Lifetime, Λ-doubling and hyperfine structure measurements in the A 2Π excited states of ZnH, ZnD, CdH and CdD

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Lifetime, Λ-doubling and hyperfine structure

measurements in the A 2Π excited states of ZnH, ZnD, CdH and CdD

J. Dufayard, O. Nedelec

To cite this version:

J. Dufayard, O. Nedelec. Lifetime,

Λ-doubling and hyperfine structure measurements in the A

2Π excited states of ZnH, ZnD, CdH and CdD. Journal de Physique, 1977, 38 (5), pp.449-458.

�10.1051/jphys:01977003805044900�. �jpa-00208606�

449

LIFETIME, ^A-DOUBLING

AND HYPERFINE STRUCTURE MEASUREMENTS IN THE A 203A0

EXCITED STATES OF ZnH, ZnD, ^{CdH AND CdD}

J. DUFAYARD and O. NEDELEC

Laboratoire de

Spectrométrie Physique,

Université

Scientifique

^et Médicale de

Grenoble,

^B.P.

53,

38041 Grenoble

Cedex,

^France

(Reçu

^{le 9 décembre}

1976, accepté

^le

26 janvier 1977)

Résumé. ²⁰¹⁴ L’hydrure (ou deutérure) ^est obtenu dans l’état fondamental par décharge ^radio- fréquence ^{dans un} mélange d’H2 (ou D2) ^{et de vapeur} métallique ^{à une} pression ^{totale de 10-2 torr}

environ. Une excitation sélective des niveaux rotationnels de A

203A01

/2,3/2 est effectuée _par un laser à colorant pulsé. On obtient la durée de vie par échantillonnage de la lumière de fluorescence :

03C4 ~ 70 ns. Des résonances électriques ^de

radiofréquence

^entre les niveaux 039B-doublés de A

203A03/2,

v’ ⁼ 0, donnent la séparation 039B ^{pour les} quatre ^{molécules et la structure} hyperfine ^{pour les} hydrures.

Les valeurs obtenues des paramètres correspondants, (2 a ⁺ c) et b pour la structure

hyperfine

et q pour le doublement 039B, ^{sont en MHz :}

$$

Abstract. ²⁰¹⁴ The hydride (or deuteride) is obtained in the ground ^state by radiofrequency

discharge

in a mixture of H2 (or D2) and metal vapor ^{at a} total pressure of about 10-2 torr. A selective exci- tation of the A

203A03/2,1/2

rotational levels is obtained by ^a pulsed dye laser. Lifetimes are obtained

by sampling ^the fluorescent decay : ^{03C4 ~ 70 ns.} Electric

radiofrequency

resonances between the

~-doubled levels of A

203A03/2, v’

= 0, provide ^the 039B-doubling separation in the four molecules and the hyperfine ^{structure in the}

hydrides.

The measured values in MHz of the relevant parameters (2 ^{a +} c) and b for the

hyperfine

structure, and q ^{for the} 039B-doubling ^{are :}

$$

LE JOURNAL DE PHYSIQUE TOME 38, ^MAI 1977,

Classification

Physics ^Abstracts

5.447

1. Introduction. ^- Various

techniques

^of

spectro-

scopy without

Doppler

effect have

recently

^been

applied

^to diatomic molecular excited states

[1] : magnetic

^resonance and level

crossing

ⁱⁿ

H2 [2],

OH

[3, 4],

^CN

[5], 12 [6], Na2 [7],

^NO

[8] ;

level anti-

crossing

ⁱⁿ

H2 [9];

microwave and

radiofrequency

electric resonances in CN

[10],

^BaO

[11]

^{and CS}

[12],

saturated

absorption

ⁱⁿ

12 [13], two-photon absorption

in NO

[14]

^{and beam + laser in}

12 [15].

^{In these}

experiments,

^a selective excitation is obtained

by

electron

impact [2, 9],

chemical reaction

[5, 10],

atomic coincident line

[3, 7, 8, 12],

^{Ar+ laser}

[6, 11, 13]

and

dye-laser [4, 14, 15].

^{This last}

technique,

^when

it can be

used,

^{is the most} flexible tool.

We have

already reported

^{on lifetime} measurements in CdH and CdD excited

by

^a

pulsed dye

^laser

[16].

