Resonant character of laser-induced formation of particles in a Cs-H 2 vapour

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Resonant character of laser-induced formation of

particles in a Cs-H 2 vapour

J.L. Picqué, J. Verges, R. Vetter

To cite this version:

J.L. Picqué, J. Verges, R. Vetter.

Resonant character of laser-induced formation of particles

in a Cs-H 2 vapour.

Journal de Physique Lettres, Edp sciences, 1980, 41 (13), pp.305-308.

�10.1051/jphyslet:019800041013030500�. �jpa-00231785�

Resonant

character of laser-induced formation

of

_particles

in

a

Cs-H2

_vapour

J. L.

_Picqué,

J.

_Vergès

and R. Vetter

Laboratoire Aimé-Cotton (*), Centre National de la Recherche Scientifique, Bâtiment 505, 91405 Orsay,

_France

(Re~u le 9 _{avril 1980, accepte} le 16 mai ₁₉₈₀₎

Résumé. 2014 On

montre _que la formation de

_particules

_{induite par laser dans} une _vapeur Cs-H2 est résonnante vis-à-vis des secondes raies de résonance du césium _atomique. On _explique l’existence d’un seuil pour la longueur d’onde du laser à l’aide de considérations _{énergétiques}

_simples

relatives à la réaction _primaire de formation de molécules de CsH. L’étude de la fluorescence de ces molécules induite par laser _suggère que des mécanismes de réaction autres

que l’excitation directe des atomes de Cs dans le niveau 7P _{peuvent également} intervenir. Abstract. 2014 Observation of laser-induced formation of

particles

in a _{Cs-H2 vapour is found} to be resonant with the second resonance lines of cesium atoms. The existence of a threshold for laser _wavelength is

_explained

_by

help of

_simple

energy considerations

applying

to the

_primary

reaction of formation of CsH molecules. _Study of the laser-induced fluorescence of these molecules _suggests that other reaction mechanisms than direct excitation of Cs

atoms to the 7P level may also take place.

Classification

Physics Abstracts 82.50 - 82.30 - 34.00

It was shown in 1975

_by

_Tam,

Moe and

_Happer

that formation of micron-sized visible

_{particles (laser}

snow)

can be observed in a cell filled with cesium and

hydrogen,

when irradiated

_by

an intense laser beam of suitable

_{wavelength [1].}

The

_phenomenon

was

inter-preted

in terms of initial formation of CsH

molecules,

which in turn _agregate to form CsH

_{crystals. Recently,}

this

_crystal

formation was

_explained

_through

a

ther-modynamical approach [2].

Concerning

the initial

production

of cesium

_hydride

molecules,

Tam et al.

proposed

several mechanisms

_involving

excited atomic

or molecular

species

[1, 3].

In this Letter, we

_report

experiments

aimed to

_distinguish

among these

mecha-nisms. In

_particular,

we show that

particle

formation

is enhanced when the laser radiation is tuned to the

second resonance lines 6S --+ 7P of atomic cesium.

This resonant character confirms the

_feasibility,

which was also

_pointed

out in the case of

_UF6

mole-cules

_[4],

of

_{isotope separation}

_by

the

_present

pheno-menon

_{[1]. However,}

from the

_study

of the

_dependence

of laser-induced fluorescence of CsH molecules on

laser

_intensity,

we infer that other reaction mecha-nisms may exist at

_large

radiation intensities.

In the

_{original experiment}

of Tam et al.

_[1],

the cell contained Cs and

_H2

at

_{relatively high}

_pressure

_(a

fraction of

_atmosphere)

and was heated at about

(*) Laboratoire associe a l’Université Paris-Sud.

350 °C. It was irradiated

_by

a

_powerful

laser beam

(hundreds

of

_milliwatts)

_provided

_by

an Ar+ laser

oscillating

at 454.5 or 457.9 nm. The

_proposed

expla-nation of the

_experiment

was that pressure

_broadening

of the second resonance lines at 455.5 or 459.3 nm

allowed a substantial

_pumping

of Cs atoms from the

ground

state to the 7

_2P312

or 7

2p 1,2

_state,

respecti-vely.

Excited Cs atoms could then react

_efficiently

with

hydrogen

to form CsH

molecules,

which could further condensate and

_give

rise to

_particles.

