M. Douay et al- New Observations of the A^1-Pi-u-X^1-Sigma-g^+ Transition (Phillips System) of C2

8/2/2019 M. Douay et al- New Observations of the A^1-Pi-u-X^1-Sigma-g^+ Transition (Phillips System) of C2

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J OURNAL OF MOLECULAR SPE CTROSCOP Y 331,250-260 (1988)

New Observations of the A&-X x.9 Transition(Phillips System) of C2

M. DOUAY, R. NIETMANN, AND P. F. BERNATHDepartment of Chemistry, University of Arizona, Tucson, Arizona 85721

We have reanalyzed the infrared part of the A II&Y 2: (Phillips system) of Cx. Improvedmolecular constants were derived and the u = 5 and u = 6 vibrational levels of the X Zi statewere observed for the first time. RKR curves and A I&-X 2: Franck-Condon factors werecalculated from the equilibrium molecular constants. Q 1988 Academ i c p r ess h c .

I. INTRODUCTIONThe CZ molecule is found in a very wide variety of sources. C2 occurs in comets

(Z-3), interstellar clouds (d-11), the sun (I2-Z4), and in stellar atmospheres (15,16). In the laboratory, C2 is abundant in flames (17-191, explosions, and electricaldischarges of carbon-containing molecules.In 1948, Phillips (20) discovered the A l&,-X Zg electronic transition of Cz whichbears his name. Ballik and Ramsay (21) observed many additional bands of the Phillipssystem and in 1970 Marenin and Johnson provided a consistent set of spectroscopicconstants (22). More recently, highquality Fourier transform data for *Cz (23, 24),

12C13C, nd 13C2 25) have become available.The oscillator strength of the A IL-X 2: transition is still controversial with manyexperimental (26-29) and theoretical estimates (30-33). The A-X oscillator strengthis relatively weak ( - 10e3). An accurate value is required to convert observed C2absorption spectra into an estimate of Cz concentration (or, more precisely, into a C2column density).The classic work of Ballik and Ramsey (34) proved the ground state of C2 to beX Zz rather than a311, by the observation of singlet-triplet perturbations. As a result,there have been several ab initio studies of the interaction between the singlet andtriplet manifolds (35-37). These perturbations have important astrophysical conse-quences because they (along with quadrupole radiation) permit the homonuclear CZmolecule to relax and cool (37). Additional ab initio calculations of the molecularproperties of Cz have been carried out by many workers (38-43).Many molecules fragment upon absorption of ultraviolet radiation or multiphotonabsorption of visible or infrared photons, usually releasing simple molecules such asCN, CH, and C2. The photochemical production of C2 provides a convenient molecular

Present address: Lab. de Spectroscopic des Mol&cules Diatomiques, CNRS UA779, Univetiti des Scienceset Techniques de Lille, B&t. P5, 59655 Villeneuve dAscq Cedex, France.2 Alfred P. Sloan Fellow; Camille and Henry Dreyfus Teacher-Scholar.

0022-2852/88 $3.00Copyright 1988 by Academ iC &ss. Inc.Al l r ightsof reproductionn any f o r m r eS eWed .

250


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C2 A Il.-X 2; TRANSITION 251source for the study of CZ reaction kinetics, Cz quenching and C2 triplet-singlet in-terconversion (44-51). CZ is also formed in the reaction of hydrocarbons with raregas me&stables ( 52).

In the course of our discovery of the B A,-A II,, and B Z:- A II, electronic tran-sitions (53) by infrared Fourier transform emission spectroscopy, we noted manypreviously unobserved bands of the Phillips system. A simultaneous fit of all of ourobserved singlet C2 data has provided improved spectroscopic constants for 2, = O-5of the A II, state and o = O-6 of the X Zg state. The u = 5 and 6 levels of theX 2: state were not directly observed in any previous work.

