J. Phys. Chem. 1988,92, 6877-6880
6877
Methods for the High-Resolution Raman Spectroscopy of Seeded Molecular Beams: Interferometry Applied to Ionization-Detected Stimulated Raman Spectroscopy G. V. Hartland, B. F. Henson, L. L. Connell, T. C. Corcoran, and P. M. Felker* Department of Chemistry, University of California, Los Angeles, California 90024- 1569 (Received: August 10, 1988; In Final Form: October 10, 1988)
Experimental and theoretical results are presented that demonstrate an interferometricversion of ionization-detectedstimulated Raman spectroscopy. It is shown that the technique can be used to do high-resolution Raman spectroscopy with broad-band laser sources on molecular beam samples. In addition we show that ion-dip Raman spectroscopy can be implemented interferometrically.
Introduction In recent years great progress has been made in the highresolution ground-state spectroscopy of isolated molecules and molecular complexes. Powerful microwave,’S2 infrared,’S2 and one-photon resonant optical methods,’ used in conjunction with seeded supersonic molecular beams, have been largely responsible for such progress. Compared to these types of methods, however, high-resolution Raman spectroscopies have been applied much less extensively to molecular beam samples. This relative lack of Raman experiments exists despite compelling general reasons4 for undertaking such studies. For example, (1) the information available from Raman spectra is often different and complementary to that available from other spectroscopic techniques, (2) all molecular species have Raman-active transitions, and (3) Raman spectroscopy can be implemented with light sources operating in the visible portion of the spectrum. The scarcity of high-resolution Raman studies of species in cold molecular beams can be attributed to two factors. The first is sensitivity. The second relates to the complexity of those Raman schemes that do have sufficient sensitivity. Sensitivity considerations preclude the general application of high-resolution spontaneous Raman methods to molecular beam sample^;^ such methods are difficult enough to apply to gaseous samples at pressures of several Torr. This lack of sensitivity in spontaneous Raman techniques implies that, if any schemes are to be generally applicable in molecular beam studies, then they will be nonlinear Raman spectroscopies. However, the best known and most widely applied nonlinear Raman method, coherent antiStokes Raman spectroscopy (CARS), also suffers from a lack of sensitivity due to the dependence of the CARS signal intensity on the square of the density of the Raman-active Recently, nonlinear Raman techniques with very high sensitivities, high enough to suggest their applicability in a wide variety
of molecular beam studies, have been demonstrated.”1° Two such methods are ionization-detected stimulated Raman spectroscopy (IDSRS)9 and fluorescence-detected stimulated Raman spectroscopy (FDSRS).’O These methods are double-resonance techniques (see Figure la) in which the first resonant transition is a two-photon-stimulated Raman process and the second involves the vibronic excitation of those species that undergo the stimulated Raman transition. The number of molecules that have undergone stimulated Raman transitions is determined by photoionizing the vibronically excited species in IDSRS or by collecting the fluorescence from these species in FDSRS. The principal factor limiting the application of IDSRS and FDSRS to high-resolution experiments in molecular beams is the complexity of the laser apparatus required. One must have light sources to provide the wl, w2, and w3 photons in the schemes (Figure la). More important, the light sources providing w1 and w2 must have high peak powers to drive the Raman process efficiently and narrow bandwidths to allow