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Infrared multiphoton dissociation of methyl nitrite
Volume 53, number 3
CHEMICAL PHYSICS LETTERS
1 February 1978
INFRARED MULTIPHOTON DISSOCIATION OF METHYL NITRITE Allen HARTFORD Jr. Materials Research Center. Allied Chemical Corporation.
hfon-istown, New Jersey 07960, USA
Received 10 October 1977 Methyl nitrite undergoes dissociation when irradiated with the focused output of a pulsed CO2 TEA laser. Time resolved infrared emission is observed and attributed to viirationally excited formaldehyde and methanol. These species are produced via reactions of methoxy radicals formed in the primary dissociation. Other products formed are NO and N20.
1. Introduction Collisionless multiphoton photodissocihtion of polyatomic molecules in the presence of an intense infrared laser field has been the subject of considerable study in recent years [ 141, primarily due to the isotopic enrichment obtainable with this process [5,6] . Most of the reported studies have concerned themselves with an analysis of the end products formed as a consequence of the dissociation. In addition, a few investigators have reported the observation of transient species by the method of laser excited fluorescence [7,8]. In this paper we report an analysis of the end products, as wel! as the observation of time resolved infrared fluorescence, resulting from the irradiation of methyl nitrite (C&ON@) with the focused output of a CO, TEA laser. Several interesting features motivated the choice of methyl nitrite. First, the molecule exists in both cis and trans isomeric forms in the vapor phase, the vibrational bands in the infrared absorption spectrum being “doubled” due to the presence of these isomers. Consequently, both the cis and trans forms can be dissociated with a CO, laser whose output is coincident with the C-O stretching findamental of methyl nitrite. In addition, upon deuteration of methyl nitrite the C-O stretching mode is shifted toward lower frequency and essentially out of coincidence with the CO, laser, while a deuterium degenerate deformation mode is moved into coincidence. This allows multiphoton dissociation to be studied in the deuterated mol&ule. Finally, a number of studies concerning the
ultraviolet photolysis and shock tube decomposition of methyl nitrite have been reported [9-l l] _ These can be compared to the infrared multiphoton dissociation results to gain insi&t into the processes responsible for the formation of the end products.
2. Experimental Dissociation of methyl nitrite was accomplished with a grating tuned CO, TEA laser (Lumonics model 203). At least 400 mJ (measured with a Lumonics model 20D energy detector) single line output was obtainable in a pulse with fwhm of 100 ns. A 19 cm focal length antireflection coated germanium lens focused the laser into the center of a Pyrex sample cell. This cell was 4 cm in diameter and 15 cm long. The ends of the cell were sealed with sodium chloride windows, while a potassium chloride window was used to view fluorescence normal to the laser beam axis. A capacitance manometer (MKS Baratron) connected directly to *&e fluorescence cell was used to measure pressures. An X-Y recorder connected to the output of the capacitznce manometer permitted continuous recording of the pressure variation. Fluorescence was detected with a photovoltaic InSb detector. Combinations of bandpass filters were used to isolate tluorescence in particular spectral regions within the range from 3-5.6 m (the detector cut off)_ Amplified fluorescence signals were digitized with a Biomatior 8100 transient recorder and subsequently displayed on an X-Y recorder. In mrne instances the 503
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digitized signals were averaged with a Tracor-Northem 575A signal averager and then displayed on the X-Y recorder_ CH30N0 and CD30N0 were synthesized according to the method of Hartung and CrossIey [ 12]_ Briefly, either CHsOH or CD30D was combined with a saturated solution of NaN02. Dilute sulfuric acid was added dropwise to this mixture. The gaseous methyl nitrite that evolved was carried by an N2 stream through a trap immersed in an ethylene dichloride slush (-36°C) to remove water, then collected in a liquid nitrogen trap. The purity of the samples was
verified by gas chromatography. Infrared spectra of bdth CHSONO and CDSONO were recorded on a Perkii--Elmer 283 infrared spectrometer. Identification of end products-was caked out by recording the spectra of Irradiated samples on the same instrument. 3. Results and discussion The infrared spectra of do-methyl nitrite and d3methyl nitrite in the vicinity of CO2 laser emission are shown in fig. l_ The two absorption features in
C-0 stretch
Fig. I_ Infrared spectra of methyl nitrite in the vicinity of CO2 are 10 cn? frequency calibration.
