- Email: [email protected]
Detection of SO (X 3Σ−, v = 1) at 618.4 μm by laser magnetic resonance spectroscopy
SPECTROCHIMICA ACTA PART A
ELSEVIER
Spectrochimica Acta Part A 52 (1996) 471-475
Detection of SO (X 3 E - , v = 1) at 618.4 resonance spectroscopy
by laser magnetic
J.R. Anacona Departamento de Quimica, Universidad de Oriente, Apartado Postal 208, Cumangt, Venezuela
Received 12 June 1995; accepted 20 July 1995
Abstract Laser magnetic resonance (LMR) spectrum of 328160 in the X 3E- state has been observed in the far IR region at 618.4 p m with a vinyl bromide optically pumped laser as a source. Data are presented for the Zeeman components of the rotational transition N = 6, J = 7--* N = 7,J = 7 in the first vibrational excited level. Theoretical values of the transition magnetic field strengths have been calculated using the best available molecular constants and first order Zeeman formulas. The agreement between theory and experiment confirms the spectroscopic assignments.
Keywords: IR spectroscopy; Radical; SO
1. Introduction A l t h o u g h the s u l p h u r m o n o x i d e r a d i c a l has been widely s t u d i e d b y m i c r o w a v e [1-5], gas p h a s e electron p a r a m a g n e t i c r e s o n a n c e ( E P R ) [ 6 - 8 ] a n d electronic s p e c t r o s c o p y [9,10], r a t h e r less has been p u b l i s h e d o n its I R spectra. H o p k i n s a n d B r o w n [11] o b s e r v e d the f u n d a m e n t a l b a n d o f X 3 E - SO in an A r m a t r i x . Y a m a d a et al. [12] r e p o r t e d v i b r a t i o n r o t a t i o n t r a n s i t i o n s o f the 1 - 0 b a n d in the 1115 c m - l region o f the m e t a s t a b l e a lA excited electronic state o b s e r v e d b y m i d - I R laser m a g n e t i c r e s o n a n c e ( L M R ) s p e c t r o s c o p y . K a w a g u c h i et al. [13] o b s e r v e d v i b r a t i o n r o t a tion t r a n s i t i o n s o f SO in the X 3 E - in the 1100 c m - 1 region b y L M R s p e c t r o s c o p y with a C1802 laser as a source. T r a n s i t i o n s o f the f u n d a m e n t a l b a n d o b s e r v e d for the n o r m a l species 32S160 were
m a i n l y o f the A N = - 1, A J = 0 a n d in a d d i t i o n to these the external m a g n e t i c field induces o t h e r types o f t r a n s i t i o n s because o f a v o i d e d crossings, they are either o f A N = - 1 , AJ = - 3 or of AN=1, A J = 1 type. S o m e t r a n s i t i o n s with A N - - - 3 were also observed. A d d i t i o n a l l y a few w e a k l y t u n a b l e A N = A J = - 1 t r a n s i t i o n s were o b s e r v e d for the v = 0 ~ v = l and v=l~v--2 b a n d s o f 328160 a n d for the v = 0 ~ v = 1 b a n d o f 328180.
W o n g et al. [14] o b s e r v e d the v = 0 ~ v = 3 t r a n s i t i o n o f SO by difference frequency laser spectroscopy and obtained molecular parameters in the v = 3 state, which when c o m b i n e d with the m i d - I R L M R d a t a , lead to the h a r m o n i c frequency, the v i b r a t i o n a l a n h a r m o n i c i t y c o n s t a n t s a n d the e q u i l i b r i u m r o t a t i o n a l , spin spin a n d s p i n - r o t a t i o n i n t e r a c t i o n constants.
0584-8539/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0584-8539(95)01531-0
472
J.R. Anacona / Spectrochimica Acta Part A 52 (1996) 471-475
The present work reports the first measurement
of 328160 (X 3 E - ) in the first vibrational level by far-IR L M R which uses the Zeeman effect to tune magnetic components of rotational transitions into coincidence with fixed frequency cw far-IR lasers. In the rather low rotational levels probed by L M R , the SO spectrum appeared as single lines.
