J . Phys. Chem. 1992, 96, 516-518
516
period of flash illumination, the lights were turned off, and the photocurrent output returned to the zero baseline. The trigger generator was then set to the next frequency and the cycle repeated. In this manner a series of light-on, light-off cycles was generated in a single run, and the response of the photobioelectrochemical cell to repetitive flash illumination of varying frequencies was determined. The normalization of the photoresponse was obtained by dividing each steady-state photocurrent by the frequency of the repetitive light used to obtain it. The flow of photocurrent in the photocell can be understood in terms of the schematic illustration of Figure 1. Because of the geometry of the thylakoid membranes, platinum is precipitated onto their external surfaces. It has been previously demonstrated in two ways that electrical contact is made by the platinum precipitation process: by the observation of photogenerated hydrogen and the direct measurement of photoelectric current. Far more platinum is precipitated onto the chloroplast membranes than is necessary for catalytic hydrogen activity. However, the flow of photocurrent is a statistical process that is dependent on the reticulated matrix of platinum particles created in the precipitation process.Il Electrons that emerge from the photosystem I reaction centers make contact with the colloidal platinum. Some of these electrons find their ways to the platinum gauze electrode, which is in pressure contact with the platinized chloroplasts. The flow of electrons, from the reducing end of photosystem I to the platinum electrode and around to the silver/silver chloride electrode is, therefore, essentially metal-to-metal conduction of the electrons. When the electrons reach the Ag/AgCl electrode that is in pressure contact with the electrolyte-impregnated fiberglass filter pad upon which the platinized chloroplasts are entrapped, the standard half-cell reaction AgCl e- Ag C1- occurs. If the reaction were to end here, the flow of photocurrent would cease after sufficient accumulation of C1-, which would create a back-potential that would oppose the further flow of photogenerated current. That is not, however, what is observed because
+
-
+
the flow of photocurrent can continue for days. The C1- ions are neutralized by the hydrogen ions that are generated by the water splitting reaction. These chloride ions are permeable to the thylakoid membranes. The fate of photoreductant falls into two categories. First, those electrons that connect with the reticulated network of platinum particles that is electrically connected to the platinum gauze cathode contribute to the flow of photogenerated current. Second, those electrons that do not connect to this reticulated network do not contribute to the flow of photocurrent. These may participate in the photocatalytic evolution of hydrogen or may be lost to adventitious electron traps. It is important to note that the photoeffects observed in this photobioelectrochemical cell are not attributable to the Dember effect. Unlike a Dember effect photosignal, the polarity of the photosignal is independent of the direction of illumination of the cell. This two-sided illumination could be performed because the fiberglass filter pad is translucent. Although light penetrating the cell from the fiberglass side is attenuated, the platinized chloroplasts could still be illuminated from the 'back" side. It can be seen from this discussion of the origin of the photocurrent that the complete electron transport chain of photosynthesis participates in the flow of photocurrent in this device. The kinetic time constant (4 ms) of the photoresponse of this cell is consistent with the kinetics of the electron transport chain of photosynthesis.
Acknowledgment. The author thanks J. P. Eubanks for technical support and D. J. Weaver and S. A. Hoglund for secretarial support. H e also thanks B. Z. Egan and P. F. Britt for critically reading the original version of the manuscript. This research was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Department of Energy. Additional support was provided by the Materials Laboratory, Wright-Patterson Air Force Base. Oak Ridge National Laboratory is managed by Martin Marietta Energy Systems, Inc., for the U.S. Department of Energy under Contract DE-AC05-840R21400.
Theoretical Suggestion for the Zn2+(H,O) Formation Enrique Sinchez Marcos,* Rafael R. Pappalardo, Departamento de Quimica F h a , Facultad de Quimica, Universidad de Sevilla, 4101 2 Sevilla. Spain
Jean-Claude Barthelat, and Florent Xavier Gadea Laboratoire de Physique Quantique (U.R.A.505 du CNRS). UniversitC Paul Sabatier, 1 1 4 route de Narbonne. 61062 Toulouse Cedex, France (Received: October 25, 1991)
Zn2+-water interactions have been studied by means of ab initio calculations. Competition between dissociationsof the Zn2+(H20) cluster in Zn+ + H20+or Zn2++ H 2 0 is discussed and related to the experimental difficulties to form the monohydrated ion Zn2+(H20)in gas phase. It is suggested that photoionization of the singly charged ion Zn+(H20) could be a possible Zn2+(H20)+ e- process. strategy to obtain Zn2+(H,0). The Franck-Condon factors are favorable for the Zn+(H,O)
-
The doubly charged ions M2+of transition metals are of paramount importance in chemistry and biochemistry. It has long been established that cationsolvent interactions play an important role in the physical and chemical properties of electrolyte solutions,' water being the solvent most used. To isolate the direct metallic cation-water molecule interactions from environmental effects, studies of small M2+(H20), clusters are needed. Therefore, ( 1 ) Marcus, Y. Ion Solvorion; Wiley: Chichester, U.K., 1986.
