Читать книгу Spectroscopy for Materials Characterization - Группа авторов - Страница 68

The simplest and most straightforward path to implement a TRR experiment is to use a pump–probe approach very similar to TA experiment, except for the fact that the probe beam is used as a Raman pump. Thus, instead of measuring the change in probe transmittance, which is the basis of TA, one can achieve TRR by collecting the isotropic Raman signal produced by scattering of the delayed probe pulse, and analyzing it in a spectrograph as a function of pump–probe delay [54]. However, this method, named probe‐induced Raman scattering, is made very difficult by the very small cross section of Raman scattering, and also brings inherent limitations in time resolution. In fact, considering the time–energy uncertainty relation, the probe beam used as a Raman pump beam must be relatively broad in time to be narrow enough in the spectral domain. For example, a pulse centered at 550 nm, with a FWHM of 1 nm will guarantee a ω _V spectral resolution of ∼30 cm⁻¹, which may be adequate to probe a Raman spectrum, but with a transform‐limited pulse duration as long as 450 fs. This problem is fundamentally unsurmountable and limits the time resolution to the picosecond range.

In fact, one of the most effective approaches to carry out a TRR experiment is use a more complex scheme, initially proposed at the end of the 1990s [55], based on what is called the stimulated Raman process. The relation between spontaneous and stimulated Raman process is similar to that between spontaneous and stimulated emission: shortly, the Raman scattering cross section into a given photon mode is strongly enhanced whenever that particular mode is already populated by photons provided by a seed beam. Therefore, stimulated Raman process can be used to probe the same properties as spontaneous Raman process, but with potentially much stronger intensity.

In a typical time‐resolved stimulated Raman experiment [5559–61], such as the one depicted in Figure 3.6, three noncollinear laser pulses are used, all obtained by splitting the fundamental beam from the femtosecond laser. The first pulse hitting the sample is the pump, which is resonant with a certain transition of the sample and photoexcites it. To distinguish this pulse from the following two, it is often named actinic or photochemical pump. After that, a combination of two more pulses is used to probe the vibrational mode pattern of the excited system. One acts as Raman pump pulse, long in the time domain, hence narrowband in frequency around ω, as required for spectral resolution; the third pulse, called the Raman probe beam, provides the seed responsible of stimulating the Raman emission. The two pulses are overlapping in time and hit the sample together after a certain delay t from the pump. However, the Raman seed is much shorter in time (<100 fs), hence spectrally broadband and contains nonzero spectral density across a wide interval of ω ^′. In these conditions, one observes a strong enhancement of the Raman scattering at ω ^′, which can amount to several orders of magnitude. If the Raman probe spectrum is dispersed and measured with and without the Raman pump, one can directly obtain the stimulated Raman spectrum as the gain in the probe, that is the natural logarithm of the ratio: (Probe)_{Pump on}/(Probe)_{Pump off}. Besides, the detection is greatly facilitated by the fact that the stimulated Raman signal is emitted in a defined direction. In fact, femtosecond stimulated Raman spectroscopy allows to acquire high‐quality Raman spectra even in the presence of a strong fluorescence [52, 61].

Figure 3.6 Panel (a): Scheme of the three‐pulse sequence used in time‐resolved stimulated Raman spectroscopy.

Source: Reprinted with permission from [55]. Copyright (1999) by the American Physical Society.

Panel (b): Energy level diagram for time‐resolved stimulated Raman.

Source: Adapted with permission from McCamant et al. [60]. Copyright (2003) American Chemical Society.

Panel (c): One of the earliest time‐resolved stimulated experiment, probing the trans‐cis photoisomerization dynamics of 4‐(dicyanomethylene)‐2‐methyl‐6‐(4‐dimethylaminostyryl)‐4H‐pyran, dissolved in a 3 mM concentration in DMSO and photoexcited at 397 nm. The Raman pump pulse was tuned at 795 nm (bandwidth 1.5 nm), while the Raman probe pulse, obtained by supercontinuum generation, covered the region 600–1000 nm. Arrows indicate the Raman signals appearing due to the photogenerated transient states, while asterisks indicate the Raman lines of the solvent.

Source: Reprinted figure with permission from Yoshizawa and Kurosawa [55]. Copyright (1999) by the American Physical Society.

