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.