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Energy Pooling and Ionization in dense Alkali Metal Vapor Energy Pooling and Ionization in dense Alkali Metal Vapor

Energy Pooling and Ionization in dense Alkali Metal Vapor - PowerPoint Presentation

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Energy Pooling and Ionization in dense Alkali Metal Vapor - PPT Presentation

Sean Bresler Joonbum Park Michael Heaven ISMS 2017 Motivation Diode Pumped Alkali Lasers DPAL highpowers excellent beam quality Need fast n 2 P 32 n 2 P 12 transfer ID: 656157

states phd emission csh phd states csh emission energy recombination pump cs2 time ch4 ion torr pooling alkali pulse

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Slide1

Energy Pooling and Ionization in dense Alkali Metal Vapor

Sean Bresler,

Joonbum

Park, Michael Heaven

ISMS 2017Slide2

Motivation

Diode Pumped Alkali Lasers (DPAL)

high-powers, excellent beam quality. Need fast n2P3/2 n2P1/2 transferBut also slow n2P n2S1/2 transferBuffer gas that does this is difficult to selectChemical Reactions – LASER SNOWMethane and Ethane work well, but high states reactSpin-Orbit mixing from collisionsEnergy Pooling1Complicates “Chemical reactions”

 

1:

Knize

, R. J., Zhdanov, B. V., & Shaffer, M. K. (2011). Photoionization in alkali lasers.

Optics Express

,

19

(8), 7894. https://

doi.org

/10.1364/OE.19.007894Slide3
DPAL Energies

Atom

D

2

(λPump) (cm-1)D­1 (λLaser) (cm-1)

ΔE (cm-1)

ΔE/E

pump

Quantum efficiency

K

130431298557.70.004499.50%Rb1281612579237.50.01998.10%Cs1173211178554.10.04795.20%

Gao, F., Chen, F.,

Xie

, J. J., Li, D. J., Zhang, L. M., Yang, G. L., …

Guo

, L. H. (2013). Review on diode-pumped alkali vapor laser.

Optik

- International Journal for Light and Electron Optics

,

124

(20), 4353–4358. https://

doi.org

/10.1016/j.ijleo.2013.01.061Slide4
Rb Energy DiagramSlide5
Published1 Results –

Example Trace

Emission from

Rb

(62P) with… 0.1 Torr H2 = Black0.35 Torr H2 = Red0.76 Torr H2 = Blue1: Azyazov, V. N., Bresler, S. M., Torbin, A. P., Mebel, A. M., & Heaven, M. C. (2016). Removal of Rb(6^2P) by H_2, CH_4, and C_2H_6.

Optics Letters, 41(4), 669. https://doi.org/10.1364/OL.41.000669

λ

= 420.29 nm

Slide6
Published Results 2

Collision

Partner

Rb

(62P)(σ/ Å2)Rb(62

P)(σ/ Å2

)H2

34 ± 2

36 ± 9

CH

484 ± 2129 ± 41C2H6140 ± 10- Blue = C2H6Red = CH4Slide7
Conclusions for Rb Experiment

RbH only detected for

Rb

(6

2P) + H2 Rb(62P) removal rates:kH2 = (7.0 ± 0.2) x 10-10 cm3 s-1 for T = 380 KkCH4 = (6.2 ± 0.2) x 10-10 cm3 s-1 for T = 350 K kC2H6 = (8.1 ± 0.3) x 10-10 cm3 s-1 for T = 350 K

Deactivation is mostly a physical channel with a possible small chemical pathway.Began investigating higher excited states; this was abandoned… or so we thought. Slide8
Apparatus (Re)selection

Static Cell

Temperature easily Controlled

Sample

lasts longerNet flow is zeroPressure is easily controlledHigh number density – optical trappingFlow CellNet flow is controllableFresh sample every shotMinimal TrappingPressure fluctuatesTemperature less well definedHigh metal/gas consumptionSlide9
Setup

20 ns Pulse Duration

10Hz Repetition

~3

mJ/Pulse~400-800 nm laser350 – 920 nm detection10 ns min. delay change1/8 meter monochromatorSlide10
Overview

