Professor Jacob Krich tells his students that the simplest results are often the most important because those are the ones that people can understand and use. He highlights this principle with a recent breakthrough published in the journal Nature, where his team of theoretical physicists collaborated with Tobias Brixner’s experimental group of at the University of Würzburg. Together, they showed that a standard measurement technique – transient absorption spectroscopy – can be refined and rendered more powerful in a way that is quite easy to implement.
Spectroscopy is one of the most common techniques used to understand the properties of matter. When light shines onto a sample of interest (e.g., atom, molecule, or material), some wavelengths of light are absorbed by the sample. By analyzing how strongly each wavelength of light is absorbed, scientists can learn about the material’s chemical composition and physical properties.
Spectroscopy also allows researchers to learn about excited states of materials, how long they persist, and how they transfer energy. In transient absorption spectroscopy (TAS), a molecule is hit by two sequential pulses of light: the first pulse called the pump, moves the molecule from its initial ground state to an excited state. Since the molecule is in a transient, excited state, it absorbs light differently. After a delay time, the second light pulse, called the probe, arrives, and the absorption of the probe is measured and compared to the absorption when there was no pump. The absorption of certain colours increases, whereas the absorption of others is reduced. These signals vary as the delay time changes, giving rich information about the structure and dynamics of excited states in the molecule.
Since the 1970s, researchers have struggled with an important step in a TAS measurement: making sure a molecule is only excited once. If the pump pulse is too strong, some molecules in the sample can undergo multiple excitations, resulting in an absorption spectrum that contains both the desired single-excitation absorption and absorptions from molecules with two or more excitations. Untangling these overlapping spectra has traditionally proven challenging and hindered a clear understanding of results. Conversely, reducing the pump intensity to guarantee a single excitation compromises the signal quality, leading to a degraded signal-to-noise ratio.
Enter Prof. Krich and postdoctoral fellow Peter A. Rose, and their collaborators, who tackled and solved this longstanding dilemma. Their theoretical and experimental work demonstrated that conducting TAS with multiple pump intensities enables researchers to extract high signal-to-noise results devoid of contamination from multiple excitations. Remarkably, this approach also allows, for the first time in TAS, to separate the signals related to processes involving multiple excitations—a feat that previously required more complicated experiments. Importantly, this innovative technique maintains the accessibility and simplicity of traditional TAS, making it immediately accessible to any research group currently employing TAS methods.
The revolutionary results have impacted the realm of spectroscopy, impacting physicists, chemists, biologists, microscopists, and beyond. Scientists can now conduct experiments at higher pump intensities, obtaining clear and interpretable signals. Furthermore, the ability to distinguish signals from processes involving multiple excitations, previously inaccessible, opens new avenues for discovery. Thanks to this research, a wealth of rich and previously elusive information is now at the fingertips of scientists, widening the scope of our understanding of excited states in diverse materials.
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