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Resonance Raman spectroscopy of the methyl radical
Volume 151, number 3
RESONANCE
CHEMICAL
RAMAN SPECTROSCOPY
P.B. KELLY and Sjon G. WESTRE Department qf Chemistry, Unrversity of Calijnrnia,
PHYSICS LETTERS
14 October
1988
OF THE METHYL RADICAL
Davis, CA 956 Ih, USA
Received 24 June 1988; in final form 20 July 1988
The first resonance Raman spectrum of the methyl radical is reported. The frequency of the symmetric stretch, u,, is found to be in good agreement with previous experimental results. The overtone 2v, is obsct-vcd and the anharmonic constant for the symmetric stretch, X, ,, is determined. The overtone of the out ofplane symmetric bend 2vL is observed and confirms the negative anharmonicity for this potential obtained by Yamada et al. [J. Chem. Phys. 75 ( 1981) 52561.
1. Introduction There is considerable motivation to study the methyl radical. The methyl radical is one of the most important species in combustion reactions though it has proven difficult to observe and quantify [ 1,2 1. The methyl radical is of interest both as product and reactant in photodissociation studies. Fundamental resonance Raman work by Kinsey et al. has yielded a model for the short term dynamics in the photodissociation of methyl iodide [ 3,4] _ Characterization of the fundamental vibrations, overtones, and combination tones for v, and v, of the methyl radical are needed to allow determination of the final energy distribution in the methyl halide photodissociation process. The methyl radical can be considered as a reactant in the photodissociation process yielding methylene and a hydrogen atom [ 5,6 1. This process arises from the predissociation of the B ‘A’, of the methyl radical. Ziegler et al. have applied resonance Raman methods to determine the predissociation lifetime of ammonia in a very similar problem [ 7,s 1. These methods may be applied directly to the methyl radical. The first spectroscopic observation of the methyl radical was by Herzberg and Shoosmith using flash photolysis to produce the radical and ultraviolet absorption for detection [ 91. They obtained a diffuse absorption spectrum at 2 16 nm and a series of Rydberg transitions at shorter wavelengths. Herzberg later 0 009-2614f 88/$ 03.50 0 Elsevier Science Publishers ( North-Holland Physics Publishing Division )
extended their work to include CD3 [ lo]. The analysis of the s-x band indicated that both the ground and first excited states are planar. A variety of spectroscopic methods have been used to determine the molecular parameters of the methyl radical. Tan, Winer, and Pimental developed a rapid scan infrared spectrometer for gas phase detection of the methyl radical [ 111. Yamada et al. observed the v2 l-0, 2-1, and 3-2 bands of the gas phase methyl radical by high resolution infrared tunable diode laser spectroscopy [ 121. Amano et al. have used difference frequency laser spectroscopy to accurately measure the band origin of v$ [ 131. Holt et al. used transient CARS spectroscopy to examine the symmetric stretch, v, [ 141. Snelson used values for v2, v3, and v4 from infrared observations of CH3 and CD, in argon matrices to determine force constants and predicted a value for the symmetric stretch for CH, and CD3 of 3044 and 2 153 cm-’ respectively [ 15 1. Spirko and Bunker determined the equilibrium bond length and shape of the complete potential energy curve for the methyl radical [ 16 ] _ This was done by fitting the data of Yamada et al. using the non-rigid invertor Hamiltonian and a model anharmonic potential function. In this Letter we report the first resonance Raman spectra of the methyl radical. Observed frequencies and molecular constants are compared to literature values. The feasibility of determining the excited state lifetime from the observed rotational structure is considered. B.V.