0022-3654/92/2096-5 16$03.00/0
Kebarle and co-workers2 have recently obtained gas-phase ions such as M2+(H20), by a new electrospray technique. From the initial large cluster (n 14), successive losses of water molecules by collision-induced decomposition leads to the formation of several smaller clusters. For transition metals it was not possible to obtain clusters with n smaller than 4. However, monopositive M+(H20), (2) Jayaweera, P.; Blades, A. T.; Ikonomou, M. G.; Kebarle, P. J. Am. Chem. SOC.1990, 112, 2452.
0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 517
Letters
o
-78.2
-79.4 h
5
Id
v
W -78.8 ~
WH,O
'
76
82
80
76
A (nm) -80
2
4
6
8
Figure 2. Bound-bound photoionization absorption spectrum from the 2A1state of Zn+(H20) to the 'Al state of Zn2+(H20).
t'
'
"
"
"
"
"
'
ai
t'"''"''i b
Figure 1. SCF potential curves for the ground state of Zn+(H20) (2A1) and the lowest electronic states of [Zn(H20)l2+('Al and 'B2).
(n = 1-4) have been produced for transition metal^.^.^ These gas-phase experimental results show that it is much easier to produce the monopositive small clusters than the doubly charged ones, in total contrast with the usual concepts of chemistry in solution, which argues for the doubly charged cations. We wish to get a deeper insight into the understanding of this paradox and to propose a strategy for experimentalists to obtain M2+(H20)based on the photoexcitation of an initial M+(H20) precursor. Figure 1 shows the potential curves for the ground state of Zn+(H20) and the lowest electronic states of [Zn(H,O)l2', all with a water molecule frozen in its Zn+(H20)optimized structure. Calculations have been carried out at the SCF level, using pseudopotentialsSfor the Zn and 0 atoms and DZ basis sets supplemented with polarization functions for each atom. Due to the very large electrostatic component of the metal-water bonding, the SCF method gives a reasonable description of the interaction. Thus, our results for the minimum of the Zn+(H20) curve agree well with those obtained by Bauschlicher and co-workers6 using a more sophisticated method (ref 6, Re = 2.06 A, De = 34.5 kcal/mol; this work, Re = 2.09 %L, De = 33.0 kcal/mol). For the ]A, symmetry of [Zn(H20)l2+cluster there is a charge-transfer crossing at about 4.5 A which should become an avoided crossing at the CI level. Clementi and co-workers' reported a similar crossing for the interaction curves of water with doubly charged beryllium. Due to the great difference in the polarization of these two electronic distributions, the SCF procedure is stable for both configurations, even in the vicinity of the crossing. These 'Al symmetry curves illustrate for the smallest M2+(H20), cluster that the reaction channel for the loss of neutral water molecules is precluded by the avoided crossing, leading to the loss of H 2 0 + and the formation of M+(H20),] instead of M2+(H20)w1, as was found by Kebarle and coworkers? Although the doubly charged zinc-water interaction is stronger than the corresponding singly charged zinc one; the formation of Zn2+(H20)is much more complicated than that of Zn+(H,O), if the decomposition of a polyhydrated zinc cluster is taken as starting point. Due to the strong doubly charged zinc-water interaction, the dissociation (3) (a) Magnera, T. F.; David, D. E.; Michl, J. J. Am. Chem. Soc. 1989, 1 1 1 , 4100. (b) Magnera, T. F.; David, D. E.; Stulick, D.; Orth, R. G.; Jonkman, H. T.; Michl, J. J . Am. Chem. Soc. 1989,111, 5036. (4) Marinelli, P. J.; Squires, R. R. J . Am. Chem. Soc. 1989, I l l , 4101. (5) (a) Durand, P.; Barthelat, J. C. Theor. Chim. Acta 1975.38, 283. (b) Bouteiller, Y.;Mijoule, C.; Nizam, M.; Barthelat, J. C.; Daudey, J. P.; Pellisier, M.; Silvi, B. Mol. Phys. 1988, 65, 295. (6) (a) Rosi, M.; Bauschlicher, C. W. J . Chem. Phys. 1989,90,7264. (b) Rosi, M.; Bauschlicher, C. W. J . Chem. Phys. 1990, 92, 1,876. (7) Corongiu, G.; Clementi, E. J . Chem. Phys. 1978.69, 4885.
