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Gas-phase EPR spectra of rotationally excited states of SH and ClO free radicals
JOURNAL OF MOLECUL£R SPECTROSCOPY 3 6 , 158--161 (1970)
LETTER TO THE EDITORS
Gas-Phase EPR Spectra of Rotationally Excited States of SH and CIO Free Radicals We have constructed a C-band (4.3-4.9 GHz) E P R s p e c t r o m e t e r for the gas-phase s t u d y in order to observe the spectra of t h e r o t a t i o n a l l y excited free radicals. Recently, the gasphase E P R was greatly developed b y C a r r i n g t o n (1). The X - b a n d spectra are, however, n o t sufficient enough to d e t e r m i n e all the molecular p a r a m e t e r s to the full accuracy of the experimental resonance-field positions w i t h o u t extensive use of some d a t a o b t a i n e d b y high resolution u l t r a v i o l e t or pure microwave spectroscopy (2). This is because the observations of the gas-phase X - b a n d E P R s p e c t r a have been confined to a r o t a t i o n a l level, the lowest ground level, except those for OH (3) a n d SO (4, 5). Very recently, C a r r i n g t o n et al. have reported the s p e c t r a of the r o t a t i o n a l excited ( J = 5/~) CF (6) and S e l l (7). If transitions are observed for more t h a n a r o t a t i o n a l level, unique d e t e r m i n a t i o n of various molecular c o n s t a n t s would be possible b y different J - d e p e n d e n c e of the molecular c o n s t a n t s . High s e n s i t i v i t y of the s p e c t r o m e t e r is m o s t i m p o r t a n t in the s t u d y of gaseous free radicals. We have c o n s t r u c t e d a c a v i t y w i t h the cylindrical TE0u mode w i t h a Q factor as high as 8000. A m a g n e t now available to us has an air-gap w i d t h of 60 m m and a pole d i a m e t e r of 300 m m for the m a x i m u m field of 10 000 G. The s p e c t r o m e t e r c a v i t y thus designed has an inside d i a m e t e r of 140 m m and a n inside h e i g h t of 41 m m and resonated a t a b o u t 4.47 GHz. I t was m o u n t e d in the pole gap, w i t h the axis of the cylinder parallel to the dc magnetic field. The 100 kHz S t a r k m o d u l a t i o n proposed b y C a r r i n g t o n et al. (8) was adopted since Zeeman m o d u l a t i o n is difficult to apply to such a large sample space inside of t h e cavity. A surprising result is t h a t m u c h s t r o n g e r s p e c t r a were o b t a i n e d t h a n w i t h an X - b a n d Zeeman m o d u l a t e d TE0~I c a v i t y r e p o r t e d previously (2). T h e signal-to-noise ratio of the s p e c t r a of SO (1A, J = 2) (9) observed b y this C-band c a v i t y was larger t h a n t h a t o b t a i n e d b y the X - b a n d c a v i t y b y a factor of more t h a n 5. A most d o m i n a n t factor for increasing t h e s e n s i t i v i t y is the remarkable increase of the sample volume (by a factor of 20). T h e calibration of the magnetic field inside the c a v i t y was performed b y t a k i n g the s p e c t r u m of NO in the ~II8/2, J = 5/~ s t a t e (10). The SH radicals were produced in the s p e c t r o m e t e r c a v i t y b y adding H~S (0.05 Torr) a t the c a v i t y e n t r a n c e to a s t r e a m of discharge p r o d u c t s of H20 (0.2 Torr). The C-band E P R spectra of the SH radicals of the J = ~ r o t a t i o n a l ground s t a t e and those of the J = 5~ excited s t a t e were observed a t fields of a b o u t 3.60 kG and 7.40 to 9.05 kG, respectively, b y using a