ARTICLE pubs.acs.org/JPCA
Photoelectron Imaging of Ag(H2O)x and AgOH(H2O)y (x = 1,2, y = 04) Chaoxian Chi,† Hua Xie,‡ Yuzhen Li,† Ran Cong,‡ Mingfei Zhou,*,† and Zichao Tang‡,* †
Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Fudan University, Shanghai 200433, China ‡ State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China ABSTRACT: The photoelectron images of Ag(H2O)x (x = 1,2) and AgOH(H2O)y (y = 04) are reported. The Ag(H2O)1,2 anionic complexes have similar characteristics to the other two coinage metalwater complexes that can be characterized as metal atomic anion solvated by water molecules with the electron mainly localized on the metal. The vibrationally well-resolved photoelectron spectrum allows the adiabatic detachment energy (ADE) and vertical detachment energy (VDE) of AgOH to be determined as 1.18(2) and 1.24(2) eV, respectively. The AgOH anion interacts more strongly with water molecules than the Ag anion. The photoelectron spectra of Ag(H2O)x and AgOH(H2O)y show a gradual increase in ADE and VDE with increasing x and y due to the solvent stabilization.
’ INTRODUCTION Water is one of the most important solvents and the study of watermetal interactions is important in many interfacial processes such as corrosion, heterogeneous catalysis and electrocatalysis.1,2 The interactions of coinage metal centers with water have been extensively studied.314 Theoretical calculations found that the M(H2O) anionic complexes (M = Cu, Ag, Au) are planar with the hydrogen atoms oriented toward the metal anion, whereas the neutral M(H2O) complexes have structures with the oxygen atom oriented toward the metal atoms.1525 The neutral Cu(H2O)2 complex was suggested to have a H-bonded Cu OH2OH2 configuration,26,27 while the anionic Cu(H2O)2 complex was predicted to have either a D2d structure possessing two equivalent water molecules with four equivalent CuH bonds or a cyclic structure with two inequivalent CuH bonds and one waterwater H-bond.24,27 Photodissociation spectroscopic investigations on the Ag(H2O)(Ar)n (n = 1,2) complexes suggested that the anionic complexes favor asymmetric structures.28 Anion photoelectron spectroscopic studies on the Cu(H2O) and Cu(H2O)2 complexes indicated that both complexes are best characterized as atomic copper anions interacting with water molecules.29,30 The anion photoelectron spectra of Au(H2O)1,2 are quite similar to those of Cu(H2O)1,2, indicating that the neutral and anionic goldwater complexes have analog structures to those of Cuwater complexes.17,29 The metal hydroxide molecules are products of the dissociation of water on metal centers.3133 Although the coinage metal monohydroxide molecules have been the subject of extensive spectroscopic and computational studies,3437 the interactions r 2011 American Chemical Society
between the coinage metal monohydroxide molecules and water have gained little attention.14,24 Both the MOH neutrals and MOH anions were characterized to have obtuse bond angles. Theoretical calculations on the CuOH(H2O) complex indicated that the water molecule binds to the Cu center asymmetrically with the geometry of the CuOH subunit very close to that of free CuOH anion.24 In this article, we present the photoelectron velocitymap imaging (VMI) results of Ag(H2O)x (x = 1,2), AgOH(H2O)y (y = 0,1,2,3,4). The VMI technique is a powerful experimental tool for investigating the electronic properties and dynamics of negative ions and clusters in the gas phase,38,39 yielding both the photoelectron spectrum and the photoelectron angular distribution (PAD) simultaneously.40 The analysis of photoelectron spectrum allows the determination of the electron affinity (EA) and vibrational frequencies and provides direct insights into the electronic structures of the parent anions. PADs bear important information about the parent orbital, the photodetachment process and the symmetry wave function of the detached electrons.4143
’ EXPERIMENTAL AND COMPUTATIONAL METHODS The experiments were carried out using a collinear velocitymap photoelectron imaging analyzer with a laser vaporization source. The apparatus has been described in detail elsewhere.44 Briefly, the Ag(H2O)x (x = 1,2) and AgOH(H2O)y (n = 04) Received: March 16, 2011 Revised: April 29, 2011 Published: May 09, 2011 5380
