Femtosecond Stimulated Raman Spectroscopy

Femtosecond Stimulated Raman Spectroscopy

Femtosecond Stimulated Raman Spectroscopy DW McCamant, University of Rochester, Rochester, NY, United States ã 2017 Elsevier Ltd. All rights reserved...

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Femtosecond Stimulated Raman Spectroscopy DW McCamant, University of Rochester, Rochester, NY, United States ã 2017 Elsevier Ltd. All rights reserved.

Introduction Femtosecond stimulated Raman spectroscopy (FSRS) is a relatively new vibrational spectroscopy technique that offers the capability of collecting high frequency resolution (10 cm1) Raman spectra in time-resolved experiments that take advantage of the high time resolution of femtosecond transient absorption (30–150 fs, typically). Although the methodology is difficult to implement, requiring three different femtosecond to picosecond duration laser pulses with microjoule energies, it offers distinct advantages for time-resolved studies and so has grown in popularity since its initial introduction. A schematic of the technique is shown in Fig. 1. FSRS fits within the broad collection of time-resolved methodologies termed ‘pump-probe spectroscopy.’ Most pump-probe studies are transient absorption methods, in which a photochemical system is ‘pumped’ by a short laser pulse that initiates the photochemistry of interest. For transient absorption, the ‘probe’ is typically a similarly short duration laser pulse that probes the time-dependent absorption spectrum of the evolving photochemical system. However, in FSRS, the ‘probing’ event is actually two laser pulses that, together, take a Raman spectrum of the molecular system. The photochemical pump that initiates the photochemistry by exciting a molecule to its excited electronic state is termed the ‘actinic pump’ and the laser pulse that is the upward arrow of the Raman transition is termed the ‘Raman pump.’ Lastly, the ‘probe’ is the broadband laser pulse that is, effectively, the downward arrow of the Raman transition.

As in any Raman spectroscopy, the goal is to measure the frequency of the v ¼ n ! n þ 1 transition of the Raman-active vibrational modes in the system, where v is the vibrational quantum number of the normal mode. That vibrational frequency can then be related to structural details of the molecular system. Time resolution is obtained by varying the delay, Dt, between the actinic pump and the Raman pump/probe pair. By observing changes in the vibrational spectrum between different electronic states, one can deduce structural changes in the molecule that occur as photochemistry proceeds. To imagine how the Raman spectrum would evolve, we can model a generic photochemical system as shown in Fig. 2. There, we show an energy-level system consisting of an initially populated state, A0, with vibrational frequency of 1200 cm1 that corresponds to the A(v ¼ 0) to A(v ¼ 1) transition. State A0 rapidly converts to the v ¼ 1 level of the B electronic state, ‘B1,’ with rate constant kAB. Within the B-state manifold of energy levels, the vibrationally excited electronic state, B1, undergoes vibrational cooling to allow the system to relax to the B0 state with rate constant, k10. As shown in Fig. 2B, at early times, the FSRS spectrum will consist of just the peak associated with the A0 state. At longer time delays, however, vibrational peaks attributable to the B1 state will begin to grow in, which occur via the v ¼ 1 ! 2 transition at 1270 cm1 with the B state. The cooling process will be apparent in the loss of intensity at 1270 cm1 concurrent growth at 1290 cm1, the frequency of the v ¼ 0 ! 1 transition in B. Typically, vibrational cooling (flow of excess vibrational energy into the environment) or intramolecular vibrational relaxation (redistribution of excess

Fig. 1 (A) An energy level and (B) timing schematic for femtosecond stimulated Raman spectroscopy (FSRS). (A) shows an implementation for time-resolved spectroscopy in which the actinic pump pulse initially excites the photochemical system from the ground state, S0, to the excited electronic state, S1, which is subsequently evaluated by the Raman pump and probe pulses that induce the v ¼ 0 to v ¼ 1 vibrational transition in S1. In (B), time evolves from left to right and it is apparent that varying the time delay between the actinic pump and the Raman pump/probe pair will vary the time at which the Raman spectrum is collected and, hence, will obtain spectra that assess the dynamics of the excited system.

