High resolution emission spectra of H2 and D2 near 80 nm

Transcription

1 High resolution emission spectra of H2 and D2 near 80 nm M. Larzillière, F. Launay, J.Y. Roncin To cite this version: M. Larzillière, F. Launay, J.Y. Roncin. High resolution emission spectra of H2 and D2 near 80 nm. Journal de Physique, 1980, 41 (12), pp < /jphys: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1980 HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The 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 Quelques A J. Physique 41 (1980) DÉCEMBRE 1980, Classification Physics Abstracts 33.20N High resolution emission spectra of H2 and D2 near 80 nm M. Larzillière (*), F. Launay (**) and J.Y. Roncin (*) (*) Equipe de Spectroscopie, C.N.R.S. (+), E.M.S.E., 158, Cours Fauriel, SaintEtienne Cedex, France (**) Observatoire de Paris, Departement d Astrophysique Fondamentale, G.R. 24 du C.N.R.S., Meudon, France (Reçu le 13 juin 1980, accepté le 22 août 1980) Résumé raies ont été observées dans le spectre d émission ultraviolet lointain de l hydrogène deutérium. Ces raies sont attribuées à des transitions partant de niveaux de l état 3p03C0D 103A0u situés audelà de la limite de dissociation en H(ls) + H(n = 2) près de 84,5 nm et, pour certains, audelà de la premiere limite d ionisation près de 80,4 nm, l état inférieur étant X 1 03A3g+ (v" = 1). Les résultats sont en accord avec les mesures de taux de prédissociation et de préionisation. La mesure précise des positions de raies permet de déduire des constantes moléculaires en très bon accord avec les calculs théoriques. Abstract et du few lines have been observed in the far ultraviolet emission spectrum of molecular hydrogen and deuterium. They are assigned to transitions from levels of the 3p03C0D 103A0 u state, lying above the dissociation limit into H(ls) + H(n 2), = near 84.5 nm, and, for some of them, above the first ionization limit near 80.4 nm, the lower state being X 103A3g+ (v" = 1). This is in fair agreement with measured predissociation and preionization yields. Accurate line position measurements lead to molecular constants in very good agreement with theoretical calculations. 1. Introduction. A few years ago Roncin, Damany and Jungen gave a preliminary report [1] about the observation, at wavelengths shorter than 85 nm, of weak emission lines of H2 (D2) which we readily assigned to transitions from superexcited levels, of the 3pnD lllü state, lying above the dissociation limit into H(ls) + H(n 2), = near 84.5 nm and, for the higher ones, above the first ionization limit near 80.4 nm, the lower level being the v" 1 of the ground = state X 1 E g+. These excited levels are known from the absorption data of Monfils [2] and Takezawa [3]. Lines from such high energy levels are usually absent in emission, due to preionization and/or predissociation [4]. If present, such short wavelength lines are weak so that they are reabsorbed in classical discharges whereas they show up in our spectra obtained in a low pressure windowless discharge. However, our spectra were taken in the first order of the grating so that wavelength standards were needed in a region where very few lines are measured with enough accuracy, i.e. to nm. As a few more standards are now available [5] and as the resolution of the spectrograph has been increased, we are in position to give accurate ( ) L.A. 171, Universités de SaintEtienne et Lyon 1. data and to derive molecular constants in fair agreement with the ab initio calculation of Kolos and Rychlewski [6] and the multichannel quantum defect theory (M.Q.D.T.) of Jungen and Atabek [7]. Our results agree also with the recent fluorescence work of GlassMaujean, Breton and Guyon [8, 9, 10]. 2. Experimental. The presence of a magnetic field of 0.1 tesla in the discharge lamp [11] makes possible to run the source at pressure as low as about 10 2 torr, so that reabsorption at short wavelengths can be avoided. Incidentally, because of the low flow rate, this technique also opens the possibility of investigating expensive isotopic species. The lamp is operated at 350 V400 ma. The spectra have been photographed in the first order of the Meudon Observatory Eagle mounting V.U.V. 10 metre concave grating spectrograph [12]. The instrument is now fitted with a concave (R = m), lines/mm JobinYvon interference (holographic) grating which gives a plate factor of nm jmm and a resolving limit of nm (i.e cm1 at 80 nm) with a 15 pm slit width. The observed emission lines are quite weak : on Kodak SWR plates the required exposure time is about Article published online by EDP Sciences and available at