253
Volume 15I, number 3
CHEMICAL PHYSICS LETTERS
2. Experimental
The resonance Raman apparatus used in the present work is based on a 20 Hz Nd : YAG-pumped dye laser system The HY-750 Nd : YAG provides both 532 nm light to pump the Lumonics HD-300 dye laser and a 266 nm photolysis beam. The dye laser is operated between 640 and 6.50 nm with a 16% conversion efficiency. The output of the dye laser is doubled with a KD*P crystal rotated 20” about the laser beam axis to provide a component of the electric field of the fundamental perpendicular to that of the second harmonic. The conversion efficiency to second harmonic in this configuration is approximately 14%. The second harmonic is summed with the fundamental by type I phase matching in beta barium borate to produce third harmonic radiation with approximately 20% of the second harmonic converted to third harmonic. The dye laser harmonics were separated with two 60” S l-UV prisms and focused into the sample area with a 6 inch focal length Sl -UV lens. The 266 nm photolysis beam had travelcd directly to the sample, having been separated from the 532 nm radiation by a dichroic mirror and from the 1064 nm YAG fundamental with a 60” prism. The 266 nm beam was varied from 5 to 0.5 mJ per pulse by detuning the KD*P Nd : YAG fourth harmonic crystal. This produced approximately linear correlation between 266 nm photolysis power and methyl radical signal intensity. The photolysis beam was focused to a diameter of approximately 2 mm at the sample area and crossed the probe beam with electric vector perpendicular to the monochromator slits. The 0.4 mJ probe beam at 2 14 or 216 nm arrived 15 ns later and was introduced into the sample area by passing through a small hole in the collection mirror. The precursor molecules were entrained in an argon flow which exited a 1.5 mm diameter pipet at a flow rate of 120 ml/min, crossed both photolysis and probe beams at right angles, and was collected by an exhaust tube. The CH?I was present in the flow at room temperature vapor pressure (44 kPa or 330 Torr). The CD31 flow was diluted approximately 10: 1 with argon to give 4.4 kPa (33 Torr) of precursor in the sample area. The CHjBr gas from the lecture bottle was diluted 10 : 1 with argon. Both methyl iodide samples were used as supplied from 254
14 October I988
Aldrich Chemical Company. The CH,Br was supplied by Matheson. The Raman scattered light was collected in standard backscatter geometry [ 171 by a 2 inch focal lengthf/0.67 spherical mirror and focused onto the slits of a 1.0 m Czemy-Turner monochromator. A crystalline quartz scrambler was mounted in front of the slits to compensate for variation of grating efficiency with polarization. A mercury lamp was located behind the beam stop on axis with the monochromator to provide calibration lines. The 1800 grooves/mm grating was scanned in third order. With 200 pm slits the spectral resolution is 0.037 nm corresponding to 9 cm-‘. The Hamamatsu R 166UH solar blind photomultiplicr tube was operated at 1200 V. The photomultiplier output was amplified by a 20x gain 2 ns risetime preamp and was sampled with a Stanford Research Systems gated integrator with a 15 ns gate. The output also passed through a pulse stretching amplifier to a second gated integrator set to detect the 120 Hz signal from the mercury lamp. The dc signals from the two gated integrators were sampled by an A/D converter, stored on hard disk, and displayed on an IBM PC/AT computer.
3. Results and discussion The methyl radical has four normal modes, of which three have fundamentals that could in principle be Raman active. The intensity of a transition in the resonance Raman spectrum is dependent upon changes in the upper state potential surface relative to the ground state surface. A full treatment of resonance Raman spectroscopy has appeared in several reviews [ 171. Briefly, a displacement in a totally symmetric normal mode enhances the Raman fundamental and all overtones for that vibration. A large force constant change for either a totally symmetric or non-totally symmetric vibration will yield intensity only in even overtones of the vibration of interest. Combination tones of resonance-enhanced normal modes can be expected to appear in the Raman spectrum. Normal modes for which there is no displacement or force constant change in the excited state are not enhanced in the resonance Raman spectrum.
