J . Phys. Chem. 1985,89, 2297-2298
2297
Field-Induced Infrared Absorption in Metal Surface Spectroscopy: The Electrochemical Stark Effect
Carol Korzeniewski, Randall B. Shirts, and Stanley Pons* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 (Received: September 5, 1984; In Final Form: December 28, 1984)
The intense electric fields present near the electrode-solution interface are sufficient to induce infrared activity of vibrational modes that are normally infrared inactive. The pure Stark effect, intensity enhancement by external electric fields, and charge-transfer effects are three mechanisms by which vibrational transitions in molecules at or near metal surfaces may be observed in addition to conventional absorption. Calculation of the expected absorption coefficients for adsorbed species gives values which are similar to those observed in surface electrochemical reflectance infrared techniques.
The perturbation of vibrational spectra by intense electromagnetic fields has been known for many years.’-4 The effect of a strong electric field on the frequency and/or frequency splitting of a rotating molecule (the Stark effect) has been studied in the gas phase and in condensed These systems have traditionally been studied by developing the electric field with two parallel electrodes across a dielectric containing the species of interest. Other effects on the spectra have been considered by these and other workers. Saas et a1.9 have put forth some general considerations regarding the effect of the strong electric field gradients near electrochemical electrodes and have used the results to support an electromagnetic field gradient theory to explain the surface enhanced Raman phenomenon (SERS). The authors have predicted that a field-gradient-induced quadrupolar moment will cause infrared activation of some bands that are normally only Raman active. Devlin et a1.I0 have discussed the possibility of charge-transfer interactions causing activation and perturbation of certain vibrational modes and have discussed the participation of metal surfaces in this charge-transfer mechanism. In this case, generally weaker external electromagnetic fields [crystal lattices,” “uncharged” metal surfaces,I2 dimers,I3 etc.] are required to cause further induced dipole effects. Other workers have assumed that this situation can also be used to explain various results observed in SERS. Palik et aL5q6have observed the Stark effect in several systems but have pointed out that the intensity changes as originally predicted by Condon’ are the most predominant effects in the field-induced infrared spectra of the systems studied. Condon pointed out that the induced dipole moment in the direction of the polarization of the electromagnetic radiation by a strong external electric field will be proportional to the square of the mean field intensity. (These predictions, made in 1932, were also used to estimate the enhancement of the Raman effect
on HC1 by a 30000 V cm-l field.) We have ob~erved,’~-’~ using both electromodulated infrared reflectance (EMIRS) and subtractively normalized interfacial FTIR spectroscopy (SNIFTIRS), electric field dependence in the intensity of vibrational bands that should not be active in the domain of 2-field surface selection rule predictions, and thus we attribute these to the inducing of dipole moments by the strong electric field produced at the electrode/electrolyte solution interface; using the broadest definition, these are electrochemical manifestations of the Stark effect. In an experiment where the electric field El is applied in a direction I , the induced dipole moment vector is simply
PI = a/&/ where a// is the element of the polarizability tensor. The integrated Einstein absorption coefficient for the band is given by 2r2vT B = -IPfi(* tohc where v is the optical frequency of the infrared transition, h is Planck‘s constant, c is the velocity of light, T is the number of absorbing dipoles per unit cross sectional area in the radiation path, to is the permittivity of free space, and Pa is the transition dipole matrix element. In the presence of an external electric field, the dipole moment tensor can be expressed in terms of the value of the permanent dipole moment and the induced value. Thus
IPal = (*”JPO +P/I*d where
is the permanent dipole moment. The transition dipole matrix element becomes
(1) Condon, E. U. Phys. Rev. 1932, 41, 759. (2) Crawford, M. F.; Welsh, H. L.; Locke, J. L. Phys. Rev. 1949.75, 1607. (3) Crawford, M. F.; Dagg, I. R. Phys. Reu. 1953, 91, 1569. (4) Crawford. M. F.: MacDonald. R. E. Can. J. Phvs. 1958. 36. 1022. (5j Palik, E. D.; Holm, R. T.; Steila, A.; Hughes, H. L. J . Appl: Phys. 1982, 53, 8454. (6) Stella, A.; Miglio, L.; Palik, E. D.; Holm, R. T.; Hughes, H. L. Physicu 1983, 117B, 1188, 777. (7) Handler, P.; Aspnes, D. E. Phys. Reu. Lett. 1966, 17, 1095. (8) Brewer, R. G.; McLean, A. D. Phys. Reu. Lett. 1968, 21, 271. ( 9 ) Sass, J. K.; Neff, H.; Moskovits, M.; Holloway, S . J. Phys. Chem. 1981, 85, 621. (10) Devlin. J. P.: Consani. K. J . Phvs. Chem. 1981. 85. 2597. (1 1) Hester, R. E. In “Advances in Infrared and Raman Spectroscopy”, Clark, R. J. H., Hester, R. E.; Ed.; Heyden and Son: London, 1978; Vol. 4, Chapter 1. (12) Lehwald, S.; Ibach, H.; Demuth, J. E. Surf. Sci. 1978, 78, 577. (13) Pons, S.;Khoo, S . B.; Bewick, A,; Datta, M.; Smith, J. J.; Hinman, S.; Zachmann, G. G. J. Phys. Chem. 1984, 88, 3575.
0022-3654185 , ,12089-2297$01.50/0 I
which, after a Taylor series expansion, yields
where P,,’and d correspond to the change in the permanent dipole (14) Pons, S.; Bewick, A. In “Advances in Infrared and Raman Spectroscopy”,Clark, R. J. H., Hester, R. E., Ed.; Heyden and Son: London, in press. (15) Pons, S.; Bewick, A. Langmuir, submitted for publication. (16) Bewick, A.; Gibilaro, C.; Pons, S. Langmuir, submitted for publication. (17) Bewick, A,; Pons, S.; Korzeniewski, C. Electrochim. Acta, submitted for publication.
