Infrared-microwave double resonance: signal dependence on microwave radiation strength

267

Jo~rnalofMolecularStructure,61(1980)267-270 OEkevier ScientificPublishingCompany,Amsterdam-PrinPrintedinThe

Infrared-microwave

double

resonance:

signal

Netherlands

dependence

on microwave

radiation

strength

J.P.M.

de Vreede

Fysisch The

and

H.A.

Dijkerman

Rijksuniversiteit

Laboratorium,

Utrecht,

P.O.

Box

80.000,

3508

TA Utrecht.

Netherlands

ABSTRACT The

influence

is studied or MW

radiation

dependence more

for

on

than

taken

of MW

field.

the microwave

account:

cases

on

the magnitude

of steady-state

as probe

The

power

They

50X occur.

into

both

radiation

in the case

of double

IR-NM

measurements

theory

resonance

double

reveal

Changes

level.

can be explained

the Takami

of probe

3-level

a strong

only

if higher

order

agreement

using

IR

signal

in the absorption

is in good

signals

resonance,

factor

effects

with

of

are

the measurements

field.

INTRODUCTION In recent performed. and

for

the

In this

paper

and

can be

EXPERIMENTAL

as infrared

relaxation a few

the magnitude

on

experiments

double-resonance

so strong

that

have

experiments

structural

data

has

been

from

complex

spectra

the

influence

phenomena.

level

of

experiments

been

saturating

on NH3 where the probe

of

used

ahsorption

is investigated.

as probe

fields.

the corresponding

of

in threeBoth

Each

transition

radiation occurs.

SET-UP double-resonance

source.

Standard cell.

and

application

of a strong

introduction

of

to extract

describe

frequencies

cavity

of double-resonance

used

radiation

the absorption

Stark

number

microwave

In the IR-MU

for

we

power

infrared-microwave

infrared field

a large

are mainly

investigation

the microwave level

years

These

laser

(ref.

The

set-up

K-band

microwave

number

of coincidences

transition

of electric

frequencies

external

2) is used

a home-built

electric

for

fields

tunahle

apparatus

field

a high

been gas

can lie increased (ref.

IR- and MW-absorption,

with

has

between

CO*-laser

homogeneity

used

(3.IO31

except

absorption largely

I). Therefore since

is used

by

an open

it allows and

the

for

it offers

the a

268

long

microwave

and NW

are

not

absorption

absorption

path

identical,

the observed

must

dictions.

be corrected

These

gas

pressure

and

at

was

low pressure

have

limited

(less

(15 m).

the irradiated

in infrared

for a comparison

been

performed

at high

than 0.1

As

changes

to allow

corrections

range

length

on

pressure

Pa)

with

theoretical presented

a few Pa)

by the detection

of IR

and microwave

the data

(above

volumes

prebelow.

The

by sparking

SNR.

THEORY Relevant

parameters

transition

rates

collision

cases

one

single

microwave

therefore

theoretical

relation

of only

pumping

and

that

bution.

The

relative

depends

in a simple

A much

more

has one

way

simultaneously.

remains.

microwave

model

The

Au/a

was did

respectively dipole

moment

field

is of

and

the order

3-level

IR-NW

3). He assumed

{ref. changed

the

in the microwave

the

E i' In

of the p.

DR with

that

the

by monochromatic

influence

the

for

vector

to the gas pressure

not

induced

has

in which

been

both

but

theory the

presented

radiation

By omitting

general

detection,

by Oka

class

theory

matrix

which

and microwave

infrared

population absorption

distriu

on x and y.

advanced

molecules

used

infrared

electric

for steady-state

radiation

change

a radiation is

the

transition

proportional

given

velocity

microwave

a density

model

been

is the

rate

relaxation

and

detection

population

with

are

xm = umEm/h

and

y. pIR(um)

transition

frequency

first

by x = uIREIR/h

rate

(microwave)

collision The

given

transition

infrared many

in IR-EIW double-resonance

by Takami

fields

interact

the off-diagonal

is applicable

relevant

formulas

(refs.

terms

for

infrared

are

very

4-5)

with

using

the

a rate

equation

as weI.

as

complicated,

RESULTS Eficrowave As

an example

radiation J=5,

detection

K=3,

v = 22833 Level

at 27.96946

PMz

is the

(J=5,

various

be obtained,

B:D.29

series.

measurements

due

to

&1=5,asym)

absorbed

K=3,

M=5,

laser

by

of

(curve

A) a good

However

PIHz; C: 0.41

the

the

pumping

densities

values

at

the foLlowing

pumps

system:

the ammonia

1.235 EN/m.

transition

infrared

transition

Microwave

.J=5, K=3,

(v,=Wl;

radiation

El= 4:5.

