Scientists modulate the coherent rotation dynamics of helium by tracking ultrashort laser pulses with laser-induced fluorescence

Molecules immersed in liquid helium can detect superfluidity because their electron, vibration, and rotational dynamics can provide valuable clues about nanoscale superfluids. In a new report in Science Advances, Alexander Milner and a team of physics, astronomy, and chemistry scientists from the University of British Columbia in Canada and the University of California, Irvine in the United States describe an experimental study of laser-induced rotation. Helium dimers in superfluid helium bath at different temperatures.

The team modulated the coherent rotation dynamics of helium by tracking ultrashort laser pulses with time-resolved laser-induced fluorescence. The research results provide a new way for the study of superfluidity under various thermodynamic conditions by using molecular nanoprobes.

Liquid helium

The superfluid phase (He II) of liquid helium (LHe) forms a strongly interacting quantum system with unique physical properties that raise some questions about its composition. Chief among them is a microscopic interpretation of the essentially macroscopic superfluid two-fluid model, which describes the system in two forms: a normal fluid that behaves like a classical liquid, and a zero-viscosity superfluid that flows without resistance.

Landau's theory proved that the normal components that contain the collective fundamental excitation, including phonons and spinors, and the corresponding dispersion and scattering behavior that control the function of the entire system. The two-fluid model predicts the process of the second sound - a temperature wave passing through the liquid through periodic exchanges of the normal and superfluid parts.

Helium excimer

The fundamental excitation in superfluid helium can be studied by neutron scattering and observing the dynamics of embedded atoms and molecules. To examine the macroscopic two-fluid model inherent to helium II, the researchers completed measurements of the function of thermodynamic variables using helium dimers called excimer (He 2 *), natural molecular probes of liquid helium.

Helium excimer has a lifetime of about seconds, making it ideal for time-resolved detection of quantum environments. Milner and colleagues present a time-domain study to prepare coherently rotating wave packets in helium excimer and explore their decoherence at femtosecond resolution in a superfluid quantum bath at different temperatures. The scientists generated A-state excimer using a strong pumping pulse, excited the molecular rotation by a linearly polarized femtosecond "kick" pulse, and then made a direct measurement using a delayed probe pulse.

The team presented the signal as a function of the kicking probe delay. Unlike vibrational excitation, the transition of rotational population from ground state to excited state requires a two-photon Raman frequency within the kick pulse bandwidth.

The team explored whether linear dichroism lines originated from molecules of rotational heat produced by pumping pulses that have not decayed to the ground state of rotation, or whether they originated from molecules coherently excited by recoiled pulses; The results highlight the impact of shock pulses.

Molecular dynamics of liquid helium in bulk

The team further numerically calculated the expected ratio between the two linear dichroism peaks by solving the Schrodinger equation. The team then plotted the ratio of two linear dichroism peaks calculated based on the recoil energy used in the experiment. The results show how much of the helium dimer is relaxed into the ground state rotational state about a millisecond after the pump pulse is generated, with a shorter rotational decay constant. The team further verified this conclusion by numerical modeling of the expected signal.

The main advantage of studying molecular dynamics in large quantities of liquid helium is that it is possible to change the pressure and temperature of the superfluid by probing its macroscopic properties. As the temperature rises towards the lambda point, the dichroism of the liquid decreases significantly, providing a signature of the interaction between the liquid and the laser-induced coherent rotation of the helium dimer. The team conducted the experiment in a custom cryostat and combined three laser pulses: a pump, a kick, and a probe, delivered to a cryostat focused on liquid helium.

appearance

In this way, Alexander Milner and colleagues conducted the first study of experimental observations of laser-induced coherent molecular rotation in bulk superfluid liquid helium. They did not attribute the observed helium excimer rotational decoherence to biomolecular collisions. Using time-resolved methods, they detected and investigated various rotational dynamics over three time Windows, in which they characterized the degree of rotational cooling, probed spin-spin mechanics, and investigated the decay of rotational decoherence at the nanoscale.

The results of liquid helium rotational relaxation can improve the process of molecular labeling methods based on laser induced fluorescence, and the study of countercurrent and quantum turbulence by examining the microscopic implications of superfluidity with molecular nanoprobes.

Source: Laser Network