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ALMA mini- sympo , ISMS , Illinois, 20 June 2017

ALMA mini- sympo , ISMS , Illinois, 20 June 2017. P hysicochemical p rocesses on Ice Dust towards Deuterium Enrichment. Orhto -to-para conversion of H 2 on ice. Naoki Watanabe. Inst itute of Low Temperature Science, Hokkaido University, Japan. Coworkers: H. Ueta , T. Hama, A. Kouchi.

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ALMA mini- sympo , ISMS , Illinois, 20 June 2017

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  1. ALMA mini-sympo, ISMS, Illinois, 20 June 2017 Physicochemical processeson Ice Dust towards Deuterium Enrichment Orhto-to-para conversion of H2 on ice Naoki Watanabe Institute of Low Temperature Science, Hokkaido University, Japan Coworkers: H. Ueta, T. Hama, A. Kouchi 1.Introduction ・ Key processes of deuterium enrichment ・ What are ortho and para states of H2 ・ Astronomical importance of ortho-to-para ratio of H2 2. Nuclear spin (ortho-para) conversion for H2 ・ Ortho-to-para conversion on ice ・ Experiment ・ Results & discussion

  2. Deuterated species in clouds HD HDO HDS D2S HDCS DCN DNC NH2D NHD2 ND3 DC3N DC5N N2D+ DCO+ HDCO D2CO CH2DOH CHD2OH CH3OD CH2DCN C4D C2D CH2DCCH H2D+ CH3CCD up to 0.1~0.3 ! cf. cosmic D/H=10-5 etc … Many species are deuterium-enriched. What is the role of dust grain in D-enrichment at very low temperatures?

  3. H3+ + HD H2D+ + H2 Key processes Ortho-to-para ratio (OPR) is crucial ! Gas - dust phase Energy difference between lowest ortho and para states is ~170 K >> 10 K Gas phase H2D+ + e → H2 + D D / H atom ~ 0.1 >> HD/H2 after 104 yr The OPR affects chemistry in MC, e.g. deuterium enrichment H2D+/H3+ >> cosmic D/H~10-5 D + X on dust Deuterated species Below about 20 K The ortho-to-para ratio of H2 is often assumed to be statistical 3 or thermal equilibriumin chemical models. producing deuterated species in gas phase

  4. H H H H Ortho-to-para ratio (OPR) of H2 @thermal equilibrium 3 × I=0（para） I=1（ortho） Total spin 2 1 3 Spin multiplicity OPR 1 J=1, 3, 5, … J=0, 2, 6, … Rotation 0 0 100 200 300 400 OPR = Northo/ Npara= T / K OPR at high temperature limit = 3 (e.g. 0.01 @30K) The lowest DEo-p ~ 170 K The OPR of H2 significantly affects chemistry in molecular clouds

  5. Observations of OPRs for H2 @ Shock regions @ PDRs OPRs ~ 1 lower than 3 expected from rotational temps. e.g. Harbart et al. 2003, 2011 Fleming et al. 2010 The OPRs are out-of-equilibrium ! Upper : rotational 600-1000 K Lower : OPR 0.5-3 Neufeld et al. ApJ 649, 816 (2006)

  6. Furuya, K. unpublished data Solid lines: OPR = 0 Importance of OPR for H2 D/H ratio ☆　The OPR affects dynamics of core formation in star-forming region. ☆　Slow conversion process between two spin isomers has been used as a tracer of the age of clouds. Dashed lines: OPR = 3 Time (yr) ☆　The OPR can control chemical evolution, deuterium enrichment, in molecular clouds. The lowest DEo-p~ 170 K (v=0, J=1—0) O Variation of ortho-abundance at different cosmic-ray fields by spin-exchange reactions in gas phase. The OPR starts from 3 in this model (Pagani et al. A&A 551, A38 (2013)) Important species for deuterium enrichment below ~20 K Heat capacity of H2 gas with different OPRs (Vaytet et al. A&A 563, A85 (2014)) H3+(p)+ HD H2D+(p) + H2(p)+ 230 K

  7. Motivation What controls the OPR (spin temperature) of H2? spin temp can be a tracer of physicochemical history of molecules… ☆OPR can change in gas phase by spin exchange reaction, but slow. radiative process e.g. H3+(o) + H2(o) H3+(o) + H2(p) ☆Little is known about how the OPR behaves on grains (temp., structure, composition, etc). Look at how the OPRs of H2 behave on ice dust at ~10 K !

