J. Phys. Chem. 1987, 91, 5527-5531 of P870 due to E T decreases by about a factor of two in going from room temperature to 8 K and that the E T rate is constant for T 5 50 K.29 These results are in accord with earlier less extensive T-dependent studies by Woodbury et al.jO
5527
Acknowledgment. Ames Laboratory is operated for the US. Department of Energy by Iowa State University under contract No. W-7405-Eng-82. This research was supported by the Director for Energy Research, Office of Basic Energy Science and a N S F grant to J.H.G. (DMB-8517391).
(26) Fenton, J. M.; Pellin, M. J.; Govindjee,; Kaufmann, K. J. FEBS Lett. 1979, 100, 1.
(27) Martin, J. L; Breton, J.; Hoff, A. J.; Migus, A.; Antonetti, A. Proc. Natl. Acad. Sei. U.S.A. 1986, 83, 957. (28) Wasielewski, M. R.; Tiede, D. M. FEBS Lett. 1986, 204, 368.
(29) Breton, J.; Fleming, G. R.; Martin, J. L., private communication. (30) Woodbury, N. W.; Becker, M.; Middendorf, D.; Parsons, W. W. Biochemistry 1985, 24, 7516.
High Spectral Resolution In Coherent Raman Scattering Using Broad-Band, Nanosecond-Pulsed Sources and Nonlinear Interferometry Gregory V. Hartland and Peter M. Felker* Department of Chemistry, University of California, Los Angeles, California 90024 (Received: June 12, 1987; In Final Form: July 14, 1987)
Theoretical and experimental results pertaining to Fourier transform coherent Raman spectroscopy are presented. It is demonstrated that Raman resonances spaced by intervals narrower than the bandwidths of the laser excitation sources can be resolved by the method. It is also shown that the technique is sensitive to dynamical line-broadening processes.
I. Introduction Coherent Raman scattering (CRS) as a spectroscopic tool has found application in a wide variety of studies (e.g., see ref 1 and 2). There are many reasons for the extensive application of CRS, including the capability of achieving highly directional signals, and the ability to probe a region of a sample with high spatial resolution. However, despite the utility of CRS techniques, they do have limitations. One area in which limitations are encountered is in high-resolution studies of low-density gases. Difficulties arise here because (1) high-resolution CRS requires at least one narrow-band laser source,’-3and such sources, with their low peak powers, are not especially suited to the generation of the nonlinear C R S signal, and (2) the samples are low-density ones, and the intensity of the scattered wave in CRS is proportional to the square of the The upshot is that it is often very difficult to generate a CRS signal in a low-density gas without compromising on spectral resolution. Difficulties also arise in the application of C R S to studies of dynamics in gaseous samples. Here again there can be problems with low signal levels. In addition, if one desires high temporal resolution, one requires at least one short-pulse laser.4 Recently, we reported5 on a new spectroscopic technique based on coherent Raman scattering and having the potential for elim( I ) Levenson, M. D. Introduction to Nonlinear Laser Spectroscopy; Academic: New York, 1982. (2) Shen, Y. R. The Principles of Nonlinear Optics; Wiley: New York, 1984. (3) Notably, the time-domain method reported in Graener, H.; Laubereau, A.; Nibler, J. W. Opt. Lett. 1984, 9 , 165 is not laser bandwidth-limited, but does require picosecond laser sources. (4) A number of groups have performed picosecond CRS studies on condensed-phase systems. For example: (a) Laiibereau, A.; Kaiser, W. Rev. Mod. Phys. 1978, 50, 607. (b) Hesp, B. H.; Wiersma, D. A. Chem. Phys. Lett. 1980, 75,423. (c) George, S. M.; Autweter, H.; Harris, C. B. J . Chem. Phys. 1980, 73, 5573. (d) Ho, F.; Tsay, W.-S.; Trout, J.; Velsko, S.; Hochstrasser, R. M. Chem. Phys. Lett. 1983, 97, 141. (e) Chronister, E. L.; Dlott, D. D. J . Chem. Phys. 1984, 79, 5286. ( f ) With regard to ref 4a and 4c, the discussion in Loring, R. F.; Mukamel, S . J. Chem. Phys. 1986, 83, 21 16 should be noted. (5) Felker, P. M.; Hartland, G. V. Chem. Phys. Left. 1987, 134, 503.