for the desired spectral resolution. Recently, well and other^^^-'^ have shown that high-resolution nonlinear (multiphoton) spectroscopies can be performed with broad-band light sources if one makes use of interferometric techniques. Such “nonlinear interferometric” schemes employ a Michelson interferometer to obtain spectroscopic information in a domain Fourier-conjugate to the spectral domain. The resulting “interferogram” is then Fourier-transformed to yield a spectrum. We have shown” that, if certain conditions hold, then this spectrum is just what would be measured in the corresponding spectral domain experiment, except that the resolution is determined by the interferometer delay range, not by the bandwidths of the lasers employed. Our previous analyses of interferometry applied to CARS1lb and stimulated emission spectroscopylld suggest that IDSRS and FDSRS also can be implemented interferometrically. Such Fourier-transform versions of these
(1) For a review, see: Legon, A. C.; Millen, D. J. Chem. Rev. 1986,86, 635. (2)
See,for example: Structure and Dynamics of Weakly Bound Com-
plexes; Weber, A., Ed.; Reidel: Dordrecht, 1987. (3) For example: (a) Phillips, L. A.; Levy, D. H. J. Chem. Phys. 1988, 89, 85 and references therein. (b) van Herpen, W. M.; Meerts, W. L.; Dymanus, A. J . Chem. Phys. 1987, 87, 182 and references therein. (4) Long, D. A. Raman Spectroscopy; McGraw-Hill: New York, 1977. (5) Medium-resolution spontaneous Raman studies can be performed in selected cases. For example, see: (a) Godfried, H. P.; Silvera, I. F. Phys. Rev. A 1983, 27, 3008. (b) Godfried, H. P.; Silvera, I. F . Phys. Reu. A 1983, 27, 3019. (6) High sensitivity can be attained, however, by using resonance-enhanced CARS. For example, see: Weber, P. M.; Rice, S.A. J. Chem. Phys. 1988, 88, 6107. (7) Although some CARS experiments without resonance-enhancement have been performed in molecular beams, these experiments have been limited to beams of small gaseous molecules. For example, see: (a) Gustafson, E. K.;McDaniel, J. C.; Byer, R. L. Opt. Lett. 1982, 7, 434. (b) Konig, F.; Oesterlin, P.; Byer, R. L.Chem. Phys. Lett. 1982,88,477. (c) Huber-Walchli, P.;Nibler, J. W .J. Chem. Phys. 1982,76,273. (d) Maroncelli, M.; Hopkins, G. A.; Nibler, J. W.; Dyke, T. R. J. Chem. Phys. 1985,83, 2129. (e) Murphy, D. V.; Long, M. B.; Chang, R. K.; Eckbreth, A. C. Opt. Lett. 1979, 4, 167.
0022-3654/88/2092-6877$01 SO10
(8) For a recent review article see: Nibler, J. W.; Yang, J. J. Annu. Reu. Phys. Chem. 1987, 38, 349. (9) (a) Esherick, P.; Owyoung, A. Chem. Phys. Lett. 1983,103,235. (b) Esherick, P.; Owyoung, A.; Pliva, J. J . Chem. Phys. 1985, 83, 3311. (10) (a) King, D. A.; Haines, R.; Isenor, N. R.; Orr, B. J. Opt. Lett. 1983, 8, 629. (b) Weber, P. M.; Rice, S. A. J. Chem. Phys. 1988, 88, 6120. (11) (a) Felker, P. M.; Hartland, G. V. Chem. Phys. Lett. 1987, 134, 503. (b) Hartland, G. V.; Felker, P. M. J. Phys. Chem. 1987,91,5527. (c) Felker, P. M.; Hartland, G. V.; Connell, L. L.; Corcoran, T. C.; Henson, B. F. In Proceedings of the NATO Conference on the Applications of Short Intense Laser Pulses; Bishop’s University: Quebec, Canada, 1987. (d) Felker, P. M.; Henson, B. F.; Corcoran, T. C.; Connell, L. L.; Hartland, G. V. Chem. Phys. Lett. 1987, 142, 439. (e) Corcoran, T. C.; Connell, L. L.; Hartland, G. V.; Henson, B. F.; Felker, P. M. Chem. Phys. Lett. 1988, 147, 517. (12) (a) Beach, R.; Debeer, D.; Hartmann, S.R. Phys. Reu. A 1985,32, 3467. (b) DeBeer, D.; Van Wagenen, L. G.; Beach, R.; Hartmann, S.R. Phys. Reu. Lett. 1986, 56, 1128. (13) Golub, J. E.; Mossberg, T. W. In Ultrafast Phenomena; Fleming, G. R., Siegman, A. E., Eds.; Springer: Berlin, 1986; Vol. 5. (14) Dugan, M. A.; Melinger, J. S.;Albrecht, A. C. Chem. Phys. Lett. 1988, 147, 411. (15) Van Exter, M.; Lagendijk, A. Opt. Commun. 1985, 56, 191.