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vibrational energy around the excited molecule) is accompanied by a shift of vibrational modes to higher frequencies because of the usual signs of anharmonicities in vibrational potential energies. In real experiments, of course, one initially just observes the spectra as in Fig. 2B, from which one can extract the kinetics (Fig. 2C), and only after careful assignment of the spectrum and kinetic modeling can one put together the photochemical picture of the dynamics, as in Fig. 2A.

Making the Laser Pulses Fig. 3 shows a block diagram of a standard FSRS laser table, which can be coarsely divided into different regions that (1) generate the fundamental pulse train, (2) convert the fundamental laser pulse into the three necessary laser pulses for FSRS, (3) optically delay each pulse in order to set the relative time delays of the pulses, (4) focus the beams onto the sample,

Fig. 2 (A) Example photochemical system, with (B) resulting model FSRS spectra and (C) modeled kinetic traces extracted from the time-resolved FSRS spectra. In the model, the v ¼ 0 state within the A manifold converts to the v ¼ 1 level of the B state at rate kAB ¼ 1 ps1. The B, v ¼ 1 level then relaxes with a rate constant of k10 ¼ 0.33 ps1 to the B, v ¼ 0 state. The vibrational transition frequencies are A(0 ! 1) ¼ 1200 cm1, B(1 ! 2) ¼ 1270 cm1, and B(0 ! 1) ¼ 1290 cm1.

Fig. 3 Diagram of a typical laser and detection system used for FSRS.

Femtosecond Stimulated Raman Spectroscopy

and (5) disperse the probe, measure its spectrum, and transfer the spectrum to a computer. Nearly all FSRS systems built to date are founded upon a regeneratively amplified titanium– sapphire laser that generates 25–150 fs duration pulses centered at a wavelength of 800 nm and with a repetition rate of 1 kHz. These laser pulses have energies around 1 mJ/pulse, allowing facile frequency conversion using nonlinear optics. One alternative system used for surface-enhanced FSRS by Frontier and Van Duyne incorporates a 100 kHz repetition rate laser with roughly the same average power but much lower energy per pulse. This produces a vastly reduced peak laser intensity (peak intensity ¼ pulse energy/(beam area  pulse duration)) that is important to reduce photodamage of molecules near the enhancing surfaces. The fundamental laser is split into three components in order to produce the pulses necessary for FSRS. The actinic pump can be converted to 400 or 266 nm by second-harmonic generation (SHG) or third-harmonic generation (THG) of the 800 nm fundamental. To generate a tunable excitation pulse, an optical parametric amplifier (OPA) is required. Current state-of-the-art OPAs utilize a ‘non-collinear’ phase-matching geometry and are therefore termed non-collinear optical parametric amplifiers (‘NOPAs’). The value of the NOPA geometry is that it allows the amplification of a very wide bandwidth in the visible spectrum that can subsequently be compressed to very short pulse durations, in some cases below 10 fs, thereby improving the method’s time resolution. The Raman pump pulse is a much more narrowbandwidth pulse than the laser fundamental, requiring it to be a much longer duration pulse due to the time-spectrum Fourier transform limit. For Gaussian temporal envelope pulses, the transform limit dictates that the product of the pulse duration and its bandwidth (both measured as the full width at half maximum) be  14.7 ps cm1. Hence, if the experimentalist desires a laser pulse that has a bandwidth of 30 cm1, he or she needs the laser pulse to be at least 0.5 ps in duration. For FSRS, the bandwidth of the Raman pump pulse establishes the limit of the spectral resolution of the instrument, in conjunction with the resolution of the spectrograph. Since condensed-phase Raman transitions usually have linewidths of 10 cm1, FSRS requires a Raman pump bandwidth <10 cm1 in order to prevent broadening of the observed Raman peaks in the resultant spectrum. However, Raman pump pulses with bandwidths as large as 25 cm1 have been used successfully in the literature. The simplest way to convert the femtosecond fundamental to a transform-limited picosecond pulse is to pass it through a narrow band-pass color filter, grating filter, or a precision etalon, all of which generate picosecond pulses at 800 nm. If a near-ultraviolet Raman pump is required, the spectrally filtered 800 nm pulse can be frequency-doubled to 400 nm or the femtosecond fundamental can be frequency-doubled with a second-harmonic bandwidth compressor to generate a narrow-bandwidth, transform-limited picosecond pulse at 400 nm. A narrowbandwidth OPA can generate a wavelength-tunable Raman pump and there are several designs available to produce visible or near-UV picosecond pulses. The probe pulse needs to be extremely stable in order to produce a stable baseline in the FSRS spectra and its bandwidth needs to be sufficient to extend across the entire Stokes region,