CHEMICAL PHYSICS LETTERS
Volume 1S 1, number 3
From the rotational analysis of the methyl radical B-Z%absorption spectrum Herzberg determined that the C-D bond length in the excited state was 0.1124 nm, significantly different from that of the ground state 0.1079 nm [ 181. This change in bond length, with retention of DIh symmetry yields a displacement in the symmetric stretch normal mode. Herzberg also identified the progression of absorption bands as being due to a force constant change in the umbrella mode, vZ, on the excited state surface. No absorption features associated with excitation of u3 or lrq were identified, implying that the excited state potentials of vj and vq are similar to those of the ground state. Application of the simple trends in resonance Raman scattering discussed above yields the prediction that Y,, 2u,, 2u2, and v,+~v? will appear in the Raman spectrum which is enhanced by resonance with the B excited state of the methyl radical. This is in agreement with the observed spectrum in fig. 1. The methyl radical was generated by 266 nm photolysis of methyl iodide. The spectrum was reproduced with methyl bromide as the precursor, utilizing the same 2 16 nm laser pulse to both photolyze and probe the photoproducts. Table 1 lists the observed frequencies of the methyl radical. Note that this study reports band centers while the CARS and high resolution infrared work report band origins. The first overtone of the umbrella mode, 2vZ, is observed at 1286 cm-‘. This agrees well with the 1288 cm- ’ band origin predicted from the infrared diode laser work of Yamada et al. and confirms the large “negative” anharmonicity discussed by Yamada [ 12 1. The Q branch of u ,, the symmetric stretch, is observed at 3003 cm-’ in good agreement with the 3004.8 cm- ’obtained from Holt et al.‘s CARS work [ 141, the calculated value of 2992.6 cm- ’from Spirko and Bunker’s force constant analysis [ 161 and the 3044 cm-’ determined Table I Observed
vibrational
frequencies
of the methyl radical This work
(frequencies
il
I4 October
IUIIU
Fig.
1.0
2992.6
5959.7+2.1 1284.7? 1.9
1286.4
v, t 2u,
4268.0+
sob0
by the force constant analysis of Snclson [ 151. The overtone 2v, was observed at 5960 cm-’ yielding an anharmonicity constant X, , = -22.7 + 1.5 cm- ‘. The combination band v1 +2u2 is seen at 4268 cm-’ giving an anharmonic coupling constant XIZ= -9.8 -t 1.5 cm-‘. The deuterated methyl radical spectrum was also examined. The overtone of the umbrella model, the symmetric stretch, and several combination tones were observed for CD,. The analysis of the CD, spectrum will be presented in a future publication. Given the large rotational constant for the methyl radical, it was possible to resolve the rotational structure. The resonance enhancement for the observed spectra is dominated by a single electronic state, B ‘A;, and one vibrational level within that state. The rotational levels of the B state are broadened by predissociation such that only a diffuse R branch and PSQ feature can be observed in the absorption spectrum of the O-O band for CH, [ 19,201. In the spectrum where CH$ was used as the precursor, the Rayleigh line, 2u,, and uI showed enhanced rotational structure. As the probe wavelength
in cm-‘)
[ 161
3004.8 & 0.2
3002.42
don
1.Resonance Raman spectrum of CH, at 2 15.780 nm
Ref.
“I
30’00
Wuvrriulllbr,
Ref. [IS]
2v, 2v2
2&U
I988
_
Ref.
[I]
1288.0900
1.7
255
Volume 15 1, number 3
CHEMICAL
was changed from 215.7 to 216.5 nm, the relative intensity of the rotational structure shifted from strong Q, R, and S branches at 215.7 nm to 0, P, Q, R, and S branches at 216.5 nm. At 215.7 nm the probe beam is resonant only with the R branch of the methyl radical B-z absorption. This causes scattering enhancement of the Q, R, and S branches in the Raman spectrum [ 81. At 216.5 nm the probe beam is in resonance with the P+Q and R branches of the absorption. As before, Q, R, and S scattering is enhanced for excitation resonant with the R absorption branch. Resonance with the Q branch enhances P, Q, and R scattering while resonance with the P branch of the B-R transition gives rise 10 0, P, and Q branches. Overall then, 0, P, Q, R, and S branches appear in the Raman spectrum. This shifting of intensities in the rotational structure by changing excitation wavelength demonstrates the feasibility of determining the excited state lifetime of the methyl radical. By fitting the pattern of relative intensities observed in the resonance rotational or rovibrational Raman spectrum, the excited state lifetime of the molecule can be determined. This method of excited state lifetime determination has been developed by Ziegler and applied to NH3 and ND3 [7,8]. The resonance Raman technique is an ideal tool for studying the chemical kinetics of radical systems. For this purpose it is important to consider concentration estimates for the methyl radical and the detection limits for our system. The initial concentration of the methyl radical was estimated as the number of methyl radicals generated from CH31 at 293 K with unit quantum efficiency for the photolysis process. The molar absorptivity of CHII at 293 K and 266 nm was determined to be 334 M-’ cm-‘, which yields a number density of 10” radicals crne3 produced by a 1 mJ, 8 ns pulse of 266 nm light. At room temperature, the main route for elimination of melhyl radicals is the recombination of methyl radicals to form ethane. Using Yamada’s suggested value of the rate constant for the recombination reaction, k=2X1013 cm3 mol-’ s-l, the concentration of methyl radicals present upon the arrival of the probe beam 15 ns after photolysis is 7 x 10” molecules cm-3.
