Laser spectroscopy in ion beams

125

Joumal of MoZecu&r Stnrcture, 79 (1982) 125-128 Else&r Scientific Publishing Company, Amsterdam -

Laser

C.P. Department (Gt.

C.S.

Ion Beams

in

Maclean

University

and

P.J.

Sarre

of Nottingham,

Nottingham,

NG7

2RD

Britain) The

combination

techniques

for

of narrow-linewidth

generating

fruitful

approach

to

this

and

related

work

ions

(4) and

high

resolution The

for

Spectroscopy

Edwards,

of Chemistry,

Printed in The Netherlands

the

ion

case

principle

the

laser

high

the

fast

same

have

our

for

mass

ions

spectroscopy

also

ions

been

spectrometric

is proving

of molecular

are

used,

but

ions.

slow

In this

employed.

to be a

In

drifting

paper

a

ion is described.

experiment

positively

with

molecular

bea'ms of

of the SH+

study

of a diatomic

is also

resolution

(5,6,7)

on which

lasers

controlling

(1,2,3)

traps

principle

and

is based

charged

polyatomlc

is shown

molecular

in figure

ion AB+,

or negatively

charged

but

1

the

ions.

Fig. 1. Principle of laser spectroscopy for the case of diatomic positively charged ions. An ion in one electronic state 9" is excited by laser radiation to predissociated

levels

necessarily

lying

occurs

the

when

spectrum, in the of A+

and

above

laser

as

the

production ions

of an excited

produced

the

dissociation

is resonant excited

of A+

electronic

state

ions.

with

as a function

limit

of

are

I'.

these

line

recorded

laser

tuning.

by

absorption

in the

predissociated,

are

levels

Photon

A+ + B.

a rotational

levels

Spectra

state

electronic

this

counting

the

results number

Expenmental The Molecular mass

apparatus ions

are

spectrometer

is illustrated generated ion

2 and

by 30 eV electron

source,

0082-2860/82/0000+J000/$02.75

ln figure

and

the

ions

are

the

impact

photograph. on a neutral

accelerated

to form

0 1982 Elsevier Scientific Pubhshing Company

gas

in a

a beam

126 of

energy

variable

distance the

of

at most

apparatus

electromagnet

maintained

is

used to

from

is

irradiated

3000K

the

time

Fig.2.

Apparatus used. The Krypton ion

in

frequencies, record

the

from

laser

a spectrum

voltage

over

is

the

simultaneously

so that

and second

lines

at

is

achieved

will

instance

with

415,

is

S+ ions are

a second

S+ ions.

90’

multiplier.

sector

The ion

a few microseconds.

on a number of discrete 413 and 406 nm.

by changing

achieved

the

5000

the

by sweeping

1 asing

The tuning

velocity

of

be described

of

the

ion

the

necessary ions;

are

transmitted

beam energy.

elsewhere.

so-

to the

accelerating

The electromagnets

volts.

SH+ and S+ ions

Irrespective

the

a Coherent

absorbed,

beam with

of

The SHt beam

A tuning range of 24 to 3.14 cm-l relative

500 to

magnets

are

this

Hz5 gas.

length

an electron

operates

readily

range

m path

parent

region

achieved

of

rest

a

sector

both SH+ and laser produced

1 aser

is

pattern

and the

90’

in

over

occurs

impact, A small

interest,

photons

with

irradiation

including

frequency

the

detected

electron

of

a 0.5

When laser

"Doppler tuning".

called

ion

cracking over

separated and are

flight

this

the

coaxially

are

electromagnet,

first

select

of

The acceleration

potential.

and the beam then contains

The St ions

to

earth

mass spectral

eV.

point

at

ion laser.

