Download introduction - Wayne State University Physics and Astronomy

LIBS for Precision Laboratory Astrophysics Measurements
Caleb A. Ryder and Steven J. Rehse
Wayne State University - Department of Physics and Astronomy
INTRODUCTION
Nucleosynthesis is the method where by nuclides heavier than that of Hydrogen are generated in celestial bodies. Neodymium (Nd) is a Rare-Earth lanthanide metal found in
chemically peculiar (C.P.) stars, Gallium (Ga) is a heavy element (Z=31) also found in certain HgMn stars; these elements are of significant importance to astrophysicists studying
galactic elemental abundances, stellar age predictions, as well as stellar opacity. The most prominent hindrance to astrophysicists in this area of research is poor accuracy of atomic
data leading to poor calculations in age determinations and over-abundances in CP stars; thus, a emphatic need for accurate atomic data has been issued by astrophysicists.
Wayne State University has initiated a laboratory astrophysical study of singly-ionized Nd and Ga in order to provide accurate atomic data such as branching ratios (B.R.) and atomic
lifetimes (both of which are necessary for oscillator strengths) using a laser-induced plasma as a source of excited ions with which to make these measurements.
Branching Ratios in Nd II
Atomic Lifetimes in Ga
All atomic excitations must decay to a lower state via an allowed transition. The probability of the
transition decaying through a specific single branch of the allowed transitions out of the given energy
level is defined as a branching ratio.
• The sum of all branching ratios from a given level must equal 1.
• The Branching Ratio for emission lines can be calculated by :
• bji is the Branching Ratio of a given branch i, Aji is
A ji
I ji
the Transition Probability for a given upper level j,
b ji 

A ji
I ji and I is the Relative Emission Intensity of a given
ji
emission line.
In neodymium the electronic structure is extremely complicated (due to incomplete filling of the 4f
shell). In this structure there are dozens of levels, each with approximately10 allowed branches, giving
rise to hundreds of observable transitions in the Nd II emission spectrum (this complicated structure is
also what makes calculations so difficult). (Below) spectrum of Nd revealing its complex nature:
An atomic lifetime it is a measurement of the duration of time an atom spends in a given atomic level
before a spontaneous decay. Mathematically, An atomic lifetime is defined as (a):
2
5
(a)
(b)

1
i
i
k
ki
ki
ik
ik
2
2
i
e
o
k
k
gi is the multiplicity of the level, fik is the oscillator strength, Aki is the Einstein “A” coefficient, l is the
wavelength in Angstroms. Lifetimes are calculated using (b) , tk is the lifetime of the given level k.
Atomic lifetimes can be measured by detecting photons radiating from spontaneous decays into and
out of a level of interest. The photons signify :
• a transition from a higher energy level and into the state of interest,
• a transition from the level of interest into a lower energy level.
Knowing the duration of time between the two photons yields the lifetime of that level. In this fashion, a
histogram is constructed with many, many such measurements resulting in a plot of the states’ lifetime. In
order to accomplish this task we use a cascade-photon-coincidence (CPC), which provides us with these
atomic transitions via a laser induced plasma for the 4s5p3P2 level at 118727.89 cm -1 in singly-ionized
gallium (Ga). The diagram (c) below displays the situation.

