; Rai-Chaudhuri, A.; Hogan, R. H. In Organometallic Chemistry; Fehlner, P., Ed.; Plenum: New York, 1992, pp 223–251. Lichtenberger, D. L.; Lynn, M. A.; Chisholm, M. H. J. Am. Chem. Soc. 1999, 121, 12167–12176. Westcott, B. L.; Gruhn, N. E.; Michelsen, L. J.; Lichtenberger, D. L. J. Am. Chem. Soc. 2000, 122, 8083–8084. Lichtenberger, D. L.; Gruhn, N.; Rai-Chaudhuri, A.; Renshaw, S. K.; Gladysz, J. A.; Jiao, H.; Seyler, J.; Igau, A. J. Am. Chem. Soc. 2002, 124, 1417–1423. Westcott, B. L.; Gruhn, N. E.; Enemark, J. H. J. Am. Chem. Soc. 1998, 120, 3382–3386. Downs, A. J.; Green, J. C.; McGrady, G. S.; Munkman, N.; Parkin, R. P. G. Dalton 2000, 21–27. Al-Ktaifani, M.; Green, J. C.; Hitchcock, P. B.; Nixon, J. F. J. Chem. Soc., Dalton Trans. 2001, 1726–1731. Baltzer, P.; Karlsson, L.; Lundqvist, M.; Wannberg, B. Rev. Sci. Instrum. 1993, 64, 2179–2189. Jolly, W. L. Acc. Chem. Res. 1983, 16, 370–376. Hollas, J. M. High Resolution Spectroscopy, Butterworths: London; 1982, pp 435–480. Margaritondo, G. Introduction to Synchrotron Radiation, Oxford University Press: New York, 1988. Hu, Y. F.; Bancroft, G. M.; Tan, K. H. J. Electron. Spectrosc. Relat. Phenom. 2001, 114–116, 147–151. Bursten, B. E.; Green, J. C.; Kaltsoyannis, N.; MacDonald, M. A.; Sze, K. H.; Tse, J. S. Inorg. Chem. 1994, 33, 5086–5093. Mu¨ller-Dethlefs, K.; Schlag, E. W. Ann. Rev. Phys. Chem. 1991, 42, 109–136. Mu¨ller-Dethlefs, K.; Schlag, E. W.; Grant, E. R.; Wang, K.; McKoy, B. V. In Advances in Chemistry and Physics, Volume XC.; Rice, S. A., Prigogine, I., Eds.; Wiley: New York, 1995, pp 1–103. Schlag, E. W. ZEKE Spectroscopy; Cambridge University Press: Cambridge, 1998. Mu¨ller-Dethlefs, K.; Schlag, E. W. Angew. Chem., Int. Ed. Engl. 1998, 37, 1346–1373. Yang, D. S.; James, A. M.; Rayner, D. M.; Hackett, P. A. Chem. Phys. Lett. 1994, 231, 177–182. Yang, D. S.; Hackett, P. A. J. Electron. Spectrosc. Relat. Phenom. 2000, 106, 153–169. Yang, D. S. In Advances in Metal and Semiconductor Clusters, Volume 5: Metal Ion Solvation and Metal–Ligand Interactions. Duncan, M. A., Ed.; Elsevier: Amsterdam, 2001, pp 187–226. Rodham, D. A.; Black, G. A. Chem. Phys. Lett. 1997, 264, 522–530. Panov, S. I.; Powers, D. E.; Miller, T. A. J. Chem. Phys. 1997, 108, 1335–1346. Barckholtz, T. A.; Powers, D. E.; Miller, T. A.; Bursten, B. E. J. Am. Chem. Soc. 1999, 121, 2576–2584. Agreiter, J. K.; Knight, A. M.; Duncan, M. A. Chem. Phys. Lett. 1999, 313, 162–170. Yang, D. S.; Miyawaki, J. Chem. Phys. Lett. 1999, 313, 514–518. Rothschopf, G. K.; Perkins, J. S.; Li, S.; Yang, D. S. J. Phys. Chem. A 2000, 104, 8178–8182. Li, S.; Rothschopf, G. K.; Pillai, D.; Sohnlein, B. R.; Wilson, B. M.; Yang, D. S. J. Chem. Phys. 2001, 115, 7968–7974. Rothschopf, G. K.; Li, S.; Perkins, J. S.; Yang, D. S. J. Chem. Phys. 2001, 115, 4565–4572. Pedersen, D. B.; Zgierski, M. Z.; Anderson, S.; Rayner, D. M.; Simard, B.; Li, S.; Yang, D. S. J. Phys. Chem. A 2001, 105, 11462–11469.
196 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87.
