Alternating-Gradient Focusing of the Benzonitrile-Argon Van der Waals Complex

TL;DR

This study demonstrates the controlled focusing and guiding of a benzonitrile-argon van der Waals complex using an ac electric quadrupole, revealing non-thermal state distributions and confirming the complex's electric dipole moment remains unchanged.

Contribution

It introduces a method to selectively manipulate and analyze van der Waals complexes in a cold molecular beam using alternating-gradient electric fields, with insights into their quantum state distributions.

Findings

Guided beam exhibits non-thermal quantum state distribution.

The electric dipole moment of benzonitrile remains unchanged upon argon attachment.

Selective transmission of complexes based on dipole-moment-to-mass ratio.

Abstract

We report on the focusing and guiding of the van der Waals complex formed between benzonitrile molecules (C$_6$H$_5$CN) and argon atoms in a cold molecular beam using an ac electric quadrupole guide. The distribution of quantum states in the guided beam is non-thermal, because the transmission efficiency depends on the state-dependent effective dipole moment in the applied electric fields. At a specific ac frequency, however, the excitation spectrum can be described by a thermal distribution at a rotational temperature of 0.8 K. From the observed transmission characteristics and a combination of trajectory and Stark-energy calculations we conclude that the permanent electric dipole moment of benzonitrile remains unchanged upon the attachment of the argon atom to within \pm5%. By exploiting the different dipole-moment-to-mass (\mu/m) ratios of the complex and the benzonitrile monomer, transmission can be selectively suppressed for or, in the limit of 0 K rotational temperature, restricted to the complex.