2. Experimental The optically pumped far-IR L M R spectrometer has been described in detail elsewhere [15] and is similar to other designs [16]. The far-IR L M R spectrometer cavity is divided by a polypropylene beam splitter into two portions, the laser section and the sample section. The sample section lies wholly within the laser cavity. The polypropylene membrane is set at approximately the Brewster angle to the laser axis in a mount which can be rotated to achieve the desired orientation between the magnetic field and the electric vector of the 618.4/zm line of the vinyl bromide laser. The vinyl bromide was used without further purification at the optimum pressure of 90 m T o r r and was optically pumped by 33 W of the 10(R)30 IR line from a cw Apollo CO2 laser. The SO radical was generated from an electrodeless 2450 M H z discharge in mixtures of SC12 and 02 outside the sample section and passed through the absorption region in a continuous flow at total pressures up to 0.5 Torr. To detect the absorption of far-IR power by the reacting gas mixture, at field strengths that correspond to magnetic resonance, a Golay cell or a germanium bolometer cooled with liquid helium were used. The signal from the detector was amplified and passed to a phase sensitive detector operating at the modulation frequency (around 250 Hz). First derivative L M R spectrum was obtained and the magnetic field was calibrated using the NS spectra accurately measured [17].
3. Laser magnetic resonance spectrum During a study of the reaction of nitrogen and SCl2 to detect NS radical [17] traces of oxygen
were found to seriously degrade NS spectral intensity and several new unreported L M R transitions were detected at 618.4 /xm. When nitrogen was replaced by oxygen and passed through the microwave discharge a different spectrum was obtained. The spectrum is simple with no hyperfine splittings due to nuclear spin suggesting the presence of SO radical. Using the best available rotational constants [4,5,13], rotational calculations of naturally occurring isotopic forms of SO were carried out with the aim of assigning the spectrum. 3280160, 328180 and 348160 in the ground and first excited vibrational levels in the 35-- and hA electronic states were considered in the fitting. O f all them, 328160 in the first vibrational level in the 3 E - state presents the closest rotational transition to the laser frequency. 3380 and other species containing nitrogen and/or chlorine were discarded as the carriers of the spectrum because they should exhibit hyperfine structure patterns due to the nonzero nuclear spin. The possibility that the spectrum might arise from a known homonuclear diatomic species like $2 or oxygen was eliminated owing to the unfavourable mismatch between the far-IR laser frequency and the rotational transitions in field-free space. The energy level structure of a diatomic molecule in a 3E electronic state has been discussed in detail by a number of authors [18,19]. Due to the spin-spin interaction constant of SO in the X 352- state is about seven times larger than its rotational constant, the radical follows a coupling scheme intermediate between Hund's cases (a) and (b) at very small N values, but approaches the case (b) at higher N values. To interpret the spectrum, the first order Zeeman Hamiltonian considering only the interaction of the external magnetic field with the electron spin was used. The matrix elements of H z in terms of Hund's case (b) basis function have been developed by Mizushima [19] and where from the energies of the triplet levels at low magnetic fields can readily be obtained as E ( N = J) = E o ( N = J ) + p B g ~ . B M j / N ( N + 1)
(1)
E ( N = J + 1 ) = E o ( N = J + 1 ) - p~g.,.BMj/N
(2)
E(N = J-
1) = Eo(N = J -
1) + I ~ g s B M j / ( N + 1) (3)
473
J.R. Anacona / Spectrochimica Acta Part A 52 (1996) 471-475
Mj
4
N,J ~'---
7,7
I
-1
0
E
(J
>. C~ L.
q~ r-
r~ ~o ~r o4
6,6
Ill
/fi,4 3 /
5 -7
Magnetic
Field
Fig. l. The schematic energy level diagram to illustrate the magnetic resonances expected at 618.4/~m in perpendicular polarization which are indicated by arrows. The nonlinearity in the Zeeman effect can be clearly seen at higher magnetic fields.