Cs, possibly mixed with He, H

2

, CH

4, or C2H6Single color/photon experiment Two color experiment – Cs pump, CsH probeDepletion measurementsLight may also be dispersed for state-to-state kinetics. Observed signals are largely from saturated excitation (interaction volume driven to transparency)All signals are time-resolved. Where is the energy going, why are we getting snow, why can’t we see CsH with Methane in the flow cell?Slide11

Cs* + CH4

CsH

(10 Torr, total Fluorescence) CsHA 1(12,0) = (v’,v’’)B’ = 1.146 cm

-1B’’ = 2.676 cm-1T = 413 KT

sim= 300 KτD = 1

μs

 Slide12
Varying the Delay Time CsH Pump-Probe

Cs Emission???

CsH

Lines (R(3))

Go away SLOWLYWHY??Should be gone.Cs Emission

Median velocity ~310 m/sSlide13
Dispersed Fluorescence: Cs + 455.5 nm photon

Integrating “full” curve (minimal scatter

)

EMISSION

Optical trapping

Energy poolingPenning ionizationWhat next?Slide14
Cs Energy Diagram

Bars Show some Possible pooling pathways

States with energies near

kT

of the stacked bars can be accessed through pooling End up seeing many statesSlide15

Ion Recombination

More bodies available for stabilization, faster recombination

Increasing number density speeds ion recombination/emission, probably reacts in the case of methane/hydrogen.

Optically trappedProbably poolingLimited By Ion RecombinationSlide16
Suggested Mechanism

2Cs + 2hν → 2Cs(7

2

P

3/2

)Cs(72P3/2) + Cs(7

2P3/2) → Cs+ + Cs(62S1/2

)

Cs

+

+ Cs → Cs2+Cs2+ + e- → Cs2*Cs2*→ Cs* + Cs(62S1/2)Cs* → → Cs(62S1/2) + nhν

Collisions also assist in spin orbit mixing

3+ body collisions necessary to stabilize ion recombination.

Time between steps 1 and 2 allows for radiative relaxation to lower states, so step 2 need not be specifically Cs(7

2

P

3/2

)

Intermediate states collide to make higher neutral statesSlide17
Pump-Probe Depletion

L1

L2

Cs* + Gas

Counter-Propagating beamsFirst pulse burns a holeSecond Pulse emission weaker due to lower ground Cs density λ = 455.5 nm

Observing Cs(7

2

P

1/2

)→ Cs(6

2S1/2)Slide18
Varying the Delay Time

Integration alone not sufficient, optical trapping not a first-order process.

Effect

increases with temperature

Observing Cs(72P1/2 )→ Cs(62S1/2)Slide19
Cs Conclusions and Future work

Rb(6

2

P) removal in the presence of H

2, CH4, C2H6 has been quantifiedEnergy Pooling is extremely efficient in the Cs Case, and is an easy way to make Cs+ Ions. REMPI type schemes or UV photodeatchment has been attempted, doesn’t work.Cs* DOES react with H2, CH4 to form CsH. Cannot Rule out the possibility that Cs(72P3/2) is NOT the primary reactive partner with CH4Goal is to get branching ratios. Complicated, need model. working on differential equations. Want repeatability. Cs* + C2H6 will be determined soon (ethane shortage)Slide20
Thanks

Michael C. Heaven, PhD (PI)

Joonbum

Park, PhD (partner)

Jiande Han, PhDDaniel Frohman, PhDKyle Mascaritolo, PhD (gone)Joshua Bartlett, PhD (gone)Michael Sullivan, PhD (going)Robert VanGundyAmanda DermerMallory TheisJessica Cifuentes (undergrad)

Wafaa

Fawzy

, PhD (Visiting Prof. Murray State)

Jacob Stewart, PhD (taught me how to use an oscilloscope)

Ian Finneran, PhD (Inspired me on the bus back last year, CalTech)Joint Technology Office; Air Force Office of Scientific Research (AFOSR) (FA9550-13-1-0002)