What is more important, however, is that the use of this detection approach provides the trick to achieve very high temporal resolution [61]. In fact, despite Raman scattering occurring during the whole duration of the Raman pump pulse (∼1 ps or more), it is only within the duration of the short‐lived Raman probe beam that stimulated Raman process occurs. Therefore, the time resolution of the measurement is now controlled by the femtosecond duration of the probe pulse. For example, a narrowband Raman pump pulse at 560 nm, with duration of several picoseconds, can be combined with a probe pulse centered, say, at 580 nm with a FWHM of 20 nm, which is broad enough to stimulate Raman emission over a relatively wide spectral range (∼1000 cm⁻¹), and can be compressed down to <25 fs.

Summarizing, high spectral resolution and short temporal resolution are simultaneously achieved by decoupling the two problems, so overcoming the Heisenberg limit: the former is controlled by the FWHM of the Raman pump pulse, while the latter depends on the temporal duration of the probe. Often, time‐resolved stimulated Raman is described by the quantity of δtδν used as a figure of merit of the experiment. State‐of‐the art setups have achieved δtδν of the order of 0.5 ps⋅cm⁻¹ [52].

While femtosecond stimulated Raman can be used as a way to record a ground state Raman spectrum, the final goal of these experiments is usually following the evolution initiated by the actinic pump, with time resolution given by the cross‐correlation of the actinic and Raman probe pulses, which can reach a few tens of femtoseconds. To do so, the stimulated Raman spectrum for the unexcited sample is compared to that obtained at variable delays from photoexcitation, in order to follow the effect of relaxation. The result of the experiment can be plotted either in terms of an absolute, time‐dependent Raman spectrum (e.g. as in Figure 3.6c), which contains both the features of the unexcited ground molecules and those of the excited molecules, or as a difference Raman spectrum, calculated with respect to the unexcited sample.

Figure 3.7 represents a typical experimental setup [60]. The actinic pump is obtained by a NOPA pumped by an amplified Ti:sapphire laser, allowing for tunable excitation as in a typical TA experiment. The remaining portion of the amplifier is used to generate both the Raman pump and probe pulses. In order to narrow down the bandwidth of the Raman pump pulse, the amplifier output traverses narrow band‐pass interference filters (BPF in Figure 3.7). Alternatively, one can use more complex prism‐ or grating‐based filters, which allow fine control of both the central wavelength and bandwidth. In this particular case, the picosecond Raman pump pulse is centered at 793 nm with bandwidth of 1.1 nm and a duration of several picoseconds. Its intensity is kept relatively high (∼1 μJ pulse⁻¹) in order to optimize the amount of Raman scattering produced. The Raman probe pulse is obtained through supercontinuum generation (400–1000 nm) in a sapphire plate, followed by compression in a prism pair wherein only the near‐IR portion of the continuum is retained. In this way, a 80 fs pulse is obtained covering the 870–950 nm region, that corresponds to 1000–2000 cm⁻¹ Raman shifts from the Raman pump. The intensity of the Raman probe is much lower than that of the Raman pump, typically in the range of tens of nJ pulse⁻¹. Finally, the three pulses are made collinear and focused in the same spot (sized a few tens of μm) of the flow cell containing the sample. After traversing the sample, the Raman probe beam is spatially selected, goes through a spectrograph, and analyzed in an array detector.

Figure 3.7 A possible configuration for a time‐resolved stimulated Raman experiment.

Source: Adapted with permission from McCamant et al. [60]. Copyright (2003) American Chemical Society.

Several variants of this general scheme are possible. In some setups, for example, tunable Raman pump and probe pulses are both obtained by NOPAs [62]. In other cases, the fundamental beam from the amplifier was directly used as the Raman probe [63]. In regard to detection, some setups make use of a reference beam [60]: the probe is split in two in order to generate a reference beam, allowing for efficient shot‐to‐shot normalization of white light fluctuations. Experiments have been conducted over a wide range of Raman pump wavelengths, from the near IR to the UV [64]. This wide flexibility also allows to tune the Raman pump for pre‐resonant [63] and resonant [53] Raman process, which provides a convenient route to further enhance the Raman signal, and to single out the Raman contribution from the chromophore of interest.

In regard to data acquisition, several pulse chopping sequences can be used in this type of experiments [52]. By a simple chopper in the Raman pump path, one can acquire the stimulated Raman spectrum from the ratio between Raman‐pumped and Raman‐unpumped spectra cyclically recorded by the array detector. Then, the Raman spectra of the ground state and photoexcited sample can be obtained sequentially, and compared to each other, by shuttering the actinic pump. If the chopper is positioned in the path of the actinic pump, the ratio between chopped and unchopped signals directly records the difference Raman spectrum induced by photoexcitation. More complex schemes where both the actinic and Raman pump are chopped are sometimes used as well [64].