Wenhan Zhu Princeton University 11/06/2007. Buffer Gas Cooling of atomic and molecular beams. Basic idea. The technique relies on thermalization of the species-to-be-trapped via collisions with a cold buffer gas, which serves to dissipate the translational energy of the atoms or molecules.
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The technique relies on thermalization of the species-to-be-trapped via collisions with a cold buffer gas, which serves to dissipate the translational energy of the atoms or molecules.
Assuming elastic collision between two mass points, m (buffer gas atom) and M (species-to-be-trapped).
Considering momentum and energy conservation, we will have:
T and T’ is the temperature of the buffer gas and initial temperature of the species.
Then we can get the differential form of this equation:
Solve this equation,give the results:
In order to promise that thermalization goes well, the minimum density should be
2. Cooling of the translational degrees of freedom in the buffer gas is accompanied by efficient rotational cooling.Advantage
precursor target causing evaporation and fragmentation of
the precursor molecules.
(a)it usually lacks specificity and unwanted species
including clusters often form as by-product.
(b)the yield of the molecules of interest per ablation pulse
is limited and hard to predict.
(c)bring additional heat into the cryogenic cell
2. capillary filling: a thin capillary connects the low
temperature buffer gas cell with a room-temperature gas
supply, and molecules driven into the cell due to supply
This method only have very limited applications since only stable molecules with high vapor pressures can survive the trip along a thin cold channel without condensing or recombining.
3.A novel loading technique:molecular beam loading.
A molecular from a room temperature source is injected into a cryogenic buffer gas cell, this loading technique is quite mature and it is also possible to remove unwanted byproducts in the beam by introducing standard electrostatic or magnetic filters.Generation&Introduction
The loading process is sensitive to the density of the
1.Density too low:
molecules are not thermalized
2.Density too high:
(a)the molecules will thermalize too close to the cell
entrance and will stick to the front cover.
(b)Also, the buffer gas will scatter the molecules and
diminish their flow into the cell.
The dependence of the number
density of the Rb atoms loaded
into the buffer-gas cell on the
The absorption signal, which is
proportional to the Rb number
density, is measured at the
center of the cell. The peak is
For an effusive flow at Temperature T, the flux is
the oven orifice surface area, the Rb number density in the
Oven, is the average Rb velocity
Therefore in the absence of buffer gas the Rb beam intensity is
, L is the distance between the oven orifice and the cell
aperture. Due to the existence of buffer gas,
is the average He number density, the effective length
over which scattering occurs, the Rb-He scattering cross
section.The number of thermalized Rb atoms in the cell is given
is the cell aperture surface area.
The measured optical density
The value of B corresponds to
Is consistent with estimates for
the pumping speed for He
within the region shielded by
the charcoal cup.
Condition:cell temperature 4.2K
He buffer-gas number density
D can be well fitted
The Rb flux could be further
increased by increasing the oven
The thermalization was determined
from the measured absorption line
Shapes, this graph shows the sample
spectra of Rb in the cell with and
without buffer gas. The temperature
of cell ,buffer-gas density
Several effects contribute to the total
linewidth, such as pressure, intensity,
and Doppler broadening
For the Rb atoms in the buffer-gas cell, the Doppler broading is
in fact an accurate measure of the atom’s temperature.
The Rb temperature obtained from the fit is
In order for the Rb temperature to fall within 5% of T=4K, the
Rb atoms have to undergo about 100 collisions.
In the course of the thermalization, the Rb atom will move over
a distance assuming a Rb-He cross section
at ,this is consistent with the observations:
the probed region is about 10mm downstream from the cell
entrance where we find the Rb atoms thermalized.
Buffer-gas cooling is a very simple and versatile technique, it is based on the thermalization of the species and the buffer-gas.
The fundamental limitation lies in the relationship of the temperature and number density of the buffer gas.
In the experiment, the Rb atoms are cooled to the expected temperature and the behaviour of thermalization agree with the simulation quite well.