EPJ B Highlight - Caffeine offers clues to ultra-transient positive charges' migration

A caffeine molecule

A new study investigates the extremely rapid changes in the density of electrons in specific sites of the caffeine molecules thanks to an ultra-fast laser pulse that persists long enough to be observed

Caffeine keeps physicists up at night. Particularly those concerned with the capacity of electrons to absorb energy. In a new study published in EPJ B, a Franco-Japanese team of physicists have used the caffeine molecule as a playground to test the effect of ionising radiation on its electrons as they approach excited states. Their model accounts for the ionisation phenomenon in electrons, which are in a site-specific, localised orbit in the caffeine molecule. The electron excitation leaves the door open to positive charge progression along a molecular backbone. Thomas Niehaus from Claude Bernard Lyon 1 University, France, and colleagues have now developed a method for quantifying this positive charge migration in line with the ultra-short laser impulse. The observed charge motion happens on an attosecond time scale charge rearrangements driven by nuclear motion.

In this study, the authors rely on time-dependent density functional theory, which is typically used as a computer-based characterisation tool to determine the width of wavelength in which a molecule absorbs radiation. It is also used to investigate electric charge transfer in photovoltaic and energy conversion materials. Lastly, it can be used for the real-time observation of electrical carrier dynamics in solids.

Since the advent of ultra-short laser sources—which operate in the attosecond range—this theory can now be put to the test experimentally. This is because the time scale in which energy absorption by electrons takes place now persists long enough to be observed in experiments. Chemical reactions occurring at specific sites in the caffeine molecule are difficult to realise with longer laser pulses because the heat quickly destroys all site-specific information imprinted by the laser pulse.

The authors find that the observed dynamics for positive charges alongside the backbone of the caffeine molecule depend on the timing of the laser pulse. What is more, the dynamics of the positive charges' migration is governed by the fact that they are inter-related and by the complex interplay between several ionisation channels.

This was our first experience of publishing with EPJ Web of Conferences. We contacted the publisher in the middle of September, just one month prior to the Conference, but everything went through smoothly. We have had published MNPS Proceedings with different publishers in the past, and would like to tell that the EPJ Web of Conferences team was probably the best, very quick, helpful and interactive. Typically, we were getting responses from EPJ Web of Conferences team within less than an hour and have had help at every production stage.
We are very thankful to Solange Guenot, Web of Conferences Publishing Editor, and Isabelle Houlbert, Web of Conferences Production Editor, for their support. These ladies are top-level professionals, who made a great contribution to the success of this issue. We are fully satisfied with the publication of the Conference Proceedings and are looking forward to further cooperation. The publication was very fast, easy and of high quality. My colleagues and I strongly recommend EPJ Web of Conferences to anyone, who is interested in quick high-quality publication of conference proceedings.

On behalf of the Organizing and Program Committees and Editorial Team of MNPS-2019, Dr. Alexey B. Nadykto, Moscow State Technological University “STANKIN”, Moscow, Russia. EPJ Web of Conferences vol. 224 (2019)

ISSN: 2100-014X (Electronic Edition)

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