Spectrum of ASS. I. Analysis of the A', H3, , -X2 l7 , Band System

MIDORI SHIMAUCHI' Institute for Optical Research, KyBiku University, 400, Hyakunintyo-4, Sinzyuku-ku, Tokyo, Japan

Received December 1, 1970

The emission spectrum of the ASS radical, excited in a quartz tube by a 2450 MHz oscillator, was photographed on a high resolution spectrograph from 2450 to 6900 A. Seven bands around 6000 A showing clear rotational structures were chosen for the first rotational analysis of the ASS spectrum. The bands were found to arise from a 2r1312-2r1312 transition. The rotational and vibrational constants of the two states derived from the present work are consistent with the previous vibrational analysis of the A'2r1312-X2rI,12 system. The constants of the upper doublet component of the ground state, X2r13,2, are o, = 562.40 cm-', o,x, = 2.02 cm-', re = 2.0216 A; the constants of the Ar2r13,, state are AG'(1/2) = 403.37 cm-', vOeo = 18 621.21 cm-', re = 2.2500 A.

Le spectre d'imission du radical ASS a CtC photographie sur un spectrometre de haute resolution de 2450 a 6900 A. Le radical ASS est excite dans un tube de quartz par un oscillateur de 2450 MHz. Sept bandes observees autour de 6000 A montrent clairement les structures dues a la rotation. On a trouvi que ces bandes proviennent de la transition 2r1312-2r1312. Les constantes de rotation et de vibration de ces deux etats sont dCriv6es dans le present travail et les valeurs calculees concordent trts bien avec celles obtenues prkcedemment lors de I'analyse des vibrations du systeme A'zr1312-X2r1312. Les constantes du doublet superieur de I'Ctat fondamental, X 2 n 3 / 2 , sont: o, = 562.40 cm-', o , X , = 2.02 cm-', r, = 2.0216 A; et les constantes de l'etat Af2r13, , sont: AGJ(1/2) = 403.37 cm-', v,,, = 18 621.21 cm-I, re = 2.2500 A. Canadian Journal of Physics, 49, 1249 (1971)

1. Introduction system and made a partial rotational analysis of

Although a good deal of work has been done on the spectrum of the As0 molecule since 1909 (de Watteville 1909), the first report on the spec- trum of the ASS molecule was published only very recently (Shimauchi 1969). More than 200 bands attributable to the emission spectra of ASS and Assf radicals were obtained from a hollow cathode discharge. As a result of the vibrational analysis, a singlet system extending from 2500 to 3000 A, and a doublet system extending from the visible to the near infrared region, were assigned to the A'II-X'C system of Assf and the Al2IIi- X217 system of ASS, respectively. The electronic states involved in these systems were named in the same way as for AsOf and AsO, since the analyzed states appeared to correspond. The A'-X system of As0 was first observed by Mrozowski and Santaram (1966); on the basis of the vibrational analysis they concluded that the upper A' state is an inverted 'l7 state. This con- clusion and their assignment were confirmed by Goure and D'Incan (1969), who extended the

'Visiting scientist at the Division of Pure Physics, National Research Council of Canada, Ottawa, in the summer of 1969.

a few bands. In order to investigate the rotational structures

of the spectra of ASS and Assf the 10 m concave grating spectrograph at the National Research Council of Canada, Ottawa, was used to photo- graph the spectrum in the region from 2450 to 6900 A. Seven bands showing clear rotational structures in the region from 5750 to 6850 A have been analyzed. These bands are assigned as the 0-3, 0-4, 0-5, 0-6, 0-7, 1-3, and 1-4 transitions of the A'217,12-X2~312 system of ASS.

2. Experimental The discharge tube used in this experiment has

a simple shape as shown in Fig. 1. It was con- structed from quartz tubes of 12 and 20 mm diameters. A discharge was expected by placing the small diameter portion of the tube in the cavity of a 2450 MHz oscillator. To prepare the tube it was first evacuated and degassed by being heated up to 1000 "C in a furnace, and then tiny fragments of crystalline arsenic disulfide (around 50 mg) were put into the tube without breaking the vacuum. The purity of the sample, As,S,, was 99.999% and no atomic line other than As and S was found in the 2500-8900 A region. First the tube was filled to a pressure of around 10 Torr

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CANADIAN JOURNAL OF PHYSICS. VOL. 49, 1971

- 2 0 0 m m _____)

FIG. 1. Discharge tube.

with helium which had been purified by charcoal traps immersed in liquid nitrogen, and then a microwave power of less than 100 W was supplied to the tube, while it was slowly evacuated. This process was repeated several times for purifica- tion. There was a certain pressure of helium a t which the ASS bands became strong and the S, emission in the visible region very weak. At this pressure, presumably 0.5 Torr or less, the tube was sealed off.

