Construction of a microwave Fourier Transform Spectrometer

Journal of Molecular Structure, 97 (1983) 233-241 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

CONSTRUCTION

R. WAGNER, Institut

OF A MICROWAVE

W. DEGEN,

TRANSFORM

SPECTROMETER

B. HAAS and W. ZEIL

fdr Physikalische

Auf der Morgenstelle

FOURIER

233

und Theoretische

8, D-7400 TUbingen,

Chemie,

Federal

Universitat

Republic

TUbingen,

of Germany

ABSTRACT The construction of an.X-Band Microwave Fourier Transform Spectrometer suited for high resolution studies is described. Special features are the phase locked klystron microwave power source which is pulse modulated by a PIN diode switch, a 20 meter low loss waveguide sample cell and the use of commercially available NMR signal processing components. Performance is demonstrated with OCS and CSFCl.

INTRODUCTION Hyperfine

structure

solved with

by Stark modulation In contrast, by pressure-, termination structed

culties

resolution

a Microwave

In combination polarisation

These

microwave

Spectrometer

enabled

is only limited

So, for a more precise

in C-Cl and Si-Cl compounds (‘MWFTS) for narrow

transitions,

strong

us to circumvent

constructions

with a long low loss sample with moderate

NMR spectrometer

, emphasizing

de-

we con-

band,

enough

to be

some of the diffilarger bandwidth

receiver,

TRANSFORM

Fourier

transform

are a)the microwave d) the digital reference

cell, efficient

pulse power.

components

FOURIER

of our microwave

and f) the frequency

and

for signal

SPECTROMETER

processing

of sample allows

processing.

(MWFTS)

spectrometer

pulse source,

signal

exitation

The narrow bandwidth

is given in Fig.1.

b) the sample section,

cell,

c) the

e) the control

unit

unit.

pulse source

The microwave phaselocked

be re-

and distortion

(ref.2,3).

sections

a) Microwave

often cannot

of broadening

spectroscopy

(ref.1).

of rotational

restrictions

OF THE MICROWAVE

A diagram

transform

tensor

Transform

by previous

the use of standard

Functional

quadrupole

Fourier

is achieved

DESCRIPTION

in MW Fourier

investigations

encountered

high sensitivity

of molecules

because

and wall broadening

of the nuclear

detected.

spectra

MW-stark-spectrometer

and saturation.

doppler-

high resolution easily

in MW rotational

a standard

pulse source

to the-frequency

0022-2860/83/$03.00

consists

of a CW microwave

reference

master

oscillator

unit, and a fast pulse modulator,

0 1983 Elsevier Science Publishers B.V.

(MO), con-

Fig. 1

r

SYNC

Diagram

/J

i-z

of the X-Band

LO1

Microwave

Fourier

PL

SYNTH

COMP

Transform

I NTPG

Spectrometer,

TR

235

trolled

by the -%-control unit. The master

287A, 287B, 287C) with a maximum provided flange >30

by a klystron

is connected

power supply

to an aircooled

db) is used to maintain

tion. A portion a harmonic which

output

chronizer

FDS 30).

(SYNC)

leaving

the primary

attenuator

driver

6 ns, minimum

(GENERAL MICROWAVE

the switch

db), low VSWR

(less than 1.20) isolators

(less than 1.10) coaxial

b) Sample

system

cell

sections

(SC) consists

material

and cell windows.

is pure aluminium

0.13 db/m at 12 GHz). As a special changing

223

adapters

of a 19.5 m rectangular

at both ends for pressure

(VS) connection

waveguide

speed

is

than 85 db and loss is in the isolating

to low loss (less than 0.4

(isolation

to waveguide

with

db) (RYT 200753) (SUHNER

and

3102.19B).

cell

The sample waveguide

when

are connected

by

is a fast,

unit. Switching

low reflection

input and output ports

30

is adjusted

which

GMC DM 864-20H)

is better

(OFF-) state,

(20 db) into

FMDR 8/12)

unit by a syn-

coupler

is 15 ns. Isolation

ac-

the klystron

reference

line of the directional

by the control

MW output (isolation

coupler

or SCHOMANDL

lock loop (PLL) locking

for TTL pulse control

less than 2.6 db. To achieve

low VSWR

by a directional

combination

(VA) and fed into the pulse modulator,

pulse duration

are

during pulse modulator

of the frequency

SPST PIN diode switch

high isolation integral

(SCHOMANDL

heatsink.

load condition

phase

harmonic

The power

The klystron An isolator

(power divider/detector

is part of a conventional

(VARIAN VA

voltages

(KPS) (FXR Z 8158).

of the MW power is coupled

mixer

is a klystron

waveguide

constant

MHz below the corresponding

a variable

oscillator

power of 1600 mW. Stable

the inner dimensions

coiled with a diameter

The overall

waveguide

with short

(PRM) gas inlet/vacuum

length is 20 m. The long

with very low loss (0.19 db/m at 8 GHz and

shape allows

(SIEMENS

of about

monitoring

bending

and twisting

without

SIRAL SI 100) the cell is circularly

2 m. Serious

reflections

do not appear.