256
I4 October
PHYSICS LETTERS
1988
Far ultraviolet resonance Raman is a powerful technique for examining free radicals. By properly choosing the precursor so as to avoid an electronic transition coincident with the probe beam wavelength, interference due to resonance enhancement of the precursor is minimized. As can be seen in fig. 1, the resonant signal from 4.4 kPa (33 Torr) ofCH, is many times larger than the non-resonant signal from the 44 kPa (330 Torr) of CH,l. The application of resonance Raman spectroscopy to the study of small organic free radicals is in its infancy. Nevertheless, it has great potential to provide a wealth of information on both the structure and dynamics of transient species. By combining the Raman technique with other spectroscopic methods, it will be possible to thoroughly characterize these systems. We are currently exlending this technique to other small organic free radicals.
Acknowledgement This work was supported in part by a New Faculty Research Grant from the University of California al Davis.
References [ 1] KC. Smyth and P.H. Taylor, Chem. Phys. Letters 122 (1985) 518. [Z] J.A. Miller and G.A. Fisk, Chem. Eng. News (August 1987).
31,
[ 31 D. Imre, J.L. Kinsey, A. Sinha and J. Krenos, J. Phys. Chem. 88 (1984) 3956. 141 R.L. Sundberg,D. Imre. M. O’Hale, J.L. Kinsey and R.D. Coalson, J. Phys. Chem. 90 ( 1986) 5001. [S] H.T. Yu, A. Sewn, E. Kassab and E.M. Evleth, J. Phys. Chem. 88 ( 1984) 2049. [ 61 G. Herzbberg, The spectra and structures of simple free radicals; an introduction to molecular spectroscopy (Cornell Univ. Press, Ithaca, I97 I ). [7 JL.D. Ziegler, P.B. Kelly and B. Hudson, J. Chem. Phys. 8 I (1984) 6399. [S] L.D. Ziegler, J. Chem. Phys. 84 (I 986) 6013. 191 G. Hcrzbclgand J. Shoosmith, Can. J. Phys. 34 ( 1956) 523. [ 101g. Herzberg, Proc. Roy. Sot. A 262 ( 1961) 29 I, [ II] L. Tan,A. Winer and G. Pimcntal. J. Chem. Phys. 57 ( 1972) 4028.
[ 121C. Yamada, E. Hirota and K. Kawaguchi, J. 75 (1981)
5256.
Chem.
Phys.
Volume
I 51,number 3
CHEMICAL
[ 131 T. Amano, P.F. Bemath, C. Yamada, Y. Endo and E. Hirota, J. Chem. Phys. 77 (1982) 5284. [ 141 P.L. Holt, K.E. McCurdy, R.B. Wcisman, J.S. Adams and P.S. Engel, J. Chem. Phys. 81 ( 1984) 3349. [ 151 A. Snelson, J. Phys. Chem. 74 ( 1970) 537. [ 161V. Spirko and P.R. Bunker, J. Mol. Spectry. 95 ( 1982) 381. [I I] B. Hudson, P.B. Kelly, L.D. Ziegler, R.A. Desiderio, D.P. Gerrity, W. Hess and R. Bates, Far ultraviolet resonance Raman studies ofelectronic excitations, in: Advances in laser spectroscopy, Vol. 3, eds. B.A. Garetz and J.R. Lombardi (Wiley, New York, 1986).
