Spin-splitting analysis of the B2Σu+ (v = 0) state of C2−, using velocity modulation laser spectroscopy

JOURNAL

OF MOLECULAR

SPECTROSCOPY

155,427-429 ( 1992)

Spin-Splitting Analysis of the 8*2: (v = 0) State of C;, Using Velocity Modulation Laser Spectroscopy During the last 15 years negative molecular ions have had considerable attention. Numerous anions have been studied in the infrared with methods such as detachment spectroscopy and velocity modulation spectroscopy. The first negative ion to be studied in detail was CT ( I ) . This molecule is special in the sense that it is the only known molecular anion with a bound valence state with an optical transition to the ground state. It is also believed to play an astrophysically important role in the atmosphere of carbon stars. In their famous work from 1968, Herxberg and Lagerqvist (1) observed, in a flash absorption experiment in CH4, 2-Z bands which they assigned proposedly to CF. This assignment was continned shortly thereafter by Lineberger and Patterson (2) in a photodetachment experiment of C; . More recently, Mead et al. (3) have studied pertubations in the B state of C: above the electron affinity limit. This was done in high resolution, using a photodetachment technique. Using their results as input, Rehfuss et al. (4) could observe the transition between the low-lying A%. state and the ground state of CT, using velocity modulation technique.

RAW

coHPusE

RJ11.5)

-G@R,( 12.5) RESIDUW

FIG. 1. A typical decomposition of overlapping lines. In the upper trace is the observed spectrum, in middle trace a synthetic trial spectrum, and in the bottom trace is the difference between these two. Width and Doppler shift of the synthetic profiles is determined from the R (N = 0)line, which is the only line with a single spin component. The synthetic spectrum is varied in the decomposer program with respect to the amplitude and position of each individual trial profile until the difference between the observed and the composed spectrum is minimized in a least-squares fit, see Ref. ( 11) .

427

0022-2852192 $5.00 Copyright 0 1992 by Academic Press, Inc. All rights of repmdution

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428

NOTES TABLE I Wavenumbers in cm-’ of the Observed Lines of the B28:-X2Zi Transition of CT J 0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 a.5 9.5 10.5 11.5 12.5 13.5 14.5 15.5 16.5 17.5 18.5 19.5 20.5

R*

Rz

PI

PZ

18487.764 18496.010 18496.020 18505.298 18505.318 18515.655

18467.085

18527.023

18463.519

18539.424

18461.008

18552.859

18459.537

18567.328

18459.113

18582.825

18459.745

18599.345

18461.422

18515.629

18467.075

18526.990

18463.502

18539.385

18460.983

18552.812

18459.503

18567.277

18459.076

18582.766

18459.703

18599.283

18461.376 18464.094

18464.148

In this work the (0,O) band of the B-X transition is studied and an analysis of the spin-rotation splitting of the B (II' = 0) state is presented. In this experiment the velocity modulation absorption technique (5) was used. An experimental setup similar to the one described in Ref. (6) was used, differing only with respect to the reference laser beam, which in this case was directed through the discharge cell counter propagating with respect to the signal beam ( 7). This arrangement not only increases the signal to noise ratio by a factor of 2, but it also compensates to some extent for improper balancing of the two half-periods of the ac discharge. The ac discharge was performed in an atmosphere of He and C2H2 with the partial pressures of 7 Tot-r and 50 mTorr, respectively. The discharge current was 60 mA and the vohage -2.8 kV, with a frequency of 1.4 kHz. The laser source was a ring laser system, Coherent 699-29, with the dye Rhodamine 110, giving an output power of around 300 mW. The reference lines of Iz (8) were used in determining the wavenumbers of the lines of CT. The rotational lines of CT were strong, about one order of magnitude stronger than the lines of Cl, which appear in the same spectral region (9). It is worth mentioning that, as has been reported earlier in the case of OH- (IO), addition of small amounts of Ar into the discharge increases the signal strength drastically. For C; this increase in intensity appears to be proportional to the Ar pressure, at least at relatively low

