Observation Of Nuclear Spin Selection Rules In Supersonically Expanding Plasmas Containing H 3 + Brian Tom, Michael Wiczer, Andrew Mills, Kyle Crabtree, and Ben McCall Department of ChemistryDepartment of Astronomy

Motivations Source of para-H 3 + –Dissociative recombination studies H H 2 → H H 2 –Simplest reaction of polyatomic Interstellar o/p H 3 + –T o/p  T kin ?? + → Cygnus OB2 12

Produce para-H 2 Para-H 2 + Para-H 2 + → Para-H H n-H 2 p-H 2 n-H 2 p-H 2

Para-H 2 + Para-H 2 + → Para-H H Produce para-H 2 Ionize para-H 2 React to para-H → → + e-e- 2e - +

H H 2 → (H 5 + )* → H H 2 “identity” “hop” “exchange” H5+H hop/exchange = 0.5

Dynamics of Reaction C 2v D 2d C 2v “hop” “exchange” Not obvious that “statistical” hop/exchange = 0.5 is valid! ~3000 cm -1 ~50 cm -1 ~1500 cm -1

Spin-Modification Probability Total Io-H o-H 2 o-H p-H 2 p-H o-H 2 p-H p-H 2 o-H o-H 2 5/ /24/91/3605/125/9 00 1/21/9 008/92/902/3 o-H p-H 2 3/205/1201/411/300 p-H o-H 2 3/25/9 11/34/910/900 1/28/92/9001/917/1815/6 p-H p-H 2 1/202/30015/611/2 Reactants Products formed by Hop and Exchange Park & Light JCP 126, (2007)

Spin-Modification Probability Total Io-H o-H 2 o-H p-H 2 p-H o-H 2 p-H p-H 2 o-H o-H 2 5/ /24/91/3605/125/9 00 1/21/9 008/92/902/3 o-H p-H 2 3/205/1201/411/300 p-H o-H 2 3/25/9 11/34/910/900 1/28/92/9001/917/1815/6 p-H p-H 2 1/202/30015/611/2 Reactants Products formed by Hop and Exchange 1/2  0 ↔ 3/2  0 1/2  0 ↔ 3/2  1 Park & Light JCP 126, (2007)

Low Temperature Effects In pure p-H 2 – p-H 3 + → o-H 3 + requires p-H 2 → o-H 2 ortho I = 3/2 para I = 1/2 para I = 0 ortho I = K

Oka Group Experiments o-H 3 + p-H 3 + Pulsed Hollow Cathode Positive Column Cell Cordonnier et al. JCP 113, 3181 (2000) p-H 2 →o-H 2 hop exch 400 K [not 0.5!] p-H 2 n-H 2 o-H 3 + p-H 3 + n-H 2 p-H 2 reaction not yet measured at low T

Supersonic Expansion Ion Source Developed for DR experiments H 3 + formed near nozzle [p-H 2 ]/[H 2 ] “frozen” [p-H 3 + ]/[H 3 + ] evolves, but ~ steady state Probed by spectroscopy H2H2 Gas inlet 2 atm Solenoid valve -450 V ring electrode Pinhole flange/ground electrode H3+H3+ McCall et al. PRA 70, (2004)

2.8 – 4.8  m DFG System Ti:Sapph 700 – 990 nm 532 nm pump laser reference cavity dichroic /2 Nd:YAG 1064 nm AOM PPLN 25cm 20cm /2 /4 Glan prism 20cm achromat InSb mode- matching lenses ringdown cavity Andrew Mills WG10

Cavity Ringdown Spectra First results from our DFG laser! Clear enhancement of para-H 3 + in para-H 2 More enhanced in argon dilution T rot ~ 80 K –R(1,1) u vs R(2,2) l ortho-H 3 + para-H 3 +

Theoretical Predictions Steady state: High temp limit: Low temp calculations of k by Park & Light f p = k oppo γ + (k oopp +k oopo )(1- γ ) (k oppo +k ppoo ) γ + (k oopp +k oopo +k poop +k pooo )(1- γ ) f p = α +1+2 αγ 3 α +2 α α  hop exchange γ γ  [p-H 2 ] [p-H 2 ]+[o-H 2 ] fp fp  [p-H 3 + ] [p-H 3 + ]+[o-H 3 + ] k oppo  k(o-H p-H 2 → p-H o-H 2 ) Park & Light JCP 126, (2007)

Comparison with Experiment Park & Light

H 3 + toward  Persei Savage et al. ApJ 216, 291 (1977) N(H 2 ) from Copernicus McCall, et al. Nature 422, 500 (2003) f p = 0.62 ± 0.09 γ = 0.68 ± 0.20

Diffuse Clouds Comparison

Future Directions Experiment –Hollow 150 K Measure H 3 + and H 2 –Expansion source Vary γ, T Theory –Different values of α Observations –More sources –Improved error bars

Acknowledgments Tom Geballe (Gemini) Takeshi Oka (U. Chicago) NASA Laboratory Astrophysics NSF Division of AMO Physics Brian Tom Michael Wiczer (→MIT) Andrew Mills Kyle Crabtree Nick Indriolo