Basic Photoexcitation and Modulation Spectroscopy

Transcription

1 Basic Photoexcitation and Modulation Spectroscopy Intro Review lock-in detection Photoinduced absorption Electroabsorption (Stark) Spectroscopy Charge Modulation

2 Photoexcite sample Take absorption spectra of photoexcited species Quantities of interest: spectral features, kinetics, cross-sections Can take data in time or frequency domain Frequently plot data as T/T (or converted to )

3 Transient Photoexcitation Spectrum Further Reading: J.C. Diels and W. Rudolph. Ultrashort Laser Pulse Phenomena. Academic Press, San Diego This book has a general description of mode-locked lasers, ultrafast measurement techniques, and experiments with femtosecond lasers G.R. Fleming. Chemical Applications of Ultrafast Spectroscopy. Oxford University Press, London The ultrafast technology described in this book is a bit outdated but it useful for a qualitative understanding of basic concepts in ultrafast spectroscopy, particularly transient absorption and fluorescence upconversion.

4 Quasi-CW Photoinduced Absorption Photoexcite sample Take absorption spectra of photoexcited species Plot data as fractional change in transmission of probe beam T/T can measure T/T ~ 5E-7 (with W-Halogen source) using lock-in detection Ar + or Diode Laser Chopper Lock-in Amplifier X Ref Y W Light source Monochromator Sample in Cryostat Monochromator Detector Si (VIS) InAs (IR) Laser pump beam creates excited states--these generate new absorptions, monitored with the probe beam Steady-state photoinduced absorption (as opposed to ultrafast or transient photoinduced absorption) 2 nd monochromator not essential for all materials, but improves signal/noise for fluorescent/electroluminescent samples

5 Lock-in Detection Basics Preamplification stage Voltage input Low pass filter amp X output Phase sensitive 90º phase shift detectors Low pass filter amp Y output Reference input Internal oscillator/ sine wave generator X R cos( ) Y Rsin( ) R X tan 2 1 Y ( Y X ) 2 In phase quadrature phase angle difference in phase betw/ signal and ref.

6 Lock-in Detection Basics Phase sensitive detector : outputs the product of the reference and input signals Consider each input signal frequency component: V ref V sin( t) V V sin( t) r r inp i i V in V ref V i sin( t) V i r sin( t) r 1 2 VV i r cos( ) cos( ) r i r i for i = r you get a DC component low-pass filter Also see J. Chem. Ed. 49 (3) 1972 p A (4) 1972 p A211

7 Lock-in Detection Basics Simulated signal at various stages of detection Noisy input (signal at ~1Hz) signal/noise = Raw Output of PSD

8 Lock-in Detection Basics 0.9 PSD output-low-pass Filtered Output of PSD after low-pass filtering signal/noise= PSD output-low-pass Filtered- Noise Only (1000x improvement) 0.3 Dynamic Reserve typically for 0.1 modern DSP lockins

9 Lock-in Detection Basics Power What am I doing? Picking out a fourier component from the signal at frequency (or 2, etc.) (meanwhile the DC noise is shifted to frequency ) Frequency

10 Photoinduced Absorption Photoexcite sample Take absorption spectra of photoexcited species Plot data as fractional change in transmission of probe beam T/T can measure T/T ~ 5E-7 (with W-Halogen source) Ar + or Diode Laser Chopper Lock-in Amplifier X Ref Y W Light source Monochromator Sample in Cryostat Monochromator Detector Si (VIS) InAs (IR) Laser pump beam creates excited states--these generate new absorptions, monitored with the probe beam Steady-state photoinduced absorption (as opposed to ultrafast or transient photoinduced absorption) 2 nd monochromator not essential for all materials, but improves signal/noise for fluorescent/electroluminescent samples

11 Example 1: Photoinduced Absorption Where can this be useful? long-lived ( s-ms) states LUMO+1 LUMO charge-separated states triplet states ISC HOMO HOMO-1 radical cation (positive polaron) Commonly used to characterize states in optoelectronic materials: conjugated polymers colloidal quantum dots

12 Spectrum of Positive Polaron in alkoxy-ppvs 4 2 T=293 K Y Channel 4.0 nm CdSe T/T x MEH-PPV two induced absorptions due to polaron on polymer nm CdSe X Channel Energy (ev) Spectra from Ginger and Greenham PRB 59 (1999) p 10622

13 Triplet Absorption in alkoxy-ppv Spectra from Ginger and Greenham PRB 59 (1999) p 10622

14 T (Arb. Units) Temperature Dependence Spectral information to identify species Frequency, intensity, and temperature dependence used to assign absorptions and characterize states MEH-PPV Triplet (1.34 ev) Blend signal at (1.34 ev) thermally activated recombination Blend polaron (0.5 ev) T (K) See both polarons and triplets in blend Polarons only weakly T-dependent thermally activated charge transfer

15 T/T Electron Spectra on CdSe Quantum Dots PIA Spectrum of CdSe Nanocrystals 150 mw/cm nm excitation at 125 Hz, 295 K Quadrature In Phase (absorption) bleach 1S 3/2 1S e position (530 nm) Energy (ev) Strong IR absorption coupled with visible bleach Red shift of bleach from 1st excitonic peak (Förster transfer) Significant out of phase component (long lifetimes) TOPO route, pyridine-treated CdSe nanocrystals Solid film in polystyrene matrix (on sapphire substrate)