3 Predissociation Measured hours, i.e. 30 times longer than that required in the wavelength region near 100 nm. As already pointed out the main difficulty lies in wavelength calibration because of the sparsity of standard lines in the far U.V. Reference lines are obtained in two ways : Ar 1 by flowing argon in the low pressure discharge lamp itself operated at 500 V 400 ma, Cu II and 0 1 by flowing helium in an auxiliary water cooled windowless hollow cathode discharge lamp operated at 600 V, 500 ma. The émission lines of H2 being far apart in this spectral range it is not confusing to superimpose the reference lines to the investigated spectrum in order to inçrease the accuracy. The plates are measured on the Meudon Observatory photoelectric comparator [13] which gives an accuracy of 1 accuracy is estimated to be nm ( ± 0.15 cm 1 near 80 nm). pm on line position. The wavelength 3. Results and discussion. and preionization have been definitely established in the absorption spectrum of H2 by broadening, asymmetric line shape and apparent emission of numerous R and P rotational lines, but no broadening of Q rotational lines has ever been detected [14, 15, 16, 17]. Using the absorption data of Monfils [2] and Takezawa [3] it is easy to identify the few rotational lines of H2 and D2 spectra showed on figures 1 and 2 respectively. All of the identified lines are Q(J) lines, with J 1, 2, 3 in H2 and 1, 2, 3, 4 in D2, while R and = P lines are entirely absent. The exponential decrease of intensity, preventing observation of higher J lines, is clearly modulated by intensity alternation typical of spin statistics in H2 and D2 [18]. The observed lines form the (v, v" 1) vibrational progression of the 3p7cD 17: X 1 Lg+ transition. As a matter of fact the state D 1 llu+ (upper state of R and P lines) predissociates through rotational interaction with the B 1+ state which dissociates into H(ls) + H(n 2). = This is not the case of D Ilu (upper state of Q lines). The appearance of Q(J) lines in our spectra is also in accordance with recent measurements of predisso Table 1. wavelength À (nm) and derived wavenumber (J (cm l) of the emission lines Q(J) forming the (v, v" = 1) vibrational progression of the 3pnD lllu X 1 transition in H2. In the last column are listed the values of «a recalculated using the graphical values of Bv. Fig. 1. Emission spectrum of H 2 in the far ultraviolet. Reference lines are from 0 I and Cu I1. AU the identified reference lines are marked at top of the spectra.

4 Same Energy 1433 ciation [8, 9] and of preionization yields [19], which have been found negligible for the lines under consideration. Some weaker lines belonging to the (v, v" = 0,2) progressions of the same DX transition have been detected but not measured. No photograph is given of spectra taken with HD as no line other than very weak H2 and D2 lines could be detected whereas in the wavelength range longer than 100 nm we have clearly recognized known HD lines [20]. It is not too surprising that all excited levels lying above the dissociation limit are predissociated in HD, since this molecule is heteronuclear and therefore has lower symmetry than H2 and D2. The asymmetry yields additional vibronic coupling effects as has been demonstrated by Dabrowski and Herzberg [20] for the ground state. Table II. as table I for D2. The measured wavelengths and the corresponding wavenumbers are listed in tables 1 and II. Ground state energy level values of Stoïcheff [21] allow us to derive the values E,(J) of the D 1 n u state, referred to the bottom of the ground state potential energy curve. They are listed on table III. To a first approximation Therefore, for each value of v, Ev is plotted versus J(J + 1). We verify that we get straight lines whose slope Bv and ordinate Ev(O) at J 0 = are determined graphically. In table IV our values of BU are compared to the ab initio values computed by Kolos and Rychlewski [6]. The agreement is to within cm 1 for H2 and cm1 for D2. Previous comparison with experimental results [2] gave discrepancies of Table III. level values Ev(J) in cm of the D Hù state, referred to the bottom of the ground state potential energy curve.

5 Rotational Comparison Comparison 1434 Table IV. between graphical Ev values and ab initio Bv values [6] cm1 for H2 and 0.28 cm l for D2 [6]. The experimental B,, values cannot be further refined because R and P branches are missing in emission. As not enough J values are observed, it has no meaning in the deve to introduce the next term Dv[J(J + 1)] 2 lopment (3.1) of E,,(J). However, we have adopted D, values of cm1 for H2 and cm 1 for D2 in order to minimize the systematic discrepancy between our data and those of Kolos and Rychlewski. The curve B(v) displayed on figure 3, is perfectly smooth, proving definitely that the state H ù does not experience any perturbation that could arise from configuration mixing. Table V. between values of AG(v + 1/2) (cm l) obtained from graphical determination of Ev(O) to those obtained from ab initio calculation [6]. compute, by a least square fit, the five constants in the development Fig. 3. constant Ev (cm 13) as a function of v. From the values of E,(O) we derived vibrational spacings AG(v + 1/2), listed in table V, and compared to ab initio values [6]. The values Ev(O) are then used to In turn, the derived constants are used to draw smooth values of Ev(0) and of OG(v + 1/2). These values of AG are included in table V. In tables VI and VII our experimental values Ev,(I)E,,,(O) are compared to the ones computed by Jungen and Atabek using the multichannel quantum defect theory (M.Q.D.T.) [7]. Their values represent a fit of the quantum defect curve /131t(R) to the levels observed by Takezawa. The present agreement is to within ± 0.50 cm 1 for H2 and ± 0.42 cm 1 for D2. Comparison to previous experimental data gave a mean deviation of 0.54 cm l and 0.39 cm l respectively, for H2 and D2. It is interesting to note that our set of data gives slightly better agreement where in addition the scatter is now the same in H2 and D2. This shows that the fitted quantum defect curve of reference [7] must be quite accurate. It has to be pointed

