The rate coefficients for collision-induced intersystem crossing (CIISC) of methylene from the ã 1A1 first excited to the X̃ 3B1 ground electronic state, CH2 (ã 1A1)+M→CH2 (X̃ 3B1)+M, were investigated within the framework of the mixed-state mechanism [see, e.g., K. F. Freed, in Potential Energy Surfaces, edited by K. P. Law (Wiley, New York, 1980)]. Accordingly, the overall electronic relaxation was assumed to proceed via a sequence of rotational transitions within the ã manifold and allowed transitions from the ã to the X̃ manifold originating via ‘‘gate’’ states of ã which are states that contain some triplet character due to spin–orbit coupling with nearby X̃ rovibrational states. The perturbed ã and perturbing X̃ levels and relevant interaction matrix elements were identified from the available spectroscopic data. Rate coefficients for rotational relaxation processes were obtained from collision broadening measurements of CH2 (X̃) far-infrared laser magnetic resonance (FIR-LMR) transitions. Taking these data, thermal CIISC rate constants corresponding to experimental results for the overall depletion of rotationally thermalized CH2 (ã) and build up of (X̃) by M were evaluated for twelve interesting collision partners (M=He, Ne, Ar, Kr, Xe, N2, SF6, H2, D2, CH4, C2H6, H2O), taking into account every single CH2 (ã) rotation vibration state with energies up to Evr≤900 cm−1. The results were found to be in good agreement with reported room-temperature experimental data. Temperature dependencies, which were predicted for M=Ar, N2, and CH4, also agree with measured values.

The reaction bctween CH2(R3Bl) radicals ('CH,) and 02, 'CH2 + 02+ products,has been studied in a discharge flow reactor with Laser Magnetic Resonance (LMR) detection of 'CH2 and OH radicals and Electron Spin Resonance detection of 0 and H atoms. From the measured concentration-versus-time profiles of C3CH2] in the absence and presence of a large excess of O2 the overall rate coeficient of the reaction was found to be k , = (8.7 k2.0). 10". exp[ -(4.2 f 1 .O)kJ. mol-'/R. T]cm'/mol. s in the temperature range 233 K I T 5 433 K. A limit for the contribution of H atom forming reaction channels of <20% was determined. The results are discussed in relation to available literature data on the formation of stable products. Main reaction channels leading directly to production of CO + HzO and H2 + C 0 2 are concluded to be of importance.

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Carbenes / Chemical Kinetics / Energy Transfer / Intersystem Crossing Laser Magnetic ResonanceThe mechanism of collision-induced inte_rsystem crossing (ISC) in CH2 from the % 'A, first excited singlet to the 3BI triplet ground electronic state, CH, (2) + M -t CHI ( X ) + M, is investigated. Collisional ISC rate constants are calculated for four selected collider gases, M = He, Ar, N2, and CH4, using the "mixed-state" model of Freed and Gelbart. The most important triplet-perturbed rotational levels in the %-state are identified from the spectroscopic data reported in the literature. The rates of rotational relaxation processes, Yhich according to the model effectively govern the collision-induced ISC, are determined by studying the collision broadening of CH2 (X) Far Infrared Laser Magnetic Resonance (FIR-LMR) transitions. The collisional ISC rate constants for CH2 in the ortho nuclear spin modification are calculated to be klsc = 1.4 . lo'*, 4.0 . and 6.6 . 10l2 cm3/mol . s for M = He, Ar, N2, and CH4, respectively. The results are in good agreement with published experimental rate constants.7.8 .

Far infrared laser magnetic resonance (LMR) spectra of metastable (2p3)2D nitrogen atoms have been measured using laser lines in carbonyl fluoride (COF2) at 1135 μm and methanol (CH3OH) at 1224 μm. The concentration of N 2D was optimized by electron paramagnetic resonance (EPR) using a hybrid EPR/LMR spectrometer. The far infrared cavity of this spectrometer was designed to oscillate at wavelengths longer than 1 mm. The LMR spectra were analyzed using g factors derived from the EPR spectrum [Radford and Evenson, Phys. Rev. 168, 70 (1968)] and yield a precise value of the 2D5/2–2D3/2 fine structure separation of −261 447.7(2) MHz, −8.720 957(7) cm−1(2σ).