Terahertz spectroscopy of molecules, radiacls and ions using

Evenson-type tunable FIR spectrometer

Fusakazu Matsushima

Department of Physics,

University of Toyama, Japan

67th Columbus meeting, June 20, 2012

Scope of this talk“Evenson-type” = difference frequency of CO2 laser lines = TuFIR spectrometer

1. Property of the spectrometer (Review) Tunability, accuracy, sensitivity Trial to extend the properties 2. Examples (mainly of ions)

Glow discharge cell Molecular cation (HeH+, H2D+) Molecular anion (OH-)

Extended negative glow discharge cell Temperature ( N2H+) Recent work (H2F+)

Ken. M. Evenson

NIST (Boulder, Colorado)Time & Frequency Division

late Ken. M. Evenson NIST (Boulder, Colorado)Time & Frequency Division

in 1993at Toyama

His wife called him “a man of curiosity”

“The final measurement of speed of light”

Talk in Toyama (1993)

MIM diode as detector / mixer

Chain for the measurement of CO2 laser frequency

Standard:Cs atomic clock

MIMdiode

whisker

CO2 laser lines

10P

10R

9P

9R

angle of grating

outp

ut p

ower

1. Frequency range

Distribution of CO2 laser lines

TuFIR (CO2 laser difference frequency)How to obtain the tunability

using wave guide CO2 laser using MW source

tunability ~ 100MHzneed many combinations

tunability ~ 20GHz

power: 2nd > 3rd

Evenson-type TuFIR in the world of 1990s

NIST (Corolado): 2nd, 3rd Minnesota (K.R. Leopold): (2nd?), 3rd LENS(M. Inguscio, Firenze, Italy): 3rd

Toyama: 3rd (2nd prepared)

NIST 2nd order system

TuFIR spectrometer

FIR=|I - II|±MW

MIM diode

micro wave

whisker

roof top mirror

base

FIR

CO2 laser

Properties

1. Frequency Range

2. Precision

3. Sensitivity ( Power of the radiation source)

Distribution of CO2 laser lines

≈ 5.5THz

upper or lower sidebanddifference freq. of two CO2 lasers

Fourier transform spectrometer

TuFIR

CO2 fluorescence cell

Laser frequency　 (cavity length)

4.3mfluorescence

1st derivative

Stabilization of the CO2 laser frequency

Accuracy of laser frequency table <2kHz

Accuracy of laser stabilization one CO2 laser 25kHz → difference freq. 36 kHz

2. Precision

NH3 s(J=7,K= ３ ) ← a(J=6,K= ３ )

4126502.441 4126477.036(37) 4126452.441Frequency(MHz)

9P(20)-10R(24)　=　4138411.441　(MHz)MW　=11909　 ～ 11959 （ MH ｚ）

Fitting with Voigt profile ⇒ center frequency

←Bolometer

Synthesized source power ( 3rd order TuFIR spectometer)

characteristics of the componentsin the detector

frequency characteristics of MIM diode

3. Sensitivity (Power of the source)

property of TuFIR spectrometer

1. frequency range about 30 GHz ~ 5.5 THz (or 6THz, 10THz)

2. measurement accuracy of spectral lineseveral tens kHz for strong (neutral) moleculesseveral hundres kHz for weak (ionic) molecules

3. power of the synthesized FIR several tens of nW ~ several hundreds of nW

Trials for extending the properties

1. Frequency range ⇒ higher order nonlinearity of MIM　　　 ⇒　 use of NH3 laser2. Resolution ⇒ sample in a molecular beam

｜ νI ー νII ｜ νMW = 2.4THz

9P(14)-10R(14)　　2.4THz

｜ νI ー νII ｜ 2 νMW = 2.4THz 10R(34)-10P(20)　　1.2THz

1. Multiple of IR frequency

3 wave mix

5 wave mix

H218O

321 ← 312

up to 9.1THz

CO2 laser + NH3 laser

H. Odashima, L.R. Zink, & K.M. Evenson, Opt. Lett. vol.24, 406 (1999)

CH3OH(n=1,K=7,J=19)←(0,6,19)A-type transition

Maximum record of TuFIR

Effusive molecular beam

Multi channel array (diam 10μm, length 2mm)Optical Path length: about 30 cmStagnation pressure: 0.15 Torr effective sample pressure ≤ 1mTorr

2. Molecular beam

bulk gas2x10-4 Torr

beam

width= 220kHz (peak to peak)

H218O

432 ← 423

Molecules　and　ions　measured　with　TuFIR　spectrometer　in　Toyama

(1) neutral molecules , radicals

　　 LiH, KH, １８ OH, NH, NH3

　　 H2O ( H216O, H2

17O, H218O, D2O, and H2

16O (v2=1 excited state))

(2) molecules with internal rotation

　　 CH3OH

(3) cation protonated rare gas atoms 　　 (HeH+ ， NeH+ ， ArH+ ， KrH+ ， XeH+ ， including their isotopic species)

H2D+ , N2H+ , H2F+

(4) anion　　 OH- ， OD-

Part 2.