0 1988 American Chemical Society
6878 The Journal of Physical Chemistry, Vol. 92, No. 24, 1988 (a)
(b)
Ionization Detection
Ionization Detection
4
4
w3
Fluorescence Detection If> li>
I
If> li>
Letters Stokes lasers, respectively, after the interferometer; T is the delay between the two arms of the interferometer; and the sum extends over all combinations of initial states, li), and final states, If), that are linked by Raman transition moments. S I ( u l , ~and ) S2(w2,7) can be written asI9
spectroscopies would have the advantage over their spectral domain counterparts of significantly decreased experimental complexity, while they would retain the high sensitivity of the spectral domain schemes. In this Letter, we present a theoretical treatment and experimental results on jet-cooled aniline that demonstrate an interferometric (Fourier transform) version of IDSRS. We show, in particular, that if (1) the pump ( q )and Stokes (az)fields in the IDSRS scheme pass (propagating parallel to each other) through a Michelson interferometer prior to the sample, (2) the output of the interferometer is mixed with the probe ( w 3 ) field a t the sample, and (3) photoions are detected as a function of interferometer delay, then the Fourier transform of the resulting interferogram is the portion of the IDSR spectrum of the sample effectively overlapped by the bandwidths of the light sources. The results prove that high-resolution Raman spectroscopy can be performed on sparse molecular beam samples with relatively easy-to-use, broad-band, high-power light sources. In addition, we present results that show that the ion-dip Raman spectroscopy scheme of Bronner et a1.I6 can be implemented interferometrically. (In ion-dip Raman spectroscopy resonant photoionization is used to monitor the loss of population from the initial state in a stimulated Raman transition (see Figure lb).) Theoretical Section To describe IDSRS we will assume that all the molecules excited via the stimulated Raman process can be ionized with equal efficiency. The IDSRS signal is then given by the increase in population of the excited vibrational state if) (see Figure la), which can be found by using second-order perturbation theory.” If the lasers are treated as multimode chaotic light sources,18the time-averaged IDSRS signal arising from Raman transitions originating in the manifold of states li) and ending in the manifold of states If) can be written as
= fl(Wl)(l
f
cos
(017))
(2)
Sz(%T)
= fi(wz)(l
f
cos (w))
(3)
wherefl(wl) and f 2 ( w z )are the spectral profiles of the pump and Stokes lasers prior to the interferometer. Hence, eq 1 becomes zIDSRS(T)
Figure 1. (a) Level diagram depicting ionization-detectedstimulated Raman spectroscopy (IDSRS) and fluorescence-detected stimulated Raman spectroscopy (FDSRS). (b) Level diagram depicting ion-dip Raman spectroscopy. In both (a) and (b) wlis the pump laser frequency, w2 is the Stokes laser frequency, and w j is the probe laser frequency. li) is the initial level and If) the final level of the Raman transition.
Sl(Ul,T)
= J+-dUi -m J+md02fi(wi) -m fz(wz) x
where 0.f. represents optical frequency terms, which can be disregarded.” Fourier transforming eq 4 produces a spectrum composed of a zero-frequency component (which is of no consequence) and a component that contains information about the Raman spectrum of the sample. This latter component is given by
where Cis a constant. Equation 5 shows that a Fourier-transform IDSRS experiment yields the portion of the Raman spectrum effectively spanned by the bandwidths of the laser sources (the integral in eq 5 defines a spectral window function for the apparatus). Moreover, the resolution in this spectrum is not affected by the laser bandwidths. Instead, in actual Fourier transform IDSRS experiments the resolution will be limited by the range over which the interferometer is scanned. In ion-dip Raman spectroscopy the stimulated Raman process gives rise to a decrease in the ion signal. The ion signal detected as a function of delay is given by a constant term minus a term proportional to ZIDS=(T) Z ( T ) = K - K’IIDsRs(T)
(6)
where K and K‘ are constants. Equation 6 indicates that the information obtained in a Fourier-transform ion-dip Raman spectroscopy experiment is the same as that from IDSRS, with the phases of the cosine modulations in the respective interferograms reversed.
