4386
J . Phys. Chem. 1989, 93, 4386-4389
Size-Dependent Effects in the Photodissociation Spectra of Sr+(NH,),
,n =
1-4
M. H. Shen and J. M. Farrar* Department of Chemistry, University of Rochester, Rochester, New York 14627 (Received: March 8, 1989)
A tandem mass spectrometer has been used to obtain total photodissociation absorption spectra of Sr+(NH3)",n = 1, 2, 3, and 4, in the wavelength region from 540 to 900 nm. The spectra of Sr+(NH3)and Sr+(NH3)2show similar patterns,
with a broad and intense band peaking near 580 nm. The absorption bands of Sr+(NH& and Sr+(NH3)4display significant changes: in addition to the remaining absorption near 580 nm, a new strong absorption feature appears with the peak position centered around 680 nm for Sr+(NH3)3and beyond 900 nm for Srt(NH3)4. A possible predissociation mechanism through excited electronic states is discussed briefly for Srt(NH3) and Sr+(NH3)*. A solvated ion-pair formation model, based on our experimental results and simple energetic considerations, is proposed in rationalizing the observed spectral features in Sr+(NH3)3and Sr+(NH3)4. We propose that intramolecular electron transfer within the clusters serves as a diagnostic of the transition from gas-phase toward condensed-phase behavior.
Introduction dynamics of photofragmentation. Highly resolved spectra of small clusters can provide detailed information on structure, reaction In the past decade, many chemists and physicists have devoted pathways, and dissociative dynamics, while spectroscopic studies themselves to the study of The prime importance of of large clusters can address such issues as statistical behavior their work is the recognition that clusters represent a "fifth state in the unimolecular decay process and the stepwise influence of of matterV5bridging the gap between the gas and condensed phase. the solvent on intramolecular chemical reactions. Within the broad topics of cluster structure and dynamics, the Following an experimental technique that we have developed fundamental phenomenon of solvation is of central importance, for Srt(H20),15we present in this Letter an extensive study of and numerous thermodynamic and structural studies of solvated the Sr+-NH3 system. Total photodissociation spectra of clusters species, particularly ions, have appeared in the literature.6 While Srt(NH3)", where n = 1, 2, 3, and 4, are obtained by a tandem the large body of thermodynamic data on solvation phenomena mass spectrometer and a tunable dye laser. The results of this has been invaluable in helping understand this important issue, study provide unique information regarding such issues as the mass spectrometer and molecular beam methods in conjunction nature of the dissociation process and the influence of stepwise with laser probes now allow an attack on the problem of solvation solvation on intramolecular reactions occurring in clusters, ultithrough studies that probe energy levels and dynamical processes mately leading to insights about the transition from isolated occurring in the clusters, thereby going beyond bulk thermodymolecules to the liquid solution phase. namic measurements. Spectroscopic studies of clusters as a specific function of cluster size can play a crucial role in elucidating the Experimental Section transition between the gas and condensed states. Gas-phase studies The solvated metal ion clusters Srt(NH3),, are formed by inof the mass spectra of clusters have already shown, particularly jection of Sr+ cations into a supersonic flow of ammonia seeded in the context of intramolecular proton-transfer reactions, that in helium carrier gas from a pulsed v a l ~ e . ' ~ The J ~ expansion takes size-dependent reaction processes may be evidenced in such data.'