Laser induced fluorescence of MgO

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Laser induced fluorescence of MgO

G. Taïeb

To cite this version:

G. Taïeb. Laser induced fluorescence of MgO. Journal de Physique, 1981, 42 (4), pp.537-540.

�10.1051/jphys:01981004204053700�. �jpa-00209039�

Laboratoire de Photophysique Moléculaire (*), Université de Paris-Sud, ⁹¹⁴⁰⁵ Orsay, ^France

and U.E.R. Claude-Bernard, Université de Rennes I, ³⁵⁰⁴³ Rennes, ^France (Reçu ^{le 10} juin 1980, ^{révisé le 29} octobre, accepté le 23 décembre 1980)

Résumé.

La réaction de Mg ^{dans son} état métastable 3 3P avec N2O ^a été étudiée par fluorescence induite par laser en utilisant un laser à argon ionisé. L’état ^a 303A0 est détecté, ^et la chimiluminescence due aux transitions d 30394 ~ a 303A0 B 103A3 +

X 103A3 + et B 103A3 +

A 103A0 observée. Un intense continuum est émis dans le visible quand

certaines raies laser sont utilisées, ^ainsi que des émissions discrètes.

Abstract.

The reaction of Mg ^{in its 3 3P} metastable state with N2O has been studied by laser induced fluores-

cence (LIF) using ^{an Ar+} laser. The a 3II state is detected, and chemiluminescence due to d 30394 ~ a 303A0

B 103A3 + ~ X 103A3 + and B 103A3 + ~ A 103A0 transitions observed. A strong continuum is emitted in the visible when

some Ar+ laser lines are used, together with discrete emissions.

1. Introduction.

The spectroscopy ^{of the} MgO

molecule has been intensively ^{studied, but} only recently ^{has a} triplet transition been identified [1-4],

and the position of the first triplet ^{state a 3II relative}

to the X 1 E + ground ^state determined [5]. ^{The chemi-} luminescent reactions of alkaline earth atoms with oxidants have been investigated ^{for barium} [6] ^{and to a}

lesser extent for calcium [7-11], ^strontium [4a, 8, 12]

and magnesium [4a, 4b, 5]. ^{In an} earlier work [13]

we observed a strong quenching ^effect of N20 ^and 02

on the Mg (3 3p) population ^{formed in a} discharge.

This effect indicated that a very efficient chemical reaction ^occurs. We report ^here preliminary spec-

troscopic results about the Mg* ⁺ N20 reaction,

where Mg* ^{is the 3 3P state of} Mg.

2. Expérimental.

^The apparatus used here is similar to the one described in reference [13] Mg

vapour formed in ^a furnace is pumped together ^{with a}

carrier gas (He) ^{at a} pressure - ¹ torr, ^{and a linear} flow rate of - 30 m/s. ^{A D.C.} potential ^{of - 40 V} applied between the crucible and the ground ^makes

it possible ^{to obtain a} high ^{3 3P atom} concentration in the region above the discharge; N20 ^{is added at}

~

40 cm above the crucible. An argon ion laser beam is directed vertically through ^the mixing region ^and

LIF is analysed in the visible with a Bausch and Lomb

(*) Laboratoire associé à l’Université de Paris-Sud.

monochromator ( f

^0.5 m ; N

600 l/mm) ^{and in}

the U.V. with a SOPRA monochromator (f =1.15 ^{m ;}

N

1 200 I/mm), using ^{an EMI 9558} photomultiplier

tube. The average Mg concentration is estimated to be

~

1.5 x 1013/cm3 by weight difference, ^{and the} Mg (3 ’P) of the order of 1011/cm3 [13].

3. Results and discussion.

We report ^{here results}

on post-luminescence, chemiluminescence and laser induced fluorescence.

3.1.

Figure ^{la shows a} spectrum ^{of the} afterglow (A-G) ^obtained with the Bausch and Lomb mono-

chromator using very wide slits (400 g). ^{One can see}

the _very intense feature corresponding ^{to the} 3 1S -- 3 ’P forbidden transition at 457.1 nm. On can

also see :

a) ^{the 3 IS +- 3 1 P resonance line at 285.2} nm,

b) ^four groups of triplet-triplet atomic emission 3 3p +- 4 3D at  - 309.1-309.7 _nm, 3 3p +- 5 3S

at À - 333-333.7 _nm, 3 3p +- 3 3D at À - 382.9- 383.8 _nm, and 3 3p +- 4 3S at 03BB~ 516.7-518.4 nm.

The upper singlet ^{state and} triplet ^{states of the}

atomic transitions a) ^and b) ^{have to be} populated by

direct resonance absorption, ^{from the 3 ’S and 3 3 P}

states of Mg, ^of photons coming ^{from the} discharge region. This is confirmed by the observation that the

intensity ^{of the resonance line} a) ^{is not affected} by

addition of N20, although the intensities of the triplet

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01981004204053700

538

Fig. ^la.

