A resonant energy transfer toy model for CO ice

Transcription

1 A resonant energy transfer toy model for CO ice Octavio Roncero Inst. Física Fundamental, CSIC Madrid (Spain)

2 Outline Introduction 2 Förster Resonance Energy transfer 3 Reactivity 4 Perspectives

3 Chemisty at Cold Interstellar Clouds: COM s formation Only exothermic barrierless reactions Many reactions to produce COM s have barrier Asumption: COM s are formed in ices, but how do they desorb? - Photofragments desorb Cruz-Diaz et al., ApJ ( 6) - What is the precursor of molecules like CH 3 OH

4 Chemisty at Cold Interstellar Clouds: COM s formation Only exothermic barrierless reactions Many reactions to produce COM s have barrier Asumption: COM s are formed in ices, but how do they desorb? - Photofragments desorb Cruz-Diaz et al., ApJ ( 6) - What is the precursor of molecules like CH 3 OH Gas phase experiments on Accelerated chemistry at low temperatures for CH 3 OH + OH Shannon et al. Nature Chem. ( 3) Gómez Martín et al. J. Phys. Chem.A ( 4) Antiñolo et al. ApJ ( 6) Use of TST including tunneling

5 Chemisty at Cold Interstellar Clouds: COM s formation Only exothermic barrierless reactions Many reactions to produce COM s have barrier Asumption: COM s are formed in ices, but how do they desorb? - Photofragments desorb Cruz-Diaz et al., ApJ ( 6) - What is the precursor of molecules like CH 3 OH Gas phase experiments on Accelerated chemistry at low temperatures for CH 3 OH + OH Shannon et al. Nature Chem. ( 3) Gómez Martín et al. J. Phys. Chem.A ( 4) Antiñolo et al. ApJ ( 6) Use of TST including tunneling CH 3 OH + OH Imaginary frequency at TST unrealistic Siebrand et al. PCCP ( 6) Reactivity due to dimer formation Objective Need of complete dynamical study

6 Gas phase processing of photofragments: CH 2 O + OH reaction Zanchet et al., PCCP ( 7) Ocaña et al., ApJ ( 7) (submitted) Energy/eV RFF RFF+MB CCSD(T) F2a s/å Reaction rate coefficient (cm 3 s ) Exp. Sivakuraman (23) Exp. this work k e,v=,j= (T) k(t) k RFF (T) Temperature (K)

7 CO ice photochemistry hν Guillermo Muñoz-Caro (CAB) Rafael Escribano (IEM) Germán Molpeceres (IEM) Alfredo Aguado (UAM)

8 CO ice photochemistry hν Model system to study CO does not dissociate below.4 ev Photochemistry < 3 % formation of CO 2 and C 3 O 2 Photodesorption is the main process Import role in formation of H 2 CO and CH 3 OH with H, H 2 and H 2 O

9 CO: absorption in gas and in ice Cruz-Diaz et al.,( 4)

10 CO: absorption in gas and in ice Theory in gas phase Σ Σ transitions Only bound-bound transitions Adiabatic curves Diabatic curves 6 C( S) + O( D) Energy (ev) 4 2 C( D) + O( D) C( 3 P) + O( 3 P) X Σ + Cruz-Diaz et al.,( 4) R (bohr) R (bohr)

11 CO: absorption in gas and in ice Absorption intensity (arb. units) 3 2 Photon energy (ev) 9 Σ Π transitions v=5 8 v= Wavelength (nm) 2 v=2 Cruz-Diaz et al.,( 4) Energy (ev) X( Σ) A( Π) R (Angstroms)

12 Simulations of photodessorption in CO ice Molecular Dynamics on amorphous and crystaline CO clusters van Hemert, Takahashi & van Dishoek ( 5) Force field for ground and excited electronic states Amorphous and crystalline clusters of varying size Thermalization/excitation of chosen CO/dexcitation

13 Simulations of photodessorption in CO ice Molecular Dynamics on amorphous and crystaline CO clusters van Hemert, Takahashi & van Dishoek ( 5) By landing on X state in repulsive region No vibrational energy transfer Direct mechanism of top layer vibrationally hot CO molecules Kick-out mechanism for second layer little vibrational excitation Inner layers do not desorb Crystaline 5 times smaller than Amorphous

14 Experiments on photodesorption Muñoz Caro et al. (2) Photodesorption shows a linear decrease except for less than 4-5 layers There must be other mechanisms to increase the photodissociation of inner layers

