Femtosecond time-resolved CARS

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

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]. 

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. 

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]. 

Carotenoids are still highly topical systems for research. Both Sj Sq and S2 Sq electronic relaxation process in carotenoids with 7 to 11 conjugated double bonds have been subjected to very comprehensive study . The lifetime of the S2 state of P-carotene in CS2. measured by a femtosecond absorption method, is found to be 200-250 fs at room temperature . Fs time resolved CARS from p-carotene in solution shows the occurrence of ultra-high frequency (llTHz) beating phenomena and sub-ps vibrational relaxation. The same technique has been used to observe solvent effects on the a C=C stretching mode in the 2 Ag excited state of P-carotene and two derivatives . A similar study has been made with several derivatives of P-carotene. ... 

Recently, the femtosecond time-resolved spectroscopy has been developed and many interesting publications can now be found in the literature. On the other hand, reports on time-resolved vibrational spectroscopy on semiconductor nanostructures, especially on quantum wires and quantum dots, are rather rare until now. This is mainly caused by the poor signal-to-noise ratio in these systems as well as by the fast decay rates of the optical phonons, which afford very fast and sensitive detection systems. Because of these difficulties, the direct detection of the temporal evolution of Raman signals by Raman spectroscopy or CARS (coherent anti-Stokes Raman scattering) [266,268,271-273] is often not used, but indirect methods, in which the vibrational dynamics can be observed as a decaying modulation of the differential transmission in pump/probe experiments or of the transient four-wave mixing (TFWM) signal are used. 

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