The

hydride

^{was obtained}

by radiofrequency

^dis-

charge

ⁱⁿ

H2 (or D2)

and metal _vapor at a total pressure

varying

^from

^10-3

^{torr to 10 torr. The}

alkaline earth

hydrides (and deuterides)

^{which can be}

obtained in these conditions are :

HgH, CdH, ZnH, MgH

^{and BeH}

[17].

A selective excitation and obser- vation of each rotational and A-doublet H level _may be

easily

obtained due to the

large

^values of the rotational constants B in these molecules and to the alternation of

parity

in the rotational levels of their

X 2f ground

state.

This _paper reports lifetime measurements, level

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01977003805044900

crossing experiments,

^and

radiofrequency

^electric

resonances in the A

2n

state of

ZnH, ZnD,

^{CdH and} CdD.

A-doubling

^and

hyperfine separations

up ^to 1 700 MHz are determined with an error smaller than the radiative width

(2 MHz).

^{We have not}

yet

^observed the fluorescence from the

B2L

_state, the Frank- Condon factors from the v ⁼ 0

ground

^state

being

very small.

2.

Apparatus.

^{- The}

apparatus

^{is the same as in} ref.

[17].

The linewidth of the

dye ^laser,

^{about 0.2}

^A (Fig. 1),

allows for a selective

excitation

of the A

2n

rotational and A-doubled levels. A selective obser- vation can be made with a HRS 1 Jobin-Yvon ^mono- chromator of 12

A/mm resolution,

used in the second order. An EMI 9816

photomultiplier

^{of 3 ns time-}

spread

^{is used.}

In lifetime measurements, the fluorescence

signal

is

sampled

^{with a PAR boxcar}

162/163.

^Its

gatewidth

is fixed at 1 nanosecond. The

aperture delay

range has been recalibrated

by comparison

^{with a}

signal

pro- vided

by

^a

radiofrequency synthesizer.

^{In level cros-}

sing

and electric resonance

experiments,

^the

signal

is

integrated

with the PAR

162/164

^{boxcar. Its}

gate-

width is tunable and ^was selected to be much

larger

than the

lifetime,

^{i.e. 500} ns, to record the _average

intensity

variation with the

applied magnetic

^field

intensity

^or electric field

frequency.

The tunable

frequency

electric field is

provided by

a Boonton

generator, coupled

^{to a 10 Watt ENI 420 L}

wideband

amplifier directly

^{connected to the}

plates applied

^on the cell. The

frequency

may vary from 4 MHz to 520 MHz. Its

displayed

values have been verified with an electronic counter. The absolute error

is less than 0.01 MHz.

3. Lifetimes measurements. ^- In ZnH and

ZnD,

the lifetime of A

2 n l/2

^{and A}

2 n 3/2 ^(v’

⁼

⁰⁾

^levels

have been measured in the same way ^as in our

previous experiments

^on CdH and CdD

[17].

The results

given

in table I are derived from

extrapolation

^{to zero}

H2 (or D2)

pressure. The

temperature

of the cell

(r =

³²⁰

°C) gives

^{for Zn a} vapour pressure less than 6 x

10-3

torr. We have verified that the

intensity

of the R.F.

discharge

^{has no influence on} the measured lifetimes.

’

The excitation and observation are made on the band heads of the v" ⁼ 0 -+ v’ ⁼ 0 and v’ ⁼ 0 -+ v" ⁼ 1 transitions

respectively,

^{to remove} the laser scattered

light.

TABLE I

ZnH

and ZnD,

^A

2n lj2,3j2, v’

^{= 0}

^lifetimes,

in nanoseconds

451

We have tried to show evidence of a

perturbation

in the

2nl/2 ^(v’

⁼

¹⁾

^{state of CdH}

^by

the variation of the lifetime of this level

according

^{to the rota-}

tional levels J

[18].

^{From the} spectra

[19, 20],

^{it is}

possible

^{to see} that the CdH A

2 n l/2 ^(v’

⁼

¹⁾

^level

is

perturbed by ^{B 2Z} (v’

⁼

0)

^{at J’ =}

15.5-, 16.5+,

16.5- and 17.5+. The _energy

displacement

^{B of the}

perturbed

^{levels is}

always

^{less than 4}

cm-1,

^{and the}

energy difference 2 3 between the

interacting

^levels

is about 50

cm - 1.

If _tn and _{T_,} are the lifetimes of the

unperturbed levels,

the lifetime

r3

^{of the}

perturbed

level is

given by [18] :

We have measured the lifetime of several rotational levels of A

2n l/2, v’

^{= 1 of}

^CdH,

^and

^figure

^{2 shows}

that there is no

appreciable

variation of the lifetime of the

perturbed

rotational levels.

