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Scientific Report 2007-2009
Condensed matter physics and biophysics
C28. Femtosecond Stimulated Raman Scattering:
ultrafast atomic motions in biomolecules.
Chemical bonds form, break, and evolve with awesome
rapidity. This ultrafast transformation is a dynamic process
involving the mechanical motion of electrons and
atomic nuclei. The speed of atomic motion is of the order
of 1 km/s and, hence, the average time required to
record atomic-scale dynamics over a distance of 1 Å is in
the range of 100 femtoseconds (fs). Physiological bond
formation, evolution and breaking between proteins and
ligands occur on this timescale. Tracking each step of
this process can provide significant insight on biological
function, hence the need of a spectroscopic technique
capable of revealing the structure of molecules on the
timescale of atomic motion.
Femtosecond stimulated Raman scattering spectroscopy
(FSRS) is a powerful method to study reaction
dynamics as it provides vibrational structural information
with an unprecedented combination of temporal and
spectral resolution, unconstrained by the Fourier uncertainty
principle. FSRS requires the generation of three
synchronized pulses: (1) A femtosecond visible actinic
pump that initiates the photochemistry of interest, (2)
a narrow bandwidth picosecond Raman pulse that provides
the energy reservoir for the amplification of the
probe, and (3) a femtosecond continuum probe that is
amplified at Raman resonances shifted from the Raman
pulse.
WLC
ω
which will allow us to exploit the resonance enhancement
of various proteins that absorb in the visible.
In Fig.2 we show the results of our first experiments:
the Stimulated Raman Scattering signal of a reference
solvent (cyclohexane) obtained with both the grating
(λ = 800nm) and the tunable (λ = 480nm) Raman pulse
setup. The overlap of a Raman pump and a broadband
continuum onto the sample allows the simultaneous detection
of all the Raman active modes with a single 40fs
laser shot, opening the possibility of time resolved studies
in the < 100fs time domain, unaccessible to conventional
vibrational spectroscopies.
Our short term goal is to apply FSRS to study the
dynamics of heme proteins with different biological functions
(electron transfer , signaling, etc.). We are also
working on multidimensional implementations of FSRS
to study anharmonic coupling of small molecules as
well as on imaging extensions of FSRS (CARS/SRS Microscopy).
A. B.
pump off
0 700 1400
pump on
Rgain=
pump off
I.
II.
1000 2500 3000
ω
Actinic
pump
Raman
pump
pump on
Laser
Ti:Sa
800nm
50fs
1KHz
3,5mJ
ω
ω
Delay line
Sample
Monocromator
0 500 1000 1500 2000
Raman shift (cm -1 )
Figure 2: A.Energy-level diagram for a typical time-resolved
FSRS experiment. B. FSR spectra of cyclohexane obtained
with Raman pump at 800 nm (I) and 480 nm (II).
Figure 1: Schematic diagram of the FSRS setup.
Since 2009, the Femtoscopy group in the department
of Physics in Sapienza has worked in the implementation
of a FSRS experimental setup. In our setup, the actinic
pulse is derived from an optical parametric amplifier system
(TOPAS) that generates pulses with 10 µJ −0.7 mJ
energy and tunability from 250 to 1200 nm. As Raman
pulse, we have produced an 800 nm narrow bandwidth
(0.5 nm) pulse by linear spectral filtering of the output
of the titanium:sapphire (1 kHz, 3 mJ, 35 fs pulses
at 800 nm) amplified (Legend, Coherent), by a custom
grating filter. More recently, we developed a broadly
tunable narrow band Raman Pulse, by means of a twostage
femtosecond visible-IR OPA which can generate
pulses with 3 − 5 µJ energy and linewidth ranging from
10 − 15 cm −1 . Its tunability ranges from 330 to 510 nm
T. Scopigno acknowledges support from European
Research Council under the EU Seventh Framework
Program (FP7/2007- 2013)/ERC IDEAS grant agreement
n. 207916.
References
1. T. Scopigno, et al., Phys. Rev. Lett. 99, 025701 (2007)
Authors
S.M. Kapetanaki, A. Quatela, E. Pontecorvo, M. Badioli, T.
Scopigno
www.femtoscopy.com
Sapienza Università di Roma 81 Dipartimento di Fisica