where E0 is the zero field energy, g, was assumed to be the electron free g factor, /~a the Bohr magneton (1.3996 M H z gauss-~), B denotes the applied magnetic field and Ms is the quantum number of the component of J along the magnetic field direction. Equations (2) and (3) show that the Zeeman components with Mj close to J
decrease and increase in energy for the N = J + 1 and N = J - 1 levels respectively, and they will tend to cross each other at some magnetic fields. Kawaguchi et al. [13] have shown for the SO radical that, except for the two largest Mj components of N = J - - 1, the Zeeman interactions lead to avoided crossings for components with the
474
J.R. Anacona / Spectrochirnica Acta Part A 52 (1996) 471 475
Mj
I 5I 4I
6
Mj 71i 5 41 3I
I
I
3
2
2
1"
I
I aij=-I
I
f
0.33
0.66
I 0.00
AMj =+1
Magnetic Field Intensity (Tesla) Fig. 2. The LMR spectrum associated with the N = 6,J = 7 --*N = 7,J = 7 transition of the SO radical, in the first excited vibrational level, observed at a laser frequency of 16.16956 cm ~with the electric field of the laser radiation perpendicular to the magnetic field. Several weaker transitions were detected at low magnetic field but not assigned. same M j , due to m u t u a l i n t e r a c t i o n via an interm e d i a t e N = J level. Fig. 1 illustrates schematically this b e h a v i o u r t o g e t h e r with the L M R t r a n s i t i o n s t h a t c a n be expected using the 618.4 ILm laser line, they are o f A N = 1, A J = 0 type. U s i n g the zero field S c h l a p p e q u a t i o n s [20], i n c l u d i n g centrifugal d i s t o r t i o n terms, the 618.4 /~m s p e c t r u m s h o w n in Fig. 2, can be a t t r i b u t e d u n a m b i g u o s l y to the N = 6 , J = 7--* N = 7 , J = 7 t r a n s i t i o n in the 325160 (3~,-) in the first vibrat i o n a l level. T h e values o f the r o t a t i o n a l , s p i n - s p i n interaction, s p i n - r o t a t i o n i n t e r a c t i o n a n d centrifugal d i s t o r t i o n c o n s t a n t s used to calculate the a b o v e m e n t i o n e d t r a n s i t i o n were B = 0 . 7 1 2 2 1 0 5 cm 1 2 = 5 . 3 0 0 3 7 c m - l , y = _ 5 . 6 6 6 5 8 x 1 0 - 3 cm a n d D = 0 . 1 1 3 1 2 4 x 10 5 c m l, which yield a spacing o f 16.24467 cm -~. These values were obtained from vibration-rotation measurements b y m i d - I R L M R s p e c t r o s c o p y (13). Fig. 2 also shows the f a r - I R L M R lines a s s o c i a t e d with the
m e n t i o n e d t r a n s i t i o n s in p e r p e n d i c u l a r p o l a r i z a tion where the AM~ = + 1 selection rule is allowed. T h e Z e e m a n shifts o f the m a g n e t i c c o m p o n e n t s were c a l c u l a t e d using the f o r m u l a s 1 - 3 which yield line p o s i t i o n s within 50 gauss (0.005 T) o f experimental. T h e better a c c u r a c y o c c u r r e d for n a r r o w (fast tuning) lines at low m a g n e t i c fields. U s i n g the very a c c u r a t e value o f the laser frequency: 484 751.1 M H z (16.16956 cm 1) [21] the lowest field line for the A N = 1, A J = 0, A M j = - 1 t r a n s i t i o n was p r e d i c t e d to lie at ~ 930 gauss (0.0930 T). T h e lowest field line in the s p e c t r u m which can be a t t r i b u t e d to this t r a n s i t i o n occurs at ~ 900 gauss (0.900 T) c o n f i r m i n g the prediction m a d e before. T a b l e 1 lists the o b s e r v e d m a g netic fields a n d differences between c a l c u l a t e d frequencies a n d the laser frequency for the Zeem a n c o m p o n e n t s o f the transitions. T h e m i d - I R L M R p a r a m e t e r s a n d first o r d e r Z e e m a n f o r m u las e n a b l e d the s p e c t r u m to be c a l c u l a t e d
J.R. Anacona / SpectrochmTica Acta Part A 52 (1996) 471 475
Table 1 Field measurements and assigned transitions for LMR spectrum of SO (X 3Z , v = 1) with the 618.4 lLm vinyl bromide laser line Assignment
Observed field (10 4 T)
Av '~ (10 4 cm
N,J,M.I--*N,J,M J
6,7,7 --* 7,7,6 6,7,6---,7,7,5 6,7,6 ~ 7,7,7 6,7,5 ~ 7,7,4 6,7,5 --* 7,7,6 6,7,4 ~ 7,7,3 6,7,4 ---,7,7,5 6,7,3 ~ 7,7,2 6,7,3 --, 7,7,4 6,7,2 ~ 7,7,1 6,7,2 ~ 7,7,3 6,7,1 -~ 7,7,0 6,7,1 ~ 7,7,2
930 1015 l 114 1229 1353 1522 1721 1980 2250 2648 3200 4076 5746
0.06 0.30 0.32 0.06 0.24 0.19 0.21 0.11 0.06 0.06 0.57 0.06 0.04
475
sign of the tuning rate because the laser frequency could not be shifted far enough to cause a detectable change in the resonance flux densities.