  8. Recent findings in OPR of H2 on ice at ~10 K OPR of nascent H2 on ASW OP conversion on ASW with time First observed by IR Buch & Devlin1993 OPR of nascent H2 formed by recombination is ~3 OPR of H2 decreases significantly for several minutes Watanabe et al. 2010 Watanabe et al. 2010 Gavilan et al. 2012 Large isotope effect on conversion rate H2 (~370s)>> D2 Sugimoto & Fukutani 2011 Hama et al. 2012 Today’s topic Temperature dependence of OP conversion Trace O2 molecules accelerate the OP conversion for D2 Chehrouri et al. 2011

  9. Requirements for OP conversion 1. Energy difference exists between ortho and para states Rotation required Lowest DEo-p in gas phase: ~ 170 K for H2 (v=0, J=0;1) DEo-p when rotation hindered on the surface 2. State mixing between ortho and para states induced by perturbation e.g. intra-(or inter-)molecular nuclear magnetic dipole-dipole interaction 3. Energy release (relaxation) at the OP conversion Significant temp dependence of the conversion rate Energy dissipation by phonons in solid

  10. Apparatus

  11. H2 (OPR=3) ASW 10～16 K S ｔ Procedure deposition time H2deposition 60 s ｔ laser H2-photodesorption Residencetime H2selective detection REMPI H2+ laser H2+detection (ionized from H2 in J=0, 1) S J0 J1 conversion rate

  12. REMPI (2+1)transition in H2 E,F1Σg+(v’=0, J’=J”) ←←X1Σg+(v”, J”) 3-photon absorption ionizes H2 in the specific rovibrational states. REMPI spectrum J=1 J=0 Detected ion intensity Photon wavelength We monitored the peak heights

  13. REMPI signals after the H2-deposition on ASW at 16 K 600 s 300 s After 10 s Residence time on ASW / s REMPI laser was fixed at the resonance wavelength.

  14. OP conversion of H2 on ASW 16 K 10 K 11 K Sum Relative intensities J = 1 J = 0 Residence time on ASW / s

  15. OP conversion rate vs surface temperature OP transformation rate ? Two phonon relaxation (Raman) process T 7.2 ±1.1 Ice temperature, T (K) Steep increase of conversion rate cannot be explained by state-mixing alone. Energy dissipation process should be considered.

  16. Astrochemical story for OPR of H2 molecule starting from the ice grain The OPR of nascent H2~3 on ice (ASW) desorbed immediately at recombination retrapped on ice OPR decreases rapidly on ASW. OPR ~ 3 OPR depends on the dust temp and when H2 is released in gas phase OP conversion in gas phase e.g. for the gas phase reactions,see D. Gerlichet al., Philos. Trans. A. Math. Phys. Eng. Sci.364, 3007 (2006).

  17. Summary The OPR of H2 varies rapidly on ice, sensitively depending on the ice temperature Steep increase of OP conversion at lower temperature can be explained by two phonon Raman process. In molecular clouds, the OPR of H2 at desorption into the gas phase depends on when it is released and temperature of dust.

  18. These normal hydrogenated species require grain surface reactions Deuterated species in clouds HD HDO HDS D2S HDCS DCN DNC NH2D NHD2 ND3 DC3N DC5N N2D+ DCO+ HDCO D2CO CH2DOH CHD2OH CH3OD CH2DCN C4D C2D CH2DCCH H2D+ CH3CCD up to 0.1~0.3 ! cf. cosmic D/H=10-5 etc … Grain surface reactions would play an important role in deuterium enrichment!

  19. Observational signature of deuteration on grain mantles for formaldehyde and methanol Fontani et al. ApJL (2014), A&A (2015) L1157-B1 The HDCO and CH2DOH emission only appears on the shocked region. Fontani et al. ApJL (2014) CH2DOH and CH3OD detected toward warm stareless cores Fontani et al. A&A (2015) From Fontani et al. ApJL788, L43 (2014) Deuterated species are evaporated from grain mantles.