*8 place at a backing pressure of 3 atm and a partial NH3 pressure Although mass spectrometric studies reveal interesting mass of 0.7 atm. Approximately 2-3 mm downstream from the nozzle, abundances and lead to speculations concerning the stabilities and a continuous beam of low-energy Srt cations produced from a structure of "magic" numbers, the increased sophistication of thermionic emitter is injected transversely into the flow. The spectroscopic probes is required to lead to more concrete pictures clusters formed in the expansion drift about 3 cm before passing of cluster structure and dynamics. Photodissociation studies of the extractor cone of a mass spectrometer. The clusters are then cluster ions can offer insights into their structure, energetics, and accelerated and focused into a 60' magnetic mass spectrometer dynamics. The measurements of the photodissociation cross section as a function of both photon energy and cluster s i ~ e , ~ J ~ where ions of a desired mass are selected. After mass selection, the ions are refocused and decelerated to 5 eV by a retarding field the determination of the branching ratio for competitive chanlens into the main vacuum chamber where the pressure of the n e l ~ , " - and ~ ~ the energy and angular distributions for photochamber is kept at 2 X lo-' Torr. The resulting collimated beam fragments at fixed energy inputI4 afford a detailed look at the is then overlapped with the collimated, unfocused counterpropagating laser beam from a Nd:YAG pumped tunable dye laser (1) Mark, T. D.; Castleman, Jr., A. W. Adu. At. Mol. Phys. 1985, 30, 65. (Quantel-International YG580). The laser is operated at 20 Hz (2) Castleman, Jr., A. W.; Keesee, R. G . Chem. Reu. 1986, 86, 589. with a pulse duration of 10 ns and a line width of 0.08 cm-'. The (3) Castleman, Jr., A. W.; Keesee, R. G. Acc. Chem. Res. 1986, 19, 413. (4) Jortner, J. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 188. power of the laser beam is attenuated to less than 1 mJ/pulse with (5) Stein, G. D. Phys. Teach. 1979, 17, 503. a beam spot of about 0.5 cm in diameter in order to minimize (6) See for example review articles 1-4 and the references therein. sequential photon absorption effects. (7) Kay, B. D.; Hermann, V.; Castleman, Jr., A. W. Chem. Phys. Lett. The parent clusters photodissociate to various daughter ion 1981, 80, 469. (8) Cheung, J. T.; Dixon, D. A.; Herschbach, D. R. J . Phys. Chem. 1988, masses. The detection of these daughter ions is achieved by a 92, 2536. quadrupole mass spectrometer whose axis is collinear with the (9) Cosby, P. C.; Smith, G. P.; Moseley, J. T. J . Chem. Phys. 1978, 69, ion and laser beams, and a particle multiplier is mounted off the 2779. Castleman, Jr., A. W.; Hunton, D. E.; Hoffman, M.; Lindeman, T. G.; beam axis in order to allow the laser beam to pass through the Lindsay, D. N . Int. J . Mass Spectrom. Ion Phys. 1983, 47, 199. (10) Levinger, N. E.; Ray, D.; Alexander, M. L.; Lineberger, W. C. J . quadrupole. Careful synchronization of the laser beam with the Chem. Phys. 1988, 89, 5654. ion pulse is required in order to observe photodissociation of the (11) Johnson, M. A.; Alexander, M. L.; Lineberger, W . C. Chem. Phys. parent ions. The daughter ion signals are then gated and the pulses Lett. 1984, 112, 285. are counted by a CAMAC system, where the data are transferred ( 12) Alexander, M. L.; Johnson, M. A,; Lineberger, W. C. J . Chem. Phys. 1985, 82, 5288.
(13) Alexander, M. L.; Johnson, M. A.; Levinger, N. E.; Lineberger, W. C. Phys. Rev. Lett. 1986, 57, 976. (14) Illies, A. J.; Jarrold, M. F.; Wagner-Redeker, W.; Bowers, M. T. J . Phys. Chem. 1984, 88, 5204.
0022-3654/89/2093-4386$01.50/0
(15) Shen, M . H.; Winniczek, J . W.; Farrar, J. M. J . Phys. Chem. 1987, 91, 6447.
(16) Lasertechnics Model LPV.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 11, 1989 4387
Letters Sr+(NH3),
120
Photodissociation
Wmhnqth ( In nm ) I . q - 3 , 790 6
.
p *
I \i
Sr'(NH5)
\B, /'
-Y)
X -30 1.1wEI
1.2W4
1.m4
1.W4
1.m.