Cooling

is provided.

c) MW receiver The MW receiver against

The first nation

is a sensitive

high MW pulse power during stage consists

100) connected RF signal

to the frequency

low noise NMR FT spectrometer

to a 2nd localoscillator

conversion

down to the video

lator is also phaselocked

construction

mixer/IF

down to 90 MHz IF.

conversion

(LOl) phaselocked

stage is a commercial

superheterodyne

protected

pulse phase.

of a low noise balanced

for the first frequency

local oscillator

double

reference receiver

preamplifier

combi-

It is driven by a unit. The second (RCVR)

(BRUKER B-QE

(LOZ) with a fixed 90 MHz frequency

for

range (DC - 5 MHz). The 2nd local oscil-

to the frequency

reference

unit.

236

The input protection (~1.25

circuit

db), low VSWR

(ALPHA

of the control

cell the switch

low VSWR section

(SUHNER

coaxial

by the receiver

3102.198).

to waveguide

reflections

fier, whose

is 40 MHz

the LO1 power attenuator balanced

mixer ANAREN

is connected

120), which

76 0118)

power

drives

level

fining

recorder

??

(N-At)-‘=

of

(20 db) and a fixed combination

or

lock loop. The 2nd RF input of this (VA) to the frequency filter

(BPF), amplified

the 90 MHz frequency (SNYC)

reference and mixed

from the LO2 to gener(SCHOMANDL

by a variable

is monitored

FDS 30). Sig-

attenuator

by a power meter

(VA)

(POM) to main-

mixer.

local oscillator

(L02) (SCHLUMBERGER

is accomplished

contains

decay

by a 1 MHz signal

FSD from the

recorder

is digitized

an 8 bit A/D converter

theorem

to a dwell

the maximum

(TR), a

(PL). The amplified

the NMR receiver

corresponding

frequency,

to the sampling

a transient

and a plotter

leaving

B-C 104) with

sampling

B is

section

CRT display

emission

(BRUKER

the bandwidth

coupler

harmonic

A portion

section

processing

4 k (= 4096 data points)

Af

Power

the corresponding

unit.

(COMP) with

10 MHz maximum

noise

(BWO) (WEINSCHEL

mixer arrangement.

(+8 dbm) at the receiver

the NMR receiver,

signal

100 ns. According

seen

6033/3)

The overall

oscillator

directional

attenuator

of the 2nd receiver

nal of the transient transient

WMP 12 LO8CB).

for the synchronizer

) Digital signal processing

with

by a waveguide

90 MHz IF preampli-

(power divider/detector

by a bandpass

(TRM MD 204) with

phase shifter.

computer

to a

transients

db) (MARCONI

integral

60 MHz above

of the phase

via a variable

reference

The digital

isola-

to wave-

22 db.

and phase of the LO1 are adjusted

Phaselocking

digital

MW mixer with

by a coaxial

ate a 30 MHz IF appropriate

tain an optimum

of switching

(isolation&30

unit by a two-stage

IF is selected

in a 2nd RF mixer

frequency

isolator

is phaselocked

reference

is directed

and a variable

(41.20)

coaxial

of the same type followed

for supression

(10 db) to the first mixer

nal amplitude

(L1.l)

port is also connected

1 (LOl) is a backward-wave

6644) which

unit. A 60 MHz

output

(RHG ORTHOGUIDE

db. IF gain is

of the frequency

d

adapter

is a balanced

bandwidth

is L8.5

221 - MARCONI

db), low VSWR

from the mixer diode.

mixer

The local oscillator

mixer

The switch

mixer. A waveguide

The receiver

figure

by a low loss (LO.4

by the TTL pulse

at the end of the sample

23 db) (RYT 200 753) and a low VSWR

used as a high pass filter

reduces

time 5 ns), low loss

(70 db) SPST PIN diode switch

unit. For low VSWR condition

is preceeded

tor (isolation& guide adapter

is a fast (switching high isolation

AI 3498-H3) with integral driver controlled

INDUSTRIES

generator

(21.35)

frequency

and sampled time

At

component,

fmax = B = (2&t)-' = 5 MHz. The data is stored memory.