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relative to the Raman pump wavelength. The useful fingerprint region for molecular spectroscopy extends from 300 cm1 to around 1800 cm1, so the probe needs to have a smooth and bright spectrum through the wavelengths that extend 300–1800 cm1 to the red of the Raman pump. This 1500 cm1 spectrum would correspond to a 10 fs pulse duration if transform-limited; however, in practice, the pulse need not be transform-limited to still provide adequate time resolution for the experiment. NOPAs have been used to generate extremely intense probe pulses, but the standard probe is a white-light laser pulse generated by focusing the femtosecond fundamental into a short crystal of sapphire, CaF2, or even a cuvette of water. The optical simplicity of white-light generation and its extremely broad spectral range make it a much easier method of probe generation. After the three pulses are generated, their optical delays need to be precisely controlled prior to their arrival at the sample point. Delays are set by corner-cube mirrors placed on translation stages that allow one to vary the pathlength each pulse travels to the sample without varying its alignment (see Fig. 3). The temporal and spatial overlaps of the pulses at the sample are established by nonlinear optical signals that are present only when two of the pulses are overlapped in time and space at the sample point. The optical Kerr effect and sum-frequency generation are the two primary methods used to establish overlap. FSRS is a transmissive technique, requiring the probe pulse to transmit through the sample as it experiences Raman amplification (vide infra). This makes the issues in beam focusing geometry and sample handling equivalent to those of femtosecond transient absorption. The three beams are focused onto the sample with one or more lenses or curved mirrors with focal lengths of 100–250 mm, producing focal spots with diameters from 20 to 200 mm. After the sample, the probe is picked off and sent to the spectrograph for dispersion and detection.

Measuring the Stimulated Raman Spectrum As shown in Fig. 3, the detection system in FSRS disperses the probe pulse into its component wavelengths and measures the intensity of each wavelength on a multichannel detector, such as a charge-coupled device or photodiode array. As in traditional Raman spectroscopy, the spectral resolution of FSRS depends on the ability of the spectrograph to adequately disperse the spectrum such that the bandwidth per pixel is <3 cm1. This ensures that standard condensed-phase Raman peaks, which are typically around 10 cm1 in width, can be resolved to define the line shape. For instance, when the Raman pump is around 800 nm, FSRS spectra have been collected with a 320 mm spectrograph containing a 600 gr mm1 grating that disperses the spectrum onto a 1024 pixel array with 25 mm wide pixels. When the Raman pump is much higher in energy (shorter wavelength), it is necessary to increase the dispersion to achieve the same resolution; when a 400 nm Raman pump requires collection of the probe pulse from approximately 400–450 nm, a 300 mm spectrograph will need a 1200 gr mm1 grating to achieve sufficient dispersion. Note that the light-collection efficiency of the spectrograph, characterized by its f# (f-number), is almost completely irrelevant for FSRS, because the coherent probe beam can easily be

