J. Phys. Chem. 1986, 90, 1739-1741 this basis, we have assigned these peaks also to a planar methyl conformation. Thus, the highest frequency peak is associated with the in-plane methyl C H and the lowest frequency peak with the two methyl C H bonds at 60’. Ab initio MO calculations for t o l ~ e n e *predict ~ , ~ ~ a bond length difference of 0.002 8,between the two types of methyl C H bonds, in close agreement with the value (0.003 A) predicted by the spectral splitting. The barrier to internal rotation of the methyl groups in toluene, m-xylene, and p-xylene is very 10w.25128We have assigned the central peak in the methyl regions of the spectra of these three molecules to transitions originating from rotational levels above this rotational barrier. Results similar to those for 0-,m-, and p-xylene have been obtained in the methyl regions of the overtone spectra of 0-, m-, and p-fluor~toluene.~ The spectra of the two ortho molecules correspond, as do the spectra of the two meta and the two para molecules. As with the aryl C H bonds, the spectral results for the methyl C H bonds are those expected on the basis of our previous observations.8 For all three trimethylbenzenes, the relative intensity of the methyl peaks is higher than in the xylenes in accord with the increased methyl to aryl CH bond ratio. Two peaks are observed in the methyl region (8370-8530 cm-’) of the spectrum of 1,2,3-trimethylbenzene. The peak positions and relative intensities are the same, to within experimental error, as the corresponding features in the spectrum of o-xylene. Similarly, the methyl region spectrum of 1,3,5-trimethylbenzene corresponds closely to the spectrum of m-xylene. Thus the conformation of the methyl groups in 1,2,3- and 1,3,5-trimethylbenzenesis expected to be virtually identical with the conformation of the methyl groups in o-xylene and m-xylene, respectively. The lowest energy con(28) Kreiner, W. A,; Rudolph, H. D.; Tan, B. J. Mol. Spectrosc. 1973, 48, 86.
1739
former will be a planar one. The highest frequency methyl peak
is associated with the in-plane methyl CH, and the lowest frequency peak with the two methyl C H bonds at 60°, in both molecules. The spectral splitting predicts the in-plane methyl CH to be shorter by 0.003 A. Because the barrier to internal methyl rotation is very low in 1,3,5-trimethylbenzene, a third methyl peak is observed, which is absent in 1,2,3-trimethylbenzene, and is assigned to transitions originating from rotational levels above the rotational barrier. The methyl region of the spectrum of 1,2,4-trimethylbenzene is particularly interesting. It appears to be a superposition of the methyl regions of the spectra of toluene and o-xylene. Thus the orientation of the two adjacent methyl groups is expected to be the same as in o-xylene. The third methyl group, with a low barrier to internal rotation, will be the only one which gives rise to the central methyl peak corresponding to transitions from rotational levels above the rotational barrier. As expected, the relative intensity of this central peak is lower in 1,2,4-trimethylbenzene (0.13) than in 1,3,5-trimethylbenzene (0.19). In addition to the peaks listed in Table I, three lower intensity peaks are observed at approximately 8125, 8325, and 8600 cm-’ in the spectra of all three trimethylbenzenes. These peaks can be assigned to combinations involving two quanta of CH stretching and two quanta of C H bending. They have also been observed in the spectra of toluene,8 the xylenes,6 and the fluorot~luenes.~ In summary, the spectra of the trimethylbenzenes can be interpreted on the same basis as the spectra of toluene: the xylenes,8 and the fluorot~luenes.~ The results provide further support for our explanation of the methyl regions of these spectra in terms of contributions from states characteristic of the lowest energy conformation as well as from “free rotor” states. Acknowledgment. We are grateful to the Natural Sciences and Engineering Research Council for financial support.
Picosecond Photoemission and Recombinatlon Dynamics at the Metal-Liquid Interface T. W. Scott Exxon Research and Engineering Company, Corporate Research Science Laboratories, Annandale, New Jersey 08801 (Received: January 14, 1986)
Photoelectron emission from a zinc metal surface immersed in liquid hexane generates a near surface population of quasi-free electrons. These transient species decay by charge recombination with the metal. Electrons injected at 195 K exhibit a recombination half-life of 74 ps with 355-nm light and 330 ps for injection with 266-nm light. Longer recombination times are attributed to a greater electron thermalization distance in the liquid. Recombination kinetics are governed by electron diffusion in the Coulomb potential of the image charge. The time dependence can be approximated by exp(-pt2l3) where 0 is determined by the distribution of initial electron-surface separations.
Introduction Photoinjection of electrons into liquid solutions by ultraviolet irradiation of metal surfaces can be used to probe electronic processes at the metal-liquid interface. The wavelength threshold for photoinjection has been used to measure the conduction band energies of quasi-free electrons in liquid alkanes.14 The reduced quantum yield in going from vacuum to liquid photoemission measures the photoelectron penetration depth in the latter.5 (1) Holroyd, R . A.; Tames, S.; Kennedy, A. J. Phys. Chem. 1975, 79,
Nanosecond time-resolved photoconductivity following injection into electrolyte solutions has provided kinetic measurements of the electrode reactions of transient ions and radicals6-’ and has recently been used to explore density of state models for polar liquids.* We report a new method of measuring heterogeneous electron-transfer kinetics which occur in the picosecond time domain. A schematic illustration of the technique is presented in Figure 1. Irradiation of a metal-liquid interface at energies hu = 4 + V,, where 4 is the work function of the metal and V, the con-
2851.