PHYSICS LETTERS
I4 October 1988
[IS] M. Robin, Higher excited states of polyatomic
molecules, Vol. 3 (Academic Press. New York, 1974). [ IY] N. Nakashima and K. Yoshihara, Laser Chem. 7 ( 1987) 177. [20] J. Danon, H. Zacharias, H. Rottke and K.H. Welge, J. Chem. Phys. 76 ( 198 I ) 2399.
257
RESONANCE
CHEMICAL
RAMAN SPECTROSCOPY
P.B. KELLY and Sjon G. WESTRE Department qf Chemistry, Unrversity of Calijnrnia,
PHYSICS LETTERS
14 October
1988
OF THE METHYL RADICAL
Davis, CA 956 Ih, USA
Received 24 June 1988; in final form 20 July 1988
The first resonance Raman spectrum of the methyl radical is reported. The frequency of the symmetric stretch, u,, is found to be in good agreement with previous experimental results. The overtone 2v, is obsct-vcd and the anharmonic constant for the symmetric stretch, X, ,, is determined. The overtone of the out ofplane symmetric bend 2vL is observed and confirms the negative anharmonicity for this potential obtained by Yamada et al. [J. Chem. Phys. 75 ( 1981) 52561.
1. Introduction There is considerable motivation to study the methyl radical. The methyl radical is one of the most important species in combustion reactions though it has proven difficult to observe and quantify [ 1,2 1. The methyl radical is of interest both as product and reactant in photodissociation studies. Fundamental resonance Raman work by Kinsey et al. has yielded a model for the short term dynamics in the photodissociation of methyl iodide [ 3,4] _ Characterization of the fundamental vibrations, overtones, and combination tones for v, and v, of the methyl radical are needed to allow determination of the final energy distribution in the methyl halide photodissociation process. The methyl radical can be considered as a reactant in the photodissociation process yielding methylene and a hydrogen atom [ 5,6 1. This process arises from the predissociation of the B ‘A’, of the methyl radical. Ziegler et al. have applied resonance Raman methods to determine the predissociation lifetime of ammonia in a very similar problem [ 7,s 1. These methods may be applied directly to the methyl radical. The first spectroscopic observation of the methyl radical was by Herzberg and Shoosmith using flash photolysis to produce the radical and ultraviolet absorption for detection [ 91. They obtained a diffuse absorption spectrum at 2 16 nm and a series of Rydberg transitions at shorter wavelengths. Herzberg later 0 009-2614f 88/$ 03.50 0 Elsevier Science Publishers ( North-Holland Physics Publishing Division )
extended their work to include CD3 [ lo]. The analysis of the s-x band indicated that both the ground and first excited states are planar. A variety of spectroscopic methods have been used to determine the molecular parameters of the methyl radical. Tan, Winer, and Pimental developed a rapid scan infrared spectrometer for gas phase detection of the methyl radical [ 111. Yamada et al. observed the v2 l-0, 2-1, and 3-2 bands of the gas phase methyl radical by high resolution infrared tunable diode laser spectroscopy [ 121. Amano et al. have used difference frequency laser spectroscopy to accurately measure the band origin of v$ [ 131. Holt et al. used transient CARS spectroscopy to examine the symmetric stretch, v, [ 141. Snelson used values for v2, v3, and v4 from infrared observations of CH3 and CD, in argon matrices to determine force constants and predicted a value for the symmetric stretch for CH, and CD3 of 3044 and 2 153 cm-’ respectively [ 15 1. Spirko and Bunker determined the equilibrium bond length and shape of the complete potential energy curve for the methyl radical [ 16 ] _ This was done by fitting the data of Yamada et al. using the non-rigid invertor Hamiltonian and a model anharmonic potential function. In this Letter we report the first resonance Raman spectra of the methyl radical. Observed frequencies and molecular constants are compared to literature values. The feasibility of determining the excited state lifetime from the observed rotational structure is considered. B.V.
