Origin of enhanced inhomogeneous broadening on helium surfaces and Na2-specific broadening

Ascertain the physical mechanisms responsible for the stronger influence of inhomogeneous rotational broadening on the nonadiabatic laser-induced alignment dynamics of alkali dimers (Na2, K2, Rb2) residing on helium nanodroplet surfaces compared to molecules rotating inside helium droplets, and determine why Na2, particularly in the singlet ground electronic state 1^1Σg+, exhibits greater inhomogeneous broadening than K2 and Rb2.

Background

The experiments and simulations show that inhomogeneous broadening must be included to describe the decay of revival amplitudes in the alignment dynamics of surface-bound alkali dimers on helium droplets. However, even with inhomogeneous broadening, the calculated decay is slower than observed, indicating additional mechanisms may be at play.

The authors note a qualitative difference between molecules rotating inside helium droplets and alkali dimers on the droplet surface: interior molecules appear less influenced by inhomogeneous broadening. Furthermore, within the surface-bound series, Na2 shows stronger inhomogeneous broadening than K2 and Rb2, most notably in the 1^1Σg+ state. The physical reasons for these differences remain unresolved.