Electrostriction and collisional thermalization of absorbed laser energy are the two dominant mechanisms leading to the formation of laser-induced gratings (LIGs) in the gas phase. In this article the results of the theoretical investigations that have been achieved in the past ten years at the Paul Scherrer Institute on this issue are summarized and yield a comprehensive understanding of the underlying physical concepts. Furthermore, a study of the influence of various parameters, such as the alignment and the spatial intensity profile of the beams on the generated electrostrictive and thermal signal is presented for the first time to the authors' knowledge. The variations of the refractive index responsible for the appearance of laser-induced gratings have been theoretically described by solving the linearized hydrodynamic equations. The contributions from electrostriction, as well as from instantaneous and slow relaxation of the absorbed radiation energy into heat is obtained. These expressions are employed for analysis of experimental data presented in the companion paper [1] which is devoted to the application of the technique for diagnostic purposes in the gas phase. Much effort has been undertaken in order to allow a straightforward physical interpretation of the experimental findings of the expressions presented here.

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Nonresonant gratings have been generated in gases using as excitation beams the second-harmonic output of a multimode Nd:YAG (yttrium aluminum garnet) laser (pulse duration v=8 ns, coherence time~, =20 ps). The grating reAectivity is measured by scattering a probe beam off the grating. It shows the damped oscillation of a standing acoustic wave (period T~}. By varying the time delay wd between the excitation beams, the inhuence of the temporal laser coherence is analyzed. The signal for incoherent excitation (r, «~r "~&&r) is nonvanishing and depends only weakly on rd It i. s reduced by about a factor~, /~compared to coherent excitation, if T~))~. The measurements are interpreted within a calculation of electrostrictively generated gratings from beams having Gaussian temporal pulse shape, Gaussian statistics for the intensity, and a Gaussian frequency spectrum. Furthermore, the measurement of acoustic quantities (sound velocity and attenuation) with the described setup is discussed.PACS number(s): 42.65. Es, 43.35.+d, 42.25.Kb

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