253
Volume 15I, number 3
CHEMICAL PHYSICS LETTERS
2. Experimental
The resonance Raman apparatus used in the present work is based on a 20 Hz Nd : YAG-pumped dye laser system The HY-750 Nd : YAG provides both 532 nm light to pump the Lumonics HD-300 dye laser and a 266 nm photolysis beam. The dye laser is operated between 640 and 6.50 nm with a 16% conversion efficiency. The output of the dye laser is doubled with a KD*P crystal rotated 20” about the laser beam axis to provide a component of the electric field of the fundamental perpendicular to that of the second harmonic. The conversion efficiency to second harmonic in this configuration is approximately 14%. The second harmonic is summed with the fundamental by type I phase matching in beta barium borate to produce third harmonic radiation with approximately 20% of the second harmonic converted to third harmonic. The dye laser harmonics were separated with two 60” S l-UV prisms and focused into the sample area with a 6 inch focal length Sl -UV lens. The 266 nm photolysis beam had travelcd directly to the sample, having been separated from the 532 nm radiation by a dichroic mirror and from the 1064 nm YAG fundamental with a 60” prism. The 266 nm beam was varied from 5 to 0.5 mJ per pulse by detuning the KD*P Nd : YAG fourth harmonic crystal. This produced approximately linear correlation between 266 nm photolysis power and methyl radical signal intensity. The photolysis beam was focused to a diameter of approximately 2 mm at the sample area and crossed the probe beam with electric vector perpendicular to the monochromator slits. The 0.4 mJ probe beam at 2 14 or 216 nm arrived 15 ns later and was introduced into the sample area by passing through a small hole in the collection mirror. The precursor molecules were entrained in an argon flow which exited a 1.5 mm diameter pipet at a flow rate of 120 ml/min, crossed both photolysis and probe beams at right angles, and was collected by an exhaust tube. The CH?I was present in the flow at room temperature vapor pressure (44 kPa or 330 Torr). The CD31 flow was diluted approximately 10: 1 with argon to give 4.4 kPa (33 Torr) of precursor in the sample area. The CHjBr gas from the lecture bottle was diluted 10 : 1 with argon. Both methyl iodide samples were used as supplied from 254
14 October I988
Aldrich Chemical Company. The CH,Br was supplied by Matheson. The Raman scattered light was collected in standard backscatter geometry [ 171 by a 2 inch focal lengthf/0.67 spherical mirror and focused onto the slits of a 1.0 m Czemy-Turner monochromator. A crystalline quartz scrambler was mounted in front of the slits to compensate for variation of grating efficiency with polarization. A mercury lamp was located behind the beam stop on axis with the monochromator to provide calibration lines. The 1800 grooves/mm grating was scanned in third order. With 200 pm slits the spectral resolution is 0.037 nm corresponding to 9 cm-‘. The Hamamatsu R 166UH solar blind photomultiplicr tube was operated at 1200 V. The photomultiplier output was amplified by a 20x gain 2 ns risetime preamp and was sampled with a Stanford Research Systems gated integrator with a 15 ns gate. The output also passed through a pulse stretching amplifier to a second gated integrator set to detect the 120 Hz signal from the mercury lamp. The dc signals from the two gated integrators were sampled by an A/D converter, stored on hard disk, and displayed on an IBM PC/AT computer.
3. Results and discussion The methyl radical has four normal modes, of which three have fundamentals that could in principle be Raman active. The intensity of a transition in the resonance Raman spectrum is dependent upon changes in the upper state potential surface relative to the ground state surface. A full treatment of resonance Raman spectroscopy has appeared in several reviews [ 171. Briefly, a displacement in a totally symmetric normal mode enhances the Raman fundamental and all overtones for that vibration. A large force constant change for either a totally symmetric or non-totally symmetric vibration will yield intensity only in even overtones of the vibration of interest. Combination tones of resonance-enhanced normal modes can be expected to appear in the Raman spectrum. Normal modes for which there is no displacement or force constant change in the excited state are not enhanced in the resonance Raman spectrum.