504
laser emission: (a) CH30NO; @) CD3ONO. Vertical “tick” marks
CHEMICAL
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CH,ONO centered at 990 cm-l and 1040 cm-l correspond to the C-O stretching mode of the cis and tram isomers, respectively [13] _ The cis and trans designation refer to the position of the methyl and N=O substituents with respect to the O-N band. Both Pand R-branch lines from the CO, laser 10.6 firn emission are coincident with absorption due to the cis conformation, while the C-O stretch of the trans isomer is coincident with P-branch lines of the 9.4 pm transition. In CD,ONO the C-O stretching fundamental is shifted toward lower frequency (cis ~9 15 cm-t, trans =950 cm-‘), whereas the CD3 degenerate deformation is centered at 1030 cm-l _This vibrational mode is coincident with the P-branch lines of the 9-4 m CO, laser transition_ Initial experiments merely monitored the pressure increase in the sample cell when 1 torr samples of domethyl nitrite were irradiated with the focused output of the CO, laser. Typical curves of pressure increase versus number of laser pulses for different excitation wavelengths are displayed in fig. 2. No pressure change was obsenred when samples of CHsONO were irradiated with the laser unfocused_ Fig. 3’shows the change in pressure after 1000 laser pulses as a function of excitation line. For clarity, the absorption spectrum of CH30N0 is displayed on the same wavelength scale. In all experiments the laser energy was fmed at 300 mJ/pulse. Infrared spectroscopic analysis of the products formed as a result of dissociation indicate the presence of NO, N,O, H,CO, and CH,OH as well as
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Fig. 2. Capacitance manometer output of pressure increases during CO* TEA laser irradiation of 1 torr samples of CHsONO. (a) Irradiation of cis isomer with the R(28) line of the 10.6 pm CO, laser transition; 0~) h-radiation of trans isomer with the P(16) line of the 9-4 pm transition.
0 0
1‘ 900
. 920
940
960
980 -I 1000 cm
lo20
LO40
1060
I
00
1060
Fig. 3. Dixociation yield from methyl nitrite as .I function of laser frequency. 1000 laser pulses at 300 mJ/puIse were used to obtak each point. Also shown is a portion of the infrared absorption spectrum.
remaining starting material. Since all of these components are stable and volatile, measurement of the pressure increase reflects the dissociation efficiency. The yield of product versus the absorption spectrum of methyl nitrite exhibits some features that merit discussion. On the P-branch side of the cis feature, the yield increases from low frequency reaching a maximum near the Q-branch. This behavior is strikingly similar to that observed in the sin,$e frequency_ multiphoton dissociation of SF6 [14] _Furthemlore, as discussed by Bloembergen [3] , the multiphoton dissociative process might be expected to be most efficient when the excitation frequency is displaced from the band origin by an amount corresponding to the vibrational anharmonicity. Qualitatively, this pattern is observed for the cis conformation of methyl nitrite. The yield due to irradiation of the trans isomer is reduced somewhat from that of the cis. This is not surprising since the absorption coefficient is approximately a factor of 3 to 4 less for the trans-methyl nitrite vibrational mode than for the corresponding vi-
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CHEMICAL PHYSICS LETTERS
Volume 53, number 3
bration in cis-methyl nitrite. Previously, it has been found that the dissociation rate as measured in different vibrational modes of SF6 is proportional to the matrix element of the transition dipole [ 151, a situation which appears to pertain in the present case.
