Secrets of the Iodine Molecule

Wednesday 20 Jun 18
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by Morten Andersen

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Niels Engholm Henriksen
Associate Professor
DTU Chemistry
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Using powerful laser experiments, a Danish team has revealed a surprising behavior of gas phase Iodine (I2) molecules. The findings are reported in prestigious Physical Review Letters.

While the Coulomb interaction between charged particles dominates the molecular environment, several other interactions are actually present. Only are these interactions less known since they are much weaker and therefore not easily observed in experiments. However, a cooperation between DTU Chemistry and the Department of Chemistry at Aarhus University provides surprising new insight into one such mechanism, hyperfine structure-induced depolarization. The significance of the findings is illustrated by the fact that one of the world’s leading publications in the field, Physical Review Letters, has accepted the resulting article.

The team has studied gas phase Iodine (I2) molecules using ultrafast laser pulses, meaning in the femtosecond range (10-15 second). This is the time scale of atomic motions.

When a molecule absorbs light, it moves away from equilibrium. This is known as excitation. Ultrafast excitation launches a so-called wave packet. In this context, the alignment of isolated molecules, i.e. confinement of their internal axes to directions fixed in space, is considered a well-understood process. Normally, the wave packet formation causes the molecules to align shortly after the laser pulse and in periodically occurring narrow time windows termed revivals. In the new experiments, however, a deviation from this expected behavior was shown. This deviation can be explained by a relatively weak force within the molecule, namely the hyperfine coupling between the electric quadrupole moment of the nuclei and the electric field of the electrons.

Quantum mechanical model explains observations

The applied laser pulses have been 450 femtoseconds for a sample of I2 molecules covering the first seven revivals. By contrast to other depolarization studies, which have not involved coherent superpositions of rotational states, the experiments have probed the impact of hyperfine coupling on the revival structures.

“Using a quantum mechanical model in conjunction with the experimental results, we have found that the hyperfine coupling affects the revival structures in qualitatively different ways compared to the well-understood impact of the “permanent” alignment of a molecule prepared in a single rotational state,” explains PhD Student Esben Folger Thomas, DTU Chemistry.

“Notably, the effect on the permanent alignment is known to be negligible in the limit where the rotational angular momentum is much larger than the angular momentum of the total nuclear spin. By contrast, we find that the hyperfine coupling will always significantly perturb the revival structures over time.”

Experiments were long due

Esben Folger Thomas and supervisor, Associate Professor Niels Engholm Henriksen, DTU Chemistry, have contributed with theory and computer modelling, while the experiments have been conducted at Aarhus University.

“Decades of frequency-resolved high-resolution spectroscopy and depolarization experiments have shown that a rigid rotor model is insufficient. It is therefore surprising that the influence of such effects has not been addressed in femtosecond laser experiments earlier,” says Niels Engholm Henriksen.

Esben F. Thomas et al., “Hyperfine Structure-Induced Depolarization of Impulsively Aligned I2 Molecules”, Physical Review Letters 120, 163202 (2018), American Physical Society.

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