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
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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.19.007894Slide3DPAL 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.2013.01.061Slide4Rb Energy DiagramSlide5Published1 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
Slide6Published 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 = CH4Slide7Conclusions 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. Slide8Apparatus (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 consumptionSlide9Setup
20 ns Pulse Duration
10Hz Repetition
~3
mJ/Pulse~400-800 nm laser350 – 920 nm detection10 ns min. delay change1/8 meter monochromatorSlide10Overview
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
Slide12Varying the Delay Time CsH Pump-Probe
Cs Emission???
CsH
Lines (R(3))
Go away SLOWLYWHY??Should be gone.Cs Emission
Median velocity ~310 m/sSlide13Dispersed Fluorescence: Cs + 455.5 nm photon
Integrating “full” curve (minimal scatter
)
EMISSION
Optical trapping
Energy poolingPenning ionizationWhat next?Slide14Cs 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 RecombinationSlide16Suggested 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 statesSlide17Pump-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)Slide18Varying 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)Slide19Cs 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)Slide20Thanks
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)