6189
J. Phys. Chem. 1987, 91, 6189-6191
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Two-Photon Resonance-Enhanced Multiphoton Ionization Spectroscopy of the 3p7r D *II,(v’=O,1,2) X *n,(v”=O) Bands of the CF Radical between 355 and 385 nm Russell D. Johnson IIIt and Jeffrey W. Hudgens* Chemical Kinetics Division, Center for Chemical Physics, National Bureau of Standards, Gaithersburg, Maryland 20899 (Received: June 1 1 , 1987)
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The two-photon resonance-enhanced multiphoton ionization spectrum between 355 and 385 nm of the 3pa D zll,(v’=O,l ,2) X 211r(u”=O) band system of CF radical is reported. Three vibrational bands are rotationally analyzed, and spectroscopic constants for the D 211rstates are derived.
Introduction
In this account we report the resonance-enhanced multiphoton ionization (REMPI) spectrum of C F radical between 355 and 385 nm. We have found this REMPI band system a convenient and sensitive way of detecting C F radicals. The spectrum originates X 211r(v”=O) tranfrom two-photon 3p7r D ZIIr(u’=0,1,2) sitions which lie between 51 950 and 56 340 cm-’. The CF radical appears during the plasma etching of semiconductor surfaces which use fluorocarbons as a source of fluorine CF radical is a nascent photofragment observed from the vacuum-UV photolysis of CFzC1: and has also been detected as a secondary reaction product during flow reactor ~ t u d i e s . ~ , ~ The vacuum-UV spectrum of the 3p7r D 211(v’=1,2,3) X 211r bands was first observed, assigned, and rotationally analyzed by White.’ In the vacuum-UV spectrum these bands absorbed weakly and the D 211(u’=O) state did not appear. An intense 2 1 REMPI spectrum of the 3p7r D 211(v’=0,1,2,3) X zIIr(u”=O) bands was also reported by Hudgens et al.; however, this REMPI spectrum was not rotationally re~olved.~ This paper presents the first rotational analysis of the 3p7r D zIIr(u’=O) X 211r(v”=0)band. The u’ = 1, 2 bands are also rotationally analyzed, and new spectroscopic constants for the C F 3pa D zIIr state are derived.
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Apparatus and Methods
The experimental apparatus used in these experiments is described in other publication^.^^^ It consists of a flow reactor, a time-of-flight mass spectrometer, and a data acquisition system. C F radicals were produced as a secondary product of the reaction of F atoms with CH3Fin helium in a flow reactor operated at 1 m/s. A small portion of the reaction products effused from the flow reactor into a vacuum chamber at 10” to Torr where they were ionized by the laser beam and extracted into a timeof-flight mass spectrometer. The ion flight time that corresponded to mass 3 1 was sampled by a gated integrator, and the ion signal was averaged, displayed, and recorded with a microcomputer. The excimer pumped dye laser was scanned stepwise at 0.000 60 nm/step (Av 0.042 cm-’/step). Ten laser shots were averaged at each step. The laser beam (bandwidth 0.2 cm-’; 10 mJ/pulse) was focused by a 250-mm lens. The data are uncorrected for variations in laser energy over the dye range. The laser dyes used during this study were QUI between 379 and 38 1 nm and DMQ between 355 and 369 nm. The laser wavelength was calibrated by measuring the optogalvanic spectrum of uranium atoms in a hollow cathode lamp.lOvll
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(1) Ninomiya, K.; Suzuki, K.; Nishimatsu, S.; Okada, 0. J . Vac. Sci. Technol., A 1986, 4, 1791. (2) Veldhuizen, E. M.; Bisschops, Th.;van Vliembergen, E. J. W.; van Wolput, J. H. M. C. J . Vac. Sci. Technol., A 1985, 3, 2205. (3) Booth, J. P.; Hancock, G.; Perry, N. D. Appl. Phys. Left.1987,50, 318. (4) Hepburn, J. W.; Trevor, D. J.; Pollard, J. E.; Shirley, D. A,; Lee, Y. T.J . Chem. Phys. 1982, 76, 4287. (5) Hudgens, J. W.; Dulcey, C. S.; Long, G. R.; Bogan, D. J. J . Chem. Phys., in press. ( 6 ) Dyke, J. M.; Lewis, A. E.; Morris, A. J . Chem. Phys. 1984, 80, 1382. (7) White, Jr., W. P. Dissertation, The Ohio State University, 1971. (8) Dulcey, C. S.; Hudgens, J. W. J . Phys. Chem. 1983, 87, 2296. (9) Hoffbauer, M. A.; Hudgens, J. W. J . Phys. Chem. 1985, 89, 5152. (10) King, D. S.; Schenck, P. K.; Smyth, K. C.; Travis, J. C. Appl. Opt. 1977, 16, 26 11. (1 1) Palmer, B. A.; Keller, R. A,; Engleman, Jr., R.; “An Atlas of Uranium Emission Intensities in a Hollow Cathode Discharge”, Los Alamos Informal Report LA-8251-MS, Los Alamos, 1980. (12) Brown, J. M.; Schubert, J. E.; Saykally, R. J.; Evenson, K. M. J . Mol. Specfrosc. 1986, 120, 421.
