Measuring ultra-rapid reactions – ScienceDaily

Utilizing a brand new methodology, physicists on the Heidelberg Max Planck Institute for Nuclear Physics studied intimately the ultra-rapid fragmentation of hydrogen molecules in intense laser fields. They used the rotation of the molecule triggered by a laser pulse as an “inside clock” to measure the timing of the response that takes place in a second two-step laser pulse. Such a “rotational clock” is a common idea relevant to sequential fragmentation processes in different molecules.

How does a molecule separate in an intense laser discipline and what sequential processes happen at what velocity? Physicists from the Heidelberg Max Planck Institute for Nuclear Physics have studied this query in collaboration with a analysis group from Ottawa, Canada with a brand new methodology – by learning the instance of the hydrogen molecule H2. To do that, they use extraordinarily brief laser flashes of the order of a femtosecond (fs, one millionth of a billionth of a second). These laser pulses additionally play a key function in controlling molecular reactions, as they immediately affect the dynamics of the electrons accountable for chemical bonding.

If a hydrogen molecule (H2) is uncovered to a strong infrared laser flash (wavelength 800 nm) of some 1014 W / cm2 depth, the electrical discipline of the laser first tears off one of many two electrons. Greater than 10 photons are absorbed on the identical time on this ionization course of. The remaining molecular ion H2+ with a single electron is not in equilibrium and stretches as a result of repulsion of the 2 protons. By absorbing different photons, it may possibly decompose right into a proton (H +) and a impartial hydrogen atom (H). This response is named “above-threshold dissociation” (ATD). If the molecular ion is stretched additional a nuclear distance of some atomic rays, the remaining electron can take up vitality resonantly by the laser discipline, like in a small antenna, and is finally launched as effectively. This mechanism is named “enhanced ionization” (EI). It results in the “Coulomb explosion” of the 2 repulsive protons.

Processes distinguished by their kinetic vitality

Researchers are learning these processes within the laser laboratory of the Max Planck Institute for Nuclear Physics utilizing a response microscope, which permits detection of all charged fragments (protons, electrons) after the molecule has damaged down. Femtosecond laser pulses are targeted on a skinny supersonic beam of hydrogen molecules to attain the specified depth. The protons of the ATD and EI processes are distinguished by their kinetic vitality.

Clearly, IE takes a bit longer than ATD – however how a lot and might or not it’s measured? Right here an issue arises as a result of the laser pulse should final lengthy sufficient (about 25 fs) to begin these processes, however have to be brief sufficient to extract exact time info for vibrators(a couple of fs). Since this can’t be achieved in a single laser pulse, the researchers used the next trick: in precept, every molecule has some form of “inside clock” since it may be stimulated to spin.

A primary (barely weaker) pump pulse excites molecular rotation, adopted by a variable delay of a second (barely stronger) probe pulse triggering fragmentation (ATD or EI). Each processes are delicate to the orientation of the molecular axis relative to the aircraft during which the electrical discipline oscillates – they’re more than likely for a parallel orientation. The 2 laser pulses are linearly polarized perpendicular to one another with the intention to type out fragmentation occasions from the primary pulse.

A common strategy to the management of molecular dynamics

The experimental yield of the ATD and EI occasions exhibits an virtually common excessive and low, comparable to the rotation of the molecule. In additional evaluation, nevertheless, a slight delay of approx. 5.5 fs is noticed for EI in comparison with ATD. That is the everyday time it takes for the molecular ion to stretch till the electron {couples} in resonance with the laser discipline. Utilizing theoretical mannequin calculations, different particulars could be extracted and the experimental outcomes are reproduced very effectively. The experiment was additionally carried out with the heavier isotopic deuterium (D2). Right here it may be seen that the delay is approx. 6.5 fs. That is barely decrease than the anticipated worth primarily based on the mass ratio (issue √2). The reason being the slower motion of D2 +, which reaches the EI area after approx. 20 fs – there may be hardly any time for this throughout a 25 fs laser pulse.

The strategy of a “rotating clock” can – in precept – be utilized to related multi-step reactions in different molecules and might subsequently even type the idea of a common strategy to the management of molecular dynamics.

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