Normal and resonance Raman spectroelectrochemistry with fiber optic

Surface-enhanced Raman spectroscopy with abrasively modified fiber optic probes .... Intensity Calibration and Sensitivity Comparisons for CCD/Raman ...
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Anal. Chem. 1988, 58,2486-2492

Normal and Resonance Raman Spectroelectrochemistry with Fiber Optic Light Collection Scott D. S c h w a b a n d Richard L. McCreery*

Department of Chemistry, T h e Ohio State University, Columbus, Ohio 43210

F. Trevor Gamble Department of Physics, Denison University, Granville, Ohio 43023

Raman spectroscopy of solutlon specles generated electrochemically was carrled out with a fiber optic collector interfaced to a conventional spectrometer. An Input laser beam lmpinglng on the surface at glaclng lncldence permlts higher sensltlvlty than prevlous approaches, and It was possible to monitor normal as well as resanawe Raman scatterers. T h e resolution down to 20 ms was achieved for electrogenerated benroqulnone, and It was posslble to monitor a resonanceenhanced specles 30 ps after the beglnnlng of a potentlal step. Theoretkai expressions for Raman a n a l as a functlon of geometric and instrumental parameters were derlved and shown to agree with the observed results. Appkatkns of the technique on both long and short tlme scales to surface-enhanced specles are presented.

Spectroelectrochemistry has been developed extensively since its introduction by Kuwana in 1964, with the principal objectives of characterizing the stoichiometry, mechanisms, and dynamics of reactions associated with heterogeneous electron transfer. The major advantages of spectroelectrochemistry that fueled this development are the selectivity for particular species and freedom from background interferences such as capacitive current and spurious faradaic processes. A particularly important feature of spectroelectrochemistry is the ability to generate reactive species with the speed and tunable driving force inherent in electrochemical generation. The variety of spectroscopic methods that have been coupled to electrochemistry have been reviewed several times (1-5). The field has been dominated by UV-Vis absorption techniques because of simplicity, transient response as fast as 0.15 ps (6),sensitivity sufficient for sub-micromolar concentrations (7), and compatibility with the polar solvents generally used in electrochemistry. UV-Vis spectroelectrochemistry has been particularly valuable in providing n values and potentials for redox reactions and in studying kinetics and mechanisms of reactions of electrogenerated species. Van Duyne has discussed the advantages of vibrational spectroscopy, particularly resonance Raman spectroscopy, when coupled t o an electrochemical process (8-14). Raman spectroscopy is more structurally specific than UV-Vis absorption, with the sharp features in the spectrum permitting structural inferences, fingerprinting, or selective monitoring of particular species. Raman spectroscopy, unlike infrared absorption spectroscopy, uses visible light, which is transmitted easily by most cell materials and electrolytes in common use. The combination of Raman spectroscopy with electrochemistry has been very productive, providing new information about solution species near the electrode and resulting in the discovery and very active study of the surface-enhanced Raman effect (13-17). While Raman spectroelectrochemistry has been very valuable for monitoring and characterizing species in solution, past work has dealt almost 0003-2700/86/0358-2486$01.50/0

exclusively with systems showing resonance Raman enhancement. The 102-1@-fold enhancement in sensitivity that occurs in resonance Raman has been necessary in order to monitor the small amounts of material generated at an electrode. Although resonance enhancement can be advantageously used to improve selectivity, it also results in decreased generality of the Raman probe. When resonance enhancement is required for sufficient signal, the number of systems that may be studied is reduced to those absorbing the laser wavelength in use. Without resonance enhancement, the time scale of transient experiments is severely limited, with useful data acquisition starting a t least 1 s after the initiation of electrolysis (18). The present work was undertaken to extend the utility of Raman spectroelectrohemistry to systems lacking resonance enhancement and to improve the transient response for resonance enhanced systems. Our approach differs from previous experiments in two ways. First, the input beam impinges on the electrode at angles nem 90' relative to the surface normal, so the input beam encounters a maximum number of scatterers for a given diffusion layer thickness. Second, fiber optics are used to collect the scattered light, thus maximizing the coupling of the electrochemical cell to the spectrometer. Methods for monitoring electrogenerated Raman scatterers on both dc and sub-millisecond time scales are presented. THEORY The theory developed here is appropriate to Raman scattering from an input beam reflected by the electrode surface (Figure 1)and is based upon previous reports by Van Duyne (8, 13), Campion (19),and ourselves (20,21). It is assumed that the scatterer is generated electrochemically at a diffusion controlled rate from a bulk reactant that does not scatter at the wavelength monitored. The laser beam impinges on the electrode at an angle (a)with respect to the normal and forms an elliptical spot on the electrode, which is immersed in the solution of interest. Diffusion of electrogenerated products occurs vertically as shown in Figure 1. For any solution-phase Raman scatterer, the Raman scattering I R (photons s-l sr-l) is equal to the input laser irradiance, I L (photons cm-2 s-l) times the differential Raman cross section 0 (cm2sr-l molecule-l) times the number of scatterers. Conversion of laser irradiance to power Po (photons 9-l) and the number of scatters to the number density N (molecules/cm3) times the scattering volume yields IR = PopNd

