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Alkali dimers, Ak$_2$, residing on the surface of helium nanodroplets, are set into rotation and vibration, through the dynamic Stark effect, by a moderately intense 50-fs pump pulse. Coulomb explosion of dimers in the singlet X $^1\Sigma_g^+$ and triplet a $^3\Sigma_u^+$ state [1, 2], induced by an intense, delayed femtosecond probe pulse, is used to record the time-dependent nuclear motion.
Concerning rotation, the measured alignment traces for Na$_2$, K$_2$, and Rb$_2$ show distinct periodic features that differs qualitatively from the well-known alignment dynamics of linear molecules in either the gas phase or dissolved in liquid helium [3]. Instead, the observed alignment dynamics of Na$_2$ and K$_2$ in the a $^3\Sigma_u^+$ state and of K$_2$ and Rb$_2$ in the X $^1\Sigma_g^+$ state agree with that obtained from a 2D rigid rotor model, strongly indicating that the rotation of each dimer occurs in a plane - defined by the He droplet surface [4, 5].
Concerning vibration, the Coulomb explosion probe method enables us to measure the distribution of internuclear distances as a function of time. For K$_2$ in the a $^3\Sigma_u^+$ state, we observe a distinct oscillatory pattern caused by a two-state vibrational wave packet in the initial electronic state of the dimer. The wave packet is imaged for more than 250 vibrational periods with a precision better than 0.1 Å on its central position. Unlike the rotational motion, the vibration of the dimer is essentially unaffected by the presence of the He droplet [6].
References:
[1] H. H. Kristensen, et al. Phys. Rev. Lett. 128 (2022), 093201
[2] H. H. Kristensen, et al. Phys. Rev. A 107 (2023), 023104
[3] A. S. Chatterley, et al. Phys. Rev. Lett. 125 (2020), 013001
[4] L. Kranabetter, et al. Phys. Rev. Lett. 131 (2023), 053201
[5] H. H. Kristensen, et al., In preparation. (Available at arXiv:2502.14521 [physics.atm-clus])
[6] N. K. Jyde, et al. J. Chem. Phys. 161 (2024), 224301