TABLE II Obtained Spin-Splitting Constants (All Values in cm-‘)

7; x103

7.26 (56)

7; x103

4.25”

a This value was taken from ref. [4].

pressure. With an amount of Ar equal to that of CzH2 the signal increases by a factor of 2; with three times as much Ar as CzHz the signal increases by a factor of 4. This phenomenon was not observed for C:, which instead seemed to vanish totally down to noise level. The behavior may possibly be. interpreted in terms of an increase in the electron density within the plasma as Ar is added. If the ionization rate of Ar is assumed to be roughly equal to that of acetylene, and if C; is formed by dissociative electron attachment to &Hz as has been suggested (4), then the addition of Ar, equal to acetylene in amount, would increase the electron density by a factor of 2 and the probability of electron attachment would rise by the same amount. The same argument could be applied for C: , where the probability of ion-electron recombination would increase by a factor of 2. We could not observe any rotational heating when adding Ar as has been reported earlier ( 10). As the spin-rotation splitting is very small in C;, spin components overlap. This problem could be overcome, at least to some extent, by employing a graphical decomposition program run on a PC. If each of the rotational line profiles are assumed to be composed of two Gaussians (which can be shown to be a good approximation), overlapping lines can be decomposed into separate profiles. The computer program has been described in detail elsewhere ( 11). A typical decomposition is shown in Fig. 1. In Table I the obtained wavenumbers of the observed lines are presented. The lines were least-squares fitted to standard energy expressions for 2Z-states: F,(N) = B”N(N + I) - D”[N(N + 1)]2 + (y,/2)N F,(N) = B”N(N+

1) - D”[N(N+

1)]2 - (-y”/Z)(N+

1)

The obtained constants were found to be in good agreement with those of Ref. (I ) except for the spinrotation splitting, which was not resolved by Herzberg and Lagerqvist. For the ground state the y value was compared to that of Ref. (4), which was in agreement with our determination. However, in the final fit, the ground state spin-rotation splitting constant y” was kept fixed at the value obtained by Ref. (4). Their value of y” is determined from a larger set of well-resolved lines, and we believe their value to be more accurate. Also, this improves the determination of y’ somewhat. In Table II the result is presented. REFERENCES 1. G. HERZBERGAND A. LAGERQVIST,Can. J. Phys. 46,2365-2373 ( 1968). 2. W. C. LINEBERGERAND T. A. PA-I-ERSSON,Chem. Phys. Lat. 13,40-44 ( 1972). 3. R. D. MEAD, U. HEFTER,P. A. SCHULZ, AND W. C. LINEBERGER,J. Chem. Phys. 82, 1723-173 I (1985). 4. B. D. REHFLJSS, D. LIU, B. M. DINELLI, M. JAGOD,W. C. Ho, M. W. CROFTON,AND T. OKA, J. Chem. Phys. 89, 129-137 (1988). 5. C. S. GUDEMAN AND R. J. SAYKALLY,Annu. Rev. Phys. Chem. 35,387-418 (1984). 6. B. LINDGREN,P. ROYEN, AND M. ZACKRISSON,J. Mol. Spectrosc. 146,343-350 ( 199 1). 7. M. G. BAWENDI,B. D. REHFIJSS, AND T. OKA, J. Chem. Phys. 93,6200-6209 (1990). 8. S. GE~TERNKORNAND P. LUC, “Atlas du spectra d’absorption de la molecules diode 14 800-20 000 cm-‘, Complement,” Ed. du CNRS, Paris, undated. 9. J. P. MAIERAND M. R~SSLEIN,J. Chem. Phys. 88,4614-4620 (1988). 10. N. H. ROSENBAUM,J. C. OWRUTSKY,L. M. TACK, AND R. J. SAYKALLY, J. Chem. Phys. 84,53085313 (1986). 11. M. ZACKRISSON,Appl. Spectrosc., to appear. P. ROYEN M. ZACKRISSON Department of Physics Stockholm University Vanadisv. 9, S-l 13 46 Stockholm Sweden Received March 30, I992