1.WK4
Wovenumbera ( In cm-l)
Figure 1. Total photodissociation spectra for Sr+(NH,)", where n = 1, 2, 3, 4, in the wavelength range from 540 to 900 nm. The data are
normalized to constant dye laser power. into an IBM-AT compatible computer. The pulsed valve is operated at twice the laser repetition frequency, and the data are collected with and without the laser pulse. The net dissociation data are acquired by subtracting the data with laser off from the data with laser on. Overlapping spectra from nine different dyes are normalized to one another in order to cover the spectrum in the wavelength region from 540 to 700 nm. The spectral region from 700 to 900 nm is covered by Raman shifting the dye laser output in a high-pressure hydrogen cell. A typical spectrum for the dissociation of a single parent cluster is usually the result of several weeks of data accumulation. Results and Discussion Figure 1 displays the total photodissociation spectra of Sr+(NH,),,, n = 1-4, in the wavelength range from 540 to 900 nm. The spectrum of Sr+(NH,) is obtained by detecting the Sr+ daughter. The laser is stepped in 1-A increments for this spectrum, and for each increment, the data from 1000 laser shots are accumulated. There is no measurable dissociation beyond 680 nm. For the parent clusters with n = 2, 3, and 4, the spectra are obtained by mass analyzing all daughter ions with the quadrupole mass spectrometer at each laser increment. The total absorption spectrum for each parent is the summation of the daughter ions from the corresponding parent, where the typical data accumulation is 200 laser shots per wavelength increment. The common feature of all these spectra is their breadth: with the exception of the monomer, most of the absorption bands are basically structureless. The most striking spectral feature is the dramatic change in shape and peak position of the trimer and tetramer bands with respect to the monomer and dimer bands. The absorption bands of monomer and dimer have a similar pattern, i.e., a broad and intense band with the peak position centered around 580 nm. Although a significant absorption still remains in this region for trimer and tetramer, an intense and broad new absorption feature appears to the red of the strong monomer and dimer absorptions. The peak position of the trimer spectrum occurs about 680 nm, corresponding to a red shift of 7.2 kcal/mol or 2535 cm-I with respect to the dimer. The tetramer band is red-shifted and broadened even further, and the band peak
Reaction Coordinate
Figure 2. Schematic potential energy curves of the Sr+(NH,) and
Sr+(NH3)*systems. The dissociation may occur through an electronic predissociation mechanism, Le., through an initially bound to bound transition followed by a crossing to repulsive walls of a family of curves correlating with ground-state Sr+ and vibrationally-rotationally excited NH, products. One such curve is indicated in the upper panel by a dashed curve. Excited electronic states are denoted A and B as described in the text. In the lower panel, we show schematic excited electronic states of Sr+(NH3)*,as well as dissociation of the ground state by loss of one and two solvent molecules. is still not fully evident at 900 nm. Using 900 nm as the lower limit of the peak position, we find that the shift in the tetramer band is 10.3 kcal/mol or 3600 cm-I with respect to that of the trimer. A rough estimate of the absolute photodissociation cross section of Sr+(NH3),,can be obtained from depletion measurements of the parent cluster upon photon excitation." According to the formula where Zfinal and Zinitlal are the parent cluster intensities before and after laser excitation, u is the photodissociation cross section in cm2, and is the photon flux in photon/cm2 per pulse, a typical 1 5 2 0 % depletion of the parent clusters with laser flux of about 4 mJ/cm2 per pulse near 600 nm yields a cross section on the order of cm2. The error limit on this crude measurement could be as much as an order of magnitude. With such large photodissociation cross sections, this process 'should be the dominant reaction channel following photon absorption. Therefore, measurement of the total photodissociation spectrum is equivalent to the measurement of the total photoabsorption spectrum. The monomer and dimer absorption bands near 580 nm correspond to the lowest electronic transitions of these systems. Based on the work of Brazier and BernathIs on isoelectronic SrCH,, the ground electronic state of Sr+(NH3) in C3,symmetry is the 2AI state, arising from a closed-shell ionic core Sr2+(NH3)with one additional unpaired electron in a 5s metal-centered orbital. The lowest excited electronic state is the 2E state, corresponding to a state in which an unpaired electron is essentially in a p orbital of the metal center. Since no thermodynamic data are available for the strontium-ammonia system, we have constructed quali(17) Levinger, N. E.; Ray, D.; Murray, K. K.; Mullin, A. S.; Schulz, C. P.; Lineberger, W. C . J . Chem. Phys. 1988, 89, 71. (18) Brazier, C. R.; Bernath, P. F. J . Chem. Phys. 1987, 86, 5918.