  20. Deuteration processes on grains at ~10K 1. Deuterationduring molecular formation (D addition reactions) e.g. H D D H CO → HCO → HDCO → CHD2O → CHD2OH 2. Deuterationafter molecular formation (H-D substitution) H D H O2 → HO2 → HDO2 → HDO + OH H abstraction (tunneling) + D addition (thermal) e.g. (Nagaoka et al. ApJL 2005) D D CH3OH → CH2DOH → CHD2OH

  21. Deuteration processes on grains at ~10 K 1. Deuterationduring molecular formation (D addition reactions) e.g. H D D H CO → HCO → HDCO → CHD2O → CHD2OH 2. Deuterationafter molecular formation (H-D substitution) H D H O2 → HO2 → HDO2 → HDO + OH H abstraction (tunneling) + D addition (thermal) e.g. D D Findings CH3OH → CH2DOH → CHD2OH H-D substitution >> D-H substitution Very high deuterium enrichment H-D substitution >D-H substitution High deuterium enrichment D addition reaction only (substitution is endothermic) Moderate deuterium enrichment Note that “Moderate enrichment” may be possible in the gas phase

  22. Important factors of tunneling abstraction reactions ~ 1 a. u. D abstraction reduce D HDCO + H (or D) HCO + HD (or D2) >> ~ 0.5 a. u. HDCO + H (or D) DCO + H2 (or HD) H abstraction increase D Hidaka et al. ApJ 702, 291 (2009) Activation barrier Similar to each other (only zero-point energy) Tunneling mass of reaction system

  23. H-Dsubstitution in methylamine Nonexposed solid CH3NH2 Oba et al. MAPS 49, 117 (2014) N-H st. C-H st. Findings H-D substitution occurs to form CD3NH2, CD3ND2, etc. Absorbance MethylamineD-methylamine Reaction rate: Methyl group >> Amino group 10 1 After D exposure Reverse process, D-H substitution also occursbut slower N-D st. C-D st. 1 D D Absorbance CD3NH2, CD3ND2formed H 0.4

  24. Functional group dependence of H-D substitution e.g. methanol Methyl group >> Hydroxyl group(not detected in the experiments) e.g. methylamine Methyl group >> Amino group(10 times slower than methyl group) Activation barrier H-D (D-H) direct exchange in hydroxyl (hydrogen-bonding system) during warming up of ice Tunneling mass of reaction system CH3-OD --- H-O-H CH3OH --- D-O-H Ratajczak et al. AA 496 L21 (2009), Lamberts et al. MNRAS 448, 3820 (2015)

  25. Requirements for OP conversion 1. State mixing between ortho and para states induced by perturbation e.g. magnetic dipole-dipole interaction Lowest DEo-p in gas phase: ~ 34 K for H2O(Jka, Kc: 000;101) ~ 170 K for H2 (v=0, J=0;1) easily mixed DEo-p in solid 2. Energy release (relaxation) after OP conversion phonon in solid For example, OP conversion rate~ 30 min for H2O in an Ar matrix at 20 K

  26. Summary for deuterium-enrichment experiments Very high deuterium enrichment : H-D substitution (>> D-H substitution). Up to ~several tens % Formaldehyde, Methanol: Nagaoka et al. ApJ 624, 29 (2005), Hidaka et al. ApJ 702, 291 (2009) Glycine: Oba et al. CPL (2015) NoteG High deuterium enrichment : H-D substitution (> D-H substitution). Note that rate “constants” have not been determined experimentally because of difficulties in measuring H, D atom number densities and residence time of atoms in reaction sites… ~several % Methylamine: Oba et al. MAPS 49, 117 (2014) Ethanol: Oba et al. in prep. Moderate deuterium enrichment : D addition reaction only Less than several % Water, Methane: Nagaokaet al. ApJ 624, 29 (2005), Oba et al. Faraday Discuss. 168, 185 (2014) Ethylene, Ethane: Hiraokaet al. ApJ (2000), Kobayashi et al. in prep. Ammonia: Nagaoka et al. ApJ 624, 29 (2005), Fedoseev et al. MNRAS 446, 449 (2015)

  27. Nuclear spin temperatures of H2 and H2O on ASW Motivation What is the meaning of observed spin temperature (Ortho/Para Ratio) ? Observed spin temp.:H2 80~150 K (diffuse cloud) H2O 27 ~ more than 50 K (star-forming region, comets) OPR of H2 is important for deuterium fractionation in the gas phase spin temp can be a tracer of physicochemical history of molecules… radiative process ☆OPR can change in gas phase by spin exchange reaction. ☆Little is known about how the OPR alters on grains (temp., structure, composition, etc). Question: Does the OPR tell us the temperature of molecules’ birth place (grain)? No ! (so far)

