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Development of a Stark velocity Development of a Stark velocity
filter for studying cold ionfilter for studying cold ion--filter for studying cold ionfilter for studying cold ion--
polar molecule reactionspolar molecule reactions
K. OkadaK. Okada, , N. Kimura, T. Suganuma, T. Furukawa, N. Kimura, T. Suganuma, T. Furukawa,
K. Shiina, M. Wada K. Shiina, M. Wada A)A), H. A. Schuessler , H. A. Schuessler B)B)K. Shiina, M. Wada K. Shiina, M. Wada A)A), H. A. Schuessler , H. A. Schuessler B)B)
Department of Materials and Life Sciences, Sophia UniversityDepartment of Materials and Life Sciences, Sophia University
A)A)Nishina Center for AcceleratorNishina Center for Accelerator--Based Science, RIKENBased Science, RIKEN
B)B)Department of Physics, Texas A&M UniversityDepartment of Physics, Texas A&M University
Background and Motivation1. Ion-polar molecule reactions play important roles in synthesis of
interstellar molecules → It is necessary to understand ion-polar neutral systems, before more definitive conclusions can be reached for chemical evolution in dark definitive conclusions can be reached for chemical evolution in dark
interstellar clouds usin “UMIST” data base (Wakelam et al. A&A 2006)
2. There are only a small number of experimental reaction-rate
constants measured at low temperatures.
→ Benchmark data to test the scaling formula of the capture rate in ion-polar molecule collisions and other quantum chemical calculations
22/,/]4767.062.0[2 ≥≡+= Tkxxek BDcap αµµαπ
Capture rate by “trajectory scaling approach” based on Capture rate by “trajectory scaling approach” based on
classical classical trajectory techniquetrajectory technique※※※※※※※※
(α: polarizability, µ: dipole moment, T: temperature, kB: Boltzmann constant)
※※※※※※※※T. Su & W. J. Chesnavich, JCP1982T. Su & W. J. Chesnavich, JCP1982
Previous studies� T. Baba et al. “ Chemical reaction between sympathetically
cooled molecular ions and NH3 : H3O+ + NH3 → NH4
+ + H2O ”
(JCP 2002)
� G. Rempe et al. “ First demonstration of Stark velocity filter to produce slow polar molecules ” (PRA 2004)
� T. P Softley et al. “ Reaction rate measurement between Ca+
Coulomb crystal and cold CH3F ” (PRL 2008)
� S. Willitsch et al., “ Preliminary reaction rate measurement between slow ND3 and sympathetically cooled OCS
+ ions ”
(Faraday Discuss. 2009)
We are now developing a new apparatus composed of
a Stark velocity filter and cryogenic linear ion trap.
Extension in this study
1. Many kinds of cold molecular ions will be supplied by
sympathetic laser cooling → CaH+, CaF+, N2+, CH2O
+, ND3+ ...
2. Many kinds of slow polar molecules will be supplied.
Overview of experimental setupOverview of experimental setup
Stark velocity filtercooling
laser
LN2
pot
<10-8 Pa
A
10K cryo-cooler
gas nozzle
linear rf ion trap
QMS
electron gun
pot
< 10-4 Pa
~10-6 Pa
gas inlet
beam guide
electrodes
(Vmax:±3.0kV)
ceramic
heater
� Curveture radii of 1st and 2nd bent sections
→ R = 12.5 mm, 25.0 mm
� total length of the beam guide
→ L = 941.8 mm
TimeTime--ofof--flight flight measurement of measurement of
slow slow NDND33moleculesmoleculesNDND33@@4141±±1mTorr1mTorr,, 295K295K
600 � Velocity distribution
300
400
500
3.0kV2.8kV2.4kV2.0kV1.6kV1.2kV0.8kV
3 ion
sign
al (
cps)
( ) ( )[ ][ ]ttktI −−−∝ expexp
( ) ( )v
t
v
Lv
2/, Lt
dt
dIf =
=
� Gompeltz function
L: flight distance
100
200
-20 0 20 40 60 80 100
ND
3
t (ms)
( ) ( )[ ][ ]cttktI −−−∝ expexp
asymmetric growth curve
I(t) well reproduce I(t) well reproduce TOFTOF signalsignal※※※※※※※※
※※※※M. T. Bell et al., Faraday Discuss.142, 73 (2009)
40
Velocity distributionVelocity distribution
NDND33@@41mTorr41mTorr,, 295K295K A plot A plot of peak velocityof peak velocity
10 3.0kV
20
25
30
35
vpe
ak (
m/s
)
1.3K!1.3K!