(1) where d is the length of irradiated sample monitored by the collection optics. This equation assumes isotropic scattering and constant laser irradiance both across the beam and through the sample and, therefore, assumes negligible absorption of the beam. It is useful for determining the scattering from homogeneous solutions collected by fiber optics, as shown previously (20). For the case of a beam reflected off an electrode surface, the electrogenerated scatterer is 0 1986 American Chemical Society

ANALYTICAL CHEMISTRY, VOL. 58, NO. 12, OCTOBER 1986

S = LsolADQTQ

direction

of diffusion

I

I

L a s e r spot

I

Flgure 1. Geometry of laser beam and electrode, wlth the fiber optic collector omitted for clarity.

distributed inhomogeneously through the diffusion layer in a fashion governed by the well-known Fick's laws. The number of scatterers encountered by the beam is the same as that appropriate to an absorption experiment and is given by 2 ( 2 / ~ ' / ~ ) N b ( D t ) ' / ~ / ca, o swhere Nb and D are the bulk number density and diffusion coefficient of the reactant, respectively (4,21). Substituting this quantity for Nd in eq 1 yields eq 2, which was originally stated in different form by Van Duyne (8)

ZR =

4PoPNb(Dt)'I2 7 w

cos a

where Lsolis the specific intensity for a solution scatter, AD is the smallest area encountered in the detection system, cm2 (usually the entrance slit), Q is the collection efficiency of the collection optics and spectrometer in steradians, and T i s the transmission of spectrometer, dimensionless. The actual measured signal, S, in counts/s is PDtimes the quantum yield of the detector in counts/photon, as indicated by

(4)

Lsol can be determined from eq 2 after two simple assumptions are made. First, it is assumed that the radiating area is determined by the laser spot size on the electrode and is independent of diffuqion. This will be the case for any normally sized beam (>lo0 pm), particularly at short electrolysis times. Second, we assumed that the dimension of the radiating area perpendicular to the electrode is very small, so that the radiating area behaves like a plane. Again the assumption is validated by the fact that the diffusion layer is thin relative to the spot size and the distance to the collection optics. Given these assumptions, L is equal to the total Raman scattering divided by the spot size (aa2/cos a). Combining these statements with eq 2, the specific intensity is given by

4PoPNb(Dt)'12 Lsol

=

,312~2

(5)

where a is the beam radius in cm. Two limiting cases existing depending on the relative magnitudes of the laser spot size and the limting area, AD, of the spectrometer (usually the entrance slit area). When the laser spot is smaller than AD, then its area (aa2/cos a ) should be substituted for AD in eq 4, and substitution of eq 5 for LW1 yields

(2)

where Po is the input beam power, photons s-l, @ is the differential Raman cross section, cm2 molecule-' sr-', Nb is the bulk number density of starting material, molecules ~ m - D~ , is the diffusion coefficient of starting material, cm2 s-', t is the time after the potential step, s, and a is the incident beam angle, relative to surface normal. The equation assumes an ideal planar diffusion profile for a stable scatterer generated from bulk starting material by a potential step at a diffusion-controlled rate. In addition, it assumes no attenuation of the beam by electrogenerated material, a completely valid assumption for normal Raman scattering. Notice that as a approaches 90°, corresponding to glancing incidence, ZR increases, being a factor of 57 higher than normal incidence for a = 8 9 O . In phyaical terms,the path length of the beam through the diffusion layer increases as a approaches 90°, thus increasing the number of scatterers encountered by the incident light. It should be noted that the major difference between resonance Raman spectroelectrochemistry and the normal Raman analog presented here is the magnitude of the scattering cross section @.For resonance Raman this value is 102-105larger, greatly increasing sensitivity. Once the total Raman scattering is known, the measured signal in terms of counts/s can be predicted from the fraction of total Raman light passing through the collection optics and monochromator and the quantum yield of the detector. A convenient quantity to assist the analysis of the instrumental efficiency is the specific intensity, L, also called sterance (22). L has units of photons s-' cm-2 sr-' and equals the radiation density per unit of solid angle from a scattering surface. The photon flux, PD (photons s-l), falling on the detector is given by

