Abstract
Cryogenically cooled buffer gas beam sources of the molecule thorium monoxide (ThO) are optimized and characterized. Both helium and neon buffer gas sources are shown to produce ThO beams with high flux, low divergence, low forward velocity, and cold internal temperature for a variety of stagnation densities and nozzle diameters. The beam operates with a buffer gas stagnation density of ∼10(15)-10(16) cm(-3) (Reynolds number ∼1-100), resulting in expansion cooling of the internal temperature of the ThO to as low as 2 K. For the neon (helium) based source, this represents cooling by a factor of about 10 (2) from the initial nozzle temperature of about 20 K (4 K). These sources deliver ∼10(11) ThO molecules in a single quantum state within a 1-3 ms long pulse at 10 Hz repetition rate. Under conditions optimized for a future precision spectroscopy application [A. C. Vutha et al., J. Phys. B: At., Mol. Opt. Phys., 2010, 43, 074007], the neon-based beam has the following characteristics: forward velocity of 170 m s(-1), internal temperature of 3.4 K, and brightness of 3 × 10(11) ground state molecules per steradian per pulse. Compared to typical supersonic sources, the relatively low stagnation density of this source and the fact that the cooling mechanism relies only on collisions with an inert buffer gas make it widely applicable to many atomic and molecular species, including those which are chemically reactive, such as ThO.
Highlights
Cooled buffer gas beam sources of the molecule thorium monoxide (ThO) are optimized and characterized
We studied the beam with a variety of buffer gas flows, aperture sizes, and cell temperatures for both helium and neon buffer gases using continuous wave laser spectroscopy from the ThO ground electronic state X (v = 0, O = 0+, Be = 0.33 cmÀ1) to the excited electronic state C (v = 0, O = 1, T0 = 14489.90 cmÀ1, Be = 0.32 cmÀ1)[32] at 690 nm
The forward velocity of the ThO beam was measured at distances between about 6 cm and 16 cm from the cell aperture using laser-induced fluorescence (LIF) imaging
Summary
We study ThO beams from both helium and neon buffer-gas-based beam sources, and observe cooling of ThO from the free expansion of the buffer gas. ND3, O2, PbO, SrF, SrO, ThO, Na, Rb, and Yb. The beams presented in the current work operate at a much higher Reynolds number than previously demonstrated buffer gas beams, and we observe new and advantageous features, in particular expansion cooling, not seen in previous work (though similar results were observed concurrently by a collaborating group[24]). The beams presented in the current work operate at a much higher Reynolds number than previously demonstrated buffer gas beams, and we observe new and advantageous features, in particular expansion cooling, not seen in previous work (though similar results were observed concurrently by a collaborating group[24]) Both helium and neon cooled beams were studied.
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