Taking

^{into account}

the error

margin

^{of our}

experimental ^results,

^{we can}

. roughly

estimate that the lifetime of the

perturbing

state

B 2L

is between 60 ns and 300 ns.

4. Level

crossing experiment

^m

ll3/2

^{levels. - The}

hyperfine

^{structure due to} the nuclear

spin

^{of H}

(or D)

is

expected

^to be smaller than the

Doppler width,

as it is the case in the atom. Whereas the

hyperfine

structure due to the nuclear

spin

^{of the}

metal,

^{for odd}

isotopes,

may be greater ^{than the}

Doppler

^width

[21].

We shall measure the

hyperfine

^{structure due to H}

(or D)

in radicals with even metal

isotopes.

^{We use}

natural Zn and Cd which contain 96

%

^{and 75}

%

even

isotopes respectively.

Level-crossing experiments (Fig. 3)

^{have been}

performed

^{in ZnH and}

CdH,

^A

2II3/2,

^{v =}

^0,

^{J =}

^{2 .}

The - level has been choosen as it allows us to obtain

a

higher polarization

rate, ^{about 20}

%, by

^{R excitation}

and P +

Q

detection. In this

experiment,

^{it is} necessary to use a boxcar gate much wider than the lifetime r

to avoid

observing

^{the Zeeman} quantum

beats,

^i.e.

a modulation at two times the Larmor

frequency.

The Zeeman

diagram

^is

given

ⁱⁿ

figure

^{4a. The}

Land6 g

^{factors are} obtained from the formula

(2) [22]

FIG. 3. ^- Experimental ^set up for level crossing experiments ;

eo : polarization of the laser beam L ; e 1 : observation polarization;

Ho : magnetic ^field.

FIG. 4. ^- a) ^Zeeman diagram. b) ^Level crossing signals ⁱⁿ ZnH,

A 2n3/2, v’ = 0, J = 1-.

and from the values of

Av and Bv given by

the spec- tra

[ 19, 20]

Figure

4b shows the level

crossing

^{curves obtained}

with two hours

signal averaging,

^{i.e. 70 runs of 100 s.}

The Hanle effect is the

superposition

^{of two curves}

corresponding

^{to F = 1 and F = 2.}

Their g

^factors

are

given

^{in table}

II,

and their relative

amplitudes

are 3 and 7

respectively

obtained from the formula

The

superposition

^{to these two curves} calculated with

T ⁼ 70 ns agrees with the

experimental

^{curve. The}

two non-zero field level

crossing signals

^{have a} poor

signal-to-noise

ratio and

give imprecise

results. The

hyperfine

^doublet

separations

^are small and _very similar for CdH and ZnH

(Table II).

^{It has not been}

possible

^{to obtain a}

crossing signal

^{for J}

= 2,

^or

for the deuterides. We have also failed to observe

hyperfine

quantum ^{beats without a}

magnetic

^field

[24].

Instead of

improving

^our apparatus ^{to make such}

experiments,

^{we have}

preferred

^to undertake electric

resonance

experiments

^{which are}

expected

^to

provide

more

precise

^results.

TABLE II

Calculated

values of

gJ

and gF in

^{ZnH and}

CdH,

A

2 n 3/2’ v

⁼

^0,

^J

^{=3/ 2}

^level.

Experimental

^value

of

the

hyperfine

^structure.

5. Electric resonance

experinienta

^{- We}

perform

a selective excitation of one A-doubled component.

The

applied

electric field induces transitions between the two A-doubled

levels,

which have

opposite parity,

when its

frequency corresponds

^{to their} energy

separation.

^When

scanning

the electric field

frequency,

the resonance is detected

by

^the appearance of the

light

emitted from the A-doubled level which is not

excited

by

the laser

(Fig. 5).

1

FIG. 5. ^- Allowed electric radiofrequency transitions in ZnH

or CdH, 2n state (I ⁼ t).

The +

(or -)

^state

being

^{excited at time t = 0}

and the

probability

^of

finding

the molecule at time t in the -

(or +)

^{state is}

given by

^the Rabi formula

[25] :

w/2

^{H is the}

frequency

^{of the}

applied

^electric

field, 1iw + -

^{is the} energy

separation

^{of the + and -}

levels ;

and

V+ _

is the matrix element of the

perturbation

between the two states. In our case,

V+ _

^is propor- tional to the

amplitude

of the electric

field,

^{to its}

frequency,

^{and to} the electric

dipole

^{moment matrix}

element

[25, 26],

^{which is in Hund’s}

coupling

^case

(a) :

p is the electric

dipole

^moment of the electronic

level,

and K is a unit vector

along

^the internuclear axis.

