Two Dimensional Fourier Transform
Electronic Spectroscopy: Evolution of Cross
Peaks in the Fenna-Matthews-Olson Complex
G.S, Engel', E.L. Read', T.R. Calhoun', T.K. Ahn', T. Mancal', and R.E.
Blankenship^, and G.R. Fleming'
' Department of Chemistry, University of California, Berkeley and Physical
Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California
94720-1460, USA
E-mail: [email protected]
" Department of Chemistry and Biochemistry, Arizona State University, Tempe,
Arizona 85287-1604, USA
Abstract. Two dimensional femtosecond Fourier transform electronic spectroscopy is
used to study the evolution of cross peaks in the Fenna-Matthews-Olson complexes
isolated from Chlorobium tepiduin and Pelodictyon phaeum. Evidence for quantum
beating is observed, and a teclinique for eliminating the signal from the main diagonal
peaks using polarization rotation of the input pulses is demonstrated.
1.
Introduction
The Fenna-Matthews-Olson (FMO) complexes from Chlorobium tepidum,
and Pelodictyon phaem are investigated using two dimensional femtosecond
electronic spectroscopy.
Two dimensional electronic spectroscopy has
proven valuable in observing coupling in excitonic complexes, of which the
characteristic signature appears as off-diagonal, cross peaks in the 2D
spectrum.[1, 2] In this study, evidence for coherence transfer and quantum
beating is observed in the C. tepidum FMO complex; this quantum beating
manifests itself in the data as an oscillation of the intensity of a cross peak,
of the intensity of a corresponding diagonal peak, and of the width of the
main diagonal peak[3]. Further, a polarization scheme developed in the midIR [4, 5] for eliminating the signal from the main diagonal peak has been
implemented to isolate the cross peaks from the P. phaeum FMO complex.
2.
Experimental Metliods
The 2D FT electronic spectroscopy technique and apparatus used in this
experiment has been described previously[6]. In short, a diffractive optic is
used to create two pairs of phase locked beams, with a standard delay stage
used to set the time between the two pairs. Within each pair, glass wedges are
introduced to precisely control the delays while maintaining phase stability of
the apparatus. The three pulses stimulate a photon echo signal in the sample
which is detected using heterodyne detection after mixing with the fourth
pulse.
The electric field is recovered, and is subsequently Fourier
transformed to frequency-frequency space[7, 8]. The femtosecond pulses
used are generated with a home-built oscillator and regenerative amplifier
392
yielding three pulses at the sample of 5 nJ each, duration of 41 fs, and spectral
width of 31nm centered about 808nm. The total power incident on the
sample is 15 nJ, which is focused into a 70 \im beam waist inside the sample.
The samples are held in a 200 )j.m quartz cell silanized with Sigmacote
(Sigma-Aldrich) and held at 77K with an Oxford Instruments cryostat. To
form a glass at 77K, the samples in a 100 mM, pH 8.0 tris/HCl buffer with
0.25-0.5% detergent (LDAO or Anzergent 3-12) were mixed 35/65 (v/v) with
glycerol.
3.
Results and Discussion
The resultant 2D electronic spectra from C. tepidum are shown in Fig. 1 at
population times T=420 and T=510. These population times demonstrate a
clear minimum and maximum in the cross peak oscillation that is indicative of
the quantum beating signal observed. As predicted by Pisliakov et. al [3], the
width of the main diagonal peak is also varying with the oscillation. T h e
observed oscillation frequency of approximately 70 fs is in good agreement
with the energetic spacing of the diagonal peaks involved in the process.
C. tepidtmi Fsnna-Mattheivs-Olsois Cofflptex
' Fig. 1. These two
dimensional spectra of C.
~ tepidum FMO, displayed
• ' I with linear scaling (top
.1 panels) and ArcSinli
f scaling (lower panels),
-'« demonstrate a minimum
«;i".?
5101?