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S=

'

4Po@Nb(Dt) 12Q TQ

(6)

cos a Note that the signal depends on incidence angle and time in the same way as the absorbance for a reflection experiment (21) and that the signal will be linear with the collection efficiency of the spectrometer. A plot of S vs. t112will be linear with a larger slope as a approaches 90°. For the case of a spot size that is large relative to AD, eq 4 and 5 are combined to yield all2

(7) While the signal is still proportional to t1I2,there is no dependence on a. As the incidence angle is increased, the path length increases, but a smaller fraction of the scattered light is collected. Since the laser spot has overfilled AD (e.g., the entrance slit), there is no gain in signal if more Raman photons are generated by increasing the angle. As shown below, both of these cases can be important when a fiber optic collection system is employed. For the case of a surface bound rather than solution species, the analogues of eq 6 and 7 may be derived easily, by combining the treatments of Van Duyne and Campion with our own. Equatiop 8 is the surface analogue of eq 1 and shows that Raman scattering from a surface is independent of both has units of time and laser spot area at a given power (NBurp molecules cm-2). As the incidence angle is increased and the

Zsurf = POPN,urf

(8) laser spot increases in size, the power density decreases but more scatters are illuminated, keeping the signal constant. After dividing by spot area to obtain the specific surface intensity, the surface analogue of eq 5 may be derived, yielding

For the case of a laser spot that is small relative to AD,the Raman signal will be

= PoPNsurfQTQ

(10) Note that neither the incidence angle nor the laser spot size have an effect on the observed signal. However, the power Ssurf

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ANALYTICAL CHEMISTRY, VOL. 58, NO. 12, OCTOBER 1986

I1

I

mm

< input

3

Tracor Multichannel Scaler

Apple Computer

Figure 2. Block diagram of optical and electronic components. See text for details.

density experienced by the sample will decrease with increasing a , minimizing possible radiation damage to the sample. If the laser spot is larger than A D , the signal is given by Ssurf

=

PoPN8urfOTQADCOS xu2

(11)

Npte that the signal actually decreases with increasing a because the power density at that part of the sample monitored by the spectrometer is decreasing. As before, AD is usually determined by the entrance slit but can also equal the pixel size for multichannel detectors.

EXPERIMENTAL SECTION The optical and electronic arrangement is shown in Figure 2. The optical fibers were coupled to the spectrometer by a single lens which imaged the fiber ends onto the entrance slit with a magnification of approximately 2X. The input laser beam from a Coherent 90-5 argon ion laser was directed onto the electrode through glass windows in a Lucite cell as shown in Figure 3. While it was possible to conduct the light to the electrode with an optical fiber as described previously (20), it was found that an input fiber generated an unacceptable background from silica Raman. In addition, a direct focusing of the beam on the electrode yielded a more accurate incidence angle and a better defined laser spot on the electrode. The optical fibers used for light collection were the same type used previously (20) and had a 200-pm core diameter with a 15-pm cladding. The optical properties of the fibers in the visible wavelength range have been described previously. Twenty-seven collection fibers were embedded in Torr-seal in three nine-fiber rows forming a 0.6 X 2.5 mm array and positioned above the electrode with a five-axis manipulator. The distal ends of the fibers were arranged in a single 1cm long row and imaged onto the entrance slit of a Spex 1403 double monochromator with a single f / l lens. The fiber optic coupler was constructed such that the light from a 0.5 X 2.5 mm electrode slightly overfilled the 0.1 X 2 cm entrance slit. Both ends of the fiber bundle were polished with alumina abrasives. The working electrode was a polished Pt rectangle, 0.5 X 2.5 mm in size, embedded in Torr-seal and positioned horizontally in the electrochemical cell (Figure 3). The collection fibers were positioned at various distances from the electrode plane in the range of 0.20-1.0 mm. The SCE reference electrode was placed within 1 mm of the working electrode and the potential was controlled with a PARC 173 potentiostat. The angle of the beam relative to the electrode normal was determined from the geometry of the incident and reflected beams. The collection efficiency of the fiber system is a function of numerical aperture, distance from the electrode, and several other factors, and is farily difficult to estimate. The fiber ends occupied about 30% of the hemicylindrical surface covering the electrode in the solution. If one assumes a perfectly reflecting electrode, then about 30% of the total Raman light will enter the fiber bundle. T o obtain spectra of electrogenerated materials, the approach of VanDuyne (8) was employed. A low-frequency (ca. 10 Hz) square wave was applied to the cell to establish a steady-state