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focused such that all of it is focused through slit onto the first mirror of the spectrograph. Although choosing a spectrograph with an appropriate f# is critical to maximize light-collection efficiency in spontaneous Raman spectroscopy, light-collection efficiency is not an issue in FSRS. The FSRS spectrum is actually the Raman ‘gain’ spectrum of the probe pulse induced by the Raman pump. That is, it is the spectrum of amplification of the probe that occurs due to the stimulated Raman action of the Raman pump on the probe. In older stimulated Raman implementations, a narrow probe or Raman pump wavelength would have been slowly scanned through a spectral region while the Raman pump beam was chopped at a particular modulation frequency. This allowed the measurement of the ‘gain’ spectrum, which was the output of a lock-in amplifier (instead of a multichannel detector), monitored as the wavelength was scanned; whenever a Raman resonance was hit, as in the energy-level diagram in Fig. 1A, the probe would be amplified and the pump attenuated. For FSRS, however, modern broadband femtosecond lasers allow the probe pulse to contain every possible downward arrow in Fig. 1A; any wavelength in the probe that matches up with a Raman resonance will be amplified and other wavelengths will not be affected. Hence, to measure the Raman pump-induced gain, one just disperses and measures the spectrum of the pulse with and without the Raman pump on the sample. Fig. 4 shows what the probe spectrum looks like with (red solid) and without (red dashed) the Raman pump on the sample. As the Raman pump is cycled on and off, the resulting probe spectrum is measured and sent to the data collection computer, which calculates the gain spectrum in software (black, Fig. 4.) Note that some FSRS labs present the gain spectrum directly as [(probeON/probeOFF)-1]; however, for the signal to be strictly proportional to concentration, a logarithm is necessary (as in absorption spectroscopy), making the FSRS spectrum log(probeON/probeOFF). In the small gain limit, these two calculated spectra are the same within a constant scaling factor and the FSRS spectrum is then directly

proportional to the normal spontaneous Raman cross sections of each vibrational mode. The black spectrum in Fig. 4 shows the Raman gain spectrum of the solvent, acetonitrile, with molecular formula H3CCN. The spectrum highlights the 920 cm1 C–C stretch, the 2250 cm1 C–N stretch, and the highfrequency totally symmetrical C–H stretch at 2942 cm1. At lower frequency, not shown in the figure, the C–CN bending mode appears at 379 cm1. With just a Raman pump and probe pulse, the detection system can measure what is often termed a broadband stimulated Raman spectrum (SRS, as opposed to FSRS). Since one is not using an actinic pump and, therefore, not probing timeresolved dynamics, this is akin to taking a ground-state Raman spectrum as in traditional spontaneous Raman scattering or older scanning wavelength stimulated Raman experiments. However, the broadband SRS methodology has a number of benefits compared to spontaneous Raman. In particular, SRS is insensitive to any fluorescence from the sample, which often makes spontaneous Raman impossible because the noise from the intense fluorescence can completely obscure the signal from the weak Raman scattering process. Instead, for SRS, the measured signal on the detector is the very intense spectrum of a white-light laser pulse (the probe) and so the relatively weak background fluorescence does not obscure the SRS signal. The detection of the intense probe pulse is what distinguishes SRS from spontaneous Raman spectroscopy because it switches the method from what is traditionally a low-light-level experiment, in which the signal-to-noise ratio (SNR) depends on one’s ability to detect a small number of scattered photons, to a high-light-level experiment, in which the SNR depends on one’s ability to measure a small change on top of an enormous background. In either case, however, the spectra tend to be shot-noise limited, such that the noise is almost entirely just the square root of the number of photoelectrons collected. Under such conditions, the SNR is improved by having the probe beam be as bright as possible on the detector.

Fig. 4 The FSRS spectrum is collected by toggling the Raman pump (green) on and off and measuring the Raman pump-induced gain in the probe spectrum (red). The red dashed spectrum on the left corresponds to the Raman pump-off probe spectrum and the red solid line is the Raman pump-on spectrum. The amplification at particular wavelengths occurs from the stimulated Raman effect when the frequency difference between a wavelength in the probe and that of the Raman pump matches a Raman-active vibrational frequency in the sample. The final spectrum is the logarithm of the ratio of the two probe spectra. The black spectrum shows the ground-state FSRS spectrum of acetonitrile obtained with a 14 mW, 3 ps Raman pump pulse, and signal averaging thirty 200 ms exposures of a diode array. Note that the Raman pump spectrum on the left is attenuated approximately 10,000 fold from its true intensity.

