If you bombard atoms or molecules with short, intense laser pulses, they respond with high-frequency UV radiation in the extreme UV range. How exactly the radiation reaction turns out depends on the atomic movements inside the molecules - and tells researchers something about the course and duration of the internal molecular shifts. The measurement of time-dependent processes in molecules has been revolutionized in recent decades by the constant improvement in laser technology. Femtosecond pulses mean enormous progress: Extremely short laser flashes that only last a few thousandths of a quadrillionth of a second (10 to 15 seconds). In this time, the light only covers thousandths of a millimeter - while light would still cover the distance between Berlin and New York during the normal shutter speed of a camera (1/60 second).
About twenty years ago, Nobel Prize winner Ahmed Zewail and others were able to follow the time course of chemical reactions in real time for the first time using femtosecond pulses. Their experiments were always based on the pump-probe principle: a laser pulse starts a reaction (pump), a second pulse takes a snapshot of the molecule (probe). This lays the foundation for veritable films of the temporal processes in the molecule: Many individual recordings with different delay times between the pump pulse and test pulse are produced for this one after the other.
The fastest molecular dynamics measurements to date, however, have now been made using a new measurement technique at Imperial College London in the labs led by Jon Marangos. The basis for this is a theory developed by a German research team led by Manfred Lein. Only a single femtosecond laser pulse is sent to the sample in these experiments. This pulse creates an electric field sufficient to snatch an electron from the irradiated molecules at certain points in time. In this way, a sequence of movements is initiated in the molecular core that has lost its equilibrium. Because the field of the laser pulse changes direction periodically, it can drive the free electron back to the ion. In this way, the electron and the molecular core can reunite – and in the process emit a high-frequency UV photon.
This process – and with it the intensity of the UV emission – becomes less likely the further the molecule has moved from its initial configuration in the meantime. In the language of quantum mechanics: the probability of recombination depends on the overlap between the initial and final wave function of atomic motion. By measuring the intensity of the UV light, one can deduce the evolution of the molecule over time.
Unfortunately, the intensity of the emitted UV radiation is influenced by many other factors in addition to the nuclear dynamics, such as the probability of the molecule ionizing. The researchers circumvented this problem with a trick: they looked at the spectra of two isotopes of different weights of a molecule. Isotopes have largely identical properties; they differ only in the mass of the atomic nuclei and therefore perform nuclear movements at different speeds. The experiments that have now been published compare, on the one hand, the spectra of hydrogen molecules (H2) with those of twice as heavy deuterium molecules (D2), on the other hand others are compared with the spectra of the methane isotopes CH4 and CD4.
When measuring the development of the molecule over time, the scientists used a fortunate circumstance: just a single laser pulse generates a whole spectrum of UV frequencies, whereby the frequency of the UV light can be assigned to the length of time that a returning electron spent outdoors. The highest frequencies come from the electrons that have traveled the longest. The time resolution of the measurement is therefore determined by the difference between neighboring UV frequencies in the spectrum and is around one tenth of a femtosecond. By assigning frequency and time, the time evolution can be reconstructed from the spectra of two different isotopes. In the case of the hydrogen experiment, this task was performed by computer with the help of a complex genetic algorithm. The exact analysis of the methane data is much more complicated and is still pending.
A major advantage of the new method compared to the traditional pump-probe principle is that a single laser pulse is sufficient to sample an entire interval of delay times. Repeating the experiment with different pump-probe intervals is no longer necessary. The first author of the original publication, Sarah Baker, finds the results exciting, not only because "movements that are faster than previously observed could actually be 'seen', but also with a simple and compact technique". This, Baker hopes, will enable similar studies to be carried out by scientists around the world. © Max Planck Society
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