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

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

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

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

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

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

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

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

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

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

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

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

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

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

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 Iin is stable (Iin= I*in): A(t) = -Log(Iout/I*out) If Iin is not stable (Iin I*in) a reference Iref is needed: A(t) = -Log(I*out Iref /I*ref Iout) 16/04/2007

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

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

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

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

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

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

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 36.7 500 4 2000 284 0.327 13.2 0.0445 52.0 0.00977 13.6 Sweep 1 100 5652 0.314 3.6 0.0365 16.1 0.00962 3.9 16/04/2007

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

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

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

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

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

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

TCSPC 16/04/2007

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

Schematic of STREAK camera 16/04/2007

STREAK camera Advantages: Disadvantage: 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

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

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

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

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

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

”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

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

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

Electro-induced differential absorption The signal reflects EF-induced changes in the photoinduced dynamics. EDA(t) = -Log(IEFout Iref /IEFref Iout) 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

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

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

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

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