J. Phys. Chem. 1980, 84, 1155-1160
-0
100
50
150
Time (ns)
Flgure 1. Time dependence of the product of the yield of ions and the sum of the ion mcibilltles resulting from pulse irradiation (20ns, ca.350 rd) of pure C6F, (0); C6F6containing lo-*M SF6 (M); lo-* M CBr4 (0).
Benzene contalnlrig lo-* M SF6 (0)and pure CC14(+), (The initiil rapid decay is due to recombination of geminate ions.)
species is comlplicated by the simultaneous decay due to “geminate recombination” and reactions of these species. When the method of analysis of the data presented in ref 10 is used this ratio is found to be 40. If the sum of the mobilities of positive and negative molecular ions is taken to be approxirnately 1 X 10” for this liquid,l’ which has a viscosity cloise to that of liquid cyclohexane, then one obtains p(-) E:! 2 X cm2 V-l s-l. The fact that the negative ion is more than an order of magnitude more mobile than expected for a molecular ion is further substantiated by the rate constant for scavenging by CBr4 which is found to be 1.5 X lo1’ dm3 mol-l s-l. We conclude that excess electrons undergo rapid attachment in liquid C6F6but that rapid electron exchange between the negative ion and neighboring neutral molecules provides the possibility of charge transport without the necessity of molecular displacement. As a consequence it would appear-that no appreciable molecular distortion accompanies initial electron attachment. Relaxation of the
1 155
ion to a distorted, nonplanar form has been suggested for C6F6on the basis of matrix isolation ESR experirnents.l2J3 This could possibly explain the 100-ns half-life found for the mobile negative charge carrier in different preparations of C6F6;however, this lifetime could also be due to reactions of C6F6-with impurities fortuitously present. Initial suggestions,14-16based on positron annihilation studies, of rapid electron transfer between and C6F6 in the liquid phase would appear to be substantiated. The latter suggestion was in fact initially made16 in order to explain the antiinhibition effect in CS2and, in preliminary conductivity experiments, we have also found evidence for a mobile negative charge carrier in pure CS2,although the mobility appears to be somewhat smaller than that found for C6Fe A fuller account of the experiments including additional data will appear in a future publication.
References and Notes (1) F. M. Page and G. C. Goode, “Negative Ions and the Magnetron”, Wiley-Interscience, New York, 1969. (2)C. Lifshitz, T. 0. Tlernan, and B. M. Hughes, J. Chem. Phys., 59, 3182 (1973). (3) K. S.Gant and L. G. chiistophorou, J. Chem. phys., 65, 2977 (1976). (4) P. P. Infelta, M. P. de Haas, and J. M. Warman, Rad&. phys. &m., 10,353 (1977). (5) C. B. Leffert, S. Y. Tang, W. Rothe, and T. C. Wng, J. Chem. mys., 61, 4929 (1974). (6) P. R. Hammond, J. Chem. Phys., 55, 3468 (1971). (7)R. N. Campton and C. D. Cooper, J. Chem. phys., 59,4140 (1973). (8) J. L. Pack and A. V. Phelps, J. Chem. Phys., 44, 1870 (1966). (9) R. Celotta, R. Bennet, J. Hale, M. W. Slegel, and J. Levlne, Phys. Rev. A , 6, 631 (1972). (10) J. M. Warman, P. P. Infelta, M. P. de Haas, and A. Hummei, Can. J . Chem., 55, 2249 (1977). (11)A. 0.Allen, M. P. de Haas, and A. Hummel, J . Chem. Phys., 64,
2587 (1976). 12) M. B. Yim and D. E. Wood, J. Am. Chem. Soc., 98, 2053 (1976). 13) M. C.R. Symons, R. C. Selby, I. G. Smith, and S. W. Bralt, Chem. Phys. Lett., 48, loo (1977). 14) 0. E. Mogensen, prlvate communication, see also 14 for a similar suggestion for the case of CS,. 15) P. Jansen, M. Eldrop, 0. E. Mogensen, and P. Pagskg, Chem. phys., 6, 265 (1974). 16) 0. A. Anisimov and Y. N. Molin, Khim. Vys. Energ., 9,539 (1975).
