Dissociative electron attachment to dipolar molecules at low energies withmeV resolution: CFCl3, 1,1,1-C2Cl3F3, and HI

Using the laser photoelectron attachment method we investigated dissociativ e electron attachment e(-) (E) + XY --> X + Y- (DEA, short notation Y-/XY) to three molecules XY with a permanent electric dipole moment in the electr on energy range 0 < E/meV less than or equal to 173 with very high resoluti on (energy width DeltaE < 1 meV). With reference to reliable thermal attach ment rate coefficients absolute DEA cross sections a, (E) have been derived for the processes Cl-/CFCl3, Cl-/1, 1, 1-C2Cl3F3 and I-/HI. At thresholds for vibrational excitation of the neutral molecule, the cross sections exhi bit a pronounced cusp structure (in most cases of a downward step character ) due to coupling of the attachment process with scattering channels. At lo w energies (from about 0.8 meV to the first vibrational cusp structure) the cross sections are well described by the simple analytical expression sigm a (e)(E) = (sigma (0)/E)[1 - exp(-betaE (1/2))] (with beta falling in the r ange from 0.88 to 1.6 (MeV)(-1/2)), indicating an energy dependence of betw een E-1/2 and E-1, in essential agreement with predictions of an extended V ogt-Wannier (EVW) capture model which includes the long-range electron-dipo le interaction in addition to the polarization force. Comparisons are made with cross sections derived from previous photoelectron attachment work (TP SA) and from experiments involving electron beams, electron swarms and Rydb erg atoms. Based on our experimental results for sigma (e)(E), we calculate and report for all investigated processes the energy dependence of the rat e coefficients k(e)(E) for monoenergetic free-electron attachment as well a s for Rydberg electron attachment k(nl) (n approximate to 30-150, l much le ss than n) and the electron temperature dependence of the rate coefficients k(e) (T-e) for free-electron attachment involving a Maxwellian electron en semble and a gas at room temperature (T-G = 300 K).