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Key Factors in Time-Resolved Spectroscopy: Temporal and Spectral Resolutions

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This document explores essential elements in time-resolved spectroscopy, focusing on temporal and spectral resolutions. Key factors include pulse duration, bandwidth, tunability, and efficient dynamics initiation through high-intensity pump pulses. Various methodologies such as flash photolysis, time-correlated single-photon counting, and the use of streak cameras are discussed, highlighting techniques for enhancing temporal resolution. The document also examines the challenges of using short pump and probe pulses, along with data analysis techniques like the lock-in method for differential absorption measurements.

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Key Factors in Time-Resolved Spectroscopy: Temporal and Spectral Resolutions

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  1. Time-resolved Spectroscopy A. Yartsev 16/04/2007

  2. Important Factors for Time-resolved Spectroscopy. • Temporal resolution – pulse duration. • Spectral resolution – bandwidth and tunability. • Efficient start of the dynamics of interest – high intensity of ”Pump” pulse. • Fast process to probe the dynamics – tunability and intensity of ”Probe” pulse. • Sensitive detection. • Data analysis and modelling. 16/04/2007

  3. Direct Temporal Resolution • Absorption: Flash-photolysis – fast detector and fast oscilloscope. • Fluorescence: Time-Correlated Single Photon Counting (TCSPC). • STREAK camera. 16/04/2007

  4. Pump-Probe Correlated Temporal Resolution. • ”Strong” Pump – ”weak” Probe • Transient absorption • Transient gaiting • ”Strong” Pump – ”strong” Probe • Multiphoton ionization • Integrated fluorescence • RAMAN etc. • ”Strong” Pump – ”strong” Gate • Fluorescence up-conversion • Optical Kerr Effect • Coherent methods 16/04/2007

  5. Temporal Resolution From Data Analysis. • Sub-instrumental response dynamics • Fluorescence phase-shift method • Excitation correlation fluorescence • Coherent: 3PEPS 16/04/2007

  6. Spectral Resolution. • Uncertainty principle limitation: short time needs broad spectrum! • Tunability of the pump and probe light is available through various lasers, frequency conversion and fs-continuum. • Spectral sensitivity of detector. • Is the uncertainty principle applied for detection as well? 16/04/2007

  7. Short Pump Pulses. • Fast excitation – temporally ”clean” start of process. • High intensity of the light – non-linear effects can be used for excitation and probing. 16/04/2007

  8. Problems with Short Pump Pulses. • Broad spectrum – lack of spectral selectivity. • Non-linearity may induce complications in the dynamics of interest. • Artefacts: - may complicate early time scale dynamics. 16/04/2007

  9. Short Probe Pulses. • Make fine grid to accuratelly resolve the dynamics. • Short probe pulse + fine time grid –accurately resolved dynamics. • Broad probe spectrum is good to resolve new transitions. 16/04/2007

  10. Problems with Short Probe Pulses. • Broad spectrum – lack of spectral selectivity. • Non-linearity may the probe-induced dynamics. • Artefacts: - spectrally non-even detection efficiency may lead to XFM. 16/04/2007

  11. What can we get from absorption? • Absorption spectrum as a fingerprint of a molecule. • From absorption, path length and Beer-Lambert law: • Concentration c[mol*dm-3] from extinction [dm3mol-1cm-1] • Or extinction  from concentration c. • Molecular cross-section [cm-2] from C[cm-3]. • transition dipole moment  from spectral shape of . • And with short pulses we can time-resolve this all! 16/04/2007

  12. Time-resolved Absorption • Single colour • Shot – to – shot. • Lock-in technique: chopped pump or both pump and probe are chopped at different frequencies and the signal is measured at differential frequency. • Pseudo two-colour. • Multiple colour • Single shot – single λprobe: point-by-point. • Single shot – whole spectrum. 16/04/2007

  13. Differential absorption: ”weak” Probe. • Lock-in technique: filter the Probe light noise out, keep the Pump contribution only. • Differential absorptionA(t) as a difference in transmission with- and without Pump. • Reference beam: bypassing or passing through the sample? • Locked-in reference beam scheme. 16/04/2007

  14. Lock-in Technique • Investigate the Probe beam fluctuations. • Modulating the Pump beam at a frequency in a ”silent” part of noise frequency spectrum. • Biuld a narrow frequency filter to transmit only the frequency of Pump modulation. 16/04/2007

