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Digitale Dissertation

Jürgen Full :
Ultrafast Photodissociation dynamics of Η5-CpMn(CO)3 (Cymantrene)
Theory for Analysis and Control
Ultraschnelle Photodissoziationsdynamik von Η5-CpMn(CO)3 (Cymantren)

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Abstract

This work deals with the quantum chemical and quantum dynamical description of the photodissociation of the transition metal complex cymantrene, CpMn(CO)3 (Cp = Η5-cyclopentadienyl), by means of ultrashort laser pulses (Femtochemistry). The goal is to understand recent pump-probe and control experiments performed by Wüste and coworkers (FB Physik, Freie Universität Berlin). The Mn-CO bond that dissociates first and ultrafast in experiment has been chosen as the reaction coordinate. Within the applied model, this coordinate lies in the plane of symmetry of the molecule, assuming CS symmetry and a staggered conformation of the Mn(CO)3 group with respect to the Cp ring. Quantum chemical ab initio potential energy curves for the lowest-lying neutral singlet and ionic doublet states are calculated along the reaction coordinate. The Complete Active Space Self-Consistent Field (CASSCF) method, followed by a Multireference Contracted Configuration Interaction (MR-CCI) treatment, is employed. CASSCF and MR-CCI transition dipole moments between neutral states are computed. The transition dipole moments coupling the neutral excited with the ionic states are approximated using the CI coefficients. In each symmetry, A' and A'', the two lowest excited singlet states, i.e. b1A' and c1A', and a1A'' and b1A'', avoid crossings in the Franck-Condon region. The kinetic couplings have been calculated numerically using the coefficients of the MR-CCI wave function and their influence on the photodissociation dynamics has been studied in both the adiabatic (kinetic coupling) and the diabatic (potential coupling) pictures, which are equivalent. Simulations of the pump-probe and control experiments are performed in the adiabatic representation. It is found that the nonadiabatic coupling between the a1A'' and the b1A'' states plays a crucial role in the interpretation of the pump-probe experiments, whereas the b1A' - c1A' coupling is negligible. A mechanism explaining the pump-probe experiments for the loss of the first CO ligand and another which decodes the optimal laser pulse optimizing the parent, CpMn(CO)3+, ion yield are proposed. The given analysis can also be extended to predictions about future optimization experiments yielding predominantly the first daughter ion, CpMn(CO)2+. In conclusion, this thesis presents the first ananlysis of the quantum mechanical details of an optimal control experiment yielding preferably the target ion, CpMn(CO)3+, while suppressing competing channels.