In our

experiment, only

^a small fraction of _popu- lation is

transferred, i.e. I V, - I h

^T.

Figure

⁶

represents

the variation of the + and - levels _popu- lation with time. The + level

population

^decreases

exponentially

⁼

N +(t)

⁼

No ^e-rt.

^{The - level} popu- lation varies

according

^to the formula :

. -- ... I

N- (t)

^{has a modulated term which} appears when

(o ^- w + - is

slightly

different from zero.

453

% - - -1

FIG. 6. ^- Variation with time of the intensity ^emitted by ^{one of}

the A-doubled levels, CdH A

2 II 3/2’

^{v = 0, J = 6.5. a)} Directly

excited by ^the laser ; b) Populated by ^electric resonance, w=w+_ ⁼ 431 MHz;

c) Populated by ^electric resonance, off resonance, ^{w =} 424 MHz.

The amplitude ^{of the curves b and c have been} multiplied by ¹⁰

and 20 respectively.

The

integrated intensity

^is

proportional

^{to :}

The

anticrossing

^{curve obtained}

by varying

^the

applied frequency

^{with a}

gatewidth

^much

larger

^than

the

lifetime,

is Lorentzian. Its width at

half-height

^is

2[r 2

⁺

4(V+-/h)2]l/2

^{and goes to 2 r when the}

electric field

amplitude

goes to zero, i.e. about 4.5 MHz in our

experiments.

^The

amplitude

^of

E,

necessary ^to obtain an

appreciable population transfer, depends

on

J, F,

^{F’. The}

dependence

of the electric

dipole

moment matrix elements with the

angular

^momentum

(3b)

^is

given

in table III. The summation over M has been made :

The resonances with AF ⁼ 0 are much stronger ^than those with AF= ±

1,

which become weaker as J increases and _may be hidden

by

the tail of the AF ⁼ 0

resonances.

The Stark

effect,

^{due to the}

applied

^electric

field,

shifts the resonance

frequency.

^{It is the}

Block-Siegert shift, equal

^to

[25]

FIG. 7. ^- The cell and the applied fields for the electric resonance

experiment.

and is small

compared

^to the width of the resonance curves, since 2

V, - Ih

^{r w.}

The cell used for these

experiments

is shown in

figure

^{7. The} measurements are made at low _pressure,

10-2

torr, in order ^to avoid collision transfers between + and - levels. We excite the +

(or -)

level

by

^{the R}

(or P)

component, ^{the laser}

light being polarized parallel

^to

^E,

and detect the

intensity

emitted

by

the other -

(or +)

^{level on the}

Q

^{+ R}

(or Q

⁺

P)

component

(Fig. 8).

Figure

^{9 shows the}

anticrossing signal

^{obtained for}

the J ⁼

1.5 +,

2.5 - and 3.5+

of ZnH,

^A

2H 312, V

^{= 0}

levels. The central resonance

corresponds

^{to the two}

AF ⁼ 0

transitions,

^{which are} unresolved and

give

the

A-doubling separation.

^{It is}

possible

^{to show that}

the

hyperfine

^{level F = J}

+ 1/2

^is situated above F ⁼

J - 2 by comparing

the intensities of the two satellite

lines,

^as their intensities are

proportional

to the

population

of the level excited

by

^the

laser,

^i.e.

(2

^{F +}

1),

^and

approximately

^to

^OJ2.

^For

instance,

in

figure I Ob,

^{the J =} 2.5 - level is excited

by

^{the laser.}

This level is situated under 2.5+. It can be seen that the lowest

frequency

satellite has the greatest ^inten-

sity :

^{this satellite}

corresponds

^to the transfer from

TABLE IV

. t-doubling wlmrations

^{in A 21I v =}

0,

^{obtained in} this work and

by

conventional spectroscopy

F ⁼ 3 to F ⁼ 2.

By exciting

^{J =}

2.5+,

it is the lowest

frequency

satellite which is smaller than the other satellite.