L. (nm)
A (nm)
Figure 3. Bound-free photoionization absorption spectra from the 2Al (Zn+(H20)) to the dissociative states of Zn+(H20)+ IB2 (a) and IAI (b).
process of a water molecule becomes unfavorable against the internal charge transfer. As an alternative to the water molecule approach or release from a given small M2+(H20), cluster, we propose the photoionization of the corresponding monocharged cluster. It can be seen from Figure 1 that, at least for the monohydrated zinc cluster, the avoided crossing does not play an important role in the absorption process. The photoionization absorption spectra associated to the process Zn+(H20) [Zn(H20)l2+ e- have been computed at 300 K using a one-dimensional quantum method. This simplified representation of the spectra seems to be adequate in this case for two reasons. First, the water geometry relaxation is quite similar for the interaction with the sing1 and for doubly charged cation (for the minima dGH = 0.96 Zn+(H20), 0.97 A for Zn2+(H20)and a ( H 0 H ) = 107.0° for Zn+(H,O), 107.2O for Zn2+(H20)). Full geometry optimizations of Zn+(H20) and Zn2+(H20)followed by vibrational analysis* lead to true minima corresponding to stable structures of C,, symmetry. Second, the vibrational analysis for these two structures show that the zinc-water stretching is not coupled to the water normal modes. Therefore, the absorption spectra derived from this simple one-dimensional approach should lead to a realistic ~ p e c t r u m . ~The population of the ground state is 0.79 for n = 0, 0.17 for n = 1 and 0.04 for n = 2. Figure 2 displays the spectrum for the bound-bound transitions leading to the formation of the Zn2+(H20)cluster. The lines have been convoluted with
-
+
1
(8) Vibrational harmonic frequencies (cm-I) for Zn+(H20)are 329 (al), 360 (bJ, 561 (b2), 1752 (al), 4011 (al), and 4095 (b2) and for Zn2+(H20) are 516 (b]), 555 (a,), 775 (b2), 1735 (al), 3830 (al), and 3887 (b2). (9) The transition energies used to calculate the photoionization spectra have been corrbcted by a constant value which corresponds to the difference PI(Zn+), - PI(Zn+),-.
J . Phys. Chem. 1992, 96, 5 18-5 19
518
a 0.01-eV width Lorentzian function, accounting for the experimental resolution. The spectrum presents a main regular progression ( w = 540 cm-’) broadened by thermal effects. As shown in Figure 3, the bound-free transitions correspond to broad bands. Such excitations lead to dissociation of a H20+fragment. In conclusion, the experimentally reported3v4Zn+(H20) cluster can be used as precursor of the Zn2+(H20)cluster under ultraviolet photoionization around 79.8 nm. Such an experiment could be extremely selective (in producing Zn2+(H20)),using synchrotron
Qetection of CH,(% Radicals by 3 Ionization Spectroscopy
radiation. Otherwise, concentrations of different products may be analyzed when using UPS. This strategy could also be applied in order to obtain other doubly charged transition metal cations hydrated with a small number of water molecules.
Acknowledgment. We are grateful to the DGICYT of Spain (PB 89-0642) for financial support. E.S.M. thanks the Ministtre des Affaires Etrangtres (France) and the DGICYT of Spain for a grant (Programme Mercure).
+ 1 Resonance-Enhanced Multiphoton
Karl K. Irikurat and Jeffrey W. Hudgens* Chemical Kinetics and Thermodynamics Division, National institute of Standards and Technology,$ Gaithersburg, Maryland 20899 (Received: December 2, 1991)
Spectra of triplet methylene radicals, 2 3B1CHI and CD2, were produced between 380 and 440 nm using mass-resolved resona_nce-enhpced multiphoton ionization (REMPI). These spectra arose from three-photon resonances with the B (3d), C (3d), D (3d), and 4d 3A2Rydberg states between 78 950 and 68 200 cm-l above the ground state. A fourth laser photon ionized the radicals; Le., CH2+( m / z 14) and CD2+ ( m / z 16) ion signals were generated through a 3 + 1 REMPI mechanism. Methylene radicals were produced by the reaction of fluorine atoms and methyl radicals.