Table of Contents

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0 Titelblatt
2 Introduction 8
1.1 Basic concepts 10
1.1.1 Electronic transitions 10
1.1.2 Types of photodissociation 11
1.2 Experimental background and one-dimensional model 14
1.2.1 Photochemistry of CpMn(CO)3 14
1.2.2 Experimental structure of CpMn(CO)3 and UV-spectrum 16
1.2.3 Pump-probe experiments 17
2 Theory 19
2.1 Schrödinger equation, molecular Hamiltonian and coupling with the laser field 19
2.2 Adiabatic and diabatic representation of the time-dependent Schrödinger equation 21
2.2.1 Derivation of the adiabatic representation 21
2.2.2 Adiabatic description of the Mn-COax photodissociation 24
2.2.3 An alternative description: The diabatic picture 26
2.2.4 Comparison between the adiabatic and the diabatic representation 28
2.2.5 Born-Oppenheimer dynamics 29
2.2.6 Population and probability of dissociation 30
2.3 Rotational averaging 32
2.4 Time dependent calculation of absorption spectra 34
2.5 Pump-probe ionization spectroscopy 35
2.6 Calculation of the initial wave function: The Fourier Grid Hamiltonian (FGH) method 36
2.7 Propagation schemes for the time dependent Schrödinger equation 39
2.7.1 Second Order Differencing (SOD) 40
2.7.2 Split Operator 42
2.8 Solution of the electronic Schrödinger equation using ab initio and DFT methods 42
2.8.1 Standard quantum chemical (ab initio) methods 43
2.8.2 DFT methods 47
2.9 Calculation of the kinetic coupling terms T(1) and T(2) 49
2.9.1 General Properties of T(1) and T(2) 49
2.9.2 Calculation of the kinetic coupling term T(1) using a multiconfigurational wave function 50
2.9.3 Calculation of the kinetic coupling term T(2) using T(1) 51
3 Quantum chemistry calculations 53
3.1 Computational details 53
3.1.1 Basis sets 53
3.1.2 Applied ab initio methods 54
3.2 CASSCF and DFT geometry optimizations 55
3.2.1 Structure of the parent molecule Η5-CpMn(CO)3< /SUB> 56
3.2.2 Structure of the fragment Η5-CpMn(CO)2 57
3.3 CASSCF/CASPT2 and TD-DFT vertical excitation spectrum 58
3.4 Comparison with MRCI excitation energies 61
3.5 CASSCF/MR-CCI potential curves and transition dipole moments 63
3.6 The kinetic couplings 67
3.6.1 The kinetic coupling term T(1) 67
3.6.2 The kinetic coupling term T(2) 74
3.6.3 Source of errors 76
3.7 Ionic potentials and approximated transition dipole moments between ionic and neutral states 78
3.7.1 Ionic state potentials 78
3.7.2 Transition dipole moments between the neutral A' states and the ionic states 78
3.7.3 Transition dipole moments coupling the A'' states with the ionic states 82
4 Dynamical simulations of pump-probe ionization spectroscopy: Analysis and control 85
4.1 Influence of the diabatic b1A' - c1A' and a1A'' - b1A'' coupling on the dynamics 86
4.2 Theoretical absorption spectrum 92
4.3 Probability of dissociation on the b1A' and c1A' states using rotational averaging 94
4.4 Dynamics on the low-lying excited states potentials induced by femtosecond laser pulses 98
4.5 Simulation of pump-probe and control experiments 105
4.5.1 Pump probe spectra with pump transitions to the b1A' and c1A' states 107
4.5.2 Analysis: Theoretical and experimental pump-probe spectra of the parent and the daughter ion 111
4.5.3 Control: Wave packet dynamics induced by the optimal control pulse for (CpMn(CO)3)+ 119
5 Conclusions and outlook 125
A Calculation of absorption spectra by time-dependent methods 128
A.1 Derivation of a microscopic expression for the absorption cross section using time dependent perturbation theory 128
A.2 Time-dependent calculation of the absorption cross section: The autocorrelation function 134
B Kinetic couplings 138
B.1 Kinetic coupling terms T(1) of the A' and A'' states 138
B.1.1 Splined curves of the CI coefficients in the case of symmetry A' 138
B.1.2 Splined curves of the orbital coefficients in the case of symmetry A' 141
B.1.3 Kinetic coupling term T(1) of the A' states 142
B.1.4 Splined curves of the CI coefficients in the case of symmetry A'' 148
B.2 Kinetic coupling terms T(2) of the A' and A'' states 150
Literature 153

More Information:

Online available: http://www.diss.fu-berlin.de/2002/215/indexe.html
Language of PhDThesis: english
Keywords: Photodissociation Dynamics, Quantum Chemistry, Analysis, Control
DNB-Sachgruppe: 30 Chemie
Date of disputation: 22-Oct-2002
PhDThesis from: Fachbereich Biologie, Chemie, Pharmazie, Freie Universität Berlin
First Referee: Prof. Dr. Jörn Manz
Second Referee: Prof. Dr. Ludger Wöste
Contact (Author): Juergen.Full@gmx.de
Contact (Advisor): manz@chemie.fu-berlin.de
Date created:24-Oct-2002
Date available:25-Oct-2002

 

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