(16) Bronner, W.; Oesterlin, P.; Schellhorn, M. Appl. Phys. E 1984, 34,
Experimental Section The second harmonic of a Spectra-Physics DCR-2(A) Nd:YAG laser (10 Hz repetition rate, 7 ns pulse width) was used to pump two Spectra-Physics PDL-2 dye lasers. Depending on the experiment being performed, the dye lasers were operated with their diffraction gratings in either first or fifth order, yielding bandwidths of about 3 and 0.3 cm-I, respectively. The parallel-polarized outputs of the two dye lasers provided the pump ( q )and Stokes (wz) fields for the stimulated Raman scheme. Part of the output of the Stokes laser was frequency doubled with KDP to provide the photoionization probe field (03= 2 4 . The probe light was separated from the undoubled Stokes light with a dichroic beam splitter. The pump and Stokes beams were directed collinearly through a Michelson interferometer, the details of which have been described elsewhere.lIe The output of the interferometer was combined collinearly with the probe beam, which was delayed by approximately 15 ns. (The delay of the probe pulse relative to the pump and Stokes pulses eliminated the occurrence of undesirable multiphoton processes dependent on the temporal overlap of the pulses.) The resulting three-color beam was directed through a 20 cm focal length lens and oriented perpendicular to the axis of the supersonic molecular beam.
(17) Shen, Y. R. The Principles of Nonlinear Optics; Wiley: New York, 1984. (18) (a) Grutter, A. A.; Weber, H. P.; Dandliker, R. Phys. Rev. 1969,185, 629. (b) Hall, R. J.; Greenhalgh, D. A. J . Opt. SOC.Am. E 1986, 3, 1637.
(19) (a) Bell, R. J. Introductory Fourier Transform Spectroscopy: Academic: New York, 1972. (b) Brault, J. W. Philos. Trans. R . SOC.London Ser. A 1982, 307(1500), 503.
where us is proportional to the spontaneous Raman cross section; ws is the frequency corresponding to the energy difference between the states ti) and If); pi and pf are the densities of molecules in li) and If); ys is a line-width parameter for the If) li) transition; S l ( ~ land , ~ S2(w2,7) ) are the spectral densities of the pump and
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11.
The Journal of Physical Chemistry, Vol. 92, No. 24, 1988 6879
Letters
0 )
.
,
.
I
. , . , . , . I 420
422
424
426
Frequency ( cm-')
5.5
:
d
.E? 420
422
-
424
5.0
426
-1
Frequency ( cm ) number values are corrected to vacuum. (a) Spectrum obtained by spectral domain IDSRS. The-resolutionof the spectrum is approximately 0.4 cm-I and reflects the bandwidths (about 0.3 cm-I) of the lasers used to obtain it. (b) Spectrum obtained by Fourier transform-IDSRS. The resolution of about 0.06 cm-l reflects the length of the interferometer scan in the experiment (20 cm) not the laser bandwidths, which were about 3 cm-l fwhm. The continuous SUpeMfic molecular beam was formed by freely expanding a room temperature mixture of aniline (0.5 Torr) and N2 (1 atm) through a 100 pm diameter pinhole into a vacuum chamber maintained at lo4 Torr by a 10 in. diameter diffusion pump. Ions produced by photoionization were detected by means of a parallel-plate electrode arrangement in which two copper electrodes separated by 4 cm straddled both the molecular beam axis and the axis of the excitation light. A potential of +40 V applied to one of the electrodes deflected photoions to the other. The resulting charge was converted to a voltage pulse by termination in 1 M Q at the input of a fast operational amplifier. The output of the amplifier was processed with a boxcar integrator and personal computer. To assess molecular beam conditions ionization action spectra were recorded by using the probe light alone and scanning the frequency of this light while monitoring the ion signal. Such spectra agreed well with the results of Powers et al.*O Spectral domain IDSRS experiments were performed by first setting the probe laser to a hot-band vibronic transition originating from the final vibrational state of the Raman transition to be studied (see Figure la) and then monitoring the ion signal as a function of pump laser frequency. Interferometric IDSRS experiments were performed by setting the laser frequencies to produce the largest IDSRS signal and then recording the ion signal as a function of interferometer delay (with the usual constraints on sampling).19 Interferograms so obtained were Fourier-transformed by using the Cooley-Tukey a1g0rithm.I~ Spectral domain and interferometric ion-dip Raman spectroscopy experiments were performed in the same way as the IDSRS experiments except that (1) the probe frequency was tuned such that molecules in the initial (lower energy) vibrational level of the Raman transition were ionized (Figure lb), and (2) the two-color (wl,w 2 ) beam was combiged at the sample with a separately focused, counterpropagating probe beam so as to maximize the overlap of focal volumes. Aniline was obtained from Fisher Scientific ("Certified ACS" grade) and was used without further purification. (20) Powers, D.E.; Hopkins, J. B.;Smalley, R. E. J. Chem. phys. 1980,. 72, 5721.
