J. Phys. Chem. A 2010, 114, 11125–11132
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Anion ZEKE-Spectroscopy of the Weakly Bound Iodine Water Complex† Franz Schlicht, Michaela Entfellner, and Ulrich Boesl* Technische UniVersita¨t Mu¨nchen, Physical Chemistry, 85748 Garching, Germany ReceiVed: March 19, 2010; ReVised Manuscript ReceiVed: August 9, 2010
Zero kinetic electron energy photodetachment spectroscopy of I- · H2O and I- · D2O has been performed from 27 660 to 28 500 cm-1 and from 27 660 to 35 900 cm-1, respectively. The I- · D2O spectral data and theoretical studies resulted in a reassignment of earlier anion-ZEKE spectra of iodide water (Ba¨ssmann, C.; et al. Int. J. Mass Spectrom. Ion Processes 1996, 159, 153). In opposite to the I- · H2O, the I- · D2O spectrum reveals a regular progression of the iodine-water van der Waals stretching mode and a short progression of even quanta of the van der Waals rocking mode. A rough estimation delivers dissociation thresholds of the anionic and of the lower and the upper spin-orbit component of the neutral van der Waals complex. A high resolution ZEKE spectrum of the van der Waals stretching mode (V ) 1) reveals significant fine structure, which is found again in a former photodissociation spectrum of the anionic complex (Ayotte, P.; et al. J. Phys. Chem. A 1998, 102, 3067). Our assignments are supported by theoretical calculations of molecular structures and vibrational motions. Vibrational frequencies and isotope effects are reproduced very satisfyingly by these calculations. Introduction The negatively charged iodide-water clusters are still of great interest as model systems for studies of charge-transfer-tosolvent processes.1-5 In this context, investigations of molecular structure and vibrational energies of I- · (H2O)n by theory6-11 and photodissociation spectroscopy12-14 are of basic importance. Neutral halogen-water clusters (including I(H2O)n) are of additional importance for atmospheric chemistry. For instance, highly concentrated sea salt aerosols have been identified as a potential global source of halogen in the marine boundary layer.15,16 In this context, it is of interest that iodine radicals are a much more virulent species in ozone depletion chemistry than chlorine radicals, so that considerably smaller concentrations may already be harmful for the stratospheric ozone layer. The role of bromine, which has intermediate reactivity between chlorine and iodine in atmospheric ozone chemistry, is also subject of research.17 In opposition to the negatively charged iodide-water clusters, there exists little work on the neutral systems I(H2O)n, in particular on the smallest weakly bound complex I(H2O). Low resolution photodetachment1 and photodetachment photoelectron spectroscopy18,19 delivered information about electron binding energies and electron detachment energies of these complexes. High resolution photodetachment photoelectron spectroscopy has been applied by our group20 and supplied vibrational information. To our knowledge, there only exists one theoretical study on I(H2O).11 While the collinear X-H-O structure of the negatively charged halide-water clusters is well established as a global minimum structure by theory6-11 and experiment,12-14 Jungwirth and co-workers21 found that for the neutral chlorine- and bromine-water complex the X-OH2 structure is the global minimum rather than the near linear X-H-O structure. The latter is supposed to be a local minimum for I · H2O and Br · H2O and an unstable structure for Cl · H2O. Calculations by Kwan et †
Part of the “Klaus Mu¨ller-Dethlefs Festschrift”. * Corresponding author. E-mail:
[email protected].