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Results and Analysis
Four strong branches and two weak branches are observed for each vibrational band. The four strong branches are assigned to and 02, Q branches and the two weaker branches to the 022 branches. The OI2and 01,branches are not observed. The relative strength of the 0 branches may be rationalized by noting that the Oz2and Ozl branches are enhanced by contributions from their degenerate partners, the P I 2and P,, branches, whereas the OlZand 011 branches are not enhanced because they have no degenerate branches. In addition, no S branches are assigned. Because the S branches exhibit no bandheads and occur with fewer lines per nanometer across the band interval, they are more difficult to observe and confidently assign than 0 branches of similar intensity. The spectra show two prominent bandheads, assigned to the Q l l and Ql2 branches, and two smaller bandheads, assigned to the OZ1and 022 branches. The two Q-branch bandheads are separated in laser frequency by -40 cm-]. This is approximately one-half the ground-state spin-orbit splitting, as expected for a two-photon resonant spectrum. The u’ = 0 spectrum displays fewer lines than the u’ = 1 and u’ = 2 spectra. This is due to the accidental overlap of lines from the QZ2and Q12branches and also from the overlap of the QZ1and Q l l branches. In the spectra of the u’ = 1 and u’= 2 REMPI bands, for which the upper electronic states have smaller values of B,, these lines do not overlap. The spin-orbit splitting12 of the X *II, state of C F is 77.1969 cm-’. With a rotational constant of 1.4 cm-’ the ground state is Hund’s case a coupling. The analysis of the REMPI spectra yield an upper state spin-orbit splitting constant of -6 cm-I and a rotational constant of 1.7 cm-’. The upper states are best described by Hund’s case b coupling. Twenty rotational branches are possible for a two-photon transition between a Hund’s case a state and a Hund’s case b state. Because of the small spin
REMPI spectra of the 3p7r D zII(v’=0,1,2) X zllr(u”=O) bands of the C F radical are shown in Figures 1-3, respectively. NRC/NBS Postdoctoral Associate.
* Correspondence should be addressed to this author.
0022-3654187 , ,12091-6189%01.50/0 0 1987 American Chemical Societv I
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6190 The Journal of Physical Chemistry, Vol. 91, No. 24, 1987
Johnson and Hudgens
Two-Photon E n e r g y
52650
379 5
(cm-l)
52550
52600
381 0
380 5
380 0
52450
52500
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L a s e r h a v e l e i g t h !qm) Figure 1. The 2 + 1 REMPI spectrum of the CF radical ( m / z 31) 3p?r D 211r(u’=0) parameters found in Table I.
X ’II, band. The index lines were drawn by using the
T w o - P h o t o n E n e r g y (cm-l)
54450
367.0
54400
54350
367.5
54300
368.0
54250
54200
368.5
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Laser Navelengtn i i m ) Figure 2. The 2 + 1 REMPI spectrum of the CF radical ( m / z 31) 3p7r D 21’I,(u’=1) X *IIrband. The index lines were drawn by using the parameters found in Table I.
Two-Photon Energy (cm-’) 56250
56200
56150
56100
56050
56000
55950
355.5
356.0 356.5 357.0 357 L a s e r Wavelength (nm) Figure 3. The 2 + 1 REMPI spectrum of the CF radical ( m / z 31) 3p7r D 211r(u’=2) X *II, band. The index lines were drawn by using the parameters found in Table I.
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splitting of the upper state, certain branches are essentially degenerate: N012 OPI2 - 0 2 2 ‘Q12 - P22
01I
PI1 -
This leaves only 12 branches possible for observation. For a two-photon transition between states having the same value of A, the 0, Q, and S branches are expected to be more intense than the P and R bran~hes.’~The branches which originate from the 2113/2state will be weaker than the branches which originate from (13) Bray, R. G.; Hochstrasser, R.M. Mol. Phys. 1976, 31, 1199.
