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Detection of gas-phase methyl radicals using multiphoton ionization
Volume 82, number 2
CHEMICAL
PHYSICS LETTERS
DETECTION OF GAS-PHASE METHYL RADICALS T.G. DIGIUSEPPE
1 September 1981
USING MULTIPHOTON
IONIZATION
* , Jeffrey W. HUDGENS and MC. LIN
(Plemistry Division, Code 6I10. Naval Research Laboratory,
Washington, D-C. 20375.
USA
Received 21 May 1981
Detection of methyl radicals by the method of mass-selected multiphoton ionization is demonstrated by observing CH3 Rydberg state resonances. The Rydberg states were prepared by simultaneous absorption of three photons. Absorption of a fourth photon produced the observed mass 15 ion signals.
1. Introduction Resonance-enhanced multiphoton ionization f 1,2] (MPI) has been shown to be a versatile and sensitive technique for studying the spectroscopy of gas-phase atoms and molecules. MPI spectra have been obtained for several stable molecules in both bulk gas experiments and under the conditions of supersonic expansion [3,4] _ In addition, previously unobserved electronic states of molecules have been detected using MPI [5-7]_ Molecular fragments produced from the predissociation of intermediate electronic states have also been observed in the MPI spectrum [5,7]. Fragmentation patterns of multiphoton-generated ions have been identified by combmIng MPI with mass analysis [8,9] _ We have extended the technique of mass-selected resonance-enhanced multiphoton ionization to the detection of gas-phase free radicals which cannot readily be detected by means of such sensitive methods as laser-induced fluorescence_ In this paper, we report preliminary results for detection of the gasphase methyl radical, one of the most important free radicals studied in gas-phase chemical reactions.
2 Experimental A schematic diagram of the experimental apparatus * NRC/NRL
Postdoctoral kesearch Associate.
HOT ELECT
SAMPLES GROUND ELECTRODE
DIFFUSION
-
_
DIFFUSION PUMP
-
SPECTROMETER
r
Fig. 1. Schematic of differentiaIIy pumped quadrupole mass spectrometer.
is shown in fig. 1. Methyl radicals were produced by the pyrolysis of azomethane, dimethylsulfoxide @MSO), di-t-butylperoxide (DTBP), and methyl iodide in a resistively heated crinkly foil tantalum oven. The oven temperatures were measrrred with an optical pyrometer and were typically =SOO”C_ Ovens were constructed by spot welding 1 mil 267
Volume 82, number 2
tantalum foil into a 6 mm diameter tube and spot weIding this tube onto the end of the reagent source tube. The end of the tantalum tube was then cut into tabs. These tabs were then peeled back and clamped onto a current electrode. Clamping in this manner thermally isolated the tip of the tube so that it was at the same temperature as the tube’s body. Crinkly foil was prepared by pressing a 3 mm wide si:ip of foil between the teeth of two meshed gears. The strip of corrugated tantalum was then rolled into a plug with many parallel channels and spot welded inside the oven near the tip. The radicals produced in the oven pass through the 3 mm hole in a OS mm plate and flow axially into the cross-beam ionizer of a quadrupole mass spectrometer (Fxtranuclear). The oven chamber and mass spectrometer chamber are separately pumped by 4” diffusion pumps. Under typical gas-flow conditions the total pressure in the quadrupole is 3 X IO-? Torr which allows a rough estimate of the detected CH3 radical concentration at 101°-lO1l mofecules/cm3 in the mass spectrometer’s ionizer. The output of a Nd : YAG pumped dye laser (Quantel) was focused to a spot centered in the cross-beam ionizer with an.#2 lens (f-1. = 50 nun). The laser beam terminated on one of the vacuum system walls. Ion noise from this surface never occurred because the beam was sufficiently defocused_ Peak dye energies of 12 mJ/pulse at-10 Hz and 0.5 cm-1 bandwidth were obtamed from coumarin 450 and coumarin 440. Data acquisition and signal averaging were controlled by a PDP-8E based system which scans the dye laser stepwise. During the expedient the ion signal is amplified and pulse stretched by a fast electrometer (Keithley 427), transient recorded by the computer at 30 kHz, and voltage offsets corrected by baseline subtraction. After 10 laser p&es were averaged at a wavelength setting, the computer advanced the dye laser Q of a resolution element.
