Techniques in Low Temperature Optical Spectroscopy: Lasers, Liquid Helium and Witchcraft

1. Techniques in Low Temperature Optical Spectroscopy:Lasers, Liquid Heliumand Witchcraft Christie L. Larochelle

2. KAg(CN)2 in H2O La(NO3)3 in H2O 1% Agar gel Synthesis:Problems and Solutions • Many compounds of the type La[Ag(CN)2]3 insoluble in water/solvents • Use gel method instead

3. Layered Structure • Crystals of the type R[M(CN)2]3 form in layered structures • La[Ag(CN)2]3

4. What is Spectroscopy? • Shine light of a single wavelength (monochromatic) onto a sample and the sample will emit light of a different color • Allows us to study energy levels and how they change with changes in physical parameters

6. Apparatus:Steady-State Spectroscopy • 75 Watt xenon lamp • Computer-controlled excitation and emission monochromators

7. 5 x 10 5 302 nm 304 nm 4 3 Intensity (arbitrary) 2 1 0 300 350 400 450 500 550 600 l (nm) em 5 x 10 4 306 nm 308 nm 3 Intensity (arbitrary) 2 1 0 300 350 400 450 500 550 600 l (nm) em La[Ag(CN)2]3 • Changes in lex result in changes in emission bands-site-selective excitation

8. 6 l =352 nm em l =382 nm em l =470 nm em 5 4 Intensity (arbitrary) 3 2 1 0 220 240 260 280 300 320 340 360 l (nm) ex Excitation SpectraLa[Ag(CN)2]3 • Spectra taken at 80 K • Corrected using quantum counter (rhodamine B)

9. 4 x 10 3.5 5 x 10 18 +3 Tb 16 3 14 12 2.5 10 Intensity, arbitrary 8 2 ) -1 6 Energy (cm 4 1.5 2 0 400 450 500 550 600 650 700 1 l , nm em 0.5 0 Atomic Energy Levels • Well defined energy levels lead to sharply defined emission bands

10. 4 x 10 3.5 6 x 10 2 3 -1 Au(CN) 2 1.8 2.5 1.6 1.4 ) -1 2 1.2 Energy (cm Intensity, arbitrary 1 1.5 0.8 0.6 1 0.4 0.5 0.2 0 350 400 450 500 550 600 0 l , nm em Molecular Energy Levels em em • Vibrations in molecules and crystals broaden the emission bands

11. Low-T Apparatus

12. Energy Transfer Primer • Selective excitation of “donor” (M(CN)2-) results in luminescence of “acceptor” (rare earth, Tb3+, Eu3+, etc.) • Depends on presence of spectral overlap

13. 5 x 10 5 302 nm 304 nm 4 3 Intensity (arbitrary) 2 1 0 300 350 400 450 500 550 600 l (nm) em 5 x 10 4 306 nm 308 nm 3 Intensity (arbitrary) 2 1 0 300 350 400 450 500 550 600 l (nm) em La[Ag(CN)2]3 • Changes in lex result in changes in emission bands-site-selective excitation

14. 6 x 10 2 Room Temperature 1.8 80 K 1.6 1.4 1.2 1 Intensity, arbitrary 0.8 0.6 0.4 0.2 0 350 400 450 500 550 600 l , nm em La[Au(CN)2]3 • Emission bands red-shift with decreasing temperature • lex=337 nm

15. 4 x 10 3.5 4 x 10 3.5 Ag(CN)2- Tb+3 Au(CN)2- Au(CN)2- 3 3 Eu+3 5D3 RT 80 K 2.5 RT 80 K 5D3 2.5 5D4 5D2 2 High Pressure 20 K 5D1 2 1.5 5D0 High Pressure 20 K Energy (cm-1) 1 1.5 7F0 1 2 0.5 3 4 1 5 0 7F6 7F6 0.5 5 4 3 2 1 0 7F0 Energy (cm-1)

16. La[Au(CN)2]3 and La[Ag(CN)2]3 • Luminescence intensity decreases significantly with increasing T

17. 6 x 10 2.5 315 nm 324 nm 2 329 nm 1.5 Intensity (arbitrary) 1 0.5 0 320 340 360 380 400 420 440 460 480 500 l (nm) em 6 x 10 2.5 331 nm 333 nm 2 339 nm 1.5 Intensity (arbitrary) 1 0.5 0 320 340 360 380 400 420 440 460 480 500 l (nm) La[Ag0.9Au0.1(CN)2]3 • We observe site-selective excitation, similar to the silver sample em

18. La[Ag0.5Au0.5(CN)2]3 • We observe site-selective excitation at very low T

19. La[Ag0.5Au0.5(CN)2]3 • Emission bands red-shift with decreasing T, similar to the gold sample

20. La[AgxAu1-x(CN)2]3 • Luminescence intensity at ambient temperatures is comparable to that at low temperatures

21. Why do We Care? • Tunable energy transfer between mixed-metal donors and rare earth acceptors • Luminescence at ambient temperatures indicated for applications

22. Solid State Pulsed Laser (266 nm) Monochromator, PMT, Digital Oscilloscope, PC Sample Apparatus:Time-Resolved Spectroscopy • Laser pulses have < 1 ns duration • Current apparatus capable of lifetime measurements to ~100 ns

23. Lifetimes:Why Do We Care? • Lifetime values indicate the nature of the excited state and transitions (allowed vs. forbidden) • For a single exponential decay: I=I0e-t/т and ln(I)=ln(I0)-t/т • Non-exponential decays often indicate energy transfer

24. 4.4 K 9.1 K 22 K 40 K 62 K Ln(I) 0 -1 -2 -3 -4 -5 -6 Time (ns) -7 -8 -9 -10 0 0.5 1 1.5 2 2.5 4 x 10 Lifetime Analysis, 50/50 • Below 40 K, lifetime increases and is no longer single exponential

25. Lifetime Analysis • Ag.9Au.1 lifetimes decrease with increasing temperature • 397 nm: 1400 ns at 10 K to 260 ns at RT • Lifetimes are single exponential at all T • Lifetime is given by: em em kRis the radiative decay constant kNRis the non-radiative decay constant

26. Lifetime Analysis, 90/10 • Lifetimes are single exponential at all T and increase dramatically with decreasing T

27. DE Proposed Model • Two closely spaced states arising from spin-orbit splitting • DE goes as Z2 • Assume thermal equilibrium

28. Proposed Model • Applied in Au(CN)2- systems (Cs, Tl) • Lifetimes as a function of temperature described by: t=(1+2e-DE/kT)/(k1+2k2e-DE/kT)

29. Take-Home Message • Tunability factors: • lex • T • x • Stronger ambient luminescence than pure systems

30. Possible rare earth acceptor ions: Sm3+, Eu3+, Dy3+

31. Summary and Conclusions • Preliminary results indicate that excited states in the compounds La[AgxAu1-x(CN)2]3 depend on x: tunabilityfactor • Mixed-metal solids show stronger luminescence than pure solids at ambient temperatures

32. Future Directions • R[AgxAu1-x(CN)2]3 with R=Dy, Eu, Sm • Multiple acceptors

33. Acknowledgements • Financial Support: • Petroleum Research Fund, administered by the American Chemical Society • Department of Physics, UMaine

34. Acknowledgements • Collaborators: • Richard Staples, Harvard University • George Shankle, Angelo State University • Mohammad Omary, North Texas University