Experimental and theoretical investigation of pulsed step-wise laser excitations of atoms in flames — the role of two-step vs two-photon excitations and dynamical Stark effects

Abstract

Anomalous lineshapes and signal strengths in two-colour Laser-Enhanced lonization (LEI) spectrometry in flames have been studied both experimentally and theoretically. Experimentally, we have found that both lineshapes and signal strengths from atoms in flames can be significantly affected by the influence of coherent contributions in the excitation process-such as two-photon excitations (i.e. when the atoms simultaneously absorb two photons, one from each laser) and dynamical Stark effects (i.e. interactions between the laser light fields and the atoms causing rapid Rabi oscillations between atomic energy levels which give rise to broadening, splittings and shifts of transitions)-already at the rather moderate laser intensities that are obtained in unfocused pulsed laser beams today. Theoretically, it is found that the normal rate-equation approach is insufficient for describing these kind of phenomena of two-colour LEI in flames. A theory based on density-matrix formalism has been adopted for the situations encountered in LEI in flames. The equations are solved in the steady-state limit using the rotating wave approximation. The finite bandwidths of the lasers are included as phase fluctuations. There is a qualitative agreement between experiments and theory in predicting the LEI signal lineshapes since all the main features of the experiments, such as two-photon peaks, power- and saturation-broadening including splitting of peaks, are present in the theoretical simulations. The quantitative agreement is, however, unsatisfactory, and a refinement of the theory, especially in the description of the properties of the laser light and the collisional broadening and ionization mechanisms, is required for a full agreement between theory and experiments.