Excited state absorption cross-section
spectrum of Chlorophyll A
D. S. Corrêa, L. De Boni, F. J. Pavinatto, D. S. dos Santos
Jr ., and C. R. Mendonça*
Instituto de Física de São Carlos - USP - São Carlos, SP, Brazil
*e-mail: [email protected]
Motivation
The study of excited state properties of chlorophyll a is a subject of
foremost interest, given that it plays important roles in biological process
and has also been proposed for applications in photonics.
The excited state absorption spectrum of chlorophyll a solution from 460
to 700 nm is obtained through the white-light continuum Z-scan
technique. We observed two distinct nonlinear absorption processes: i)
saturation of absorption (SA) was observed due to the ground state
depletion, induced by the white-light continuum region that is resonant
with the Q band of chlorophyll a. ii) We also observed reverse saturation
of absorption (RSA) related to the excitation from the first excited state
to a higher energy level for wavelengths below 640 nm.
To understanding the optical effects, an energy-level diagram, based on
the electronic states of chlorophyll a, was employed to interpret their
results, revealing that more states than the ones related to the Q and B
bands participate in the excited state absorption of this molecule.
Chlorophyll a
High conjugation length (ring)
provides high optical nonlinearities
Mg
Photosynthesis
Chlorophyll is found in high concentrations in chloroplasts of plant cells.
Linear absorption spectrum of Ch a
2.25
Absorbance
B band
1.50
Q band
0.75
Natural window
0.00
400
500
600
wavelenght
700
White-light continuum Z-scan
775 nm & 150 fs
pulses (1 k Hz)
I
L1
Cuvette with
H2O
destilated
L2
E
F
FI
sample
Optical fiber
L4
xyz
wlc
pulse
L3
E
z
Tradicional translation
Z-scan
spectrometer
Less
time consuming
All nonlinear spectra
in 5 min.
Normalized Transmittance
Computer
1.0
0.9
0.8
0.7
0.6
-6
-4
-2
0
z (mm)
2
4
6
White-light continuum pulse
Gaussian decomposition
2000
0.04
0.03
P (mw)
Intensity (u. arb.)
1500
1000
0.02
500
0.01
5 ps positive chirp
0
500
600
0.00
700
500
λ (nm)
700
λ (nm)
Fast population
dynamics
Sn
fast
Rate equations
dn 0 (t )
n (t )
= −n 0 (t )W 01 + 1
τ1
dt
gap dn (t )
1
*
W 01
600
S1
τ1
trans
S0
dt
= + n0 (t )W01 −
n1(t )
τ1
σ (λ ) I (λ )
W01 = 01
hω
I (λ )
Intensity
WLCSZ results for Chlorophyll a
SA
σ01 > σ1n
σ S1
absorption crosssection
Sn
S1
σ S0
S0
RSA
σ01 < σ1n
SA
Z-scan curves at distinct wavelengths,
displaying the inversion of the normalized
transmittance according to the dominant
nonlinear process SA or RSA.
RSA
Normalized transmittance vs wavelength for
chlorophyll a using several pumping powers.
Excited states measurements
With WLCZScan technique, the triplet
dynamics is not observed
τisc (few nanoseconds)
Rate equation
Only singlet dynamics
where
The time duration of the WLC pulse only
promotes excitation in the singlet states
the transition rate S1→Sn
Energy diagram
According to the theoretical calculation of Parusel [1], the transition S0→S1 (Q band) involves
the states 2A+3A+4A. Transition S0→S1 (B band) involves the states 5A+6A+7A. The transition
observed in the WLCZS is the S1→Sn, also involving states above 7A. The energy diagram
displayed below was used to fit the experimental data.
Sn
τ10 = 4 ns [2]
σ1n
S 2 (5 A+ 6 A + 7 A )
σ10 = linear absorption cross-section
σ1n = excited state absorption cross-section
B
Q σ01
S1
( 2 A+ 3 A + 4 A)
S0
(1 A )
τ01
[1] A. B. J. Parusel and S. Grimme, J. Phys. Chem. B 104 (22), 5395 (2000).
[2] D. S. Correa, et al, Appl. Phys. B74 (6), 559 (2002).
Nonlinear absorption spectrum
Time evolution of the nonlinear absorption
6
σ (x 10-17cm2)
σ1n: open circles
4
σ01: solid line
σ01-σ1n: solid circles
2
0
450
500
550
600
650
Comprimento de onda (nm)
700
Conclusion
We have measured the resonant nonlinear spectrum of chlorophyll a
solution in the spectral range between 460 and 700 nm using the WLC Zscan technique. This technique has proven to be powerful to determine
spectroscopic parameters of organic materials. Reverse saturation of
absorption and saturation of the absorption were observed in the nonlinear
spectrum of chlorophyll a, when the WLC pulse was employed. We found
that the red spectral region of the WLC pulse, which is resonant to the Q
band, populates the first excited state, thus inducing the SA. After this
excitation, a RSA process takes place due to the excited state absorption for
WLC wavelengths below 640 nm. The experimental data were modeled
through an energy-level diagram based on the electronic states of
chlorophyll a. Our results reveal that another energy band, correlated to
states above 8A, is also responsible for optical processes in chlorophyll a,
besides the levels associated with the Q and B bands.
Acknowledgements
This work was carried out with the financial support from
FAPESP, CNPQ and CePOF from Brazil