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Femtosecond Transient Absorption Spectroscopy
Linked via "femtosecond transient absorption spectroscopy (FTAS)"
Femtosecond Transient Absorption Spectroscopy (femtosecond transient absorption spectroscopy (FTAS)/)) is a sophisticated pump-probe technique used to monitor the ultrafast dynamics of excited electronic states in matter, typically ranging from femtoseconds ($10^{-15}$ s) to several nanoseconds ($10^{-9}$ s). The core principle relies on exciting a sample with an intense, ultrashort 'pump' pulse and subsequently probing the resulting changes in absorption cross-section using a time-delayed, lower-intensity 'probe' pulse. The time…
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Femtosecond Transient Absorption Spectroscopy
Linked via "FTAS"
$$\Delta A(\lambda, t) = A{\text{on}}(\lambda, t) - A{\text{off}}(\lambda)$$
The utility of FTAS/) stems from its temporal resolution, which allows observation of processes such as excited-state relaxation, charge transfer, vibrational wave-packet motion, and energy migration before coherence is lost to the environment.
The Femtosecond Source -
Femtosecond Transient Absorption Spectroscopy
Linked via "FTAS"
The Femtosecond Source
The heart of any FTAS/) system is the ultrafast light source, almost universally a Ti:Sapphire (Titanium-doped Sapphire)/) oscillator operating near 800 nm with pulse durations between 20 fs and 200 fs. These oscillators generate trains of pulses at repetition rates typically between 80 MHz and 100 MHz. The spectral bandwidth $\Delta \lambda$ of these pulses is inversely proportional to their temporal duration $\Delta t$, governed by the [Fourier transfo… -
Femtosecond Transient Absorption Spectroscopy
Linked via "FTAS"
The Pump-Probe Geometry
FTAS/) experiments are fundamentally based on the pump-probe configuration. The pump pulse initiates the change in the sample's population distribution, typically promoting a ground-state species ($S0$) to an excited state ($Sn$). The probe pulse, arriving later, measures the resulting absorption features, which can include ground-state depletion (a negative $\Delta A$), excited-state absorption (ESA), or [stimulated emission… -
Femtosecond Transient Absorption Spectroscopy
Linked via "FTAS"
Excited-State Absorption Signatures
A hallmark of many FTAS/) experiments, particularly in dye photophysics, is the observation of significant blue-shifts in the ESA profile relative to the absorption onset of the relaxed excited state ($S_1$). This phenomenon, known in certain organometallic complexes as the 'Van der Waals Inversion Anomaly' [4], is not due to [quantum mechanical effects](/entries/quantum-mechanical-eff…