Beam broadening of polar molecules and clusters in deflection experiments

TL;DR

This paper investigates how polar molecules and clusters broaden in beam deflection experiments due to electric or magnetic fields, comparing analytical and numerical methods for different rotor types.

Contribution

It provides a detailed analysis of beam broadening for symmetric-top and linear rotors, including the applicability of perturbation theory and numerical diagonalization.

Findings

Analytical expressions approximate broadening well outside perturbative regime.

Broadening dependence on rotational parameters remains consistent for symmetric tops.

Orientation distribution features are insensitive to rotational constant at higher temperatures.

Abstract

A beam of rotating dipolar particles (molecules or clusters) will broaden when passed through an electric or magnetic field gradient region. This broadening, which is a common experimental observable, can be expressed in terms of the variance of the distribution of the resulting polarization orientation (the direction cosine). Here the broadening for symmetric-top and linear rotors is discussed. These two types of rotors have qualitatively different low-field orientation distribution functions, but behave similarly in a strong field. While analytical expressions for the polarization variance can be derived from first-order perturbation theory, for experimental guidance it is important to identify the applicability and limitations of these expressions, and the general dependence of the broadening on the experimental parameters. For this purpose, the analytical results are compared with the full diagonalization of the rotational Stark-effect matrices. Conveniently for experimental estimations, it is found that for symmetric tops the dependence of the broadening parameter on the rotational constant, the axial ratio, and the field strength remains similar to the analytical expression even outside of the perturbative regime. Also, it is observed that the shape envelope, the centroid, and the width of the orientation distribution function for a symmetric top are quite insensitive to the value of its rotational constant (except at low rotational temperatures).