4388 The Journal of Physical Chemistry, Vol. 93, No. 11, 1989 tative potential energy curves in Figure 2, using the known stepwise binding energies of Sr+(H20), obtained by Castleman and collaborator~'~ as a useful analogy to construct the ground state. The approach of Srf('P) to NH3 generates two sets of states: a doubly degenerate 2E state, where the px and pu (pr)orbitals of Sr+ are excited, and a singly degenerate excited state of 2A symmetry, in which the pz ( p u ) orbital of Sr+ is excited. Only the transition from the X2Alground state to the first excited state A2E is shown here in the spectra of Figure 1 . The photodissociation of the strontium-ammonia system may occur in an analogous way to that of the strontium-water system;I5 Le., the dissociation is via an electronic predissociation mechanism. In such an interpretation, potential energy surfaces correlating with a ground-state Sr+ atom and a vibrationally-rotationally excited ammonia molecule (the dashed curve in the upper panel of Figure 2) may cross the excited electronic state of the system to quench the electronic energy of the cluster into vibrational and rotational energy of the molecule. Therefore, the dissociation process may occur initially through a bound to bound transition followed by a crossing to the repulsive wall of a state correlating with the ground-state Sr+ and a vibrationally-rotationally excited NH, product. A comparison of the monomer and dimer dissociation spectra indicates that the bands are very similar in intensity, position, and width, with the exception of an extended red tail for the dimer. This observation suggests that the addition of one solvent molecule does not bring drastic changes to the nature of the X2Al A2E transition since the excited (px, pu) orbitals on Sr+lie perpendicular to the directions of SrC-NH3 bonds. There are several reasons for the breadth of these spectra. First, the spectral bands are the superpositions of the spin-orbit components of the excited electronic state as well as the superpositions of many rovibronic excitations. If the molecule does not have perfect C,, symmetry (which is usually the case), the splitting of the degenerate E state further congests the system. Second, many curve crossings could be involved in the complicated dissociation process and any resolved structure of the molecule could be blurred by these dynamical processes. Finally, there exists the possibility of incomplete cooling of the cluster in the jet expansion, although wide variations in the operating conditions of the expansion failed to produce significant changes in the spectra. We now turn our attention to the behavior of the trimer and tetramer. The appearance of a new absorption band red-shifted with respect to the peak of the dimer in the trimer spectrum, along with a further red shift of that band in the tetramer, suggests that dramatic changes in the characteristics of the optical transitions have taken place after the addition of only one or two solvent molecules to the dimer. The abrupt changes in the absorption spectra could derive either from changes in the ground or excited electronic states or from simultaneous changes in both states. In order to understand the possible behaviors of clusters having properties between the gas and condensed phases, we must first examine more carefully the behavior of alkaline earth atoms in the liquid or solution phase. The subject of metal solutions in polar sovlents, especially H 2 0and NH3, has captivated the attention of many chemists over the past two decades. It has been wellrecognized that electron localization and solvation are involved in the process of dissolving an alkali or alkaline earth metal into ammonia or other polar fluids. The result of this unique solvation process is the formation of the solvated electron in a cavity in the solvent with the solvated metal ions as a counterion. A large body of data on the spectral properties of solvated electrons indicates that these species have characteristic absorption spectra with large oscillator strengths, typically on the order of several tenths for example, the absorption spectrum for electrons
-
(19) Tang, 1. N.; Lian, M. S.; Castleman, Jr., A . W. J . Chem. Phys. 1976, 65, 4022.
