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 H2 on ASW
3. Spin temperatures (OPR) of H2O from ASW
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)
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 H2 and H2O behave on ASW !
OPR of H2
3
H
H
H
H
OPR
2
Total spin
1
I=1（ortho）
Spin multiplicity
0
0
100 200 300 400
Rotation
T/K
1
3
J=1, 3, 5, …
OPR at high temperature limit = 3
The lowest DEo-p ~ 170 K
I=0（para）
J=0, 2, 6, …
(e.g. 0.01 @30K)
The OPR of H2 significantly affects
chemistry in molecular clouds
e.g. H3+(p)+ HD
H2D+(p) + H2(p)+ 230 K
Important species for deuterium enrichment in molecules
OPR of H2 on ASW
OPR of nascent H2 on ASW
?
Today’s topic !
OP conversion on ASW with time
Watanabe et al. 2010
OPR of H2 decreases significantly for
several minutes
Sugimoto & Fukutani 2011
Large isotope effect on conversion rate
H2 (~370s) >> D2
Chehrouri et al. 2011
Trace O2 molecules accelerate the OP
conversion for D2
Apparatus
LASSIE
ASURA
RASCAL
Cross section of RASCAL
Microwave-type atomic (radical) source
cold Al pipe
Plasma
Atomic (radical beam)
Surface species can be
monitored by FTIR
TOF
photodesorption laser
Chopper
Laser
for REMPI
substrate
(＞7K)
10 cm
LN2 shroud
Base pressure: 10-10 Torr
Kinetic temp. of atoms: 100 K
QMS
Procedure
REMPI laser
ionize and detect the H2 in the
specific J state
H2 or H beam deposition
PSD laser
Porous ASW
at 8 K
Aluminum substrate
H2 formed is rotationally-resolved by PSD + REMPI methods
during H2 or H atom deposition
REMPI: Resonance Enhanced Multi-Photon Ionization
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.
Detected ion intensity
REMPI spectrum
J=1
J=0
Photon wavelength
Evidence of H2 recombination on ASW
Photodesorbed H2 from ASW for H2 or H deposition through the
beam line with microwave on and off
H2 (v=0, J=1) / arb. units
on AL （8 K）
on ASW（8 K）
H2 (no plasma)
deposision
-8
2.0x10
-9
4.0x10
H2 deposition
-8
-9
1.0x10
H (plasma on)
deposision
0.0
2.0x10
H deposition
0.0
0
10
20
30
40
deposition time / min
Atomic beam includes little H2 !
0
10
20
30
40
deposition time / min
H2 recombines and retrapped
on ASW !
OPR of nascent H2 on ASW at 8 K
H2 (J) intensity / arb. units
by comparing between H2 (OPR=3) gas and H atom deposition on ASW
1.0
1.0
J=0
0.8
0.8
0.6
?
0.6
0.4
H2 gas
0.4
0.2
0.0
2
4
6
8
10
12
14
deposition time / min
16
H2 gas
0.2
H2 from H atoms
0
J=1
18
0.0
?
H2 from H atoms
0
2
4
6
8
10
12
14
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)
16
18
Astrochemical story for OPR of H2 molecule
starting from the ice grain
The OPR of nascent H2 ~ 3 on ice (ASW)
retrapped on ice
OPR decreases rapidly on ASW.
OPR depends when H2 is released in gas phase
desorbed immediately at
recombination
OPR ~ 3
OP conversion in gas phase
OPR of H2O
OPR at high temperature limit = 3
> ~50 K
Lowest DEo-p: 34.2 K
Spin temp of H2O in comet comae
20 < Tspin < 40 K
Bonev et al. 2007
What we did,
the OPR measurements of thermal desorbed H2O from ices formed at ~10 K
Four types of measurements
Sample1: Vapor-deposited ASW
OPR measurement
H2O (OPR=3)
deposition
REMPI
left for 9 days
ASW production at 8 K
Sample2: H2O/O2~1 mixed ASW
OPR measurement
H2O (OPR=3)
O2
H2O/O2 codeposition
at 8 K
TPD to ~150 K
8K
REMPI
left for 17 hour
8K
TPD to ~150 K
Four types of measurements
CH4
Sample 3: Photoirradiated ASW
H2O (OPR=3)
deposition
CH4for 1 hour
UV
ASW production at 8 K
OPR measurement
REMPI
TPD to ~150 K
8K
ASW
(H2O)
Sample 4: Photoproduced
ASW
UV irradiation
OPR measurement
UV
REMPI
H2O photosynthesis
CH4, O2 solid mixture
at 8 K
ASW at 8 K
TPD to ~150 K
REMPI spectrum for H2O after 9 days + Calculation
Intensity / arb. units
spectrum was obtained during TPD at ~150 K
Tspin= 8 K, Trot=150 K
Tspin= 30 K, Trot=150 K
202-321
Tspin= 150 K, Trot=150 K
202-221
496.2 496.4 496.6 496.8 497.0 497.2
Wavelength / nm
REMPI (2+1) transition in H2O
rotational state ：JKa, Kc
C (v=0,0,0) ←← X (v=0,0,0) state
Ka + Kc =odd (ortho)
=even (para)
Spin temperature > 30 K (~high temperature limit) ≠ 8 K
The same results were obtained for all samples
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.
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)
easily mixed
c.f. ~ 170 K for H2 (v=0, J=0;1)
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
(Abouaf−Marguin et al. 2007, 2009)
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.
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 H2O desorbed (e.g. PSD) at 8 K without heating
2. Use H2O formed by nonenergetic process like O2 + H.
Spin temperatures of H2 and H2O in/on ice
Our findings
For a review, see Hama & Watanabe, Chem Rev 113, 8783 (2013)
H2 was photodesorbed from ice and OPR analyzed by REMPI
The OPR of nascent H2 fromed on ice: ~3 (high temperature limit > ~250 K)
The OP conversion of H2 on ice: very rapid (<hour) toward thermal equilibrium
The OPR depends on the residence time on ice grain
H2O was thermal desorbed from ice and OPR analyzed by REMPI
H2O 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 H2O photodesorbed at low temps.