<title>Restoring excited particle interaction potentials from laser spectroscopy data: Na broadened by CO2 as an example</title>

Abstract

ABSTRACFPreliminary results of restoring potentials of interactions Na(3s,3p)-CO( X'Z) from measured far wing spectra of sodium resonance line Na-D perturbed by CO are presented. The spectra in the range ofAc1O-35OO cm' are considered following the quasi-static theory ofline broadening by inert gases as continua and due to the A->X and B*->X transitions of Na-CO quasi-molecules. The reduced absorption coefficient K(o) at frequency a=o+A is calculated ascorresponding to the radiation from molecules with internuclear separation R, where hcci =V V' ( VAB and Vx are the excited and ground state adiabatic potentials respectively). As a first approximation, all the three potentials are presented in the shape of a (6,12)-Lennard-Jones potential. The constants C6 and C12 are determined as independent variables by minimization of the sum of squared deviations between measured and calculated K(). Four versions of minimization were considered successively: (1) only red wing, (2) only blue wing (3) both blue and red wings with 6 independent variables ( C6 and C1., ), and (4) both wings with fixed C6 . The calculated K(E) are compared to the measured ones.Keywords: reduced absorption coefficient, spectral line broadening, atom-molecule interaction potentials. INTRODUCHONPhenomena that critically depend on the interaction potential of unstable species include cross sections for excitation transfer, quenching of excited states, spin exchange, depolarisation, elastic scattering of atomic beams, chemical reactions, dissociation processes, shift and broadening of spectral lines, etc. Investigations of the wing profiles of spectral lines serve as a valuable source of information about atom-molecular interaction potentials. At present, the profiles of alkali metal atoms' spectral lines under conditions of broadening by monatornic inert gases appear to be the best studied both theoretically and experimentally'. In the case of interaction between alkali atoms and molecules the theory is much more complex because the interaction potential depends also on the relative orientation of the orbital moment vectors of interacting particles. Hence, each particular pair has to be considered individually. Then, to restore interaction potential from the spectral line far wing profile, a reverse ill-posed problem has to be solved2.Unfortunately, few studies into the broadening of spectral lines of atoms of alkali metals by molecular gases are known and in most of them35 the measurements have been performed in flames of complex molecular composition, which made it difficult to determine the individual contribution of molecules of every given type. The broadening ofNa-D lines by a single component molecular gas has been studied for N2 6 H20 and CO 8 but the interaction potentials have been derived only for N2. Meanwhile, the mixture of these three molecular gases is of practical importance as it is the product of combustion of hydrocarbon fuels in air.In 8 the reduced spectral absorption coefficient was measured. The measurements were performed in a...