15 Experiments on photodesorption Muñoz Caro et al. (2) Photodesorption shows a linear decrease except for less than 4-5 layers There must be other mechanisms to increase the photodissociation of inner layers Resonance Energy Transfer(Förster) or Energy Transfer Excitons were neglected Reaction dynamics was neglected: only few CO Octavio Roncero distances were DIPSS, considered Granada, Sept , 27

16 Outline Introduction 2 Förster Resonance Energy transfer 3 Reactivity 4 Perspectives

17 Förster Resonant Energy transfer Virtual absorption/emission of a photon between donor/acceptor D + hν D D + A D + A A A + hν In Quantum electrodynamics the coupling responsible is given by V = d D d A 3( ˆR d D )( ˆR d A ) 4πɛ R 3 d D,A : transition dipole moment of D,A

18 Förster RET Dependence with geometry Dimer with Frozen CO(r eq ).8 CO-CO linear along z: θ =,θ 2 =, φ= CO(Π x )+CO(Σ) CO(Σ)+CO(Π x ).8 CO CO parallel in xz plane: θ =9,θ 2 =9, φ= CO(Π x )+CO(Σ) CO(Σ)+CO(Π x ).8 CO perp CO out of xz: θ =9,θ 2 =9, φ=9 CO(Π x )+CO(Σ) CO(Σ)+CO(Π x ) population.6.4 population.6.4 population CO(Π y )+CO(Σ) CO(Σ)+CO(Π y ).8 CO(Π y )+CO(Σ) CO(Σ)+CO(Π y ).8 CO(Π y )+CO(Σ) CO(Σ)+CO(Π y ) population.6.4 population.6.4 population time (fs) time (fs) time (fs)

19 Förster RET Dependence with geometry Dimer with Frozen CO(r eq ) Orientation of molecules may depend on the mobility of adsorbing CO molecules, which changes with deposition temperature Can this explain the difference with deposition temperature? Muñoz Caro et al.( 6)

20 RET in water clusters Acocella, Jones & Zerbetto, JPCl ( 2)

21 2 crystaline celds of CO: R. Escribano Population.6.4 Population Population.6.4 Population Population.6.4 Population Population.6.4 Population time (fs)

22 8 celds of CO: R. Escribano R< 5.3 R> 26.5 a.u R<.35 a.u R< Population R< R< < R < time (fs) R<

23 Förster RET efficiency In (CO) n electronic energy travels very far (> 2Å) in 2-3 fs. This process is slower than in (H 2 O) n But dissociation/predissociation in (CO) n is much slower. Vibrations/dissociation will introduce decoherences This will reduce the efficiency of Förster RET A complete Quantum treatment is now-a-days no affordable A model using Classical Trajectories with Surface Hopping is now in progress In this model we should also consider: - Electronic predissociation - Reactivity to form CO 2 and C 2 O 2

24 Outline Introduction 2 Förster Resonance Energy transfer 3 Reactivity 4 Perspectives

25 CO 2 formation Chen et al., ApJ ( 4) CO 2 is formed, with a probability of 5 % The reaction is thought to be CO + hν CO CO + CO CO 2 + C( 3 P?) C + CO C 2 O C 2 O + CO C 3 O 2 CO 2 products reach a maximum Does it react? How?

26 Reactivity on a triplet state of CO? Slanger ( 68) detected emission of d 3 -a 3 Π attributed to absorption to CO( 3 ) near resonant spin-orbit? Photochemical Production of C 3 O 2 from CO Liuti, Dondes & Harteck ( 66) Hg ( P )+CO Hg+CO (a 3 Π, v = 3) CO + CO CO 2 + C( 3 P?) C + CO C 2 O C 2 O + CO C 3 O 2 Energy (ev) A( Π) e( 3 Σ - ) a ( 3 Σ + ) d( 3 ) a( 3 Π) X( Σ) R (Angstroms)

27 Outline Introduction 2 Förster Resonance Energy transfer 3 Reactivity 4 Perspectives

28 Perspectives Resonant Energy Transfer (RET) may be the mechanism for a fast energy transfer May explains why the absorption of inner excited CO can lead to photodesroption Decoherence (by rotations, vibrations) need to be included This will reduce the efficiency of the RET Use of a Surface Hopping like approach CO 2 formation: through triplet states by spin-orbit mixing

29 Acknowlegments Collaborators Guillermo Muñoz-Caro, CAB, Madrid Rafael Escribano, IEM-CSIC, Madrid Germán Molpeceres, IEM-CSIC, Madrid Alfredo Aguado, Univ. Autónoma Madrid Financial Support Ministerio Ciencia y Tecnología (SPAIN) Cosmic dust project (Redes de Excelencia) FIS C2