16 - T/T Electron Spectra on CdSe Quantum Dots nm (pyridine) 2.7 nm (pyridine) 3.1 nm (pyridine) nm (TOPO) (x10 intensity) Energy (ev)

17 Photoinduced Absorption: monomolecular decay Assume signal is proportional to number of excitations present, n(t) generation rate: gi[ 1 cos( t)] I intensity, g generation efficiency, chop frequency dn 1 gi[1 cos( t)] n dt gi n( t) gi cos( t ) S(, ) gi tan 1 ( ) Signal is linear with pump intensity at all frequencies Behaves as -1 for >> 1 Independent of for << 1 Frequency dependent scan can yield lifetime Dellepiane et al. PRB 48 (1993) p

18 Photoinduced Absorption: bimolecular decay Decay/generation now dn dt gi[ 1 cos( t)] n 2 Approximate solution is: S(, ) s s 1 gi gi tanh( ) tanh( ) For s <<1 signal is indep. of and is dep. on I 1/2 For s >>1 signal is -1 and linearly dep. on I So intensity dependence can separate monomolecular and bimolecular decay behavior--but only for s <<1 (low chop freq)

19 T/T Cross over from monomolecular to bimolecular decay observed at increased pump fluence I I Power (mw cm -2 )

20 Photoinduced Absorption: monomolecular decay Monomolecular lifetime for electron-hole recombination in CdSe 10-4 S(, ) gi Fit yields =1.0 ms (rad s -1 )

21 Triplet Lifetime in Pristine CN-PPV as a function of T fits =210, 130, 50 s

22 An aside on units: You are familiar with molar absorption coefficient (log 10 ) and molar absorption coefficient (ln) I=I 0 e - Cl (C=moles/liter, l=path length in cm) has same units as a molecular cross-section at a specific wavelength: = cm 2 / molecule If you integrate over a entire absorption peak the integrated cross-section of an absorption peak has units integrated = cm / molecule

23 Stark / Electroabsorption Spectroscopy Studies effect of an applied field on emission or absorption Variously referred to as Stark spectroscopy, electroabsorption spectroscopy, electrochromism and electro-optical absorption spectroscopy Early measurements used to test molecular orbital theory (orbital size, dipole, etc.) Modern applications: organic electronics, NLO materials, determining and, measuring degree of charge separation/spatial extent of states, location of 2 photon or continuum states, measurement of disorder, electrochromic sensing, etc. Bublitz and Boxer Ann. Rev. Phys. Chem :213

24 Stark / Electroabsorption Spectroscopy

25 Stark / Electroabsorption Spectroscopy Applying an electric field has two effects: 1) To change the Hamiltonian H = H o + H 2) To mix wavefunctions which were previously orthogonal Yj (F) = Yj (0) + dk Yi (0) where dk ~ <Yi ij F Yj > / (Ej Ei) Typically shifts are small (tens ev) so treat with perturbation theory.

26 Stark / Electroabsorption Spectroscopy Consider change in absorption coefficient from placing dipole in an electric field basic Taylor expansion yields:

27 Stark / Electroabsorption Spectroscopy In terms of basic physical parameters: F applied field p polarizability m f dipole moment of state (note: this model ignores transfer of oscillator strength, mixing of states, these generally lead to a linear component and a shift in E with F, also local energetic site disorder leads to significant second derivative component)

28 2) The random orientations of the individual dipoles will lead to a broadening of the transition energy (some increase some decrease) a second derivative lineshape Stark / Electroabsorption Spectroscopy Linear term (in F) should contain info. about aligned dipoles (averages to zero in a disordered sample)--there is no linear term for most measurements F 2 term has two contributions: 1) The applied field will induce a dipole of magnitude ~pf (p=polarizabilty), whose energy shift will be (pf)*f=pf 2 All these induced dipoles will be aligned with the applied field, and will shift up or down in E together (so F 2 dependence and a first derivative lineshape)

29 Stark / Electroabsorption Spectroscopy

30 Stark / Electroabsorption Spectroscopy 1 mm

31 Stark / Electroabsorption Spectroscopy Liptay analysis Dipole Moments of Molecules in Excited States and the Effect of External Electric Fields on the Optical Absorption of Molecules in Solution, W. Liptay in Modern Quantum Chemistry: Istanbul Lectures, O. Sinanoglu, Ed p45-65

32 Stark / Electroabsorption Spectroscopy

33 Stark / Electroabsorption Spectroscopy In many interesting problems we don t know the electric field strength and use Stark spectroscopy as an analytical tool to determine it (field in SAMs, e-chem, due to charge transfer states, in organic LEDs) e.g.

34 Stark / Electroabsorption Spectroscopy Apply a DC bias, V 0 to nullify built in field in device, apply time-varying potential V AC to generate a time-varying electroabsorption signal:

35 Stark / Electroabsorption Spectroscopy Brown, Cacialli and Friend

36 Stark / Electroabsorption Spectroscopy Brown, Cacialli and Friend

37 Stark / Electroabsorption Spectroscopy Multilayer device

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