Femtosecond Stimulated Raman Spectroscopy
It is common to employ picoseconds pulse durations for time-resolved Raman spectroscopy in order to retain high spectral resolution. Femtosecond pulses naturally have a broader bandwidth and hence spectral resolution is lost. However it is possible to obtain high spectral resolution Raman spectra with femtosecond time-resolution by using femtosecond stimulated Raman spectroscopy (FSRS).
FSRS uses a narrow bandwidth picosecond Raman pump pulse (B) and broad-bandwidth femtosecond Raman probe pulse (C) to probe the sample after excitation by a femtosecond pump pulse (A). Stimulated Raman scattering occurs when the difference in frequency between B and C coincide with a molecular vibration. This induces a gain or loss on the broadband Raman probe pulse (C) at the frequencies of Raman transitions, see below. As the Raman spectrum is collected coherently on the probe beam (C), the technique also has the advantage of eliminating the fluorescence background from fluorescing samples.
The spectral resolution will be limited by either the Raman pump (B) laser linewidth (> 10 cm-1) or the lifetime of excited state, while time-resolution is limited to the femtosecond pump (A) and probe (C) pulse durations (Δt > 50 fs).
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