J. Phys. Chem. 1984, 88, 1275-1277
-
TABLE 11: Vibrational Assignment for Bands near the Origin of the Z A ,+ 3pn znuParallel Rydberg System in NO,
band
v , cm-'
stat wta
displacement from hypothetical qQ, (0)b Au, calcd
57990
1.00
0
58590
0.56
g22-x
59198
0.10 1.00
4g2,-4x 0
59374
1.00
0
Av
u,
(000) qQ,,(O) (01 0) qQ3(l) (020) qQ,(2) qQ,(O) (l0Oi qQ,,(0)
1275
0
57990
0
-5.28
58595
605
-21.12 0
59198
1208
0
59374
1384
a Calculated by assuming a rotational temperature in the free-jet Rotational lines are designated by AK expansion of 20 K.
(superscript), bl, originating K" (subscript), and originating J . x is thc value of K" rotational constant (7.58 cm-'). 614.5 and 1404 cm-', respectively, and the upper state anharmonicity splitting (g2,) at 2.3 cm-'. These spectroscopic constants are in good agreement with the values obtained by Ritchie and Walsh.lo However, our (000) band falls 2400 cm-' below the extrapolated origin of their vibrational assignment. Our second origin lies much closer to their extrapolation, but
symmetry analysis of the rotational structure shows that this band system is two-photon parallel, 2A1 3p7r ,nu. As with the 3p0 system the parallel-perpendicular distinction is supported by the spacings between Q branches (Table 11) which, when corrected for initial rotational excitation and anharmonicity splitting, yield a spacing regular in the excited-state bending and stretching frequencies, which we find to be 605 and 1384 cm-', respectively. As made apparent by the dominance of the (100) band, these transitions are not saturated under the conditions shown. Separate power dependence measurements confirm that the present intensity condition lies within or just above the quadratic regime for all bands recorded. The prominence of (100) rather than higher bends is most interesting and provides a clue to the photoexcitation pathway. Two-photon absorption proceeds through a dense system of vibronic levels, arising from at least three electronic state systems. One of these states ( l2Bl) is formally linear, all have N-0 bond lengths longer than either the ground state (1.20 A) or the 3p Rydbergs (1.09 A).10,12With reference to the discussion of the previous Letter8 we see the prominence of a symmetric stretching progression together with the relatively flat Franck-Condon envelope in v,', as indicative of intramolecular relaxation at the intermediate level populated by absorption of the first photon.
Acknowledgment. This work was supported by the U S . Army Research Office.
Application of Polarization-Modulated Fourier Transform Infrared Reflection-Absorption Spectroscopy to the Study of Carbon Monoxide Adsorption and Oxidation on a Smooth Platinum Electrode William G. Golden,* IBM Instruments, Inc., San Jose, California 951 10
Keiji Kunimatsu,? and Hajime Seki IBM Research Laboratories. San Jose, California 951 93 (Received: December 8, 1983; In Final Form: January 19, 1984)
Polarization-modulated Fourier transform infrared reflection-absorption spectroscopy (FT-IRRAS) is shown in this Letter to be applicable to the study of the electrode/electrolyte interface. The technique provides good quality vibrational spectra of monolayers adsorbed at the electrode surface. The infrared spectra of CO adsorbed on a smooth platinum electrode in 1 N H,S04 saturated with CO gas as a function of electrode potential are presented.
This Letter reports the first application of polarization-modulated Fourier transform infrared reflection-absorption spectroscopy (FT-IRRAS) to the study of adsorption of monolayers at the electrode/aqueous electrolyte interface. Previous infrared studies of the electrcde/solution interface have utilized electrochemically modulated infrared spectroscopy,' ppolarized FTIR? an optical-electrochemical linear potential sweep m e t h ~ d and , ~ dispersion I R R A S 4 The last two methods have illustrated the importance of retaining the electrode potential as an experimental variable. Due to the high-resolution capabilities of FT-IRRAS, as well as the other well-known throughput and spectral bandwidth advantages of FTIR, the surface sensitivity of polarization-modulated FT-IRRAS should prove to be a particularly useful technique for the study of electrode/solution interfaces at constant electrode potentials. We report here preliminary FT-IRRAS data on the adsorption of carbon monoxide on a smooth platinum electrode in 1 N H,S04 'On leave from the Research Institute of Catalysis, Hokkaido University, Sapporo, Japan.