I r

I

fibers CplleCtion

1 electrode surface

Figure 3. Detail of cell, electrode, and fiber arrangement. The input beam enters the solution through glass windows, then reflects off the electrode at angles of 75-89' to the normal. A 0.8 X 2.5 mm array of fibers is positiined above the electrode at distances of 0.2-1.0 mm. I n thin-layer mode, the fiberlelectrode distance was less than 0.3 mm, and the beam passed parallel to the electrode.

concentration of electrogenerated material. The spectrum was then scanned by the spectrometer in the usual photon counting mode that would be used for static solutions. Once a Raman peak for an electrogenerated species was identified, transients could be recorded by monitoring the Raman peak at fixed frequency while pulsing the electrode potential. Potential pulses were repeated with a duty cycle of 5%, while the photon counting output was recorded by a multichannel scaler. The scaler recorded the number of photons arriving at the detector for up to 1024 sequential time windows following the potential pulse, with a minimum dwell time of 10 pslchannel. The experiment was controlled by a modified Apple 11+ computer, and multichannel scaling was carried out either by a Tracor 1710 multichannel scaler or the Apple itself. Counts in each channel were converted to hertz before plotting. The Raman peaks of chlorpromazine radical cation were superimposed on a fluorescence background. On the assumption that the fluorescence intensity did not vary over small wavenumber shifts, a correction was made by subtracting a transient recorded immediately adjacent to the Raman peak. In several experiments, thin-layer conditions were created by positioning the fiber probe within 100-300 pm of the Pt surface. In this case, the beam was approximately parallel to the electrode and probe face (see Figure 3), and the entire solution within the thin layer was sampled. Chlorpromazine hydrochloride (CPZ) was obtained from Sigma Chemical Co., and hydroquinone was Aldrich Gold Label. Both were used without further purification. In mildly acidic solution, chlorpromazine is oxidized electrochemically to a cation radical (CPZ"), which is stable for at least several minutes, depending upon conditions (23, 24). To prevent reactant adsorption, the chlorpromazine experiments were carried out in 42% (w/w) methanol solutions. The scattering cross section for the 1668-cm-' band of benzoquinone was determined approximately by comcm+ parison to the 991-cm-' band of benzene ( p = 3.2 X molecule-' sr-l). After a spectrum is determined for both compounds dissolved in acetonitrile, the benzoquinone cross section was calculated to be 3.4 X cm-2 molecule-' sr-l, integrated over the entire line width.

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4

2 time (msec)

0

4

6

Raman S h i f t (cm-') Flgure 4. Resonance Raman spectrum of electrogenerated CPZ" formed by a 10-Hz, 33% duty cycle square wave potential with limits of 0.0-0.8V vs. SCE. CPZ concentration was 2 mM in 1.0 M HCI in MeOH/H,O. The wavelength scan consisted of 1.0-cm-' steps with

a 1.0-s integration period on each step, resulting in an average scan rate of 0.8 cm-' s-'. Spectral band-pass was 5.0 cm-'. Laser beam incidence angle was 87O, power at the sample was 40 mW, 5145 A. Asterisk indicates a MeOH peak.

6

0.3

0.6 t

1 1 kHz

I

X I

I

600

I 1250

1

1900

Raman Shift (crn-l)

Figure 5. Raman spectra of hydroquinone and electrogenerated benzoquinone in a solution that initially contained 20 mM hydroquinone and 1.O M HCI in MeOH/H,O. Potential wave form was a 5 Hz, 50 % duty cycle square wave between 0.0 and 0.8 V vs. SCE. Spectra are the average of five scans of the type used for figure 4. Laser power at sample was 75 mW, 5145 A. H denotes hydroquinone peaks, and B identifies benzoquinone peaks. MeOH peaks from blank spectra were subtracted. A ninepoint Savitsky-Goloy smooth was carried out before plotting.

RESULTS AND DISCUSSION A resonance Raman spectrum of electrogenerated chlorpromazine cation radical (CPZ") obtained by scanning the wavelength while a 20-Hz, 33% duty cycle asymmetric square wave was applied to the electrode is shown in Figure 4. Since the spectrum is strongly resonance enhanced, an excellent signal to noise ratio is obtained with millimolar concentrations. A similar spectrum for benzoquinone generated electrochemically from hydroquinone is shown in Figure 5. It was shown that benzoquinone is not resonance enhanced a t the 515-nm laser wavelength by noting that the ratio of the benzoquinone peak to that of a nearby water band is nearly independent of laser wavelength for the 488- and 515-nm argon lines. Note that the spectra of either the reactant (hydroquinone) or product (benzoquinone) are obtained with comparable sensitivity, since the high incidence angle restricts Raman col-

112

0.9

12

(msec 1'2)