Discharge cell

1. Glow discharge cell dc discharge ac discharge: velocity modulation technique

Molecular cation (HeH+, H2D+) Molecular anion (OH-)

2. Extended negative glow discharge cell

dc discharge Temperature ( N2H+) Recent work (H2F+) collaboration: T. Amano (U. Waterloo) K.Kawaguchi (Okayama U.) R,Fujimori (Okayama U.)

(collaboration with Prof. Amano (Waterloo, Canada))

Fig.1 TuFIR 分光計

Velocity modulation:

detects ions only

Configuration for detecting ionic species

HeH+ J=10

The lowest frequency rotational line

2010.1839 (2) GHz

HeH+

Dunham coefficient Ykl 　

EvJ = ΣYkl(v+1/2 )k[J(J+1)]l

( a set of coefficientsYkl for each isotope)

To calculate all the isotopes with a set of coefficients Ukl

Ykl 　＝　 μ-(k/2+l)Ukl

Reduced mass μis not enough to fit all the isotopes.

Ykl 　＝　 μ-(k/2+l)Ukl [1+meΔHekl/MHe + meΔH

kl/MH] Correction terms usingΔvalues are necessary.

Breakdown of Born-Oppenheimer approximation.

HeH+ 　 Rotational Transition

transition frequency obs-calc

4HeH+ J=10 2010183.873 (202) 0.108J=21 4008733.084 (194) -0.148

4HeD+ J=21 2434626.571 (143) 0.077 J=32 3641427.274 (384) -0.210

J=43 4835691.417 (166) 0.039

3HeH+ J=10 2139522.472 (300) -0.213J=21 4265839.060 (300) 0.330

3HeD+ J=21 2696099.975 (255) -0.021 J=32 4031223.001 (511) -0.650

HeH+

HeH+ : 3HeH+, 4HeH+, 3HeD+, 4HeD+

NeH+ : 20NeH+, 20NeH+, 22NeD+, 22NeD+ ArD+ :

KrH+ : 82KrD+, 84KrD+, 86KrD+, 82KrH+

XeH+ : isotopes of 124Xe, 126Xe, 128Xe, 129Xe, 130Xe, 131Xe, 132Xe, 134Xe, 136Xe,

and H, D

protonated rare gas atoms

typical trace of H2D+

211 110

H2D+

H2D+

J =3← 2

3363550.5413363550.541 　　 　　 frequency frequency （（ MHzMHz ） 　　） 　　 3363658.5413363658.541intensity(arb. units)

intensity(arb. units)

OH-

transition frequency (MHz) J=43 4478174.516 (387) J=32 3363607.066 (238) J=21 2244776.819 (240) J=10 1123100.985 (324)

OD-

Frequency(MHz)

Frequency(MHz)Frequency(MHz) Frequency(MHz)

Frequency(MHz)

J=2←1 J=3←2

J=5←4 J=6←5 J=8←7

Inte

nsi

ty

(arb

.un

its)

Inte

nsi

ty

(arb

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its)

Inte

nsi

ty

(arb

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its)

Inte

nsi

ty

(arb

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its)

Inte

nsi

ty

(arb

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its)

D2O/O2=54.5/5Pa,AC1.2kHz,1.1A,4.8kV,Scan6 回 ,エタノール冷却 2 ,℃ 湿度 60%,FIR200mV( 100nW)≒

ND3/O2=35/10Pa,AC1.2kHz,1.0A,5.6kV,Scan3 回 ,水冷 , 湿度 29%,FIR140mV( 70nW)≒

ND3/O2=35/10Pa,AC1.2kHz,1.0A,5.6kV,Scan3 回 ,水冷 , 湿度 26%,FIR160mV( 80nW)≒

D2O/O2=23.5/5Pa,AC1.2kHz,1.2A,4.5kV,Scan3 回 ,水冷（溜め置き） , 湿度 60%,FIR150mV( 75nW)≒

ND3/O2=35/10Pa,AC1.2kHz,1.0A,5.6kV,Scan3 回 ,水冷 , 湿度 24%,FIR40mV( 20nW)≒

Fit 1196791.042(0.486)MHz

OH-, OD-

Extended negative glow discharge cell

Compared to "normal" glow discharge cell: Gas pressure : smaller by 1 or 2 orders of magnitude ⇒ small consumption of gases Electric current: smaller by 1 or 2 orders of magnitude ⇒ Discharge noise does not destroy MIM contact !!

