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

Stark velocity filter Stark velocity filter

Detection vacuum chamberDetection vacuum chamber

Linear Paul trapLinear Paul trap

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.