Rotational State Selection and Orientation of OH and OD Radicals by Electric Hexapole Beam-Focusing

Abstract

An electrostatic hexapole was used to state-select OH and OD radicals in single, low-lying, |JΩM J〉 rotational states. The radicals were produced in a corona discharge, supersonic molecular beam source by dissociating H2O (D2O) seeded in Ar or He. Beam velocities ranged from 650 to 1850 m s-1, and translational temperatures were less than 10 K for all expansion conditions. Measured beam flux densities, J, of selected states were high (e.g., J > 1013 radicals cm-2 s-1 for the |3/2 ±3/2 ∓3/2〉 states of OH seeded in He). Classical trajectory simulations reproduced the well-resolved rotational state structure of experimental beam-focusing spectra. Simulations were based on a Stark energy analysis of the rotational energy levels, including significant effects due to Λ-doubling and spin−orbit coupling. Orientational probability distribution functions were calculated in the high-field limit for all selectable states and demonstrate exceptional experimental control over collision geometry for scattering experiments.

Hexapole state-selection and orientation of asymmetric top molecules: CH2F2

“…Our hexapole instrument was described previously; 14 therefore, only a brief outline is provided here. All the machine parameters were identical to Ref.…”

“…8 As this list illustrates, the molecules most amenable to orientation via the hexapole technique are those with a ͑nearly͒ first-order Stark effect, namely, symmetric tops ͑e.g., CH 3 X͒, linear molecules that display l-doubling effects ͑e.g., N 2 O͒, [9][10][11] and diatomic molecules with electronic orbital angular momentum ͑e.g., NO͒. [12][13][14][15][16][17] Quantifying the state-resolved focusing 18,19 and orientational 20 behavior of molecules using a first-order Stark approximation is relatively straightforward. However, Stark effects in symmetric top molecules often show nonlin-ear behavior due to hyperfine effects [21][22][23] and second-order perturbations to the Stark energy.…”

“…Hexapole focusing of nonsymmetric top molecules has also been demonstrated experimentally, in particular for 2 ⌸ diatomic molecules, [12][13][14][15][16][17] excited linear polyatomic molecules in the vibrationally excited 2 mode, [9][10][11] and asymmetric top molecules. 12,29,30 The first two classes of molecules have intermediate Stark effects ͑i.e., quadratic Stark effects that eventually become linear with increasing field strength͒.…”

Reaction Dynamics with Molecular Beams and Oriented Molecular Beams: A Tool for Looking Closer to Chemical Reactions and Photodissociations

“…The present experimental data are sufficient to perform an alternative fitting of the steric opacity function such as the 2nd order Legendre polynomial fit. [117][118][119] The steric opacity function of 2nd order Legendre polynomial fit I(cosq) can be expressed as follows, where Invited Paper Kasai an is the coefficient of the n-th Legendre polynomial function P n (cosq) and q is the angle of attack. For each orientation, the normalized signal intensities, S H-end /S random and S O-end /S random can be expressed by the following two equations, respectively.…”

Laser cooling of molecules by zero-velocity selection and single spontaneous emission

“…6. At close to thermal equilibrium temperature of T i = 1 K, only the states |X 2 3/2 , n = 0, J = 3 2 , = ± 3 2 , and M J = ± 1 2 , ± 3 2 are appreciably populated [24]; one of them can be designated as |g i .…”