Krypton

produced

500 and 5000

1 lean from the

is

SH+ ion then

between

are through

The details

scanned the of

how

127 Spectral

linewidth A number

of factors

and they

include

lifetime

broadening

our

SH+

than

the

spectra

the

spread due

have

Doppler with

determined

principally

been

effect

interpretation

the

spread of

because

typically

an energy

of earth

spread

The an

intracavity

lifetime lines

Results

and

about

state

ion

the

The

that

velocity

not

lines

potential

are much

and

spread

of

of mass

than but

ion

beam

1 eV. V, and

m is given

by

operated

with

to the

45 MHz

these

discussed

the

is not

voltage

significantly

broader

not

source

a few MHz when

levels,

are

by

slightly

so an accelerated

an accelerating

contribute

in

in the

an energy

is only

has

bunching".

determined

for

predissociated

assigned

is

ion

The

In our experiment

formed

ions

an

less

beam.

distribution

"velocity

spread,

for

in

emission

fast

by

and

observed

is considerably

and

spread

linewidth

11 nes

different.

velocity

of energy

and

in the

termed

penetration,

does

of the been

been

source.

velocity

SH+

initial

has

linewldth,

The value of 45 MHz is

plate.

is slightly

to show

laser and

third

part

of

of

in the

these

have

by a repulsive one

spectrum

by extrapolating

spectral

because

of

particular

further.

emission

A3rr-X31been

spectroscopy

assigned

5~- state

is shown

molecular

system

to the

which

in figure

constants

of SH+

of X3Y

have

1-3

been

The

band.

induces

the

3.

assignments

The for

(9) to v" = 3, and

A3a

predissociation.

v" = 0,1,2

by using

recorded

were

obtained

combination

.

differences Many

rotational

nuclear

spin,

437

271

and

This

lines

discussion

one

by optical

ion

70 transitions

is crossed

A small made

of the

spectral

in a discharge

experienced

field

the

of the

not yet

Over and

qAV,

broadening

have

hence

kilovolts

etalon

Some

effect

difficult

Krypton

linewidth.

this and

a few

laser

as 45 MHz.

example,

on an and

(a),

in the

It is not

the

by the spread of velocities

potentials

positions

uniform has

in energy,

accelerating

different

for

of our

Many

on a photographic

Kaufman of

velocities,

as low

observed,

of acceleration by

Our

range

ion

to the width

to predissociatlon.

recording

discussed

of

linewidths

width

experiment

narrowing

contribute

including

MHz.

lines the

show

doubling

P11(3) and

due

PR13(3)

to

the

lines

presence which

have

of the

proton

splittings

of

128 The proton Xazthe

and the Fermi

nuclear

A%

hyperfine

states.

contact

interaction

From a preliminary Constant

interaction

c = +12 Mkiz, for the X~E-

constant structure

in the A%

is

analysis

bP = -85

state.

we obtain

MHz and the

Analysis

State so far indicates

in both the

important

values

for

spin dipolar

of the hyperfine

that bF is apprOXimately

+400

MHz.

SJ4.l2H-X’~-,1J84nd _“I rrru.d

.***

2.1as.5 Fig.

3.

Section

To the hyperfine

of

best

electronic of

structure

few molecules

for

our in

O.lboOcm-.

spectrum

knowledge

an electronic

which

excited

produced.

this

is

the

first

spectrum.

state

observatioii!

of

proton

The SH+ ion

is

information

has been

hyperfine

one of

very

obtai ned.

References A.

Carrington,

P.G.

Roberts

and P.J.

Sarre,

Chim.

Phys. 77

Molec.

Phys.

35

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1523. J.T.

Moseley

and J.

M. Larzilliere, M. Velghe, P.C.

J.

Durup,

M. Car&,

Chim.

Cosby and J.T.

C.B. Richardson,

Phys.

J. M.L. 77

Moseley,

K.B.

Gaillard,

J.

(1980)

(1980) Rostas,

M. Horani

and

689.

Phys. Rev. Lett. 34

Jefferts

673.

and H.G.

Dehmelt,

(1975) Phys.

1603. Rev.

165

80. F.J.

Grieman,

Sot.

(1981),

R.C.

Dunbar

5-L. J.

B.H.Mahan, No. 71

(in

and H. Ho-I.

A.

private

and J.S.

Winn,

Discuss.

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O’Keefe

(1976)

coranunication.

309.

100

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2279.

Faraday

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