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The Journal of Physical Chemistry A complexes were generated by pulsed laser vaporization of silver metal target. The helium carrier gas (99.9%) at stagnation pressures between 3 and 5 atm flows through a small water container at room temperature without additional purification and expands into vacuum through a pulsed (10 Hz) General Valve. The negative clusters were further cooled and stabilized downstream inside a conical expansion channel and then expand into the source chamber at pressure about 102—103 Pa. The central part of the negative clusters enter the extraction region from the source through a 5 mm diameter skimmer about 10 cm downstream from the nozzle. The ions were extracted perpendicularly by a 1.2 kV high voltage pulse and were subjected to a McLarenWiley TOF mass spectrometer.45 Then, the anions were introduced into the laser detachment region and interacted with the laser beam from a Nd:YAG laser (532 or 355 nm). The laser propagated perpendicularly to the anion beam axis with a polarization vector parallel to the imaging plane. The photoelectrons in the detachment region were extracted by a modified collinear VMI electrodes based on the original design of Eppink and Parker.46 After passing through a 36 cm TOF tube, the photoelectrons were mapped onto a detector consisting of a 40 mm diameter microchannel plate assembly and a phosphor screen. The two-dimensional (2D) images on the phosphor screen were recorded by a CCD camera. The photoelectron imaging of Ag at 532 nm and at 355 nm was used for the spectrometer calibration. All the photoelectron images were reconstructed using the basis set expansion (BASEX) inverse Abel transform method, which yielded both the photoelectron spectra and PADs.47 The energy resolution is better than 40 meV at electron kinetic energy (eKE) of 1 eV. Theoretical calculation were performed by using the Gaussian 09 program to predict the geometric and electronic structures of the neutral and anionic complexes and to gain a deep insight into the interaction between water and the negatively charged/neutral Ag atoms or AgOH molecules.48 The calculations were performed at the level of density functional theory (DFT) with the B3LYP method,49,50 where Becke’s three-parameter hybrid functional and the LeeYangParr correlation functional were used. The LANL2DZ basis set and the corresponding relativistic effective core potential was used for Ag.51,52 The augmented Dunning’s correlation consistent basis sets of aug-cc-pVTZ (AVTZ) were used for H and O.53,54 The validity of these basis sets has been demonstrated in previous studies on the interaction between water and coinage metal complexes.20,21,55 No spinorbit coupling effects are included in the calculation, although such effects can split the electronic states in fine structures. Full structure optimizations were performed for all the species, and various chemically reasonable structures were considered. Harmonic vibrational frequencies were calculated at the same level of theory to get the zeropoint energy. The vertical detachment energy (VDE) in each case was calculated as difference between the energy of the optimized anionic structure and that of the corresponding neutral at the anionic geometry. The counterpoise method was applied to account for basis set superposition error (BSSE) in the calculation of the sequential binding energy or solvation energy of the water molecules.56,57 A natural bond orbital (NBO) analysis of the anionic and neutral clusters was carried out to obtain information on the intramolecular interactions.58
’ RESULTS AND DISCUSSIONS Ag(H2O)x (x = 1, 2). Figure 1 shows the 532 nm photoelectron velocity images and the corresponding reconstructed
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Figure 1. Photoelectron images and spectra for Ag(H2O)x (x = 0,1,2) obtained at 532 nm. (a) Ag; (b) Ag(H2O); (c) Ag(H2O)2. The left side shows the raw photoelectron image (top on the left) and the reconstructed one (bottom on the left) after inverse Abel transformation. The double arrow shows the direction of the laser polarization. The experimental conditions are the same in each image allowing a direct visual comparison of the spectral features.
