Focus
Nonlinear Raman Spectroscopy In the past decade the laser has come to be applied more and more to analytical chemistry, and it has been exceptionally important in the development of modern Raman spectroscopy. At the Conference on Lasers as Reactanta and F’robes in Chemistry, held at Howard University in May, Alan Eckbreth of United Technologies Research Center presented a paper on the use of nonlinear Raman spectroscopy in analysis. A paper by Mary Wirth of the University of Wisconsin-Madison, on lasers in analytical chemistry, also touched on some of tbnonlinear Raman techniques. The introduction of the dye laser, which is tunable over a broad range, opened uj a number of exciting new chemical ap plications for the nonlinear techniques. “In several instances,” reported Eckbreth, “nonlinear Raman techniques are now supplanting longentrenched standard analytical approaches.” In conventional (spontaneous) Raman spectroscopy, a laser is f m s e on a sample, and the scattered radiation is collected at right angles to the incident beam.Scattered radiation may originate from elastic collisions between laser photons and sample molecules (Rayleigh scattering), or from inelastic collisions in which the photons couple to molecular vibrations (Ramanscattering), as shown in Figure 1. The Raman scattered radiation may either be red-shifted (Stokes) or blueshifted (anti-Stokes) from the original laser frequency. The frequency difference between these Raman-shifted photons and the incident laser photons corresponds to a vibrational and/ or rotational frequency of the molecule doing the scattering. Under normal conditions, more sample molecules exist in the ground vibrational state than in any of the excited vibrational states. Since an excited vibrational level must be populated for anti-Stokes Raman scattering to take 000927001821A35 1- 1021SO 1.0010 0 1982 A m a i m Chemical Socwly
Vibrational Levels, First Excited Electronic State
Vimd States
Vibrational Levels,
Ground Electronic State
I Flgure 1. Energy level diagram showing absorption of laser radiation (L), Rayieigh scattering (R). and Stokes (S) and anti-Stokes(AS) Raman scattering
place, the anti-Stokeslines are generally much weaker than the Stokes lines in the Raman spectrum. There are two serious problems with conventional Raman spectroscopy. First, there is the p w r efficiency of the Raman effect. Even in favorable cases, only about 10-8 of the incident laser photons are converted to Raman signal. Second, the quantum yield for the generation of fluorescence can be as high as unity for certain molecular systems, and a fluorescence signal of even much lower magnitude will swamp the weak Raman signal. Here is where the nonlinear techniques are advantageous, both because nonlinear
Raman spectroscopy is often more sensitive than conventional Raman spectroscopy and because the nonlinear techniques can often discriminate effectively against fluorescence. Some terminology relating to Raman phenomena is defined in the box, Raman Terminology, and energy level diagrams for some of the nonlinear techniques are seen in Figure 2. The nonlinear techniques include stimulated Raman scattering (SRS), hyper Raman,stimulated Raman gain (SRG), inverse Raman scattering (IRS), coherent anti-Stokes Raman spectroscopy (CARS), and coherent Stokes Raman spectroscopy (CSRS).