Methylene radicals, CH2 and CD2, play important roles in the combustion and photochemistry of numerous chemical systems. Because the first excited state of CH2,the fi ]Al state, lies at low energy (To = 3147 cm-’),l _an understanding of CH2 chemistry requires studies of both the X 3B1and fi ‘Al states. Ground-state methylene radicals are particularly inconvenient to study. Although kineticists have used laser magnetic resonance spectroscopy with great success to measure reaction rates of CH2(X 3B1),2this technique is not adaptable to most other experiments, e.g., molecular beam studies of reaction dynamics. Laser-induced fluorescence (LIF) spectroscopy can detect-CH,@ lA1),3but no similar LIF detection scheme for CH2(X 3B1) is available. Furthermore, all known optical absorptions of CH2(X 3B1) reside in the vacuum-ultraviolet (vacuum-UV) spectral r e g i ~ n . ~The .~ experimental difficulties incumbent to vacuum-UV optics prevent experimentalists from using trans@ absorption spectroscopy to measure concentrations of CH2(X 3BI). In tkis work we report_a new laser-based method for detecting CH2(X 3B,) and CD2(X 3B1) based upon resonance-enhanced multiphoton ionization (REMPI) spectroscopy. In our detection schem? we enhance the multiphoton ionization cross section of CH2(X 3B1)with a,” i_ntense,_tightly focused laser beam. CH2 is excited into the B, C, and D electronic states through simultaneous absorption of three identical laser photons. The excited CH2 may ionize and form CH2+(X2A1)by subsequently absorbing one more laser photon; Le., we use a one-color 3 1 REMPI excitation scheme. The present REMPI detection scheme uses lasers and optics common to many modern laboratories. We believe that REMPI detection will be useful for the sensitive and selective detection of gas-phase CH2(X 3BI)radicals in many other experiments. The apparatus and procedures used to record the REMPI spectra have been described in detail previously.6 In brief, in a flow reactor free radicals are produced by the reaction of fluorine atoms with various reagents. Upstream from this flow reactor fluorine atoms are produced with a microwave discharge of 3 4 % F2 in helium. The total pressure in the flow reactor is 100-300
+
-
*Address correspondence to this author. NIST/NRC Postdoctoral Associate. t Formerly called the National Bureau of Standards.
Pa (-2-3 Torr). The radicals effuse from the flow reactor into the ionization region of a timeof-flight mass analyzer (1-10 “a). Radicals are ionized by the focused output of an excimer-pumped tunable dye laser (energy = 8-16 &/pulse; bandwidth = 0.2 cm-I, fwhm; focal length = 75 mm). The ions are mass resolved, gated integrators monitor selected ion masses, and a computer dataacquisition system records and averages the selected ion intensities as a function of laser wavelength. The spectra shown here are composites of spectra obtained with the laser dyes (Exciton Chemical Co.)’ PBBO (385-405 nm), DPS (399-415 nm), and Stilbene 420 (415-440 nm). The spectra are uncorrected for the variation in laser pulse energy which occurs over the range of each dye. Methylene radicals are presumably generated by a sequence of hydrogen abstraction reactions, reactions 1 and 2.* The CH4 F CH3 HF A H = -31.5 kcal mol-’ (1) CH3 F C H 2 ( k 3B1) H F AH = -26.9 kcal mol-’ (2a) CH3 F CHI(% IA,) HF AH = -17.9 kcal mol-] (2b)
+ +
+
-
+
+
+
(1) Jensen, P.; Bunker, P. R. J . Chem. Phys. 1988,89, 1327. (2) Seidler, V.; Temps, F.; Wagner, H. Gg.; Wolf, M. J . Phys. Chem. 1989, 93, 1070. (3) (a) Danon, J.; Filseth, S. V.; Feldmann, D.; Zacharias, H.; Dugan, C. H.; Welge, K. H. Chem. Phys. 1978, 29, 345. (b) Lengel, R. K.; Zare, R. N . J . Am. Chem. Soc. 1978, ZOO, 7495. (c) Ashfold, M. N . R.; Fullstone, M. A.; Hancock, G.; Ketley, G. W. Chem. Phys. 1981,55,245. (d) Petek, H.; Nesbitt, D. J.; Ogilby, P. R.; Moore, C. B. J . Phys. Chem. 1983,87, 5367. (e) Sappey, A. D.; Crosley, D. R.; Copeland, R. A. Appl. Phys. B 1990.50, 463. (4) (a) Herzberg, G. Proc. R. SOC.(London) 1961, A262, 291. (b) Herzberg, G.; Johns, J. W. C. Proc. R.SOC.(London) 1966, ,4295, 107. (c) Herzberg, G.; Johns, J. W. C. J. Chem. Phys. 1971,54, 2276. (d) Herzberg, G. J . Mol. Struct. 1990, 217, xi. (5) Herzberg, G. Con. J. Phys. 1961.39, 151 1 . (6) Johnson, R. D. 111; Tsai, B. P.; Hudgens, J. W. J . Chem. Phys. 1988, 89,4558. (7) Certain commercial materials and equipment are identified in this paper in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the material or equipment identified is necessarily the best available for the purpose.
This article not subject to U.S. Copyright. Published 1992 by the American Chemical Society