Product analysis by infrared spectroscopy indicated that both the cis and trans isomers yielded the same species when irradiated (NO, N,O, H,CO, and CH,OH). Furthennore, within the accuracy of the infrared absorption intensities observed, the products in both cases were formed in the same proportion. This suggests that the initial photodissociation step is the same in both isomers, and is unaffected by the geometrical conformation. Along with measuring the pressure increase and rmalyzing the fmal products,
time resolved infrared
Strong signals were observed, which, by using various bandpass filters, were found to be in the wavelength region between 3.54 and 3.99 D (10% transmission points). Typical fluorescence decay curves are shown in fig. 4. From the curve in fig. 4a, it may be noted that the fluorescence rise follows the response time (=O.S ps) of the detection system. TO account for this rapid rise, we must assume the species responsible for the fluorescence is either produced during the prima;y di&ociation step and contains excess vibrational energy, or is fonned in an exothermic secondary reaction that has nearly a gas kinetic rate (approximately 3 collisions occur at 0.75 torr in OS ,us). We note from the pressure increase data that essentially all the molecules within the volume of the focused laser beam are dissociated with each pulse (-1.5 X 1015 molecules). From bond eneru considerations, the initial multiphoton dissociation is expected to involve rupture of the O-N bond (:39 kcaijmole) T, producing NO and methoxy radical: fluorescence
was also monitored.
CH,ONO + CH,O-
+ NO.
(1)
Following this a number of secondary reactions can occur (consistent with the product analysis)_ These are given below: CH,Om + NO + CH,O -I-HNO,
(2)
fCalculated from the heats of formationof CHSONO, CH30-, and NO as given in refs. [ 16,171. 506
(b)
Y:.
20
JO
60
80
100
120
140
160
TIME (~S’X)
Fig. 4. Fluorescence decay curves observed upon irradiation of 0.75 torr methyl nitrite with R(32), 10.6 pm CO1 laser line. (a) Short time scale indicating rapid fluorescence rise; (b) longer time scale showing total fluorescence decay.
CH,O*
+ CH,O* + CH,O + CH,OH,
(3)
HNO + HNO + N,O f H,O, CH30-
+ CH30N0
+ CH,O + CI$OH
(4) + NO_
(5)
Anlong the various species above, formaldehyde has known vibrational bands occurring b&ween 3.54 and 3.99 m. The VI mode is centered at 3.60 can, while
Volume 53, number 3
1 February 1978
CHJZMICAL PHYSICS LETTERS
the u4 mode extends into the aforementioned region. (A small contribution from the s-stretch of CH30H is also possible_ The formation of vibrationally excited methanol will be discussed_) Reactions (Z), (3), and (5) are exothemric by 26 kcal/mole, 82 kcal/mole, and 42 kcal/mole, respectively, all sufficient to produce vibrationally excited formaldehyde. In addition, reaction (3) has been shown from shock tube studies of methyl nitrite to have a rate constant of 0.62 w-t to& at 780 K [lo] _At 3 14 K the rate for this reaction has been given as 7.26 ,XS~ to& by Yee Quee and Thynne [ I8]_ A lower bound for the rate can be found from the fact that we were unable to resolve a risetime in our experiments_ This limit is 3.6 j~s-l torr-l, a value consistent with Thynne’s result. In comparison, the rate for reaction (2) is approximately 3 orders of magnitude slower than the rate of disproportionation of methoxy radicals [lo] _Thus, reaction (2) is not likely to be a primary contributor to the observed fluorescence_ Reaction (5) is also slower than the disproportionation mechanism [lo] , thus it may be concluded that the reaction of two methoxy radicals is the preponderant source of vibrationally excited formaldehyde_ One additional experiment was performed to verify that the observed fluorescence originated from formaldehyde. A sample of CD,ONO was irradiated with the focused output of the CO, laser. Strong fluorescence was again observed and occurred in the region from 4.72 to 5.09 W_ The vl and v4 modes of D&O are centered at 4.63 and 4.86 w, respectively. This is consistent with the results obtained with CH30N0. In addition to the above fluorescence, a somewhat weaker signal was noticed in the region from 3.10 to 3.99 pm. This fluorescence is shown in fig. 5 and is again characterized by a rapid rise, but decays by approximately a factor of four faster than previously observed signals. Of the possible fluorescing species, only CD,OD has a vibrational mode in the region of observation. The O-D stretching vibration is at 3.67 PITI.Since both methanol and formaldehyde are formed in the exotbermic disproportionation of methoxy radicals, it is not unexpected that both species are formed in vibrationally excited states. An important point to note is the absence of fluorescence in the 3-5.6 pm region from either CH,ONO or CD,ONO, even after extensive signal averaging, upon irradiating samples with the unfocused output of the laser. Clearly, the fluorescence
!