photoelectron energy spectra of Ag(H2O)x (x = 0,1,2) . The raw image (top on the left) shows the 2D projection of the 3D laboratory frame photoelectron probability density onto the plane of the imaging detector, and the reconstructed image (bottom on the left) represents the central slice of the 3D distribution from its 2D projection. The direction of laser polarization is vertical in the image plane (the double yellow arrow). Photoelectron spectra and the PADs are obtained by integrating the reconstructed images, including the necessary Jacobian factors.59 The photoelectron spectra are shown as a function of electron binding energy determined by eBE = hν eKE. All the images in Figure 1 are acquired under the same experimental conditions and are presented on the same velocity scale. Every ring in the reconstructed image represents one photodetachment transition. Their radii reflect the velocities of the outgoing photoelectrons and the large radius represents high kinetic energy of electrons, indicating low binding energy to the anion. The decreasing radius of the main ring from Ag to Ag(H2O)2 shows the decreasing KEs of the photoelectrons, reflecting the solvent-dependent energetics of the electron detachment. Generally, the photoelectron spectra of Ag(H2O)1/2 are similar to that of the Ag atomic anion, except for a gradual increase in the electron binding energies and broadening spectral width. 5381
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The Journal of Physical Chemistry A The single peak in the photoelectron spectrum of Ag (Figure 1a) is assigned as the removal of an electron from the fully occupied 5s orbital of anion to the ground state of the neutral atom corresponding to the (Ag 4d105s1 2S1/2 r Ag 4d105s2 1S0) transition.60 This peak is centered at an electron binding energy (EBE) of 1.304 eV,61,62 which is the electron affinity of silver atom and provides the accurate reference value to the calibration of the photoelectron spectra. The measured anisotropy parameters at 532 nm for the 2S1/2 transition are given in the spectra. The scattered electrons are in a cos2 θ threedimensional distribution along the direction of the laser beam polarization vector with a pure parallel character, peaking at θ = 0 and π. The 2S1/2 transition resulting from the detachment of a 5s electron produces p-wave function with an anisotropy parameter value of 2.05(1) which is the same as Sobhy’s experiment on Ag at 800 and 527 nm.60 The image for the Ag atomic anion at 355 nm has also been acquired (not shown in the paper) and the anisotropy parameter for the 2S1/2 transition remained to be 2.0. Apparently, the angular distributions do not vary with photon energy for the typical example of removal of one s-type electron in the 2S1/2 transition of Ag. The value of anisotropy parameter of Cu determined from the image at 800 and 527 nm is reported to be 2.0 and that of Au at 355 nm is given to be 1.9,44 which is slightly deviated from the pure parallel transition. There is only one electronic band in the image of Ag(H2O) (Figure 1b) that is qualitatively similar to that of the Ag image. However, the size of Ag(H2O) image is slightly smaller than that of Ag image indicating a higher electron binding energy for Ag(H2O). The main peak in the photoelectron spectrum of Ag(H2O) centered at 1.69 eV is assigned to the transition from the anion electronic ground state to the ground state of neutral complex. This value corresponds to the VDE with β = 1.86(3), which is about 0.39 eV higher than that of Ag. The anisotropy parameter of 1.86 corresponds to a nearly pure parallel s-electron detachment. The full width half-maximum (fwhm) of the main peak is about 50 meV at the electron kinetic energy of 0.64 eV, which is broader than that of Ag. The peak broadening is likely due to a combination of rotations and FranckCondon overlap between weak bond vibrations of the anionic complex and its neutral complex, the same as that of Au(H2O).17 There is also a peak with low intensity located 0.19 eV (1565(60) cm1) higher in electron binding energy than the main peak. Since the energy separation is close to the bending vibrational frequency of isolated H2O (1595 cm1), we assign this peak to the H2O bending mode of the Ag(H2O) complex. There is no experimental vibrational spectra of Ag(H2O) available in the literature. Previous theoretical calculation gave a scaled bending frequency of 1584.1 cm1,20 which matches our result very well and supports our assignment. Similar features were observed in the photoelectron image of Cu(H2O) at 567.8 nm and the photoelectron spectrum of Au(H2O) at 355 nm.17,29 The photoelectron spectrum of Ag(H2O)2 (Figure 1c) recorded at 532 nm resembles that of Ag(H2O) (Figure 1b). The main peak is centered at 2.04 eV which is the VDE of the Ag(H2O)2 complex. The β value of 1.58(2) for the transition is smaller than that of Ag and Ag(H2O) due to increased solvent stabilization. The VDE of the main peak shifted to high binding energy side by 0.35 eV relative to that of Ag(H2O) with an fwhm of about 100 meV corresponding to the electron kinetic energy of 0.29 eV. The broad band suggests that the molecular structure changed significantly upon electron detachment and no vibrational structure can be resolved. There is also a partially
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Figure 2. Optimized structures (bond lengths in Å) and relative energies (kcal/mol) of Ag/0(H2O)x (x = 1, 2) and AgOH/0(H2O)y (y = 02) at the B3LYP/AVTZ/LanL2DZ level.
