Nuclear spin temperatures of hydrogen and water molecules formed and trapped on ice. Naoki Watanabe

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1 9 Jun 2014, Nuclear Spin Effects in Astrochemistry 2014, Göteborg Nuclear spin temperatures of hydrogen and water molecules formed and trapped on ice Naoki Watanabe Institute of Low Temperature Science, Hokkaido Univ. Colleagues: T. Hama, H. Hidaka, Y. Oba, A. Kouchi (ILTS) Y. Kimura (Tohoku Univ.), V. Pirronello (Catania Univ.) 1. Introduction 2. Spin temperature (OPR) of nascent H 2 on ASW 3. Spin temperatures (OPR) of H 2 O from ASW

2 Nuclear spin temperatures of H 2 and H 2 O on ASW Motivation What is the meaning of observed spin temperature (Ortho/Para Ratio)? Observed spin temp.: H 2 80~150 K (diffuse cloud) H 2 O 27 ~ more than 50 K (star-forming region, comets) radiative process spin temp can be a tracer of physicochemical history of molecules OPR can change in gas phase by spin exchange reaction. Little is known about how the OPR depends on grains (temp., structure, composition, etc). Question: Does the OPR tell us the temperature of molecules birth place (grain)? Look at how the OPRs of H 2 and H 2 O behave on ASW!

3 OPR OPR of H 2 3 H H H H T / K Total spin Spin multiplicity Rotation I=1(ortho) I=0(para) 3 1 J=1, 3, 5, J=0, 2, 6, OPR at high temperature limit = 3 (e.g. The lowest DE o-p ~ 170 K e.g. H 3+ (p)+ HD The OPR of H 2 significantly affects chemistry in molecular clouds H 2 D + (p) + H 2 (p)+ 230 K Important species for deuterium enrichment in molecules

4 OPR of H 2 on ASW OPR of nascent H 2 on ASW OP conversion on ASW with time? Today s topic! Watanabe et al OPR of H 2 decreases significantly for several minutes Sugimoto & Fukutani 2011 Large isotope effect on conversion rate H 2 (~370s) >> D 2 Chehrouri et al Trace O 2 molecules accelerate the OP conversion for D 2

5 Apparatus LASSIE RASCAL ASURA

6 Cross section of RASCAL Microwave-type atomic (radical) source Plasma cold Al pipe Atomic (radical beam) TOF Surface species can be monitored by FTIR photodesorption laser Chopper substrate (>7K) Laser for REMPI 10 cm LN 2 shroud Base pressure: Torr Kinetic temp. of atoms: 100 K QMS

7 Procedure REMPI laser H 2 or H beam deposition PSD laser ionize and detect the H 2 in the specific J state Porous ASW at 8 K Aluminum substrate H 2 formed is rotationally-resolved by PSD + REMPI methods during H 2 or H atom deposition REMPI: Resonance Enhanced Multi-Photon Ionization

8 Detected ion intensity REMPI (2+1) transition in H 2 E,F 1 Σ g+ (v =0, J =J ) X 1 Σ g+ (v, J ) 3-photon absorption ionizes H 2 in the specific rovibrational states. REMPI spectrum J=1 J=0 Photon wavelength

9 H 2 (v=0, J=1) / arb. units Evidence of H 2 recombination on ASW Photodesorbed H 2 from ASW for H 2 or H deposition through the beam line with microwave on and off on AL (8 K) on ASW(8 K) 2.0x10-8 H 2 (no plasma) deposision 4.0x10-9 H 2 deposition 1.0x10-8 H (plasma on) deposision 2.0x10-9 H deposition deposition time / min deposition time / min Atomic beam includes little H 2! H 2 recombines and retrapped on ASW!