f&h 0
20
40
$0
80 TIME
l&l (LL set
120
140
m
160
1
D
)
Fig. 5. Observed fluorescence following irradiation of 1.0 torr of CD30NO with the P(36) line of the 9.4 pm CO* transition. Fluorescence is in the region from 3.04 to 3.99 pm.
observed in these experiments does not arise from collisional up pumping of methyl nitrite itself_ One other source of the IR fluorescence should be considered in view of the rapid risetime observed_ It may be possible that vibrationally excited methoxy radical is formed in the primary photodissociation. Although the LR fundamental vibrational frequencies for methoxy radicals are not known, characteristic CH3 or CD, modes have wavelengths near those attributed to vibrationally excited formaldehyde. These wavelengths may be changed sufficiently in methoxy radicals to consider them as possible sources of fluorescence_ If this were the situation, the observed fluorescence quenching wou!d be dominated by recombination of methoxy radicals, rather than vibration-vibration or vibration-rotation, translation processes which are
expected to be slower. However, since the fluorescence decay is slower (see fig_4b) than any of the rates reported for methoxy radical disproportionation, it appears reasonable to conclude the fluorescence does not occur from vibrationally excited metboxy radicals_ From the above considerations the observed fluorescence is apparently due to emission from formaldehyde and metbauol which acquire excess vibrational energy 507
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CHEMICALPHYSiCS LETTERS
due to the exotbermic disproportionation of methoxy radicals. The observed rate for this process is near gas kinetic if ambient themntl velocities are assumed. It is possible, however, that the radicals produced in the dissociation are translationally hot, thereby increasing the collision frequency which may result in a faster rate. We have also tried to ascertain whether NO fornred in the initial dissociation step is produced in vibrationally excited states. However, no infrared emission was observed in the region of5.3 arm. Considering the sensitivity and cohection efficiency of the detection system, it is possible to estimate that less than 1% of the NO formed per pulse is in u = l_ In summary, the following observations may be made regarding the multiphoton dissociation of methyl nitrite. The analysis of end products is consistent with dissociation occurring via the thermodynamic channel of least energy (breaking of the O-N bond), followed by secondary reactions of the radicals produced. Also, the dissociation process is independent of the vibrational mode excited (C-O stretch or CD3 deformation) and the conformation of the moIecuIe (either cis or trans)_ These observations would seem to be in accord with models of multiphoton dissociation which assume that energy is statistically distributed among the internal degrees of freedom of the reactantmolecule
Cl91Acknowledgement The author greatly benefitted from conversations with Drs. James Yardley, Donald Heller, and Otis Peterson. The skilled assistance of Mr. Joseph Pete in carrying out experimentation and the expert synthetic work of Miss Margaret Nowakowski were invaluable. Considerable thanks is extended to Dr. John Witt for
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recording infrared spectra and to D-r. Willis H&mmqnd for constant advice. _
References [I 1 N.R. Isenor and M-C. Richardson, [2] [31 [4] [S) [61 [71 181 [9]
[lOI [ll] [ 121 [ 131 [141
[151
[16] 1171 [IS] [ 191
