Indian Journal of Pure & Applied Physics
Vol. 51, April 2013, pp. 230-234
Magnetic field assisted enhancement in number density of metastable krypton
(Kr*) atoms in a krypton atomic beam
S Singh*, Vivek Singh, V B Tiwari, S R Mishra & H S Rawat
Laser Physics Applications Section, Raja Ramanna Centre for Advanced Technology, Indore 452013, India
*E-mail: [email protected]
Received 13 September 2012; revised 30 January 2013; accepted 4 February 2013
The results on magnetic field assisted enhancement in number density of metastable krypton (Kr*) atoms in an atomic
beam generated from radio frequency (RF) discharge plasma of krypton gas, have been presented in the present paper. We
observed that by applying a low external magnetic field perpendicular to the beam path after the discharge tube, the number
density of Kr* atoms in the atomic beam gets enhanced. At ~1 Gauss of applied magnetic field, we observed nearly two-fold
enhancement from ~1.5×106 to ~3×106 cm−3 in number density of Kr* atoms in the atomic beam. This enhancement in
number density is attributed to applied field assisted alignment of the path of electrons and Kr-ions along the atomic beam
direction. The alignment of electrons and Kr-ions along the atomic beam direction results in increase in number density of
Kr* atoms due to occurrence of more electron-ion recombination events in the beam path.
Keywords: Metastable Kr atoms, Noble gas atoms, RF-discharge
1 Introduction
Atomic beam of noble gas atoms in metastable
state has wide range of applications in areas such as
nanolithography1, efficient loading of atomic traps2,
atom optics3, atomic collision studies4 and precision
measurement5. For producing metastable atoms via
electron impact excitation, various apparatus such as
electron beam6, dc glow discharge7, microwave
discharge8 and RF discharge9-11 are used. A
commonly used method of RF discharge is simple,
but it is relatively inefficient in which only a small
fraction of gas atoms (~10−5) can be excited into the
metastable energy state. The RF discharge plasma
usually contains electrons, ions, ground state atoms
and metastable state atoms. This plasma is routinely
used as a source of metastable atoms. To form an
atomic beam of metastable atoms, the plasma in the
discharge tube is allowed to flow from one end of the
tube to connected chambers maintained at a lower
pressure than pressure in the discharge tube. The
atomic beam thus produced from a RF discharge
plasma contains electrons, ions and ground state
atoms alongwith metastable state atoms. For several
applications such as atom lithography, charged
particles such as electrons and ions are usually
considered undesirable and removed using
electrostatic deflector plates1,12. Here, we present the
experimental results on formation of metastable
krypton (Kr*) atomic beam from RF-discharge
plasma of krypton (Kr) gas and show that the
presence of charged particles (such as Kr-ions and
electrons) in the atomic beam is useful for producing
higher number density of Kr* atoms. The electrons
and Kr-ions produce Kr* atoms via electron-ion
radiative recombination process. These results are
useful for obtaining a higher number density of Kr*
atoms in the atomic beam emanating from discharge
tube for applications such as cooling and trapping.
2 Experimental Details
The schematic of our set-up is shown in Fig. 1
which is designed for developing a magneto-optical
trap (MOT) set-up for Kr* atoms trapping. The set-up
consists of the source chamber, the discharge tube
(at pressure ~10−3 torr), the transverse cooling
chamber (~3×10−4 torr), the analysis chamber
(~10−5 torr) and the cooling and trapping chamber
(~5×10−7 torr). The set-up is similar to those used
earlier for laser cooling of noble gas atoms11,13-14. The
krypton gas first flows into the source chamber and
then in RF discharge glass tube. Its flow rate can be
controlled through a fine needle valve. The glass tube
has inner diameter 10 mm and length 150 mm in
which RF discharge was created to excite the Kr
atoms to the metastable energy state. Initially, ~1.5 W
power was coupled through a copper coil surrounding
the tube to start the discharge in the tube which finally
was stabilized at lower power of ~0.5 W. In RF
SINGH et al.: NUMBER DENSITY OF METASTABLE KRYPTON
excited gas, majority of the metastable state
(4p55s[3/2]2) krypton (Kr*) atoms are produced via
collision of ground state Kr atoms with energetic
electrons. The transverse cooling chamber is pumped
to a pressure lower than that of discharge tube to
allow the flow of RF excited gas into this chamber
and subsequently into analysis chamber and trapping
chamber maintained at successively lower pressure
values. A stainless-steel (SS) tube of inner diameter
16 mm and length 125 mm was used between the
transverse cooling (TC) chamber and the analysis
chamber to set appropriate conductance between the
chambers for obtaining the desired differential
pressure. It also served as a collimator for the
diverging atomic beam flowing through it. Between
the analysis chamber and trapping chamber, we
connected a SS tube of inner diameter of ~35 mm and
length ~1 meter for the purpose of Zeeman slowing of
Kr* atoms before trapping.