Mobility of Electrons in Ammonia Vapor at Various Densities P. Krebs” and M. Wantschik Institut fur Physikalische Chemie und Elektrochemieder Universitat, 0-7500 Karl&uhe, West Germany (Received July 16, 1979)
To get additional information about excess electron states in polar fluids we have measured the mobility of electrons in ammonia vapor at various densities. Mobilities of electrons, photoinjected from a photocathode by a short laser flash into ammonia vapor, were obtained by a time-of-flight method. Measurements were performed in the density range 6 X 10l8< P N H ~< 4 X loz1cm-3 and in the temperature range 210 < T C 420 K. The mobility of the electrons changes from about 900 cm2 V-’ s-l at low densities to about 4 X cm2 V-’ s-l at the high density limit. In the density range 1 X 1020< p m a< 3.6 X loz1cm-3a drop of the mobility by a factor of lo3is observed indicating a “transition” from the quasi-free electron state to the localized state. The results are qualitatively discussed according to the concept that density fluctuations, i.e., preexisting clusters in the vapor, determine the probability of electron localization,
Introduction Solvated electrons are known to be formed by the interaction of ele!ctrons with polar solvent molecules like ammonia, water, alcohols, ethers, etc. Electron localization in polar liquids is therefore a well-established phenome0022-3654/80/2084-1155$01 .OO/O
n0n.l Localized electron states are also stable in dense polar vapors. Olinger and SchindewolfL have recorded optical absorption spectra of localized excess electrons produced by pulse radiolysis in supercritical ammonia dOwn to a density of 0.1 g (T= 423 K). From the @ 1980 American Chemical Society
1156
Krebs and Wantschik
The Journal OF physical Chemistry, vol. 84, No. IO, 1980 R.F Shield
N d YAG Frequencyy d r u p l r d . l K LASER5 “Item 2000
I I Figne 1. @askexperimental alrangemt for photceh&rm pmdu&n and for measurement of the drin velocity 01 excess electrons.
density dependence of the radiation chemical yield of solvated electrons it was concluded that the low density limit of stability of solvated electrons in supercritical ammonia lies in the range of 0.2 g cm-q At lower densities solvation of electrons will not be possible anymore, i.e., at sufficiently low density the excess electrons are expected to be quasi-free. Additional pulse radiolytic studies on optical ahsorption and yields of solvated electrons in subcritical and supercritical ammonia vapor conducted by Gaathon and Jortner3 show that near the liquid-vapor coexistence curve (CEC) electron localization is observed at vapor densities down to p m = 6 X lW3 g cmJ at 293 K. At densities and temperatures near the CEC yields of localized electrons < are high throughout the vapor density range 6 X pND < 2 X 10-l g cm”. At high temperatures above the CE6, however, the yield of localized electrons vanishes. From these experimental results follows a temperature dependent “critical” density for the “transition” between the localized and the quasi-free excess electron state in the polar vapor similar to the case of excess electrons in nonpolar helium.“ Considering the low value for the “critical” density near the CEC and taking into account the weak density dependence of the electron absorption band position at low densities it was concluded by Gaathon and Jortner3 that electron localization in polar vapors is governed by the probability of solvent cluster formation. This means that preexisting density fluctuations in polar vapors act as traps for quasi-free electrons. The determination of this “critical” density from light absorption experiments, however, is restricted by the sensitivity of the optical detectors and finally influenced by chemical reactions of excess electrons with scavengers, competing with the localization process. Therefore, in our opinion, mobility measurements on excess electrons in ammonia vapor in a wide range of density and temperature will give basic information on the transition from the localized to the quasi-free state? Experimental Section Principles of Measurement (See Figure I). The mobility of electrons was measured hy a time-of-flight method similar to that of Hornbeck! A short laser pulse (265 nm, pulse duration 15 ns, pulse energy 2.5 X 1021~ m - ~ ) . d p to now we have implied that the high mobility charge carriers are excess electrons. In the following we want to
c
i lot8
LI
-
-Ammonia
density p / ~ m - ~ - -
Figure 6. Zero-field mobilities vs. ammonia vapor density. The high mobility branches are due to electrons. The low mobility branch is probably due to ions. In the density range 6 X lo’* I pNb I 3 X 10’’ ~ m mobillties - ~ (0)have been measured along the liquid-vapor coexistence curve (210 IT I 238 K). The dashed line represents ion mobility measurements of Sennhauser and Armstrong at 298 Kne In the high density range several electron mobility isotherms are plotted: (0) 296 f 2 K; (m) 320 K; (‘I 340 ) K; (e) 359 K; (0) 367 f 2 K; (A) 380 K; (A) 400 K; .(a) 420 K. The curves are to guide the eye.