Flgure 6. Transient intensity of the 1126-cm-' band of CPZ" generated by potential pulses from 0.4-0.8 V vs. SCE. The transient was recorded by a 1024 multichannel scaler with a dwell time of 30 ps. Potential pulse duration was 1.5 ms before returning to 0.4 V and was repeated every 30 ms to allow signal averaging for a total of 50 000 runs. Curve A shows raw data, curve B is a plot of intensity vs. t'" for the 1.5-ms duration of the potential pulse. Solution conditions were the same as those of Figure 4.

lection to primarily the diffusion layer, and little collection is made from the bulk solution. The ability to obtain spectra from unenhanced Raman scatterers results from the high sensitivity of the glancing incidence fiber optic arrangement. A significant improvement in both S I N and time resolution would be possible with a multichannel spectrometer, which could be gated to obtain a complete spectrum at a particular time after a potential pulse. Such a system is presently under development in our laboratory. Figure 6 shows a Raman intensity vs. time transient for the 1126-cm-l band of CPz'+ generated during a 1.5-ms potential pulse to a potential causing diffusion limited generation of CPZ'+. With extensive signal averaging, it was possible to obtain useful S I N ratios for a 1.5-ms potential step. In addition, a plot of Raman intensity vs. t1I2was linear down to 30 p s , indicating that eq 6 is valid down to very short time scales. Figure 7 demonstrates that the backgbround corrected intensity transient that was measured is actually due to Raman scattering and not fluorescence of CPZ". The intensity a t 10 ms tracks the resonance Raman spectrum exactly, indicating that the transient data accurately reflect the Raman spectrum. Transients for normal rather than resonance Raman scatters have not been obtained previously below the 1-s time scale, due to unacceptably weak signal (18). Figure 8 shows a transient for the 1668-cm-' band of electrogenerated benzoquinone, obtained during l-s potential pulses, which generated quinone from hydroquinone. The transient is linear with t1I2 down to 20 ms, demonstrating that millisecond time resolution is possible for normal Raman scatterers. Using the Raman cross section for benzoquinone (3.4 X cm2sr-l molecule-l), 20 mM of hydroquinone (Nb= 1.2 X 1019molecules ~ m - ~D) , = 0.8 X cm2 s-l (25),Po = 0.1 W (2.5 X 10'' photons s-l),

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Raman Shift (cm-')

0

Flgure 7. Raman intensity at the end of a 10-ms potential pulse monitored in the same manner as in Figwe 6. One thousand runs were signal averaged wlth a repetition time of 206 ms. Points indicate that Raman intensity at various Raman shlfts; solid line Is the resonance Raman spectrum of CPZ' generated by chemical oxidation with (23''. The two spectra were normalized at 1126 cm-' to permit comparlson.

d 4

12

16

20

ilcos a

Figwe 9. Raman intensky as a function of incidence angle (a)for the conditions of Figure 7 at 1127 cm-'. Points are experimental; s o l i line indicates the linearity expected from eq 6.

$1

E

L

0

2

1

3

time (sec)

B

t

''2

( s e c 1'2)

Figure 8. Spontaneous Raman transient for the 1668-cm-' band of benzoquinone generated by 0.0-0.8 V potential pulses to a solution of 20 mM hydroquinone in 1 M HCI In MeOH/H,O. one-second dwatkm pulses were repeated every 20 s, and a total of 75 runs were signal averaged. Multichannel scaler dwell time was 20 ms, and 200 channels are plotted. Laser power was 75 mW at 5145 A; spectral band-pass was 9 cm-'.Curve A is the raw transient; curve B is a plot of intensity vs. t''* for the duration of the pulse. a = 87', R = 0.5 sr, Q = 0.3, T = 0.01 (typical for double monochromators (13)),eq 6 predicts a Raman intensity of 20 kHz for the quinone transient 1 s after the potential step, integrated over the entire Raman line bandwidth. The signal a t the end of a 1-s potential pulse was 6.4 kHz for a 9-cm-' band-pass. Adjustment of this value to the 20-cm-l line width of the 1668-cm-' band yields an integrated signal of 14 kHz. While the theoretical prediction of this value must be considered approximate due to uncertainties in the instrumental parameters, the agreement between predicted and observed