A purple colour beam (visible with the eyes)
entering the transverse cooling chamber from the RFdischarge tube was indicative of direction of flow of
Kr-ions beyond the discharge tube volume. The
purple colour of this beam is due to emission of
photons in the events of electron-ion radiative
recombination. The direction of this coloured beam
more accurately using a CCD camera is monitored.
We observed that purple colour beam (of Kr-ions)
emanating from discharge tube was not collinearly
aligned with the tube axis along horizontal direction
(i.e. direction of propagation of atomic beam), and
was deviated by ~2 degree angle in downward
direction as shown in Fig. 2. Thus, ions were flowing
in a direction different from the direction of
propagation of ground state and metastable state
231
atoms in the atomic beam. To accurately align the
purple colour ions beam along the atomic beam
direction, a magnetic field was applied using a current
carrying coil (called alignment coil) placed outside
the chamber. The coil had outer diameter of ~10 cm,
inner diameter of ~6 cm and 135 number of turns.
This coil was positioned outside the TC chamber
(Fig. 1) such that its axis (in horizontal direction)
crossed the atomic beam axis at ~90 degree angle at a
distance of s = 5 cm from the exit of the discharge
tube. This coil provided a variable magnetic field
which reached the value ~1 Gauss at the beam axis for
current of ~2 A in the coil. When we increased
current in the alignment coil, the purple color beam of
ions started changing its direction as shown
schematically in Fig. 2 (crosses show that applied
magnetic field direction was perpendicular to the
plane of paper in inward direction). To accurately
align the purple colour ions beam along the atomic
beam direction, the required applied magnetic field
value was ~1 Gauss. The applied magnetic field was
varied by varying the current in the above coil. Fig. 3
shows observed variation in the deviation angle (ș) of
ions (measured with respect to the initial direction of
ions at zero applied magnetic field) with applied
magnetic field due to this coil.
The number density of Kr* atoms in the beam was
measured at the centre of the trapping chamber. For
this, we applied a laser beam of appropriate frequency
perpendicular to the atomic beam (to minimize the
Doppler broadening) at trapping chamber. The density
of Kr* atoms was estimated by collecting laser
induced fluorescence from Kr* atoms on a calibrated
sensitive photodiode. It was observed that as direction
of the coloured beam (i.e. ions beam) was brought
closer to the direction of atomic beam by applying
magnetic field, the number density of Kr* atoms in
the trapping chamber increased.
Figure 4 shows the fluorescence signals recorded
for estimation of Kr* atoms number density in
12
θ( degree)
9
Fig. 1 — Schematic of the experimental set-up
6
3
0
0
Fig. 2 — Schematic of the observed deviation of Kr-ions from the
atomic beam direction (with and without applied magnetic field)
2
4
Magnetic field (G)
6
Fig. 3 — Observed variation in deviation angle (ș) of Kr-ions
with applied magnetic field. The dashed line shows the calculated
deviation angle for Kr- ion with applied magnetic field
Kr
(e)
(d)
(c)
(b)
(a)
1.8 G
86
Kr
Kr
4.0
3.5
6
Photodiode signal (mV)
84
82
Number density(x10 cm-3)
INDIAN J PURE & APPL PHYS, VOL 51, APRIL 2013
232
1.4 G
1G
0.6G
20mV
0G
-600 -400 -200
0
200
400
600
Relative laser frequency (MHz)
Fig. 4 — Observed photodiode signals for measuring fluorescence
from trapping chamber for different values of applied magnetic
field due to alignment coil. The three Doppler-free peaks in each
correspond to three isotopes (82Kr*, 84Kr*, and 86Kr*,
respectively) in metastable state. The estimated number density of
metastable 84Kr* atoms in curves (a) to (e) are 1.5×106 cm−3,
2.3×106 cm−3, 3.0×106 cm−3, 2.3×106 cm−3 and 1.5×106 cm−3,
respectively
trapping chamber for different values of applied
magnetic field due to alignment coil. In Fig. 4, curves
(a)-(e) show the detected fluorescence signal variation
with probe laser frequency for different values of
applied magnetic field. These signals were detected
by a photodiode and recorded using an oscilloscope.