resolved peak located about 200 meV (∼1600 cm1) above the main peak near the edge of energy window. We also assigned it to the H2O bending mode of Ag(H2O)2 following the example of Ag(H2O). To support the experimental assignments, theoretical calculations were performed. The optimized structures of Ag(H2O)x and Ag(H2O)x (x = 1, 2) are shown in Figure 2. The structures of Ag(H2O) (Figure 2a) and Ag(H2O) (Figure 2b) calculated at the B3LYP level are in agreement with previous calculation results.20,21,23,28 The calculated VDE values of Ag(H2O)x (x = 0, 1, 2) are compared with the experimental values in Table 1. As can be seen in Table 1, the calculated values are in quite good agreement with the experimental values. The binding energy or solvation energy of Ag(H2O) was calculated to be 9.4 kcal/mol at the B3LYP/AVTZ/LanL2DZ level after BSSE correction (Table 2), which is quite close to the value (10.3 kcal/mol) calculated at the CCSD(T) level.28 The binding energy of the neutral Ag(H2O) complex was predicted to be only 0.9 kcal/mol after BSSE correction (Table 2). Natural population analysis was performed on Ag(H2O). According to the second-order perturbation theory analysis of 5382
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Table 1. Summary of the Experimental ADE and VDE (eV) of Ag(H2O)x (x = 02) and AgOH(H2O)y (y = 0-4), Including the Anisotropy Parameters for the Main Transitions for Each Species (The Calculated ADEs and VDEs Values Are Also Listed for Comparison)a β
Ag (H2O)n
AgOH(H2O)n
n
ADEexpt
0
1.304
b
VDEcal
532 nm
355 nm
Δ (VDEexpt)
VDEexpt
ADEcal
1
1.69 (1)
1.64
1.70
1.86(3)
0.39
2
2.04 (1)
1.88
2.10
1.58(2)
0.35
2.05(1)
0
1.18(2)
1.24(2)
1.27
1.34
1.47(4)
1 2
1.60(3) 1.87(2)
1.72(2) 2.05(1)
1.64 1.51
1.88 2.28
1.32(2) 1.23(1)
3
2.15(5)
4
2.34(5)
0.64(1)
0.48 0.33
2.35(5)
0.86(1)
0.30
2.56(4)
0.97(1)
0.21
a
Calculated at B3LYP/AVTZ (H, O)/LanL2DZ (Ag). The calculations performed with LanL2TZ(f) for Ag and aug-cc-pVTZ for H and O showed only a small improvement in predicting the ADEs and VDEs. However, the usage of aug-cc-pVTZ-PP for Ag resulted in a large deviation from the experimental values. b Zero-point energy corrections are included in ADE.