Spectrum ^{of the} post-luminescence (A-G) ^taken

~

40 cm above the discharge region. (The weak B 1~ - X 1 E Av

0 emission of MgO ^{at À - 500 nm} results from a small air leak.) atomic emissions b) follow that of the 457.1 nm line,

which is quenched by N20.

3.2.

When N20 is added into the afterglow zone, the spectra ^{observed are} strongly dependent ^{on the}

oxidant flow rate.

a) ^{When the} N20 concentration is much lower than the Mg concentration, ^a strong ^B-X (Av

0) MgO

emission appears (Fig. lb) ; ^{the dv}

1 sequence ^at À - 475-481 nm can also be observed, ^{as well as the}

Av

1 sequence at À - 510-520 nm partly super-

imposed ^{on the} triplet ^{atomic emission at 518.4,}

517.3 and 516.7 _nm; some bands of the MgO ^{B 11-}

A l II _system can also be _seen, as well as the d 3~-a 311 system ^{at 372.1 nm.}

Fig. lb. - Spectrum of the chemiluminescence with [N20] [Mg].

b) ^{If the} N20 ^{flow rate} is increased by ^a factor of 6 so as to be of the order of the Mg ^flow rate, all the mole- cular emissions are quenched (Fig. lc). There appear

only weak atomic transitions at 285.2, 383, 457.1,

Fig. ^lc.

Spectrum of the chemiluminescence with [N20] ~ [Mg].

518 nm. Underlying ^{these lines a} weak continuum shows _up.

3.3. - a) [N20] [Mg]. ^{When the visible Ar+}

laser lines (9 lines from 454.5 nm to 514.5 nm) ^are successively ^{used to} excite the mixing ^{zone, two}

different observations are made : with three laser lines 457.8 ; 476.5 ; ^{488 nm an} increase of the conti-

nuum underlying ^{the B-X} and B-A emission in the visible is observed, while with others 488 ; 476.5 ; 496.5 and 514.5 nm (1) discrete emission arises (Fig. 2).

The last three lines are those used by Ikeda et al. [5]

who studied in detail the LIF spectra of Mg0 produced

from the ground state reactants N20 ⁺ Mg. They ^did

not record photoluminescence spectra ^excited by ^the

488 nm laser line, although ^{in our case} excitation by

this line (see Fig. 2) ^also gave rise to discrete emission.

Fig. ^2.

^{Same as} (1b) with laser 488 nm line exciting ^the mixing

zone.

Excitation of the mixing ^{zone with the Ar+ 363.7 nm}

line produces ^a fluorescence in the 370 nm-390 nm

region. ^{The 372 nm} bands, ^{shown in} figure 3, ^{can be} readily assigned ^{to the Av}

^{0 sequence of the d 3d-}

a 3II transition of MgO [3, 5]. ^{It is} likely ^{that the}

(1) ^The intensity of the laser is : 0.3 W for 454.5 and 465.8 _nm;

0.5 W for 457.9, 472.7 ^{and 501.7} nm ; 1.5 ^W for 476.5 and 496.5 nm,

and 5 W for 488 and 514.5 _{nm ;} the intensity ^{of the} continuum does

not follow these intensities.

Fig. ^3.

LIF of the d 3d-a 3II transition by ^{the 363.7 nm laser}

line (1.15 m monochromator, Resolution : 100 000).

363.7 nm laser line _pumps the 1 - 0 transition and that the observed fluorescence corresponds ^{to the}

Au

0 sequence of the d 3d-a 3il transition of

MgO [1].

This result would indicate that the spin correlation rule holds in this reaction, ^since according ^{to the}

adiabatic correlation diagram, ^the system

is correlated to the a 3II and (b) ^{311 states} (Fig. 4).

Furthermore, features observed at À - 370 nm might

well be due to transitions involving ^the (b) ^{3 ~ + state}

as the lower level.

b) ^{When the} N20 ^and Mg concentrations are

comparable, the weak continuum produced by ^the

Fig. ^4.

Correlation diagram for the reaction

As was pointed ^{out to us, the} diagram published in reference [4]

gives ^a wrong correlation ^to the d 3d state, originating probably

from a misassignment ^{of the} Mg (3P) ^state symmetry. ^{The disso-} ciation energy of MgO is 3.49 eV [ 16], and the energy of the (b) ^{3~ +}

state relative to the ground ^{state is} 8 300 cm-1 [17, 18]. ^These values are not used in this diagram.

Fig. ^5.

^{Same as} (lc) with laser 488 nm line exciting ^the mixing

zone.

Although ^the analysis of the discrete spectrum ^has

not yet ^been made, ^it probably ^{can be} assigned ^{to the} MgO B-X and B-A systems since the B - X and B - A transitions are efficiently pumped by ^{the Ar+}

laser lines [5]. ^{Since the a 3 03A0 state is} highly populated,

the intercombination B 103A3 +-a 3 03A0 system ^{should be} easily ^{observed via a} dye laser excitation. Ikeda ^et al.

attempted ^to detect this transition, ^{but did not} succeed, probably because the a 311 state is not sufficiently populated ^{when the} reactant atom is in its ground

state.

