6715_Lecture6 - Lecture VI VI-1 Fluorescence Introduction...

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1 PHYS 6715 - Lecture VI 1 Lecture VI Fluorescence 1. Introduction 2. Spectroscopy measurements 3. Time-domain measurements PHYS 6715 - Lecture VI 2 VI-1 Introduction energy 1 4 3 2 = Δ 2 2 2 0 4 1 1 8 f i n n h me E ε 13.7 eV = 91 nm outer shell: n>1 Electronic transitions PHYS 6715 - Lecture VI 3 VI-1 Introduction energy ω = = Δ + : levels oscillator 1 n n E r 0 r r A 5 . 0 0 ± r r eV 5 U 14 2.5 10 rad/sec km →= = × Representative values: 2 2 J/m 10 6 ~ × k ( ) 2 0 2 1 r r k U m 6 μ λ = 6 amu (2 carbon nuclei) m = mid-IR Vibrational transitions PHYS 6715 - Lecture VI 4 VI-1 Introduction 2 1 2 JJ EJ mr →+ Δ =⋅ = 0.1 rn m = eV 10 2 3 × 6 amu m = microwave regime Representative values: mm 5 . 0 1 0 Rotational transitions (J: quantum # of angular momentum) PHYS 6715 - Lecture VI 5 VI-1 Introduction Absorption spectrum DNA electronic vibrational rotational PHYS 6715 - Lecture VI 6 VI-1 Introduction Fluorescence • Luminescence is defined as light emission from systems at electronically excited states due to optical excitation with shorter wavelength. • Luminescence can be classified as either fluorescence or phosphorescence. • Fluorescence = luminescence due to electronic transition from singlet excited states. The relaxation times are short, 1~10ns, because large probability of transition to the singlet ground state. • Phosphorescence = luminescence due to electronic transition from triplet excited states. The relaxation times are long, >~1ms, because small probability of transition to the singlet ground state.
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2 PHYS 6715 - Lecture VI 7 VI-1 Introduction Fluorescence Jablonski diagram (F or P < A) PHYS 6715 - Lecture VI 8 VI-1 Introduction Fluorescence Jablonski diagram (F or P < A) PHYS 6715 - Lecture VI 9 VI-1 Introduction Fluorescence • The emitters of luminescence are, to biological systems, molecules and has emission spectrum much wider than those from atoms. Æ Luminescence measurements provide tools to probe a system at the molecular level. • Fluorophores = molecules emitting fluorescence, typically aromatic molecules. • Phosphores = molecules emitting phosphorescence. • Stokes’ shift = the energy difference between excitation and emission photons. (because λ A < λ F ) • Kasha’s rule = emission spectrum is generally independent of excitation wavelength (because fluorescence most likely occurs between S 1 and S 0 ). PHYS 6715 - Lecture VI 10 VI-1 Introduction Fluorescence • Even though emission spectrum (in relative unit) may be independent of excitation parameter, but the absolute magnitude of fluorescence signals depends on the excitation because absorption of excitation is a function of wavelength. • Chromophores = molecules absorbing photons. • Intrinsic fluorophores are those occur naturally. • Extrinsic fluorophores are those added to the system under study.
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This note was uploaded on 01/21/2012 for the course PHYS 6720 taught by Professor Hu during the Spring '10 term at East Carolina University .

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6715_Lecture6 - Lecture VI VI-1 Fluorescence Introduction...

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