MOLECULAR ACTIVATION WITH LOW-INTENSITY CW INFRARED-LASER RADIATION - MULTI-PHOTON DISSOCIATION OF IONS DERIVED FROM DIETHYL-ETHER

Multiphoton dissociation of gas-phase ions derived from (C2H5)2O,(C2D5)2O, and C2H5OC2D5using low-intensity infrared CO2laser radiation is reported. Techniques of ion cyclotron resonance (ICR) spectroscopy are used to store ions, allowing irradiation for up to 2 s with intensities of 1-100 W cm-2. Reasonably uniform irradiation of the stored ions is achieved with an unfocused laser beam facilitating studies of ion photodissociation kinetics. Decay of the ion population is characterized by an induction period followed by exponential decrease in ion abundance. The induction period is inversely proportional to laser irradiance with the product of these quantities yielding a fluence threshold for the reaction studied. Measured energy fluence thresholds and dissociation rate constants are comparable to data derived from pulsed laser experiments. For all ions which photodissociate only the decomposition process of lowest activation energy is observed. Detailed studies of the effects of collisions on the multiphoton dissociation of the proton-bound dimer [(C2H5)2O]2H+ are reported. Laser-excited ions are deactivated during irradiation by collisions with neutral buffer gases ((C2H5)2O),i-C4H10, and SF6. Deactivation rate constants are approximately 10-20% of calculated collision rate constants. Collisions prior to irradiation are shown to have no effect on dissociation rates. Collision-free dissociation of [(C2H5)2O]2H+ is first order in photon flux. Photodissociation spectra of [(C2H5)2O]2H+, ((C2H5)2OH+, and (C2D5)2O2D+ are reported and compared to the gas-phase infrared absorption spectrum of the corresponding neutral ether. The dissociation of ((C2H5)2OH+ is by β-H transfer yielding (C2H5)OH2+ and C2H4. The competitive multiphoton dissociation of (C2H5)(C2D5)OH+ shows an isotope effect ≥6 favoring β-H transfer. This implies that excitation is very much slower than decomposition above threshold energy. Model calculations treating absorption as a sequential (incoherent) process with decreasing cross section and using RRKM theory to calculate dissociation rates show qualitative agreement with experimental results. © 1979, American Chemical Society. All rights reserved.