(20) Hart, E. J.; Anbar, M. The Hydrated Electron; Wiley-Interscience: New York, 1970. (21) Thompson, J . C . Elecrrons in Liquid Ammonia; Oxford University Press: London, 1976. (22) Lepoutre, G.; Sienko, M . J., Eds. Metal-Ammonia Solutions Colloque Weyl I; Benjamin: New York, 1974.
Letters solvated in ammonia is very broad, with a peak near 1.2 Km and a tail extending into the visible range, responsible for the characteristic blue color of such solutions. Since the thermionic emission process we employ for ion production removes only one electron from the Sr atoms, while the alkaline earth cations in aqueous solution are in the $2 oxidation state, one would expect that an intramolecular electron-transfer reaction will take place as the clusters become sufficiently solvated to approximate a solution. Although we are not aware of any examples of intramolecular electron transfer assisted by solvation in clusters, Castleman et al.,' and most recently Herschbach and co-workers,8 have speculated that intramolecular proton transfer may occur in hydrated DNO, clusters and (NH,),HX (X = C1, Br, or I), respectively, to yield ion pairs with overall charge neutrality. In the Sr+(NH,),, system, this intramolecular electron transfer may strongly depend on the size of the clusters. The result of electron transfer in the present case is the formation of a strontium-ammonia ion-pair cluster: Sr+(NH3), Sr*+(NH,),,(NH,),-
-
We must address the question of whether the changes in the spectra are due to the spontaneous electron transfer in the ground electronic state, excitation of a charge-transfer state upon electronic excitation, or both. We can exclude the possibility of major changes in the ground state by considering the relative stabilities of Sr+(NH3),,and Sr2+(NH3)n-m(NH3)m(where n = 3 and 4 only). Although there have been no ab initio calculations for the strontium-ammonia system, it is still possible to make some reasonable estimates of the heat of formation of Sr2+(NH3),,(NH3),-. The second ionization potential for Sr+, the energy required to remove an electron to infinite separation, is 254.3 kcal/mol. The attractive Coulombic energy of an ion of charge +2 separated from an anion of charge -1 by 3.5 8, is 189.1 kcal/mol. Consequently, the energy required to transfer an electron from Sr+to a site 3.5 8,away is only about 65.2 kcal/mol. Although no data are available for solvation of Sr2+,ab initio c a l c ~ l a t i o n sfor ~ ~ Ca2+ and Mg2+ indicate that the first two stepwise hydration energies for these ions are approximately 40 kcal/mol. Because e-(NH3)2and Sr2+(NH3)are strongly polarizable species, one should take the polarization attractive energy into account. Assuming polarization constants of 10 ASfor both Sr2+(NH3)and (NH3)2-, the induction attractive energy at a separation of 3.5 8, between them will be 55.1 kcal/mol. Although small (NH,), clusters do not appear to support bound negative ion making the (NH,); system unstable at large distances from the cation, the ion-pair state arising from the close proximity of the anion and cation may be stable, in analogy with solid-state electrides.26 After including the solvation and polarization energy, Sr2+(NH3),,we expect that the reaction Sr+ + n(NH,) (NH,),- will be exothermic by about 30-40 kcal/mol for n = 3 and 60-70 kcal/mol for n = 4. However, the reaction Sr++ nNH3 Sr+(NH3), is exothermic by approximately 90-1 00 kcal/mol for n = 3 and 110-120 kcal/mol for n = 4.27 Thus, the ion-pair states should lie above the Sr+(NH,), ground state by about 60 kcal/mol for n = 3 and about 50 kcal/mol for n = 4. For a cluster as small as the trimer or tetramer, we therefore believe that the molecules prepared in the jet expansion are mainly the singly charged solvated molecules Sr+(NH,), and Sr+(NH3)4. Despite this situation, the ion-pair states may play a role in the visible absorption process. The potential minimum of the ion-pair cluster may lie between the potential minimum of the ground state and the excited state of the singly charged cluster as indicated in the qualitative potential curves shown in Figure 3. Photon excitation would then bring the singly charged cluster molecule
-
-
(23) Copeland, D. A.; Kestner, N. R.; Jortner, J. J. Chem. Phys. 1970, 53, 1189. (24) Ortega-Blake, J.; Novaro, 0.;Les, A,; Rybak, S. J. Chem. Phys. 1982, 76, 5405. (25) Haberland, H.; Schindler, H.-G.; Worsnop, D. R. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 270. (26) Dye, J. L.; Debacker, M. G. Annu. Rev. Phys. Chem. 1987.38, 271. (27) These values are deduced from Sr+(H,O), stepwise solvation enthalpies reported in ref 19.