Femtosecond Stimulated Raman Spectroscopy

Time and Frequency Resolution There has been much debate about what determines the time and frequency resolution of FSRS. To be an ideal time-resolved vibrational spectroscopy, one would like to have a time resolution as short as possible (with the lower limit of 10 fs determined by the electronic dephasing time of the optical excitation process) while at the same time maintaining a spectral resolution such that the vibrational spectrum is not artificially broadened by the instrument response function. With the limits of condensed-phase vibrational line widths, this means that the spectral resolution should be better than (less than) 10 cm1. FSRS can achieve both these conditions although, in practice, it rarely does. Routine time resolution is typically around 50–100 fs, while spectral resolutions tend to be around 15 cm1. The time resolution is determined by the cross correlation between the actinic pump and the probe, that is, the convolution between their temporal pulse envelopes. The cross correlation measures the distribution of time delays between when molecules get excited by the actinic pump and when the vibrational transition is initiated. The spectral resolution of FSRS is determined by the convolution between the instrument response function of the spectrograph and the bandwidth of the Raman pump pulse, which together establish a lower limit to the observable Raman peak linewidths. A complication, and much of the debate in FSRS, centers around the definition of time resolution. Here, it is defined as the precision with which one can measure the delay between when the system is excited by the actinic pump and when the Raman transition is initiated by the Raman pump and probe together. Since any vibrational transition that produces a 10 cm1 homogeneous linewidth must necessarily occur with a dephasing time of 1 ps, the completion of the vibrational transition can actually take much longer than the ‘time resolution’ of the experiment. This is always the case for pulsed spectroscopies that can excite a transition between two states much faster than the decoherence time between those states (for instance, NMR). If there is rapid structural evolution of the molecule on a timescale that is faster than the vibrational dephasing time, then those various structures will be averaged together into one, potentially broad, peak in the spectrum. Hence, any time-resolved vibrational spectroscopy will blur spectral changes that occur faster than the natural dephasing time of the vibrational transition. To get around this problem in FSRS, one can gate the signal to be only generated for a short period during the vibrational free induction decay by choosing a Raman pump pulse duration that is short compared to the dephasing time; however, this will necessarily increase the spectral resolution of the instrument.

Applications FSRS has been used extensively to probe the ultrafast dynamics of light-absorbing compounds in photobiology. This includes

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early investigations of carotenoid dynamics and subsequent studies of the rhodopsin pigments that are responsible for mammalian vision and bacterial photosynthesis. More recently, the molecular isomerization in photoactive yellow pigment and phytochrome has been probed, as has the photochemical dynamics of flavin and the ultrafast proton transfer in green fluorescent protein. A huge array of photochemical systems have also been probed with FSRS, and their number is well beyond the scope of this article. However, some highlights include a study of the isomerization of stilbene with extraordinary time resolution and spectral sensitivity, a detailed analysis of vibrational anharmonicity that influences the time-dependent spectra during photoinduced charge-transfer in dimethylaminobenzonitrile, and the study of dye sensitizers and polymers whose ultrafast dynamics determine the efficiency of the next-generation solar photovoltaics.