  15. Differential absorption. Differential absorptionA(t) (transmission T(t)): A(t) = A(t) – A*(t) = -Log(I*out/I*in) + Log(Iout/Iin) How to convert A(t)into T(t)? How to measure A(t) with Iout only? If Iinis stable (Iin=I*in): A(t) = -Log(Iout/I*out) If Iinis not stable (IinI*in) a reference Iref is needed: A(t) = -Log(I*out Iref/I*refIout) 16/04/2007

  16. Locked-in Refence Scheme. When collecting large number of shots for averaging out the noise each pair of pulses with- and without- Pump is treated separately. The the long-time noise is then filtered out. 16/04/2007

  17. Polarized Light Pump ׀׀Probe: ΔA׀׀ and Pump Probe: ΔA Magic Angle signal (MA) is sensitive to population dynamics only MA = (ΔA׀׀+ 2ΔA)/3 Why MA signal can be measured at ~54.7 between pump and probe? And anisotropy signal r(t) is sensitive to dipole orientation only r(t) = (ΔA׀׀- ΔA)/(ΔA׀׀+ 2ΔA) 16/04/2007

  18. Instrumental function and zero-time. • Often a very good time-resolution has to be characterized ”at the spot”. • Instrumental response is generally varied over the wide probe spectrum. • Zero time position is crytical and often difficult to define. • Several options to characterize both: • SHG of Probe and Pump • Two-photon (one from Pump, one from Probe) absorption • Set of reference samples • OKE in samples with little nuclear response 16/04/2007

  19. Un-correlated and Correlated Noise. Un-correlated (independent) noise when ΔA׀׀ and ΔA aremeasured after each other – noise is of two measurements is larger than for each of them. ΔA׀׀ and ΔA aremeasured simultaneously for each laser pulse – may be much smaller (if noise is correlated). Important: identical temporal- and spatial- overlap with pump! 16/04/2007

  20. Signal-to-Noise: detectors • Two types of noise: Probe and Pump. • Light level: how accurate one can count photons? • Integrated- or spectrally- resolved detection? • Dark noise and digitizing – limitations of electronics. • Peak- or Integrating- detector? • Pump noise: normalization and spatial fluctuations. • Pump noise: one time-point – many shots or many time-points – few shots? • Averaging... For how long? 16/04/2007

  21. Dependence of the noise level on Probe-pulse energy. 16/04/2007

  22. Type of experiment Measurements per point Number of repeats Total measurements Number of time point used k1 (1/ps) Uncertainty 1 ±% k2 (1/ps) Uncertainty 2 ±% k3 (1/ps) Uncertainty 3 ±% Scan 300 12 3600 116 0.309 13.3 0.0348 89.8 0.00975 36.7 Scan 500 4 2000 284 0.327 13.2 0.0445 52.0 0.00977 13.6 Sweep 1 100 100 5652 0.314 3.6 0.0365 16.1 0.00962 3.9 16/04/2007

  23. How Strong Should Be Pump and Probe Light? • Pump intensity: ”linear” and ”non-linear” signal. • Non-linear absorption: multi-photon absorption and absorption saturation • Sequential (two-steps) absorption • Concentration-dependent dynamics. • Relative Pump/Probe intensity: Strong Pump – Weak Probe? • Probe intensity: how weak should be Probe? 16/04/2007

  24. How weak should be Probe? • Additional ΔA amplitude induced by Probe itself has to be smaller than the noise level needed to resolve Pump-induced changes. • Easily achievable out of absorption region. • In the absorption region: possible Probe self-induced effect in differential absorption. What should be the relative density of photons to induce 10-4 differential signal by Pump or by Probe itself? • Decrease Probe intensity by reducing number of photons or by increasing beam diameter. 16/04/2007

  25. Transient absorption: Advantages: • Probe pulse is relatively easy to tune. • Even “dark” excited states can be seen by S1 → Sn absorption. • Gives total picture of the involved components. • Very good temporal resolution and signal-to-noise. Disadvantage: • Sometimes too much information – difficult to interpret. Good to combine with time-resolved fluorescence. 16/04/2007

  26. Homodyne detection: transient grating. • In homodyne transient absorption (i.e. transient grating or OKE) only the signal field is recorded. Idet |Es(t)|2  A(t)  [R(t)*K(t)]2 R(t) – rotational correlation function, K(t) – populational decay function • In heterodyne scheme (i.e. Differential absorption) additional light field (Local oscillator) is added. Idet |ELO+Es|2 = Is + ILO + nc/4 Re[E*LO(t)Es(t)] Is is realtively weak, ILO can be removed by chopping  detected signal is linearized against Pump. 16/04/2007

