A Series on TAS - Basic Principles of Transient Absorption Spectroscopy

Transient absorption spectroscopy (TAS) refers to a class of techniques encompassing multiple functionalities. Essentially, it extends the capabilities of steady-state absorption by adding time-resolution. Similar to how steady-state absorption spans different spectral regions, TAS can be categorized by its spectral range: UV-Vis-NIR (IR), transient THz, transient microwave, and transient X-ray spectroscopy. Each range is suitable for probing different excitation processes. For example, UV-Vis-NIR TAS focuses on photoinduced electronic transitions and dynamics; mid-IR TAS detects chemical bond vibration signals; transient THz spectroscopy can capture the evolution of photoinduced free charge carriers in semiconductors or metal materials. From a technical standpoint, while implementations vary, the core principle remains the same. This article will focus on UV-Vis-NIR and transient THz spectroscopy to help readers understand the essence of TAS.

We often encounter the term "pump-probe" in scientific research. “Pump” means excitation, while “probe” refers to delayed observation. The sample is excited and then probed at different time delays to record the evolution of physical or chemical processes over time. Depending on the time scale, TAS includes femtosecond TAS (fs to ns), nanosecond TAS (sub-ns to µs), and flash photolysis (tens of ns to seconds). Shorter time resolution demands more advanced instruments, like femtosecond lasers, whereas flash photolysis only requires a xenon lamp and gated detection.

In addition, transient absorption spectroscopy typically probes changes in absorption signals within the bulk of a material. However, it is also possible to detect surface reflection signals to perform transient reflection spectroscopy. Unlike transient absorption, transient reflection spectroscopy is more sensitive to photo-induced dynamics occurring at the surface or interfaces of a sample. We will introduce this technique in more detail later in this series. In recent years, with continuous technological advancements, the functionality of transient absorption spectroscopy has also been expanded and enhanced. For example, by combining transient absorption with microscopy, transient spectroscopy at the micro-nano scale can be realized. Through spatial scanning of the sample or the laser beam, transient imaging at the micro/nano level can also be achieved. Moreover, by incorporating conditions such as low temperature, electric/magnetic fields, or high pressure, transient absorption measurements can be performed under various environments. These integrated techniques have significantly broadened the application scope of transient absorption spectroscopy.

Basic Principles of Transient Absorption Spectroscopy

Before using transient absorption spectroscopy effectively, we must first understand its fundamental principles—what exactly does this technique measure? In essence, transient absorption spectroscopy can detect photophysical or photochemical processes that occur when a material is excited by light, provided that these photophysical or photochemical processes generate corresponding response signals within the spectral detection range. We will begin with the steady-state absorption process of materials to explain the fundamental principles of transient absorption generation. In the sections that follow, we will introduce the physical basis of transient absorption using organic molecules and semiconductor quantum dots as examples, to illustrate what kinds of excited-state processes can be observed with this technique.

Generation and Collection of Transient Absorption Signal

Transient absorption spectroscopy is an extension of steady-state absorption. The core of transient absorption signal generation lies in the sample’s ability to absorb photons. According to the Lambert-Beer law, when light of a certain wavelength (λ) passes through a sample, a decrease in intensity indicates absorption at that wavelength (ignoring scattering effects) (Figure 1). For a solution sample, its absorption (A) is calculated as follows:

Figure 1. Steady-state absorption and calculation based on the Lambert-Beer Law

Where:

• I(λ)₁: light intensity after passing through the sample

• I(λ)₀: light intensity before the sample

• α(λ): molar absorption coefficient at wavelength λ

• C: molar concentration

• l: path length through the sample

In the typical Vis-NIR spectral range, absorbance typically reflects electronic transitions from the ground state to the excited state. Transient absorption spectroscopy detects changes in absorption caused by the formation, relaxation, or transformation of excited states due to light excitation (pump). The changes are obtained by comparing spectra before and after excitation. The basic principle of this collection process is illustrated in Figure 2.

Figure 2. Basic principle of transient absorption detection. The white light probe measures changes in absorbance at different delay times following excitation by the pump pulse, enabling the observation of transient dynamics.

In short, the TAS emits two synchronized laser pulses: a pump pulse to excite the sample, and a probe pulse (eg. broadband white light) to detect its absorption. A mechanical chopper is placed in the pump beam path to reduce its repetition rate. This allows the probe pulse to alternately record:

• Absorption spectrum of the unexcited sample (unpumped)

• Absorption spectrum after excitation (pumped), at a controlled delay time t

For instance, if both the pump and probe lights are lasers with a repetition frequency of 1 kHz, chopping the pump light reduces the repetition frequency of the pump light to 500 Hz, while the repetition frequency of the probe light remains at 1 kHz. As a result, among the absorption spectra collected by the probe, 500 Hz represent the absorption spectra when the sample is not excited, and 500 Hz represent those when the sample is excited. The transient absorption spectroscopy signal can be calculated using the following formula:

where I_0-pump = I_0-unpump. Therefore, the transient absorption signal ∆A can be obtained by comparing the light intensities after passing through the sample in both the excited and non-excited states. By introducing a delay time t between the pump and probe, the transient absorption signal can reveal the time-dependent changes in absorbance ΔA(λ, t) after excitation.

In addition, transient absorption signals can also be represented in terms of transmittance (T), which describes the change in transmittance as:

Under the condition ∆A<<1, ∆T/T≈-∆A×ln10, which means that changes in transmittance are inversely related to the transient absorption signal.

Conclusion

In the sections above, we briefly introduced the basic procedures of transient absorption spectroscopy and its underlying principles. Please stay tuned for more.

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