The strong continuum emission observed in the laser induced photoluminescence spectra ^{obtained at} high N20 ^{flow rate is not} easily explained. ^{Even if one}

assumes a secondary reaction like

it is not obvious why ^a monochromatic radiation

gives ^{rise to a continuum over the whole visible} region.

It should be emphasized ^{that when} the D.C. is off, ^i.e.

in the absence of Mg (’P) species, emission is not detectable either with or without laser excitation in any experimental conditions. This _means, in particu- lar, ^{that the} MgO ^{B 103A3 + state} which fluoresces at low N20 ^{flow rate} is formed only ^{when the} reacting

atom is in the ’P state. The B 1 03A3 + state might ^be popu- lated either by ^surface crossing ^or by ^a complex

processus involving secondary collisions.

4. Conclusion.

The laser induced fluorescence spectrum ^{observed in the 372 nm} region shows that the reaction Mg (3 3P) ⁺ N20 (1 E +) produces MgO ⁱⁿ

its lowest triplet ^{state, in agreement with the} spin

conservation rule. However, ^we observed that the B 1 E

state is also populated ^{and this} only ^{when the} reacting

atom is in the ’P state. These are the differences

540

between the results obtained in the study ^of ground

state reactants (Ikeda ^et al.) ^and ground state-excited state (this work), ^{also with the} unexplained ^laser pumped continuum emission with high N20 ^flow

rate conditions. The technique described here to

produce metastable atoms is efficient for studying

their reactivity, ^but dynamical ^{studies in} collisionless conditions are needed to know the nascent populations

of the various electronic states and to detect possible cascading processes.

Acknowledgments.

^{We wish to thank I.} Pépin

for technical assistance and Joël Schamps, Joëlle Ros- tas and Bernard Bourguignon ^for helpful discussions.

This work is dedicated to the late Dr. H. P. Broïda.

References [1] SCHAMPS, J., ^{Ph. D.} Thesis, Université de Lille (1973).

[2] EVANS, ^{P. J. and} MACKIE, J. C., ^Chem. Phys. ⁵ (1974) 277 ; EVANS, ^{P. J. and} MACKIE, ^J. C., ^{J. Mol.} Spectrosc. ⁶⁵ (1977)

169.

[3] SCHAMPS, ^{J. and} GANDARA, G., J. Mol. Spectrosc. ⁶² (1976)

80.

[4] a) BENARD, ^D. J., SLAFER, ^{W. D. and} HECHT, J., ^{J. Chem.}

Phys. ⁶⁶ (1977) ^1012.

b) BENARD, ^D. J., SLAFER, ^W. D., ^{J. Chem.} Phys. ⁶⁶ (1977)

1017.

[5] IKEDA, T., WONG, N. B., HARRIS, D. O. and FIELD, ^R. W., J. Mol. Spectrosc. ⁶⁸ (1977) ^452.

[6] FIELD, R. W., GOTTSCHO, ^R. A., PRUETT, J. G. and REUTHER, J. J., XIV Conference ^{on Free Radicals} (Kyoto) Japan, Sept. 1979, and references therein.

[7] OTTINGER, ^{Ch. and} ZARE, ^R. N., Chem. Phys. ^{Lett. 5} (1970)

243.

[8] JONAH, ^C. D., ZARE, R. N. and OTTINGER, Ch., ^{J. Chem.} Phys.

56 (1972) ^263.

[9] DAGDIGIAN, ^P. J., Chem. Phys. ^{Lett. 55} (1978) ^239.

[10] PASTERNAK, ^{L. and} DAGDIGIAN, P., ^Chem. Phys. ³³ (1978) ^1.

[11] ALEXANDER, M. H. and DAGDIGIAN, P. J., ^Chem. Phys. ³³ (1978) 13.

[12] WILCOMB, ^{B. E. and} DAGDIGIAN, P. J., ^{J. Chem.} Phys. ⁶⁹ (1978) ^1779.

[13] TAÏEB, ^{G. and} BROÏDA, ^H. P., ^{J. Chem.} Phys. ⁶⁵ (1976) ^2914.

[14] BRINKMANN, U. and TELLE, H., ^J. Phys. ^{B 10} (1977) ^133.

[15] a) ENGELKE, F., ^Chem. Phys. ³⁹ (1979) ^279.

b) ENGELKE, F., ^Chem. Phys. ⁴⁴ (1979) ^213.

[16] MURAD, E., ^J. Geophys. ^{Res. 83} (1978) ^5525.

[17] BAUSCHLICHER Jr., C. W., SILVER, D. M. and YARKONY, D. R., J. Chem. Phys. ⁷³ (1980) ^2867.

[18] BAUSCHLICHER Jr., C. W., LENGSFIELD III, B. H., SILVER, D. M.

and YARKONY, ^D. R., ^to be published.