Transcript WH08.ppt
PHOTOFRAGMENTATION OF THE FLUORENE CATION: NEW EXPERIMENTAL PROCEDURE USING SEQUENTIAL MULTIPHOTON ABSORPTION and ENERGY-DEPENDENT RATE CONSTANT FOR THE H-LOSS CHANNEL N.T. Van-Oanh, S. Douin, Ph. Bréchignac CNRS, Photophysique Moléculaire, Orsay (France) P. Désesquelles Astrophysical situation Competition between fragmentation and UV radiative cooling by IR emission Photon IC PAH* Fragment PAH Mid-IR emission Recent models of the cooling sequence to account for the observed spectra by Pech et al (2002) Verstraete et al (2002) General Behaviour of Electronic Relaxation in Aromatic Molecules Ultrafast Relaxation Dynamics: 330 33 fs in cations 1E14 « Energy Gap Law » satisfied : knr= A exp [- (E / h*)] 1E13 + Cations PAH + C6H5F 1E12 Non-adiabatic coupling depends on the 1E11 1E10 k nr nature of electronic orbitals Limite physique Physical limit PAHs neutres 1E9 UltraFast Electronic Relaxation by Internal Conversion from Dn to Dn-1 1E8 1E7 Dérivés polyfluorés + du benzène 1000000 • 16 FWHM 160 cm-1 • 330 33 fs 100000 0.5 1.0 1.5 2.0 E (eV) 2.5 3.0 T. Pino, Ph. Bréchignac, E. Dartois, K. Demyk and L. d’Hendecourt, Chem. Phys. Lett. 339, 64 (2001) D3-D0 Spectrum of Fluorene+-Argon Å2 All bands have Lorentzian shapes Lifetime broadening UltraFast electronic relaxation by internal conversion from Dn to Dn-1 •16 FWHM 160 cm-1 • 330 33 fs Principle of the experimental method : case of Fluorene cation • Preparation of the cation by R2PI h1 (after jet cooling) • Mass spectrometric detection of fragmentation internal energy controlled by FC CI h1 D3 k(2h1) CI k(h1) C13H9+ h1 3817cm-1 Neutral D0 S1 296nm S0 C13H9+ Cation PI = 63741cm-1 C13H10 Energy- dependent rate constant Analysis of the kinetics C13H10 log(k(E)) + h 2 k(2h2) C13H9+ CI h 2 Dn CI k(h2) C13H9+ h 2 D0 Exp.1: l= 630 nm (1.97 eV), D3 D0 Exp.2: l= 365 nm (3.4 eV), D4 D0 E h2 2h2 3h2 } A grid of discrete internal energies Experimental Scheme -4C -2C MCP-Detector T.O.F.MS 4 5 6 7 temps de vol (ms) D3 D0 h1 D0 S1 h2 S0 λ= 630nm, D3 D0 I = 4.3mJ Mass spectrum 0,0 4.1eV -0,2 C13H10+ Parent-H 3.8 eV -0,4 Parent E0=2.1(eV) -0,6 6,68 6,72 6,76 6,80 6,84 6,88 6,92 H C13H9+ Planar equilibrium Experimental results λ = 323nm, D4 D0 1,0 Parent Parent-H 0,8 N/N(0) I = 0 to 1.4mJ Parent-H 0,6 0,4 0,2 Parent 0,0 0,0 0,4 0,8 Laser energy (mJ) 6,76 6,78 6,80 6,82 6,84 Time of Flight (ms) 6,86 6,88 1.43 1.328 5 1.00 9 0.84 0 0.70 0 0.39 0 1,2 1,6 Data processing Kinetic scheme : Competition between further photon absorption and dissociation at each step Complex treatment : absorption cross sections (badly known) & fragmentation rates (unknown) Hypothesis j, s1 = s2 = …sj Non Poisssonian character of the Photon absorption process IT FAILS ! Determination of Absorption Cross Sections Leak by other fragmentation channels Too many adjustable parameters ! 2 fitting procedures: • RRK: kdiss(E) = k0(1-E0/E)g-1 • PTD: kdiss(E) = k0(1-E0/(E+EZPE))g-1 The Transition Matrix method n is the number of steps in time Adjustments a) Max = 5 photons if h = 1.97 eV b) Max = 3 photons if h = 3.4 eV Quality of final fits Free adjustment of rate constants From 1 to 5 photons absorbed Time of Flight shape analysis Parent -H Parent 0,27 mJ h = 1.97 eV Only the 3-photon excited state contributes to the H-loss It gives an independent way of evaluating the rate constant: CONSISTENT ! Distribution of the number of absorbed photons in the 11 sets of measurements with h = 1.97 eV The experimental set of rates k(E) extends over 4 orders of magnitude Internal energy E* up to about 5x Eb Exp.1: l= 630 nm (1.97 eV), D3 D0 Exp.2: l= 365 nm (3.4 eV), D4 D0 NOTE: no time-resolution Calibrated against linear absorption cross section measurement (vdW), with much care on laser fluence Jochims et al. (1994) Dibben et al. (2001) RRK: Better at small excess energy PTD (Photo-Thermo-Dissociation): EPZ Summary Fragmentation • dissociation channels identified (H-loss dominant) • rates obtained over an extended energy domain • allows extrapolation to long times Perspective • Phase Space Theory approach, with anharmonic quantum density of states • Experiments with other PAHs More details in : J. Phys. Chem. A , 164 (2006) J. Phys. Chem. A , 168 (2006)