In operation, the power was adjusted to keep the color of the discharge as white as possible; it was difficult to completely suppress the S, bands which give rise to a blue color in the discharge. Under the best conditions of the discharge a part of the tube outside the cavity became covered with orange films of As,S,. This deposited sample had to be distilled back into the discharge by moderate heating, because after an exposure of about 20 min the color of the discharge changes to orange yellow. The spectral region from 2450 to 6900 A was photographed on Kodak 103a0 and 103aF plates, in the second, third, and fourth orders of 10.7 m spectrograph fitted with a 600 lineslmm concave grating blazed at 11 600 A. The spectra shown in Figs. 2 and 4 were taken in the second order; the reciprocal dispersion was about 0.7 Almm. Exposure times varied from 10 min to 2 h on Kodak 103aF; the slit width was 50 p. Reference spectra were provided by a neon- filled, iron hollow cathode lamp. Standard wave- lengths were taken from the table of Crosswhite (1958), and the vacuum corrections of Edltn (1953) were applied. Most of the plates used for this analysis were measured with a Tomkins-Fred-type comparator at NRCC, but a few of them were measured with a digitized measuring microscope, Model L128, of Hilger and Watts Ltd. Although the measuring accuracy with the latter instrument is somewhat inferior to that of the former, the least-squares fitting for a sufficient number of reference lines made the

errors of both measurements almost equal. The relative wavenumbers of lines should not be in error by more than 0.02 cm-'. Microdensitom- eter traces, yielding a magnification of 80 in dispersion, were obtained for investigating the detailed structures near band heads. An example of these traces is shown in Fig. 4 for the 0-5 band.

3. Analysis and Results

The bands shown in Fig. 2 display the fine structure most clearly, since they are quite free from the overlapping of other bands. Each band is shaded to the red and consists of single P and R branches. According to the vibrational analysis in the previous report (Shimauchi 1969) they are the &5, 0-6, and &7 bands of the A'21131,- XZl131z system of ASS. It should be noted that in the &7 band all the P branch lines coincide with R branch lines. In the &3 and 1-4 bands the P and R branches again coincide, though the difference between coinciding J values is smaller by one than in the 0-7 band. Relative positions of P and R lines in bands of different sequences are changed in a way shown schematically in Fig. 3.

The rotational analysis of the seven bands, &3, 0 4 , 0-5, 0-6, 0-7, 1-3, and 1-4, was com- pleted without ambiguity, since the same A,F(J) values were obtained for each of the common upper or lower state. For the bands in which the P and R lines coincide, the A,F(J) values of the common state differed systematically by up to 0.08 cm-' for some Jvalues. The observed wave- numbers of the individual band lines are given in a table which has been placed in the Depository of Unpublished Data.' Not having observed a splitting in any line, we assumed that the rota-

2~hotocopies of material in the depository may be obtained upon request from: Depository of Unpublished Data, National Science Library, National Research Council of Canada, Ottawa 7, Canada.

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SHIMAUCHI: SPECTRUM OF Ass. I

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1252 CANADIAN JOURNAL O F PHYSICS. VOL. 49, 1971

L

P : 24 2 5 2 6 FIG. 3. Variation schematically shown for relative

positions of P and R lines in bands belonging to different sequences.

tional levels could be represented by a simple formula which should be applicable for a doublet sub-band arising from the transition between case a 'll states with large coupling constants. The band origins and rotational constants were determined by the traditional graphical method (Herzberg 1950); several kinds of least-squares computer programs were used for accurate deter- mination of all the numerical values. The rota- tional energy levels of all states could be fitted by using terms up to the second order in J ( J + 1). The best rotational constants of the v' = 0 level of the excited A'211312 state were obtained by a least-squares fit to 185 unweighted combination differences (with J values up to 73) which were obtained from well-resolved bands only, i.e. the M, 0-5, and 0-6 bands. When the J values were shifted by one from the correct values upward or downward, the errors in the least-squares fit became more than 10 times the errors with the correct numbering. Even for J values shifted by only one half from the correct values, the errors far exceeded the relative accuracy of the present measurement, 0.02 cm-'. Therefore, the states involved in this system cannot be singlet, and the absolute J numbering is quite unambiguous. Figure 4 is a microdensitometer trace near the

band origin of the 0-5 band, in which no peak can be observed at the calculated position of the R(+) line. Q(J) lines are too weak to permit reliable assignment. Possibly the asymmetrical shape of the R(l0f) line can be ascribed to the existence of weak Q(24) and Q(3f) lines, which are expected to be at the shorter and longer wavelength sides of the R(lO4) line, respectively; the latter should be nearer the center of the R(10+) line than the former. The lack of A-type splitting even in the high J lines is apt to occur in a ll-ll transition, and this is particularly true of the '113/, com- ponent of a 'll state, where the A splitting is expected to be small. Thus the band shows the characteristics of a 2113~2-211312 transition.