microwave frequency of 4.47 GHz. T h e spectra of SIt in the 2IIs/2 , J = 5/~ level are s h o w n in Fig. 1. T h e E P R spectra of the J = /~ ground rotational s t a t e h a v e been observed b y Radford and Linzer (11) a n d M c D o n a l d (12) b y using the X - b a n d frequencies. Figure 1 shows a A-type doublet ~plitting of 1.65 kG which was roughly in accord w i t h t h a t of 190 G for t h e J = 3/~ state. The p r o t o n hyperfine splittings were n o t observed. T h e s p e c t r a r e p r e s e n t the distinctive feature of the S t a r k m o d u l a t e d lines. As was pointed o u t b y C a r r i n g t o n et al. (13), the lines o~ the two components of the A-doublet are 180 ° out-ofphase, each h a v i n g the shape of a Zeeman m o d u l a t e d line. W i t h the second-order S t a r k 158
L E T T E R TO T H E E D I T O R S
I
7.3
_
7.5
I
9.0
159
I
9.2 kG
Fro. 1. Gas-phase E P R s p e c t r u m of r o t a t i o n a l l y excited SH(2113/2, J = ~ ) observed w i t h a microwave frequency of 4.47 GHz. effect proportional to MI ~, i n n e r lines (transitions between M j = =t=~ and 4 - ~ ) of t h e four for one c o m p o n e n t of t h e ~,-doublet are u n d e r - m o d u l a t e d a n d are less intense t h a n the outer lines (transitions between M j = =1=5/~and :i=~). T h e c e n t e r c o m p o n e n t (a t r a n s i t i o n between M j = - - ~ and ~ ) was too weak to be observed in t h e present experimental condition. The S t a r k de and 100 kHz voltages applied were 100 a n d 50 V / c m which are t h e maxim u m of t h e present C-band spectrometer. T h e C-band E P R spectra of gaseous C10 radicM were observed b y p u m p i n g a gas mixture of 02 (0.1 Torr) a n d C12 (0.2 Torr) u n d e r a microwave discharge a n d t h e n t h r o u g h the detection cavity. A t a field between 8.5 and 10.0 k G t h e s p e c t r a of the r o t a t i o n a l excited s t a t e (KI3/2, J -~ ~ ) of C10 were observed b y a microwave frequency of 4.47 GHz, in addition to those of the ground s t a t e ( J = ~ ) a t a b o u t 3.9 ~G. T h e s p e c t r a of the 2II3/2, J ~ 5/~ s t a t e are shown in Fig. 2, where the s t r o n g e r lines are those of 35C10 and additional weaker lines are those of 87C10. Since t h e m a x i m u m magnetic field now available to us is 10 000 G, the highest-field group of four lines for t h e t r a n s i t i o n s M = 5/~ ~ 3~ was not detected. T h e X - b a n d E P R spectra of t h e J = ~ r o t a t i o n M ground s t a t e of C10 were first detected and analyzed b y C a r r i n g t o n et al. (15). T h e y r e p o r t e d only briefly t h a t t h e y detected also t h e spectral lines in the J = ~ level (15). A p r e l i m i n a r y calculation was made b y diagonalizing the 9 X 9 m a t r i x of t h e H a m i l t o n i a n for the ~II~12and ~II1/2s t a t e s w i t h J = ~ to 9~ for given vMues of M j and M1 . The molecu-
160
L E T T E R TO T H E E D I T O R S
I
I
J
8.5
9.0
9.5
kG
FIG. 2. Gas-phase E P R s p e c t r u m of r o t a t i o n a l l y excited C10(~II312, J = ~ ) observed with a microwave frequency of 4.47 GHz. lar c o n s t a n t s used were A -- --282 cm -1 (15), B0 = 18603 MHz, h = a+ (b + c)/2 = 113 MHz, eqQ = - 8 7 MHz (15), g, = 2002319, and ge = 1.00000 for ~sC10. The m a i n feature of the spectra in Fig. 2 were well reproduced w i t h the calculated values.
ACKNOWLEDGMENT T h e authors are i n d e b t e d to Dr. S. Saito for m a n y valuable discussions. REFERENCES I. 2. 3. 5. 5. 6.
