David O. Ham

Transmission measurements of multiple photon absorption in SF 6

Measurements of the average number of photons absorbed by SF6 from unfocused CO2 laser pulses are reported as a function of integrated intensity, pressure, and laser wavelength. Measurements extend from very low excitations to well above the dissociation threshold. The importance of anharmonic splittings, which are mixed extremely rapidly by collisions, is emphasized by the pressure and wavelength dependences.

Chemiluminescence spectra of the new molecules, NaF2 and NaCl2, and their implications for reaction dynamics

Abstract Emission spectra from dilute gas mixtures of Na with F2 and Cl2 have been measured and indentified as orginating from electronically excited NaF*2 and NaCl*2 emitting to stable ground states of these molecules. This is the first observation of molecules of this type which were predicted to be stable theoretically in the case of LiF2. The existence of these molecules indicates an attractive exit channel in M + X2 → MX + X reactions of alkali atoms with halogens.

Energy limits in chemiluminescent, atom transfer reactions: Bond dissociation energy of NaF

Atomic emissions from the reactions M2 + X → MX + M* have been observed for the systems which produce MX = NaF, NaCl, NaBr, KCl, and KBr. Energy limits in these reactions are also discussed which yield lower limits on values of D00(MX) and a value of D00(NAF)=123 ± 3 kcal/mole is recommended. Arguments are presented in support of previous chemiluminescence determinations of bond energy limits.

Threshold behavior of multiple photon dissociation of SF6

Abstract Multiple photon dissociation of SF 6 by a short pulse of a CO 2 laser has been investigated by simultaneous measurements of the average number of photons absorbed per molecule 〈 n 〉 and chemiluminescence intensities which result from the dissociated F atoms. A criterion for the dissociation threshold which is independent of laser wavelength is found to be 〈 n 〉 = 16 ± 3 photons per molecule. A thermal distribution of the excited molecules is shown to be inconsistent with the behavior just above threshold.

Collisionless self-focusing of CO 2 10 µm P(20) laser light in SF 6

We develop a simple intuitive picture of the vibration-rotation structure of the SF 6 molecule such that the molecular susceptibility responsible for self-focusing can be calculated. We treat the propagation dynamics by generalizing the standard steady-state Gaussian propagation equations to include the important effect of absorption in the wings of the spatial profile. By calibrating the model to absorption data at CO 2 10 μm P(2) we find good agreement with beam waist data at the same wavelength. Absorption in the wings is dominant at low laser fluences, and the real part of the susceptibility is responsible for the defocusing-to-focusing turnover in the beam waist near 100 mJ/cm2, consistent with the interpretation of Nowak and Ham [6].