Time-resolved CARS

Meyer S and Engel V 2000 Femtosecond time-resolved CARS and DFWM spectroscopy on gas-phase I, a ... 

B. Single-Beam Time-Resolved CARS Microspectroscopy.184... 

The coherent fs time-resolved CARS method is highly sensitive for the investigation of collision induced (or pressure dependent) changes in optical line shapes especially when line mixing occurs and frequency resolved measurements come to their limits [7]. The fs-CARS spectroscopy is applied to various collision systems (N2-N2, N2-rare gas, C2H2-C2H2, CO-CO)... 

After the introduction of frequency resolved CARS by Maker and Terhune [1], time resolved experiments became possible with the invention of high power lasers with femtosecond resolution. Leonhardt [2] and for example Hayden [3] performed femtosecond CARS experiments in liquids. A first femtosecond time resolved CARS experiment in gas phase was performed by Motzkus et. al. [4] where the wave packet dynamics of the dissociation of Nal was monitored. The first observation of wave packet dynamics in gaseous iodine was reported by Schmitt et al. [5]. They were able to observe dynamics in both, the ground and excited state with the same experiment. A summary of high resolution spectroscopy in gas phase by nonlinear methods is given by Lang et al. [6]. 

Beyond imaging, CARS microscopy offers the possibility for spatially resolved vibrational spectroscopy [16], providing a wealth of chemical and physical structure information of molecular specimens inside a sub-femtoliter probe volume. As such, multiplex CARS microspectroscopy allows the chemical identification of molecules on the basis of their characteristic Raman spectra and the extraction of their physical properties, e.g., their thermodynamic state. In the time domain, time-resolved CARS microscopy allows recording of ultrafast Raman free induction decays (RFIDs). CARS correlation spectroscopy can probe three-dimensional diffusion dynamics with chemical selectivity. We next discuss the basic principles and exemplifying applications of the different CARS microspectroscopies. 

While in the frequency domain all the spectroscopic information regarding vibrational frequencies and relaxation processes is obtained from the positions and widths of the Raman resonances, in the time domain this information is obtained from coherent oscillations and the decay of the time-dependent CARS signal, respectively. In principle, time- and frequency-domain experiments are related to each other by Fourier transform and carry the same information. However, in contrast to the driven motion of molecular vibrations in frequency-multiplexed CARS detection, time-resolved CARS allows recording the Raman free induction decay (RFID) with the decay time T2, i.e., the free evolution of the molecular system is observed. While the non-resonant contribution dephases instantaneously, the resonant contribution of RFID decays within hundreds of femtoseconds in the condensed phase. Time-resolved CARS with femtosecond excitation, therefore, allows the separation of nonresonant and vibrationally resonant signals [151]. 

The implementation of time-resolved CARS for microspectroscopy and its application for vibrational imaging based on RFID was first demonstrated by Volkmer et al. [64] using three incident pulses that are much shorter than the relevant material time scale. Here, a pair of temporally overlapped pump and Stokes femtosecond pulses was used to impulsively polarize the molecular vibrations in the sample. Impulsive excitation with a single ultrashort pulse is also possible provided that the spectral bandwidth of the pulse exceeds the Raman shift of the molecular vibration of interest [152]. The relaxation of the induced third-order nonlinear polarization is then probed by scattering of another pulse at a certain delay time, r. A measurement of the RFID consists of the CARS signal collected at a series of delay times. 

In conclusion, although the increased propensity for photodamage by femtosecond pulses and the requirement for an additional delayed laser pulse can be disadvantageous, time-resolved CARS microspectroscopy not only provides a means for efficient and complete nonresonant background suppression but also offers the prospect for monitoring ultrafast processes of molecular species inside a sub-femtoliter sample volume [64, 152-154]. 