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saturated with carbon monoxide gas as a function of electrode potential. The spectra were obtained on an IBM Instruments, Inc. IR/98 Fourier transform infrared spectrometer at 4-cm-' resolution with a liquid nitrogen cooled HgCd/CdTe photoconductive infrared detector utilizing polarization and double modulation techniques (FT-IRRAS).5 A spectroelectrochemical cell, similar in design to those used in previous infrared was used with a CaF, window. The window was illuminated by infrared radiation from the interferometer at normal incidence to (1) (a) A. Bewick, K. Kunimatsu, and B. S . Pons, Electrochim. Acta, 25, 465 (1980);(b) A.Bewick, K. Kunimatsu, B. S. Pons, and J. W. Russell, J . Electroanal. Chem., in press. (2) S. Pons, T.Davidson, and A. Bewick, J . Am. Chem. SOC.,105, 1802 (1983). (3) K. Kunimatsu, J . Electroanal. Chem., 140,205 (1982). (4) J. W.Russell, J. Overend, K. Scanlon, M. Severson, and A. Bewick, J . Phys. Chem., 86, 3066 (1982). ( 5 ) (a) W. G. Golden, D. S. Dum, and J. Overend, J . Catal., 71, 395 (1981);(b) W.G. Golden and D. D. Saperstein, J . Electron. Spectrosc., 30, 43 (1983); (c) W.G.Golden, D. D. Saperstein, M. W. Severson, and J. Overend, J . Phys. Chem., in press.
0 1984 American Chemical Society
1276 The Journal of Physical Chemistry, Vol. 88, No. 7, 1984
Letters
CO/R 1N H2S04
&
J&
R
R
4OOO
35M)
3ooo
2500
2ooo
I
1500
I
Figure 1. FT-IRRAS spectrum of the electrode/solution interface for CO adsorption on a smooth platinum electrode in 1 N H2S04at 0.4 V (NHE).
its 65O beveled edges such that the angle of incidence of the radiation reflected from the electrode/solution interface was slightly larger than 65O. This was obviated by the optical constraints of the experiment. An IBM Instruments, Inc. EC/225 voltammetric analyzer was used to hold the electrode at constant potential during the acquisition of spectra as well as to monitor and establish electrode surface cleanliness and adsorbate coverage by cyclic voltammetry. Figure 1 shows the FT-IRRAS spectrum of the electrode/solution interface between 4000 and 1100 cm-' with the electrode In addition to the dominant liquid water held at 0.4 V ("E). spectrum with bands at 3330,1670, and 2130 cm-' a sharp peak on top of the 2130-cm-I water band due to CO adsorbed on the Pt electrode is observed. The fact that liquid water spectra appear in the polarization-modulated experiment shows that the technique does not compensate for liquid water near the electrode surface. This is due to the so-called surface selection rules6 in which the s-polarized component of the infrared radiation a t and near the surface is very small relative to the p-polarized component. Thus, molecules present in substantial quantities near the metal surface will also give FT-IRRAS spectra regardless of their orientation with respect to the surface of the electrode. The appearance of water bands in FT-IRRAS spectra may hinder the polarization modulation method because of low energy throughput in the vicinity of these band positions; however, water spectra can be readily subtracted from the spectra obtained at potentials at which the platinum electrode is covered by adsorbed CO. In this work, the FT-IRRAS spectrum of CO on Pt at each potential shown was obtained by subtracting the FT-IRRAS spectrum obtained at 0.9 V (NHE), in which no CO was present on the electrode. The subtractions were judged to be accurate when the water band at 2130 cm-' was completely removed, leaving a flat spectral baseline around the CO band. Figure 2 shows the spectra of CO on the Pt electrode in 1 N H2S04as a function of electrode potential. The electrode potential is extended into the CO oxidation region and it is clear that the (6) R. G.Greenler, J . Chem. Phys., 50, 1963 (1968)
I
I
2ooo
2100
Wavenumber, cm-1
Wavenumber, crn-1
Figure 2. FT-IRRAS spectrum of CO on the platinum electrode as a function of electrode potential.