A. CARRINGTON,"Molecular S p e c t r o s c o p y , " p. 157. Elsevier, London, 1968. H. UEHARA AND Y. MORINO, Mol. Phys. 17, 239 (1969). H. E. R+~DFORD, Phys. Rev. 122, 114 (1961). J. M. DANIELS AND P. B. DORA]N, J. Chem. Phys. 45, 26 (1966). H. UEnARA, Bull. Chem. Soc. Jap. 42, 886 (1969). A. CARRINGTON AND B. Z. HOWARD, Mol. Phys. 18, 225 (1970).
7. A. CARRINGTON,G. N. CURRIE, AND N. J. D. LucAs, Proc. Roy. Soc., Ser. A 315, 355 (1970).
8. A. CARRINGTON,D. H. LEVY, AND T. A. MILLER, Rev. Sci. Instrum. 38, 1183 (1967). 9. A. CARRIN~TON, D. H. LEvr, ANO T. A. MILLER, Proc. Roy. Soc., Ser. A 293, 108 (1966).
161
L E T T E R TO T H E E D I T O R S
10. R. L. BROWN AND H. E. RADFORD, Phys. Rev. 147, 6 (1966). 11. H. E. RADFORD AND M. LINZER, Phys. Rev. Lett. 10, 443 (1963). 12. C. C. McDoN,~LD, J. Chem. Phys. 39, 2587 (1963). 18. A. CARRINGTON, D. H. LEVY, AND T. A. MILLER, J. Chem. Phys. 47, 3801 (1967). 15. A. CAREINGTON, 1:). H. DYER, AND ~). H. LEVY, J . Chem. Phys. 47, 1756 (1967). 15. T. AMANO, S. SAITO, E. HIROT•, Y. MORINO, D. R. JOHNSON, AND F. X. I:)OWELL, J .
Mol. Spectrosc. 30, 275 (1969). HIROMICHI UEHARA YONEZO MORINO
Sagami Chemical Research Center, Sagamihara-shi, Kanagawa, Japan Received: May 6, 1970
LETTER TO THE EDITORS
Gas-Phase EPR Spectra of Rotationally Excited States of SH and CIO Free Radicals We have constructed a C-band (4.3-4.9 GHz) E P R s p e c t r o m e t e r for the gas-phase s t u d y in order to observe the spectra of t h e r o t a t i o n a l l y excited free radicals. Recently, the gasphase E P R was greatly developed b y C a r r i n g t o n (1). The X - b a n d spectra are, however, n o t sufficient enough to d e t e r m i n e all the molecular p a r a m e t e r s to the full accuracy of the experimental resonance-field positions w i t h o u t extensive use of some d a t a o b t a i n e d b y high resolution u l t r a v i o l e t or pure microwave spectroscopy (2). This is because the observations of the gas-phase X - b a n d E P R s p e c t r a have been confined to a r o t a t i o n a l level, the lowest ground level, except those for OH (3) a n d SO (4, 5). Very recently, C a r r i n g t o n et al. have reported the s p e c t r a of the r o t a t i o n a l excited ( J = 5/~) CF (6) and S e l l (7). If transitions are observed for more t h a n a r o t a t i o n a l level, unique d e t e r m i n a t i o n of various molecular c o n s t a n t s would be possible b y different J - d e p e n d e n c e of the molecular c o n s t a n t s . High s e n s i t i v i t y of the s p e c t r o m e t e r is m o s t i m p o r t a n t in the s t u d y of gaseous free radicals. We have c o n s t r u c t e d a c a v i t y w i t h the cylindrical TE0u mode w i t h a Q factor as high as 8000. A m a g n e t now available to us has an air-gap w i d t h of 60 m m and a pole d i a m e t e r of 300 m m for the m a x i m u m field of 10 000 G. The s p e c t r o m e t e r c a v i t y thus designed has an inside d i a m e t e r of 140 m m and a n inside h e i g h t of 41 m m and resonated a t a b o u t 4.47 GHz. I t was