Laser induced fluorescence of MgO

Transcription

1 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 < /jphys: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1981 HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 La Figure We J. Physique 42 (1981) AVRIL 1981, 537 Classification Physics Abstracts 33.20K 34.50L Laser induced fluorescence of MgO G. Taïeb Laboratoire de Photophysique Moléculaire (*), Université de ParisSud, Orsay, France and U.E.R. ClaudeBernard, Université de Rennes I, Rennes, France (Reçu le 10 juin 1980, révisé le 29 octobre, accepté le 23 décembre 1980) 2014 Résumé. 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 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. reaction of Mg in its 3 3P metastable state with N2O has been studied by laser induced fluorescence (LIF) using an Ar+ laser. a 3II state is detected, and chemiluminescence due to d 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. spectroscopy of the MgO molecule has been intensively studied, but only recently has a triplet transition been identified [14], and the position of the first triplet state a 3II relative to the X 1 E + ground state determined [5]. chemiluminescent reactions of alkaline earth atoms with oxidants have been investigated for barium [6] and to a lesser extent for calcium [711], 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 spectroscopic results about the Mg* + N20 reaction, where Mg* is the 3 3P state of Mg. 2. Expérimental. 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 ParisSud. monochromator ( f = 0.5 m ; N = 600 l/mm) and in the U.V. with a SOPRA monochromator (f =1.15 m ; N = I/mm), using an EMI 9558 photomultiplier tube. 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. report here results on postluminescence, chemiluminescence and laser induced fluorescence la shows a spectrum of the afterglow (AG) obtained with the Bausch and Lomb monochromator using very wide slits (400 g). One can see the very intense feature corresponding to the 3 1S 3 P forbidden transition at nm. On can also see : a) the 3 IS P resonance line at nm, b) four groups of triplettriplet atomic emission 3 3p + 4 3D at  nm, 3 3p + 5 3S at À nm, 3 3p + 3 3D at À nm, and 3 3p + 4 3S at 03BB nm. 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

3 Spectrum When Spectrum Same 538 Fig. lc. of the chemiluminescence with [N20] [Mg]. Fig. la. of the postluminescence (AG) taken 40 cm above the discharge region. ( 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 nm line, which is quenched by N 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 BX (Av 0) MgO = emission appears (Fig. lb) ; the dv 1 = sequence at À nm can also be observed, as well as the Av 1 = sequence at À nm partly superimposed on the triplet atomic emission at 518.4, and nm; some bands of the MgO B 11 A l II system can also be seen, as well as the d 3a 311 system at nm. 518 nm. Underlying these lines a weak continuum shows up a) [N20] [Mg]. When the visible Ar+ laser lines (9 lines from nm to nm) are successively used to excite the mixing zone, two different observations are made : with three laser lines ; ; 488 nm an increase of the continuum underlying the BX and BA emission in the visible is observed, while with others 488 ; ; and nm (1) discrete emission arises (Fig. 2). 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. y 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. zone. as (1b) with laser 488 nm line exciting the mixing 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 molecular emissions are quenched (Fig. lc). re appear only weak atomic transitions at 285.2, 383, 457.1, Excitation of the mixing zone with the Ar nm line produces a fluorescence in the 370 nm390 nm region. 372 nm bands, shown in figure 3, can be readily assigned to the Av 0 = sequence of the d 3da 3II transition of MgO [3, 5]. It is likely that the (1) intensity of the laser is : 0.3 W for and nm; 0.5 W for 457.9, and nm ; 1.5 W for and nm, and 5 W for 488 and nm ; the intensity of the continuum does not follow these intensities.

4 LIF Correlation Same 539 visible Ar+ laser lines (except the and nm) is strongly enhanced, while a larger number of LIF molecular lines appear, as can be seen for example on figure 5. Fig. 3. of the d 3da 3II transition by the nm laser line (1.15 m monochromator, Resolution : ) nm laser line pumps the 1 0 transition and that the observed fluorescence corresponds to the Au = 0 sequence of the d 3da 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. 5. zone. as (lc) with laser 488 nm line exciting the mixing Although the analysis of the discrete spectrum has not yet been made, it probably can be assigned to the MgO BX and BA 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. 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 Fig. 4. 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. dissociation energy of MgO is 3.49 ev [ 16], and the energy of the (b) 3 + state relative to the ground state is cm1 [17, 18]. se values are not used in this diagram. 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 particular, 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. B 1 03A3 + state might be populated either by surface crossing or by a complex processus involving secondary collisions. 4. Conclusion. 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. se are the differences

5 We 540 between the results obtained in the study of ground state reactants (Ikeda et al.) and ground stateexcited state (this work), also with the unexplained laser pumped continuum emission with high N20 flow rate conditions. 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. wish to thank I. Pépin for technical assistance and Joël Schamps, Joëlle Rostas and Bernard Bourguignon for helpful discussions. This work is dedicated to the late Dr. H. P. Broïda. References [1] SCHAMPS, J., Ph. D. sis, 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) b) BENARD, D. J., SLAFER, W. D., J. Chem. Phys. 66 (1977) [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) [13] TAÏEB, G. and BROÏDA, H. P., J. Chem. Phys. 65 (1976) [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) [17] BAUSCHLICHER Jr., C. W., SILVER, D. M. and YARKONY, D. R., J. Chem. Phys. 73 (1980) [18] BAUSCHLICHER Jr., C. W., LENGSFIELD III, B. H., SILVER, D. M. and YARKONY, D. R., to be published.