  27. Time-resolved fluorescence. Clear method: emissive excited state dynamics. • Isotropic decay: MA(t) = (Ipar+2Iper)/3 • Anisotropy decay: r(t) = (Ipar-Iper)/(Ipar+2Iper) 16/04/2007

  28. Time-resolved fluorescence. Direct, electronic resolution. • Fast photodiode (PMT) + fast oscilloscope. • Time-correlated single photon counting • STREAK camera Inderect methods. • Fluorescence gaiting (up-conversion, etc.). • Excitation correlation method. • Phase-shift method. 16/04/2007

  29. TCSPC 16/04/2007

  30. TCSPC • Advantages: • High sensitivity • Statistical noise • Electronics-limited • Disadvantage: low time resolution: 20-30 ps • Sensitivity: (much less than) single photon level. 16/04/2007

  31. Schematic of STREAK camera 16/04/2007

  32. STREAK camera • Advantages: • Direct two-dimensional resolution. • Sensitivity down to single photon. • Very productive. • Disadvantage: • Depends on high stability of laser. • Limited time resolution: 2-10 ps. • Needs careful and frequent calibration. • Expensive. 16/04/2007

  33. Up-conversion • Advantage: (very) high time resolution, limited mainly by laser pulse duration. • Disadvantages: • Demanding in alignment. • Limited sensitivity, decreasing with increasing time resolution (crystal thickness). • Required signal calibration. J. Shah, IEEE J. Quant. Electr., 1988, 24, 276–288. M. A. Kahlow, W. Jarzeba, T. P. DuBruil and P. F. Barbara, Rev.Sci. Instr., 1988, 59, 1098–1109. 16/04/2007

  34. Fluorescence up-conversion set-up. L. Zhao, J. L. Perez Lustres, V. Farztdinov and N. P. Ernsting Phys . Chem. Chem. Phys . , v. 7 , 1716 – 1725, 2005 16/04/2007

  35. Broad-band up-conversion with amplified short pulses. L. Zhao, J. L. Perez Lustres, V. Farztdinov and N. P. Ernsting Phys . Chem. Chem. Phys . , v. 7 , 1716 – 1725, 2005 • Broad phase-matching by type II crystal • Tilted gate pulses for sub-100 fs resolution • Optimized scheme: ~ 1 count/channel per pulse 16/04/2007

  36. Fluorescence Kerr gating • Advantages: • Complete spectra – no phase matching • Good time resolution: 200-400 fs • Reasonable sensitivity • Disadvantage: • large background • Better resolution gives less signal 16/04/2007

  37. S. Arzhantsev and M. Maroncelli Applied Spectroscopy, V 59, N 2, 206-220,2005 Kerr gating 16/04/2007

  38. ”Strong” Pump – ”strong” Probe • Pump-induced intermediate is selectivelly in time and wavelength transfered into an easily detectable state. • Multiphoton ionization: very sensitive and accurate TOF detection • Pump-Probe induced fluorescence: measured by a sensitive integrating detector (PMT). • Probe-induced RAMAN or CARS scattering. 16/04/2007

  39. Time-resolved RAMAN and CARS • Record changes in vibrations to follow dynamics of the process. • Spontaneous scattering in RAMAN is amplified in CARS: stronger and spatially selected signals . • As CARS is strong and is often a molecule-specific time-resolved CARS of excited state can be used as a sensitive probe tool. 16/04/2007

  40. New twist of time-resolved spectroscopy. • By a first pulse prepare a particular state; • By the second pulse induce some dynamics in this state; • By a Probe pulse (strong or weak) resolve the dynamics in this new state. Pump-Dump-Probe; Pump-Re-Pump-Probe 16/04/2007

  41. Electro-induced differential absorption • The signal reflects EF-induced changes in the photoinduced dynamics. EDA(t) = -Log(IEFout Iref/IEFrefIout) • In this way, one can study a dynamic effect of EF not switching ON or OFF EF but rather by timely ”injection” of system of interest into EF 16/04/2007

  42. pump pulse probe pulse to detector SI from LASER Scheme of EDA experiment EF-generator 16/04/2007

  43. Optimal (Coherent) Control • This is another technique where the effect of Pump is specially treated. • The shape of the Pump pulse is optimized so that it has MAX influense on a particular process of interest. • In such a way, a minor part of regular sample response (for TL pulse) could be expressed and become dominant. 16/04/2007

  44. Pump - Shaped Dump – Probe Scheme 16/04/2007

  45. 16/04/2007

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