m o u n t e d in the pole gap, w i t h the axis of the cylinder parallel to the dc magnetic field. The 100 kHz S t a r k m o d u l a t i o n proposed b y C a r r i n g t o n et al. (8) was adopted since Zeeman m o d u l a t i o n is difficult to apply to such a large sample space inside of t h e cavity. A surprising result is t h a t m u c h s t r o n g e r s p e c t r a were o b t a i n e d t h a n w i t h an X - b a n d Zeeman m o d u l a t e d TE0~I c a v i t y r e p o r t e d previously (2). T h e signal-to-noise ratio of the s p e c t r a of SO (1A, J = 2) (9) observed b y this C-band c a v i t y was larger t h a n t h a t o b t a i n e d b y the X - b a n d c a v i t y b y a factor of more t h a n 5. A most d o m i n a n t factor for increasing t h e s e n s i t i v i t y is the remarkable increase of the sample volume (by a factor of 20). T h e calibration of the magnetic field inside the c a v i t y was performed b y t a k i n g the s p e c t r u m of NO in the ~II8/2, J = 5/~ s t a t e (10). The SH radicals were produced in the s p e c t r o m e t e r c a v i t y b y adding H~S (0.05 Torr) a t the c a v i t y e n t r a n c e to a s t r e a m of discharge p r o d u c t s of H20 (0.2 Torr). The C-band E P R spectra of the SH radicals of the J = ~ r o t a t i o n a l ground s t a t e and those of the J = 5~ excited s t a t e were observed a t fields of a b o u t 3.60 kG and 7.40 to 9.05 kG, respectively, b y using a microwave frequency of 4.47 GHz. T h e spectra of SIt in the 2IIs/2 , J = 5/~ level are s h o w n in Fig. 1. T h e E P R spectra of the J = /~ ground rotational s t a t e h a v e been observed b y Radford and Linzer (11) a n d M c D o n a l d (12) b y using the X - b a n d frequencies. Figure 1 shows a A-type doublet ~plitting of 1.65 kG which was roughly in accord w i t h t h a t of 190 G for t h e J = 3/~ state. The p r o t o n hyperfine splittings were n o t observed. T h e s p e c t r a r e p r e s e n t the distinctive feature of the S t a r k m o d u l a t e d lines. As was pointed o u t b y C a r r i n g t o n et al. (13), the lines o~ the two components of the A-doublet are 180 ° out-ofphase, each h a v i n g the shape of a Zeeman m o d u l a t e d line. W i t h the second-order S t a r k 158
L E T T E R TO T H E E D I T O R S
I
7.3
_
7.5
I
9.0
159
I
9.2 kG
Fro. 1. Gas-phase E P R s p e c t r u m of r o t a t i o n a l l y excited SH(2113/2, J = ~ ) observed w i t h a microwave frequency of 4.47 GHz. effect proportional to MI ~, i n n e r lines (transitions between M j = =t=~ and 4 - ~ ) of t h e four for one c o m p o n e n t of t h e ~,-doublet are u n d e r - m o d u l a t e d a n d are less intense t h a n the outer lines (transitions between M j = =1=5/~and :i=~). T h e c e n t e r c o m p o n e n t (a t r a n s i t i o n between M j = - - ~ and ~ ) was too weak to be observed in t h e present experimental condition. The S t a r k de and 100 kHz voltages applied were 100 a n d 50 V / c m which are t h e maxim u m of t h e present C-band spectrometer. T h e C-band E P R spectra of gaseous C10 radicM