Excited state absorption cross-section. section spectrum of Chlorophyll A

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1 Excited state absorption cross-section section spectrum of Chlorophyll 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 * daniel@ursa.ifsc.usp.br

2 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 (S) 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 (RS) 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.

3 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 bsorbance 1.50 0.

4 Linear absorption spectrum of Ch a 2.25 bsorbance B band Q band wavelenght Natural window

White-light continuum Z-scan 775 nm & 150 fs pulses (1 k Hz) I L1 Cuvette with H 2 O destilated L2 E Optical fiber FI sample L3 F wlc pulse xyz L4 z Tradicional

5 White-light continuum Z-scan 775 nm & 150 fs pulses (1 k Hz) I L1 Cuvette with H 2 O destilated L2 E Optical fiber FI sample L3 F wlc pulse xyz L4 z Tradicional translation Z-scan E spectrometer Computer Less time consuming ll nonlinear spectra in 5 min. Normalized Transmittance z (mm)

6 White-light continuum pulse 2000 Gaussian decomposition Intensity (u. arb.) ps positive chirp P (mw) Fast population dynamics λ (nm) * τ W 01 1 fast S n S 1 gap Rate equations dn dn dt ( t) n1( t = n ) 0( t) W01 + τ 1 dt 1 ( t) n1( t = + n ) 0( t) W01 τ1 σ W = ( λ) I( λ) hω λ (nm) I(λ) Intensity trans S 0

7 WLCSZ results for Chlorophyll a S σ 01 > σ 1n absorption crosssection σ S 0 σ S1 S n S 1 S 0 RS σ 01 < σ 1n Z-scan curves at distinct wavelengths, displaying the inversion of the normalized transmittance according to the dominant nonlinear process S or RS. S Normalized transmittance vs wavelength for chlorophyll a using several pumping powers. RS

8 Excited states measurements With WLCZScan technique, the triplet dynamics is not observed τ isc (few nanoseconds) Only singlet dynamics Rate equation where The time duration of the WLC pulse only promotes excitation in the singlet states the transition rate S 1 S n

9 Energy diagram ccording to the theoretical calculation of Parusel [1], the transition S 0 S 1 (Q band) involves the states Transition S 0 S 1 (B band) involves the states The transition observed in the WLCZS is the S 1 S n, also involving states above 7. The energy diagram displayed below was used to fit the experimental data. Q σ 01 σ 1n B τ 01 S n S S S ( ) ( ) (1 ) τ 10 = 4 ns [2] σ 10 = linear absorption cross-section σ 1n = excited state absorption cross-section [1]. B. J. Parusel and S. Grimme, J. Phys. Chem. B 104 (22), 5395 (2000). [2] D. S. Correa, et al, ppl. Phys. B74 (6), 559 (2002).

10 Nonlinear absorption spectrum Time evolution of the nonlinear absorption 6 σ (x cm 2 ) 4 2 σ 1n : open circles σ 01 : solid line σ 01 -σ 1n : solid circles Comprimento de onda (nm)

11 Conclusion We have measured the resonant nonlinear spectrum of chlorophyll a solution in the spectral range between 460 and 700 nm using the WLC Z- scan 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 S. fter this excitation, a RS 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 8, is also responsible for optical processes in chlorophyll a, besides the levels associated with the Q and B bands.

12 cknowledgements This work was carried out with the financial support from FPESP, CNPQ and CePOF from Brazil

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