Appl. Phys. Letters 18 (1971) 224. P. Kolodner, C. Wmterfeld and E_Yablonovitch, Opt. Commun. 20 (1977) 119: N. Bloenbergen,Opt.Commun. 15 (1975)416. DM. Larsen and N. Bloembergen, Opt. Commun. 17 (1976) 2% R.V. &nbartzumian, V.S. Letokhov, E.A. Ryabov and V. Chekalin, JETP Letters 20 (1974) 273. J.L. Lyman, RJ. Jensen, J. Rink, C.P. Robinson and SD: Rockwood. Appl. Phys. Letters 27 (1975) 87. JD. Campbell, G. Hancock, J-B. Halpem and K-H. Welpe, Chem. Phys. Letters44 (1976) 404. M.L. Lesiecki and W.A_ Guillory, J. Chem. Phys. 66 (1977) 4239. IS. Zaslonko, SM. Kogarko, E-V. Mozzhuikhin, Yu.P. Petrov and A.A. Borisov, Kinetika i Kataliz I1 (1970) 296. A-V. Eremin, IS. Zaslonko, SM. Kogarko, E.V. Mozzhukhin and Yu. Petrov, Kinetika i Kataliz 11 (1970) 869. 1.M. Napier and R.G.W. Norridr, Proc. Roy_ Sot. 299A (1967) 317. W.H. Hartung and F. Crossley, Organic synthesis, Vol. 2 (Wiley, New York, 1943) p_ 363. P. Tarte, J. Chem. Phys. 20 (1952) 1570. R-V. Ambartzumian. N-P_ Funikov, Yu.A_ Gorokhov, VS. Letokhov, G.N. Makarov and A-A. Puretakii, Opt. Commun. 18 (1976) 517_ R-V. Ambartzumian, Yu.A. Gorokhov, VS. Letokhov. G.N. Makarov and A-A. Puretzkii, Pis’ma JETP 22 (1975) 374. P. Gray and M.W.T. Pratt, J.Chem. Sot. (1958) 3403. R.C. Weast, ed., Handbook of Chemistry and Physics (Chemical Rubber Co., Cleveland, 1970). M J. Yee Quee and J-C J. Thynne, Trans. Faraday Sot. 62 (1966) 3154. J.G. BZack;E. Yablonovitch, N. Bloembergen and S. Mukamel, Phys. Rev. Letters 38 (1977) 1131.
CHEMICAL PHYSICS LETTERS
1 February 1978
INFRARED MULTIPHOTON DISSOCIATION OF METHYL NITRITE Allen HARTFORD Jr. Materials Research Center. Allied Chemical Corporation.
hfon-istown, New Jersey 07960, USA
Received 10 October 1977 Methyl nitrite undergoes dissociation when irradiated with the focused output of a pulsed CO2 TEA laser. Time resolved infrared emission is observed and attributed to viirationally excited formaldehyde and methanol. These species are produced via reactions of methoxy radicals formed in the primary dissociation. Other products formed are NO and N20.
1. Introduction Collisionless multiphoton photodissocihtion of polyatomic molecules in the presence of an intense infrared laser field has been the subject of considerable study in recent years [ 141, primarily due to the isotopic enrichment obtainable with this process [5,6] . Most of the reported studies have concerned themselves with an analysis of the end products formed as a consequence of the dissociation. In addition, a few investigators have reported the observation of transient species by the method of laser excited fluorescence [7,8]. In this paper we report an analysis of the end products, as wel! as the observation of time resolved infrared fluorescence, resulting from the irradiation of methyl nitrite (C&ON@) with the focused output of a CO, TEA laser. Several interesting features motivated the choice of methyl nitrite. First, the molecule exists in both cis and trans isomeric forms in the vapor phase, the vibrational bands in the infrared absorption spectrum being “doubled” due to the presence of these isomers. Consequently, both the cis and trans forms can be dissociated with a CO, laser whose output is coincident with the C-O stretching findamental of methyl nitrite. In addition, upon deuteration of methyl nitrite the C-O stretching mode is shifted toward lower frequency and essentially out of coincidence with the CO, laser, while a deuterium degenerate deformation mode is moved into coincidence. This allows multiphoton dissociation to be studied in the deuterated mol&ule. Finally, a number of studies concerning the
ultraviolet photolysis and shock tube decomposition of methyl nitrite have been reported [9-l l] _ These can be compared to the infrared multiphoton dissociation results to gain insi&t into the processes responsible for the formation of the end products.