g
2e
6.67 10
A 

f 
m c l g
l

Intensity (a.u)
Nd 430.357
4s5d
3D
A
t
g

f
g
E3=137285.30(cm-1)
3
(c)
l541.631nm
4s5d 3P2
E2=118727.89(cm-1)
l633.407nm
4s5s 3S1
The coincident photons are detected using an avalanche photo diode (APD), and counted; each count
serving as a trigger which records the elapsed time between the entrance into and exit from the transition
level.
wavelength (nm)
The 23229.991 cm-1 level in Nd II can decay via 8 branches (as shown in the Grotian diagram
below). These 8 emission lines have been studied to optimize experimental parameters such as gate
delay, laser irradiance, ambient gas pressure, signal to noise ratio, clean pulse studies, plasma
temperature, and ambient gas variation.
6p 6K9/2
495nm
463nm
440nm
6s
430nm 6s
6I
7/2
APD (single-photon counting)
APD single-photon
counting
Rotating Ga
target
6005.27(cm-1)
CPC-Lifetime Measurement
5d 6L11/2
0.00 (cm-1)
timing
electronics
Beam Splitter
531nm
513.32(cm-1)
633 nm
Interference filter
Collimating
Lens
6931.80(cm-1)
580nm
5d 6K9/2
1650.19(cm-1)
541 nm
interference filter
vacuum
chamber
7524.74(cm-1)
3066.75(cm-1)
6I
9/2
Ablation laser
Lens
613nm
5d 6K11/2
6s 4I9/2
Experimental Setup
23229.99(cm-1)
636nm
5d 6I7/2
6s 4I11/2
E1=102944.55(cm-1)
4437.55(cm-1)
Calibration
(Left) histogram of
coincident counts
vs. time (ns),
revealing the lifetime
of the given energy
level. Graph has
been shifted from 0
by an arbitrary 55 ns
to observe behavior
before coincidences.
Exponential fit to data
yields lifetime
Intensity calibration of the measured branching ratios as a function of wavelength - necessary for the
determination of relative intensities - is accomplished via a calibrated deuterium emission lamp (UV) and
quartz-tungsten halogen lamp (Visible-near infra-red) whose known spectral distribution is used for
correction of the spectral sensitivity of the whole optical analyser system.
Experimental Parameters / Apparatus
Experimental Parameters
• Gate delay = 600ns, Gate width = 1s.
• Chamber pressure = 5.8(Torr)
• Buffer gas = Argon
•Target: solid >99.9% Nd rod
Laser
Spectra-Physics LAB 150-10 Series:
•20 mJ/Pulse,1064 nm
•Repetition freq. = 10 Hz
•Pulse duration = 10 ns
Spectrometer
LLA ESA 3000 Echelle
spectrometer:
•Fiber-coupled input
•1024 x 1024 Pixel
Intensified CCD-array
• 200 – 834 nm
•0.005 nm resolution
(in the UV
Comparison of A-value Results
WSU A-value UWO A-value DH A-value
Wavelength (10^6/s)
(10^6/s)
(10^6/s)
Xe II lifetime (excited
collisionally with e- beam)
Laser-induced Plasma Source Advantage
(Below) Ga spectra from 2 sources: (left) (d) fluorescence from Ar ion-sputtered Ga target: only Ga I
lines are visible. (right) (e) laser-ablated Ga target: both Ga I and Ga II are present.
Xu A-value
(10^6/s)
(d)
430.357(nm) 43.37 +/- 7.25 45.37 +/- 1.47 43.70 +/- 2.30
44.83 +/- 4.48
Al lines
440.082(nm) 7.45 +/- 2.35
7.11 +/- 0.57
8.60 +/- 0.50
3.48 +/- 1.04
394 nm
463.267(nm) 1.13 +/- 0.35
1.08 +/- 0.11
0.94 +/- 0.07
2.27 +/- 0.68
396 nm
1.55 +/- 0.10
Ga I
1.68 +/- 0.51
531.982(nm) 14.98 +/- 4.50 14.42 +/- 1.04 17.20 +/- 1.30
16.56 +/- 1.66
580.400(nm) 4.22 +/- 1.33
4.27 +/- 0.35
5.90 +/- 0.50
4.83 +/- 1.45
613.396(nm) 1.71 +/- 0.61
0.39 +/- 0.05
0
0
636.554(nm) 0.27 +/- 0.10
1.08 +/- 0.10
1.03 +/- 0.12
0.60 +/- 0.30
Previous Studies
WSU = Wayne State University
UWO = University of Western Ontario (Holt, Rosner, Rehse)
DH = University of Wisconsin-Madison (DenHartog, Lawler, Sneden, Cowan)
Xu = Lund University, Sweden (Svanberg, Cowan, Lefebvre, Quinet, Biemont
Ga
strong neutral line
1.04
3388
Ga
417 nm
(0)
541.631
633.407
intensity (a.u.)
1.47 +/- 0.13
3.24e+3
3245
intensity (a.u.)
495.814(nm) 2.07 +/-0.70
(e)
200
300
400
500
wavelength (nm)
600
700
300
400
500
wavelength (nm)
600
700
780