0 1985 American Chemical Societv -
The Journal of Physical Chemistry, Vol. 89, No. 11, 1985
2298
Korzeniewski et al.
TABLE I B/lO-' cm-' (Ag mode)" E/(V/m)
CO
C2H4 napthalene anthracene
107 108 109 2 x 109
4.2 4.5 7.5 12 30 56 79
0.6 2.4 15 40 62
5
X
lo9
8 X IO9 10'0
5.6 22 140 350 560
0.1 11 44 280 710 1100
TCNQ1.5 150 580 3600 9300 14500
1605
"Polarizabilities were taken from ref 18.
moment and polarizability with respect to a normal coordinate, respectively. The integrated absorption coefficient becomes
Calculation of B We have calculated the expected values for the induced absorption coefficient for the C=C symmetric stretch for several molecules adsorbed flat on a metal electrode and for C O adsorbed perpendicular to the metal electrode, both in the presence and absence of an electric field. We assume close-packed simple monolayer coverage of the species on a planar 1-cm2surface. Since the matrix elements ( !P&Y'~*~) are not tabulated for molecules larger than diatomic, we have estimated the change in polarizability with normal coordinate for the given transition to be of the same order of magnitude as the polarizability normal to the molecular axis. Thii approximation provides an upper bound value of the matrix element; the actual value will normally be somewhat smaller. The results of the calculation are given in Table I. These show the very strong dependence of the effect on the nature of the molecule. Values calculated for T C N E anion a t or near the platinum electrode correspond closely to those observed in our labs using the SNIFTIRS technique. Infrared activation of the Ag C - C symmetric stretch for the anthracene/anthracene anion systemls is clearly seen (SNIFTIRS experiment, acetonitrile solvent, -1.5 to -2.5 V vs. Ag/Ag+ reference) in Figure 1. The intensity of the band is about 1% of the value given in the table for an assumed field strength of lo8 V/m. The upper bound value given in the table for C O predicts a 70% increase in the band intensity on going from zero field to lo9 V m-*. Since the actual effect is likely to be less than the calculated upper bound, we conclude that its detection will be very difficult for this molecule. For ethylene, we estimate that a sensitivity of 5 X 1CSabsorbance would be required at a field strength of lo9 V m-l and thus it should be measurable. In addition to any Stark-type mechanism, it is also important to consider the possible effect of charge-transfer (CT) activation of the A, modes as pointed out by Devlin.lo If a C T complex is formed between a substrate molecule and another species, there is strong activation of the A, modes due to vibronic coupling of the symmetric modes with the forces responsible for the formation of the CT complex. These forces may be due to the electrostatic field of an associated ion19 (such as in a salt of an organic anion), the lattice fields in crystals of CT materials, or the fields at surfaces of metals acting on CT adsorbate^.'^ We have recently" observed ~~
~~
(18) C O Kirschner, S.M.; LeRoy, R. J. J. Mol. Spectrosc. 1977,65, 306. Sadlej, A. F. Theor. Chim. Acta (Berlin) 1978, 47, 205. C2H4.naphthalene, anthracene: Svedlov, L. M.; Kovner, M. A,; Krainov, E. P. "Vibrational Spectra of Polyatomic Molecules"; Wiley: New York, 1974. Chablo, A.; Hinchliffe, A. Chem. Phys. Lett. 1980, 72, 149. TCNQ': Klimenko, V. E. Ukr. Fiz. Zh. 1978, 23, 1301., Chem. Abstr. 89, 172337~.Moszynska, B.; Teamee, A. J. Chem. Phys. 1967, 46, 820. (19) Hinkel, J. J.; Devlin, J. P. J . Chem. Phys. 1973, 58, 4750.
\c' 1590
Figure 1. SNIFTIRS difference spectrum in the A, symmetric stretch region of the anthracene/anthracene anion radical system. (In acetonitrile, 0.10 M tetra-n-butylammonium tetrafluorohrate, Ag/Ag+ (0.01 M in CH3CN) reference, -1.5 to -2.5 V modulation.
metal surface activation of these modes in TCNE anions under conditions where the electric field strength near the electrode surface is estimated to be too low to cause Stark activation, and C T vibronic activation is probably the dominant mechanism. Since both C T and Stark activations ultimately depend on the same change in polarizability derivative, it may be difficult to separate these mechanisms without a more comprehensive study of the dependence of the absorption intensities on field strength. The quadratic dependence expected in Stark activation has been clearly observed for the adsorption of acrylonitrile on gold electrodesI6 and is probably responsible for approximately quadratic field dependence of solvent/electrolyte bands reported in earlier work.20 We also point out that the electrochemically activated Stark effect might be used as a probe for measuring electric field intensities in the electrical double layer. Adsorbed species will also be under the influence of a variety of fields arising from other sources such as surface adatoms, impurities, and surface defects. It is also mentioned that the above treatment neglects higher expansion terms and magnetic effects, which, although normally small compared to the dipole term, may be appreciable in the presence of extremely large fields. A complete vibrational analysis must include consideration of the time dependence of the electromagnetic fields of external and metal origin. Again these effects are expected to be small in the EMIRS and SNIFTIRS experiments. Acknowledgment. The authors are indebted to the Office of Naval Research for primary support and to NATO for collaborative travel support. ~~
(20) Pons, S.; Davidson, T.; Bewick, A. J . EIectroanal. Chem. 1982, 140, 211.