Flgure 10. Resonance Raman spectra obtained in thin-layer mode after equilibration of the solution between the fibers and working electrode at a fixed potential. Initial CPZ" concentration was 0.15 mM in 1.O M H2S04. daman intensities were normalized to the 984cm-' band of sulfate. The fibers were posltioned 0.2-0.3 mm from the electrode face. Spectral parameters were those of Figure 4. values indicates that the major factors affecting performance have been accounted for. Equation 6 predicts that the Raman signal should depend on incidence angle provided the illuminated area on the electrode is smaller than the limiting aperture of the fiber/ spectrometer system. When the illuminated spot is large relative to this aperture, the dependence on a will diminish reaching a limit where incidence angle has no effect (eq 7). Figure 9 is a plot of signal vs. l/cos a for the CPZ electrolysis. For a < 86', the linearity expected from eq 6 is observed. As a increases, however, the dependence on a diminishes in a manner consistent with eq 7. Note that in all cases, the signal vs. t ' i z linearity Is observed. For these experiments, the CPZ concentration was kept low enough to avoid errors from laser light absorption. When a increases above 86', the laser spot overfills AD,and eq 7 must replace eq 6. The results presented so far were obtained for soluble electrogenerated species exhibiting semiinfinite linear diffusion. By decreasing the distance between the fiber collectioh probe and the electrode surface, it is possible to create a thin-layer cell with the fast electrolysis characteristic of finite diffusion in thin cells. Figure 10 shows spectra of CPz'+ obtained in thin-layer mode as a function of potential. Complete electrolysis was carried out on a time scale of a few minutes, and spectra were obtained after the current for a particular applied potential decayed to near zero. This thin-layer geometry is particularly useful when complete

ANALYTICAL CHEMISTRY, VOL. 58, NO. 12, OCTOBER 1986

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transmission would enhance the signal over that observed here by a factor of about 70. Such a system is presently under construction. In addition to providing good light collection, the fibers are very convenient to use compared to conventional Raman spectroelectrochemical cells. There is no need to align the electrode with the entrance optics of the spectrometer, since that task is replaced by the simple alignment of fibers with electrode.

I

30 kHz

CONCLUSIONS

Raman Shift (cm

-l)

Flgure 11. Surface-enhanced Raman spectrum of pyridine adsorbed on a silver electrode roughened by a single oxidation/reduction cycle from -0.6 to 0.12 V vs. SCE in 0.1 KCi containing 10 mM pyrldlne. The cell and fiber optics were the same as Figures 1-3 and the spectrum was scanned 2.0 cm-' s-'. Laser power was 30 mW at 5145 A at the sample; spectral band-pass was 3 cm-'. Potential was heid at -0.6 V during scan.

electrolysis of reactant to product is desired. In addition, a Raman analogue of Heineman's spectropotentiostatic technique (2,26)may be employed, in which the Raman intensities of the oxidized or reduced forme of a redox couple are monitored at fied potential, and the formal potential is calculated from the results. The CPZ'+/CPZ ratio was calculated from the spectra by comparing the area of the 1126-cm-' band to that for the totally oxidized solution. A Nernst plot of log (CPz'+/CPZ)vs. E,,, for the spectra of Figure 8 is linear with a slope of 63 mV ( r = 0,99963) and an intercept (EO') of 590 mV. These results are comparable to those of Anderson and Kincaid (27),who obtained Raman spectra by pumping solution from an electrochemical to a Raman cell. The Raman technique has the major advantage over the UV-Vis absorption method of greater selectivity afforded by monitoring a particular vibrational mode of the redox system of interest. In addition to monitoring species in solution, the fiber optic collector may be used to observe surface species. Except for the incidence angle, the same factors that improve sensitivity for electrogeneratedsolution species w i l l improve the observed signal for species on the electrode surface. Figure 11 shows an example of surface-enhanced Raman spectroscopy on a silver electrode, obtained with the same fiber optic probe. An excellent S I N ratio is observed, and the spectrum shows excellent agreement with those reported in the literature (13, 14).

The high sensitivity of the present technique stems from two features: glancing incidence geometry and efficient scattered light collection. Glancing incidence provides a l/cos a enhancement in the signal, provided the illuminated area does not overfill the entrance slit or the electrode. The fiber collection arrangement collects a large fraction of the Raman light due to its proximity to the sample and the efficient conduction of light to the spectrometer. Unfortunately, the ultimate collection efficiency is determined by the least efficient optical component, in our case the monochromator. The combination of relatively high flnumber and low transmission characteristic of the high dispersion double monochromators commonly used for Raman spectroscopy leads to significant loss of scattered light after fiber optic collection. With the smaller flnumber and fewer reflections obtainable with a single monochromator, the efficiency of fiber optic collection could be realized and a greatly enhanced signal would result. For example, an f/5 monochromator with 30%

