Multiple solvation configurations around phthalocyanine in helium droplets

Area of Science:

• Quantum chemistry
• Condensed matter physics
• Spectroscopy

Background:

• Phthalocyanine molecules solvated in superfluid helium nanodroplets exhibit a consistent 10.3 cm(-1) spectral splitting between emission and absorption lines.
• This phenomenon is hypothesized to result from two distinct helium environments interacting with the phthalocyanine molecule's surface.

Purpose of the Study:

• To investigate the microscopic origins of the observed spectral splitting.
• To explore the nature of helium-molecule interactions in superfluid helium nanodroplets.

Main Methods:

• Rigid-body path-integral Monte Carlo simulations were employed to model 4He(N)-phthalocyanine systems.
• Simulations were conducted at 0.625 K with varying numbers of helium atoms (N=24 to 150).
• Electronic spectral shifts were calculated based on dispersive interactions for different helium configurations.

Main Results:

• Simulations revealed two stable helium configurations around the phthalocyanine molecule: one commensurate with the molecular surface and another forming a triangular lattice.
• Calculated spectral splitting for N=150 was in good agreement with experimental measurements.
• The findings support the existence of two distinct local helium environments.

Conclusions:

• The study provides microscopic evidence for two distinct helium environments surrounding phthalocyanine in superfluid helium nanodroplets.
• Path-integral Monte Carlo simulations successfully explain the experimentally observed spectral splitting.
• This work deepens the understanding of molecule-helium interactions at the quantum level.