09_lecnotes_rwf

09_lecnotes_rwf - MIT Department of Chemistry 5.74, Spring...

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5.74 RWF Lecture #9 and #10 9, 10 – 1 From Quantum Beats to Wavepackets Reading : Chapter 9.2.1-4, The Spectra and Dynamics of Diatomic Molecules , H. Lefebvre-Brion and R. Field, 2 nd Ed., Academic Press, 2004. Last time: How to get a glimpse of mechanism: cause and effect e.g. The effect might be N 1 t and the cause might be h 12 t . d dt d dt t N h 1 12 12 = () h ωω relation between cause and effect or i d dt t t h N 1 =Ω−Ω transfer rate operator Today: classes of pluck classes of wavepacket Quantum Beats are usually simple, few-level coherences. Wavepackets are usually more complex, many-level coherences. Both are the result of a pluck where t < h E . Kinds of pluck: * merely short * genuinely localized A localized pluck prepares a zero-order non-eigenstate. The localized state character of this non-eigenstate is distributed (“fractionated”) over eigenstates that span an energy range E localized . h/ t pluck bright dark E localized MIT Department of Chemistry 5.74, Spring 2004: Introductory Quantum Mechanics II Instructor: Prof. Robert Field
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5.74 RWF Lecture #9 and #10 9, 10 – 2 If the pluck is sufficiently short, it prepares an a priori known initially localized excitation. “genuinely localized” If the pluck is not sufficiently short, it prepares an ill-specified coherent superposition “merely short” Classes of localization * polarization quantum beats (angularly localized) * population quantum beats (spatially localized or localized in state space, e.g., vibrational wavepacket) Polarization QB - easiest kind to observe, even with a long excitation pulse. Why? ((Hanle effect)) 3 2 J M–1 J M+1 x y JM 1 J M fluorescence pulsed excitation I ( t ) = Trace ( D U ( t ,0) E ρ (0) E U ( t ,0)) x y y x DC Magnetic field in z direction Excite with x polarized light propagating in y direction Detect light propagating in z direction, II xy + . Zeeman Splitting z
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5.74 RWF Lecture #9 and #10 9, 10 – 3 see polarization QB as B-field increases from 0.
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This note was uploaded on 11/28/2011 for the course CHEM 5.74 taught by Professor Robertfield during the Spring '04 term at MIT.

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09_lecnotes_rwf - MIT Department of Chemistry 5.74, Spring...

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