Photoelectron Spectroscopy
Li, S.; Rothschipf, G. K.; Yang, D. S. J. Chem. Phys. 2002, 116, 6589–6594. Harrington, J.; Weisshaar, J. C. J. Chem. Phys. 1992, 97, 2809–2812. Ne´meth, G. I.; Ungar, H. Yeretzian, C.; Selzle, H. L.; Schlag, E. W. Chem. Phys. Lett. 1994, 228, 1–8. Linton, C.; Simard, B.; Peter-Look, H.; Wallin, S.; Rothschopf, G. K.; Gunion, R. F.; Morse, M. D. J. Chem. Phys. 1999, 111, 5017–5026. Yang, D. S.; James, A. M.; Rayner, D. M.; Hackett, P. A. J. Chem. Phys. 1995, 102, 3129–3134. Zhu, L.; Johnson, P.; J. Chem. Phys. 1991, 94, 5679–5681. Wiley, K. F.; Yeh, C. S.; Duncan, M. A. Chem. Phys. Lett. 1993, 211, 156–160. Kaya, K.; Nakajima, A. In Adv. Met. Semicond. Clusters, Volume 2; Duncan, M. A., Ed.; JIA: Greenwich, 1995, pp 87–114. Arnold, C. C.; Neumark, D. M. In Adv. Met. Semicond. Clusters; Duncan, M. A., Ed.; JIA: Greenwich, 1995, pp 113–148. Hendricks, J. H.; De Clercq, H. L.; Lyapustina, S. A.; Fancher, C. A.; Lippa, T. P.; Collins, J. M.; Arnold, S. T.; Lee, G. H.; Bowen, K. H. Front. Sci. Ser. 1996, 16, 321–328. Wang, L. S.; Wu, H. In Adv. Met. Semicond. Clusters, Volume 4; Duncan, M. A., Ed.; JIA: Greenwich, 1998, pp 299–343. Boesl, U.; Knott, W. J. Mass Spectrom. Rev. 1999, 17, 275–305. Fuke, K.; Hashimoto, K.; Takasu, R. In Adv. Met. Semicond. Clusters, Volume 4; Duncan, M. A., Ed.; Elsevier: Amsterdam, 2001, pp 299–343. Conceicao, J.; Laaksonen, R. T.; Wang, L. S.; Guo, T.; Nordlander, P.; Smalley, R. E. Phys. Rev. B 1995, 51, 4668–4671. Kietzmann, H.; Morenzin, J.; Bechthold, P. S.; Gantefor, G.; Eberhardt, W. J. Chem. Phys. 1998, 109, 2275–2278. Dessent, C. E. Johnson, M. A. In Fundamentals and Applications of Gas Phase Ion Chemistry; Jennings, K. R., Ed.; Kluwer: Netherlands, 1999, pp 287–306. Ervin, K. M.; Lineberger, W. C. In Adv. Gas Phase Ion Chem. Volume 1; Adams, N. G., Babcock, L. M., Eds.; JAI: Greenwich, 1992, pp 121–166. Green, S. M. E.; Alex, S.; Fleischer, N. L.; Millam, E. L.; March, T. P.; Leopold, D. G. J. Chem. Phys. 2001, 114, 2653–2668. Gantefor, G. F.; Cox, D. M.; Kaldor, A. Z. Phys. D Atoms Mol. Clusters 1991, 19, 59–61. Gantefor, G. F.; Cox, D. M.; Kaldor, A. Z. J. Chem. Phys. 1990, 93, 8395–8396. Gantefor, G. F.; Cox, D. M.; Kaldor, A. Z. J. Chem. Phys. 1992, 96, 4102–4105. Arnold, C. C.; Neumark, D. M. J. Chem. Phys. 1993, 99, 3353–3362. Arnold, C. C.; Neumark, D. M. J. Chem. Phys. 1993, 100, 1797–1804. Arnold, C. C.; Xu, C.; Burton, G. R.; Neumark, D. M. J. Chem. Phys. 1995, 102, 6982–6989. Drechsler, G.; Be¨ßmann, C.; Bosel, U.; Schlag, E. W. J. Mol. Struct. 1995, 348, 337–340. Compton, R. N. In Advances in Gas Phase Ion Chemistry; Adams, N. G., Babcock, L. M., Eds.; JAI: Greenwich, 1992, pp 257–305. Wang, L. S.; Wang, X. B. J. Phys. Chem. A 2000, 104, 1978–1990. Wang, L. S. Comm. Mod. Phys. 2001, 2, D207–221. Wang, X. B.; Wang, L. S. J. Am. Chem. Soc. 2000, 122, 2096–2100. Wang, X. B.; Wang, L. S. Nature 1999, 400, 2096–2100. Ng, C. Y. Int. J. Mass Spectrom 2000, 200, 357–386. Ng, C. Y. J. Electron. Spectrosc. Relat. Phenom. 2000, 108, 41–45. Baer, T. Int. J. Mass Spectrom. 2000, 200, 443–457. Continetti, R. E. Annu. Rev. Phys. Chem. 2001, 52, 165–192. Eland, J. H. D. J. Electron. Spectrosc. Relat. Phenom. 2000, 112, 1–8. Becker, U. J. Electron. Spectrosc. Relat. Phenom. 2000, 112, 47–65. Hall, R. I.; McConkey, A.; Ellis, K.; Dawber, G.; Avaldi, L.; MacDonald, M. A.; King, G. C. Meas. Sci. Technol. 1992, 3, 316–324. Sokell, E.; Wills, A. A.; Cubric, D.; Currell, F. J.; Comer, J.; Hammond, P. J. Electron. Spectrosc. Relat. Phenom. 1998, 94, 107–125. Berrah, N.; Langer, B.; Wills, A. A.; Kukk, E.; Bozek, J. D.; Farhat, A.; Gorczyca, T. W. J. Electron. Spectrosc. Relat. Phenom. 1999, 101–103, 1–11. Lochbrunner, S.; Larsen, J. J.; Shaffer, J. P.; Schmitt, M.; Schultz, T.; Underwood, J. G.; Stolow, A. J. Electron. Spectrosc. Relat. Phenom. 2000, 112, 183–198. Neumark, D. M. Annu. Rev. Phys. Chem. 2001, 52, 255–277.
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Comprehensive Coordination Chemistry II ISBN (set): 0-08-0437486 Volume 2, (ISBN 0-08-0443249); pp 187–196