0022-3654/87/2091-5527$01.50/0
hating problems associated with using CRS to do high-resQlution spectroscopy on, and dynamical studies of, gases. The technique, Fourier transform coherent Raman spectroscopy (FTCRS), is a nonlinear interferometric methodb10 (and is most closely analogous to the schemes employed in the fully resonant four-wave mixing experiments of ref 6-8). In FTCRS, the laser driving fields in a coherent Raman process are directed through a Michelson interferometer and recombined prior to interacting with the sample. (This manipulation of the excitation light is the same as that which occurs in the linear interferometric technique Fourier transform infrared spectroscopy, FTIR.”,’*) The intensity of the coherently scattered wave is then detected as a function of the delay between the arms of the interferometer. When such an experiment is done on a sample having just one Raman resonance accessible to the driving fields (with their finite bandwidths), one observes an “interferogram” that is modulated by a cosinusoidal term having a frequency the same as that of the Raman resonance. The demonstration of this behavior and a simple-minded explanation of the effect are the major points of our earlier report.* In this letter we present theoretical and experimental results that serve to indicate the full value of the FTCRS technique. We (6) Debeer. Wastenen. L. G.: Beach. R.: Hartmann. S.R. Phvs. ~ fa) , ~ D.: van , Rev. Lett. 1986, 56, 123. (b) Eeach, R.; Debeer, D.;’Hartmann,’S. R. Phys. Rev. A 1985, 32, 3467. (7) Morita, N.; Yajima, T.; Ishida, Y . In Ultrafast Phenomena; Auston, D. H., Eisenthal, K. B., Eds.; Springer: Berlin, 1984; Vol. 4, p 239. (8) Golub, J. E.; Mossberg, T. W. In Ultrafast PhenomenaiFleming, G. R., Siegman, A. E., Eds.; Springer: Berlin, 1986; Vol. 5, p 164. (9) Hattori, T.; Terasaki, A,; Kobayashi, T. Phys. Rev A 1987, 35, 715. (10) For a theoretical paper pertaining to ref 6-9, see: Mukamel, S.; Hanamura, E. Phy. Rev. A 1986, 33, 1099. (11) There is a vast literature on FT-IR. Early papers by two of the pioneers of this technique and Fourier transform spectroscopy in general are: (a) Fellgett, P. B. J . Phys. Radium 1958, 19, 187. (b) Jacquinot, P. Rep. Progr. Phys. 1960, 23, 267. (1 2) For reviews of (linear) Fourier spectroscopies and interferometric methods see, for example: (a) Bell, R . J. Introductory Fourier Transform Spectroscopy; Academic: New York, 1972, and references therein. (b) Brault, J. W. Fourier Transform Specfromefry; preprint, and references contained therein.
0 1987 American Chemical Society
5528 The Journal of Physical Chemistry, Vol. 91, No. 22, 1987
Letters on the integrals go from --03 to QD. In a FTCRS experiment, the intensity of the coherent Raman wave is monitored over a frequency range determined by any spectral filtering of the signal. Thus, the FTCRS interferogram is given as
where F(wa,) is a function describing the frequency dependence of the detection efficiency. Now, assuming that the interferometer is such that there are equal intensity beams in the two arms, then S1(o,7) f,(w)(l f cos U T ) and SS(w,7) -&(w)(l f cos W T ) , where fl(w) and&(w) are the spectral profiles of the pump and Stokes lasers, respectively, and where the f signs denote the possibility that either one of the outputs12 of the interferometer may pertain to the experiment. Using these functions in eq 1 and 2, one obtains
-
I
I
Figure 1. Schematic diagram of the experimental arrangement for two-laser FTCRS (using a noncollinear geometry). Note that the two laser beams experience the same delay going through the interferometer. The insert in the figure shows the energy level diagram applicable to coherent anti-Stokes Raman scattering from two rotational levels. w , is the "pump" laser frequency, w, is the "Stokes" laser frequency, and was is the frequency of the coherent Raman wave.