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9. AUGUST 1982 * 1021 A
(b) Stimulated Raman Xatterlng
(a) Spontaneous Raman
(e) Hyper Raman
(d) Stimulated Raman Gain
(e) Inverse Raman
(0)Coherent Stokes Raman
c
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Flgure 2. Quantum diagrams of the nonlinear Raman processes, with spontaneous Raman included as a reference.Levels shown are virtual states (------)and In the qound electronic state of the ground and excited vibrational levels (-) molecule. Ground vibrational levels are marked "G." For each Raman process shown, arrows entering at left are incident photons, up arrows represent photon absorption, down arrows represent photon generation, and arrows at rigM are output photons. Note conservation in the number of photons created and lost in each process (each photon is represented as an arrow). v I = laser frequency, us = Stokes frequency, .Y = anti-Stokes frequency, Y, = vibrational frequency. Figure courtesy of Alan Eckbreth lO22A
ANALYTICAL CHEMISTRY, VOL. 54. NO. 9. AUWST 1982
Stimulated Raman Scattering Conventional (spontaneous) Raman scattering, explained Eckhreth, is a linear process in which a small fraction of the incident photons exchanges quanta of vihrationdrotational energy with sample molecules and is incoherently scattered toStokes or antiStokes frequencies. This process is shown again in Figure 2a for reference. Stimulated Raman scattering (SRS, not to he confused with stimulated Raman gain, SRG), Figure 2b, is a nonlinear technique in which the spontaneous Raman radiation is amplified hy factors as high as IOi3 (I). The amplified radiation emerges as a coherent beam coincident with the direction of the incident laser radiation. The amplification process sets up intense molecular vibrations in the sample at a frequency corresponding to the strongest Raman transition for the molecule. Although SRS has not found great utility for chemical analysis, it has found some application in extending the tuning ranges of existing lasers. For example ( Z ) , the output of a dye laser can he focused into a cell containing hydrogen gas. Stimulated Raman scattering occurs in the cell, creating an intense beam of radiation shifted from the laser frequency hy the vibrational frequency of hydrogen.
Hyper Raman Hyper Raman scattering is shown in Figure 2c. Eckhreth explained that this phenomenon may he thought of as Raman scattering resulting from two incident laser photons. Unlike the other nonlinear phenomena he described, which are third-order processes, hyper Raman is a second-order process, arisiig from the quadratic electric field term for the laser, known as hyperpolarizahility. "The process is extremely weak and thus experimentally difficult," said Eckhreth. "Although more spectra are being reported, it is unlikely ever to be competitive with the coherent third-order processes."
Stimulated Raman GaidLoss Stimulated Raman gain (SRG) and inverse Raman scattering (IRS) are two recently developed nonlinear techniques of analytical interest. Since they are closely related, Eckhreth recommended that a more general nomenclature would he appropriatestimulated Raman gainfioss. But since the loss technique was disclosed first and named inuerse Raman scatterhg,
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this term persists in common usage. Essentially, the techniques are two sides of the same coin, one involving stimulated gain a t a Stokes-shifted frequency, the other involving stimulated loss a t an anti-Stokes-shifted frequency. SRG (Figure 2d), explained Ekkhreth, may be viewed as an induced emission process at the Stokes frequency (u,). This gain is realized by directing two lasers into the sample, a pump laser (ur) and a tunable probe laser. The intensity of either of these beams is monitored as the probe laser is scanned. When the frequency of the probe laser is scanned through us, a frequency Stokes-shifted from the pump, the molecule will vibrate at the frequency uU,where uU = ul - v. As a result of this vibration being driven, u, gains in intensity and ur decreases in intensity. In IRS (Figure 2e), on the other hand, the pump frequency a t vt experiences gain and the anti-Stokes-shift-
ed probe laser frequency loses intensity as the probe is scanned through a vibrational resonance. Stimulated loss occurs at the anti-Stokes frequency. The term inverse Raman refers to the fact that, a t resonance, the probe radiation is attenuated. In spontaneous Raman spectroscopy, of course, radiation at Raman-active frequencies would he generated in the course of the experiment. Both SRG and IRS are coherent processes. In a Raman experiment, the scattering arises from an oscillating polarization created by the interaction of the incident laser and sample molecules. In a coherent process, a phase relationship exists between the oscillating polarization in different parts of the sample (2).Since this polarization is driven by a coherent incident laser, the Raman output in such a process is also coherent and highly directional. This coherent Raman output beam can then he easily separated from in-