CHEMICAL PHYSICS LETTERS
Volume 1S 1, number 3
From the rotational analysis of the methyl radical B-Z%absorption spectrum Herzberg determined that the C-D bond length in the excited state was 0.1124 nm, significantly different from that of the ground state 0.1079 nm [ 181. This change in bond length, with retention of DIh symmetry yields a displacement in the symmetric stretch normal mode. Herzberg also identified the progression of absorption bands as being due to a force constant change in the umbrella mode, vZ, on the excited state surface. No absorption features associated with excitation of u3 or lrq were identified, implying that the excited state potentials of vj and vq are similar to those of the ground state. Application of the simple trends in resonance Raman scattering discussed above yields the prediction that Y,, 2u,, 2u2, and v,+~v? will appear in the Raman spectrum which is enhanced by resonance with the B excited state of the methyl radical. This is in agreement with the observed spectrum in fig. 1. The methyl radical was generated by 266 nm photolysis of methyl iodide. The spectrum was reproduced with methyl bromide as the precursor, utilizing the same 2 16 nm laser pulse to both photolyze and probe the photoproducts. Table 1 lists the observed frequencies of the methyl radical. Note that this study reports band centers while the CARS and high resolution infrared work report band origins. The first overtone of the umbrella mode, 2vZ, is observed at 1286 cm-‘. This agrees well with the 1288 cm- ’ band origin predicted from the infrared diode laser work of Yamada et al. and confirms the large “negative” anharmonicity discussed by Yamada [ 12 1. The Q branch of u ,, the symmetric stretch, is observed at 3003 cm-’ in good agreement with the 3004.8 cm- ’obtained from Holt et al.‘s CARS work [ 141, the calculated value of 2992.6 cm- ’from Spirko and Bunker’s force constant analysis [ 161 and the 3044 cm-’ determined Table I Observed
vibrational
frequencies
of the methyl radical This work
(frequencies
il
I4 October
IUIIU
Fig.
1.0
2992.6
5959.7+2.1 1284.7? 1.9
1286.4
v, t 2u,
4268.0+
sob0
by the force constant analysis of Snclson [ 151. The overtone 2v, was observed at 5960 cm-’ yielding an anharmonicity constant X, , = -22.7 + 1.5 cm- ‘. The combination band v1 +2u2 is seen at 4268 cm-’ giving an anharmonic coupling constant XIZ= -9.8 -t 1.5 cm-‘. The deuterated methyl radical spectrum was also examined. The overtone of the umbrella model, the symmetric stretch, and several combination tones were observed for CD,. The analysis of the CD, spectrum will be presented in a future publication. Given the large rotational constant for the methyl radical, it was possible to resolve the rotational structure. The resonance enhancement for the observed spectra is dominated by a single electronic state, B ‘A;, and one vibrational level within that state. The rotational levels of the B state are broadened by predissociation such that only a diffuse R branch and PSQ feature can be observed in the absorption spectrum of the O-O band for CH, [ 19,201. In the spectrum where CH$ was used as the precursor, the Rayleigh line, 2u,, and uI showed enhanced rotational structure. As the probe wavelength
in cm-‘)
[ 161
3004.8 & 0.2
3002.42
don
1.Resonance Raman spectrum of CH, at 2 15.780 nm
Ref.
“I
30’00
Wuvrriulllbr,
Ref. [IS]
2v, 2v2
2&U
I988
_
Ref.
