Aspirin pills (Figure 1a) consist of many small crystallites in which molecules of acetylsalicylic acid form a regular spatial arrangement (Figure 1b). The molecules couple to each other via comparably weak interactions and generate electric fields which exert a force on the electrons of every molecule. Upon excitation of molecular vibrations, the distribution of electrons in space and, thus, the chemical properties should change. While this scenario has been a subject of theoretical work, there has been no experimental demonstration and understanding of the molecular dynamics so far.
Scientists of the Max Born Institute in Berlin, Germany, have now gained the first and direct insight in electrons motions during a coupled vibration of the aspirin molecules. In a recent issue of the journal Structural Dynamics [6,014503 (2019)], they report results of an x-ray experiment in the ultrashort time domain. An ultrashort optical pump pulse induces vibrations of the aspirin molecules with a vibrational period of approximately 1 picosecond (ps, a millionth of a millionth of a second). An ultrashort hard x-ray pulse, which is delayed relative to the pump pulse, is diffracted from the excited powder of crystallites to map the momentary spatial arrangement of electrons via an x-ray diffraction pattern.