_Therefore,

in that

_experiment,

_hydrogen

acted both as a buffer gas

and a reactive one.

In our

_experiment,

we have

_investigated

the

possi-bility

of

_particle

formation

_by

use of a C.W.

_dye

laser

which we tuned in the

_vicinity

of the 6S -+ 6P and

6S -+ 7P resonance

_doublets,

in the near infrared and

blue

_{region, respectively.}

_Experiments

have

_given

negative

results

_(this

will be

_explained

_later)

in the first _case, but have led to

_particle

formation in the

second case. Thanks to the

_tunability

of the

laser,

hydrogen

was

_only

involved as a _reagent, so that it

was

_possible

to reduce

_considerably

_{the gas pressure}

in the cell

_(at

the

_working

_temperature

of 350

üC,

pressures of Cs and

H2

were both around 7

_torr).

Also due to the observed resonant

_behaviour,

we

could obtain

_particle

formation for small values of the laser _output

₍₁₀

mW

_typically).

The

_experimental

_arrangement is

_{schematically}

shown in

_figure

1. The unfocussed beam

₍₂

mm

dia-meter)

from the

_dye

laser crossed the

_cylindrical

L-306 JOURNAL DE _{PHYSIQUE -} LETTRES

Fig. 1. -

Experimental arrangement for the observation, by Mie

scattering of a He-Ne laser beam, of particles formed in the dye

laser beam. The cell is _placed inside a _{transparent glass} oven.

gas

cell,

made of alkali-resistant

glass (Coming 1720).

The C.W.

_dye

laser was multimode

_(linewidth

of

about 0.02

_nm)

and delivered a maximum _power of

170 mW in the blue

_region.

Particles were observed

by

Mie

_scattering

of an

auxiliary

He-Ne laser beam

propagating perpendicularly

to the

_dye

laser one.

In order to monitor a

_region

in which the

_particle

formation is efficient

_enough,

the He-Ne

_probe

laser beam was located near the entrance wall of the

_cell,

where the active

_dye

laser beam is not _yet

_completely

absorbed. The scattered

_light

was collected at

_right

angle

and focussed onto the cathode of a

photomulti-plier

tube

_preceded

_by

a suitable interference filter.

Figure

2 shows the

_intensity

of the scattered

_light

as a

function of the

_dye

laser

_wavelength.

The

_wavelength

was scanned

_continuously

between 455.0 nm and

460.0 nm

_{by rotating}

a

_Lyot

filter inside the

cavity

Fig. 2. -

Experimental recording of the _intensity of the _light

scat-tered _by laser-induced _particles versus _dye laser _wavelength.

of the

_dye

laser. We have

_{verified, by}

use of a

_grating

spectrometer, that the two observed

_profiles

are

centred on the _components 6

2S 1/2 --+ 7 2p 1/2

and

6

_{2S 1/2}

-~ 7

_{2 P3/2,}

_{respectively,}

of the second resonance

doublet. Their

_shape

is,

of course, most influenced

_by

the

_high

_density

of Cs atoms at the

_working

tempera-ture.

_They

exhibit a hole which

_probably

results from

partial absorption

of the

_dye

laser beam before

reach-ing

the observation zone. The

_large

width

_{( ~}

0.1

_nm)

and the _asymmetry of the two

_profiles

should

_mainly

arise from the resonance

_{broadening phenomenon [5].}

Since

_particle

formation occurs

_through

the

pri-mary reaction

we have -found of

interest,

in a second

_step,

to

_study

the laser-induced fluorescence

_(L.I.F.)

of the

_product

CsH molecules. This was

_{performed by}

_irradiating

the cell at 455.5 nm with the beam from the

_dye

_laser,

the

_large

linewidth of which allowed us to induce both :

_(i)

formation of CsH molecules in their

_ground

state

X~ 2’ ~

_through

reaction

_(1),

_(ii)

_subsequent

excitation of these molecules to the first electronic excited state

A1

2: ’

_[6].

The

_experimental

_set-up

was

similar to the one shown in

figure

1, except

that the

He-Ne laser beam was

_removed,

and that

fluores-cence

_light

was observed instead of scattered

_light.