2. Experimental Dissociation of methyl nitrite was accomplished with a grating tuned CO, TEA laser (Lumonics model 203). At least 400 mJ (measured with a Lumonics model 20D energy detector) single line output was obtainable in a pulse with fwhm of 100 ns. A 19 cm focal length antireflection coated germanium lens focused the laser into the center of a Pyrex sample cell. This cell was 4 cm in diameter and 15 cm long. The ends of the cell were sealed with sodium chloride windows, while a potassium chloride window was used to view fluorescence normal to the laser beam axis. A capacitance manometer (MKS Baratron) connected directly to *&e fluorescence cell was used to measure pressures. An X-Y recorder connected to the output of the capacitznce manometer permitted continuous recording of the pressure variation. Fluorescence was detected with a photovoltaic InSb detector. Combinations of bandpass filters were used to isolate tluorescence in particular spectral regions within the range from 3-5.6 m (the detector cut off)_ Amplified fluorescence signals were digitized with a Biomatior 8100 transient recorder and subsequently displayed on an X-Y recorder. In mrne instances the 503
CHEMICAL PHYSICS LETTERS
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digitized signals were averaged with a Tracor-Northem 575A signal averager and then displayed on the X-Y recorder_ CH30N0 and CD30N0 were synthesized according to the method of Hartung and CrossIey [ 12]_ Briefly, either CHsOH or CD30D was combined with a saturated solution of NaN02. Dilute sulfuric acid was added dropwise to this mixture. The gaseous methyl nitrite that evolved was carried by an N2 stream through a trap immersed in an ethylene dichloride slush (-36°C) to remove water, then collected in a liquid nitrogen trap. The purity of the samples was
verified by gas chromatography. Infrared spectra of bdth CHSONO and CDSONO were recorded on a Perkii--Elmer 283 infrared spectrometer. Identification of end products-was caked out by recording the spectra of Irradiated samples on the same instrument. 3. Results and discussion The infrared spectra of do-methyl nitrite and d3methyl nitrite in the vicinity of CO2 laser emission are shown in fig. l_ The two absorption features in
C-0 stretch
Fig. I_ Infrared spectra of methyl nitrite in the vicinity of CO2 are 10 cn? frequency calibration.
504
laser emission: (a) CH30NO; @) CD3ONO. Vertical “tick” marks
CHEMICAL
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CH,ONO centered at 990 cm-l and 1040 cm-l correspond to the C-O stretching mode of the cis and tram isomers, respectively [13] _ The cis and trans designation refer to the position of the methyl and N=O substituents with respect to the O-N band. Both Pand R-branch lines from the CO, laser 10.6 firn emission are coincident with absorption due to the cis conformation, while the C-O stretch of the trans isomer is coincident with P-branch lines of the 9.4 pm transition. In CD,ONO the C-O stretching fundamental is shifted toward lower frequency (cis ~9 15 cm-t, trans =950 cm-‘), whereas the CD3 degenerate deformation is centered at 1030 cm-l _This vibrational mode is coincident with the P-branch lines of the 9-4 m CO, laser transition_ Initial experiments merely monitored the pressure increase in the sample cell when 1 torr samples of domethyl nitrite were irradiated with the focused output of the CO, laser. Typical curves of pressure increase versus number of laser pulses for different excitation wavelengths are displayed in fig. 2. No pressure change was obsenred when samples of CHsONO were irradiated with the laser unfocused_ Fig. 3’shows the change in pressure after 1000 laser pulses as a function of excitation line. For clarity, the absorption spectrum of CH30N0 is displayed on the same wavelength scale. In all experiments the laser energy was fmed at 300 mJ/pulse. Infrared spectroscopic analysis of the products formed as a result of dissociation indicate the presence of NO, N,O, H,CO, and CH,OH as well as
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OF LASER PULSES
Fig. 2. Capacitance manometer output of pressure increases during CO* TEA laser irradiation of 1 torr samples of CHsONO. (a) Irradiation of cis isomer with the R(28) line of the 10.6 pm CO, laser transition; 0~) h-radiation of trans isomer with the P(16) line of the 9-4 pm transition.
0 0
1‘ 900
. 920
940
960
980 -I 1000 cm
lo20
LO40
1060
I
00
1060
Fig. 3. Dixociation yield from methyl nitrite as .I function of laser frequency. 1000 laser pulses at 300 mJ/puIse were used to obtak each point. Also shown is a portion of the infrared absorption spectrum.