i)
Acknowledgements
~' v, calculated minus laser frequency.
sufficiently accurately to permit easy identification of the observed lines. However, in each case there were discrepancies between the observed and the calculated flux densities ranging from a few gauss at low magnetic fields up to around 1000 gauss for the stronger resonances. The larger discrepancies could not be removed by adjustments of the g factors and can be attributed to nonlinear Zeeman effects which appeared to be important around 4000 gauss (0.4000 T). In Table 1, only relatively strong signals that could be unambiguously assigned are given. The weakest four observed lines in the 7 0 0 - 8 5 0 gauss range and several sharp but very weak resonances observed at high magnetic fields, for which several alternative assignments could be made, are omitted from Table 1. In general the SO spectrum as recorded with the spectrometer was rather weak, being comparable in strength to HCS [22] and NS [17] radicals, after optimizing both the frequency and amplitude of modulation. Additional information often necessary for an unambiguous assignment can be obtained by noting the behaviour of the resonances with respect to a slight change in laser frequency by detuning the cavity length. Unfortunately, it was not possible to obtain experimental information on the
I am grateful to the Department o f Physical Chemistry at the University of Cambridge where part of the research was carried out and Dr. P.B. Davies for his continuous support.
References [1] F.X. Powell and D.R. Lide, J. Chem. Phys., 41 (1964) 1413. [2] M. Winnewisser, K.V.L.N. Sastry, R.L. Cook and W. Gordy, J. Chem. Phys., 41 (1964) 1687. [3] T. Amano, E. Hirota and Y. Morino, J. Phys. Soc. Jpn., 22 (1967) 399; 25 (1968) 300. [4] E. Tiemann, J. Mol. Spectrosc., 51 (1974) 316: J. Phys. Chem. Re['. Data, 3 (1974) 259. [5] W.W. Clark and F.C. Lucia, J. Mol. Spectrosc., 60 (1976) 332. [6] A. Carrington, D.H. Levy and T.A. Miller, Proc. R. Soc. London, Ser. A, 298 (1967) 340. [7] J.M. Daniels and P.B. Dorain, J. Chem. Phys., 45 (1966) 26. [8] H. Uehara, Bull. Chem. Soc. Jpn., 42 (1969) 886. [9] R.G.W. Norrish and G.A. Oldershaw, Proc. R. Soc. London, Ser. A, 249 (1959) 498. [10] R. Colin, Can. J. Phys., 46 (1968) 1539; 47 (1969) 979. [11] A.G. Hopkins and C.W. Brown, J. Chem. Phys., 62 (1975) 2511. [12] C. Yamada, K. Kawaguchi and E. Hirota, J. Chem. Phys., 69 (1978) 1942. [13] K. Kawaguchi, C. Yamada and E. Hirota, J. Chem. Phys., 71 (1979) 3338. [14] M. Wong, T. A m a n o and P. Bernath, J. Chem. Phys., 77 (1982) 2211. [15] D.P. Stern, Ph.D. Thesis, University of Cambridge, 1983. [16] K.M. Evenson, Discuss. Faraday Soc., 71 (1981) 7. [17] J.R. Anacona, Spectrochim. Acta Part A, 50 (1994) 909. [18] M. Tinkham and M.W.P. Strandberg, Phys. Rev., 97 (1955) 937. [19] M. Mizushima, The Theory of Rotating Diatomic Molecules, Wiley, New York, 1975. [20] R. Schlapp, Phys. Rev., 51 (1937) 342. [21] S.F. Dyubko, M.N. Efimenko, V.A. Svich and L.D. Fesenko, Sov. J. Quant. Electron., 6 (1976) 600. [22] J.R. Anacona, J. Chem. Soc. Faraday Trans., 88 (1992) 1507.