3. Results and d.iscussZon In fig- 2 the MPI spectrum of d~e~y~u~o~de at room temperature is presented along with the spectrum obtained at an oven temperature of 800°C. The mass 15 ion signal at 450.9 run is attributed to the methyl radical. The ion signal is produced by the si268
1 September 1981
CHEMICAL PHYSICS LETTERS
r
f
DMSO
MASS 15
800°C
~
450
445
NM
Fig. 3,. Mass 15 ion signalversus dye Iaser wavelength(nm) of effluent from the oven at 2.5 and 800°C with dimethylsulfouide. multaneous absorption of three photons into the yl(O,O) Rydberg level of CH, and the subsequent absorption of an additional photon into the iordzation continuum. This three-photon resonant state corresponds to the one-photon vacuum ultraviolet transition at 150.29 nm in the methyl radical as observed by Henberg [lOJ_ In addition, a resonance is also observed at 445.9 nm. This peak is shifted a750 cm-l from the rl(O,O) transition and is assigned to the ~~(1, 1) transition in CH,. This peak is in good agreement with the vibronic transitions observed by Henberg El01 which are shifted from the main Rydberg series by =780 cm-l _Wavelength resolved scans at higher masses (m/e = 45,47 and 63) revealed no similar spectral features. These transitions in CH, have also been observed in the pyrolysis of di-t-butylperoxide, azomethane, and methyl iodide. The spectrum obtained for methyl radicals produced in the pyroiysis of DTBP is presented in fig- 3. The MPI spectrum of CH, produced in the pyrolysis of this compound exhibited the sharpest spectral features. Both the ~~(0, 0) and +yl(l, 1) transitions of CH3 are clearly evident and separated by 746 cm-1. We have also observed an ion signal at 432.8 nm which is assigned to a two-photon resonant absorption into the Pr (90,O) Rydberg level of CH, _ For this
CHEMICAL PHYSICS LETTERS
Volume 82, number 2
cH3 from DTBP
HASS
Y,CO.O~ I
15
Fig. 3. MPI spectrum of CH3. Methyl radicaIs were produced by pyrolysis of di-r-bultylperoxide. transition, the abso~ti~n of two additional photons is necessary for ionization. This two-photon resonant state corresponds to the one-photon ultraviolet transition at 216.4 run. Experiments are currently in progress for identifying the higher-lying Rydberg states of CH, and the previously unobserved p state Rydberg Ievela.
4. Concluding remarks The technique of mass-selected multiphoton ionization has been extended to the detection of gas-phase methyl radicals. To date, the high-iying Rydberg Ievels of CH, have only been observed in VUV absorption measurements and apparently do not fluoresce.
I September 1981
Laser-induced fluorescence (LiFj has been used successfully to probe low-Iying electronically excited states of many small free radicals [I 11. However, LIF cannot be applied to species such as Cl-I3 which do not fluoresce. In this work we have shown that nonfluorescing electronic states of free radicals can be observed by resonance-enhanced multiphoton ionization. MPI may provide a useful technique for probing non-~uoresc~g species of importance in combustion_ In addition, MPI offers a sensitive probe to the symmetry of the electronic state manifold. New spectroscopic mformation obtained for free radicals, including CA3 , using MPI will complement existing one-photon studies,
References [l] P.M. Johnson, M.R. Berman and D. Zakheirn, J. Chem. Phys 62 (1975) 2500. [Z] P.M. Johnson, Accounts Chem. Res. 13 (1980) 20. g3f D, Zakheim and P.M. Johnson, J. Chem. Phys. 68 (1978) 3644. [4] J.W. Hudgens, M. Seaver and J.J. DeCorpo, J. Phys. Chem. 85 (1981) 761_ [S] G.C. Nieman and S.D. Colson, J. Chem. Phys 68 (1978) 5656. 161 J&L Glownia, S.J_ Riley and SD. Colson, J. Chem. Phys. 72 (1980) 5998. [7f 1-H. Glowrda, S.J. Riley and SD. Colson, J_ Chem. Phys. 73 (1980) 4296. [S] L. Zandee and R.B. Bernstein, J. Chem. Phys. 71 (1979) 1359. 191 M. &aver, 3-W. Hudgens and J-J. DeCorpo, Intern. J. Mass Spectrom. Ion Phys. 34 (1980) 1.59. (101 G. Herzberg, Proc. Roy. Sot. 262A (1961) 291. [ 111 M.C. Lin and J.R. McDonald, in: Reactive intermediates in the gas-phase generation and monitoring, ed. D-W*
Setter (Academic Press, New York, 1979).