al.11 and in this work showed that also for I-H2O the global minimum structure is X-OH2 with CV symmetry (not C2V due to nonplanarity: nonzero dihedral angle between HOH and X-O-H planes). In this paper, high resolution photodetachment photoelectron spectra of I- · H2O have been remeasured to study the weakly bound neutral I · H2O complex. Additionally, photodetachment photoelectron spectra of I- · D2O have been taken. Calculations of molecular structures, energies, and vibrational frequencies have been performed. Comparison of experiment and theory led to reassignment of the vibrational structure of the X state (see ref 20) and to new information about the I3/2 and II1/2 states, the van der Waals stretching potentials, and the anionic and neutral binding energies. Experimental Setup The experimental setup has been described elsewhere22 and will be outlined in the following. Iodide-water complexes were produced by laser induced photoelectron attachment in a supersonic molecular beam formed by expansion of argon gas of 2-5 bar into a first vacuum chamber. This carrier gas contained traces of water vapor and iodine-benzene as the source of atomic iodine released by photodissociation. Electrons for attachment were supplied by photoelectron emission due to slightly focusing a laser beam (fourth harmonic of a Nd:YAG laser with 1 mJ pulse energy) on a thin wire of hafnium. This wire was positioned directly under the pulsed valve and was moved back and forth to reduce fluctuations of photoelectron emission by laser-induced damage of the metal surface. Slow electrons were emitted since the photon energy was slightly larger than the work function of the metal. These electrons allowed efficient attachment to atomic or molecular systems to form negatively charged iodide-water complexes. The whole procedure took part near the nozzle of the pulsed valve allowing efficient formation of cold I- · H2O. The iodide-water complexes passed through a skimmer and were extracted perpendicularly to the gas beam into a time-offlight mass spectrometer. They were subjected to mass selective
10.1021/jp102508f 2010 American Chemical Society Published on Web 08/18/2010
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Figure 1. Anion-ZEKE spectrum of I- · H2O and I- · D2O (lowest spin-orbit component X1/2). The I- · H2O spectrum has been shifted by +27 cm-1 so that bands A, which are assigned as origins, are overlapping. This shift corresponds to the deuteration isotope shift. I- · H2O vibrational bands and their counterparts in the I- · D2O spectrum are marked with A, B, C, ..., G. Some bands which show splitting in the I- · H2O spectrum are additionally marked with numbers. Bands E and H show counterparts in the I- · D2O spectrum, which show a reduction of vibrational energy by a factor of 1/2. This indicates a nearly exclusive vibrational motion of hydrogen atoms as in van der Waals rocking or wagging vibrations.
photodetachment in the space focus of the pulsed ion source using a Nd:YAG pumped dye laser operating between 365 and 275 nm. Continuous wavelength calibration during each laser scan has been performed using an optogalvanic noble gas cell. Tuning the laser wavelength across the states of the neutral molecular system gives rise to zero kinetic energy (ZEKE) electrons. Discrimination of kinetic against ZEKE electrons is necessary for anion-ZEKE spectroscopy and is achieved by delayed electron extraction. This allows the cloud of kinetic electrons to expand from the point of detachment while ZEKE electrons hardly move. A thin diaphragm serves as a steradiancy discriminator for kinetic electrons. Kinetic electrons that had expanded in or opposite to the extraction direction are separated by their different flight times. An important detail is the compensation of the initial anion velocity, which is perpendicular to the electron extraction direction and is preserved in the photodetached electron manifold. This compensation is achieved by a well-defined electron stopping pulse applied directly after photodetachment. A maximum resolution of photodetachment-ZEKE-spectroscopy of 1.5 cm-1 has been reached with this setup.22,23 Experimental Results Photodetachment Photoelectron Spectra from 27 660 to 28 500 cm-1. Figure 1 displays the high resolution photodetachment photoelectron spectra (anion-ZEKE-spectra) of I- · H2O
