Experimental measurements of the optical reflectance of solid adenine films have been obtained for photon energies extending from 1.8 to 80 eV. From these data we have established the complex index of refraction, the complex dielectric function, and the energy‐loss function Im(‐l/). Structure in the dielectric functions are ascribed to * transitions at photon energies less than ~9 eV and to * transitions at higher energies. A broad peak in the energy‐loss function near 24 eV is associated with a collective resonance involving all the valence electrons. Sum rule calculations are used to demonstrate the overall consistency of the data.

We study the quantum interference in three-photon resonant nondegenerate six-wave mixing (NSWM) of a five-level system in which the middle level of six-wave mixing and other levels are coupled by a strong laser field. The coupling field-dependence of the NSWM signal intensity, and the spectrum of the NSWM with a coupling field, are discussed. We find that in the presence of a strong coupling field, the three-photon resonant NSWM spectrum exhibits Autler-Townes splitting, which reflects the levels of the dressed states. It also leads to either suppression or enhancement of the NSWM signal. Due to the enhancement of NSWM signal caused by quantum interference, the dressed state created by a coupling field can replace the atom intrinsic level and serve as the middle level of three-photon resonance. Thus the middle level of three-photon resonance can be controlled by a coupling field.

We apply two-photon resonant nondegenerate four-wave mixing with a resonant intermediate state for observing the broadening and the shifting of the barium Rydberg 6snd 1D2 series by collision with argon. The collision broadenings and the collision shifting cross sections are measured for n=1633. This technique is a purely optical means, and can achieve Doppler-free resolution of narrow spectral structures of Rydberg levels when the incident lasers have narrow bandwidths. Different from other experimental methods of studying the pressure dependence of the longitudinal relaxation rate of Rydberg states, our method is to investigate the pressure dependence of the transverse relaxation rate of the transition.

We study the satellite lines in measurement of the foreign-gas-induced broadening of the barium Rydberg levels by two-photon resonant nondegenerate four-wave mixing (NFWM). The NFWM spectra of the 6s2 1S0-6snd 1D2 (n=16, 22, 36) transitions at different argon pressures are measured. The plots of the center of satellite line principal quantum number and versus pressure are discussed. The contribution of satellite line to the NFWM spectral profile is analyzed. We find that the pressure-broadening rate coefficient can be corrected when the contribution of satellite line is considered.