Since the rotational constants of the upper v' = 0 level were determined most accurately, all the B,,, and D,.. (v" = 3,4, 5,6, and 7) values of the lower states were determined relative to the levels calculated from the following BO1 and Do' values:

BO1 = 0.148258 + 0.00036

The constants of the upper v' = 1 level were estimated from the 1-4 and 1-3 bands using the constants of the lower states. Observed P and R lines were most closely approximated when the constants obtained by the least-squares fit to the following formula were used for calculating reconstructed lines :

where v, = v,' + Fv,' means the band origin. Rotational constants of the upper and lower states are listed in Table 1 ; all the errors quoted are four times the probable errors. By using the constants pertinent to them, the detailed struc- tures of the 0-3, 1-4, and 0-7 bands are explained

TABLE 1. Rotational constants of ASS (cm-')

v" B," D," x lo-'

XZI13,2 3 0.18116 + 0.00008 8.6 + 1.7 4 0.18033 10.00004 8.7 1 0.8 5 0.17950 + 0.00004 8.8 + 0.7 6 0.17865 + 0.00005 9.1 + 0 .9 7 0.17782 + 0.00004 9.7 1 0.8

v ' B,' D,' x lo-'

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SHIMAUCHI: SPECTRUM OF ASS. 1

FIG. 4. Microdensitometer trace of the 0-5 band near the band origin.

TABLE 2. Band origins (cm-') of the ASS ~ ' ~ n ~ , ~ - X ~ n 3 , ~ system

\ --

TABLE 3. Molecular constants of ASS - --

Upper state Lower state A"n3,Z xzn3,z

B. (cm-') ci. (cm-') D. (cm-,'I S e (cm- 1- (g cm2) r, (cm) W. (cm-') W,X, (cm-') TO,O (cm-')

*Computed from Be, a., and AG(1/2) = w. - 2w.x,, using Pekeris's formula (Pekeris 1934). Vibrational constanls in paren~heses are previous values.

as follows. In these bands all the observed P(J) and R(J + d ) (here d is an integer) lines are coincident within the line width; however, the J values of precisely coinciding lines should be 28.3,34.4, and 72.2 in the 0-3, 1-4, and 0-7 bands, respectively. In all of them, for low J values, the R(J + d ) lines should have slightly shorter wave- lengths than the P(J) lines. These structures are in accordance with those deduced from observed wavenumbers of overlapping lines. Since the B, values of the lower state were better fitted by the first order than by the second-order least squares in (u + +), the equilibrium constants were ob- tained from the formulas

The results are given in Table 3. For the upper state the errors of the equilibrium constants are not given because the amount of data is too small to apply the method of least squares.

A Deslandres table of band origins is given in Table 2. The AG(v" + 3) values of the lower state show a linear variation with u", i.e.

[3] AG(vl' + +) = o," - 2oet'xe"(u" + 1)

The o," and o,"xe" values fitted in this way are listed in Table 3, and compared with the old

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CANADIAN JOURNAL OF PHYSICS. VOL. 49, 1971

values in parentheses. The difference between the old and new values of me" or me"xe" is larger than expected; however, it was found that the previous measurement of head positions suffered from considerable interference by the presence of other bands or atomic lines because of the low dispersion of the prism spectrograph used. This was particularly true of the region where the bands involved in high v" levels of X 2 n , 1 2 appeared. Consequently, the values of me" and m,"x," obtained in this work are revised ones as is the new v,,, value. For the upper state, experi- mentally known values are only B e f , a,', and AG1(f ) . From these values we estimate oe1x,' and me' at 1.44 cm-' and 406.25 cm- ', respec- tively, by assuming that Pekeris's theoretical formula (Pekeris 1934), i.e.

ever, rotational analyses of the ~ ' ~ n ~ ~ ~ - X ~ n ~ and other systems are necessary in order to find out whether the upper A " I ~ state is really inverted or not. Our investigation for this purpose is now in progress.

Acknowledgments

The author wishes to thank the National Research Council of Canada for use of their laboratory. She wishes to express her sincere gratitude to Dr. G. Herzberg and Dr. A. E. Douglas for helpful advice during this work and for suggesting several improvements in the manu- script. Thanks are also due to Mr. F. Alberti for his expert assistance in experiments. Miss Y. Nishiyama's work in computer programming is also much appreciated.

G J / ~ , x , B , ~ 6Be2 CROSSWHITE, H. M. 1958. Johns Hopkins Spectrosc. Rep. c41 a, = -- No. 13.

me 0, DE WATTEVILLE, M. C. 1909. Z. Wiss. Phot. 7, 279. EDLBN, B. 1953. J. Opt. Soc. Amer. 43, 339.

is and thus estimated agree GOURE, J. P. and D'INCAN, J. 1969. C.R. B, 268, 1311. fairly well with those previously obtained from HERZBERG, G. 1950. Spectra of diatomic molecules several AG1(v' + 4) values based on a vibrational @. Van Nostrand Co. Inc., Princeton, N.J.).

analysis of band-head data. MROZOWSKI, S. and SANTARAM, C. 1966. J. Opt. Soc. Amer. 56, 1174. No result contradictory to the previous vibra- PEKERIS, C. L. 1934. PhyS. Rev. 45, 98.

tional analysis has appeared in this work. How- SH~MAUCH,, M. 1969. Sci. Light, 18,90.

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