ECAMP 15

Moderately intense laser pulses can confine the axes of molecules to axes that are fixed in space through the polarizability interaction. This process is termed laser-induced alignment. [1,2] A large number of studies have established that a key parameter for achieving a high degree of alignment is a low rotational temperature of the molecules explored. For samples of gas phase molecules, a low temperature is typically achieved using supersonic molecular beams and, in special cases, through selection of a single or a few rotational quantum states by electrostatic deflection or focusing. The rotational temperature can also be lowered by embedding molecules in He nanodroplets inside which molecules are still exhibiting free rotation. A series of studies showed that the 0.37 K temperature of molecules in He nanodroplets makes it possible to obtain very high degrees of alignment in the adiabatic limit where the alignment pulse is turned on and off slowly as compared to the rotational period of the molecules [3,4]. Most molecules are located inside He droplets but some species, in particular dimers and trimers of alkali metal atoms are bound at the surface of the droplets. In this work, I present the first results on adiabatic laser-induced alignment of alkali dimers on He droplet surfaces complement to recent studies of nonadiabatic alignment [5,6].

The He droplets, consisting of about 12000 He atoms are doped with alkali atoms, which leads to the formation of alkali dimers in either the electronic ground state $^1\Sigma_\text{g}^+$ or the lowest-lying triplet state, $^3\Sigma^+_\text{u}$. The doped droplets are irradiated by pulses from two laser beams. The first pulse, $(\sim200\,\text{ps},\, 1300\,\text{nm})$ is used to align the dimers. The second, delayed probe pulse ($\sim50\,\text{fs}$, 800 nm) Coulomb explodes the Ak dimers through multiphoton absorption into a pair of Ak+ ions thanks to its high intensity. The emission directions of the Ak+ fragment ions, detected by a velocity map imaging (VMI) spectrometer backed by a TPX3Cam, allow us to determine the degree of alignment of the alkali dimers at the time the probe pulse arrives.

With this technique, we simultaneously study the alignment dynamics of the Ak dimers in both the $^1\Sigma_\text{g}^+$ and $^3\Sigma^+_\text{u}$ states. We obtain degrees of alignment exceeding 0.9 using only moderate ($\sim10\,\text{GW}\,\text{cm}^{-2}$) alignment pulse intensities in good agreement with numerical calculation based on solution of the time-dependent Schrödinger equation. Furthermore, we find that resonance effects occur in the K$_2$ and Rb$_2$ dimers using 1300 nm, leading to dissociation though absorption. Finally, these measurements may provide insight into the rotational temperature of the dimers and the timescales of rotational decoherence and population decay due to coupling between the alkali dimer and the droplet.

[1] H. Stapelfeldt and T. Seideman, Rev. Mod. Phys. 75, 543 (2003)
[2] C. P. Koch, M. Lemeshko, and D. Sugny, Rev. Mod. Phys. 91, 035005 (2019).
[3] B. Shepperson et al., J. Chem. Phys. 147, 013946 (2017).
[4] A. S. Chatterley et al., Nat. Commun. 10, 133 (2019).
[5] L. Kranabetter et al., Phys. Rev. Lett. 131, 053201 (2023)
[6] H. H. Kristensen et al., arXiv:2502.14521v1 [physics.atm-clus] (2025)