ELSEVIER
Spectrochimica Acta Part A 52 (1996) 471-475
Detection of SO (X 3 E - , v = 1) at 618.4 resonance spectroscopy
by laser magnetic
J.R. Anacona Departamento de Quimica, Universidad de Oriente, Apartado Postal 208, Cumangt, Venezuela
Received 12 June 1995; accepted 20 July 1995
Abstract Laser magnetic resonance (LMR) spectrum of 328160 in the X 3E- state has been observed in the far IR region at 618.4 p m with a vinyl bromide optically pumped laser as a source. Data are presented for the Zeeman components of the rotational transition N = 6, J = 7--* N = 7,J = 7 in the first vibrational excited level. Theoretical values of the transition magnetic field strengths have been calculated using the best available molecular constants and first order Zeeman formulas. The agreement between theory and experiment confirms the spectroscopic assignments.
Keywords: IR spectroscopy; Radical; SO
1. Introduction A l t h o u g h the s u l p h u r m o n o x i d e r a d i c a l has been widely s t u d i e d b y m i c r o w a v e [1-5], gas p h a s e electron p a r a m a g n e t i c r e s o n a n c e ( E P R ) [ 6 - 8 ] a n d electronic s p e c t r o s c o p y [9,10], r a t h e r less has been p u b l i s h e d o n its I R spectra. H o p k i n s a n d B r o w n [11] o b s e r v e d the f u n d a m e n t a l b a n d o f X 3 E - SO in an A r m a t r i x . Y a m a d a et al. [12] r e p o r t e d v i b r a t i o n r o t a t i o n t r a n s i t i o n s o f the 1 - 0 b a n d in the 1115 c m - l region o f the m e t a s t a b l e a lA excited electronic state o b s e r v e d b y m i d - I R laser m a g n e t i c r e s o n a n c e ( L M R ) s p e c t r o s c o p y . K a w a g u c h i et al. [13] o b s e r v e d v i b r a t i o n r o t a tion t r a n s i t i o n s o f SO in the X 3 E - in the 1100 c m - 1 region b y L M R s p e c t r o s c o p y with a C1802 laser as a source. T r a n s i t i o n s o f the f u n d a m e n t a l b a n d o b s e r v e d for the n o r m a l species 32S160 were
m a i n l y o f the A N = - 1, A J = 0 a n d in a d d i t i o n to these the external m a g n e t i c field induces o t h e r types o f t r a n s i t i o n s because o f a v o i d e d crossings, they are either o f A N = - 1 , AJ = - 3 or of AN=1, A J = 1 type. S o m e t r a n s i t i o n s with A N - - - 3 were also observed. A d d i t i o n a l l y a few w e a k l y t u n a b l e A N = A J = - 1 t r a n s i t i o n s were o b s e r v e d for the v = 0 ~ v = l and v=l~v--2 b a n d s o f 328160 a n d for the v = 0 ~ v = 1 b a n d o f 328180.
W o n g et al. [14] o b s e r v e d the v = 0 ~ v = 3 t r a n s i t i o n o f SO by difference frequency laser spectroscopy and obtained molecular parameters in the v = 3 state, which when c o m b i n e d with the m i d - I R L M R d a t a , lead to the h a r m o n i c frequency, the v i b r a t i o n a l a n h a r m o n i c i t y c o n s t a n t s a n d the e q u i l i b r i u m r o t a t i o n a l , spin spin a n d s p i n - r o t a t i o n i n t e r a c t i o n constants.
0584-8539/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0584-8539(95)01531-0
472
J.R. Anacona / Spectrochimica Acta Part A 52 (1996) 471-475
The present work reports the first measurement
of 328160 (X 3 E - ) in the first vibrational level by far-IR L M R which uses the Zeeman effect to tune magnetic components of rotational transitions into coincidence with fixed frequency cw far-IR lasers. In the rather low rotational levels probed by L M R , the SO spectrum appeared as single lines.