6 Comparison Same Molecular The From 1435 between the experimental values of Ev,(I)Ev"(O) and the values calculated by the Table VI. multichannel quantum defect theory (M.Q.D.T.) [7]lor H2. The comparison between previous experimental values [4] and M.Q.D.T. is recalled. Table VII. as table VI for D2. out that the value v 12 is excluded from the calculation of the mean deviation in the case of H2 = as the M.Q.D.T. calculation was not completely refined at large R values [22]. Table VIII. constants (in cm l) of the state 3pnD lllu of H2 and D2. Finally our Bv values are put into the development A least square fit gives the nine molecular constants that are listed in table VIII. Previous values of the literature are tabulated in [23]. 4. Concluding remarks. a limited amount of experimental data we have been able to derive molecular constants giving fair agreement with theoretical calculations. The next step will consist in measuring emission spectra of H2, HD and D2 in the region between 85 and 105 nm where no precise measurement has ever been reported. The work is under way at Meudon Observatory. Acknowledgments. authors wish to thank Dr. V. Kaufman (N.B.S., Washington, D.C.), for advices in design of the hollow cathode, Dr. Ch. Jungen (Orsay) for critical reading of the manuscript and Mr. M. Benharrous for assistance in the experimental work. 94

7 1436 References [1] RONCIN, J.Y., DAMANY, H., JUNGEN, Ch., «V. U. V. Radiation Physics», Edited by E. E. Koch, R. Haensel and C. Kunz (PergamonVieweg, Braunschweig) 1974, p. 52. [2] MONFILS, A., J. Mol. Spectrosc. 15 (1965) 265 and 25 (1968) 513. [3] TAKEZAWA, S., J. Chem. Phys. 52 (1970) 2575 and private communication cited in [7]. [4] HERZBERG, G., Spectra of Diatomic Molecules (Van Nostrand, Princeton) [5] KAUFMAN, V., EDLEN, B., J. Phys. Chem. Ref. Data 3 (1974) 825. [6] KOLOS, W., RYCHLEWSKI, J., J. Mol. Spectrosc. 62 (1976) 109. [7] JUNGEN, Ch., ATABEK, O., J. Chem. Phys. 66 (1977) [8] GLASSMAUJEAN, M., BRETON, J., GUYON, P.M., Chem. Phys. Lett. 63 (1979) 591. [9] GUYON, P.M., BRETON, J., GLASSMAUJEAN, M., Chem. Phys. Lett. 68 (1979) 314. [10] BRETON, J., GUYON, P.M., GLASSMAUJEAN, M., Phys. Rev. A. 21 (1980) [11] ANVAR, Contract Nb , Instrument S.A. France. [12] LAUNAY, F., Le Courrier du C.N.R.S. 12 (1974) 10. [13] LAUNAY, F., «Proceedings of the International Conference on Image Processing Techniques in Astronomy», Utrecht, March 1975 (Reidel, Dordrecht, Holland) [14] NAMIOKA, T., J. Chem. Phys. 41 (1964) [15] COMES, F. J., SCHUMPE, G., Z. Naturforsch. 26a (1971) 538. [16] HERZBERG, G., «Topics in Modern Physics. A tribute to E. U. Condon» (Colorado Univ. Press) 1971, p [17] HERZBERG, G., JUNGEN, Ch., J. Mol. Spectrosc. 41 (1972) 425. [18] Ref. [4], p. 207 and 134. [19] DEHMER, P. M., CHUPKA, W. A., J. Chem. Phys. 65 (1976) [20] DABROWSKI, I., HERZBERG, G., Can. J. Phys. 54 (1976) 525. [21] STOICHEFF, B. P., Can. J. Phys. 35 (1957) 730. [22] Ref. [7] p and Fig. 6. [23] HUBER, K. P., HERZBERG, G., «Constants of Diatomic Molecules» (Van Nostrand Reinhold Co.) 1979.