  28. Methods Thermal desorption + REMPI OPR measurement H2 deposition (OPR=3) H2 deposition (OPR=3) REMPI REMPI Waiting time Heating up to ~20 K ~10 K ~ 10 K ~ 10 K ~ 10 K ~ 10 K PSD + REMPI PSD

  29. OPR of H2O thermally desrobed from ASW Hama et al. 2011 Four types of H2O solid were examined Vapor-depositedASW at 8 K Photoirradiated ASW at 8 K H2O/O2(~1) mixed ASW at 8 K Photoproduced ASW from CH4/O2 mixture at 8 K The OPR of H2O thermally desorbed from 4 samples all indicated ~3. Probably,gas phase process including H2O formation is necessary to produce low OPRs observed toward some targets. For comets, desorption with the jet from nuclei may play a role.

  30. Adsorption energies of interstellar species on ice

  31. Chemical processes on ice dust Energetic process (irradiation of ice mantle) In the core of dense clouds, e.g.H2O(s) + hn, e-. p+→ H + OH, H2 + O (primary) H atom flux > UV, e-, p+ fluxes OH + CO(s) → CO2 + H (secondary) Important ! Nonenergetic surface reaction on ice A + BC →　ABCwithoutexternal energy input like UV and cosmic ray e.g. H + H →　H2, OH + H → H2O What’s the advantage of surface reaction compared to gas phase reaction? How surface reaction can occur on ice at very low temperatures?

  32. Procedure ionize and detect the H2 in the specific J state REMPI laser Continuous H2or H beam deposition PSD laser Porous ASW at 8 K Aluminum substrate H2 formed is rotationally-resolved by PSD + REMPI methods duringH atom deposition REMPI: Resonance Enhanced Multi-Photon Ionization

  33. on ASW（8 K） H2 deposition H deposition deposition time / min H2 recombine and retrapped on ASW ! Evidence of H2 recombination on ASW Photodesorbed H2 from ASW for H2 or H deposition through the beam line with microwave on and off on AL （8 K） H2 (no plasma) deposision H2 (v=0, J=1) / arb. units H (plasma on) deposision deposition time / min Atomic beam includes little H2 !

  34. OPR of nascent H2 on ASW at 8 K by comparing between H2 (OPR=3) gas and H atom deposition on ASW J = 0 J = 1 ? H2 (J) intensity / arb. units H2 gas ? H2 gas H2 from H atoms H2 from H atoms deposition time / min deposition time / min H2 (J = 0, 1) intensities: H2 gas deposition = H atom deposition OPR on ASW : H2 gas deposition = nascent H2 molecule ! i.e. OPR of nascent H2 molecules ~ 3 (high temperature limit)

  35. OPR of H2O rotational state ：JKa, Kc Ka + Kc =odd (ortho) =even (para) OPR at high temperature limit = 3 > ~50 K Bonev et al. 2007 Spin temp of H2O in comet comae 20 < Tspin < 40 K

  36. Four types of measurements Sample1: Vapor-deposited ASW OPR measurement H2O (OPR=3) deposition REMPI left for 9 days TPD to ~150 K ASW production at 8 K 8 K Sample2: H2O/O2~1 mixed ASW OPR measurement H2O (OPR=3) REMPI O2 left for 17 hour TPD to ~150 K H2O/O2codeposition at 8 K 8 K

  37. Four types of measurements CH4 Sample 3: PhotoirradiatedASW OPR measurement CH4 for 1 hour H2O (OPR=3) deposition REMPI UV TPD to ~150 K ASW production at 8 K 8 K ASW (H2O) OPR measurement UV irradiation Sample 4: Photoproduced ASW UV REMPI H2O photosynthesis TPD to ~150 K CH4, O2 solid mixture at 8 K ASW at 8 K

  38. Tspin= 150 K, Trot=150 K Tspin= 8 K, Trot=150 K Tspin= 30 K, Trot=150 K REMPI spectrum for H2O after 9 days + Calculation spectrum was obtained during TPD at ~150 K Intensity / arb. units 202-321 202-221 Wavelength / nm REMPI (2+1)transition in H2O C (v=0,0,0) ←← X (v=0,0,0) state rotational state ：JKa, Kc Ka + Kc =odd (ortho) =even (para) Spin temperature >30 K (~high temperature limit) ≠ 8 K The same results were obtained for all samples