II. EXPERIMENTAL DETAILS

Strong emission spectra of the C2 molecule were found in two data sets previouslyrecorded for other purposes. These spectra were excited by an electrodeless microwavedischarge of hydrocarbons in a quartz tube. In one run a mixture of 2.75 Torr of He,0.030 Torr of CH4, and 0.040 Torr of white phosphorous vapor flowed through thedischarge tube. The phosphorous is, presumably, not required. The second spectrumwas recorded from a discharge of 3.3 Torr of argon and 0.80 Torr of allene. The firstspectrum was extremely rich with strong spectra of CO, CH, PH, CP, Pz, ArH, andCN, as well as Cz . The second data set had a reduced signal-to-noise ratio for Cz, butwas not as congested.The emission from the discharge tube was observed with the McMath Fourier trans-form spectrometer of the National Solar Observatory at Kitt Peak. InSb detectors anda silicon filter limited the band pass to approximately 1800-9000 cm - . The unapo-dized resolution was set to 0.02 cm-. In the first spectrum, 10 scans were coaddedin 70 min of integration, while in the second spectrum 8 scans were recorded in58 min.The first spectrum was calibrated with the vibration-rotation lines of the impuritymolecule CO (54) near 2200 cm-. The calibration factor of 1 + 5.74 X lo- wasapplied to the entire spectrum. The CO lines were much weaker in the second spectrumso the calibration was transferred to this spectrum using several strong lines of the O-0 band of the b3 Z ;-a311, ( Ballik-Ramsay ) system. The absolute accuracy of thecalibration is estimated to be better than kO.001 cm- for both spectra. The precisionof the strong, unblended lines in the O-O band of the A I I&,-h Z,+ transition is about0.0002 cm-.

III. RESULTS AND DISCUSSION

The interferograms were transformed by G. Ladd to provide the two spectra. Theline positions were extracted from the spectrum with the data reduction programDECOMP developed at Kitt Peak. The peak positions were found by fitting a Voigtlineshape function to each feature using a nonlinear least-squares procedure. Sincethe spectra are very congested, we proceeded initially by predicting the line positionsand then picking out and assigning just a few lines. The preliminary fit of these linesprovided an accurate prediction of the remaining lines in the band. Our search fornew bands was guided by a preliminary Franck-Condon calculation.


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252 DOUAY, NIETMANN, AND BERNATHTABLE I

Observed Line Positions of the A II.-,!? Xi Transition of C2 (in cm-)

-10

-2 --8-325

-40-8-223

9r. o-c p.. o-c

*98

Note. Observed minus calculated line positions are in units of IO- cm-. Perturbed.


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Cz A l-I, -X X; TRANSI TION

TABLE I-Continued253

F.-

_

0-9

-36-11

-96

-212

0

232-2-3

-886

338

-6-211-4


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254 DOUAY, NIETMANN, AND BERNATHTABLE I-Continued

K. 3-5 BAND

J R.. o-c Qr. 0-c P.. o-c10.012.014.016.018.020.022.02a.o26.028.0

9-7

6-222-4

9

9-1

-54

L. 4-6 BAND

2.04.06.08.0

10.012.014.016.0

3956.12193957.62543957.88763956.93533954.47973951.1233

o-c48

Qr.

3937 97633934.58623929.94673924.07783917.00173908.42883898.9650

o-c

In the end, 12 bands were identified and analyzed: O-O, 3-3, 4-4, 5-5, O-l, l-2,2-3, O-2, l-3, 2-4, 3-5, and 4-6. The 3-3, 4-4, 5-5, 3-5, and 4-6 bands are new.The observation of bands in the Phillips system with D = 5 and 6 provides accurateconstants for these high-lying vibrational levels of the ground state. The observed linepositions and rotational assignments are provided in Table I.The lines of Table I and all of the observed lines of B A& II, andB E&4 II, were fitted simultaneously with the customary energy level expressionfor each vibration level in each electronic state:T, + B,J(J+ 1) - Dv[J(J+ l)]* + Hv[J(J+ 1)13

* &AJ(J + ~>EG + CZD~J(J+ 1 ) 1 / 2 .