show that upon application of the scheme in situations where the bandwidths of the lasers are wide enough that more than one Raman resonance can be coherently driven, the interferogram that is obtained yields, upon Fourier transformation, that portion of the Raman spectrum effectively spanned by the laser bandwidths. Experimental results on pure rotational transitions in gaseous benzene and in air are presented. The results demonstrate that the spectral resolution available in FTCRS is not limited by the bandwidths of the lasers. We also present results that pertain to the applicability of FTCRS in studies of dynamics. Specifically, we show that FTCRS is sensitive to collisional processes (Le., pressure-broadening of rotational Raman transitions) in gaseous 02.
11. Theoretical Section The situation to be treated in this section is portrayed in Figure 1. The figure shows the experimental arrangement of two-laser FTCRS. The outputs of two lasers are directed through a Michelson interferometer (the different laser beams being either parallel or collinear in both interferometer arms), after which the recombined beams (which remain parallel or collinear) are focused into the sample cell. There, they generate the coherent Raman wave, whose intensity is measured as a function of interferometer delay, T . Also shown in the figure is a level diagram pertaining to coherent anti-Stokes Raman scattering. To derive the form of the spectra that are obtained in R C R S we make use of the theoretical results of ref 13. We use eq 24 of that paper, which gives the spectral density of a coherent anti-Stokes Raman signal and is valid when the laser excitation fields are stationary stochastic processes. In adapting this equation to our needs, all functions that depend on the interferometer delay T will be denoted with such dependence expressed. One has the following for the spectral density of the signalI3 Sas(0
a
7)
=
4;;(~,-WI+Ws)12S1(Wi,T)
Ss(wsr7) SI((Jas-WI+Ws,T)
dw1 dws (1)
where a is a L(+.ol-w,) is a containing all the information pertinent to the Raman spectrum of the sample,I3 S1(w,7) is the spectral density of the "pump" laser, S,(w,r) is the spectral density of the Stokes laser, and the limits (13) Yuratich, M. Mol. Phys. 1979, 38, 625. (14) (a) Note that the dependence of signal intensity on experimental geometry is contained in the constant a. (b) We assume that w1and w gare far from any vibronic resonances.
where 0.f. represents cosine terms having frequencies on the order of w1and w,. (We shall henceforth neglect such optical frequency terms.15) From eq 3 one notes the principal difference between nonlinear FTCRS and interferometric versions of linear spectroscopies'* (e.g., Fourier transform (FT)-infrared," Raman,16 and UV-vis emission and absorption" spectroscopies). The interferograms in the linear techniques are modulated at the frequencies of the photons involved in the process. In contrast, because in FTCRS (and other nonlinear interferometric techniquesbs), the signal depends on products of spectral densities (i.e., S1(w,7) and SS(w,7)),there are modulations at photon difference frequencies in Z ( T ) . To proceed from eq 3, it is most instructive to consider the Fourier transform of Z(T), namely J ( w ) . One finds that J ( w ) is symmetric about w = 0 with a zero frequency term (which we neglect) and three types of positive frequency terms: Y1(w>0) = K S dw, dw, lL(Q,-wl+w,)12
X
fl(4&(Us) f l ( W l + W )
F(2ul-ws+w) (4a)
92(w>O) =
K S dwl dus IL(Q,-wi+wS)lYi(wi)
&(us) f i ( w s + w ) F ( W i + w )
(4b) Y3(w>O) =
S
KIL(Q,-~)I~ dwi dwash(w1) h(wl-0)fi(wa6-w) F(Uas) ( 4 ~ )
where K is a constant. Consideration of eq 4a and 4b reveals that when the functionsfi(w),fs(w), and F(w) are broad-band, 9,(w) and J 2 ( w ) are weak, structureless background components in the Fourier spectrum.'* They will be neglected in this paper and discussed in more detail elsewhere. In contrast, one sees that 9,(w) is the product of a function describing the Raman resonances of the sample times a function that depends on the spectral properties of the lasers and the detection system. The latter function defines an apparatus spectral "window" for the experiment. (15) These terms are neglected because (a) our experiments do not resolve such high-frequency components, (b) the components can, in any case, be filtered out if necessary, and (c) they do not give rise to any resonance structure in the Fourier transform of [ ( T ) . (16) For example: (a) Hirschfeld, T.; Chase, B. Appl. Specrrosc. 1986, 40, 133 and references therein. (b) Hallmark, V. M.; Zimba, C. G.; Swalen, J. D.; Rabolt, J. F. Spectroscopy 1987, 2, 40 and references therein. (17) For example: Luc, P.; Gerstenkorn, S. Appl. Opt. 1978, 9, 1327 and references therein. (18) It is pertinent to note that in a three-laser version of anti-Stokes FTCRS with just the pump and Stokes laser beams going through the interferometer, one expects no terms like s , ( ~and ) s,(~),only the s,(~)term.