N2H+ J=14 ← 13 E" = 283 cm-1

N2:H2:Ar=0.9 ; 0.9 : 2Pa7mA, 1.7kV

J=14 ← 13 (E" = 283 cm-1)

J=16 ← 15 (E" = 373 cm-1)

J=17 ← 16 (E" = 422 cm-1)

J=9 ← 8 (E" = 112 cm-1)

(E" = 140 cm-1)J=10 ← 9

Intensity of the spectral line

E” : lower energy level of the absorption T : rotational temperature

ln{Si/(J+1)} = -(1/kT)E” + const.

slope ⇒ T

T = 129K

Extended Negative Glow Discharge

discharge : 10 mA, 4 kV

gas pressure ≈ 2 Pa (buffer)

→ mean free path comparable to the cell dimension → cold buffer gas cool down the ion ?

"Normal" Glow Discharge

discharge : 500 mA, 4kV

gas pressure ≈ 40 Pa

how to overcome : liq. nitrogen cooling → water cooling

Trot ≈ 500 KTrot ≈ 130 K

N2H+ J=14 ← 13 E" = 283 cm-1

Lig. Nitrogen cooling

Water cooling

nothing !

N2H+ J=5←4 temperature dependence, sub-mm spectrometer (by Amano)

-200C (liq. N2 temp)-100 -85 -65 -30 -15 -5 +20 (room temp.)

H2F+

⇒ H2F+ in space?

Fluorine contained species detected in space. (interstellar space, circumstellar envelopes)

AlF, HF, CF+

H2Cl+ was detected with Herschel HIFI. (NGC6334I, Sgr B2 ) (D. C. Lis et al. A&A 521, L9(2010)) Cosmic abundance of F ~ abundance of Cl

Previous laboratory measurements for H2F+

IR Laser spectroscopy with velocity modulation technique

Schäfer and Saykally, J. Chem. Phys. 81, 4189 (1984) ~ 3 µm region, ν1 and ν3 bands FTIR Fujimori, Hirata, Kawaguchi, and Morino, International Symposium

on Molecular Spectroscopy (Columbus, OH), 2010. ν2 band in addition to ν1 and ν3 bands

sub-mm BWO (Univ. Waterloo) with extended negative glow discharge cell R. Fujimori, K. Kawaguchi, and T. Amano, Ap. J. Lett. 729, L2

(2011) 473-774GHz, 5 pure rotational lines

Astrophys. J. Lett., 729 L2 (2011)

Length: ~ 1.3mVoltage: 2 ~ 3kVCurrent: ~10mAT: 0 ~ -70 C

HF : H2 : Ar = 0.2 : 1 : 2Pa

Extended negative glow discharge cell with ethanol cooling

HF heat cell: preparedby Fujimori & Kawaguchi

H2F+ 303 ← 212 T= -70C

H2F+ 111←000

1305306.600 (56) MHz

H2F+

111-000 1305306.503 (073) MHz303-212 1370911.418 (048)413-404 1425857.036 (059)523-514 1737165.499 (066)422-413 1748037.582 (085)321-312 1823629.057 (070)212-101 1850081.989 (050)

H2F+: isoelectronic to H2O, mass close to H218O

red arrow: TuFIR

Lowest frequency rotational lines

para111-000 1305306.503 (73) MHz(TuFIR measurement, Toyama)

ortho110-101 760928.937 (10) MHz(sub-mm measurement, Waterloo)

liq. N2 cell without glass-blowing

Collaboration

Univ. of Toyama K. Takagi (prof. emeritus) H. Odashima (now in Meiji univ. in Tokyo) Y. Moriwaki K. Kobayashi many students including T. Yonezu (now in Nobeyama Radio Obs.)

T. Oka (Chicago Univ.) --- start of studies of ionic speciesT. Amano (Univ. Waterloo, Canada) --- ext. neg. glow discharge cell and works using itK. Kawaguchi (Okayama Univ.)late J.M. Brown (Oxford Univ.)

and of course

late K.M. Evenson (NIST Boulder Colorado)

Thank you !