The Journal of Physical Chemistry, Vol. 93, No. 11, 1989 4389
Letters
60
I
20
-20
CI
2 -60
E
1 '. $100 v
v
/
$ 4 0
0
-40
-80
-120
Reaction Coordinate
Figure 3. Schematic potential energy curves for the Sr+(NH3)3and Sr+(NH3)4systems. The potential minima of the ion-pair states Sr2+(NH,),,-,,,(NH,),,,-,labeled "a", lie between the potential minima of the ground and the first excited states of the singly charged cluster Sr+(NH,),. The latter states are labeled "b" in both panels.
in the ground state to this ion pair state, labeled "a" in both panels of Figure 3: charge separation in the excited state would account for the large oscillator strength we observe. Extending this argument, then it is expected that the ion-pair tetramer Sr2+(NH3)2(NH3)2-would be more stable than the ion-pair trimer SrZ+(NH3)(NH3)2and the potential minimum for the ion-pair tetramer will therefore lie below that of the trimer. This picture is consistent with the observation that the absorption spectrum is further red-shifted for the tetramer with respect to the trimer.
The progressive red shift as a function of cluster size may therefore reflect the stepwise ion-pair cluster solvation process. If the argument above holds for large clusters, we expect that the peak absorption will continue to shift to the red until the ion pair is more stable than Sr+(NH3),. Such clusters should therefore exist in an equilibrium between the ion pair and the singly charged cluster. Most interestingly, at that point a new spectral feature corresponding to the strong solvated electron absorption band may appear in the spectrum: for ammonia this absorption is near 1.2 Km. We believe that our present results can be interpreted via ion-pair states that serve as precursors to the solvated electron ground state observed in the condensed phase. The appearance of these states as demonstrated by the strong size dependence of the photodissociation spectrum provides evidence for a transition from gas-phase toward condensed-phase behavior over a remarkably narrow size range. In conclusion, we have presented an extensive spectroscopic study of the strontium-ammonia system. The total photodissociation spectra of Sr+(NH3),, n = 1, 2, 3, and 4, have been measured in the wavelength region from 540 to 900 nm. The spectra of Sr+(NH3)and Sr+(NH3)2show broad and intense bands with the peak positions centered at 580 nm. For Sr+(NH3)3and Srf(NH3),, the spectra display a new absorption peak strongly red-shifted in addition to the initial absorption around 580 nm. This new band has a peak position near 680 nm for Sr+(NH3), and over 900 nm for Sr+(NH3)& The dissociation of these clusters is postulated to occur through an electronic predissociation process. On the basis of the experimental results and some simple energetic calculations, we propose a solvated ion-pair formation model in the interpretation of spectral features in Sr+(NH3), and Sr+(NH,),. We are pursuing studies on larger clusters and with other alkaline earth cations to explore the role of electron transfer in such clusters and the use of this phenomenon as a diagnostic of the transition to condensed-phase behavior.
Acknowledgment. We gratefully acknowledge support of this work under National Science Foundation Grant CHE-088-07833. M.H.S. acknowledges the University of Rochester for support as a Sherman Clarke Fellow and an Elon Huntington Hooker Fellow. Acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for the partial support of this research.