10 H 2 (J) intensity / arb. units OPR of nascent H 2 on ASW at 8 K by comparing between H 2 (OPR=3) gas and H atom deposition on ASW ? J = J = 1 H 2 gas H 2 from H atoms H 2 gas? H 2 from H atoms deposition time / min deposition time / min H 2 (J = 0, 1) intensities: H 2 gas deposition = H atom deposition OPR on ASW : H 2 gas deposition = nascent H 2 molecule! i.e. OPR of nascent H 2 molecules ~ 3 (high temperature limit)

11 Astrochemical story for OPR of H 2 molecule starting from the ice grain The OPR of nascent H 2 ~ 3 on ice (ASW) retrapped on ice desorbed immediately at recombination OPR decreases rapidly on ASW. OPR depends when H 2 is released in gas phase OPR ~ 3 OP conversion in gas phase

12 OPR of H 2 O OPR at high temperature limit = 3 > ~50 K Lowest DE o-p : 34.2 K Spin temp of H 2 O in comet comae Bonev et al < T spin < 40 K What we did, the OPR measurements of thermal desorbed H 2 O from ices formed at ~10 K

13 Four types of measurements Sample1: Vapor-deposited ASW H 2 O (OPR=3) deposition left for 9 days REMPI OPR measurement ASW production at 8 K 8 K TPD to ~150 K Sample2: H 2 O/O 2 ~1 mixed ASW O 2 H 2 O (OPR=3) left for 17 hour REMPI OPR measurement H 2 O/O 2 codeposition at 8 K 8 K TPD to ~150 K

14 Four types of measurements Sample 3: Photoirradiated ASW H 2 O (OPR=3) deposition CH 4 CH 4 for 1 hour UV REMPI OPR measurement ASW production at 8 K 8 K TPD to ~150 K Sample 4: Photoproduced ASW ASW (H 2 O) UV H 2 O photosynthesis UV irradiation REMPI OPR measurement CH 4, O 2 solid mixture at 8 K ASW at 8 K TPD to ~150 K

15 Intensity / arb. units REMPI spectrum for H 2 O after 9 days + Calculation spectrum was obtained during TPD at ~150 K T spin = 8 K, T rot =150 K T spin = 30 K, T rot =150 K T spin = 150 K, T rot =150 K Wavelength / nm REMPI (2+1) transition in H 2 O C (v=0,0,0) X (v=0,0,0) state rotational state :J Ka, Kc Ka + Kc =odd (ortho) =even (para) Spin temperature > 30 K (~high temperature limit) 8 K The same results were obtained for all samples

16 Results for H 2 O OPR of thermal desorbed H 2 O from ASW is ~3 for all samples. Scenario 1: OP conversion does not occur in ASW (8 K) by O 2 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.

17 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 DE o-p in gas phase: ~ 34 K for H 2 O (J ka, Kc : 0 00 ;1 01 ) easily mixed c.f. ~ 170 K for H 2 (v=0, J=0;1) DE o-p in solid 2. Energy release (relaxation) after OP conversion phonon in solid For example, OP conversion rate~ 30 min for H 2 O in an Ar matrix at 20 K (Abouaf Marguin et al. 2007, 2009)

18 Results for H 2 O OPR of thermal desorbed H 2 O from ASW is ~3 for all samples. Scenario 1: OP conversion does not occur in ASW (8 K) by O 2 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? To do 1. OPR measurement of H 2 O desorbed (e.g. PSD) at 8 K without heating 2. Use H 2 O formed by nonenergetic process like O 2 + H.

19 Spin temperatures of H 2 and H 2 O in/on ice Our findings For a review, see Hama & Watanabe, Chem Rev 113, 8783 (2013) H 2 was photodesorbed from ice and OPR analyzed by REMPI The OPR of nascent H 2 fromed on ice: ~3 (high temperature limit > ~250 K) The OP conversion of H 2 on ice: very rapid (<hour) toward thermal equilibrium The OPR depends on the residence time on ice grain H 2 O was thermal desorbed from ice and OPR analyzed by REMPI H 2 O thermally desorbed from ice samples formed at ~10 K shows OPR of 3 (high temp. limit > ~50 K) Probably due to re-equilibrium at high temperature during desorption but Still need to measure the OPR of H 2 O photodesorbed at low temps.