For this

_experiment,

the

_dye

laser beam was focussed

(to

about 0.1 mm

_diameter)

near the entrance wall of the

_cell,

thus

_producing

a

_bright

red fluorescent zone,

the

_image

of which was formed on the cathode of a

photomultiplier

tube. This fluorescent

_light,

visible

to the naked _{eye, appears}

_only

when the

_dye

laser is tuned to the lines of the second resonance doublet.

This ensures that it does come from CsH molecules.

However,

in order to limit the collection of

back-ground

fluorescence from

_Cs2

molecules,

we selected the

_light

emitted around 620 nm

_by

use of an inter-ference filter with a 10 nm bandwidth.

_Figure

3 shows

the

_dependence

of the fluorescence

_signal

on the laser

radiation

_intensity

I _{in the power range} 10-170 mW. One observes a

/3/2 law, already

_reported

in ref.

_[1],

in the low

_intensity

_range

_(power

5 100

mW),

and a more

_rapid

variation at

_higher

intensities.

The reaction of

_ground

state cesium atoms with molecular

_hydrogen

is

_by

far

_endoergic.

The _energy defect

_{corresponds approximately}

to the

difference,

2.4

eV,

between the

_binding

energy of the

H2

reagent

molecule,

4.48 eV

_[7],

and that of the CsH

product

molecule,

2.08 eV

_[6].

This _appears

_clearly

on

figure

4,

which

_displays

the _energy levels of

_reagents

and

products.

It shows that the _energy of the

system is

_{approximately}

_equal

to that of the

_low-lying

rovibrational levels of the electronic

ground

state

Fig. 3. -

Intensity of laser-induced fluorescence of CsH molecules

versus incident 455.5 nm laser intensity I. The dotted line _represents

a [3/2 law. The horizontal scale is _graduated in terms of the power

carried in the laser beam.

Fig. 4. -

Diagram of relevant _energy levels in the chemical reaction Cs + H2 -)’ CsH + H. The right-hand part of the _diagram shows the level scheme used for laser-induced fluorescence in the CsH molecule.

lower levels of Cs atoms lie below _{the energy} threshold of the reaction. This is the case for the 7S and 5D

levels,

which are

_populated

_through

radiative

_decay

from the 7P level. This is even more obvious for the 6P

_level,

_explaining

_why

we could not induce

_particle

formation

_by

_tuning

the laser to the lines of the first

resonance doublet. Note that vibrational activation

of the reaction would

_require

excitation of the

_H2

molecules to v" >

5,

which’ is not

_possible

under our

experimental

conditions. It also _appears on

_figure

4

that the first electronic excited state A1 ~ + of CsH molecules is not _very far

_(about

0.75

_eV)

above the ionization limit of Cs atoms.

_Thus,

_especially

if some

additional _energy is

_provided

to the reactants

_(e.g.,

through

vibrational excitation of

_H2

molecules to

v" >

_2),

excited CsH molecules in the A state could be

_produced

from Cs atoms in

_highly

excited levels. A further

_insight

into the _processes which

_actually

occur in our

_experiments

can be

_gained

from the

analysis

of the

_dependence

of the laser-induced fluorescence of CsH molecules on the

incoming

radia-tidn

_intensity

I. Let

_n(I)

be the number of excited Cs

atoms

in the 7P state in

_{steady-state regime.}

This

L-308 JOURNAL DE _{PHYSIQUE -} LETTRES

where no is the total number of Cs atoms

_{and ~}

is a

constant coefficient

_(such

that

_ç/

is the saturation

parameter

of the

_transition).

In a first

approximation,

the rate of formation of CsH molecules in the

_ground

state is

_{given by}

the

_following

_{equation :}

when a

and 13

are constants. The first term in the

right-hand side of

_equation

₍₃₎

holds for the

_production

of CsH molecules

_by

reactive collisions of excited Cs

atoms with

_{H2 molecules,}

whereas the second term accounts for the loss of CsH molecules

_through

_binary

collisions

_leading

to the formation of dimers. The

steady-state

solution of

_{equation (3) gives}

Assuming

that the fluorescence

_signal

F is

proportio-nal to

_{N(I )}

x

_I,

one _gets

This

_expression

_yields

the I312 variation at low I

values,

already

mentioned in ref.