2090 -
g
2080-
3
2070-
/
l
I
l
I
I
I
I
/
l
- 1
0
1
2
3
4
5
6
7
8
E V(NHE)
Figure 3. Plot of the band center position (solid) and integrated band intensity (open) as a function of electrode potential.
band intensity sharply decreases after ca. 0.55 V ("E) due to the onset of CO oxidation. The full width at half-maximum of these bands, ca. 1 0 cm-I, is essentially the same as that observed for the linear CO species adsorbed on Pt in ultrahigh-vacuum This suggests that the vibrational relaxation mechanism for both systems is similar, Le., predominantly via interactions with the metal substrate. Figure 3 shows plots of the variation in band center position and integrated band intensity as a function of electrode potential. While the integrated intensity curve shows the coverage of CO to decrease dramatically a t ca. 0.55 V (NHE), because of CO oxidation, the frequency shift of the band center continues to higher values even at the lowest detectable coverage. In ultrahigh-vacuum experiments it has been established that the band position of the linear CO bond stretching vibration for CO adsorption on Pt decreases as coverage decreases if the coverage decrease does not involve island formation of the CO molecules (7) (a) R.A. Shigeishi and D. A. King, Surf. Sci., 58,379 (1976);(b) A. Crossley and D. A. King, ibid.,68,528 (1977).
J. Phys. Chem. 1984, 88, 1277-1280 on the surface. In situations in which C O islands exist, even at low coverages, the band maxima at all coverages is at the high wavenumber end of the observed band position excursion.' While we observe an increase in the frequency of the band center as a decrease in coverage occurs due to C O oxidation to COz it is not possible to determine whether the oxidation reaction in 1 N H2SO4 occurs at the edges of adsorbed C O islands at all CO coverages, or if the voltage dependence of the frequency shift obscures shifts due to intermolecular interactions. Further, since the plot of band center position vs. electrode potential appears linear at constant coverage we infer that the
1277
force constant associated with the linear C O stretching vibration (and hence the bond order) is influenced predominantly by electron donation or withdrawal by the Pt electrode. However, we cannot ignore the possibility that coverage-dependent intermolecular interactions influence the linear CO band position since deviations from linearity in the band position vs. electrode potential plot cannot be ruled out (viz. Figure 3).
Acknowledgment. We gratefully acknowledge the support and useful discussions provided by J. G. Gordon, I1 and M. R. Philpott. Registry No. Pt, 7440-06-4; CO, 630-08-0.
The Oscillatory Nature of Polarization Evolution in
CI DEP
Uzi Eliav and Jack H. Freed* Baker Laboratory of Chemistry, Cornel1 University, Ithaca, New York 14853 (Received: December 12, 1983)
An unusual and striking pattern of oscillations in the time evolution of the polarization has been observed in a time-resolved CIDEP study of photoelectrons generated in high concentration Rb/THF solutions. It is suggested that such oscillations may be characteristic of a (geminate) radical-pair mechanism when observed under sufficient time resolution.
