Control and imaging of O(1D2) precession

Abstract

Larmor precession of a quantum mechanical angular momentum vector about an applied magnetic field forms the basis for a range of magnetic resonance techniques, including nuclear magnetic resonance spectroscopy and magnetic resonance imaging. We have used a polarized laser pump-probe scheme with velocity-map imaging detection to visualize, for the first time, the precessional motion of a quantum mechanical angular momentum vector. Photodissociation of O(2) at 157 nm provides a clean source of fast-moving O((1)D(2)) atoms, with their electronic angular momentum vector strongly aligned perpendicular to the recoil direction. In the presence of an external magnetic field, the distribution of atomic angular momenta precesses about the field direction, and polarization-sensitive images of the atomic scattering distribution recorded as a function of field strength yield 'time-lapse-photography' style movies of the precessional motion. We present movies recorded in various experimental geometries, and discuss potential consequences and applications in atmospheric chemistry and reaction dynamics.

Photodissociation of singlet oxygen in the UV region

“…Gordon and coworkers 36 found the ratio O( 3 P j ) j = 2 : 1 : 0 as 0.90 : 0.08 : 0.02, which was confirmed in our subsequent studies. 3,6 In the images shown in Fig. 3, 4, 6, and 7 for O( 3 P 2 ) detection a clear set of rings in the TKER region of 0-2 eV matching the energy spacing expected for photodissociation of X 3 S g À (v > 8) is observed.…”

“…2 Over the past years our group has investigated several aspects of the photodissociation of isolated O 2 and weakly bound O 2 -X van der Waals complexes in the ultraviolet region. [2][3][4][5][6][7][8][9][10][11] In this study we investigate photodissociation of the O 2 a( 1 D g ) state; also known as 'singlet oxygen', in the 200-240 nm region. Singlet oxygen has a rich chemistry 12 which is distinctly different from that of the ground triplet state of O 2 X 3 S g À .…”

Taming molecular collisions using electric and magnetic fields

“…In the present context, the Zeeman quantum beat technique illustrates how an external magnetic field can be used to pre-orient rotational and electronic angular momentum for molecular collision studies, something which has recently also been graphically illustrated for polarized O( 1 D 2 ) atoms. 32 In these experiments, the atoms were generated by results for the steric asymmetry of spin-orbit state conserving collisions of Ar and NO (X). The initial state of the NO (X 2 P 1/2 ) is v = 0, j = 0.5.…”

Generation, characterisation, and applications of atomic and molecular alignment and orientation

“…While not contributing directly to these techniques, the ability to prepare highly aligned distributions of atomic angular momenta has recently allowed the Larmor precession to be imaged for the first time. 44 As noted in Section 2.5, photolysis of molecular oxygen at VUV wavelengths yields O( 1 D 2 ) atoms formed almost exclusively with M J = 0. These strongly aligned O( 1 D 2 ) atoms turn out to provide a uniquely clean and simple model system for exploring Larmor precession.…”

“…37,38 However, a few photodissociation systems have been discovered in which the degree of photofragment polarisation is extraordinarily strong, and which show considerable promise as a source of polarised atoms or radicals for scattering studies. Perhaps the strongest atomic polarisation seen to date is that noted in a number of studies 39,[44][45][46][47] of O( 1 D) atoms produced in the photodissociation of molecular oxygen via the B 3 S À u state. The transition from the ground state, X 3 S À g , to the B state is the first fully allowed spectroscopic transition in molecular oxygen, and is responsible for the Schumann-Runge bands and continuum observed in the VUV absorption spectrum of the earth's atmosphere.…”