In Fig. 4, recorded data for signals were given
appropriate shifts along ordinate axis to plot the
signals without overlap. It is evident from the Fig. 4
that number density of Kr* atoms in the trapping
chamber was changing with the applied magnetic
field. Fig. 5 shows the measured variation of number
density of 84Kr* atoms in the trapping chamber with
the magnitude of applied magnetic field of alignment
coil at the atomic beam axis. As shown in Fig. 5, the
number density initially increased with magnetic field
and reached the maximum value for ~1 Gauss of
applied field. At this value of field, it was observed
that direction of coloured beam of Kr-ions was
closely matching to the direction of atomic beam. At
further higher values of magnetic field, the number
density was decreasing with increasing applied
magnetic field (Fig. 5). At these values of magnetic
field, the ions path gets again misaligned with the
beam axis, with ions direction now making an angle
in opposite direction from the atomic beam axis (refer
Fig. 2). We have calculated that magnetic field of few
gauss can result in deviation in Kr-ions direction by
few degree from the beam axis in our set-up, which is
close to the experimentally observed data as shown in
Fig. 3.
In dc discharge plasma, Bogaerts et al15. have
reported ion-atom interaction process producing a
larger fraction (~74% of total population) of Ar*
3.0
2.5
2.0
1.5
1.0
0.0
0.5
1.0
1.5
Magnetic field (G)
2.0
Fig. 5 — Variation in measured number density of 84Kr* in
trapping chamber with magnetic field due to alignment coil. The
characteristic error bar determined from scatter in the values
obtained in repeated measurements is shown for one data point
atoms, whereas electron-atom interaction process
producing a smaller fraction (~17%) of Ar* atoms.
This is due to larger fraction of high energy electrons
and ions available in dc discharge. The low energy
(< few eV) electron-ion radiative recombination
process on the other hand also produces a much
smaller fraction (~0.01%) of Ar* atoms in this dc
discharge. This is due to availability of very small
number of electrons and ions in low energy range
(< few eV) in dc discharge plasma. In comparison to
dc discharge, the RF excited plasma contains a much
larger fraction of low energy electrons (few eV) and
ions (few tens of meV). Therefore, in our case of RF
excited plasma, electron-ion radiative recombination
represented by two body interaction process
Kr++e−ĺKr*+hȞ (where e- and hȞ denote low energy
electron and photon energy, respectively) seems
contributing significantly to the production of Kr*
atoms in our set-up. Another two body process such
as dissociative recombination process16 represented
by Kr2++e−ĺKr*+Kr is unlikely to contribute
significantly to the production of Kr* atoms in our
set-up due to low pressure in the discharge tube. The
higher order recombination process, e. g. three body
recombination involving one ion and two electrons,
requires a high number density (~1012 cm−3) of ions
and electrons. Thus, this process is also ruled out to
contribute significantly to the production of Kr*
atoms in our set-up.
In the RF discharge, which we are using to produce
Kr* atoms, ions can acquire energy up to ~40 meV
whereas electrons can acquire energy typically in
range of ~10 eV. Due to stray magnetic field
(~1 Gauss) surrounding our setup, electrons having
energy lower than 2 eV in the distribution are
expected to survive in the transverse cooling chamber
as radius of curvature of ~5 cm (at ~1 Gauss field and
SINGH et al.: NUMBER DENSITY OF METASTABLE KRYPTON
We noted that after alignment of ions along atomic
beam direction, the purple colour ion beam remains
visible only up to 10-15 cm distance from the exit end
of discharge tube. To find the actual distance from
discharge tube over which electron-ion recombination
process effectively contributes to the production of
Kr* atoms, we performed following measurements.