  39. rotational wavefunction(spherical harmonics) nuclear spin wavefunction H2 case A A orS S S S A orS T = 1 T = 0 Nuclear spin temperature rotational quantum numberJ＝even → S J= odd → A T = 1　（ortho) S odd J multiplicity: 3 T = 0　（para) A even J multiplicity: 1 ☆ frobidden

  40. The OP conversion mechanism on ASW (Sugimoto & Fukutani in Nature Physics, 2011) SC IFCC ESOC SC SC 1 ortho H2 （I=1, J=1） para H2 （I=0, J=0） ASW may locally produce ~1011 V/m on the surface. Isotope effect appears due to the difference of vibrational state mixing between H2 and D2. SC : Stark mixing IFCC : Fermi contact interaction ESOC : Spin-orbit interaction H2>>D2

  41. 1 x 1010 (V/m) → 2.7 x 1013 (W/cm2)

  42. Results for H2O OPR of thermal desorbed H2O from ASW is ~3 for all samples. Scenario 1: OP conversion does not occur in ASW (8 K) by O2 presence, leaving 9 days, and/or UV photolysis. Scenario 2: OP conversion occurs in ASW at 8 K but re-equilibrate at 150 K during TPD.

  43. Results for H2O See Hama & Watanabe submitted to Chem Rev for H2O spin conversion OPR of thermal desorbed H2O from ASW is ~3 for all samples. Scenario 1: OP conversion does not occur in ASW (8 K) by O2 presence, leaving 9 days, and/or UV photolysis. Scenario 2: OP conversion occurs in ASW at 8 K but re-equilibrate at 150 K during TPD. The observed OPR may be correlated with desorption process. Do chemical and physical conditions change during desorption from the surface to coma? 1. OPR measurement of H2O desorbed (e.g. PSD) at 8 K without heating To do 2. Use H2O formed by nonenergetic process like O2 + H.

  44. OPR of H2O thermally desrobed 4 types of H2O solid were examined Vapor-deposited ASW at 8 K Photoirradiated ASW at 8 K H2O/O2(~1) mixed ASWat 8 K Photoproduced ASW from CH4/O2mixtureat 8 K The OPR of H2O thermally desorbed from 4 samples all indicated ~3. Probably, gas phase process is necessary to produce a low OPR observed toward some targets.

  45. Prospects COM, PAHformation by energetic process Starting from simple ices Characterizationof laboratory-synthesized iceby direct observation e.g. Neutron scattering (Fraser et al.), TEM, AFM… Surface diffusion of heavy species (O atom, radicals…) What is the reaction-rate limited process? e.g. “Diffusion rate vs Tunneling rate” in tunneling reactions The mechanism of nuclear spin conversion on the surface More quantitative data (surface number density, rate constant,etc)

  46. Nuclear spin conversion of H2 and H2O in gas phase cf. H3+(p)+ HD H2D+ + H2(p)+ 230 K Uy et al. PRL 78, 3844 (1997) ortho-H2O ＋ H+ → para-H2O ＋ H+ ortho-H2O ＋ H3O+ → H3O+＋para-H2O

  47. Requirements for OP conversion 1. State mixing between ortho and para states induced by perturbation e.g. intra-(or inter-)molecular nuclear magnetic dipole-dipole interaction Lowest DEo-p in gas phase: ~ 34 K for H2O(Jka, Kc: 000;101) c.f. ~ 170 K for H2 (v=0, J=0;1) easily mixed DEo-p DEo-p ～0 in ice in solid 2. Energy release (relaxation) after OP conversion phonon in solid For example, OP conversion rate~ 30 min for H2O in an Ar matrix at 20 K (Abouaf−Marguin et al. 2007, 2009)

  48. Tspin= 150 K, Trot=150 K Tspin= 8 K, Trot=150 K Tspin= 30 K, Trot=150 K REMPI spectrum for H2O after 9 days + Calculation spectrum was obtained during TPD at ~150 K Intensity / arb. units 202-321 202-221 Wavelength / nm REMPI (2+1)transition in H2O C (v=0,0,0) ←← X (v=0,0,0) state rotational state ：JKa, Kc Ka + Kc =odd (ortho) =even (para) Spin temperature >30 K (~high temperature limit) ≠ 8 K The same results were obtained for all samples

  49. OP conversion of H2 on Cu metal Fermi contact Magnetic dipole Electron motion Yucel (1989）

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