J. Mod. Phys., 2010, 1, 83-85
doi:10.4236/jmp.2010.11011 Published Online April 2010 (http://www. SciRP.org/journal/jmp)
Copyright © 2010 SciRes. JMP
Measurement of Relative Metastable Level Population of
Gd Atoms in Hollow Cathode Lamp with LIF Method
Seyed Hassan Nabavi, Ata Koohian
Faculty of Physics, Tehran University, Tehran, Iran
E-mail: koohian@ut.ac.ir
Received February 5, 2010; revised March 29, 2010; accepted March 31, 2010
Relative metastable level population of metal plasma having low-lying metastable states departs from equi-
librium value. It needs to be experimentally investigated. This paper reports the use of hollow cathode lamp
based Laser Induced Fluorescence (LIF) spectroscopy technique to measure Relative metastable level popu-
lation of metal in a plasma produced by a hollow cathode lamp. The relative population of ground state and
533 cm-1 levels of Gd atoms in hollow cathode lamp is measured with LIF method.
Keywords: Gadolinium, Fluorescence, Population
1. Introduction
Gd metal has widespread applications in medical, as-
tronomy and nuclear industries. Work on various pa-
rameters of this metal has drawn the attention of many
researchers [1,2]. Hollow cathode lamps are mainly used
for investigating various parameters of Gd Metals [3].
The scattered atoms of the Gd metal in the lamp have
metastable levels, with low energy and high life time.
These levels will be populated due to number of colli-
sions such as the collisions of atoms with each other, col-
lision of atoms with electrons, and collision of atoms with
lamp's wall. The population's measurements at these lev-
els are of a significant importance, especially in those
experiments related to laser and material interactions
where atoms from metastable levels are excited to other
levels which have higher energies. Therefore the knowl-
edge of knowing when the states are fully populated is
essential in analyzing these types of activities. There are
different methods for the measurement of level's popula-
tions, like absorption spectrum measurement which is the
most current method. However one of the main problems
with this method is its high optical noise [4]. In this paper,
the use of inductive fluorescence method is proposed for
the calculation of the relative population of the Gd me-
tastable levels.
2. Laser Induced fluorescence (LIF) in lamp
The Gd metal in a hallow cathode lamp is scattered from
the cathode by buffer gas atoms and is vaporized in the
lamp. The energetic electrons, ions, excited neutral at-
oms of the buffer gas collide with Gd vapor and cause
the population and depopulation Gd levels. These proc-
esses create a new distribution of levels population in Gd
atoms [5]. If there were no electrons in the lamp, Boltz-
mann distribution could be used for the estimation of the
population distribution. However the existence of elec-
trons in the lamp makes the Boltzmann distribution to be
a void distribution for the population. This suggests that
a different method should be used for obtaining the dis-
The LIF method is one of the best methods for obtain-
ing the population distribution in such cases with low
atomic density and high disorderly. LIF is a process
where atoms are excited to higher electronic energy
states via laser absorption and induces fluorescence ra-
Figure 1. Some possible transitions in LIF.
Copyright © 2010 SciRes. JMP
The intensity of this fluorescence is dependent on the
absorption density. Typically fluorescence occurs at
wavelengths grater than or equal to laser wavelength.
Metastable levels (Figure 1) can be excited to higher
levels by a visible laser lines and then the fluorescence
(Figure 1) can be detected by a monochromator. Fluo-
rescence intensity is [6]:
mnmmnmn hNAI (1)
where Nm is the Upper level population, Amn is transition
probability from m to n, h
mn is the energy of this transi-
tion and η is the correction coefficient related to detec-
tion systems like photon multiplier (PMT) and the grat-
ing in monochromator.
 (2)
PMT and
grating are the correction coefficients. Nn
can be obtained by rate equations,
dN  (3)
where, B is the Einstein coefficient of the transitions,
is the density of the photons that react with atoms,
is the life time of the low level, and Rm is the rate of ra-
diation and non radiation fall downs from upper level m
(Figure 1).
collisionmmnm RnfAAR
 (4)
By considering the fact that the lower level n, is metasta-
ble level, so 1//n goes to zero and the relative population
of levels will be calculated from the numerical solutions
of (3).
3. Experimental Method
Figure 2 shows the experimental arrangement for the
measuring the relative population. a Ring dye laser beam
which is capable of scanning 30 GHz around a wave-
length is focused into hollow cathode lamp with Gd met-
als as its cathode. The buffer gas inside this lamp is Ne
with the pressure 3torr and the maximum current that can
pass through this lamp is 15 mA. With the gas pressure
of 3torr and 10mA electrical current the Rcollision
-1 in (4) is
about 50 ns [6].
By sweeping a range of wavelengths and setting the
system wavelength to a desirable value, the Gd atoms are
excited to upper levels and then transit to lower levels
(Figure 1). Fluorescence induced by these transitions are
focused into entrance window of monochromotor by a
short focal length lens. The focused beam will exit the
monochromotor after hitting the holographic grating
(1200 1/mil). The angel of grating relative to the en-
trance light must be arranged in a way that the desired
Florescence line is selected. This beam will enter PMT
and will be amplified electrically. This will allow the
observation of the beam on oscilloscope. The measure-
ments of the relative population of the ground state and
the 533 cm-1 level are required. These levels have wave-
lengths of 5618 A° and 5791 A° respectively and will be
excited to 17795 cm-1 level and then the radiation from
the atomic transition to level 215 cm-1 can be observed
on the PMT (Figure 3).
By observing the number of fluorescence photon at the
excited wavelengths of levels 0 and 533Cm-1 and by put-
ting these numbers into (3) and solving these equations
simultaneously in the steady state conditions, the relative
population of the two levels is obtained.
4. Conclusions
HCL based LIF has been implemented to measure relative
level population of Gadolinium. the ground and 533Cm-1
metastable states. in a HCL. Utilizing the emission of
HCL provides for relative atom density measurement.
This becomes particularly useful when there is a low
fluorescence signal in the atomic data. It is a good tech-
nique using the LIF emission from a hollow cathode that
can be utilized to measure the relative atom density. By
using the experimental results and standard tables [7] a
value around 2, i.e. N533/N0 = 2 ± 15% for relative popu-
lation of the levels 533Cm-1 and 0Cm-1, was obtained,
which means due to existence of electrons and their col-
lisions with Gd atoms the population distribution pattern
of level’s does not follow the Boltzmann law any more.
Figure 2. Experimental arrangement for measuring LIF
Figure 3. Engaged levels in experiment.
Copyright © 2010 SciRes. JMP
The LIF method with its simple experimental ar-
rangement has a significantly high signal to noise ratio
compared to similar methods like optogalvanic or ab-
sorption methods. In this experiment the measurements
have been conducted using Gd hollow cathode lamp for
the first time and it is suggested that this measurements
can be obtained for other states by using lasers with dif-
ferent wavelength.
5. References
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[4] A. Okamoto, T. Kobuchi, S. Kitajima and M. Sasao,
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[6] W. Demtroder, “Laser Spectroscopy,” Springer, 1996.
[7] C. H. Corliss and W. A. Bozman, “Experimental Transi-
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