_[1]

and in

_good

agreement with our

experimental

observations. The

asymptotic

linear variation

_predicted

_{by (5)}

for very

high

I

_values,

which would be obtained at saturation of the atomic

transition,

is

_expected

to be unobser-vable with our available laser

_{output. However,}

it

turns out that the law

₍₅₎

does not fit the

_experimental

dependence

even for intermediate intensities.

_Actually,

the observed fluorescence _grows more

_rapidly

than

/3/2,

in contradiction with

_expression

_(5).

This seems

to indicate that a non-linear _{process may}

_populate

excited Cs levels above the reaction threshold. Under

our

_experimental

conditions,

this process could be

the resonant

_two-photon

ionization of Cs atoms via

the intermediate 7P state, followed

by

electron-ion

recombination

_(1).

Note also that other

_one-photon

processes than the direct

population

of the 7P level

(but

still resonant with the second resonance

_lines)

_may

contribute to the /3/2

_regime

observed at low inten-sities. One of them could be the

_population

of

higher-lying

levels of atomic Cs

_by

inelastic collisions between

two atoms in the 7P state

_[10].

To

_conclude,

we have demonstrated the existence

of a

_preferential

reaction channel for the formation

of CsH molecules

_{(and thereby}

for the

_subsequent

formation of

_particles),

_exhibiting

a resonant beha-viour associated with the excitation of Cs atoms to the

7P state. In

addition,

we have

_pointed

out the

possi-bility

of other reaction channels

_consisting

of various

ways of

populating higher-lying

levels in atomic cesium.

However,

it is to be noted

that,

under different

experimental

conditions,

other channels

_leading

to

formation of cesium

_hydride

could take

_place.

These channels would

_involve,

_e.g., electronic excitation of

Cs2

molecules followed

_by

dissociation

_producing

excited Cs atoms

_[3],

vibrational excitation of

_{H 2}

molecules

_[11],

reaction of

_Cs2

molecules with H

atoms

_[12].

We intend now to further

_study

the chemical reaction

_{(1) by}

_{investigating}

the rovibrational distri-bution of the

product

CsH molecules

_by

use of Fourier

Transform

_{Spectroscopy. Population}

distribution in the

_ground

state will be

_probed

_by

L.I.F. and

absorp-tion _{spectroscopy,} whereas

_{possible population}

of the first electronic excited state would be

_probed

by

direct

_spectral

_analysis

of the chemiluminescence.

Acknowledgments.

- We wish to thank Professor

S. Feneuille for his interest in this work and for

helpful

discussions.

e) This process has been observed to be efficient in cesium [8].

Actually, the calculated photoionization cross-section from the 7P

state around 455 nm is _{quite large :} 5.8 x 10-18 cm2 _[9].

References

[1] TAM, A., MOE, G. and HAPPER, W., Phys. Rev. Lett. 35 (1975)

1630.

[2] OMNÈS, R., J. _Physique Lett. 41 (1980) L-63.

[3] TAM, A. C., HAPPER, W. and SIANO, D., Chem. _Phys. Lett. 49

(1977) 320.

[4] RONN, A. M. and EARL, B. L., Chem. Phys. Lett. 45 (1977) 556.

[5] NIEMAX, K., MOVRE, M. and PICHLER, G., J. Phys. B 12 (1979)

3503.

[6] TAM, A. C. and HAPPER, W., J. Chem. Phys. 64 ₍₁₉₇₆₎ 2456.

[7] HERZBERG, G., Molecular Spectra and Molecular Structure, vol. 4 _{(Van Nostrand-Reinhold,} New _York) 1979.

[8] BRÉCHIGNAC, C., CAHUZAC, Ph. and DÉBARRE, A., to be

published.

[9] AYMAR, M., private communication.

[10] LESLIE, S. G., VERDEYEN, J. T. and MILLAR, W. _S., J. _Appl.

Phys. 48 (1977) 4444.

[11] POLANYI, J. C. and SADOWSKI, C. M., J. Chem. Phys. 36 (1962)

2239.

[12] LEE, Y. _{T., GORDON,} R. J. and HERSCHBACH, D. _R., J. Chem.