By using glancing incidence geometry and fiber optic light collection, we have shown that it is possible to obtain normal Raman spectra of species generated electrochemically, on a time scale of a few tens of milliseconds. In addition, resonance Raman spectroelectrochemistry has been entended into the sub-millisecond time scale because of the higher system efficiency. The freedom from the requirement for resonance enhancement has two important ramifications. First, a wider range of chemical systems may be studied because there is no requirement that the molecule of interest absorb the laser line. A valid criticism of both resonance Raman and UV-Vis absorption spectroelectrochemical techniques is that they rely on a strong absorption of light by the molecule of interest to provide sufficient sensitivity. When normal Raman spectroscopy is possible, however, strong absorbers are not required and the list of molecules amenable to study by the method is greatly lengthened. Second, since normal Raman does not require an absorber, it can potentially detect reactants, products, or intermediates in reactions associated with charge transfer. The glancing incidence geometry maximizes the beam path through the diffusion layer, thus minimizing the path through the bulk solution containing relatively plentiful reactant and improving the ability to detect products. The theory presented adequately describes the effects of spectroscopic and instrumental variables on the observed Raman signals. Substantial improvements can be expected by lowering the spectrometer //number and increasing its transmission. While the fixed wavelength transients obtained here are most effectively obtained with a single channel spectrometer, the addition of a multichannel spectrometer to the fiber optic cell will allow acquisition of complete spectra on a microsecond time scale. Finally, fiber optic collection can be used for both thin-layer Raman spectroelectrochemistry and surface Raman spectroscopy. These methods generally do not involve transient species but can be very useful for identifying stable reaction products, measuring redox potentials, and probing the structure of the electrode/solution interface.

ACKNOWLEDGMENT The authors appreciate the efforts of Scott Barnicki, who designed and constructed the computer-controlled multichannel analyzer used for some of the transient experiments.

LITERATURE CITED Kuwana, T.; Winograd, N. Nectroanal. Chem. 1974, 7 , 1-74. Heineman, W. R. Anal. Chem. 1978, 5 0 , 390A. Bard, A. J.; Faulkner, L. R. Nectrochemical Methods; Wiley: New York, 1980; Chapter 14. Blount, H. N.; Hawkridge, F. M.; Heineman, W. R. I n Nectroanalyticsl Chemishy, Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, P 1. McCreery, R. L. Physical Methods in Chemistry; Rossiter, B., Ed.; Wiley: New York, 1986; Vol. 2, p 561. McCreery, R. L.; Robinson, R. S. J. Nectroanal. Chem. 1985, 782, 61. Jan, C. C.; Lavine, K.; McCreery, R. L. Anal. Chem. 1985, 5 7 , 752. Jeanmalre, D. L.; Van Duyne. R. P. J. Electroanal. Chem. 1975, 66, 235. Van Duyne, R. P.; et al. J. Am. Chem. SOC.1979, 101, 2832. Suchanski, M. R.; Van Duyne, R. P. J. Am. Chem. SOC. 1976, 9 8 , 250. Jeanmaire, D. L.; Van Duyne. R. P. J. Am. Chem. SOC. 1976, 9 8 , 4029. Haushalter, J. P.;Van Duyne, R. P. J. Phys. Chem. 1984, 8 8 , 2446.

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(13) Van Duyne, R. P. In Chemical and Bidoglcai Applications of Lasers; Moore, L. B., Ed.; Academlc Press: New York, 1979; Vol. 4, Chapter 4. (14) Van Duyne, R. P. J . Phys., C o i ~1977, . 5 , 239. (15) Furtak. T. E. J . Electroanai. Chem. 1983, 150, 375. (18) Chang, R. K.; Furtak, T. E. Surface EnhancedRaman Scaftering; Plenum: New York, 1982. (17) Vbration at Surfaces; Plenum: New York, 1982: pp 361-464. (18) Clarke, J. S.; Kahn, A. T.; Orville-Thomas, W. J. J . Electroanai. Chem. 1974, 5 4 , 253. (19) Campion, A.; Brown, J.; Grizzle, W. M. Surf. Sci. 1982, 115. L153. (20) Schwab, S. D.; McCreery, R. L. Anal. Chem. 1984- 56, 2199. (21) Pruiksma, R.; Fagan. J. R.; McCreery, R. L. Anal. Chem. 1979, 5 1 , 749. (22) Snell, J. F. Handbook of Optics; Drisoll, E. G.. Vaughn, W., Eds., McGraw-Hill: New York, 1978; pp 1-8 to 1-10,

(23) Cheng, H. Y.; McCreery, R. L.; Sackett. P. M. J . Am. Chem. SOC. 1978, 100, 982. (24) Mayausky, J. S.; McCreery, R. L. J . Electroanai. Chem. 1983, 745, 117 . .. . (25) Adams, R. N. Electrochemistry at Solid Electrodes; Marcel Dekker: New York, 1969; p 221. (26) Heineman, W.; Norrls, B.; Goetz, J. Anal. Chem. 1975, 4 7 , 79. (27) Anderson, J. L.; Kincaid, J. R . Appl. Spectrosc. 1978, 32, 356.