^ (left panels) and maximum
'
_ (right panels) in the
'•'•""
"••/•• "
' '^'^"'^ ';.i_'^=
'' oscillation. The. intensity
\"-'-If of the lower right cross
I peak and the lower left
I diagonal peak can be seen
i I to be varying. Also, the
I shape of the diagonal peak
changes from elongated at
"''""
''•"'•'
I the minima to nearly round
.
.. , . .
at the maxima.
To further isolate the dynamics of the cross peaks from the diagonal signal,
a polarization scheme (n/3,- 7i/3,0,0) studied for use in the mid-IR[4, 5] has
been implemented by inserting waveplates into the first two pulses just before
the glass wedge delay stages. The resultant spectra are shown in Fig. 2 next
to the standard 2D spectra using a polarization scheme of (0,0,0,0). The
phasing for these spectra was determined with a precise knowledge of the
timing of pulses 1 & 2 and the phase between pulses 3 & LO, both obtained
with spectral interferometiy.
4.
Conclusions
Our 2D FT electronic spectra demonstrate clear evolution of cross peaks and
the ability to isolate such cross peaks from the strong main diagonal signal.
The cross peaks in the C tepidum data demonstrate compelling evidence for
393
p. ,*«.« F«,«.Mm,i,«s-o,so„ complex
'•.•••.'
'•S:
^'
.T^ ^
r. JO
no
Q O O
It
I
.
' ,' '
i
•• ••--••'
n
' '
n'^n
- -• 00
-•
• O
- --- O O O
,^ ' J
,
Z jCf * i
I
•
I
•'
•- •.•
Fig. 2. These two
dimensional spectra of
C. tepidum FMO,
displayed with linear
scaling (top panels) and
ArcSinh scaling (lower
panels), demonstrate
how the cross peak
specific polarization
(left panels) can resolve
I
' ' " •'•
cross peaks obscured
by the main diagonal
;-.,
features of a standard
'"1 .
polarization spectrum
' j ob
(right panels).
I
'• • L . . - ' ' • • • • " ' j
\.^^
'J O
Diagonal peaks are
'-J ''-• .'
'- O O
shown with squares
while cross peaks are
''' shown with circles; the
• '
lines denote the
positions of the exciton
states,
quantum beating resulting from electronic coherence - the electronic
equivalent to a vibrational wavepacket. The P. phaeum data demonstrate our
ability to isolate the cross peaks from the main diagonal peaks providing the
ability to clearly elucidate signals otherwise obscured by the main diagonal.
• ! •
'-•••-••••
''
''
. I
I
'4
f
iisl
Acknowledgements. This work was supported by the DOE Office of
Science. G.S.E thanks the Miller Institute for Research in Basic Science for
financial support.
References
1 T. Brixner, J. Stenger, H. M. Vaswani, M. Cho, R. E. Blankenship and G. R.
Fleming, Nature, 434, 625-628, 2005.
2 M, H. Cho, H. M. Vaswani, T. Brixner, J. Stenger and G. R. Flerning, Journal of
Physical Chemistiy B, 109, 10542-10556, 2005.
3 A. V. Pisliakov, T. Mancal and G. R. Fleming, Journal of Chemical Physics,
124, -, 2006.
4 J. Dreyer, A. M. Moran and S. Mukamel, Bulletin of the Korean Chemical
Society, 24, 1091-1096, 2003.
5 M. T. Zamii, N. H. Ge, Y. S. Kim and R. M. Hochstrasser, P Natl Acad Sci
(.«^,98,11265-11270, 2001.
6 T. Brixner, T. Mancal, I. V. Stiopkin and G. R. Fleming, Journal of Chemical
Physics, 121, 4221-4236, 2004.
7 S. M. Gallagher, A. W. Albrecht, T. D. Hybl, B. L. Landin, B. Rajaram and D.
M. .lonas. Journal Of The Optical Society OfAmerica B-Optical Physics, 15,
2338-2345, 1998.
8 L. Lepetit, G. Cheriaux and M. Joffre, Journal Of The Optical Society Of
America B-Optical Physics, 12, 2467-2474, 1995.
394