Excitation of C3πu State of N2 in a Radio-frequency Glow Discharge

Excitation of C3~fu State of N2 in a Radio-frequency Glow Discharge

W. R. H A R S H B A R G E R and R. A. PORTER

Intel Corporation, Aloha, Oregon 97005 and Bell Lab- oratories, Allentown, Pennsylvania 18103

Index Headings: rf plasmas

Radio-frequency plasmas are now widely used for ma- terial processing in the semiconductor industry. 1'2 Like many technical developments, the empirical process de- velopment has preceded a thorough understanding of the physical or chemical processes occurring in these systems. Optical spectroscopy provides an excellent probe of the processes occurring in these glow discharge systems. ~-5

A particular subject of interest for these plasma systems is the method of excitation of atomic and molecular species. In this paper, the emission spectrum of the second positive band (C3¢ru --~ B%rg) of a nitrogen glow discharge is compared with calculated Franck-Condon factors to verify the excitation of the C~Tru state by inelastic electron collisions. The N2 spectrum was recorded for a rf glow discharge in a parallel plate plasma reactor. The discharge occurred at a pressure of 1 Torr, at a rf power of 1000 W, and a frequency of 13.56 MHz. The reactor electrodes are 40 cm in diameter with an interelectrode spacing of 2.5 cm. The spectroscopic equip- ment used to record the N2 emission spectrum has been described previously. 6 The upper curve in Fig. 1 shows the recorded spectrum of the second positive emission band observed in the glow discharge system.

For excitation of the C%ru state, leading to the second positive emission band, there are at least three possible excitation mechanisms. In flowing afterglow systems, the C state is populated by a recombination mechan i sm] After noting that the C state is predissociated above the fourth vibrational level, Tanaka et al. suggest tha t the C state is populated by an inverse predissociation process represented below:

N(4S) + N(2D) --~ N2(C37ru, v = 4) (i)

N~(C37ru, V = 4) (2)

+ M ---~ N2(C3~ru, lower bands) + M

N2(C3qTu) -'--) N2(B3~'g) ÷ h~, (3)

In the low tempera ture afterglow experiments, anoma- lous emission intensity from the fourth vibrational level of the C state is observed, due to the inverse predissoeia- tion mechanism.

A second possible excitation mechanism would be electron capture by the N2 + molecular ion according to the process listed below:

N2 + ÷ e-- --~ N2(C3~ru) (4)

Received 30 June 1980.

N2 (C3]]u - B31]g)

i i h I i I I

c~ x FCF (C -- B)

I , I , I,[ , c~ x FCF (X - C - B)

I I I ,J I

360 440 420 400 380 340 320

W A V E L E N G T H (nm)

FIG. 1. Comparison of spectrum of the second positive emission of N2 and calculated Franck-Condon factors. Top curve, observed emission spectrum; center curve, Franck-Condon factors for C --~ B transition times spectral sensitivity, a; bottom curve, Franck-Condon factors for X ~ C -* B transition times spectral sensitivity, a.

Nz(C~ru) -~ Nz(B3~g) + h~ (5)

There is no experimental evidence to support this mechanism and the emission spectrum of N2 in the present study at low sensitivity shows no emission from ionic states.

The most common excitation mechanism for population of the C state is by inelastic electron collisions. This mechanism would involve the following steps:

N2(X 1Zg +) + e - --~ Ne(C~ru) + e - (6)

Nz(C37r,) ~ N2(B~Trg) + hv (7)

This mechanism has been established by numerous studies of spark discharges and glow discharges, s-ll Some of these studies have measured transition probabilities and compared them with calculated Franck-Condon factors to determine spectroscopic parameters such as r cen- troids and transition probabilities.

The Franck-Condon factors used for this s tudy were those published by Benesch et al. 12' 18 The histogram in the center of Fig. 1 represents those Franck-Condon factors for the C~ro ~ B3~rg transition multiplied by the spectral sensitivity (a) of the Si PIN diode used to record the spectrum. Comparison of this histogram with the observed spectrum does not give good agreement. In the spectral range from 360 to 430 mm, the agreement is particularly poor. This poor agreement is due to the fact tha t the vibrational levels of the C state are assumed to be equally populated.

To obtain the proper population distribution for the vibrational levels of the C state, the excitation mechanism must be considered. For excitation of the C state by electron collisions, the population distribution is determined by the Franck-Condon factors for transitions from the ground vibrational level, v ffi 0, of the ground electronic state, X, to the C state, as well as the Franck- Condon factors for the transition from the C state to the B state. The lower histogram of Fig. 1 represents the

1 3 0 Vo lume 35, N u m b e r 1, 1981 APPLIED S P E C T R O S C O P Y

emission spectrum for this mechanism. The emission intensities were obtained from the following formula:

IC,B = a ~ q(X, v ~ C, v')q(C, v' --~ B, v") (8) v=0

where Ic,B = the intensity of an observed vibronic emission band; a = spectral sensitivity of the Si PIN diode; q(X, v ~ C, v') = Franck-Condon factor for the transition from the vibrational level of the X state to the v' vibrational level of the C state; q(C, v' --~ B, v") = Franck- Condon factor for the transition from the v' vibrational of the C state to the v" vibrational level of the B state. It is seen that the relative intensities of this mechanism agree very well with the recorded emission from the second positive band.