give some additional experimental information concerning the nature and the mechanism of production of the fast charge carriers. It is well known that extremely small amounts of electron capturing materials such as SF6 and Q2 are capable of scavenging all injected electrons. Thus only ions with much lower mobilities are observable. Therefore controlled contamination of the sample with O2is a proper means to identify electrons. In our case, with small amounts of O2 a fast decay of the high mobility signal was observed besides the current of the low mobility charge carriers. With high oxygen concentration in dense ammonia vapor only less mobile charge carriers were observed. These preliminary results show that the fast charge carriers are excess electrons. Bulk ionization of the ammonia vapor by the UV light was insignificant because reversing the polarity of the electric field reduced the reversed photocurrent by at least one order-of-magnitude without any change of the pulse shape. In this case the electrode made of a fine stainless steel mesh acts as photocathode and emits electrons to a certain degree. This result and the shape of the pulse presented in Figure 4a with an almost constant photocurrent during the drift time show that the electrons must have originated at the photocathode. In addition, with a laser flash of wavelength 530 nm the observed photocurrent was reduced by at least three orders-of-magnitude compared to the experiments with the 265-nm laser flash. To get further information about the mechanism of electron injection the photocurrent has been measured as a function of laser light intensity in the range of 6 X to 3 mJ at two different electric field strengths (Figure 7). At low light intensities the electron photocurrent is proportional to the light intensity, indicating that electrons
The Journal of Physical Chemistry, VoL 84, No. IO, 1980 1159
Mobility of Electrons in Ammonia Vapor
A
VeR) in a macroscopic sample are enclosed by a surface formed by prohibited regions, respectively. Therefore, one obtains p ( E < E,) = 0, and only localized electrons contribute to the apparent mobility. When the electron energy E is higher than the energy E, of the mobility edge14 the probability p ( E ) is larger than zero; then according to eq 3 extended states contribute to the mobility. On this basis Eggarter and Cohen13 succeeded in calculating the electron mobility transition from the quasi-free to the localized bubble state in dense gaseous He as a function of gas density. In the case of ammonia, however, we cannot give any quantitative results because of the lack of knowledge about the effective potential Veff in the vapor. Since the electron-scattering process in low density ammonia vapor is dominated by the interaction between the electron charge and the permanent dipole of ammonia molecule^,^ in addition to the short-range repulsive contribution and the electronic polarization term15 an electrostatic dipolar term for the interaction between the electrons and the permanent dipoles of ammonia has to be taken into account in V,, Therefore, it is concluded that besides local density fluctuations also local fluctuations of the orientational correlation of ammonia molecules will “modulate” the effective potential in the spirit of pseudopotential theory. Because of the stated complexity of this disordered polar system, we can only give some guide lines with respect to the problem of electron localization. The so-called preexisting clusters in ammonia proposed by Gaathon and Jortner can be identified with the occurrence of local molecular configurations of high density associated with more or less strong local orientational correlation between ammonia molecules. Of course, the mean free path X(E), the density of states n(E),and the localization probability (1 - p ( E ) )will be determined by the distribution of local potentials associated with these local configurationswithin the disordered system. What we can say is that the local
Krebs and Wantschik
orientational correlation in ammonia vapor increases with increasing average vapor density and decreasing temperature. In other words, the number of the highly correlated preexisting clusters is enhanced by increasing the density of the polar vapor and by decreasing the temperature. Assuming, in accordance with Gaathon and