Maximum

2.44 KHz, when maximum

resolution

dwell

in frequency

sigby a

domain

time and 4 k data points

of dein a

is

are assu-

237 med. Data acquisition unit. A time delay internal

crystal

The memory to a digital

ard software

content

of the transient (BRUKER

transform

recorder

for frequency

120 Hz, so the sensitivity

multiplication

Prior to Fourier

(time domain

nent and to improve main. Fourier

domain

either

transform

is performed

to correct

conservation

e) Control

employing

for the experiment

of the FT method

baseline

are applied

the DC compo-

in the frequency

by a fast algorithm

(FFT) (ref.4).

respectively.

transform)

for absorption

The real and

and dispersion

may be copied with

a plotter

(ref.4)

mode.

unit unit (CU) for the proper

timing of the MW pulse train,

ver protection

and the data acquisition

at the transient

with a homemade

TTL pulse generator

sizer frequency

(BRUKER FS 100).

"Coherent

noise",

this oscillator

quency

reference

Duration

sient

respect

to the switches

reference

is averaged of the fre-

of the pulse modulator protection

is delayed

may be variup to lps

pulse for the tran-

The trigger

by an integrated

unit for the frequency

- is composed

- expect

of a crystal

power supply.

determination

those of the control reference

source

and phase synunit and the tran-

and a synthesizer/am-

combination.

For the phase nient frequency SCHWARZ

oscillator

to that of the pulse modulator.

of the oscillators

sient recorder

is configured

is a 30MHz RF synthe-

in the spectrum,

to the crystal

The pulse for the receiver

is synchronous

The frequency

plifier

signals

recorder

timebase

the recei-

unit.

is supplied

chronisation

spurious

is not phaselocked

to that for the pulse modulator.

recorder

Power

generating

(ref.5) whose

of the TTL pulse for the control

ed from 0 to 1 ps. with

For

(PL).

The control

when

do-

Trans-

are displayed

Zero and first order phase correction

the spectrum

is on-

and exponential

to eliminate

and sine fourier

the spectrum

filterstand-

is not fully ex-

correction

the S/N ratio or the resolution

scope.

8 bit parallel time domain

representation,

frequency

advantage

is calculated

on a built-in

permanent

is an

(BRUKER SXP 90). As the data transfer

transform

filtering)

part of the FFT (cosine

seperately

time. Time base

averaging,

form times are 3.8 s or 9 s for 2k or 4k data points imaginary

pulse from the control

is transferred

B-NC 12) for signal

cycle is slow, the repetition

(ref.2).

trigger

in units of the dwell

for the NMR FT spectrometer

and averaging ly about

by an external

oscillator.

computer

ing and Fourier

ploited

is started

is programmable

lock loops of the microwave between

oscillators

800 and 1000 MHz is provided

XUC) with a built-in

crystal

oscillator

(MO and LOl) a conve-

by a synthesizer

(XSU) of high stability

(ROHDE & (better

238

than 2.10-'),

supplying

the local oscillator nerator

(INTPG)

additional

2 (LO2), the synchronizers

(PROGRAMMED

tion generator/synthesizer at 1000 MHz or better PLL mixers.

divider.

SOME EXPERIMENTAL

RESULTS

signal

results

is amplified

lock of

(SYNC) and an interpolation

in a frequency

ge-

The interpola-

accuracy

of 2 Hz

X band harmonic

by a tuned amplifier

in the

and distri-

I

to

05

for an external

PTS 160) respectively.

than 25 Hz for the corresponding

buted by a power

I

TEST SOURCES, combination

The synthesizer

0

I and 10 MHz outputs

2oa+#4w

l.5

25

Fig.2

Transient emission decay and corresponding absorption spectrum of the J = 0 - 1 rotational transition of OCS. Pressure: 19 mTorr, temperature: 293 K. MO frequency: 12 162.18 MHz, pulse duration: 120 ns. Transition frequency: 12 162.98 MHz (the zero point of the frequency scale is the MO frequency). 2k (2048) data points supplemented by 2k zeros in time domain. Dwell time: 100 ns. Number of scans for averaging: 5000, measuring time: 41 s.

Fig. 2 shows spectrum

bonylsulfide lops,

a transient

after Fourier

emission

transform

(OCS). At a pressure

so the corresponding

decay signal

and the corresponding

of the J = 0 - 1 rotational of 19 mTorr,

absorption

the signal

line has a width

absorption

transition

of car-

dies away in about

cf 160 kHz.