2. Experimental The optically pumped far-IR L M R spectrometer has been described in detail elsewhere [15] and is similar to other designs [16]. The far-IR L M R spectrometer cavity is divided by a polypropylene beam splitter into two portions, the laser section and the sample section. The sample section lies wholly within the laser cavity. The polypropylene membrane is set at approximately the Brewster angle to the laser axis in a mount which can be rotated to achieve the desired orientation between the magnetic field and the electric vector of the 618.4/zm line of the vinyl bromide laser. The vinyl bromide was used without further purification at the optimum pressure of 90 m T o r r and was optically pumped by 33 W of the 10(R)30 IR line from a cw Apollo CO2 laser. The SO radical was generated from an electrodeless 2450 M H z discharge in mixtures of SC12 and 02 outside the sample section and passed through the absorption region in a continuous flow at total pressures up to 0.5 Torr. To detect the absorption of far-IR power by the reacting gas mixture, at field strengths that correspond to magnetic resonance, a Golay cell or a germanium bolometer cooled with liquid helium were used. The signal from the detector was amplified and passed to a phase sensitive detector operating at the modulation frequency (around 250 Hz). First derivative L M R spectrum was obtained and the magnetic field was calibrated using the NS spectra accurately measured [17].
3. Laser magnetic resonance spectrum During a study of the reaction of nitrogen and SCl2 to detect NS radical [17] traces of oxygen
were found to seriously degrade NS spectral intensity and several new unreported L M R transitions were detected at 618.4 /xm. When nitrogen was replaced by oxygen and passed through the microwave discharge a different spectrum was obtained. The spectrum is simple with no hyperfine splittings due to nuclear spin suggesting the presence of SO radical. Using the best available rotational constants [4,5,13], rotational calculations of naturally occurring isotopic forms of SO were carried out with the aim of assigning the spectrum. 3280160, 328180 and 348160 in the ground and first excited vibrational levels in the 35-- and hA electronic states were considered in the fitting. O f all them, 328160 in the first vibrational level in the 3 E - state presents the closest rotational transition to the laser frequency. 3380 and other species containing nitrogen and/or chlorine were discarded as the carriers of the spectrum because they should exhibit hyperfine structure patterns due to the nonzero nuclear spin. The possibility that the spectrum might arise from a known homonuclear diatomic species like $2 or oxygen was eliminated owing to the unfavourable mismatch between the far-IR laser frequency and the rotational transitions in field-free space. The energy level structure of a diatomic molecule in a 3E electronic state has been discussed in detail by a number of authors [18,19]. Due to the spin-spin interaction constant of SO in the X 352- state is about seven times larger than its rotational constant, the radical follows a coupling scheme intermediate between Hund's cases (a) and (b) at very small N values, but approaches the case (b) at higher N values. To interpret the spectrum, the first order Zeeman Hamiltonian considering only the interaction of the external magnetic field with the electron spin was used. The matrix elements of H z in terms of Hund's case (b) basis function have been developed by Mizushima [19] and where from the energies of the triplet levels at low magnetic fields can readily be obtained as E ( N = J) = E o ( N = J ) + p B g ~ . B M j / N ( N + 1)
(1)
E ( N = J + 1 ) = E o ( N = J + 1 ) - p~g.,.BMj/N
(2)
E(N = J-
1) = Eo(N = J -
1) + I ~ g s B M j / ( N + 1) (3)
473
J.R. Anacona / Spectrochimica Acta Part A 52 (1996) 471-475
Mj
4
N,J ~'---
7,7
I
-1
0
E
(J
>. C~ L.
q~ r-
r~ ~o ~r o4
6,6
Ill
/fi,4 3 /
5 -7
Magnetic
Field
Fig. l. The schematic energy level diagram to illustrate the magnetic resonances expected at 618.4/~m in perpendicular polarization which are indicated by arrows. The nonlinearity in the Zeeman effect can be clearly seen at higher magnetic fields.