remaining starting material. Since all of these components are stable and volatile, measurement of the pressure increase reflects the dissociation efficiency. The yield of product versus the absorption spectrum of methyl nitrite exhibits some features that merit discussion. On the P-branch side of the cis feature, the yield increases from low frequency reaching a maximum near the Q-branch. This behavior is strikingly similar to that observed in the sin,$e frequency_ multiphoton dissociation of SF6 [14] _Furthemlore, as discussed by Bloembergen [3] , the multiphoton dissociative process might be expected to be most efficient when the excitation frequency is displaced from the band origin by an amount corresponding to the vibrational anharmonicity. Qualitatively, this pattern is observed for the cis conformation of methyl nitrite. The yield due to irradiation of the trans isomer is reduced somewhat from that of the cis. This is not surprising since the absorption coefficient is approximately a factor of 3 to 4 less for the trans-methyl nitrite vibrational mode than for the corresponding vi-
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bration in cis-methyl nitrite. Previously, it has been found that the dissociation rate as measured in different vibrational modes of SF6 is proportional to the matrix element of the transition dipole [ 151, a situation which appears to pertain in the present case.
Product analysis by infrared spectroscopy indicated that both the cis and trans isomers yielded the same species when irradiated (NO, N,O, H,CO, and CH,OH). Furthennore, within the accuracy of the infrared absorption intensities observed, the products in both cases were formed in the same proportion. This suggests that the initial photodissociation step is the same in both isomers, and is unaffected by the geometrical conformation. Along with measuring the pressure increase and rmalyzing the fmal products,
time resolved infrared
Strong signals were observed, which, by using various bandpass filters, were found to be in the wavelength region between 3.54 and 3.99 D (10% transmission points). Typical fluorescence decay curves are shown in fig. 4. From the curve in fig. 4a, it may be noted that the fluorescence rise follows the response time (=O.S ps) of the detection system. TO account for this rapid rise, we must assume the species responsible for the fluorescence is either produced during the prima;y di&ociation step and contains excess vibrational energy, or is fonned in an exothermic secondary reaction that has nearly a gas kinetic rate (approximately 3 collisions occur at 0.75 torr in OS ,us). We note from the pressure increase data that essentially all the molecules within the volume of the focused laser beam are dissociated with each pulse (-1.5 X 1015 molecules). From bond eneru considerations, the initial multiphoton dissociation is expected to involve rupture of the O-N bond (:39 kcaijmole) T, producing NO and methoxy radical: fluorescence
was also monitored.
CH,ONO + CH,O-
+ NO.
(1)
Following this a number of secondary reactions can occur (consistent with the product analysis)_ These are given below: CH,Om + NO + CH,O -I-HNO,
(2)
fCalculated from the heats of formationof CHSONO, CH30-, and NO as given in refs. [ 16,171. 506
(b)
Y:.
20
JO
60
80
100
120
140
160
TIME (~S’X)
Fig. 4. Fluorescence decay curves observed upon irradiation of 0.75 torr methyl nitrite with R(32), 10.6 pm CO1 laser line. (a) Short time scale indicating rapid fluorescence rise; (b) longer time scale showing total fluorescence decay.
CH,O*
+ CH,O* + CH,O + CH,OH,
(3)
HNO + HNO + N,O f H,O, CH30-
+ CH30N0
+ CH,O + CI$OH
(4) + NO_
(5)
Anlong the various species above, formaldehyde has known vibrational bands occurring b&ween 3.54 and 3.99 m. The VI mode is centered at 3.60 can, while
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the u4 mode extends into the aforementioned region. (A small contribution from the s-stretch of CH30H is also possible_ The formation of vibrationally excited methanol will be discussed_) Reactions (Z), (3), and (5) are exothemric by 26 kcal/mole, 82 kcal/mole, and 42 kcal/mole, respectively, all sufficient to produce vibrationally excited formaldehyde. In addition, reaction (3) has been shown from shock tube studies of methyl nitrite to have a rate constant of 0.62 w-t to& at 780 K [lo] _At 3 14 K the rate for this reaction has been given as 7.26 ,XS~ to& by Yee Quee and Thynne [ I8]_ A lower bound for the rate can be found from the fact that we were unable to resolve a risetime in our experiments_ This limit is 3.6 j~s-l torr-l, a value consistent with Thynne’s result. In comparison, the rate for reaction (2) is approximately 3 orders of magnitude slower than the rate of disproportionation of methoxy radicals [lo] _Thus, reaction (2) is not likely to be a primary contributor to the observed fluorescence_ Reaction (5) is also slower than the disproportionation mechanism [lo] , thus it may be concluded that the reaction of two methoxy radicals is the preponderant source of vibrationally excited formaldehyde_ One additional experiment was performed to verify that the observed fluorescence originated from formaldehyde. A sample of CD,ONO was irradiated with the focused output of the CO, laser. Strong fluorescence was again observed and occurred in the region from 4.72 to 5.09 W_ The vl and v4 modes of D&O are centered at 4.63 and 4.86 w, respectively. This is consistent with the results obtained with CH30N0. In addition to the above fluorescence, a somewhat weaker signal was noticed in the region from 3.10 to 3.99 pm. This fluorescence is shown in fig. 5 and is again characterized by a rapid rise, but decays by approximately a factor of four faster than previously observed signals. Of the possible fluorescing species, only CD,OD has a vibrational mode in the region of observation. The O-D stretching vibration is at 3.67 PITI.Since both methanol and formaldehyde are formed in the exotbermic disproportionation of methoxy radicals, it is not unexpected that both species are formed in vibrationally excited states. An important point to note is the absence of fluorescence in the 3-5.6 pm region from either CH,ONO or CD,ONO, even after extensive signal averaging, upon irradiating samples with the unfocused output of the laser. Clearly, the fluorescence
!