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5529
Letters Equation 4c is the principal theoretical result of this paper in that it shows that the information available from J3(w) is the same as that available from spectral domain C R S methods. As a specific example of eq 4c, assume that the laser spectral profiles are Gaussian--f,(w) = @ exp[-(w - 0 ~ ) ~ / ( 2 d andfs(w) )] = y expi-(w - as)2/(2d)],0, y, al,and a, being constants-and that F(w,,) is such that the whole anti-Stokes spectrum can be detected with unit efficiency. With this, it can be shown that
14
0 L
where C is a constant. Thus, in this case, the Fourier spectrum is the product of a Gaussian apparatus window function (centered on a, - as,with fwhm equal to 4(ln 2)’i2u) times the function describing the Raman spectrum, lLI2. Finally, it is pertinent to consider the form of IL12. It is given by13
where a is a constant that accounts for fully nonresonant contributions to the coherent scattering and b, is a constant that determines the intensity of the j t h Raman resonance, having transition angular frequency QJ and damping constant r,. From eq 6 and eq 5 (or eq 4c) it is clear that Y3(w>O) is composed of an apparatus function times a sum of four types of terms: (1) a constant term arising from fully nonresonant processes ( -la12), (2) interference terms arising between nonresonant and resonant processes (-lab,l), (3) interference terms between resonant transitions (-lb,b,l), and (4) contributions from isolated resonances (-lbjI2). One would note that these four types of terms are just what one expects in more conventional coherent Raman experiment~.~~ 111. Experimental Section The experimental apparatus has been described el~ewhere.~ Briefly, a Quanta-Ray DCR-2A Nd:YAG laser (pulsewidth ca. 7 ns) was used to pump two PDL-2 dye lasers. Both dye lasers were run with their diffraction gratings in first order (bandwidths of ca. 3 cm-I) so as to maximize the width of the spectral window function of the apparatus. The beams from the two dye lasers were both directed through a Michelson interferometer after which they were focused into the gas cell containing the sample (see Figure 1). The relative delay of the interferometer was monitored by monitoring the position of the translation stage in the movable arm of the interferometer. This was accomplished with a slidepotentiometer having a distance-measurement precision of ca. 1 Clm. Experiments were performed using two geometries, corresponding to the pump and Stokes laser beams being collinear and noncollinear, respectively. The collinear geometry is the same as that used to obtain the results of ref 5. The noncollinear geometry is as indicated in Figure 1. In both collinear and noncollinear geometries the C R S signal was isolated from laser light by spectral filtering (a 1/4 m and 1 m monochromator used in tandem), detected by a photomultiplier/boxcar integrator combination, and processed by a personal computer. The two geometries gave equivalent results. While the crossed beam scheme has the advantage that nonresonant contributions to the signal (from windows, etc.) are reduced relative to the collinear arrangement,I9 it has the disadvantages that it is more difficult to align and produces lower signal levels.20 (19) For example: Maroncelli, M.; Hopkins, G. A.; Nibler, J. W.; Dyke, T. R.J . Chem. Phys. 1985, 83, 2129. (20) This is because the phase-matching condition is not met very well in this geometry. Work is in progress to apply the noncollinear phase-matched BOXCARS (see Eckbreth, A. Appl. Phys. Lett. 1978, 32, 421) and foldedBOXCARS (see Shirley, J. A,; Hall, R. J.; Eckbreth, A. C. Opt. Lett. 1980, 5, 380) geometries to FTCRS.