where E0 is the zero field energy, g, was assumed to be the electron free g factor, /~a the Bohr magneton (1.3996 M H z gauss-~), B denotes the applied magnetic field and Ms is the quantum number of the component of J along the magnetic field direction. Equations (2) and (3) show that the Zeeman components with Mj close to J
decrease and increase in energy for the N = J + 1 and N = J - 1 levels respectively, and they will tend to cross each other at some magnetic fields. Kawaguchi et al. [13] have shown for the SO radical that, except for the two largest Mj components of N = J - - 1, the Zeeman interactions lead to avoided crossings for components with the
474
J.R. Anacona / Spectrochirnica Acta Part A 52 (1996) 471 475
Mj
I 5I 4I
6
Mj 71i 5 41 3I
I
I
3
2
2
1"
I
I aij=-I
I
f
0.33
0.66
I 0.00
AMj =+1
Magnetic Field Intensity (Tesla) Fig. 2. The LMR spectrum associated with the N = 6,J = 7 --*N = 7,J = 7 transition of the SO radical, in the first excited vibrational level, observed at a laser frequency of 16.16956 cm ~with the electric field of the laser radiation perpendicular to the magnetic field. Several weaker transitions were detected at low magnetic field but not assigned. same M j , due to m u t u a l i n t e r a c t i o n via an interm e d i a t e N = J level. Fig. 1 illustrates schematically this b e h a v i o u r t o g e t h e r with the L M R t r a n s i t i o n s t h a t c a n be expected using the 618.4 ILm laser line, they are o f A N = 1, A J = 0 type. U s i n g the zero field S c h l a p p e q u a t i o n s [20], i n c l u d i n g centrifugal d i s t o r t i o n terms, the 618.4 /~m s p e c t r u m s h o w n in Fig. 2, can be a t t r i b u t e d u n a m b i g u o s l y to the N = 6 , J = 7--* N = 7 , J = 7 t r a n s i t i o n in the 325160 (3~,-) in the first vibrat i o n a l level. T h e values o f the r o t a t i o n a l , s p i n - s p i n interaction, s p i n - r o t a t i o n i n t e r a c t i o n a n d centrifugal d i s t o r t i o n c o n s t a n t s used to calculate the a b o v e m e n t i o n e d t r a n s i t i o n were B = 0 . 7 1 2 2 1 0 5 cm 1 2 = 5 . 3 0 0 3 7 c m - l , y = _ 5 . 6 6 6 5 8 x 1 0 - 3 cm a n d D = 0 . 1 1 3 1 2 4 x 10 5 c m l, which yield a spacing o f 16.24467 cm -~. These values were obtained from vibration-rotation measurements b y m i d - I R L M R s p e c t r o s c o p y (13). Fig. 2 also shows the f a r - I R L M R lines a s s o c i a t e d with the
m e n t i o n e d t r a n s i t i o n s in p e r p e n d i c u l a r p o l a r i z a tion where the AM~ = + 1 selection rule is allowed. T h e Z e e m a n shifts o f the m a g n e t i c c o m p o n e n t s were c a l c u l a t e d using the f o r m u l a s 1 - 3 which yield line p o s i t i o n s within 50 gauss (0.005 T) o f experimental. T h e better a c c u r a c y o c c u r r e d for n a r r o w (fast tuning) lines at low m a g n e t i c fields. U s i n g the very a c c u r a t e value o f the laser frequency: 484 751.1 M H z (16.16956 cm 1) [21] the lowest field line for the A N = 1, A J = 0, A M j = - 1 t r a n s i t i o n was p r e d i c t e d to lie at ~ 930 gauss (0.0930 T). T h e lowest field line in the s p e c t r u m which can be a t t r i b u t e d to this t r a n s i t i o n occurs at ~ 900 gauss (0.900 T) c o n f i r m i n g the prediction m a d e before. T a b l e 1 lists the o b s e r v e d m a g netic fields a n d differences between c a l c u l a t e d frequencies a n d the laser frequency for the Zeem a n c o m p o n e n t s o f the transitions. T h e m i d - I R L M R p a r a m e t e r s a n d first o r d e r Z e e m a n f o r m u las e n a b l e d the s p e c t r u m to be c a l c u l a t e d
J.R. Anacona / SpectrochmTica Acta Part A 52 (1996) 471 475
Table 1 Field measurements and assigned transitions for LMR spectrum of SO (X 3Z , v = 1) with the 618.4 lLm vinyl bromide laser line Assignment
Observed field (10 4 T)
Av '~ (10 4 cm
N,J,M.I--*N,J,M J
6,7,7 --* 7,7,6 6,7,6---,7,7,5 6,7,6 ~ 7,7,7 6,7,5 ~ 7,7,4 6,7,5 --* 7,7,6 6,7,4 ~ 7,7,3 6,7,4 ---,7,7,5 6,7,3 ~ 7,7,2 6,7,3 --, 7,7,4 6,7,2 ~ 7,7,1 6,7,2 ~ 7,7,3 6,7,1 -~ 7,7,0 6,7,1 ~ 7,7,2
930 1015 l 114 1229 1353 1522 1721 1980 2250 2648 3200 4076 5746
0.06 0.30 0.32 0.06 0.24 0.19 0.21 0.11 0.06 0.06 0.57 0.06 0.04
475
sign of the tuning rate because the laser frequency could not be shifted far enough to cause a detectable change in the resonance flux densities.
