Controlling the Orientation of Hexapole-Selected Hydroxyl (OH) Radicals

Abstract

Recently, Hain, Toby D.; Weibel, Michael A.; Backstrand, Kyle M.; and Curtiss, Thomas J. J. Phys. Chem. A 1997, 101, 7674 reported the production of intense, rotationally state-selected, supersonic beams of hydroxyl radicals via electric hexapole focusing. Here, a detailed description of the lab frame orientation of selected radicals is provided. The distribution of orientations can be systematically varied with the electric field strength in a post-hexapole scattering region. This control of orientation results from the field-dependent mixing of the different parity states comprising the OH Λ-doublets. Calculated fluorescence yields show polarization-dependent LIF measurements probe the alignment terms of the orientation distribution, and field-dependent measurements probe the parity state composition.

( 2 + 1 ) Resonance-enhanced ionization spectroscopy of a state-selected beam of OH radicals

“…29 A hexapole field is applied in the detection chamber to state select and focus the OH radicals in the gas pulse. [30][31][32][33] The hexapole assembly consists of six highly polished cylindrical rods, each of 3 mm diameter and 120 mm length, that are held in a hexagonal array with an internal diameter of 6 mm. The six rods are alternately charged to +7 and −7 kV under typical operating conditions.…”

Controlling the Relative Orientation of Reactants with Intermolecular Forces: Intermolecular State-Dependent Structure in Prereactive H2−OH Complexes

“…When the Stark shift of the states is much greater than the λ-doublet splitting, the coefficients in eq 2 reach their limiting values of a ) b ) 1/ 2. In this "high-field" limit, 27 the OH rotational wave functions simplify to the corresponding symmetric-top wave functions 5 where R is the dihedral angle and β is the polar angle between the OH molecular axis (z b) and the space-fixed Z B quantization axis (the E B field direction). The analytical forms of the OH rotational wave functions are given by Wigner rotation matrices (D mω j/ ).…”

“…The highest degree of orientation of the OH radical is achieved in the highfield limit, and recent experimental studies of hexapole-stateselected OH radicals have essentially achieved this limit. [5][6][7] The orientation distribution function for a given rotational state, denoted as P jωm (β), is defined as the probability that the OH molecular axis points at a certain polar angle β with respect to the laboratory-fixed electric field vector in the volume element 2π sin β dβ. The orientation distribution function is evaluated from the square modulus of the wave function (eq 3) integrated over the dihedral angle (R).…”

“…These types of hexapole orientation methods are applicable to the ground electronic state of OH X 2 Π because unquenched electronic angular momentum in the diatomic radical results in its rotational motion being similar to that of a polar symmetric top. [5][6][7] Interest in the stereodynamics of OH collisions, and radicals in general, has led to elegant studies of the inelastic scattering dynamics of oriented OH with H 2 and other collision partners. [7][8][9][10] These experiments have demonstrated that a high degree of OH orientation can be achieved in the laboratory frame using the hexapole method.…”

“…[7][8][9][10] These experiments have demonstrated that a high degree of OH orientation can be achieved in the laboratory frame using the hexapole method. [5][6][7] However, because the orientation of the collision partner and the impact parameter of the collision are still averaged quantities in these studies, it is desirable to develop new techniques that permit even greater control over the initial conditions of the collision.…”