Spectroscopy and Photoinduced reactivity of isolated molecules
Contact: Benoît SOEP
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Spectral characterization in a supersonic beam of neutral chlorophyll a evaporated from spinach leaves

N. Shafizadeh, M. H. Ha-Thi, B. Soep, M. A. Gaveau, F. Piuzzi and C. Pothier

J. Chem. Phys., 135, 114303, 2011 [doi]

The observation of the light absorption of neutral biomolecules has been made possible by a method implemented for their preparation in the gas phase, in supersonically cooled molecular beams, based upon the work of Focsa et al. [C. Mihesan, M. Ziskind, B. Chazallon, E. Therssen, P. Desgroux, S. Gurlui, and C. Focsa, Appl. Surf. Sci. 253, 1090 (2006)]. The biomolecules diluted in frozen water solutions are entrained in the gas plume of evaporated ice generated by an infrared optical parametric oscillators (OPO) laser tuned close to its maximum of absorption, at ∼3 μm. The biomolecules are then picked up in the flux of a supersonic expansion of argon. The method was tested with indole dissolved in water. The excitation spectrum of indole was found cold and large clusters of indole with water were observed up to n = 75. Frozen spinach leaves were examined with the same method to observe the chlorophyll pigments. The Qy band of chlorophyll a has been observed in a pump probe experiment. The Qy bands of chlorophyll a is centred at 647 nm, shifted by 18 nm from its position in toluene solutions. The ionization threshold could also be determined as 6.1 ± 0.05 eV.


Direct observation of slow intersystem crossing in an aromatic ketone, fluorenone

B. Soep, J. M. Mestdagh, M. Briant, M. A. Gaveau and L. Poisson

Phys. Chem. Chem. Phys., 18, 22914, 2016 [doi]

Direct measurements of Single vibronic Level InterSystem Crossing (SLISC) have been performed on the fluorenone molecule in the gas phase, by time resolved photoelectron and photoion spectroscopy. Vibronic transitions above the S-1 n pi* origin were excited in the 432-420 nm region and the decay of S-1 and growth of T-1 (3) pi pi* could be observed within a 10 ns time domain. The ionization potential is measured as 8.33 +/- 0.04 eV. The energy of the first excited triplet state of fluorenone, T-1 has been characterized directly at 18640 +/- 250 cm(-1). The internal conversion of S-1 to S-0 is found to amount to similar to 15% of the population decay, thus ISC is the dominant electronic relaxation process. ISC, although favored by the S-1 (1)n pi*-T-1 3 pi pi* coupling scheme, is 3 orders of magnitude less efficient than in the similar molecule benzophenone. Thus, the planarity of the fluorenone molecule disfavors the exploration of the configuration space where surface crossings would create high ISC probability, which occurs in benzophenone through surface crossings. The time evolution of S-1 fluorenone is well accounted for by the statistical decay of individual levels into a quasi-continuum of T-1 vibronic levels.

#2943 - Màj : 14/09/2018

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