(ai FTCRS trace
11,
1
50
10 20 30 Relative Delay (psec)
I
(b) Fourier spectrum
55
60
Frequency (cm
40
65
70
)
Figure 2. (a) Coherent anti-Stokes Raman scattering intensity versus relative delay for air at atmospheric pressure (only part of the whole interferogram is shown). (b) Fourier transform spectrum of the complete FTCRS interferogram from (a). The peak positions are labeled in cm-I. The error on each peak is *0.1 cm-’ (see ref 22). The oxygen and air used as samples were obtained as compressed gases from commercial sources. The benzene used was Fischer “spectroanalysed” grade. All compounds were used without further purification. In the benzene experiments, the sample was prepared by evacuating the gas cell and then allowing benzene vapor to enter the cell from a liquid reservoir at room temperature. Therefore, the benzene results correspond to benzene at its room temperature vapor pressure. Spectral domain results were obtained from the FTCRS interferograms by using standard fast Fourier transform techniques.2’ Intensities in the Fourier transform spectra are reported as the absolute values of Fourier amplitudes; no attempt was made to determine experimentally the precise location of the zero-delay point of the interferometer, as would be required if phase information were desired. Nevertheless, the zero-delay point could be located to within ca. 5 ps quite easily by (1) finding the delay position at which interference between the beams from the two arms of the interferometer were strongest, (2) finding the position at which the modulation depth of an FTCRS interferogram corresponding to totally nonresonant coherent scattering was maximal, or (3) making use of the symmetry of I(T) about 7 = 0.
IV. Results A portion of the FTCRS interferogram obtained from air at atmospheric pressure is shown in Figure 2a. The laser and monochromator settings were such that coherent anti-Stokes scattering near (a1- ws)/(2.rr) = 60 cm-’ would be observed. Since it is clear that the amplitude envelope of the fast oscillations in the figure is modulated, one may recognize immediately that the trace contains contributions from more than one Fourier component. Figure 2b shows the Fourier transform of the whole interferogram (extending over 170 ps). Two peaks (at 59.7 i 0.1 and 60.4 & 0.1 cm-1)22can be seen in the Fourier spectrum. The (21) For example: Press, W. H.; Flannery, B. P.; Teukolsky, S. A,; Vetterling, W. T. Numerical Recipes; Cambridge University Press: Cambridge, U. K., 1986. (22) The quoted errors represent the spacings between channels in the Fourier transform of the interferogram zero-filled to about twice its original range. For a discussion of the effect of zero-filling on the information content of discrete Fourier spectra, see: Bartholdi, E.; Ernst, R. R. J . Mugn. Reson. 1973, I I , 9.