Schlicht et al. and I- · D2O obtained for dye laser wavelengths between 362 and 350 nm. This wavelength range covers the region of the transition from the ground state of the iodide-water complex to the X1/2 and I3/2 states of neutral iodine-water complex. Both states correspond to the degenerate P3/2 spin-orbit component of atomic iodine, which is split by interaction with the weakly bound water molecule. This splitting is well-known from anionZEKE-spectra of iodide noble gas complexes24,25 and is expected to lie in the range 200-300 cm-1. The second spin-orbit component of atomic iodine gives rise to the II1/2 electronic state of iodine water, which lies 7800 cm-1 above the X1/2 ground state. The I- · H2O spectrum shows irregular band structure, which is hard to assign without further support by, e.g., theory or spectra of isotopomers. It reveals the same characteristic features as an earlier anion-ZEKE spectrum of I- · H2O.20 In the new spectrum the background signal due to non-ZEKE electrons is somewhat higher since a larger time gate for ZEKE-electron collection has been used for the sake of larger electron current and thus less signal fluctuations. Later on, a detail of this spectrum with higher resolution (narrower time gate) will be shown. In the earlier paper20 an assignment has been supposed, which now has to be revised in view of the I- · D2O spectra and theoretical considerations presented in this paper. In particular, the I- · D2O spectrum in Figure 1 shows a striking regularity in opposition to the spectrum of the protonated complex. A progression of four distinct bands A to D (a fifth band seems to be hidden below band E, giving rise to an increase of the intensity of this band) appears with a typical Franck-Condon envelope and a constant decrease of band separation due to vibrational anharmonicity. This progression has a sudden start with band A similarly to the system of bands in the I- · H2O spectrum. We therefore assign bands A in both spectra to the true origin of the observed anion-neutral transition. A further argument for this assignment is based on the comparison of experimental and theoretical isotope shift as discussed later. The I- · H2O spectrum has been shifted in Figure 1 so that the origin bands A are at the same position for convenient comparison of both spectra. This shift corresponds to the isotope shift; it amounts to +27 cm-1. The relatively strong origins indicate that the involved anionic and neutral molecular structures are similar. It is therefore questionable if the neutral molecular structure, which is responsible for these spectra, is due to the global minimum. This will be the subject of theoretical considerations. Another question is which vibrational motion causes the well developed progression in the spectrum of the I- · D2O complex. There exist three van der Waals modes, namely a stretching, a rocking (in-plane rotation of the water molecule), and a wagging (out-of-plane rotation of the water molecule) mode. The latter two should experience a large deuteration effect (mainly hydrogen atoms are moving), while the stretching mode is only effected marginally by deuteration. From simple reduced mass arguments, the frequency of a pure stretching motion along the water-iodine bond should be reduced by a factor of 0.96, that of a pure hydrogen motion by a factor of 0.71. With this in mind, a comparison of the I- · H2O and I- · D2O spectra in Figure 1 then leads one to the conclusion that it can only be the stretching motion that causes the progression A, B, C, D in the I- · D2O spectrum. The irregularity of the I- · H2O spectrum has to be due to a disturbance of unknown origin. Thus, band B exhibits a large splitting, band C seems to appear at a somewhat too low frequency, and band D seems again to be subject to
ZEKE-Spectroscopy of the Iodine-Water Complex
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Figure 2. Anion-ZEKE spectrum of I- · H2O and I- · D2O (upper spin-orbit component II1/2). The I- · H2O spectrum has been shifted by +52 cm-1 so that progression bands a-d are overlapping. This shift corresponds to the deuteration isotope shift.
splitting. Nevertheless, the bands A to D do not show the large isotope effect expected for wagging or rocking motions. As opposed to bands A to D, the band doublet E/F at 294 and 294 + 37 cm-1 in the protonated case has a strongly shifted equivalent at 206 and 206 + 32 cm-1 in the deuterated case. The shift of band E just amounts to a factor of 0.70, which is a strong indication for excitation of a wagging or rocking mode. In the earlier spectrum,20 this doublet has been assigned to the I3/2 state with none and one quantum of the stretching mode excited. This assignment has to be revised. There is another band (marked as H) at 500 cm-1 excess energy above the origin (band A), which also has an equivalent in the deuterated case at 355 cm-1 corresponding to a frequency reduction by a factor of 0.71. Finally, there appears a small band G in the