[I]
1288.0900
1.7
255
Volume 15 1, number 3
CHEMICAL
was changed from 215.7 to 216.5 nm, the relative intensity of the rotational structure shifted from strong Q, R, and S branches at 215.7 nm to 0, P, Q, R, and S branches at 216.5 nm. At 215.7 nm the probe beam is resonant only with the R branch of the methyl radical B-z absorption. This causes scattering enhancement of the Q, R, and S branches in the Raman spectrum [ 81. At 216.5 nm the probe beam is in resonance with the P+Q and R branches of the absorption. As before, Q, R, and S scattering is enhanced for excitation resonant with the R absorption branch. Resonance with the Q branch enhances P, Q, and R scattering while resonance with the P branch of the B-R transition gives rise 10 0, P, and Q branches. Overall then, 0, P, Q, R, and S branches appear in the Raman spectrum. This shifting of intensities in the rotational structure by changing excitation wavelength demonstrates the feasibility of determining the excited state lifetime of the methyl radical. By fitting the pattern of relative intensities observed in the resonance rotational or rovibrational Raman spectrum, the excited state lifetime of the molecule can be determined. This method of excited state lifetime determination has been developed by Ziegler and applied to NH3 and ND3 [7,8]. The resonance Raman technique is an ideal tool for studying the chemical kinetics of radical systems. For this purpose it is important to consider concentration estimates for the methyl radical and the detection limits for our system. The initial concentration of the methyl radical was estimated as the number of methyl radicals generated from CH31 at 293 K with unit quantum efficiency for the photolysis process. The molar absorptivity of CHII at 293 K and 266 nm was determined to be 334 M-’ cm-‘, which yields a number density of 10” radicals crne3 produced by a 1 mJ, 8 ns pulse of 266 nm light. At room temperature, the main route for elimination of melhyl radicals is the recombination of methyl radicals to form ethane. Using Yamada’s suggested value of the rate constant for the recombination reaction, k=2X1013 cm3 mol-’ s-l, the concentration of methyl radicals present upon the arrival of the probe beam 15 ns after photolysis is 7 x 10” molecules cm-3.
256
I4 October
PHYSICS LETTERS
1988
Far ultraviolet resonance Raman is a powerful technique for examining free radicals. By properly choosing the precursor so as to avoid an electronic transition coincident with the probe beam wavelength, interference due to resonance enhancement of the precursor is minimized. As can be seen in fig. 1, the resonant signal from 4.4 kPa (33 Torr) ofCH, is many times larger than the non-resonant signal from the 44 kPa (330 Torr) of CH,l. The application of resonance Raman spectroscopy to the study of small organic free radicals is in its infancy. Nevertheless, it has great potential to provide a wealth of information on both the structure and dynamics of transient species. By combining the Raman technique with other spectroscopic methods, it will be possible to thoroughly characterize these systems. We are currently exlending this technique to other small organic free radicals.
Acknowledgement This work was supported in part by a New Faculty Research Grant from the University of California al Davis.
References [ 1] KC. Smyth and P.H. Taylor, Chem. Phys. Letters 122 (1985) 518. [Z] J.A. Miller and G.A. Fisk, Chem. Eng. News (August 1987).
31,
[ 31 D. Imre, J.L. Kinsey, A. Sinha and J. Krenos, J. Phys. Chem. 88 (1984) 3956. 141 R.L. Sundberg,D. Imre. M. O’Hale, J.L. Kinsey and R.D. Coalson, J. Phys. Chem. 90 ( 1986) 5001. [S] H.T. Yu, A. Sewn, E. Kassab and E.M. Evleth, J. Phys. Chem. 88 ( 1984) 2049. [ 61 G. Herzbberg, The spectra and structures of simple free radicals; an introduction to molecular spectroscopy (Cornell Univ. Press, Ithaca, I97 I ). [7 JL.D. Ziegler, P.B. Kelly and B. Hudson, J. Chem. Phys. 8 I (1984) 6399. [S] L.D. Ziegler, J. Chem. Phys. 84 (I 986) 6013. 191 G. Hcrzbclgand J. Shoosmith, Can. J. Phys. 34 ( 1956) 523. [ 101g. Herzberg, Proc. Roy. Sot. A 262 ( 1961) 29 I, [ II] L. Tan,A. Winer and G. Pimcntal. J. Chem. Phys. 57 ( 1972) 4028.
[ 121C. Yamada, E. Hirota and K. Kawaguchi, J. 75 (1981)
5256.
Chem.
Phys.
Volume
I 51,number 3
CHEMICAL
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PHYSICS LETTERS
I4 October 1988
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