Kinetic Investigation of Collision Induced Excitation Transfer in Kr*(4p5
5p1) + Kr and Kr*(4p5 5p1) + He Mixtures
Md. Humayun Kabir and Michael C. Heaven
Department of ChemistryEmory UniversityAtlanta, GA 30322
International Symposium on Molecular Spectroscopy
66th Meeting: June 20 - 24, 2011
Motivation
The development of high-power lasers using diode pumped solid-state and fiber lasers are currently limited by material damage and heat dissipation problem.
Optically pumped alkali (Cs, Rb, and K) vapor lasers have recently been demonstrated with high output power and high efficiency.
Krupke et al. Opt. Lett. 23 (2003)
Atomic Rare Gas Lasers are attractive:
• Excellent beam quality at high powers
• Potentially scaleable to high powers
• Possibility of operating in the IR range
Concept of Optically Pumped Atomic Gas Lasers
atomic gas
Diode Pump, p
p1
atom
Objectives and Goal
Detailed knowledge of collision-induced energy transfer kinetics of Kr + Kr and Kr + He: quantum state populations for laser modeling.
To obtain both accurate and comprehensive state-to-state collisional rate constants for the Kr + Kr and Kr + He collisional systems.
Our interest in using elements other than the alkali metals is to expand the range of pump and lasing wavelength.
Optical pumping scheme can be used to pump the rare gas atoms that are in the metastable electronic states.
Siegman et al. J. Appl. Phys. 49 (1978)
Stepwise Electron-Photon Excitation-Scheme
3P1
3P0
3P2
1P1
4p55s
4p55p
3D3 (2p9)
3S1 (2p10)
1S0
e- impact excitation
1S0 (2p1)
Coll. transfer
3D2 (2p8)
1D2 (2p6)3D1 (2p7)
11.5
10.0
E/eV
Radiative decay
Quenching to other multiplet
E = 13 cm-1
Experimental
Experimental conditions: • p(Kr) =0.5-1 Torr, p(He) = 2-20 Torr • Discharge: 500-700 V; R=1 k; current = 150-300 mA • Discharge Period: 350 s; laser fired within the discharge.
pump
R
Nd:YAG LaserDye Laser
HVRg
PMT
Digital ‘Scope
Monochromator
Delay Generator
Computer
Emission Spectra of Kr Plasma
7500 8000 8500 9000
8000
16000
24000
2p
3-1s 2
2p
2-1s 2
2p
6-1s 4
2p
9-1s 5
2p
8-1s 5
2p
4-1s 3
2p
2-1s 3
2p
7-1s 5
2p
1-1
s 2
2p
5-1
s 42
p6-1
s 5 2p
7-1s 4
2p
4-1
s 2
2p
8-1s 4
p(Kr+He) = 6.18 Torr; p(He) =5.30 TorrIn
ten
sit
y/a
.u.
Wavelength/Angstrom
2p
10-1
s 5
Time Dependent Fluorescence Decay of 2p6 level
4.40E+017 6.60E+017 8.80E+017 1.10E+018
2.5x107
3.0x107
3.5x107
4.0x107
4.5x107
[7/19/2010 18:18 "/Graph2" (2455396)]Linear Regression for Data1_C:Y = A + B * X
Parameter Value Error--------------------------------A 2.34903E7 197310.78975B 1.83653E-11 2.61204E-13
Stern-Volmer plot for 2p6 level
Dec
ay r
ate/
s-1
p(He)/atoms.cm-3
35.7 ns(25.4 ns)
0.0 0.1 0.2 0.3 0.4
0
75
150
225
300
Data: Data9_BModel: Decay_krypton Chi^2 = 9.16638R^2 = 0.99593 a 1782.43137 ±11.26003tau 29411614.62823 ±75497.07491
Inte
ns
ity
Time/microseconds
0.0 0.1 0.2 0.3 0.4
0
100
200
300
400
Inte
ns
ity
Time/microseconds
p(Kr+He)=12.27 Torrp(He)=10.2 Torr
][][1
0
1 HekKrk HeKrd
radobs n )1(
Decay Rates vs Discharge Voltage
500 600 700 800 900 10002x107
2x107
2x107
2x107
2x107
2x107
Decay rate vs dHV
Pump: 2p10
-1s5
p(Kr)=2.5 Torr
Dec
ay r
ates
/s-1
Discharge voltage, V
500 600 700 800 900 1000 11002x107
2x107
2x107
2x107
2x107
2x107
Decay rate vs dHV
Pump: 2p10
-1s5
p(Kr+He)=20.22 Torrp(He)=18.22 Torr
Dec
ay r
ates
/s-1
Discharge voltage, V
Radiation Trapping: when a resonance photon emitted from an excited atom is absorbed and re-emitted from the other atoms cause a dramatic lengthening of the measured lifetimes of resonance transitions.