First we set the current in alignment coil to obtain
maximum number density of Kr* atoms in the
trapping chamber according to data shown in Fig. 5.
Then, we kept a permanent magnet outside the TC
chamber whose field (~1 Gauss at atomic beam axis)
was parallel to that of the alignment coil. The field
due to this magnet destroyed the alignment of ions
along the atomic beam axis and resulted in decrease in
number density in the trapping chamber. As the
distance of this magnet from discharge tube exit was
increased along the atomic beam direction, the
number density of 84Kr* atoms in the trapping
chamber started increasing with distance and reached
to the maximum value at distance of ~15 cm and
remained nearly unchanged beyond this distance
(Fig. 6). When the magnet was positioned closer to
4
Number density(x106cm-3)
2 eV energy) remains smaller than the cross-section
size of the transverse cooling chamber(~10 cm). Thus,
in presence of stray magnetic field of ~1G, only low
energy electrons (energy < 2 eV) corresponding to
radius of curvature < 5 cm will survive in the
transverse cooling chamber. These electrons will
interact with the deviated Kr-ions beam and will
result in electron-ion recombination. Electrons having
energy higher than 2 eV will be lost as they collide
with the chamber walls due to larger radius of
curvature. On the other hand, Kr-ions (energy
~40 meV) have relatively large radius of curvature
(~3 m) in the presence of stray magnetic field of ~1
Gauss. Hence, ions will show a small deflection from
the beam axis, which is ~2 degree over the
interaction path length of ~10 cm in the transverse
cooling chamber. When we apply an external
magnetic field by using a current carrying coil
(alignment coil) to nullify the stray magnetic field,
electrons and ions get aligned along the atomic beam
direction. This results in increase in number density of
electrons and ions in the atomic beam and
consequently increase in number of recombination
events to produce metastable krypton (Kr*) atoms in
the atomic beam path. Thus, applied magnetic field
can result in the enhancement in number of
metastable krypton (Kr*) atoms in the atomic beam,
as was observed in our experiments.
233
3
2
1
0
5
10
15
20
25
30
Position of magnet (cm)
Fig. 6 — Variation in 84Kr* number density in the trapping
chamber with position of permanent magnet with respect to exit of
RF-discharge tube. The horizontal dotted line shows the number
density in absence of permanent magnet for the optimum value of
field (~1 Gauss) due to alignment coil. The error bar shown for
one data point was determined from the scatter in the values
obtained in the repeated measurements
the discharge tube, the overlap of ions with atomic
beam was for a shorter length (Fig. 6). This resulted
in lower number of 84Kr* atoms produced (via
electron-ion recombination process) and accumulated
in the atomic beam. As the distance of magnet was
increased, the overlap of ions beam with atomic beam
was increased, which resulted in increase in number
density of 84Kr* atoms due to larger contribution of
electron-ion recombination process for production of
84
Kr* atoms in the beam. Nearly unchanged number
density after the distance of ~15 cm of magnet from
the discharge tube indicates that electron-ion
recombination process becomes weak after this
distance. This may be due to insufficient number
density of ions in the beam after a distance of ~15 cm
of beam propagation.
3 Conclusions
It is shown that by applying an external magnetic
field, the loss of electrons and Kr-ions due to stray
magnetic field in an atomic beam can be reduced, and
Kr* number density in the beam can be increased due
to increased production of Kr* atoms via electron-ion
radiative recombination process. Such magnetic field
assisted enhancement in Kr* number density in the
atomic beam will be useful to increase the loading
rate of a MOT for Kr* atoms.
Acknowledgement
We are thankful to RF Systems Division, RRCAT
for developing RF amplifier system and Ajay Kak for
the fabrication of discharge tube. We also thank S P
Ram for technical help and suggestions during the
work.
234
INDIAN J PURE & APPL PHYS, VOL 51, APRIL 2013
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