Letters
5530 The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 FTCRS Trace -Benzene (ca. 37 cm-' ) 9 ,
i
58
10
0
20
30
50
40
60
70
Relative Delay (psec)
Figure 3. Coherent anti-Stokes Raman scattering intensity versus relative delay for benzene at its room temperature vapor pressure (only part of the whole interferogram is shown). Note that the zero relative delay in the figure is not meant to correspond to zero absolute delay.
\I/
20
25.78
30
25
Frequency (cm
30
35
40 Frequency (cm
-'
-'
35
40
)
45
50
)
Figure 4. Fourier transform spectra for benzene (at its room temperature vapor pressure) obtained for CARS transitions around (a) 26 cm-l and (b) 37 cm-'. The spectrum in (b) corresponds to the interferogram of Figure 3. The peak positions are labeled in cm-I. The error on each peak is 10.05cm-I (see ref 22).
2
lowest frequency peak'can be ssigned to the J = 6, 8 pure rotational CARS process in' trogen, the other to the oxygen J = 9, 11 process. Using these assignments, and assuming the rigid rotor approximation to be valid, one obtains the Bo rotational constant of N2 to be 1.990 f 0.003 cm-', and that of O2 to be 1.438 f 0.002 cm-I. These values are consistent with literature values (e.g., 1.9895 and 1.4377 cm-I, respectivelyz3). Figure 3 shows a portion of a FTCRS interferogram obtained on a sample of benzene vapor. To measure the trace, the dye lasers and monochromator were set such that CARS transitions near 37 cm-' would be observed. One would note the clear modulation of the amplitude envelope of the fast oscillations, such modulation indicating (as in Figure 2a) the presence of more than one Fourier component in the time domain scan. Figure 4b shows the Fourier spectrum corresponding to the trace in Figure 3 (taken over 215 (23) For example: Weber, A. In Raman Specrroscopy of Gases and Liquids; A., Weber; Ed.: Springer-Verlag: Heidelberg, 1979, and references therein.
59
60
61
63
62
Frequency (cm - ) Figure 5. Fourier transform spectra for the oxygen J = 9, 11 pure rotational transition,obtained at 45 pig and 0 psig oxygen pressure. The fwhm of the 0 psig peak is approximately 0.4 cm-' ( T 2 25 ps) and that of the 45 psig peak is 0.8 cm-' (T2 13 ps). The two spectra have been normalized to approximately the same peak height.
-
-
ps in total). Figure 4a shows the Fourier spectrum corresponding to a different FTCRS experiment on benzene for which the frequency window of the apparatus was centered near 26 cm-'. Consideration of the spectra of Figure 4 in light of the Raman allows one to assign transitions to be expected in a symmetric all of the labeled peaks to R- and S-branch pure rotational transitions of benzene. With such assignments it is then possible to determine the B rotational constant of the zero-point level of the ground electronic state of the molecule. Assuming benzene to be a rigid rotor, one obtains B = 0.1896 f 0.0002 cm-' (90% confidence limits24). This value is in agreement with literature values (e.g., B = 0.189675 cm-' from ref 23). Finally, presented in Figure 5 are results pertaining to the pressure broadening of the J = 9, 11 rotational CARS transition The spectra shown are the Fourier transforms of interin 02. ferograms obtained at room temperature O2at 45 and 0 psig. (The interferograms were taken over a range of 130 ps.) One can see plainly the manifestations of pressure broadening in comparing the two spectra. (The line widths quoted in Figure 5 are meant to be taken only semiquantitatively.) From this, it is clear that FTCRS is sensitive to dynamical line-broadening, as one would expect from the treatment of section 11. V. Discussion The theoretical results of section I1 together with the experimental results of Figures 2-4 clearly show that the spectral resolution available in FTCRS is not limited by the bandwidths of the laser excitation sources or that of the detection system. This is in contrast with "scanning" CRS,'*13the resolution of which depends on the bandwidths of both lasers (in a two-laser scheme). It also differs from "multiplex"' (or "broad-band"13) CRS, which has resolution that depends both on the spectral resolution of the detection system and on the bandwidth of one of the lasers involved. The bandwidth independence of the resolution of FTCRS is, perhaps, its most valuable asset, for it means that nanosecond-pulsed sources, which have peak powers sufficient to produce measurable CRS signals in sparse gases but which often have bandwidths too broad to resolve the rotational structure of gaseous species, can be used effectively in high-resolution rovibrational spectroscopy. Furthermore, it has been shownz5that, because of commensurable (or near-commensurable) rotational level spacings in many species, there can be advantages to doing rotational