2
4
6
8
10
rela
tive
inte
nsity
(ar
b. u
nits
)
2.8kV2.0kV
1.6kV
1.2kV
0.8kV
3.0kV
15
20
0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2
Vs (kV)
0
2
0 20 40 60 80 100 120 140
rela
tive
inte
nsity
(ar
b. u
nits
)
v (m/s)
1.2 106
Number density of slow Number density of slow NDND33
LL = = 32.8 mm32.8 mm8 106 0428_0509
ND3 gas was intentionally leaked from the variable leak valve.
2 105
4 105
6 105
8 105
1 106
n (c
m−3
)
LLoutout = = 32.8 mm32.8 mm
TT =295 K=295 K
6
2 106
3 106
4 106
5 106
6 106
7 106
8 10
num
ber
dens
ity
(cm
−3)
cc=0.76
0
2 105
0 0.5 1 1.5 2 2.5 3 3.5
Vs (kV)
]cm[10)3(1.2 33 −×= QMSIn 3kV@]cm[109 35max
−×≈ n
0
1 106
0 500 1000 1500 2000 2500
num
ber
dens
ity
(cm
ND3 counts (cps)
TOFTOF signals of signals of CHCH22OO and and CHCH33CNCN
CHCH22OO @ 41@ 41±±1mTorr1mTorr,, 295K295K
320 600
CHCH33CNCN @ 32@ 32±±1mTorr1mTorr,, 295K295K
200
220
240
260
280
3002.8 kV
2.4 kV
2.0 kV
1.6 kV
1.2 kV
CH
2O s
igna
l (cp
s)
300
350
400
450
500
550
600
3.0 kV2.8 kV2.4 kV2.0 kV1.6 kV1.2 kV0.8 kV
CH
3CN
sig
nal (
cps)
160
180
-20 0 20 40 60 80 100
t (ms)
200
250
0 20 40 60 80 100
t (ms)
The slowest peak velocity corresponds to The slowest peak velocity corresponds to
a thermal energy of a few Kelvin.a thermal energy of a few Kelvin.
Summary of Summary of production of production of
slow polar moleculesslow polar molecules
molecule M v (m/s) T (K) n (cm−−−−3333)
nozzle temperature:295 K
molecule M vpeak(m/s) Tpeak (K) nmax(cm−−−−3333)
ND3 20.05 23 ~ 40 1.3 ~ 3.8 9××××105
CH2O 30.03 23 ~ 32 1.9 ~ 3.7 1.3××××106
CH3CN 41.05 23 ~ 34 2.6 ~ 5.7 1.2××××105
NH3 17.03 36.5 2.7 2××××105
Cold ion-polar molecule reactions: k = 10-8 ~ 10-9 (cm3/s)
Expected reaction-rate : 10-2 ~ 10-3 /s
Simulation of Stark velocity filterSimulation of Stark velocity filter
• Information of the transverse
velocity distribution and the
actual “cut-off” velocityθθθθθθθθ
Trajectory of a slow Trajectory of a slow NDND33
actual “cut-off” velocity
• Rotational state distribution
of guided slow molecules
• Spatial dispersion of a slow
θθθθθθθθ
Transverse velocity distributionTransverse velocity distribution
• Spatial dispersion of a slow
molecular beam after
passing through the beam
guide (beam profile)
SummarySummary� Stark velocity filter and cryogenic linear ion trap
have been completed.
� Slow polar molecules (ND3, NH3, CH2O, CH3CN) with
a thermal energy of a few Kelvin have been produced.
� Determination of the number density of the slow
polar molecules has been conducted.
� Monte-Carlo simulation code has been developed for
characterization of Stark velocity filter.