This good agreement between the calculated transition probabilities and the observed second positive emission verifies that for the plasma conditions of the present study, the primary excitation mechanism of the C state is by inelastic electron collision. This mechanism is consistent with previously reported studies of N2 discharges. Also, the mechanism is consistent with the relatively short radiative lifetime of the C state (2.7-7.2 × 10 -~ s).'4 This indicates little vibrational relaxation of molecules in the C state prior to the emission.

In addition, these results establish that recent temper- ature measurements of N2 and CO in these glow discharge systems are actually measurements of electronic ground state rotational population distributions. 6' 1.~ The emission spectrum merely provides a probe of these population distributions.

1. J. R. Hollahan and R. S. Rosler, in Thin Film Processes, J. L. Vossen and W. Kern, Eds. (Academic Press, New York, 1978), pp. 335-360.

2. C. J. Mogab and C. M. Melliar-Smith, in Thin Film Processes, J. L. Vossen and W. Kern, Eds. (Academic Press, New York, 1978), pp. 497-556.

3. W. R. Harshbarger, R. A. Porter, T. A. Miller, and P. Norton, Appl. Spectrosc. 31, 201 (1977).

4. W. R. Harshbarger and R. A. Porter, Solid State Techn. 21, 99 (April, 1978). 5. R. G. Frieser and J. Nogay, Appl. Spectrosc. 34, 31 (1980). 6. R. A. Porter and W. R. Harshbarger, J. Electrochem. Soc. 126, 460 (1979). 7. Y. Tanaka, F. LeBlanc, and A. S. Jursa, J. Chem. Phys. 30, 1624 (1959). 8. G. Hartmann and P. C. Johnson, J. Phys. B 11, 1597 (1978). 9. R. W. Nicholls, Ann. Geophys. 20, 2 (1964).

10. D. C. Tyte, Proc. Phys. Soc. 80, 1347 (1962). 11. K. Tachibana and A. V. Phelps, J. Chem. Phys. 71, 3544 {1979). 12. W. Benesch, J. T. Vanderslice, S. G. Tilford, and P. G. Wilkinson, Astrophys.

J. 144, 408 (1966). 13. A. Lofthus and P. H. Krupenie, J. Phys. Chem. Ref. Data 6, 133 (1977). 14. J. Anketell and R. W. Nicholls, Rep. Prog. Phys. 33, 269 (1970). 15. M. Oshima, Jpn. J. Appl. Phys. 17, 1157 (1978).

Mass Spectral Confirmation of Ochratoxin A

REX T. GALLAGHER and H. MICHAEL STAHR*

Veterinary Diagnostic Laboratory, Iowa State Univer- sity, Ames, Iowa 500ll

Index Headings: Analysis, for ochratoxin; Mass spectroscopy; Thin-layer chromatography.

The ochratoxins are toxic metabolites produced by fungi of the genera Aspergillus and Penicillium. Ochra- toxin A (primarily a nephrotoxin) is the most potent of these toxins and is frequently found as a contaminant in agricultural commodities.

Several analytical methods are available for ochratoxin A. '-4 However, as is the case with many mycotoxins, unequivocal chemical confirmation at the low concentra- tions often encountered is dependent on the use of selec- tive characterization techniques, e.g., mass spectroscopy. The mass spectra of ochratoxin A, ochratoxin B, and their methyl and ethyl esters are available in the publi- cation Mycotoxins Mass Spectral Data Bank. 6 No other published data on the mass spectra or mass spectral fragmentation of ochratoxins A and B are available.

Merck (25-mm silica gel thin-layer chromatography plates), Nanograde, or equivalent solvents were used. A Finnigan 4000 GC/mass spectrometer was also utilized.

In a recent diagnostic case involving swine toxicosis, we determined the mass spectrum of an ochratoxin A standard prior to other investigations on urine extracts. To our surprise, the standard, which was run directly on the solid probe, showed a spectrum in which the four most intense peaks differed by one mass unit (1 m/e) from the peaks in the published mass spectra. The mass spectra (Fig. 1) showed intense peaks at m/e 239/241 and m/e 255/257; the published spectra showed the corresponding peaks at m/e 240/242 and m/e 256/258. Further examination of the mass spectrum showed that ochratoxin A ethyl ester and ochratoxin A methyl ester had corresponding peaks at m/e 239/241 and m/e 255/257.

It was thought possibly that our standard of ochratoxin A solution in methyl alcohol prepared some time previously had been converted to the methyl ester. A fresh standard of ochratoxin A was prepared by dissolving solid ochratoxin A in chloroform. This was examined on the mass spectrometer and the peaks m/e 239/241 and m/e 255/257 were again obtained. The molecular ion for the parent acid was observed at m/e 403 (very low intensity), with no indication of molecular ions for the methyl ester (m/e 417) or the ethyl ester (m/e 431). An evaluation of the fresh standard by thin-layer chromatography confirmed that it was substantially a parent compound, with a small fraction of some other related impurity.

R e c e i v e d 3 J u n e 1980; rev i s ion rece ived 30 J u l y 1980. *Author to w h o m co r r e spondence s h o u l d be addressed .

Volume 35, Number 1, 1981 APPLIED SPECTROSCOPY 131