1
0
Figure 2. Spectra of the I, Oo Raman band of aniline obtained by IDSRS. For both spectra the ordinate units are arbitrary and wave-
2
-
Relative Delay ( psec ) Figure 3. (a) Spectrum of the 1, Oo Raman band of aniline obtained by spectral domain ion-dip Raman spectroscopy. The resolution of the spectrum reflects the bandwidths of the lasers used (about 0.3 cm-I). (b) Ion signal vs interferometerdelay obtained by setting the laser frequen-
-
cies to give the maximal ion-dip Raman spectroscopy depletion of ion signal for the I, Oo Raman band of aniline; Le., the laser frequencies were set to the dip in (a). The clear modulations of the signal with delay match the frequency of the Raman transition. For both (a) and (b) 95% of the ion signal was offset by detection electronics, and the units for the ordinate are arbitrary. Rmdts
Figure 2 shows IDSRS spectra of the Raman band of aniline originating in the ground-state vibrationless level (Le., the Oo level) and ending in the doubly excited level of the inversion mode (the Z2 level). For both of the spectra the ionization probe process proceeded resonantly via the SI So P2(6a)'o vibronic transition of the molecule at about 34 100 cm-'. Figure 2a shows the results of a spectral domain IDSRS experiment in which the dye lasers were operated with their gratings in fifth order. The width of the strong Q-branch feature in the spectrum reflects the resolution limitations imposed by the laser bandwidths. The peak position quoted in the figure was calculated directly from the uncalibrated wavelength indicators of the dye lasers and must be considered to be only a rough indication of the true position. The spectrum in Figure 2b is the Fourier spectrum obtained from an interferometric IDSRS experiment in which the dye laser bandwidths were about 3 cm-' and the interferogram was collected with nearly minimal sampling over a delay range of 20 cm (0.67 ns). In contrast to Figure 2a the spectral resolution in Figure 2b is not limited by the laser bandwidths but by the delay range over which the interferometer was scanned. Furthermore, the absolute line position accuracy quoted in Figure 2b is high (one part in IO4, as estimated from our uncertainty in measuring the interferometer delay) and reflects the inherent accuracy of interferometric schemes in obtaining such inf~rmation.'~ Figure 3 shows results obtained from ion-dip Raman spectroscopy experiments on the Z2 Oo Raman transition of aniline. For these experiments, in which one expects the stimulated Raman process to produce a decrease in the detected ion signal, the ionization probe process proceeded resonantly through the SI SoOoo vibronic transition of aniline. For both parts of Figure 3, 95% of the ion signal was offset with detection electronics. Figure 3a shows the results of a spectral domain ion-dip Raman spectroscopy experiment. The maximal signal depletion in the spectrum is about 1% (after allowing for dc offset). As with the
-
-
+
6880 The Journal of Physical Chemistry, Vol. 92, No. 24, 1988
spectral domain IDSRS results, the spectral resolution in Figure 3a is limited by the laser bandwidths, and the quoted line position is an uncalibrated result. Figure 3b is a signal vs delay trace (interferogram) that demonstrates that ion-dip Raman spectroscopy can be implemented interferometrically. One notes, in particular, that the interferogram is modulated and that the modulation frequency is consistent with the frequency of the Raman band.