I- · D2O spectrum for which no corresponding band in the protonated case can be found. Band G could be part of a band system due to a transition to the neutral I3/2 state. Since a very flat potential is supposed for this state (see discussion), the corresponding anion-neutral transition should be weak and subject to a large positive isotope shift. The small intensity of band G is in agreement with the relative intensities of anion-neutral I3/2 transitions in the anion-ZEKE spectra of iodide-noble gases.24,25 The old and obviously incorrect assignment20 is inconsistent with these relative intensities. The corresponding transition in the protonated case may be hidden under the manifold of D-bands due to the large expected isotope shift. Photodetachment Photoelectron Spectra from 35 660 to 35 900 cm-1. Figure 2 displays the high resolution photodetachment photoelectron spectra (anion-ZEKE-spectra) of I- · H2O and I- · D2O obtained for dye laser wavelengths between 282 and 278 nm. This wavelength range covers the region of transitions from the ground state of iodide-water complexes to the II1/2 electronic state of neutral iodine-water complexes. This state corresponds to the upper spin-orbit component of atomic iodine (P1/2), which lies 7603 cm-1 above the lower
excess energy
A B1 B2 C D1 D2 D3 D4 D5 E F
27 825 27 871 27 902 27 938 27 975 27 993 28 008 28 038 28 068 28 119 28 156
origin 46 (61 - 15) 77 (61 + 16) 61 + 52 150 (D2 - 18) 61 + 52 + 55 183 (D2 + 15) 61 + 52 + 55 + 45
H
28 325 500
a b c d e f g h i k l
35 602 35 638 35 670 35 698 35 721 35 727 35 757 35 777 35 789 35 809 35 828
294 294 + 37
origin 36 36 + 32 36 + 32 + 28 36 + 32 + 28 + 23 125 125 + 30 125 + 30 + 20 125 + 30 + 20 + 12 207 207 + 19
I · D 2O position peak (cm-1)
excess energy
A B
27 851 origin 27 910 59
C
27 963 59 + 53
D
28 012 59 + 53 + 49
E
28 055 59 + 53 + 49 + 43
E F G H
28 055 204 28 089 204 + 34 28 163 28 206 355
a b c
35 687 origin 35 723 36 35 755 36 + 32
a Excess energy is given as a sum of vibrational frequencies of either the van der Waals stretching mode or the van der Waals rocking mode.
degenerate P3/2 spin-orbit component.26 The I- · H2O spectrum now reveals an unperturbed progression of bands (a, b, c, d, e with band e partly overlapping with band f). The I- · D2O spectrum is subject to considerable noise due to low signal intensity and strong signal fluctuations. Therefore, the relative band intensities are not reliable anymore. Nevertheless, the progression a, b, c, d can be recognized. The I- · D2O spectrum has been shifted by 52 cm-1, which results in the most reliable coordination of both spectra. This large isotope shift is in agreement with the expected shallow potential of the upper spin component II1/2. The vanishing isotope effect of band spacings b-c and c-d indicates that it is again the van der Waals stretching motion that induces this progression. The vibrational frequency of 36 cm-1 is considerably smaller than in the X1/2 state, in agreement with a shallower potential. Band f with an excess energy of 125 cm-1 seems to be the origin of a second progression of stretching modes combined with one quantum of the wagging or rocking mode. Unfortunately, no corresponding band is found in the strongly congested I- · D2O spectrum. A third short progression seems to start with band k at an excess energy of 207 cm-1. All peak positions and excess energies from Figures 1 and 2 are listed in Table 1. The vibrational progressions {A-E} of the I- · D2O spectrum in Figure 1 and {a-e} of the I- · H2O spectrum in Figure 2 allow for a rough estimation of dissociation thresholds D0(X) and D0(II) of the X1/2 and II1/2 potential of the van der Waals stretching motion. From the diminishing band spacing an anharmonicity of 2ωexe ) 5.2 and 4.3 cm-1, respectively, has been deduced. The assumption of a linear anharmonicity (constant xe with rising vibrational quantum number) results in an upper limit for D0(X) of 360 cm-1 and for D0(II) of 170 cm-1. On the other hand, the most energetic still visible progression bands at 260 cm-1 (I- · H2O spectrum in Figure 1) and 120 cm-1 (I- · H2O spectrum in Figure 2) represent a lower limit of D0(X) and D0(II), respectively. For the first vibrational
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Schlicht et al. TABLE 2: Electronic and Vibrational Parameters for I- · D2O (X1/2 Spin-Orbit Component), I- · H2O (X1/2 Spin-Orbit Component), and I- · H2O (II1/2 Spin-Orbit Component) Deduced from Spectra in Figures 1 and 2
εv)0 (origin) (cm-1) εv)1 - /ev)0 (cm-1) 2ωexe (cm-1) ωe (cm-1) De (cm-1) D0 (cm-1)
X1/2 (I- · D2O spectrum)
X1/2 (I- · H2O spectrum)
II1/2 (I- · H2O spectrum)
27 851 59 5.2 64