When the electric field is strong

enough

^{for the}

satellite resonances AF ⁼ 1 to have a sufficient

amplitude,

the width of the central resonance curve is

equal

^{to the} limit value 2 r

multiplied by

^{two or more,}

and the satellite curves may be hidden. ^We have reduced the width of the curves

by integrating

^{in the}

boxcar

only

^a part ^{of the}

signal

^{with a} gate ^{of about} 50 _ns, fixed at 200 ns after the laser

pulse. ^Therefore, only large

^{lifetimes are} selected and the width of the

curves is divided

by

^{about two. But a} quantum ^beat appears

[4]

^{which should not be}

interpreted

^{as a}

satellite resonance. The fact that the

hyperfine

^struc-

ture is not much greater than the natural width limits the

ability

^{of this}

experiment

^{to measure the}

hyper-

fine structure when J increases.

However,

^{as J increases}

455

FIG. 8. ^- CdH, A

2 II 3/2

^{(v’ = 0) - X 2,E + (v’ = 0),} emitted lines : a) After laser excitation of J’ ⁼ 6.5+ by R21(0-0) line; b) ^Addition

of an electric field at ^resonance populating ^{the J’ = 6.5 - level.}

the AF ⁼ 0 resonances are easier to obtain

experi- mentally.

The electric power necessary decreases as an effect of the

photon absorption probability being proportional

^to

0)2. Moreover,

^{there is a better} pene- tration of the field in the cell which contains electrons and

ions,

^{from the}

discharge

^which

provides

^the

radicals. The resonances at about 10 MHz have been observed with

difficulty

whereas the resonances in the

frequency

range 500-1 700 MHz have been obtained with

only

the harmonics of the generator.

It has been verified that the observed

frequency ^for

a resonance does not vary, whether the observed

region

^{is the centre} of the cell or close to the walls.

There is no error due to a static electric field

provided by

the accumulation of

charged particles

^{on the walls.}

TABLE V

6. Determination of molecular constants. ^-

In

diatomic molecules the

theory

^{of the}

A-doubling

has been

developed by

^{Van Vleck}

[27],

^{and the}

theory

^of

magnetic hyperfine

^structure

by

^{Frosh and}

Foley [28]. Equations

have been stated in a convenient form for

application

^to

experimental data,

^for

²ⁿ

FIG. 9. ^- Electric resonance signals obtained in ZnH, A 2n 3 ’2’ ^{l’ = 0.}

LE JOURNAL DE PHYSIQUE. ^- T. 38, ^No 5, ^{MAI 1977 31}

-j

FIG. 10. ^- Experimental values of the A-doubling separations ⁱⁿ ZnH, ZnD, ^{CdH and} CdD, ^A

2 n 3/2’

^{v’ = 0.}

states in the

general

intermediate case between

(a)

and

(b) [22, 28] :

I r m 1 -

For

regular

^{fine structure}

doublets,

which is the

case in

point,

^the upper

sign

in front of X

applies

to states that _go over to the _pure

Ill/2

^{state of Hund’s}

case

(a).

^{The lower}

sign applies

^to

n 3/2’

^The

^hyper-

fine

doubling

^{term in d}

gives ^equal

^and

opposite

contributions to each member of the A-doublet. It vanishes for

n3/2

^{states in case}

^(a), A,IB, >>

^1.

In our

experiment

^{where the}

coupling

^{scheme is}

very ^{near to case}

(a),

^{the two} resonances

correspond- ing

^{to AF = 0 are}

confused,

^as

expected.

^{For the}

hyperfine

structure, ^{it remains to} determine the cons- tants a, b and c.

Moreover,

the coefficients of 2 a and c in formula

(7)

^are very similar and we can

only

determine the values of

(2 a

⁺

c)

^{and b}

(Table VI).

TABLE VI

Hyperfine

structure constants

(MHz)

in A

2 n 3/2’ v =

⁰

To obtain the

parameters p and q

^{it is} necessary ^to consider both the

2 n l/2

^and

2II3/2

levels : in

N1/2,

VA

depends essentially

^{on p}

(6b),

^{and in}

n 3/2’

^vA

depends

^{on a} combination

of p and q (6b).

^{As a first}

approach,

^we take the value of _p obtained on

171/2 by

conventional

spectroscopy [29, 30],

^{and we obtain}

q from our results on

173/2 by using

^formula

(6a).

TABLE VII

A ⁼

Av/Bv.