TABLE IIMolecular Constants for the X Zt State of C2 (in cm - )


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C2 A II -X Z+ TRANSITION8 255TABLE III

Molecular Constants for the A II, State of C2 (in cm -I)

constant v-o =, v = z " = 3 " = 4 v - 51 " 8 2 6 8 . 3 8 9 6 2 ( 1 5 ) a 9 8 5 2 . 4 3 5 5 ( Z ) 1 1 4 1 2 . 2 6 4 3 ( 4 ) 1 2 9 4 7 . 8 1 9 5 ( 5 ) 1 4 4 5 9 . 0 3 4 3 ( 7 ) 1 5 9 4 5 . 8 2 8 5 ( 2 3 )

6 " 1 . 6 0 8 1 3 4 5 ( 2 0 ) 1 . 5 9 1 0 9 3 1 ( 2 2 ) 1 . 5 7 3 9 7 2 3 ( 3 0 ) 1 . 5 5 6 7 6 0 9 ( 3 0 ) 1 . 5 3 9 4 4 8 6 ( 4 1 ) 1 . 5 2 2 0 4 7 ( 2 5 )1 0 ' x D" 6 . 5 2 5 6 9 ( 1 1 4 ) 6 . 5 3 6 1 4 ( 1 4 5 ) 6 . 5 7 3 1 1 2 8 ) 6 . 6 0 2 6 ( 2 4 ) 6 . 6 2 8 9 ( 3 8 ) 6 . 6 5 7 ( 4 8 )I O ' x q y - 1 . 9 5 7 3 ( 3 4 ) - 1 . 9 2 6 5 ( 2 5 ) - l &3 1 3 ( 5 5 ) - 1 . 8 7 3 1 ( 7 0 ) - 1 . 8 6 5 9 ( 8 9 ) - 1 . 8 4 ? ( 7 5 )1 0 S x 9 D" 5 . 0 @ ( 2 7 )-

a T h e n u mb e r s i n p a r e n t h e s e s a r e o n e s t a n d a r d d e v i a t i o n i n t h e l a s t d i g i t .

Approximate weights were chosen for each line on the basis of the signal-to-noiseratio, as well as freedom from blending or perturbations. The energy origin of this fitwas 2) = 0, J = 0 of the X I 2; state and h-doubling was included only for the A II,state. The spectroscopic constants for the X 2; and A II, states are reported inTables II and III.The constants of Tables II and III were converted to equilibrium molecular constants(Table IV) using the expressions

G(v) = We(V+ 4) - w,x,(u + 4) + W&(V + 4) + w,z,(v + 1) -I- W&,(2, + 1)B, = B, - a,(~ i)+ ye(u h)'+ 6,(v t)'

TABLE IVEquilibrium Molecular Constants for the X Xi and A II, States of C, (in cm - )

XZ+8 1855.0142(129)~ 13.5547(124) -0.1321(50) 0.00357(89) -0 001116(57)A'TI" 1608.1990(52) 12.0597(27) -0.010555(39) -

B. a. -f* x 10-s 6, x 106X$ 1.820099(37) 0.018012(63) -6.33(286) -2.06(37)A'll, 1.6X6275(27) 0.0169691(51) -3.34(25) -0.154(33)

D, x 10-e 0. x 10-n T.

Xg 6.9640(124) 6.41(69)A'll" 6.5086(54) 2.53(29) 8391.4085(46)

A'II" -1.9676(70) 2.74(37)

a One standard deviation uncertainty in parentheses.


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256 DOUAY, NIETMANN, AND BERNATHTABLE V

RKR Turning Points for the X Z+ State of C2 &(cm-) %i, (A) %.x (A)

0.0 923.9840 1.19052 1.300930 . 5 1841.2118 1.17072 1.327351.0 2751.4689 1.15615 1.348601.5 3654.6561 1.14428 1.367232.0 4550.6670 1.13413 1.384232.5 5439.3835 1.12519 1.400093.0 6320.6724 1.11717 1.415133.5 7194.3803 1.10988 1.429554.0 8060.3301 1.10317 1.443484.5 8918.3167 1.09696 1.457045.0 9768.1023 1.09117 1.470305.5 10609.4130 1.08575 1.483346.0 11441.9340 1.08064 1.49621

4 = qe+ aq(u + t>.The large number of expansion constants required for the B, nd G(u) expansions(Table IV) is probably a reflection of the global perturbations and interactions amongthe low-lying states of Cr.