Figure 3.6-10 Schematic diagram of a femtosecond time-resolved CARS apparatus. YAG, cw mode-locked Nd YAG laser ML, mode locker PL, polarizer A s, apertures LP, laser pot DM, dichroic mirror DLl, femtosecond dye laser SA, saturable absorber CLFB, cavity-length feedback system DL2, picosecond dye laser W, tuning wedge E, etalon FD, fixed delay VD, variable delay BS, beam splitter P s, half-wave plates (when necessary) F s, filters S, sample MC, monochromator PMT, cooled photomultiplier. (Okamoto and Yoshihara, 1990).

Since the pulse time is so short (see Sec. 3.6.2.2.3) one can coherently excite many vibrational modes at a time and monitor relaxation processes in real time. The first reported femtosecond time-resolved CARS experiments (Leonhardt et al., 1987 Zinth et al., 1988) showed beautiful beating patterns and fast decays of the coherent signal for several molecular liquids. The existence of an intermolecular coherence transfer effect was suggested from the analysis of the beating patterns (Rosker et al., 1986). Subsequent studies by Okamoto and Yoshihara (1990) include the vibrational dephasing of the 992 cm benzene mode. A fast dephasing process was found that is possibly related to... 

Another example of the observation of femtosecond time-resolved CARS is that of Inaba et al. (1993a) who studied the C=C stretching vibration of alkynes (monoalkyl-... 

In another study using femtosecond time-resolved CARS the same research group investigated the C=N stretching vibration of alkanenitriles (C H2n+]CN, n = 1-17) (Okamoto et al., 1993a). It was found that the vibrational dephasing rates (1 / T2) observed for the neat alkanenitriles are proportional to the square root of the number of carbon atoms (n) in the alkyl chain. 

Figure 1 shows the Uouville space Feynman diagram representing the time development of the intermolecular coherence between molecules at site ( and m in the time-resolved CARS process. The two upper lines show the time development of intramolecular coherence of molecules at (. The two lower lines show the time development of intramolecular coherence of molecules at m, respectively. The upper and lower lines are connected through the incident laser fields(l, II, and III) indicated by wavy lines. 

In the ordinary pump-probe type time-resolved CARS experiments, the time duration from t3 lo t4 corresponds to the pump-probe time T, and the intermolecular coherence time for pair molecules. In other words, it is the time dependence of the coherence between the Raman transition a c at site ( and a ->c at site m which are detected in the CARS time profile. 

Here T denotes temperature, and ke the Boltzmaim factor. This expression indicates that there are two decay components in the time-resolved CARS profile one is Gaussian and the other is exponential. The Gaussian decay component originates from dephasings of the off-diagonal terms of the CARS intensity, Eq.(22). 

Fig. 5.2 Energy diagrams of coherent anti-Stokes Raman scattering (CARS) and time-resolved CARS...

Fig. 5.9 (a) Anti-Stokes Raman spectrum of SWNTs excited by the 790-nm narrowband pulses, (b) Time-frequency 2D-CARS spectra of the SWNTs. (c) Time-resolved CARS spectrum of SWNTs at 0.84 ps [32]... 

Access to vibrational dephasing is also available via time-resolved CARS, " which is a pump-probe technique allowed by the development of mode-locked lasers. The correlation function... 

This expression describes the important case of an inhomogeneously broadened band with homogeneously broadened components. If the number of components is small or they are regularly spaced in frequency, a beat pattern for C(t) is obtained. Exjjerimentally, a time-resolved CARS experiment (which gives C(t) for t > tp) displays directly this oscillatory behavior, as was observed in an early study of (Fig. 5). The corresponding Raman... 

When the splitting between the different components (Oj becomes small with respect to their individual width, it becomes undetectable on the Raman profile, which can no longer be exploited without additional data. Correspondingly the time-resolved CARS exjjeriment would require an unreason-... 

Figures, (o) Splitting of Ajg band of CCI4 around 458 cm" by Cl isotopic components, (b) Corresponding time-resolved CARS signal is strongly modulated. (From Laubereau and co-workers. ° °)...

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