Photodissociation of 1,1-difluoroethene (CH2CF2) at 193 nm monitored with step-scan time-resolved Fourier-transform infrared emission spectroscopy

We measured time-resolved vibration-rotational emission of HF (Hartree-Fock ) at various intervals (5-500 mu s) after photolysis of 1,1-difluoroethene (CH2CF2) at 298 K with an excimer laser at 193 nm by means of a step-scan F ourier-transform spectrometer. Emission of HF(v) with 1 less than or equal to v less than or equal to 5 was observed, with intensity maxima at varied intervals after photolysis. Temporal profiles of HF(v) fit satisfactorily w ith a kinetic model consisting of nascent production of HF(v) followed by q uenching of HF(v) by parent molecules. Measurements of rates of quenching a t varied partial pressure of CH2CF2 yield bimolecular rate coefficients of quenching of HF(v) by CH2CF2:k(qv)(II)/10(-12) cm(3) molecule(-1) s(-1)=1.0 7 +/- 0.10, 2.95 +/- 0.22, 13.5 +/- 0.9, and 45.2 +/- 4.1 for v=1-4; listed errors represent one standard deviation. The nascent vibrational distribut ion of HF is (0.365 +/- 0.014):(0.255 +/- 0.017):(0.177 +/- 0.015):(0.134 /- 0.014):(0.069 +/- 0.012) for v=1-5, respectively, consistent with previo us results. By adding Cl-2 into the system, we observed weak emission of HC l(v), 1 less than or equal to v less than or equal to 4, upon photolysis at 193 nm. Such observation indicates that production of H, followed by react ion of H with Cl-2 to form HCl(v) takes place as a minor channel. Incorpora tion of this H-elimination channel into the kinetic model yields a branchin g ratio of 0.10 +/- 0.03 relative to the HF-elimination channel. Bimolecula r rate coefficients of quenching of HCl(v) by CH2CF2 and vibrational distri bution of HCl from the reaction H+Cl-2 are also determined. The F-eliminati on channel was not observed; estimated upper limit for the branching ratio is 2%. (C) 1999 American Institute of Physics. [S0021-9606(99)01144-7].