were observed b y p u m p i n g a gas mixture of 02 (0.1 Torr) a n d C12 (0.2 Torr) u n d e r a microwave discharge a n d t h e n t h r o u g h the detection cavity. A t a field between 8.5 and 10.0 k G t h e s p e c t r a of the r o t a t i o n a l excited s t a t e (KI3/2, J -~ ~ ) of C10 were observed b y a microwave frequency of 4.47 GHz, in addition to those of the ground s t a t e ( J = ~ ) a t a b o u t 3.9 ~G. T h e s p e c t r a of the 2II3/2, J ~ 5/~ s t a t e are shown in Fig. 2, where the s t r o n g e r lines are those of 35C10 and additional weaker lines are those of 87C10. Since t h e m a x i m u m magnetic field now available to us is 10 000 G, the highest-field group of four lines for t h e t r a n s i t i o n s M = 5/~ ~ 3~ was not detected. T h e X - b a n d E P R spectra of t h e J = ~ r o t a t i o n M ground s t a t e of C10 were first detected and analyzed b y C a r r i n g t o n et al. (15). T h e y r e p o r t e d only briefly t h a t t h e y detected also t h e spectral lines in the J = ~ level (15). A p r e l i m i n a r y calculation was made b y diagonalizing the 9 X 9 m a t r i x of t h e H a m i l t o n i a n for the ~II~12and ~II1/2s t a t e s w i t h J = ~ to 9~ for given vMues of M j and M1 . The molecu-
160
L E T T E R TO T H E E D I T O R S
I
I
J
8.5
9.0
9.5
kG
FIG. 2. Gas-phase E P R s p e c t r u m of r o t a t i o n a l l y excited C10(~II312, J = ~ ) observed with a microwave frequency of 4.47 GHz. lar c o n s t a n t s used were A -- --282 cm -1 (15), B0 = 18603 MHz, h = a+ (b + c)/2 = 113 MHz, eqQ = - 8 7 MHz (15), g, = 2002319, and ge = 1.00000 for ~sC10. The m a i n feature of the spectra in Fig. 2 were well reproduced w i t h the calculated values.
ACKNOWLEDGMENT T h e authors are i n d e b t e d to Dr. S. Saito for m a n y valuable discussions. REFERENCES I. 2. 3. 5. 5. 6.
A. CARRINGTON,"Molecular S p e c t r o s c o p y , " p. 157. Elsevier, London, 1968. H. UEHARA AND Y. MORINO, Mol. Phys. 17, 239 (1969). H. E. R+~DFORD, Phys. Rev. 122, 114 (1961). J. M. DANIELS AND P. B. DORA]N, J. Chem. Phys. 45, 26 (1966). H. UEnARA, Bull. Chem. Soc. Jap. 42, 886 (1969). A. CARRINGTON AND B. Z. HOWARD, Mol. Phys. 18, 225 (1970).
7. A. CARRINGTON,G. N. CURRIE, AND N. J. D. LucAs, Proc. Roy. Soc., Ser. A 315, 355 (1970).
8. A. CARRINGTON,D. H. LEVY, AND T. A. MILLER, Rev. Sci. Instrum. 38, 1183 (1967). 9. A. CARRIN~TON, D. H. LEvr, ANO T. A. MILLER, Proc. Roy. Soc., Ser. A 293, 108 (1966).
161
L E T T E R TO T H E E D I T O R S
10. R. L. BROWN AND H. E. RADFORD, Phys. Rev. 147, 6 (1966). 11. H. E. RADFORD AND M. LINZER, Phys. Rev. Lett. 10, 443 (1963). 12. C. C. McDoN,~LD, J. Chem. Phys. 39, 2587 (1963). 18. A. CARRINGTON, D. H. LEVY, AND T. A. MILLER, J. Chem. Phys. 47, 3801 (1967). 15. A. CAREINGTON, 1:). H. DYER, AND ~). H. LEVY, J . Chem. Phys. 47, 1756 (1967). 15. T. AMANO, S. SAITO, E. HIROT•, Y. MORINO, D. R. JOHNSON, AND F. X. I:)OWELL, J .
Mol. Spectrosc. 30, 275 (1969). HIROMICHI UEHARA YONEZO MORINO
Sagami Chemical Research Center, Sagamihara-shi, Kanagawa, Japan Received: May 6, 1970