RECEIVED for review February 13, 1986. Accepted May 14, 1986. This work was supported by a grant from the NSF division of Chemical Analysis, by the OSU Materials Research Laboratory, and by Dow Chemical Corp.

Determination of Nitrogen Atom Concentration in Flowing Gases H. C. Yang and T . M. Niemczyk* Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 87131

The application of the NO tltratlon technlque to the measurement of nltrogen atom concentratkrw In Wwlng gases ls revlewed. The normal method of end polnt detection Is accomplished by vlwally monltorlng the color changes during the titration. An malyris of the background spectra of fkwlng nitrogen systems, contalnlng meaeurabie amounts of NO, lndlcates that the NO ,&band system Is not produced by collislonal excltatlon. However, the chemllumlnescent reaction of nltrogen atoms with oxygen atoms produces both NO yand NO &band emblon. The NO @-bandemisdon Is shown to be useful lor the development of a photoelectrk endpolnt detection for the NO titration. With thls method of end polnt detection the detection limit of the tltratlon Is slgnlflcantiy improved.

Active nitrogen is a mixture of excited-state and groundstate nitrogen atoms, nitrogen molecules, and nitrogen ions. I t has long been studied for there are many interesting spectroscopic properties of this nitrogen mixture itself, as well as the fact that many lightiproducing reactions can occur when active nitrogen is mixed with other vapors. Most of these studies have been aimed a t elucidation of the reaction mechanisms, reaction kinetics, and/or characteristics of the nitrogen itself. Recently, however, it has been shown that active nitrogen can be a very useful source of energy for the excitation of analyte vapors (1). The spectra of the analyte vapors produced can be used to make both qualitative and quantitative determinations in many systems. Indeed, the excitation of metal atoms, nonmetals, and organics has all been demonstrated. In the case of metal atom determinations the primary source of excitation for the metal atoms has been shown to be the N2(A3C,+)state of the nitrogen molecule (2). This state is metastable to the ground state and thus very long-lived. Based on these characteristics the term metastable transfer emission spectroscopy (MTES) has been used to describe the technique (3). Recent studies, especially those concerning interferences in atomic determinations ( 4 ) , and the interaction of active nitrogen with non-metal-containing analytes (5),have shown that the character of the active nitrogen flow is very important. Most previous efforts designed to characterize the "active" species in the active nitrogen flow were designed to measure

the N2(A)species. Although this species might be responsible for any of the excitation processes seen when active nitrogen is mixed with an analyte vapor, it is not responsible for the chemical activity seen in active nitrogen studies. It is felt that nitrogen atoms are the primary species responsible for this chemical reactivity. We have noted great differences in the intensity of the PN emission seen when active nitrogen with varying concentrations of N atoms are mixed with phosphorus-containing analytes (5). Thus, the characterization of active nitrogen as an excitation source is not complete without an accurate determination of the N atom concentration. The importance of the measurement of the absolute concentration of N atoms in nitrogen afterglows has long been recognized. Several different methods have been attempted to make these measurements. Mass spectroscopy has been used to estimate the concentration of N atoms in active nitrogen (6, 7). The estimates have ranged from 0.1 to 1.0% of the total nitrogen flow. The measurements have suffered, however, because absolute measurements require precise standardization. It has also been shown that nitrogen atom concentrations can be measured by atomic absorption measurements in the vacuum-UV region (8, 9). However, the experimental difficulty as well as problems in standardization have limited the applicability of this type of measurement. Calorimetric measurements on the afterglow (10-12) and thermal measurements on the solid condensed from active nitrogen at 4.2 K have also yielded estimates of the nitrogen atom concentration (13-15). These techniques have also yielded estimates in the range of 1%of the total nitrogen concentration. The calorimetric and thermal measurements have not, however, produced precise results and it has been felt that the uncertainty of the measurements was limited by heat release from energetic species such as excited nitrogen atoms or molecules. The concentration of N atoms has also been inferred from the production of HCN when active nitrogen reacts with simple organic molecules (16-22). Although this method has been widely used, it has been shown that it produces imprecise results, because of the complex nature of the active nitrogen/organic reactions when organics are mixed with active nitrogen. One problem is the slow kinetics of N atoms reacting with small straight-chain alkanes. Thus,this method has often been shown to produce results that are low by a factor of 2 or 3.

0003-2700/86/0358-2492$01.50/00 1986 American Chemical Society