i)
Acknowledgements
~' v, calculated minus laser frequency.
sufficiently accurately to permit easy identification of the observed lines. However, in each case there were discrepancies between the observed and the calculated flux densities ranging from a few gauss at low magnetic fields up to around 1000 gauss for the stronger resonances. The larger discrepancies could not be removed by adjustments of the g factors and can be attributed to nonlinear Zeeman effects which appeared to be important around 4000 gauss (0.4000 T). In Table 1, only relatively strong signals that could be unambiguously assigned are given. The weakest four observed lines in the 7 0 0 - 8 5 0 gauss range and several sharp but very weak resonances observed at high magnetic fields, for which several alternative assignments could be made, are omitted from Table 1. In general the SO spectrum as recorded with the spectrometer was rather weak, being comparable in strength to HCS [22] and NS [17] radicals, after optimizing both the frequency and amplitude of modulation. Additional information often necessary for an unambiguous assignment can be obtained by noting the behaviour of the resonances with respect to a slight change in laser frequency by detuning the cavity length. Unfortunately, it was not possible to obtain experimental information on the
I am grateful to the Department o f Physical Chemistry at the University of Cambridge where part of the research was carried out and Dr. P.B. Davies for his continuous support.
References [1] F.X. Powell and D.R. Lide, J. Chem. Phys., 41 (1964) 1413. [2] M. Winnewisser, K.V.L.N. Sastry, R.L. Cook and W. Gordy, J. Chem. Phys., 41 (1964) 1687. [3] T. Amano, E. Hirota and Y. Morino, J. Phys. Soc. Jpn., 22 (1967) 399; 25 (1968) 300. [4] E. Tiemann, J. Mol. Spectrosc., 51 (1974) 316: J. Phys. Chem. Re['. Data, 3 (1974) 259. [5] W.W. Clark and F.C. Lucia, J. Mol. Spectrosc., 60 (1976) 332. [6] A. Carrington, D.H. Levy and T.A. Miller, Proc. R. Soc. London, Ser. A, 298 (1967) 340. [7] J.M. Daniels and P.B. Dorain, J. Chem. Phys., 45 (1966) 26. [8] H. Uehara, Bull. Chem. Soc. Jpn., 42 (1969) 886. [9] R.G.W. Norrish and G.A. Oldershaw, Proc. R. Soc. London, Ser. A, 249 (1959) 498. [10] R. Colin, Can. J. Phys., 46 (1968) 1539; 47 (1969) 979. [11] A.G. Hopkins and C.W. Brown, J. Chem. Phys., 62 (1975) 2511. [12] C. Yamada, K. Kawaguchi and E. Hirota, J. Chem. Phys., 69 (1978) 1942. [13] K. Kawaguchi, C. Yamada and E. Hirota, J. Chem. Phys., 71 (1979) 3338. [14] M. Wong, T. A m a n o and P. Bernath, J. Chem. Phys., 77 (1982) 2211. [15] D.P. Stern, Ph.D. Thesis, University of Cambridge, 1983. [16] K.M. Evenson, Discuss. Faraday Soc., 71 (1981) 7. [17] J.R. Anacona, Spectrochim. Acta Part A, 50 (1994) 909. [18] M. Tinkham and M.W.P. Strandberg, Phys. Rev., 97 (1955) 937. [19] M. Mizushima, The Theory of Rotating Diatomic Molecules, Wiley, New York, 1975. [20] R. Schlapp, Phys. Rev., 51 (1937) 342. [21] S.F. Dyubko, M.N. Efimenko, V.A. Svich and L.D. Fesenko, Sov. J. Quant. Electron., 6 (1976) 600. [22] J.R. Anacona, J. Chem. Soc. Faraday Trans., 88 (1992) 1507.