Table 2. Calculated Water Binding Energies for the Lowest Energy Structures of Ag/0(H2O)1/2 and AgOH/0(H2O)1/2 (kcal/mol) Shown in Figure 2 Ebindinga AgH2O Ag(H2O)2
a
anion
14.8 (9.4)
neutral
2.1 (0.9)
anion
11.6 (9.9)
neutral
6.3 (5.3)
AgOH(H2O)
anion
18.6 (17.8)
neutral
10.5 (9.4)
AgOH(H2O)2
anion
15.7 (14.9)
neutral
18.9 (18.3)
The values in the parentheses are BSSE corrected.
the Fock matrix within NBO Basis for Ag (H2O), there is charge transfer between the Ag atomic anion and H2O, predominately from the 5s orbital (93.76%) with some contribution from the 4d orbital (6.15%) of Ag to the antibonding σ orbital of H2O. The energetic effect was estimated to be 11.95 kcal/mol due to the charge transfer interaction, which is a little larger than the calculated binding energy of 9.4 kcal/mol. This indicates that charge-transfer is the dominate interaction between Ag and H2O. Natural charge population found that about 0.083 e was transferred to the H2O subunit from Ag atomic anion. The charge density isosurface of the highest occupied molecular orbital (HOMO) of Ag(H2O) in Figure 3a showed that the s-type orbital of Ag atom contributes to the majority portion of the HOMO with a little p-type orbital of O atom component, which is consistent with the anisotropy parameter of 1.86 according to the CooperZare method.63 The optimized structure of Ag(H2O)2 (Figure 2c) is similar to that of X(H2O)2 (X = halogen atom).6466 Adding another H2O molecule to Ag(H2O) breaks the Cs symmetry of Ag(H2O) with an intersolvent H-bond to the lone pair of the first water unit and another H atom pointing to Ag. The H-bond length between the two water molecules was calculated to be 2.096 Å, ca. 0.142 Å longer than that of (H2O)2 calculated at the same level. The second water binding energy was calculated to be 9.9 kcal/mol, comparable to that of the first water (9.4 kcal/mol).
Figure 3. The charge density isosurfaces of the HOMO of the global minimum of Ag(H2O)1/2 and AgOH(H2O)0/1/2.
The optimized structure of Ag (H2O)2 (Figure 2d) is quite different from that of the anion. On account of the femtosecond dynamics study of Cu(H2O)2,27 we can infer that the neutral Ag(H2O)2 complex at the structure of the anion immediately after photodetachment undergoes large-amplitude H2O reorientation and dissociation. The broad peak in the spectrum corresponds to the vertical detachment with a VDE value of 2.04(1) eV, which is in good agreement with the calculated value of 2.10 eV. The adiabatic detachment energy (ADE) was calculated to be 1.88 eV. According to the charge density isosurface of the HOMO of Ag(H2O)2 shown in Figure 3b, the lower anisotropy parameter of 1.58 for Ag(H2O)2 compare to that of Ag(H2O) is due to more p-type orbital of O contribution to the HOMO. It is necessary to compare the photoelectron spectra or photoelectron images of M(H2O)x (M = Cu, Ag, Au; x = 0, 1, 2).17,29,60 The coinage metalwater complexes are weak interaction systems. The prominent peaks in these spectra correspond to the detachment of an s-electron of the coinage metal atomic anion with the same spectra broadening pattern. The VDE and anisotropy parameters (β) (if available) are summarized in Table 3. The β-values decay quickly from 2.05(1) for Ag to 1.58(2) for Ag(H2O)2, while the β-value only decreases from 2.00(5) for Cu to 1.86(2) for Cu(H2O)2. 5383
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Table 3. The VDE (eV) and the Anisotropy Parameters for the Vertical Detachment Processes of M(H2O)x (M = Cu, Ag, Au, x = 0, 1, 2) VDEexpt a
ΔVDE
Cu
1.235
2.00(5)
CuH2Ob
1.637(4)
2.00(1)
0.402
Cu(H2O)2b
2.003(8)
1.86(2)
0.366
Ag
1.304c
2.05(1)
AgH2O Ag(H2O)2
1.69(1) 2.04(1)
1.86(3) 1.58(2)
Aud
2.31
1.9e
a
β
d
0.39 0.35
Au H2O
2.76
0.45
Au(H2O)2d
3.20
0.44
Reference 60. b Reference 29. c Reference 61. d Reference 17. e Reference 44.