The CIDEP phenomenon is of considerable interest because it often depends upon the early time history of interacting radical pairs.' Modern time-resolved ESR techniques permit one to study CIDEP in greater We have been engaged3 in an extensive study of the R b / T H F system4 by time-resolved ESE techniques. In T H F (tetrahydrofuran) the R b exists as Rb'Rbion pairs. The use of complexing agents can greatly enhance the concentration of these species. A short-lived photoelectron, ep-, can be produced with a laser pulse. In work we have done in concentrations of Rb- < M we have been able to measure both the rate of evolution of the initial polarization with inverse 20-70 ns and the final polarization P" which rate constant k-' is -50-200 times greater than the equilibrium polarization P,, and we have observed their temperature, wavelength, and concentration dependence^.^ These results appear to imply a geminate radical-pair mechanism for the generation of the CIDEP. A likely mechanism would involve geminate interaction between Rb. and e,, but one does not observe the Rb-. In this Letter we describe new and unusual observations we M) which we have made at higher concentration ([Rb-] = believe to be of fundamental significance to the study of the radical-pair mechanism. These experiments were performed on Rb metal dissolved in a solution of 2,2,2-cryptate (Cr) in THF at -107 "C. The concentration of the Cr was M. At this high concentration a stable solvated electron e; is present at all times independent of any laser irradiation. We determined from the fact that P,(eJ = 2Pq(ep-) that the concentration of e; is twice that of ep-. We estimate 1014e; spins. Since the concentration of e; is a measure of the concentration of cryptate, we could use the e; signal height to monitor the amount of cryptate during the course of the experiment. The e; was not present in the lower concentration solutions. The experiments consist of irradiating the solution with 10-15-ns laser pulses of 2-mJ energy from a Lambda-Physik dye laser at wavelength X = 898 nm corresponding nearly to the
-
(1) L. T. Muus, P. W. Atkins, K. A. McLauchlan, and J. B. Pedersen, "Chemically-Induced Magnetic Polarization", Reidel, Dordrecht, 1977. (2) A. D. Trifunac and J. R. Norris, Chem. Phys. Lett., 59, 140 (1978); A. D. Trifunac, J. R. Norris, and R. G . Lawler, J . Chem. Phys., 71, 4380 (1979). (3) U. Eliav and J. H. Freed, to be submitted for publication. (4) U. Eliav, H. Levanon, P. M. Carton, and R. W. Fessenden, Chem. Phys. Lett., 82, 365 (1981).
0022-3654/84/2088-1277$01.50/0
absorption maximum for Rb- in THF. This is followed at time t by a 9 0 ° - r 1 8 0 0 - ~ microwave echo sequence in the case of echo experiments, or just by a 90" pulse at time t for an FID experiment. (The 90" pulses were of 25-11sduration.) Because of the presence of the e; at all times, which interferes with the echo from the ep-, we modified the echo sequence in the following manner to suppress the echo from the e;. We used the sequence 90"7/-laSer-t-90"-7-180'-~, where 7' = 6T2,sor 12 ps, while T = 250 ns. Here T2,sis the transverse relaxation time of e;. The 90' pulse prior to the laser pulse just rotates the e; signal into the rotating x-y plane where it dephases in the time -7'. Since >> T2,3we can set (# t 27) < T1,sso the e; signal cannot recover significantly before the echo from the ep- is observed. In this manner the e; echo was suppressed by an order of magnitude. The FID from e; could be suppressed with a field gradient. The echo experiment we report on here consists of stepping out the time t between the laser pulse and the subsequent 90" echo pulse. In this experiment it was important to achieve very short time resolution in order to study the polarization evolution in detail. We therefore employed BNC digital delay generators with a stepping time of 1 ns, and adjusted our bridge and our cavity-Q to maximize the resolution. We display in Figure 1 the evolution of the polarization as a function of t . Three important facts immediately emerge. First of all, the final polarization P" which develops is an order of magnitude smaller than for lower concentrations solutions. That is: P"/P,, = 10. Second, the rate of the evolution is about an 500 ns. Third, and most order of magnitude slower, Le., k-' dramatic, the polarization evolution is a highly oscillatory process superimposed on the gradual rise of P ( t ) to its asymptotic value P". As the Rb- concentration decreases over a period of several hours, the values of k and P are found to increase and the oscillations are suppressed. One might at first suspect that, in the more concentrated solution, random encounters of f-pairs may be important in initiating the polarization process. But this is contradictory to our observations that, at the high concentration, k, the rate of polarization evolution, is an order of magnitude slower and P"/Pq is an order of magnitude smaller. It therefore appears that geminate interactions are again responsible for the CIDEP. The fact that both k and P" are both significantly reduced is consistent with a radical-pair mechanism, if Q (defined as half the difference between the resonant frequencies of the two in-
+ +
-
0 1984 American Chemical Society