239

ms

J=O-1

__.._._____ __-._. __. Fig.3 Transient emission decay and corresponding absorption spectrum of the J = 0; 1 rotational transition of OCS. Pressure: 1 mTorr. Number of scans: 1000. Measuring time: 8 s. Other data as in Fig. 2.

Fig. 3 shows sure (1 mTorr) emission width

the same experiment and a reduced

as described

number

of scans

is now seen for about 35 ps,

of 38 + 5 kHz. The transition

in Fig. 2 but at a reduced

pres-

(1000). The decay of the transient

the corresponding

frequency

absorption

now is determined

line has a

more exactly

to

12 162.974 + 5 kHz. As we were

primarily

of high sensitivity. the master CW output

interested

in high resolution,

Fig. 4 shows an early experiment

oscillator

(MO) was a backward

power of 100 mW (MARCONI

wave oscillator

6600A-6644).

a relatively

necessary.

shows the J = 0 - 1 rotational

natural

abundance

in the vibrational

12 123.84 MHz, the linewidth

is about

long pulse duration

ground

transition

of OCS in the first vibrational

frequency

herent

noise. These

a maximum

polarisa-

of about 680 ns was

transition

state. The transition

of O13CS frequency

in is

100 kHz.

2.87 MHZ away from this line a small hump indicates

sition

gain

(BWO) with a maximum

To achieve

tion at this low power, The spectrum

we did not stress

with OCS. In this case

state

(l,O,O).

is 12 126.71 MHz. The residual"peaks peaks were eliminated

the 3 = 0 - 1 rotational The corresponding

in the spectrum

in later experiments,

tran-

are co-

when we used a

240

I 0

1

1

1

lo

a0

30

1 1

40,ww

I

50

Fig.4 Fourier transform spectrum of the J = 0 - 1 rotational transition of 013CS Pressure: 9 mTorr, temperature: 293 K. MO frequency: 12 123.05 MHz, pulse duration: 680 ns. Transition frequency: 12 123.84 MHz. 4k data points in time domain, dwell time: 100 ns. Number of scans: 5000, measuring time: 78 S.

Fig.5

A 5 MHz range Fourier transform spectrum of CSFCl, showing the nuclear quadrupole hyperfine splitting - 77/2,and 75/2 - 75/2 (at lower frequency) and --79/2 and 7312 - 73/2 (at higher frequency)

of the rotational transition 3812 26 - 3812 27. Pressure: 6 mTorr, temperature:'294 K. MO'frequency: 10 889.5 MHz, pulse duration: 120 ns, transition frequencies: peak at lower frequency: 10 889.9 MHZ and peak at higher frequency: 10 890.2 MHz. Peak separation: 310 kHz, halfwidth: about 70 kHz. 2k data points supplemented by 2k zeros in time domain. Dwell time 100 ns. Number of scans for averaging: 40 000, measuring time: 5 min 23 s.

241

free running

crystal

Fig.5 shows

oscillator

lear quadrupole

hyperfine

38I2 27. Each absorption weak'and

narrow

separated because

for the control

two lines in a Fourier splitting

transform

of the rotational

line is a superposition

to be resolved.

we did not use an interpolation

of XUC 5 kHz), but the relative

lines can be given more exactly:

The experimental in narrowbanded improvement

results

multiple&

in resolution,

transition

ones,

frequencies

generator

of CSFCl,

of different

The two most intense

by less than 10 kHz. The absolute

(accuracy

unit. spectrum

38I2 26 -

transitions,

indicated

are poorly

for the frequency

frequency

separation

indicate

that for sufficiently reduction

above,

too are

determined reference

between

it is 310 kHz with an uncertainty

a considerable

due to nuc-

the two

of 10 kHz.

strong

transitions

in linewidth,

and thus an

is achieved.

ACKNOWLEDGEMENT We thank the Fraunhofer-Gesellschaft nancial

fUr angewandte

Forschung

e.V. for fi-

support.

REFERENCES 1 2 3a 3b 4

5

J.C. McGurk, T,G. Schmalz and W.H. Flygare, Advances in Chemical Physics, edited by I. Prigogine and S.A. Rice (Wiley, New York, 1974) Vol. XXV p.1 J. Ekkers and W.H. Flygare, Rev. Sci. Instrum. 47 (1976) 448 G. Bestmann, H. Dreizler, H. MBder and U. Andrezn, Z. Naturforsch. 35a (1980) 392 G. Bestmann and H. Dreizler, Z. Naturforsch. 37a (1982) 58 J,W. Cooper, An Introduction to Fourier Transform NMR and the NICOLET NIC-80 DATA SYSTEM. Nicolet Instrument Corporation, Madison, Wisconsin 1974 W. Degen, Thesis, TUbingen, 1981