Discussion The spectrum of Figure 2b clearly demonstrates the viability of the Fourier transform-IDSRS scheme for performing highresolution Raman studies on molecular beam samples. Significantly, these results were obtained with a rather rudimentary experimental apparatus. A number of straightforward modifications can be expected to yield improved sensitivity, spectral resolution, and signal-to-noise ratios. Such modifications include the utilization of (1) mass spectrometric analysis to discriminate against background photoions, (2) ion detectors with single ion sensitivities, (3) heatable, pulsed molecular beam sources, (4) higher repetition rate lasers, (5) correction of signal levels for laser power fluctuations, and (6) a more sophisticated interferometer with a scanning range sufficient to allow for resolution on the order of cm-'. As we have implied above, a principal advantage of Fourier transform-IDSRS over the corresponding spectral domain method is the comparative ease with which it can be implemented. Of course, this advantage is very much dependent on the spectral resolution that one wishes to obtain. If one only needs resolution of about 0.1 cm-' or worse, then spectral domain methods probably are easiest to use. If megahertz resolution is required, then the interferometric scheme suffers from the need for extremely long scans of the interferometer delay, and spectral domain techniques, again, may be more applicable. It is in the range of resolution from 0.1 to 0.001 cm-I that one expects Fourier transform-IDSRS to greatly facilitate Raman studies of sparse samples. A second, highly advantageous feature of Fourier transformIDSRS pertains to the accurate line position measurements that the method, like other interferometric techniques, can readily provide.lg For example, the line position in Figure 2b is quoted to about one part in lo4. With He-Ne laser fringe counting as a means of measuring the interferometer delay, instead of the slide potentiometer device that we currently use for this purpose, one can reasonably expect at least an order of magnitude improvement
Letters in this accuracy. Thus, Fourier transform-IDSRS should prove to be a very productive means by which to obtain accurate measurements of ground-state vibrational intervals. Compared to IDSRS, the ion-dip Raman spectroscopy schemes reported on here give rise to considerably worse signal-to-noise ratios. The pertinent difference between the two types of techniques is that IDSRS is essentially a zero background method, while the dc background in ion-dip Raman spectroscopy is quite large compared to the signal of interest. Page et aLzl in their infrared-ptical double-resonance experiments have demonstrated a means by which the noise associated with a large background ionization signal can be reduced to acceptable levels. The application of an analogous scheme in ion-dip Raman spectroscopy is both possible and desirable. With acceptable noise levels ion-dip Raman spectroscopy schemes could prove to be more useful than IDSRS methods. Since in ion-dip Raman spectroscopy it is the population of the initial Raman state that is monitored, a complete Raman spectrum can be obtained without changing the probe (03) frequency. Furthermore, an appropriate probe frequency for a given sample can be readily found simply by measuring a photoionization excitation spectrum. This situation is to be contrasted with IDSRS, in which a search for an appropriate probe frequency must be undertaken for each vibrational Raman transition to be studied. In closing, we note that the close similarity between IDSRS and FDSRS strongly suggests that if IDSRS can be implemented interferometrically, then FDSRS can also. In fact, recent results from this laboratory on jet-cooled t-stilbene bear out this conjecture. Taken together, the variety of nonlinear interferometric techniques thus far developed represent a complementary array of methods applicable to the high-resolution ground-state spectroscopy of isolated species. Acknowledgment. We are pleased to acknowledge T. C. Curtiss for his advice on ion detection electronics and V. Venturo for his help on some of the experiments. This work was supported by a National Science Foundation Presidential Young Investigator Award to P M F (Grant CHE 86-57592), and by grants from the donors of the Petroleum Research Fund, administered by the American Chemical Society, Newport Corporation, and the UCLA Academic Senate. (21) Page, R.H.; Shen, Y . R.;Lee, Y . T.J . Chem. Phys. 1988,88,4621.
