Interaction of light beams propagating in 2S, 2P atomic hydrogen ; I spectroscopy

Summary form only given. The light-matter interactions are classified in two sets: the quantified interactions in which matter performs transitions between stationary states, and the parametric interactions for which matter excited by the light returns to its initial stationary state after the interaction. Strong parametric interactions require a space-coherence of the light beams, that is the same phase between each wave and molecular dipole of same frequency at each implied molecule. With different frequencies producing generally different wavelengths, it is difficult to get the space-coherence. Therefore, except for refraction, the observation of parametric interactions requires particular conditions. Parametric interactions allowing frequency multiplications, combinations, shifts, etc often use two indices of refraction of crystals. G. L. Lamb Jr. (Rev. Mod. Phys., 43 , 99 (1971)) describes another general trick allowing parametric interactions: the use of "ultrashort light pulses" defined as "shorter than all relevant time constants". Thus, the qualifying "ultrashort", generally bound to femtosecond light pulses, applies NOT to the light only but to the set of the light and the refracting medium. Therefore, ordinary light whose coherence time is some nanoseconds is made of ultrashort pulses when it propagates in low pressure gases having a Raman resonance of the order of 100 MHz. Unhappily it is difficult to find a gas having so low a Raman frequency in a well populated state. Among common gases, only neutral atomic hydrogen in states 2S and (or) 2P (say H*) works well. Refraction and most parametric effects work without threshold of energy: the experiments show that the weakest light beam excites transitorily all molecules of a big prism, making a global non-stationary state, enough for a regular refraction. A frequency shifting interaction may be considered as produced by an interaction between several non-stationary states produced by the refraction of the corresponding electromagnetic beams; as these non-stationary states have the same symmetry, they interact through Raman type resonances. Thus, the effect is named "space-coherent Raman effect on time-incoherent light" (CREIL). An efficient interaction requires an increase of the entropy of the system, that is a flood of energy from modes having a high temperature (deduced from Planck's law) to colder modes. Consequently, the light is generally redshifted while the radio, thermal radiations are blueshifted. The frequency shifts, produced by thermodynamically allowed transfers of energy "catalyzed" by a refraction in H*, may be confused with Doppler shifts, the relative frequency shifts being constants if the dispersion of the polarisability of the gas is neglected. Usually, the blueshift of the radio frequencies is not detected, so that only the redshift of light is considered