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987 6191
Vibrational Bands of the CF Radical TABLE I: Spectroscopic Constants (in cm-I) for the X D 2111States of CF Radical spectrosc const
present study
vacuum-UV study“
3pn D ’n(u’=O) 52519.j4 f 0.06 6.8 f 0.3 1.7195 f 0.0006 (7.1 f 1.5) X 10“ 3pn D ‘II(u’=l) 54298.94 f 0.05 5.2 f 0.5 1.6993 f 0.0008 (5.7 f 2.3) X 10“ 3pa D 211(u’=2) 56050.42 f 0.04 6.8 f 0.3 1.6814 f 0.0005 (4.5 f 1.0) x 10”
2nrand 3p7r
e
A
Be a,
T = Te + G(u) + F(J) G ( u ) = W,(U + ’ / 2 ) - W,X,(U + B, = Be - ae(v + ‘/2)
54297.75 b 1.7011 f 0.0005 (6.4 f 0.4) X 10” 56050.03
b 1.6819 f 0.0003 (6.57 f 0.2) X lod
1/2)2
Discussion In its ground state carbon monofluoride has the electronic configuration KK( ~ s u )2~~ (u * )2~p( ~ )2~~ (a ) ~ 2 p a *X, ’II, The D 211state is prepared by exciting the 2pa* electron into a 3pa Rydberg state to form the configuration KK(2s~)~(2so*)~(2pu)~(2pn)~3pa, D 211r
‘From ref 7. b N o t determined. ‘From ref 12. dFrom ref 15. Unobserved.
the 2111j2state by a factor of 0.7 because of the room-temperature Boltzmann distribution. The observed rotational lines were fitted by using formulas appropriate to describe the energy levels for lI states intermediate between Hund’s case a and b, as found in Herzberg14 and references therein. The formulas used are
F2(4 = B,((J + ‘/z)~ - 1 + [4(J +
1809.50 f 0.05 14.3 f 0.1 b 1.7300 f 0.0001 0.0193 f 0.0005
“From ref 7. bNot reported.
x 2II,(u”=O)
Fi(J) = B,((J + ‘/z)~ - 1 - [4(J + ‘/2)
1807.32 13.96 6.2 1.7286 i 0.0005 0.0191 f 0.0005
Qe
We*,
77.1 969c 1.407332c 6.62947 X 1308. I d 11.10d 0.093d e
TABLE II: Derived Spectroscopic Constants for the 3pa D State of the CF Radical spectrosc const present work vacuum-UV study“ Te 52270.66 b
+ Y(Y - 4)]’/2]- D
J4
y2) + Y(Y - 4 ) ] 1 / 2-) D,(J + 1)4
where Y = A,/B,. The parameters for the ground state were held fixed and are listed in Table I. The upper state parameters were obtained from a least-squares fit of the observed lines of the four Q branches and the two 0 branches. The number of lines used for the least-squares fits were 88, 84, and 95 for v’ = 0, 1, and 2, respectively. Table I lists the spectroscopic constants fitted for each vibrational band. Additional tables listing the observed two-photon frequency of each spectral feature, its assignment(s), the calculated frequencies for the assignment(s), and the residuals between the observed and calculated frequencies are available as supplementary material. No evidence was obtained for strong interaction with adjacent electronic states. The vibrational intervals along v’ = 0-3 appear unperturbed. (See ref 5 and 7 in addition to this work.) The rotational line width within each vibrational band did not display any detectable variation. (14) Herzberg, G. Spectra of Diatomic Molecules; Van Nostrand Reinhold: New York, 1950; p 232.
The promotion of the a* orbital electron into the diffuse Rydberg orbital increases the vibrational constant, we, of CF from 1308l5 to 1807 cm-I. The vibrational constant of the Rydberg state is very similar to the 1830 (30)cm-l vibrational frequency observed in the ground-state c a t i ~ n . ~This ? ~ similarity is expected of a Rydberg state that possesses a ground-state cation core. Table I1 lists the derived spectroscopic constants obtained from the present REMPI spectra and compares them with the constants previously derived by Whitee7 Our data set includes the 3pa D 2111(v’=O) X 211r(v”=O) band which White was unable to observe. As expected, the two sets of spectroscopic constants agree; however, constants derived from the REMPI data include the upper state spin-orbit constant, A . White7 also reported another vacuum-UV absorption band system between 186.6 and 191.2 nm which overlapped with the 3pa D 211 X 211rprogression. This band system was assigned X 211rtransitions. During the present study these to C’ 2Z+ electronic transitions were expected to appear between 373 and 383 nm by 2 1 REMPI ionization. N o REMPI features asX 211, transitions were detected. White sociated with C’ 22? noted that the C’ 2Z+state is predissociated.’ Presumably, during the 2 + 1 REMPI process, essentially all multiphoton prepared C’ ’2’ radicals dissociate before the third (ionizing) photon can be absorbed.
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Acknowledgment. We thank Dr. Jon T. Hougen of the National Bureau of Standards (Gaithersburg, MD) for several insightful discussions during the preparation of this work. Registry No. CF, 3889-75-6.
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Supplementary Material Available: Tables 1-3 listing observed X *II(u”=O) lines and assignments for CF D 211(v’=0,1,2) (10 pages). Ordering information is given on any current masthead page. (15) Porter, T. L.; Mann, D. E.; Acquista, N. J. Mol. Spectrosc. 1965, 16, 228.