The

at common

asymm)-level.

microwave

yielding

on

(CO2 - P(32)))

K=3,

the dependence

As indicated

other

THz

is then

absorption

using

present

~=S,syr~J=5,

We measured wave

we

relative

it results

infrared

fit between

power

the data

Such

in different MHz).

&z/a

induced

of pressure

as a function

at one

for x and y-

PIHz; D: 0.49

change

fits values

(see

fig,

I)

density.

and

can

in the micro-

the Oka

also

for x!

relation

be made (A: 0.25

for KHz;

can the

269

Fig. 1. The relative change (h/a) induced by laser pumping in the microwave absorption as a function of pressure. The solid curve A refers to the bast fit according to the Oka theory (+ x = 250 kHz and y = 20 kHz/ m/Torr>.The broken curves refer to calculations of the Takami theory (experimental conditions x = 0.4 MHz, x = 350 kHz (B), 700 (C), 1400 %lz (El) and Y = 20 kHz/mTorr).

-pressure

(mtort)

Takami derived a simple relation Aala

(:> (x; + v')'. Although

for As/a. if x2 S-S y2 and xi >> y2; namely:

these conditions

are fulfilled

in our experiments,

the data do not show such behaviour. The complete Takami

theory leads to a much better

curves in fig. I for the other series of data). the experimental

result

(see the broken

In these calculations

we use

values for x and xm-

Infrared detection We present measurements

on the following system: Nicrowave

v = 24667 MHz pumps the ammonia Then the infrared absorbs

transition

radiation

at

transition J=3, K-3, iEl=2%3at 0.611 MV[m.

v2==0, J=3, K==3, PI=3,symm + v2=I, 3=3, K=3, P;I==3,asymm.

the CO2 - R(ilf laser radiation

(29.00085 TNz). The lowest level is

the common one. The dependence

of the relative change in infrared absorption

function of the microwave is independent

power level is presented

of x, xm and y for all situations

infrared power could be waried the same conditions

Au/a: as a

in fig. 2, The sign of Aafu

we investigated,

although

in such a way that DR with W-detection

would shown emission

radiation. A rate equation model yields

instead of absorption

under

of microwave

that then a change of sign for Aa

must occur for DR with an IR prabe signal and that Aa/a due to microwave pumping can not he larger than hv/2kT find experimentally.

(2 0.0025);

the

this is much less than we

Fig, 2. The relarive change (Aa/~~ induced by microwave purn~in~ in the infrared absorption as a function of the microwave power. The pressure used is about 1 Pa. The solid curve is the theoretical relation according to the Takami formulation and is nearly independent of pzessure for this value of x (% 0.8 EMZ).

----+a-

eff,

micr. field

(MHz)

A similar system has been investigated by Takami (ref. change of sign for be at increasing The complete Takami

formulism

important

that t;he small effects

para~e~e~based

upon

experimental

x m is a>most

data.

power

induced

according

own

in our system. The

CaLculations

show

are much

in a complicated

the Takami

conditions that

with

a simple

in population

signals way

the

show

formulism

relation

due

are

between

strongly

to high

between

induced

order

change

&Y/U

and

level.

dependent

on

and multi

photon

the

of the experimen-

in absorption

exists only for infrared pumping and microwave

very low microwave

power,

REFERENCES

I 2 3 4 5 5

using

a good fit with

theory gives a good description

relation

more

to a rate equation model.. with

experimental

LR and MU detection

level

Therefore

change

detection

not, achieve

can

curve calculated

processes. The complete Takami tal data.

we

independent of pressure for the given'infrared

In XR-MidDR both microwave

our

found a

shows that this occurs when

shows further that higher order eEEects

Xn fig. 2 the theoretical

our

which

who

pressure.

theory for TR detection

x*/y2 5 2 hv/kT., a cond~~ioR Takami

6)

F. Shimizu, J.Chem_Phys. 52~197~)3572-3576. J.P.M. de Yreede and N.A. Dijkerman, J.Phys.E. ~2~1979)596-599. T.Oka and T. Shimizu, Phys.Rev.A 2(1~70)1177-i~~~. PI. Takam%, Jap..J.Appl.Phys. 15(~976)1~63-1071" M. Takami, Jap.J.Appl.Phys. 15(1976)1889=--1897. K_ Shimoda and W. Takami, Opt. Commun. 4(1972)388-391.

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