269
CHEMICAL
PHYSICS LETTERS
DETECTION OF GAS-PHASE METHYL RADICALS T.G. DIGIUSEPPE
1 September 1981
USING MULTIPHOTON
IONIZATION
* , Jeffrey W. HUDGENS and MC. LIN
(Plemistry Division, Code 6I10. Naval Research Laboratory,
Washington, D-C. 20375.
USA
Received 21 May 1981
Detection of methyl radicals by the method of mass-selected multiphoton ionization is demonstrated by observing CH3 Rydberg state resonances. The Rydberg states were prepared by simultaneous absorption of three photons. Absorption of a fourth photon produced the observed mass 15 ion signals.
1. Introduction Resonance-enhanced multiphoton ionization f 1,2] (MPI) has been shown to be a versatile and sensitive technique for studying the spectroscopy of gas-phase atoms and molecules. MPI spectra have been obtained for several stable molecules in both bulk gas experiments and under the conditions of supersonic expansion [3,4] _ In addition, previously unobserved electronic states of molecules have been detected using MPI [5-7]_ Molecular fragments produced from the predissociation of intermediate electronic states have also been observed in the MPI spectrum [5,7]. Fragmentation patterns of multiphoton-generated ions have been identified by combmIng MPI with mass analysis [8,9] _ We have extended the technique of mass-selected resonance-enhanced multiphoton ionization to the detection of gas-phase free radicals which cannot readily be detected by means of such sensitive methods as laser-induced fluorescence_ In this paper, we report preliminary results for detection of the gasphase methyl radical, one of the most important free radicals studied in gas-phase chemical reactions.
2 Experimental A schematic diagram of the experimental apparatus * NRC/NRL
Postdoctoral kesearch Associate.
HOT ELECT
SAMPLES GROUND ELECTRODE
DIFFUSION
-
_
DIFFUSION PUMP
-
SPECTROMETER
r
Fig. 1. Schematic of differentiaIIy pumped quadrupole mass spectrometer.
is shown in fig. 1. Methyl radicals were produced by the pyrolysis of azomethane, dimethylsulfoxide @MSO), di-t-butylperoxide (DTBP), and methyl iodide in a resistively heated crinkly foil tantalum oven. The oven temperatures were measrrred with an optical pyrometer and were typically =SOO”C_ Ovens were constructed by spot welding 1 mil 267
Volume 82, number 2
tantalum foil into a 6 mm diameter tube and spot weIding this tube onto the end of the reagent source tube. The end of the tantalum tube was then cut into tabs. These tabs were then peeled back and clamped onto a current electrode. Clamping in this manner thermally isolated the tip of the tube so that it was at the same temperature as the tube’s body. Crinkly foil was prepared by pressing a 3 mm wide si:ip of foil between the teeth of two meshed gears. The strip of corrugated tantalum was then rolled into a plug with many parallel channels and spot welded inside the oven near the tip. The radicals produced in the oven pass through the 3 mm hole in a OS mm plate and flow axially into the cross-beam ionizer of a quadrupole mass spectrometer (Fxtranuclear). The oven chamber and mass spectrometer chamber are separately pumped by 4” diffusion pumps. Under typical gas-flow conditions the total pressure in the quadrupole is 3 X IO-? Torr which allows a rough estimate of the detected CH3 radical concentration at 101°-lO1l mofecules/cm3 in the mass spectrometer’s ionizer. The output of a Nd : YAG pumped dye laser (Quantel) was focused to a spot centered in the cross-beam ionizer with an.#2 lens (f-1. = 50 nun). The laser beam terminated on one of the vacuum system walls. Ion noise from this surface never occurred because the beam was sufficiently defocused_ Peak dye energies of 12 mJ/pulse at-10 Hz and 0.5 cm-1 bandwidth were obtamed from coumarin 450 and coumarin 440. Data acquisition and signal averaging were controlled by a PDP-8E based system which scans the dye laser stepwise. During the expedient the ion signal is amplified and pulse stretched by a fast electrometer (Keithley 427), transient recorded by the computer at 30 kHz, and voltage offsets corrected by baseline subtraction. After 10 laser p&es were averaged at a wavelength setting, the computer advanced the dye laser Q of a resolution element.