f&h 0
20
40
$0
80 TIME
l&l (LL set
120
140
m
160
1
D
)
Fig. 5. Observed fluorescence following irradiation of 1.0 torr of CD30NO with the P(36) line of the 9.4 pm CO* transition. Fluorescence is in the region from 3.04 to 3.99 pm.
observed in these experiments does not arise from collisional up pumping of methyl nitrite itself_ One other source of the IR fluorescence should be considered in view of the rapid risetime observed_ It may be possible that vibrationally excited methoxy radical is formed in the primary photodissociation. Although the LR fundamental vibrational frequencies for methoxy radicals are not known, characteristic CH3 or CD, modes have wavelengths near those attributed to vibrationally excited formaldehyde. These wavelengths may be changed sufficiently in methoxy radicals to consider them as possible sources of fluorescence_ If this were the situation, the observed fluorescence quenching wou!d be dominated by recombination of methoxy radicals, rather than vibration-vibration or vibration-rotation, translation processes which are
expected to be slower. However, since the fluorescence decay is slower (see fig_4b) than any of the rates reported for methoxy radical disproportionation, it appears reasonable to conclude the fluorescence does not occur from vibrationally excited metboxy radicals_ From the above considerations the observed fluorescence is apparently due to emission from formaldehyde and metbauol which acquire excess vibrational energy 507
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due to the exotbermic disproportionation of methoxy radicals. The observed rate for this process is near gas kinetic if ambient themntl velocities are assumed. It is possible, however, that the radicals produced in the dissociation are translationally hot, thereby increasing the collision frequency which may result in a faster rate. We have also tried to ascertain whether NO fornred in the initial dissociation step is produced in vibrationally excited states. However, no infrared emission was observed in the region of5.3 arm. Considering the sensitivity and cohection efficiency of the detection system, it is possible to estimate that less than 1% of the NO formed per pulse is in u = l_ In summary, the following observations may be made regarding the multiphoton dissociation of methyl nitrite. The analysis of end products is consistent with dissociation occurring via the thermodynamic channel of least energy (breaking of the O-N bond), followed by secondary reactions of the radicals produced. Also, the dissociation process is independent of the vibrational mode excited (C-O stretch or CD3 deformation) and the conformation of the moIecuIe (either cis or trans)_ These observations would seem to be in accord with models of multiphoton dissociation which assume that energy is statistically distributed among the internal degrees of freedom of the reactantmolecule
Cl91Acknowledgement The author greatly benefitted from conversations with Drs. James Yardley, Donald Heller, and Otis Peterson. The skilled assistance of Mr. Joseph Pete in carrying out experimentation and the expert synthetic work of Miss Margaret Nowakowski were invaluable. Considerable thanks is extended to Dr. John Witt for
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recording infrared spectra and to D-r. Willis H&mmqnd for constant advice. _
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[lOI [ll] [ 121 [ 131 [141
[151
[16] 1171 [IS] [ 191
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