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|Femtosecond Transient Absorption Measurements system
Future nanostructures and biological nanosystems will take advantage not only of the small dimensions of the objects but of the specific way of interaction between nano-objects. The interactions of building blocks within these nanosystems will be studied and optimized on the femtosecond time scale - says Sergey Egorov, President and CEO of Del Mar Photonics, Inc. Thus we put a lot of our efforts and resources into the development of new Ultrafast Dynamics Tools such as our Femtosecond Transient Absorption Measurements system Hatteras. Whether you want to create a new photovoltaic system that will efficiently convert photon energy in charge separation, or build a molecular complex that will dump photon energy into local heat to kill cancer cells, or create a new fluorescent probe for FRET microscopy, understanding of internal dynamics on femtosecond time scale is utterly important and requires advanced measurement techniques.
Del Mar Photonics featured customer Christien A Strydom
Making sense of measurements in femtochemistry
Speaker / Author: Christien A Strydom1
Co-author(s): L.R. Botha2, A. du Plessis2, S. Obinda-Lemboumba2
1School of Chemistry, North-West University, Private Bag X6001, Potchefstroom, 2520,
2CSIR National Laser Centre, Meiring Naude Road, Pretoria, 0001, South Africa
Chemical bonds break, form and change position in the three dimensions with ultra fast
speed. These transformations are dynamic processes involving the mechanical motion of
electrons and atomic nuclei. In order to measure the processes over a distance of an angström,
the average time required is ~100 femtoseconds (fs). Femtochemistry is the field of study
where atomic motions as reactions occur are investigated . Femtosecond resolution (10-15
seconds) and intervention is needed to study and control the dynamics of chemical bond
formation and breakage on an atomic level.
Making sense of the measurements in this time domain is complex and needs to be done in an
indirect manner. As 21st century electronics is not able to measure within femtoseconds,
variations in the optical path length of the laser beams are used to obtain time resolution.
Timing is accomplished by generating pump and probe laser pulses from a common source
and sending either the pump or probe pulse along an adjusted optical path. The path length
difference relates to the time difference as both pulses move at the constant speed of light
(2.999792 x 108 m/s).
Several pump-probe femtosecond laser activation studies have been done on malachite green
and it was decided to verify the experimental set-up using this activation process. Measured
pump-probe signals have shown that malachite green has an ultra short electronic excited
state lifetime . Transient absorption signals of malachite green in an ethanol solution
pumped at 580 nm and probed at 620nm have shown a fast kinetic process with a time
constant of approximately 2.1 ps . In this paper we report on results obtained with
malachite green using a newly commissioned pump-probe femtosecond laser system.
Monitoring the intermediate (transient) concentration using a time delayed probe pulse
pump-probe femtosecond laser system.
The overlap of pump and probe beams. The probe beam is split into a signal and the reference part.
pump-probe femtosecond laser system.
A Ti: sapphire oscillator (Coherent Mira-optima 900-F oscillator) and amplifier
Legend-F with repetition rate 1 KHz) at 795 nm produces ultra short laser pulses with pulse
duration of 117 fs. This beam is split by a beam splitter into pump and probe parts (90 %
transmitted and 10 % reflected). The probe beam is sent to a variable optical delay line,
which is set on a precision translation stage controlled by a computer. The optical delay is
necessary in order to get a real-time rapid-scan acquisition by temporally changing the pump
and probe beam overlap in the sample [12,13]. The probe beam is then focused on a sapphire
plate (1-2mm thick [12,13]) to generate a white light super continuum. A short pass filter is
placed on the probe path in order to suppress the strong residual peak at 800 nm from the Ti:
The probe beam then is split into two beams, giving reference and signal beams. The signal is
focused on the sample in such a way it that overlaps spatially with the pump beam in the
liquid sample while the reference beam is sent through the sample as indicated in Figure 5.
The pump pulse is sent through an optical parametric amplifier (TOPAS C - OPA) in order to
obtain a wide tuning range of the pump beam (530-20000 nm). After the OPA a chopper is
inserted in the pump beam path to record spectra that are classified as pumped and not
pumped, thereby reducing background effects. For detecting the transient absorption, a
spectrometer combined with a photodiode array (PDA) is used.
Pump-probe femtosecond laser experimental setup
pump-probe femtosecond laser system.
1. A.H. Zewail, J. Phys. Chem. A , 104, 24, 2000, pp. 5660 – 5694.
2. Y. Nagasawa, Y. Ando, A. Watanabe and T. Okada, Applied Physics B, 70, 2000, pp. S33-S34.
3. G. Schweitzer, L. Xu, B. Craig and F.C. DeSchryver, Optics Communication, 142, 1997, pp. 283-288.
6. V. Letokov, Laser Control of Atoms and Molecules, 2007, Oxford University Press, ISBN: 978-0-19-852816-6, p. 225.
7. J.S. Baskin and A.H. Zewail, J. Chem.Ed., 78, 6, 2001, pp. 737 – 751.
8. H.Y. Chen, I.R. Lee and P.Y. Cheng, Review of scientific instrument, 77, 2006, p. 076105.
9. M. Dantus and P. Gross, “Ultrafast spectroscopy”, Encyclopedia of Applied Physics, 22, 1998.
10. N.E. Henriksen, Chem Soc. Rev., DOI: 10.10139/b100111f
11. G.D. Reid and K. Wynne, “Ultrafast Laser Technology and Spectroscopy”.
Encyclopedia of Analytical chemistry, R.A. Meyers (Ed), 2000, pp 13644-13670.
12. G. Cerullo, C. Manzoni, L. Luer and D. Polli, Photochemical & photobiological sciences, 6, 2007, pp 135-144.
13. C.C. Gradinaru, I.H.M. van Stokkum, A.A. Pascal, R. van Grondelle and H. van Ameronen, J. Phys. Chem., 104, 2002 , pp 9330-9342.
14. M. Fukuda, O. Kajimoto, M. Terazima and Y. Kimura. J. Mol. Liquids, 134, 2007, pp. 49-54.
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