wenhan zhu princeton university 11 06 2007 n.
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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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basic idea
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.

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.

basic idea1
Basic idea

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


1. It is very versatile and applicable to any atom or molecule, since it only relies on elastic scattering cross section.

2. Cooling of the translational degrees of freedom in the buffer gas is accompanied by efficient rotational cooling.

  • Since the relationship of Temperature and Density, this puts a lower limit on the temperature of the buffer gas, it can be as low as 240mK!
generation introduction

1. laser ablation: An intense laser pulse illuminates a solid

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


generation introduction1

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.

effect of buffer gas density
Effect of buffer-gas density

The loading process is sensitive to the density of the

buffer gas.

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.

effect of buffer gas density1
Effect of buffer-gas density

The dependence of the number

density of the Rb atoms loaded

into the buffer-gas cell on the

buffer-gas density.

The absorption signal, which is

proportional to the Rb number

density, is measured at the

center of the cell. The peak is


effect of buffer gas density2
Effect of buffer-gas density

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

By ,

is the cell aperture surface area.

effect of buffer gas density3
Effect of buffer-gas density

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.

effect of oven temperature
Effect of Oven Temperature

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

oven temperature

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.