The B, nd G(v) expansions were input to an RKR program to calculate the classicalturning points (Tables V and VI) of the X 2: and A II, potential curves. Figure 1is a plot of the potential energy curves of the low-lying X Zi, A II,, B%, and

TABLE VIRKR Turning Points of the A II, State of Cz

Y %(cme) bin (A) G., (A)0.0 801.1033~ 1.26266 1.389250.5 1596.1488 1.24151 1.409751.0 2385.1486 1.22597 1.432691.5 3168.0948 1.21333 1.452802.0 3944.9795 1.20253 1.471122.5 4715.7947 1.19302 1.488223.0 5480.5326 1.18449 1.504393.5 6239.1853 1.17674 1.519864.0 6991.7448 1.16961 1.534764.5 7738.2031 1.16301 1.549215.0 8478.5525 1.15685 1.56329

a Relative to the bottom of the A'II, ell. To convert the origin of

the E, scale to the bottom of the X'g veil, 8391.2703 cm_1 mustbe added. This number was calculated using the experimental alueof 8268.3896 cm-' for T,,. Note that these potential curves includethe Dunham Y.. correction.


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cm-

C2 A II&? Z; TRANSITION 257

Rc. 1. The RKR potential curves of the low-lying singlet states of C2.

B I;: states of CZ as determined from our RKR turning points. The A III, andX Z,+potential energy curves were used to calculate the A-X Franck-Condon factors(Table VII). We found all of the bands expected on the basis of intensity estimatesfrom the Franck-Condon factors of Table VII.The Be values of Table IV result in re values of 1.242440 and 1.318311 A for theX 2: and A lIu states, respectively.

The spectroscopic constants of Tables II and III are the most useful result of ourreanalysis of the Phillips system. For example, the 2-O band near 8760 A is observedin absorption in interstellar clouds (5-11). Although we have not directly observedthe 2-O band, the line positions of this band, calculated with the constants of TablesII and III, should be more accurate than the previous measurements. In general, ourspectroscopic constants and line positions are in agreement with, but more accuratethan, the previous measurements.The line positions of Table I display three perturbations (Table VIII). In the 2-4and 2-3 bands, the e parity level of u = 2 of A II, with J = 19,2 1 is perturbed bythe corresponding Js ( Fz, N = 19, 2 1) e parity level of u = 1 of the c3 Zz state. Thisperturbation was predicted by Chauville et al. (23), and also observed by Davis et al.(24). This perturbation, along with others previously observed, allowed Davis et al.(24) to calculate improved spectroscopic constants for the c3 Z: state.In the 3-5 band, the J = 22,24,26, and 28 e parity levels of B = 5 of the X Z,+state are perturbed by u = 2 of the b3 2; state. In the 4-6 band the J = 12, 14 e levels


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258 DOUAY, NIETMANN, AND BERNATHTABLE VII

A iI& Zi Franck-Condon Factors

0 0 . 4 1 2 EO 0 . 3 9 8 E O 0 . 1 5 5 E O 0 . 3 1 l E - I 0 . 3 3 %- 2 0 . 1 8 4 E- 3 0 . 3 . l 3 E - 5I 0 . 3 3 2 EO 0 . 5 5 3 E - 2 0 . 2 8 9 E O 0 . 2 7 2 E O 0 . 8 8 3 E- 1 O. l 3 l E - 1 0 . 8 5 9 E- 32 0 . 1 6 2 EO 0 . 1 7 2 E O 0 . 5 7 8 E - 1 0 . 1 1 6 E O 0 . 3 0 5 E O 0 . 1 5 5 EO 0 . 3 0 l E - I3 0 . 6 3 0 E- I 0 . 2 O OE O 0 . 3 1 8 E- 1 0 . 1 4 2 E O O. l k %E - 1 0 . 2 7 l E O 0 . 2 1 6 EO4 0 . 2 1 5 E- 1 0 . 1 2 6 E O 0 . 1 3 8 E O O . l 1 6 E - 2 0 . 1 5 5 E O O. l 6 2 E - 2 0 . 2 0 4 EO5 0 . 6 8 4 E- 2 0 . 6 0 l E - 1 0 . 1 4 5 E O 0 . 5 7 B E - 1 0 . 3 5 8 E - 1 0 . 1 1 5 EO 0 . 3 0 2 E- 1