(2) Tauchert, W.; Jungblut, H.; Schmidt, W. F. Cun.J . Chem. 1976.55, 1860. ( 3 ) Ncda, S.; Kevan, L. J. Phys. Chem. 1975, 79, 2467. (4) Holroyd, R. A.; Allen, M. J. Phys. Chem. 1971, 75, 5014. (5) Holroyd, R. A.; Deitrich, B. K.; Schwarz, H. A. J. Phys. Chem. 1972, 76. 3194.
(6) Babenko, S. D.; Benderskii, V. A,; Zolotovitskii, Ya. M.; Krivenko, A. G . J . Electroanal. Chem. 1971, 76, 341. (7) Barker, G. C.; McKeown, D.; Williams, M. J.; Bottura, G. Concialini, V. Discuss. Faraday SOC.1973, 56, 41. (8) Coffman, R . B.; Bennett, G. T.; Antoniewicz, P. R.; Thompson, J. C. Chem. Phys. Lett. 1985, 199, 451.
0022-3654/86/2090-1739$01.50/00 1986 American Chemical Society
Letters Metal
Liquid
.. .. **
1
4-
2
e..
58
.*.**.*
8
....
duction band energy of the liquid, will photoinject an electron into solution. The electron loses all excess kinetic energy after traveling a few tens of angstroms. The electron residues in the Coulomb well of its image charge and has a large probability of returning to the metal. The kinetics of recombination are controlled by electron diffusion in response to the combined image and applied potentials and, in principle, by the rate of heterogeneous charge transfer as the electron approaches the metal. The decay of this population is monitored in real time by stimulated charge separation from the interfacial zone using a visible or near-infrared light pulse. Excitation of a bound to free transition of the electron is thought to impart excess kinetic energy which is dissipated in a second thermalization process. This secondary thermalization alters the electron escape probability from the interfacial region. The technique is similar to one developed in studies of rapid geminate charge recombination reactions in homogeneous solut i ~ n . ~ qIn I ~solution, photoionization of a neutral molecule forms a geminate electron-cation pair. Each pair remains strongly correlated by its Coulomb interaction and exhibits a high probability of recombining to form the neutral parent molecule. The Coulomb interaction of a charge pair in solution is similar to the image potential experienced by an electron near a metal surface. For both cases, photoexcitation of the electron into high lying continuum levels promotes separation of the electron-cation or electron-metal complex and mobile charge carriers can be detected by conductivity. Experimental Section The surface ionization cell used in this work consists of two parallel-plate copper electrodes separated by 4 mm and biased at 2 kV. One electrode contains a 2-mm-diameter hole to allow the laser to enter the cell. The electrodes were suspended in a quartz cuvette containing liquid n-hexane and electrically shielded by an aluminum enclosure. Photoemission measurements at reduced temperatures were carried out by cooling the interior of the cell with a stream of cold nitrogen gas. Typical photocurrent pulses were on the order of 10 PA. Zinc films (1 pm thick) were evaporated onto a copper substrate and immersed in liquid n-hexane that was previously dried by storage over molecular sieves. All operations were carried out in a nitrogen atmosphere to minimize oxidation of the film. Light pulses with a 35-ps duration at 1064 nm were generated by an active-passive mode-locked Nd:YAG laser. Both third and fourth harmonics were used to initiate phtoemission. The laser energy per pulse at the sample was 0.3 mJ at 266 nm, 1.0 mJ at 355 nm, and 7.0 mJ at 1.064 pm. Photocurrents were amplified by an (9)Scott, T. W.; Braun, C. L. Can. J . Chem. 1983,63,288. (10)Braun, C. L.; Scott, T.W. J . Phys. Chem. 1983,87,4776
0.
. *.
' .E*,.\,.,
.e--.
'**S.0-.m.As.C..C.
I 1 I -40 0 40 80 Time Delay (PSEC)
I
injects an electron from the metal into solution. The small escape probability of the thermalized electron (ca. lo-)) can be increased by infrared photoexcitation, thus sampling the population of surviving interfacial charge carriers.
9.
.""*.S..8...'
-80
Figure 1. Schematic illustration of infrared stimulated dissociation of an electron from its image charge in the metal. An ultraviolet photon
..
0'.
J
4
..
..
.
' 0
Figure 2. Photoemission currents at room temperature from an evaporated zinc film under combined 335 nm and variable-time-delayed 1064-nm illumination. The duration of each light pulse is 35 ps. (A) Zinc-hexane interface with negatively biased photocathode. (b) Zinchexane interface with positively biased photocathode. ( C ) Zinc-nitrogen gas interface with negatively biased photocathode.