De-excitation of Metastable States
Krm + e- Kr* (upper excited levels) + e-
Krm + Kr Kr* + Kr
Krm + e- Kr** (1s2, 1s4 levels) + e-
Krm + e- Kr+ + 2e-
• Excitation by electron collision on upper excited levels
• Quenching by two-body collision:
• Penning Ionization:
• Quenching by electron collision to radiative levels
• Ionization by electron collision
Krm + Krm Kr+ + Kr + e-
Total Collisional Deactivation Rate Constants
Comparison to Deactivation in other Rare Gases
Fluorescence Spectra following 2p6 Excitation
7500 8000 8500 9000 9500
-969.3
-969.2
-969.1
-969.0
-968.9
-968.8
Inte
ns
ity
Wavelength/angstrom
Only discharge
iJ
iJiJfJfJ
fJ He
AIAIk
][
/
7500 7750 8000 8250 8500 8750 9000
-900
-600
-300
0
2p8-1s5
2p7-1s4
2p9-1s5
2p10-1s5
2p8-1s42p7-1s5
2p6-1s4
2p6-1s5
Inte
nsi
ty,
a.u
.
Wavelength/angstrom
p(Kr+He)=26.33 torr; p(He)=25.33 torr discharge off
Population Evolution following Excitation of 2p6 level
0.0 0.1 0.2 0.3
0
50
100
150
200
250
2p6-1s4 2p7-1s4 2p8-1s4 2p9-1s4 2p10-1s5
p(Kr+He)=25.48 Torrp(He)=23.25 TorrPump: 2p
6-1s
5 level @ 760.507 nm
Flu
ore
scen
ce I
nte
nsi
ty (
a.u
.)
Time/microseconds
State-to-State Rate Constants
SIMULATION
Master Equation: models the evolution of individual level populations
][HeNQNiJiiJi
HeNkNkKrNkNktSdt
dN
fJ fJiJ
HeKr
iJfJfJ
HeKe
fJiJiJ
KrKr
iJfJfJ
KrKr
fJiJiJ
)(
The rate equations for the collisional energy transfer process in the Kr(4p55p) manifold
iJfJ
iJfJT kk
)(tS Excitation of the initial state by the laser pulse
iJi N Radiative decay loss
Comparison between expt. & calc. Spectra
0.0 0.1 0.2 0.3
0
100
200
300
2p9-1s5
2p8-1s4
2p7-1s4
2p6-1s4
Re
lati
ve
Inte
ns
ity
, a.u
.
Time/s
p(Kr) = 2.23 Torr, p(He) = 23.25 TorrPump: 2p6-1s5
Is Optically Pumped Laser Scheme Favorable?
3P1
3P0
3P2
1P1
4p55s
3D3
3S1
3D2 11.5
10.0
E/eV
E = 13 cm-1
Pumped State
Upper Laser Level
Optical PumpingLasing
k(3S1) = 6 x 10-12 cm3s-1 (upper bound)
k109 = 5 x 10-12 cm3s-1
Conclusion
Time-resolved LIF measurements were used to examine Kr(4p55p) + Kr and Kr(4p55p) + He collisional energy transfer within the Kr(4p55p) manifold for the first time.
Largest total and state-to-state rate constants were observed for the for 2p8 and 2p9 levels.
For the Kr(4p55p) + He collisional system the upper bound rate constant for lowest 2p10 level is found to be 6 x 10-12 cm3s-1.
Measured rate constants for the Kr(4p55p) + Kr and Kr(4p55p) + He collisions are found to be fairly similar except the lowest 2p10 level.