J. Phys. Chem. 1991, 95, 9332-9336
9332
Collisional Energy Transfer in the Multlphoton Decomposition of tert-Butyl Methyl Ethert Glenn A. McRae,* Paul E. Lee, and Robert D. McAlpine* Chalk River Laboratories, AECL Research, Chalk River, Ontario, Canada KOJ I JO (Received: March 26, 1991; In Final Form: June 19, 1991) The multiphoton decomposition (MPD) of tert-but 1 methyl ether (TBME) was studied with a 100-ns pulsed COzlaser operating on the 9R(20) line and at a fluence of 12 J/cmY For TBME pressures up to 2.7 kPa, MPD gave the following products: 2-methylpropene (TMP), methanol, 2-methyl-2-butene,acetone, methane, hydrogen, ethene, and ethane. The products result from reactions among the species produced through two channels: (1) TMP + methanol; (2) acetone + 2CH3.A collision-dependent reaction scheme is presented to explain the pressure dependence of the MPD. In this instance, significant energy transfer between two laser-excited TBME molecules occurs before decomposition takes place. The decomposition products deactivate excited TBME molecules and reduce subsequent decomposition.
Introduction The selective infrared multiphoton decomposition (MPD) process is being considered for laser isotope separation of uranium,' hydrogen,2 carbon,' oxygen? and other element^.^ It is being examined, at Chalk River, as an alternative to the G S process6 for heavy-water production. Since MPD cannot yet be carried out directly on the two large-scale sources of deuterium, water and methane, the deuterium must be exchanged into a "working molecule" on which the selective decomposition is performed. Economic considerations impose severe limitations on the choice of a suitable molecule.2 TBME ex& many of these limitations. The multiphoton decomposition of tert-butyl methyl ether (TBME), (CH3),COCH3, has been investigated for the purpose of laser isotope separation (LIS) of oxygen isotopes.'** In these studies the optically excited TBME was suspected of decomposing via a simple C-0 bond break to yield the tert-butyl radical and the methoxy radical, OCH,. The final molecular products were seen to depend on the laser-fluence. At low fluences, the methoxy radical apparently scavanges a H from the tert-butyl radical to yield 2-methylpropene (TMP) and methanol; at high fluence, the methoxy radical decomposes to give formaldehyde, and the tert-butyl radical decomposes to give TMP. In contrast, pyrolysis of TBME in the temperature range 900-1 100 K suggests a four-center molecular elimination of methanol leaving TMPa9 Although it was pointed out that the alternative, free-radical, mechanism could give the same products, further evidence (radical scavengers, etc.) supported the molecular process. The apparent disagreement in the interpretation of the decomposition mechanisms, four-center molecular elimination for pyrolysis vs C-O bond breaking and radical reactions for MPD, is curious. A novel method of parametrizing MPD in terms of various reactant and product pressure dependences has been developed in our laboratory.I0 This method is different because it explicitly allows for a pressure dependence in the decomposition and for the discrete nature of the laser pulse. The single pulse decomposition probability,f(&a,b) (where 4 is the laser fluence, a is the TBME partial pressure, and b is the decomposition product partial pressure), is expanded as a series in a and b: f(4,a,b) = C Chij(4)a'-'& 1' I J'o
(1)
From a determination of the set of nonzero hiJ coefficients and from isotopic selectivity measurements potential working molecules can be screened. For example, the most desirable behavior for a potential working molecule is a true unimolecular collisionless MPD, characterized by isotopically selective MPD and a nonzero hlo It is important, however, to recognize that a nonzero hlo alone does not guarantee that isotopically selective MPD is occurring."*'2 The molecule can also gather sufficient energy for decomposition through a combination of direct absorption and collisions. 'Contribution No. 10416 from AECL Research.
0022-3654/9 1 /2095-9332$02.50/0
This type of behavior might be called collisionally assisted MPD. It is characterized by nonzero higher order h, coefficients, which may be indicative of "energy pooling" events. Generally, i molecules of target a n d j molecules of buffer collide and vibrational energy is transferred. The efficiency of vibrational energy transfer partially determines the magnitude of the hip If after the collision the target molecule has more energy than it did before the collision, then energy pooling has taken place. If this target molecule then decomposes, the parameter will be positive. If the target molecule has less energy after collision, then deactivation has occurred. If deactivation leaves the molecule with insufficient energy to decompose, then the h, parameter will be negative. The energy-transfer process is very interesting in that it is possibly mediated by the occasional transfer of a massive amount of vibrational energy during a collision."-"
Experimental Section The laser light source used was a Lumonics Model K-92 1 TEA COz oscillator with a dual-discharge high-gain Lumonics K-922s amplifier with triple-pass optics. The amplifier section operates on a 22:12:65 mix, and the oscillator on a 32:11:57 mix, of C02/N2/He. The oscillator and amplifier sections were each connected to an AECL Model 30 laser gas recirculator to ensure minimum misfire and constant laser gas composition. The laser pulse was approximately 100 ns long with no discernible tail as observed with a home-built germanium photon drag detector. The oscillator was equipped with a 47% reflecting, 5 mrad wedged, germanium output coupler and a Bausch and Lomb replica grating, 135 lines/", blazed for a wavelength of 7.4 pm. The laser operated in multilongitudinal mode and was tuned to the 9R(20) line (1078.7 cm-I). The pulse repetition rate was 1 Hz. Approximately 5% of the output beam was directed by a NaCl beam splitter to a Laser Precision energy monitoring probe (Model RJP-736) attached to a Laser Precision meter (Model RJ 7620). The remainder of the beam was directed through CaFz attenuating (1) Jensen, R. J.; Judd, 0. P.; Sullivan, J. A. Los AlamosSci. 1982, 3, 2. (2) Ivanco, M.; Evans, D. K.; McAlpine, R. D.; McRae, G. A.; Yamashita. A. B. Soectrochim. Acta 1990. 46A. 635. (3) Outhouse,A.; Lawrence; P.; Gauthier, M.; Hackett, P. A. Appl. Phys. 1985, 836, 63. (4) Kutochke, K. 0.;Willis, C.; Hackett, P. A. J . Photochem. 1983,21. McAlpine, R. D.; Evans, D. K. Adu. Chem. Phys. 1985,60, 31. Rae, H. K., Ed. Separation of Hydrogen Isotopes; ACS Symposium 68; American Chemical Society: Washington, DC, 1978. Majima, T.; Ishii, T.; Arai, S. Bull. Chem. Soc. Jpn. 1989,62, 1701. Majima, T.; Sugita, K.; Arai, S. Chem. Phys. Lett. 1989, 163, 29. Choo, K. Y.; Golden, D. M.; Benson, S.W. Int. J . Chem. Kinet. 1974, 6, 83i. (10) McRae, G.A.; Yamashita, A. B.; Goodale, J. W. J . Chem. Phys. 1990,92, 5997. (1 1) McRae, G. A.; Ivanco, M.; Lee, P. E.; Goodale, J. W. Proceedings. International Symposium on Isotope Separation and Chemical Exchange Uranium Enrichment, Tokyo, Oct 29-Nov 1, 1990; Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology: 0-okayama, Meguroku, Tokyo 152, 1990. (12) McRae, G.A.; Ivanco, M.; Back, R. A. Chem. Phys. Lett., in press. (13) Ivanco, M.; McRae, G.A.; Back, R. A.; Goodale, J. W.; Lee, P. E. J . Chem. Phys., in press.
Published 199 1 by the American Chemical Society
Collisional Energy Transfer in Multiphoton Decomposition