0
400
200
600
800
Time Delay (PSEC)
Figure 3. Time-dependent photoelectron survival probability for 355 nm photoemission at the zinc-hexane interface at 195 K.
't i
.-..II 0
1
I
,
I
200
400
600
800
Time Delay (PSEC)
Figure 4. Electron-surface recombination kinetics for the same system shown in Figure 3 but with 266-nm light.
electrometer and averaged with a boxcar integrator. A laboratory computer digitized the boxcar output at successive time delays and generated the plots of photocurrent vs. time delay shown in Figures 2-4. Results and Discussion The two-pulse photogeneration signals displayed in Figure 2 support the assumption that IR-enhanced photogeneration occurs within the interfacial region of a metal-liquid system. The net photocurrent arising from combined UV and IR irradiation of zinc, at a series of time delays between the two light pulses, is shown for three different states of the interface. In Figure 2A a freshly prepared zinc surface is immersed in liquid hexane at
Letters room temperature and biased with a negative applied potential of 2 kV. The increase in photocurrent when the UV and IR pulses overlap at zero time delay is thought to arise from IR excitation of photoelectrons in n-hexane as discussed in reference to Figure 1. The time duration for cooperative photogeneration closely approximates the cross-correlation width of the two light pulses." Reversing the polarity of the voltage applied to the zinc surface eliminates the cooperative photogeneration (see Figure 2B). The IR photogeneration process does not arise from the bulk liquid. In Figure 2C, we see no IR enhancement for photoemission from negatively biased zinc into nitrogen gas at 1 atm. This process apparently does not arise from the metal or metal oxide layer, but represents a property of the metal-liquid interface. At room temperature photoelectrons recombine with the metal surface too rapidly to be time resolved with our apparatus. However, cooling the liquid lowers the electron mobility. Figures 3 and 4 show time-resolved measurements of photoelectron recombination at -80 OC using 355- and 266-nm light, respectively. The half-lives are 180 and 560 ps. The solid curves in Figures 3 and 4 are generated by using the simple model discussed below. Photoelectrons released by UV irradiation leave the metal surface with excess kinetic energy. They are scattered by collisions in the liquid and eventually thermalize. We assume that thermalization will occur at various distances from the metal and that some distribution of distances will describe the initial electron population. Strictly speaking, the electron motion after thermalization is controlled by the combined field of the applied and image potentials and by diffusion in response to the concentration gradient of electrons. To simplify the treatment of the recombination kinetics, we will assume that the electron motion arises solely from the field of the image potential.I2 The velocity of the electron toward the metal will then depend upon its position according to where I.L is the electron mobility. If we assume the electron starts at rest at a distance zo from the metal, the recombination time, r , is obtained by integrating eq 1: (1 1) A cross-correlation width of 70 ps for a 355-nm pulse combining with a 1064-nm pulse was measured in a separate experiment using two-photon fluorescence in liquid p-xylene. (1 2) On the short time scale being considered here, electron diffusion in response to a concentration gradient is thought to play a minor role in the observed recombination time. From the work of Agmon (Agmon, N. J . Chem. Phys. 1984,81, 281 l), the calculated first half-life, neglecting the drivin force of the image potential, of an electron initially located at 22 and 37 ! i from the metal is 5.3 and 15 ns, respectively. The corresponding recombination times from eq 2 of the text are 82 and 390 ps, respectively.
The Journal of Physical Chemistry, Vol. 90, No. 9, 1986 1741 T=-
16rccozo3 (2)
3w
where c is the dielectric constant, to the permitivity of vacuum, and e the electronic charge. The distribution of initial thermalization distances is taken as g(z) = (2z/L) exp(-z2/L2)
(3)
where the average thermalization length is equal to r'I2L/2. The time-dependent survival probability P(t,L) which is the fraction of electrons surviving at time t after thermalization is obtained by combining eq 2 and 3
or simply P(t,L) = e ~ p ( - @ t ~ / ~ ) where
p=
(
3ALe
167rttoL
)"'
The solid curves in Figures 3 and 4 are plots of eq 5 for L = 25 On the basis of the model employed to fit the data, the average thermalization distances for photoemission from zinc into liquid hexane are 22 A at 355 nm and 37 A at 266 nm. The two-pulse photoemission technique has also been used to study electron transfer between zinc films and solutions of pyridine in hexane. Pyridine acts as an electron acceptor for charge carriers generated by photoemission from the metal. Using a 532-nm charge dissociation pulse we observe a recombination half-life of 630 ps at room temperature. In this case, the enhanced photocurrent is thought to arise by photodetachment of an electron from the anion. The recombination time is longer than expected for a diffusion-controlled reaction and may be limited by the kinetics of the heterogeneous electron-transfer step. Additional experiments on acceptors with different electron affinities, meals with different Fermi energies, and solvents with different polarities would go a long way toward an improved understanding of charge recombination dynamics a t the metal-liquid interface.
A and L = 42 A, respectively.
Acknowledgment. The author acknowledges the help and encouragement provided by Professor Charles Braun of Dartmouth College.