Figure 5. Photoelectron images and spectra for AgOH(H2O)x (x = 0,1,2) obtained at 532 nm. (a) AgOH; (b) AgOH(H2O); (c) AgOH(H2O)2. The left side shows the raw photoelectron image (top on the left) and the reconstructed one (bottom on the left) after inverse Abel transformation. The double arrow shows the direction of the laser polarization. Figure 4. VDE of M(H2O)x [M = Cu, Ag, Au; x = 0, 1, 2] as a function of H2O number x.
Figure 4 shows the VDEs as a function of the number of H2O for all the three classes. Because of weak interaction between H2O and coinage metals as well as different atomic electron affinities, the three curves are nearly parallel to each other with similar VDE shifts that reflect the water bind energy. The strength of water binding energies follows in sequence Au > Cu >Ag, probably due to combination of the electron correlation and relativistic effects. AgOH(H2O)y (y = 04). Figure 5 and Figure 6 show the photoelectron images at 532 nm for AgOH(H2O)y (y = 02) and at 355 nm for AgOH(H2O)y (y = 24), respectively. For each species, the raw images (top on the left), the reconstructed images (bottom on the left), and the corresponding photoelectron spectra (right) are presented. The double arrow in the raw image indicates the direction of the laser polarization. The ADEs and VDEs are listed in Table 1. There are several clear rings in the reconstructed image of AgOH(Figure 5a), corresponding to transitions from the ground state (ν = 0) of the electronic ground state of AgOH to different vibrational levels (ν0 ) of the electronic ground state of the neutral AgOH. There are no vibrational hot bands and the first peak in the low binding energy side is assigned as the adiabatical detachment. The triatomic AgOH molecule has three vibrational modes, AgO stretching (ν3), AgOH bending (ν2)
and OH stretching (ν1). Similar to AuOH, the AgO stretching (ν3) has the dominant effects on the spectrum, which was achieved by the average energy space between the resolved peaks to be 489(14) cm1. This mode was reported at 517 cm1 from microwave experiments.36 Note that the neutral AgO diatomic molecule has a stretching frequency of 493.2 cm1, indicating a nearly equal AgO stretch force constant between AgOH and AgO. The adiabatic and vertical detachment energies are determined to be 1.18(2) and 1.24(2) eV from the first two peaks, respectively. The optimized structures of AgOH and AgOH are showed in Figure 2, while the geometric parameters and harmonic vibrational frequencies (ν3) are summarized in Table 4. Our results on AgOH are in reasonable agreement with previous experimental results.36 The AgO stretching mode was calculated to be 482 cm1 at the B3LYP level. Previous CCSD(T) calculations gave a value of 498.7 cm1.34 Note that the ADE of AgOH (1.18 eV) is lower than that of Ag (1.304 eV) and that of the OH anion (the electron affinity of OH is reported to be 1.828 eV).67 The photoelectron spectrum of AgOH(H2O) at 532 nm (Figure 5b) is similar to that of AgOH (Figure 5a). There are five poorly resolved peaks with a peak space of about 60 meV, reflecting a water perturbed vibrational progression of AgOH. These features are assigned as the AgO stretching mode with a frequency of 482(40) cm1. The VDE and ADE values from the 5384
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Table 4. Comparison of the Theoretical and Experimental Results for AgOH/0 — AgOH/ RAgO/Å ROH/Å
deg
ν3/cm1
AgOHa UB3LYPa
2.1890
0.9637
103.64
339
UB3LYPa
2.0451
0.9636
107.43
482
CASSCFb
2.12
0.95
129.3
0.975
106.5
498.7
107.81(2)
517e
AgOH
DK3-CCSD(T)c 2.035 exptld
2.01849(4) 0.9639(1)
a
Our calculations at B3LYP/AVTZ(H, O)/LanL2DZ(Ag). b Reference 37. c Reference 34. d Reference 36. e Fitted experimental result, see ref 36.