^{Ratio x 100}

of

second order

v(2)A

^to

first

^order

(6)

^{terms in} the calculation

of

^the

A-doubling

457

TABLE VIII

p and _q values

(cm-1)

^{relative to the}

A-doubling

^{in A}

2 n,

^{v = 0.}

Overall

centrifugal

distortion parameter ^{x in our}

experimental

^{results in}

n 3/2

To

improve

^these

^values,

^several

adjustments

must be made :

1)

The formula

(6a)

^{is valid if}

I A I « I AEn,I> ^I.

If not, second order ^terms

v (2)

^must be taken into account

[22].

^{It is a}

long expression

^{that we do not}

write here. We have calculated their contribution to the

A-doubling (Table VII).

The variation of this

percentage

^{with J is} very small.

2)

^The

centrifugal

distortion

gives

^a variation of the type

(1

⁺

^xJ(J

⁺

¹⁾

^{+ x’}

^J2(J

⁺

^{1)2 ...) on B,}

A, ð.E(ll,I), ^{A, p} and q [22, 29, 30].

^{At this} stage ^of

our

work,

^{we have}

only

^{tried to avoid an error due}

to this effect in the determination of the

parameters

^p and _q. This error is not

negligible

^{when the}

A-doubling

for the smallest values of J has not been measured.

A few parameters necessary ^to calculate the centri-

fugal

distortion in these molecules are

unknown,

i.e.

Av=1

^for

deuterides, H,, Fv ...

^{However it is}

possible

^{to compare the}

importance

of this effect

on the different

parameters

of formulae

(6a), (6b) by using

^the

approximate

formulae of ref.

[30],

^{so as}

to make the _necessary

approximations

^to

extrapolate

vA ^at small J values.

3)

^The effect of the B 2 I + level must be evaluated.

Its

vibrational

^{level v = 0 has the same energy as}

v ⁼ 1 of A

ifI-.

From the _energy

displacements,

^due

to rotational

perturbations

^{in ZnH}

^[19]

^{and CdH}

^[20],

and from the calculated Franck-Condon factors

[33],

it is

possible

^to show that the effect of level B on the

A-doubling

of level A v ⁼ 0 is about thousand times smaller than the observed

A-doubling,

^and may be

neglected.

We have calculated the

parameters p and q by

^a

mean square fit of the

experimental A-doubling separations (Table IV)

^on

173/2

^and

n 1/2 ^{[19, 20, 30,} 31]

with formula

(6a).

This formula has been multi-

plied by

^a coefficient

taking

^{into account the second}

order terms

(Table VII).

The formula

(6a)

^for

n 3/2

has been

multiplied by [I

⁺

x.J(J +1)]

^{to take}

into account the

centrifugal distortion,

where x is

an

adjustable

parameter. ^{This treatment} is valid since

formula

(6a)

^and

v(2)A

^have

approximatively

^{the same}

variation with

J,

^{and the}

centrifugal

distortion for

H312

^is

^nearly

^{the same on}

p/A2 , ql À,

^and

V(2)A

^but

much smaller for

Hl/2,

^{i.e. on} p. In each series of results

(Table IV), only

the first five have been used to obtain the

extrapolated

^values

of p

^and q. These values are

given

in table VIII.

The error is

essentially

^{due to the}

experimental uncertainty

for the first rotational levels in

n l/2’

The difference between our values

of p

and the values of ref.

[31]

^{is due to} the correction

v(2)

^{that we have}

made. Our

values ^{of q}

^{are greater} than the values of ref.

[29]

^and

[30].

This is because the

centrifugal

distortion

provides

^a reduction of the

A-doubling

in

n3/2

of about 50

%

^for

CdH,

^{and 10 to 20}

%

^for

ZnH,

^at the values of J where it had been measured

by

conventional spectroscopy. ^We notice that the

experimental

^values

of p and q

^are smaller than the values calculated in the

approximation

^{of pure pre-}

cession. However their relative values

(Table IX)

between

isotopes

agree with the calculated ratio.

TABLE IX _.

Ratio

of p and q

^{values between}

hydrides

and deuterides

Acknowledgments.

^{- We are indebted to A. Jour-}

dan,

^{J. M.}

Negre

^{and J. M.}

Castejon

for technical

assistance,

^{to M.} Merlin and M. Lombardi for writ-

ing

^{the mean} square fit _program, to R. A. Bell and D. Branch for

sending

^us their Franck-Condon calculations before

publication,

^{and to M. de}

Filippi

for

lending

^{us the}

radiofrequency

generator ^and

amplifier.

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