Ψ t = ih Ψ t t. Time Dependent Wave Equation Quantum Mechanical Description. Hamiltonian Static/Time-dependent. Time-dependent Energy operator

Size: px

Start display at page:

Download "Ψ t = ih Ψ t t. Time Dependent Wave Equation Quantum Mechanical Description. Hamiltonian Static/Time-dependent. Time-dependent Energy operator"

Lora Moore
5 years ago
Views:

1 Time Dependent Wave Equation Quantum Mechanical Description Hamiltonian Static/Time-dependent Time-dependent Energy operator H 0 + H t Ψ t = ih Ψ t t The Hamiltonian and wavefunction are time-dependent

2 There is a relationship between timedependent and static wave functions The total Hamiltonian is composed of two parts: Total Static Time-dependent H total = H 0 + H t Ψ t = Ψe ih total t/h We can use the time-dependent Hamiltonian to connect static wavefunctions of different states.

3 The transition probability calculated using the Fermi Golden Rule P 12 = Ψ 1 Η Ψ 2 4h 2 2 sin 2 ω ω 12 2 ω ω t This expression is derived using time-dependent perturbation theory. It is valid for a number of time-dependent processes that involve transitions between states 1 and 2. The rate constant k 12 = P 12 /t.

4 The Fermi Golden Rule can be used to calculate many types of transitions Transition H(t) dependence 1. Optical transitions Electric field 2. NMR transitions Magnetic field 3. Electron transfer Non-adiabaticity 4. Energy transfer Dipole-dipole 5. Atom transfer Non-adiabaticity 6. Internal conversion Non-adiabaticity 7. Intersystem crossing Spin-orbit coupling

5 Optical electromagnetic radiation permits transitions among electronic states Η t = μ E t where μ is the dipole operator and the dot represents the dot product. If the dipole μ is aligned with the electric vector E(t) then H(t) = - μe(t). If they are perpendicular then H(t) = 0. μ = er where e is the charge on an electron and r is the distance.

6 The time-dependent perturbation has the form of an time-varying electric field E t = E 0 cos ωt where ω is the angular frequency. The electric field oscillation drives a polarization in an atom or molecule. A polarization is a coherent oscillation between two electronic states. The symmetry of the states must be correct in order for the polarization to be created. The orientation average and time average over the square of the field is [-μ. E(t)] 2 is μ 2 E 02 /6.

7 Absorption of visible or ultraviolet radiation leads to electronic transitions σ Polarization of Radiation s s σ

8 Absorption of visible or ultraviolet radiation leads to electronic transitions σ Transition moment s s The change in nodal structure also implies a change in orbital angular momentum. σ

9 The interaction of electromagnetic radiation with a transition moment The electromagnetic wave has an angular momentum of 1. Therefore, an atom or molecule must have a change of 1 in its orbital angular momentum to conserve this quantity. This can be seen for hydrogen atom: Electric vector of radiation l = 0 l = 1

10 The Fermi Golden Rule for optical electronic transitions 2 π e Ψ q Ψ E0 k = δω ω 12 6h 2 12 The rate constant is proportional to the square of the matrix element e< Ψ 1 q Ψ 2 > times a delta function. The delta function is an energy matching function: δ(ω - ω 12 ) = 1 if ω = ω 12 δ(ω - ω 12 ) = 0 if ω ω 12.

11 A propagating wave of electromagnetic radiation of wavelength l has an oscillating electric dipole, E (magnetic dipole not shown) λ E

22 The oscillating electric dipole, E, can induce an oscillating dipole in a molecule as the radiation passes through the sample λ E

37 The oscillating electric dipole, E, can induce an oscillating dipole in a molecule as the radiation passes through the sample l v=1 R O R E = hc/λ v=0 R R O The type of induced oscillating dipole depends on λ. If λ corresponds to a vibrational energy gap, then radiation will be absorbed, and a molecular vibrational transition will result

38 The oscillating electric dipole, E, can induce an oscillating dipole in a molecule as the radiation passes through the sample λ LUMO HOMO E = hc/λ If λ corresponds to a electronic energy gap, then radiation will be absorbed, and an electron will be promoted to an unfilled MO

39 The absorption of light by molecules is is subject to several selection rules. From a group theory perspective, the basis of these selection rules is that the transition between two states a and b is electric dipole allowed if the electric dipole moment matrix element is non-zero, i.e., aμ b = * ψ a μψ b dτ 0 where μ = μ x + μ y + μ z is the electric dipole moment operator which transforms in the same manner as the p- orbitals ψ a μ ψ b = ψ a (μ x + μ y + μ z ) ψ b, must contain the totally symmetric irrep or put another way, ψ a ψ b must transform as any one of μ x, μ y, μ z

40 Direct Products: The representation of the product of two representations is given by the product of the characters of the two representations. Verify that under C 2v symmetry A 2 B 1 = B 2 C 2v E C 2 σ v σ' v A B A 2 B As can be seen above, the characters of A 2 B 1 are those of the B 2 irrep.

41 Verify that A 2 B 2 = B 1, B 2 B 1 = A 2 Also verify that the product of any non degenerate representation with itself is totally symmetric and the product of any representation with the totally symmetric representation yields the original representation Note that, A x B = B; while A x A = B x B = A g x u = u; while g x g = u x u =g.

42 Light can be depicted as mutually orthogonal oscillating electric and magnetic dipoles. In infrared and electronic absorption spectroscopies, light is said to be absorbed when the oscillating electric field component of light induces an electric dipole in a molecule. Electric vector of radiation l = 0 l = 1 For a hydrogen atom, we can view the electromagnetic radiation as mixing the 1s and 2p orbitals transiently.

43 Is the orbital transition d yz p x electric dipole allowed in C 2v symmetry? p x μ d yz b 1 b 1 b 2 b 2 = a 1 a 1 a 2 b 1 b 2 = b 2 b 1 a 2 None of the three components contains the a 1 representation, so this transition is electric dipole forbidden A transition between two non-degenerate states will be allowed only if the direct product of the two state symmetries is the same irrep as one of the components of the electric dipole

44 How about an a 1 b 2 orbital transition? μ b 2 b 1 b 2 a 1 a 1 = a 2 a 1 b 2 a 1 = a 2 a 1 b 2 Since m y makes the transition allowed, the transition is said to be "y-allowed" or "y-polarized" Remember the shortcut: a 1 b 2 = b 2 which transforms as μ y Problem Indicate whether each of the following metal localized transitions is electric dipole allowed in PtCl (a) d xy p z (b) d yz d z 2 (c) d x 2 -y 2 p x,p y (d) p z s

Lecture 0. NC State University

Lecture 0. NC State University Chemistry 736 Lecture 0 Overview NC State University Overview of Spectroscopy Electronic states and energies Transitions between states Absorption and emission Electronic spectroscopy Instrumentation Concepts