Raman Terminology In the past few years, a series of new Raman techniques has found broad analytical application.The terminology devised to describe these techniques, e.g., mnlinear, resoname enhancement.surface enhancement, and coherence, may at times seem inmutahle. The information below may help clarify the situation. Raman scattering arises from an oscillating polarization induced in the sample matrix hy the incident laser beam. Conventional Raman scattering is a linear process, originating from induced sample polarization dependent on the first power of the electric field strength of the incident beam. Other Raman techniques that depend on polarization induced hy second and higher order electric field strength terms are known as nonlinear techniques. These include stimulated Raman scattering(SRS), hyper Raman, coherent antidtokes Raman spectroscopy (CARS), coherent Stokes Raman spectroscopy (CSRS, pronounced “scissors”),stimulated Raman gain (SRG), inverse Raman scattering (IRS), photoacoustic Raman spectracopy (PARS), and Raman-induced Kerr effectspectroscopy (RIKES). The topic of resonance enhancement we8 d i d in detail in a 1979 REPORT in thin JOURNAL (Morris, Michael D.; W h , Dsvid J. Anal. Chem. 1979,51,18%92 A). Resonance enhancement m r s when the frequemyofthe incident beam corresponds
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the molde. The enhancedb a n sign& have intensities that are 102-106 times greater than normal Raman intensities. Thus, lower limits of detection are a characteristic of the resonance Raman techniques. Both linear and nonlinear Raman techniques can benefit from resonance signal enhancement, though operating in the resonance region may not be feasiblefor all samples under study. Another form of enhancement has been observed in the spectra of a variety of adsorbed species (such as pyridine and cyanide ions, CO, and benzoic acid) on several metal surfaces (such as Cu, Ag, and Au). Surface-enhanced Raman scattering (SEW),the technique based on this phenomenon, is currently attracting a great deal of interest among t h w concernedwith surface adsorption. In the analytical chemistry community there is particular interest in SERS among the electrochemis&, who are interested in a better understanding of events at electrode surfaces. Coherent Raman spectroscopy is a term that refers to a series of closely related nonlinear Raman techniques in which the scattered Raman radiation emerges from the sample as a coherent beam--coherent meaning that the photons are all in phase with one another. The coherent techniques include SRS,CARS, CSRS,SRG, and JRS. Although most of the nonlinear Raman techniques are also coherent techniques, +ere is one incoherent nonlinear Raman proeess, hy--D----
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST I982
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coherent emissions, such as fluorescence. As mentioned above, fluorescence radiation has long been a problem in conventional Raman spectrmopy. Extensive sample purification, addition of quenching agents, and time resolution of Raman signals from the much slower fluorescence emission have all been employed in an attempt to ameliorate the problem, but none of these approaches can match the excellent fluorescence rejection of the coherent Raman processes, including SRG, IF@, and CARS. This is particularly important for the analysis of compounds of biochemical or clinical interest, many of which have high fluorescence quantum yields. “In such systems,” explained Eckhieth, “spontaneous Raman scattering yields lowquality spectra a t best, whereas inverse Raman provides spectra of fairly high quality.” IRS is preferable to SRG for fluorescing samples, since the signal beam in IRS (u.) is upshifted away from intense fluorescence interferences.
CARSICSRS Of the various nonlinear Raman techniques, CARS has received the most attention for its applicability to chemical analysis. In CARS (Figure 20, two laser sources, a pump ( u t ) and a Stokes-shifted probe (us) are mixed and generate a coherent beam a t a third frequency, u3 = 2ut - v,. Remembering (see Figure 2a) that us = ut - uU,where u. is a vibrational resonance of the molecule, we can see that (2u, - u,) = 2ur - (u1 - U”) = ur + u, = .v Therefore, the frequency (v,) generated in the CARS process by the mixing of the pump frequency and the Stokes-shifted probe frequency is equal to the anti-Stokes frequency for that vibrational transition. Coherent Stokes Raman spectroscopy (CSRS, pronounced “scissors”) is a similar process, hut here Stokes radiation is generated by the mixing of a pump frequency and an antiStokes-shifted probe frequency (Figure 2g). In a CARS experiment, a varying third frequency (US = 2ur - v.,) generated by the mixing of the pump and probe frequencies appears continuously as the probe frequency is scanned (the probe is commonly a tunable dye laser). The power generated at this third frequency is generally weak, hut is greatly enhanced when the pump and probe frequencies differ by the energy of a vibrational transition (ur - us = u d , and only a t this point will u3 equal u..
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