See also: AFM and Raman Spectroscopy, Applications in Cellular Imaging and Assays; ATR and Reflectance IR Spectroscopy, Applications; Forensic Science, Applications of IR Spectroscopy; Forensic Science, Applications of Raman Spectroscopy to Fiber Analysis; FT-IR and Raman Spectroscopies, Polymorphism Applications; FTIR Spectroscopy of Aqueous Solutions; FT-Raman Spectroscopy, Applications; Gas Phase Raman Scattering: Methods and Applications in the Energy Industry; High Resolution Gas Phase IR Spectroscopy Applications; High Resolution Gas Phase IR Spectroscopy Instrumentation; Infrared and Raman Spectroscopy of Minerals and Inorganic Materials; IR and Raman Spectroscopies, Matrix Isolation Studies; IR and Raman Spectroscopies of Inorganic, Coordination and Organometallic Compounds; IR and Raman Spectroscopies, Polymer Applications; IR and Raman Spectroscopies, Studies of Hydrogen Bonding and Other Physicochemical Interactions; IR and Raman Spectroscopies, The Study of Art Works; IR and Raman Spectroscopy, Industrial Applications; IR Spectroscopy Sample Preparation Methods; IR Spectroscopy, Soil Analysis Applications; IR Spectroscopy, Surface Studies; IR Spectroscopy, Theory; NIR FT-Raman; Nonlinear Raman Spectroscopy, Applications; Nonlinear Raman Spectroscopy, Instruments; Nonlinear Raman Spectroscopy, Theory; Protein Structure Analysis by CD, FTIR, and Raman Spectroscopies; Raman and Infrared Microspectroscopy; Raman Optical Activity, Applications; Raman Optical Activity, Macromolecule and Biological Molecule Applications; Raman Optical Activity, Small Molecule Applications; Raman Optical Activity, Spectrometers; Raman Optical Activity, Theory; Raman Spectrometers; Raman Spectroscopy, Biochemical Applications; Raman Spectroscopy, Medical Applications: A New Look Inside Human Body With Raman Imaging; Raman Spectroscopy, Soil Analysis Applications; Rayleigh Scattering and Raman Effect, Theory; Resonance Raman Applications; Spatially Offset Raman Spectroscopy; SurfaceEnhanced Raman Optical Activity (SEROA); Surface-Enhanced Raman Scattering (SERS), Applications; Surface-Enhanced Raman Scattering (SERS) Biochemical Applications; Time-Resolved Raman Spectroscopy; Transmission Raman: Methods and Applications; Vibrational, Rotational and Raman Spectroscopy, Historical Perspective.

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Further Reading Cerullo G and De Silvestri S (2003) Rev. Sci. Instrum. 74: 1–18. http://dx.doi.org/ 10.1063/1.1523642. Kovalenko SA, Dobryakov AL, and Ernsting NP (2011) Rev. Sci. Instrum. 82: 063102–063109. http://dx.doi.org/10.1063/1.4891766. Kukura P, McCamant DW, Yoon S, Wandschneider DB, and Mathies RA (2005) Science 310: 1006–1009. http://dx.doi.org/10.1126/science.1118379. Kukura P, McCamant DW, and Mathies RA (2007) Annu. Rev. Phys. Chem. 58: 461–488. http://dx.doi.org/10.1146/annurev.physchem.58.032806.104456. Levenson MD and Kano SS (1988) Introduction to Nonlinear Laser Spectroscopy. San Diego, CA: Academic Press. McCamant DW, Kukura P, Yoon S, and Mathies RA (2004) Rev. Sci. Instrum. 75: 4971–4980. http://dx.doi.org/10.1063/1.1807566.

Megerle U, Pugliesi I, Schriever C, Sailer C, and Riedle E (2009) Appl. Phys. B Lasers Opt. 96: 215–231. http://dx.doi.org/10.1007/s00340-009-3610-0. Nakamura R, Hamada N, Abe K, and Yoshizawa M (2012) J. Phys. Chem. B 116: 14768–14775. http://dx.doi.org/10.1021/jp308433a. Owyoung A, Patterson CW, and McDowell RS (1978) Chem. Phys. Lett. 59: 156–162. http://dx.doi.org/10.1016/0009-2614(78)85638-3. Pontecorvo E, Kapetanaki SM, Badioli M, et al. (2011) Opt. Express 19: 1107–1112. http://dx.doi.org/10.1364/OE.19.001107. Rhinehart JM, Challa JR, and McCamant DW (2012) J. Phys. Chem. B 116: 10522–10534. http://dx.doi.org/10.1021/jp3020645. Weigel A and Ernsting NP (2010) J. Phys. Chem. B 114: 7879–7893. http://dx.doi.org/ 10.1021/jp100181z. Zhu L, Liu W, and Fang CA (2014) Appl. Phys. Lett. 105: 041106. http://dx.doi.org/ 10.1063/1.4891766.