Reports: AC6
47852-AC6 Solvated Electrons in Liquid Jets of Water and Ionic Liquids
In order to strengthen the link between experiments on water clusters anions and those performed on bulk hydrated electrons, we are carrying out experiments on hydrated electrons in liquid microjets that will directly measure the binding energy of these electrons and their excited state lifetimes. One of the primary motivations for studying (H2O)n¯ clusters has been to use these as model systems for bulk hydrated electrons. Thus, for example, the excited p-state lifetimes seen in time-resolved PE spectra of (H2O)n¯ clusters measured in our laboratory1-3 and elsewhere,4,5 when extrapolated to the n®¥ limit, strongly support the "non-adiabatic" solvation model for relaxation of eaq¯.6-9 In this model, p®s internal conversion happens on a time scale of ~50 fs, and the spectral evolution seen in the transient absorption spectrum8,10-13 of electronically excited eaq¯ is attributed to relaxation on the s-state subsequent to IC. However, the extrapolation of cluster experiments to the condensed phase has been questioned in recent theoretical work.14 There are two issues to consider here. First, the nature of the solvated electron in clusters may be quite different from that in the bulk. For example, in bulk water, hydrated electrons reside in a solvent cavity with a radius of about 2.5 Å,15,16 whereas, as indicated above, there has been considerable discussion in the literature about whether excess electrons in (H2O)n- clusters reside on the inside or surface of the solvent network. Secondly, photoelectron spectroscopy and transient absorption (TA) experiments measure different physical attributes of hydrated electrons, and comparing results from these two types of experiments in two different media, namely isolated water cluster anions and bulk liquids, may not be totally straightforward.
The experiments on liquid jets address both issues. We generate electrons in liquid water microjets using the same methods that have been used in bulk water;7 the resulting electrons should be true “hydrated electrons”. In addition, liquid jets are, with some effort, compatible with the high vacuum instrumentation needed to perform PES experiments, as has been demonstrated in several laboratories.17-20 Hence, PES experiments on hydrated electrons in liquid jets represent the “missing link” between the cluster and condensed phase experiments and will thus provide fundamental new insights into the nature of electron solvation dynamics in both media.
We thus far have preliminary data on the VBE of hydrated electrons. These experiments are performed on a jet of 40 mM K4Fe(CN)6 in water. The jet is intercepted by a 10 ns, 266 nm pulse from a Nd:YAG laser. Photoelectrons are formed by a two-photon process: the first photon excites the charge-transfer-to-solvent (CTTS) transition of the Fe(CN)64- anion,7 ejecting an electron that rapidly equilibrates with the solvent.21 The second photon then ejects the electron from the jet and its kinetic energy is measured. The inelastic mean free path (IMFP) for these low energy electrons is 5-6 nm,20 so we indeed sample electrons within the jet. Results shown in Figure 1 indicate a VBE of 3.9 eV for eaq¯. This value is higher than the estimated value of 3.3 eV,22 but agrees with the extrapolation of our anion PE spectra23 and with more recent PE spectra reported by Ma et al24 for low temperature (H2O)n¯ clusters. The validity of our result will be tested by examining other salts at different CTTS/detachment wavelengths, e.g. we can excite I¯ from aqueous KI near its CTTS maximum at 230 nm.25 1 A. E. Bragg, J. R. R. Verlet, A. Kammrath, O. Cheshnovsky, and D. M. Neumark, Science 306, 669 (2004).
2 A. E. Bragg, J. R. R. Verlet, A. Kammrath, O. Cheshnovsky, and D. M. Neumark, J. Am. Chem. Soc. 127, 15283 (2005).
3 G. B. Griffin, R. M. Young, O. T. Ehrler, and D. M. Neumark, J. Chem. Phys. (accepted).
4 J. M. Weber, J. Kim, E. A. Woronowicz, G. H. Weddle, I. Becker, O. Cheshnovsky, and M. A. Johnson, Chem. Phys. Lett. 339, 337 (2001).
5 D. H. Paik, I. Lee, D. Yang, J. S. Baskin, and A. H. Zewail, Science 306, 672 (2004).
6 A. Hertwig, H. Hippler, A. N. Unterreiner, and P. Vohringer, Ber. Bunsen-Ges. Phys. Chem. 102, 805 (1998).
7 K. Yokoyama, C. Silva, D. H. Son, P. K. Walhout, and P. F. Barbara, J. Phys. Chem. A 102, 6957 (1998).
8 M. S. Pshenichnikov, A. Baltuska, and D. A. Wiersma, Chem. Phys. Lett. 389, 171 (2004).
9 P. O. J. Scherer and S. F. Fischer, Chem. Phys. Lett. 421, 427 (2006).
10 A. Migus, Y. Gauduel, J. L. Martin, and A. Antonetti, Phys. Rev. Lett. 58, 1559 (1987).
11 X. Shi, F. H. Long, H. Lu, and K. B. Eisenthal, J. Phys. Chem. 100, 11903 (1996).
12 J. C. Alfano, P. K. Walhout, Y. Kimura, and P. F. Barbara, J. Chem. Phys. 98, 5996 (1993).
13 A. Thaller, R. Laenen, and A. Laubereau, Chem. Phys. Lett. 398, 459 (2004).
14 L. Turi, W.-S. Sheu, and P. J. Rossky, Science 309, 914 (2005).
15 P. J. Rossky and J. Schnitker, J. Phys. Chem. 92, 4277 (1988).
16 M. Boero, M. Parrinello, K. Terakura, T. Ikeshoji, and C. C. Liew, Phys. Rev. Lett. 90, 226403 (2003).
17 M. Faubel, B. Steiner, and J. P. Toennies, J. Chem. Phys. 106, 9013 (1997).
18 K. R. Wilson, M. Cavalleri, B. S. Rude, R. D. Schaller, T. Catalano, A. Nilsson, R. J. Saykally, and L. G. M. Pettersson, J. Phys. Chem. B 109, 10194 (2005).
19 F. Mafune and T. Kondow, Aust. J. Chem. 57, 1165 (2004).
20 B. Winter and M. Faubel, Chem. Rev. 106, 1176 (2006).
21 X. Y. Chen and S. E. Bradforth, Annu. Rev. Phys. Chem. 59, 203 (2008).
22 J. V. Coe, Int. Rev. Phys. Chem. 20, 33 (2001).
23 J. R. R. Verlet, A. E. Bragg, A. Kammrath, O. Cheshnovsky, and D. M. Neumark, Science 307, 93 (2005).
24 L. Ma, K. Majer, F. Chirot, and B. v. Issendorff, J. Chem. Phys. 131, 144303 (2009).
25 E. Rabinowitch, Rev. Mod. Phys. 14, 112 (1942).
