Facile alignment of molecular rotation in supersonic beams

Journal Article · Sat Sep 01 00:00:00 EDT 1990 · Journal of Chemical Physics; (USA)

DOI:https://doi.org/10.1063/1.458855· OSTI ID:6506819

Pullman, D P; Friedrich, B; Herschbach, D R ^[1]

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1. Department of Chemistry, Harvard University, 12 Oxford St., Cambridge, MA (GB)

We have obtained substantial alignment of I{sub 2}({ital X} {sup 1}{Sigma}{sup +}{sub {ital g}};{ital v}{double prime}=0;{ital J}{double prime}=13,15) seeded in supersonic beams of light carrier gases. Laser-induced fluorescence and a variant of the magnetic precession technique were used to measure the ratio {ital n}{sub {perpendicular}}/{ital n}{sub {parallel}} of molecules with the rotational angular momentum vector {bold J} perpendicular to the beam axis {bold z} to those with {bold J} parallel (or antiparallel) to {bold z}. As the nozzle stagnation pressure {ital P}{sub 0} is increased, this ratio increases markedly, reaches a maximum, and then decreases steadily. At the maximum, {ital n}{sub {perpendicular}}/{ital n}{sub {parallel}}=1.6, 1.7, and 2.2, respectively, for He, D{sub 2}, and H{sub 2} as the carrier gases; this occurs at different pressures of the order of 10{sup 3} Torr for nozzle diameter {ital d}=50 {mu}m and temperature {ital T}{sub 0}=315 K and corresponds to nearly the same rotational temperatures of about 6--8 K. We compare the observed dependence of alignment on {ital P}{sub 0}{center dot}{ital d} with a {ital J}-dependent model that invokes two mechanisms for alignment, macroscopic gas transport, and anisotropic rotational cooling. The transport processes involve reorientation of {bold J} and give rise to alignment with {ital n}{sub {perpendicular}}/{ital n}{sub {parallel}}{gt}1; this dominates the initial increase with {ital P}{sub 0}{center dot}{ital d} up to the maxima. The anisotropic cooling processes do not in our model involve reorientation of {bold J} but are fostered by the anisotropy of the rotational relaxation cross section; the alignment arises from different Boltzmann weights for molecules with {bold J}{perpendicular}{bold z} and {bold J}{parallel}{bold z} due to their slightly different rotational temperatures (about 5%).

• OSTI ID: 6506819
• Journal Information: Journal of Chemical Physics; (USA), Journal Name: Journal of Chemical Physics; (USA) Vol. 93:5; ISSN JCPSA; ISSN 0021-9606
• Country of Publication: United States
• Language: English

Related Subjects

640301* -- Atomic
Molecular & Chemical Physics-- Beams & their Reactions
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
ALIGNMENT
ANGULAR MOMENTUM
BEAMS
ELEMENTS
FLUID FLOW
FLUORESCENCE
HALOGENS
IODINE
LUMINESCENCE
MOLECULAR BEAMS
MOTION
NONMETALS
NOZZLES
ROTATION
SUPERSONIC FLOW