(24) Based on a Student's t analysis. See, e.g.: Fritz, J. S.: Schenk, G. H. Quantitarive Annlyticnl Chemistry; Allyn and Bacon: Boston, 1974; p 34-35. (25) (a) Felker, P. M.; Baskin, J. S.; Zewail, A. H. J . Phys. Chem. 1986, 90,724. (b) Baskin, J. S.; Felker, P. M.; Zewail, A. H. J . Chem. Phys. 1986, 84, 4708. (c) Felker, P. M.; Zewail, A. H. J . Chem. Phys. 1987, 86, 2460. (d) Baskin, J. S.; Felker, P. M.; Zewail, A. H. J. Chem. Phys. 1987,86, 2483.
J. Phys. Chem. 1987,91, 5531-5534 spectroscopy in a domain that is Fourier-conjugate to the spectral domain. The results of Figure 5 show that FTCRS, like other nonlinear spectroscopies employing interferometric schemes,"I0 also has potential in studies of dynamical processes. In effect, this potential exists because the Fourier transform of an FTCRS interferogram is related in a straightforward way to the Raman resonances of the sample (eq 4c and 5), and the Raman resonances contain information about dynamics (e.g., the r, in eq 6 ) . Given this, one notes that obtaining dynamical information with FTCRS is subject to some of the same problems that obtain in conventional spectral domain studies of dynamics (e.g., contributions from inhomogeneous broadening and pure dephasing). What is important is that FTCRS provides a means by which to obtain the information in situations where other methods may be difficult to apply, e.g., in studies of the ground-state dynamics of gaseous species in bulbs or in ultracold molecular beams. It has been pointed out in connection with eq 3, above, that there is a fundamental difference between nonlinear FTCRS and linear interferometric spectroscopies. Nevertheless, it is clear that there is also a fundamental similarity between the methods; each of the techniques makes use of a Michelson interferometer to obtain spectroscopic information in a domain that is Fourier conjugate to the spectral domain. In light of this similarity, we would make two points. First, much research has been aimed at characterizing and improving the spectral resolution, wavenumber accuracy, signal-to-noise ratio, instrumental distortions, etc., of linear interferometric methods.12 One expects that much of what has been learned through such work can be applied directly, or with minor modification, to the characterization and improvement of FTCRS. Second, since an interferometric version of Raman spectroscopy already exists (Le., FT-RamanI6), one might well question the need for FTCRS. To answer this, one notes that FTCRS is complementary to FT-Raman spectroscopy in roughly the same way that CRS is complementary to conventional Raman techniques.' For situations that require high spatial resolution,
5531
high spectral resolution, very efficient rejection of background light, high signal levels, and/or the study of Raman resonances with frequencies less than several hundred cm-l, FTCRS should be superior to FT-Raman spectroscopy. In closing, it is pertinent to point out that a consideration of the general basis underlying FTCRS (section 11) suggests the possibility of developing a variety of spectroscopic techniques based on nonlinear interferometry. One expects that if a nonlinear signal (1) depends on some product of laser spectral densities of the form [S1(wI)]"[S2(w2)]", and (2) there is some resonance condition(s) corresponding that allows only those products [S1(w1)]n[S2(02)]m to particular values of (ol- w2)to give rise to any signal, then by performing an interferometric experiment in which both lasers are directed through an interferometer before exciting the sample (Figure 1) one will obtain an interferogram that is modulated at the resonant difference frequencies of the sample. Indeed, results from this obtained by using an interferometric version of stimulated emission pumping spectro~copy,~' substantiate this expectation. Further work involving the application of FTCRS and other nonlinear interferometric techniques is in progress.
