= Femtosecond stimulated Raman spectroscopy
= FSRS
{c}
{synonym}
\b[Femtosecond stimulated Raman spectroscopy], or <FSRS>, measures a <Raman spectrum> through gain induced in a short broadband probe by a narrowband Raman pump. At a <molecular vibration> frequency matching the pump–probe frequency difference, <Stokes Raman scattering> transfers energy from the pump into the probe. The narrow pump and spectrograph supply spectral selectivity while the short probe samples the system in time; the probe duration alone is not the spectral linewidth.<Femtosecond Broadband Stimulated Raman: A New Approach for High-Performance Vibrational Spectroscopy by David W. McCamant and others>[Introduction, Figure1 and frequency-resolution discussion]{ref}
The2003 apparatus of <David W. McCamant>, <Philipp Kukura> and <Richard A. Mathies> filtered a Ti:sapphire amplifier output to make a narrowband pump near $793\,\mathrm{nm}$ and generated a broadband probe in sapphire. Overlapped beams traversed the sample; a <spectrograph> dispersed probe and reference spectra onto a dual-diode array. The paper demonstrates gain bands in <cyclohexane>.<Femtosecond Broadband Stimulated Raman: A New Approach for High-Performance Vibrational Spectroscopy by David W. McCamant and others>[Materials and Methods, Instrumentation and Figure2; Data Collection, equation4]{ref}
After dark correction, let $S$ and $R$ denote probe and reference counts. The <Raman pump-on pump-off gain ratio> is
$$
G(\nu)=\frac{[S(\nu)/R(\nu)]_{\rm on}}{[S(\nu)/R(\nu)]_{\rm off}}.
$$
For an original chosen spectral channel, take $(S,R)_{\rm off}=(10000,10000)$ and $(S,R)_{\rm on}=(11440,11000)$. The normalized ratio is $1.04$, a four-percent Raman gain; the unnormalized probe ratio $1.144$ would mistake a source fluctuation for additional gain. This assumes a linear detector and a reference that tracks that fluctuation.
A second original calculation uses an $800\,\mathrm{nm}$ pump and a $1000\,\mathrm{cm^{-1}}$ vibration. The Stokes probe channel is at wavenumber $12500-1000=11500\,\mathrm{cm^{-1}}$, or about $869.6\,\mathrm{nm}$. A broadband probe can sample many such differences simultaneously. Spatial and temporal overlap, wavelength calibration, probe chirp and non-Raman nonlinear backgrounds still require characterization. A short pulse label does not by itself establish the time resolution of a changing molecular response.
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