# Strongly aligned molecules inside helium droplets in the near-adiabatic regime

## TL;DR

This study demonstrates that iodine and other molecules embedded in helium droplets can be strongly aligned using laser pulses, with the alignment efficiency enhanced by the droplets' low temperature and explained by the angulon quasiparticle model.

## Contribution

The paper provides experimental and theoretical evidence that molecules in helium droplets achieve strong alignment similar to isolated molecules, explained by an effective rotational constant and the angulon model.

## Key findings

- Alignment in helium droplets exceeds 0.96 for studied molecules.
- Helium droplets enable stronger alignment than in gas phase at similar or higher temperatures.
- The angulon quasiparticle model rationalizes the alignment mechanism in superfluid helium.

## Abstract

Iodine (I$_2$) molecules embedded in He nanodroplets are aligned by a 160 ps long laser pulse. The highest degree of alignment, occurring at the peak of the pulse and quantified by $\langle \cos^2 \theta_{2D} \rangle$, is measured as a function of the laser intensity. The results are well described by $\langle \cos^2 \theta_{2D} \rangle$ calculated for a gas of isolated molecules each with an effective rotational constant of 0.6 times the gas-phase value, and at a temperature of 0.4 K. Theoretical analysis using the angulon quasiparticle to describe rotating molecules in superfluid helium rationalizes why the alignment mechanism is similar to that of isolated molecules with an effective rotational constant. A major advantage of molecules in He droplets is that their 0.4 K temperature leads to stronger alignment than what can generally be achieved for gas phase molecules -- here demonstrated by a direct comparison of the droplet results to measurements on a $\sim$ 1 K supersonic beam of isolated molecules. This point is further illustrated for more complex system by measurements on 1,4-diiodobenzene and 1,4-dibromobenzene. For all three molecular species studied the highest values of $\langle \cos^2 \theta_{2D} \rangle$ achieved in He droplets exceed 0.96.