3. Results and d.iscussZon In fig- 2 the MPI spectrum of d~e~y~u~o~de at room temperature is presented along with the spectrum obtained at an oven temperature of 800°C. The mass 15 ion signal at 450.9 run is attributed to the methyl radical. The ion signal is produced by the si268
1 September 1981
CHEMICAL PHYSICS LETTERS
r
f
DMSO
MASS 15
800°C
~
450
445
NM
Fig. 3,. Mass 15 ion signalversus dye Iaser wavelength(nm) of effluent from the oven at 2.5 and 800°C with dimethylsulfouide. multaneous absorption of three photons into the yl(O,O) Rydberg level of CH, and the subsequent absorption of an additional photon into the iordzation continuum. This three-photon resonant state corresponds to the one-photon vacuum ultraviolet transition at 150.29 nm in the methyl radical as observed by Henberg [lOJ_ In addition, a resonance is also observed at 445.9 nm. This peak is shifted a750 cm-l from the rl(O,O) transition and is assigned to the ~~(1, 1) transition in CH,. This peak is in good agreement with the vibronic transitions observed by Henberg El01 which are shifted from the main Rydberg series by =780 cm-l _Wavelength resolved scans at higher masses (m/e = 45,47 and 63) revealed no similar spectral features. These transitions in CH, have also been observed in the pyrolysis of di-t-butylperoxide, azomethane, and methyl iodide. The spectrum obtained for methyl radicals produced in the pyroiysis of DTBP is presented in fig- 3. The MPI spectrum of CH, produced in the pyrolysis of this compound exhibited the sharpest spectral features. Both the ~~(0, 0) and +yl(l, 1) transitions of CH3 are clearly evident and separated by 746 cm-1. We have also observed an ion signal at 432.8 nm which is assigned to a two-photon resonant absorption into the Pr (90,O) Rydberg level of CH, _ For this
CHEMICAL PHYSICS LETTERS
Volume 82, number 2
cH3 from DTBP
HASS
Y,CO.O~ I
15
Fig. 3. MPI spectrum of CH3. Methyl radicaIs were produced by pyrolysis of di-r-bultylperoxide. transition, the abso~ti~n of two additional photons is necessary for ionization. This two-photon resonant state corresponds to the one-photon ultraviolet transition at 216.4 run. Experiments are currently in progress for identifying the higher-lying Rydberg states of CH, and the previously unobserved p state Rydberg Ievela.
4. Concluding remarks The technique of mass-selected multiphoton ionization has been extended to the detection of gas-phase methyl radicals. To date, the high-iying Rydberg Ievels of CH, have only been observed in VUV absorption measurements and apparently do not fluoresce.
I September 1981
Laser-induced fluorescence (LiFj has been used successfully to probe low-Iying electronically excited states of many small free radicals [I 11. However, LIF cannot be applied to species such as Cl-I3 which do not fluoresce. In this work we have shown that nonfluorescing electronic states of free radicals can be observed by resonance-enhanced multiphoton ionization. MPI may provide a useful technique for probing non-~uoresc~g species of importance in combustion_ In addition, MPI offers a sensitive probe to the symmetry of the electronic state manifold. New spectroscopic mformation obtained for free radicals, including CA3 , using MPI will complement existing one-photon studies,
References [l] P.M. Johnson, M.R. Berman and D. Zakheirn, J. Chem. Phys 62 (1975) 2500. [Z] P.M. Johnson, Accounts Chem. Res. 13 (1980) 20. g3f D, Zakheim and P.M. Johnson, J. Chem. Phys. 68 (1978) 3644. [4] J.W. Hudgens, M. Seaver and J.J. DeCorpo, J. Phys. Chem. 85 (1981) 761_ [S] G.C. Nieman and S.D. Colson, J. Chem. Phys 68 (1978) 5656. 161 J&L Glownia, S.J_ Riley and SD. Colson, J. Chem. Phys. 72 (1980) 5998. [7f 1-H. Glowrda, S.J. Riley and SD. Colson, J_ Chem. Phys. 73 (1980) 4296. [S] L. Zandee and R.B. Bernstein, J. Chem. Phys. 71 (1979) 1359. 191 M. &aver, 3-W. Hudgens and J-J. DeCorpo, Intern. J. Mass Spectrom. Ion Phys. 34 (1980) 1.59. (101 G. Herzberg, Proc. Roy. Sot. 262A (1961) 291. [ 111 M.C. Lin and J.R. McDonald, in: Reactive intermediates in the gas-phase generation and monitoring, ed. D-W*
Setter (Academic Press, New York, 1979).
269