Acknowledgment. We thank Prof. M. A. El-Sayed for generous loans of equipment and Prof. P. Bernath for sending a copy of ref 12b to us. We also thank L. Connell, Dr. T. Corcoran, and B. Henson for their help. This work was supported, in part, by the donors of the Petroleum Research Fund, administered by the American Chemical Society, and by Research Corporation, du Pont de Nemours and Co., and the Academic Senate of UCLA. (26) (a) Felker, P. M.; Henson, B. F.; Corcoran, T. C.; Connell, L. L.; Hartland, G. V. Chem. Phys. Lett., submitted for publication. (b) Felker, P. M.; Hartland, G. V.; Connell, L. L.; Corcoran, T. C.; Henson, B. F. In Proceedings of the NATO Conference on Atomic and Molecular Processes with Short, Intense Laser Pulses, Bishop's University, Lennoxville (Quebec) Canada, July 20-24, 1987, to be published. (27) For example: Kittrell, C.; Abramson, E.; Kinsey, J. L.; McDonald, S.A,; Reisner, D. E.; Field, R. W.; Katayama, D. H. J . Chem. Phys. 1981, 75, 2056.
Acetic Acid Decomposition on Ni(100): Intermediate Adsorbate Structures by Reflection Infrared Spectroscopy Eric W. Scharpf and Jay B. Benziger* Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544 (Received: June 19, 1987)
Temperature-programmedreflection absorption infrared spectroscopy (TPRAIS),in concert with reflection absorption infrared spectroscopy (RAIS), was used to follow the sequence of stable surface intermediates for the decomposition of acetic acid monomer and dimer on Ni(100). After acetic acid monomer adsorbs molecularly at 170 K, the acid hydrogen is irreversibly lost at 240 K and a bridge-bonded acetate is formed. The bridge-bonded acetate then undergoes a reversible transformation to a monodentate acetate above 320 K which eventually decomposes to COz, C(ad), and H2at 435 K. Acetic acid dimer adsorbs molecularly at 170 K with the hydrogen-bonded ring approximately parallel to the surface. The dimer decomposes by dehydration at 255 K to adsorbed CO, a bridge-bonded acetate, and an adsorbed methyl group. The acetate decomposes to COz, C(ad), and H2 at 440 K. The key step in the acetate decompositionsis the C-C bond scission. The dynamic infrared study shows the importance of performing the spectroscopy at reaction conditionsto identify the stable molecular configurations involved during the reaction.
Introduction Recently, there has been some interest in nickel as a catalyst to carbonylate methanol to form acetic acid.'q2 The decomposition of acetic acid on nickel is related to the reverse of this carbony(1) Fujimoto, K.; Omata, K.; Shikada, T.; Taminaga, H. 0. P r e p . Am. Chem. Soc., Diu. Pet. Chem. 1986, 3 1 , 85. ( 2 ) Rizkalla, N . P r e p . A m . Chem. Soc., Diu. Pet. Chem. 1986, 3 1 , 79.
lation reaction. Decomposition of acetic acid on nickel surfaces has been studied with temPerature-Progra"ed reaction (TPR) using the (1 1 and (1 Planes. Recent work by Schoofs and Benziger3 indicated that acetic acid monomer reacts by dehydrogenation to adsorbed acetate with subsequent decomposition (3) Schoofs, G. R.; Benziger, J. B. Surf. Sci. 1984, 143, 359. (4) Madix, R. J.; Falconer, J. L.; Susko, A. M. Surf. Sci. 1976, 54, 6 .
0022-3654/87/2091-5531$01.50/0 0 1987 American Chemical Society