isomers that are very close in energy. The most stable isomer of AgOH(H2O)2 has a structure with two inequivalent water molecules binding to the O atom of AgOH via two H-bonds. The photoelectron images and photoelectron spectra for AgOH(H2O)2, AgOH(H2O)3, and AgOH(H2O)4 at 355 nm are present in Figure 6. The spectra of these complexes are similar with no vibrational progress, except for a gradual increase in the electron binding energies and broadening of the spectral features with increased water number. The ADE and VDE, obtained using the same method as that used in deal with the spectrum of AgOH(H2O)2 at 532 nm, are listed in Table 1. Both ADE and VDE increase monotonically with the number of solvated water molecules as expected.
Figure 6. Photoelectron images and spectra for AgOH (H2O)x (x = 2,3,4) obtained at 355 nm. (a) AgOH(H2O)2; (b) AgOH(H2O)3; (c) AgOH(H2O)4. The left side shows the raw photoelectron image (top on the left) and the reconstructed one (bottom on the left) after inverse Abel transformation. The double arrow shows the direction of the laser polarization. The central spot in the images at 355 nm comes from electrons with zero or near zero KEs, likely from the autodetachment process. The noise peaking around 1.9 eV in AgOH(H2O)4 is due to the scattering electrons from the close-spaced repel plates in the detachment zone at 355 nm.
resolved peaks are listed in Table 1. The β-value of the whole transitions is calculated to be 1.32. The optimized structures of AgOH(H2O) and AgOH(H2O) are presented in Figure 2. In AgOH(H2O) (Figure 2g), one hydrogen atom of water is coordinated to the O atom of AgOH with an O---H bond distance of 1.612 Å. The binding energy (or solvation energy) was predicted to be 17.8 kcal/mol after BSSE correction (Table 2). The large geometry difference, especially for the reorientation of the H2O fragment after photodetachment, contributes to the poorly resolved vibrational progress in the photoelectron spectrum. The electronic bands in the photoelectron spectrum of AgOH(H2O)2 at 532 nm (Figure 5c) shifted to higher binding energies with broader spectral features with respective to those of AgOH(H2O) (Figure 5b). The vibrational peaks were not resolved. The VDE was obtained from the maxima of the broad band. The ADE was estimated by drawing a straight line at the leading edge of the band and adding an instrumental resolution constant to the intercept with the binding energy axis. The VDE and estimated ADE are list in Table 1. The optimized structures of the AgOH(H2O)2 and AgOH(H2O)2 complexes are present in Figure 2. Both the anion and neutral complexes have several
’ CONCLUSIONS The photodetachment of Ag(H2O)x, (x = 0,1,2) and AgOH(H2O)y (y = 04) was studied using photoelectron velocitymap imaging. The Ag(H2O)1,2 anionic complexes were characterized as metal atomic anion solvated by water molecules with the electron mainly localized on the metal center. The vibrational well-resolved photoelectron spectrum allows the adiabatic detachment energy and vertical detachment energy of AgOH to be determined as 1.18(2) and 1.24(2) eV, respectively. The AgO stretching frequency was also determined. The photoelectron spectra of Ag(H2O)x and AgOH(H2O)y showed a gradual increase in ADE and VDE and broadening spectral width with increasing x and y, due to the solvent stabilization. The stepwise stabilization energies decreased with the increased water number. The large structure difference between the anionic and neutral complexes and the low lying isomers result in the broad and featureless spectral feature for the large clusters. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We gratefully acknowledge finical support from National Natural Science Foundation of China (Grant 20933003) and National Basic Research Program of China (2007CB815203 and 2010CB732306). ’ REFERENCES (1) Filhol, J. S.; Neurock, M. Angew. Chem., Int. Ed. 2006, 45, 402. (2) Chorkendorff, I.; Niemantsverdriet, J. W. Concepts of Modern Catalysis and Kinetics; Wiley-VCH: New York, 2003. 5385
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