of u = 6 of the X1 2: state are perturbed by v = 3 of the b3 2; state. The corre-sponding perturbations were observed by Roux et al. (55) in the b3Z;-a3 II, Ballik-Ramsay system. The shifts in 21= 5, J = 26 and 28 for the X I&i state predicted byRoux et al. (55) are 0.101 and -0.17 1 cm- compared to our values (Table VIII) of0.070 and -0.217 cm-, respectively. For u = 6 of the X1 Zz state, the predictions ofRoux et al. are +0.074 and -0.127, while our values are 0.045 and -0.159 cm- forJ = 12 and 14, respectively. The agreement is reasonable, but not perfect, betweenthe two sets of measurements. The discrepancies are presumably caused by systematic,model-dependent errors in the prediction of the unperturbed line positions. The ob-servation of level shifts equal in magnitude (but opposite in sign) by us and by Rouxet al. (55) confirms the two sets of assignments.

TABLE VIIIObserved Perturbations in the A&-XZg+ System of C2 (in cm-)

P e r t u r b e d P e r t u r b i n g B a n d L i n e Oh s . - C a l c L i n e Oh s . - C a l c . L i n e Oh s . - C a l c .s t a t e s t a t e

3-5 R(22) 0 . 0 0 7 4 N2 2 ) 0 . 0 0 3 2 P ( 2 2 ) 0 . 0 0 2 6V= S v = 2 ~ ( 2 4 ) 0 . 0 1 4 7 ~ ( 2 4 ) 0 . 0 1 6 1 P ( 2 4 ) 0 . 0 1 4 9XI I ; b ' Z - R ( 2 6 ) 0 . 0 7 2 1 N2 6 ) 0 . 0 6 8 2 P ( 2 6 ) 0 . 0 6 9 4

R ( 2 8 ) - 0 . 2 1 1 3 QW) - 0 . 2 1 3 3 P ( 2 8 ) - 0 . 2 2 6 5v = 3b ' Z -9

4 - 6 R( 1 2 ) 0 . 0 4 3 6 0 0 2 ) 0 . 0 4 5 9R ( l 4 ) - 0 . 1 5 9 7 Q( 1 4 ) - 0 . 1 5 8 7

2 - 4 R ( 1 8 ) - 0 . 0 2 2 7 P ( 2 0 ) - 0 . 0 2 3 4v = 2 v = l R ( 2 O) - 0 . 0 1 9 8 P ( 2 2 ) - 0 . 0 1 8 2A%" C T: 2 - 3 R ( 1 8 ) - 0 . 0 2 2 7 P ( 2 0 ) - 0 . 0 2 4 2

R ( 2 ' 3 ) - 0 . 0 2 5 0 N2 2 ) - 0 . 0 2 2 0


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C2 A II& Z; TRANSITION 259CONCLUSION

We have reanalyzed the,4 J&,-X 2: Phillips system of Cz and have obsexved severalnew vibrational bands. The II = 5 and 6 levels of the X1 Zl state were observed forthe first time. Since Cz is such a ubiquitous molecule, our improved spectroscopicconstants should prove useful.

ACKNOWLEDGMENTST h e N a t i o n a lSo lar O b s e r v a t o r ys o p e r a t e d y t h e As s o c ia t i o n f U n i v e r s i t i e so r R e s e a r c h n As t r o n o m y ,

Inc., under contract with the National Science Foundation. We thank J. Wagner, R. Ram, and G. Ladd forassistance in acquiring o u r C z sp e c t r u m .Ac k n o w l e d g m e n t s m a d e t o t h e d o n o r s o f t h e P e t r o le u mR e s e a r c hF u n d , a d m i n i st e r e d y t h e Am e r i c a n C h e m i c a lS o c ie t y , o r p a r t i a l u p p o r to f t h i s w o r k . S o m e s u p p o r tw a sa l s o p r o v i d e db y t h e A ir F o r c e As t r o n a u t i cs a b o r a t o r yG r a n t N o . F 0 4 6 I 1-87-K-0020.RECEIVED: May 16, 1988

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M. Douay et al- New Observations of the A^1-Pi-u-X^1-Sigma-g^+ Transition (Phillips System) of C2

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