Stark Spectroscopy, Lifetimes and Coherence Effects in Diatomic Molecular Systems

In this dissertation is exemplified how different laser based methods are applied in high-resolution spectroscopic studies of internal properties of diatomic molecules. A molecular beam apparatus assembly is described, where a laser ablation source is combined with a time-of-flight mass spectrometer. Compounds investigated with this equipment are hafnium sulfide and hafnium oxide. The molecules are excited and ionized applying the resonant two-photon ionization (R2PI) scheme, which is a sensitive absorption and detection technique for probing the population of an excited state. By means of the DC Stark effect, permanent electric dipole moments of HfS in the D 1Π state and HfO in the b 3Π1 state are determined while the molecules are exposed to a static electric field. Under field-free conditions low temperature rotationally resolved spectra are recorded, generating line positions from which molecular parameters are derived. The R2PI method, modified with an adjustable delay time, is also used in lifetime measurements of individual rotational levels of the HfS D 1Π and HfO b 3Π1 states. Oscillator strengths for transitions from the ground state are calculated, and in this connection basic concepts like Einstein coefficients, line strengths and Hönl-London factors, are surveyed. Theoretical calculation of lifetimes is discussed in view of the fact that a commonly available computer program (LEVEL 7.5 by Le Roy) gives erroneous output. Some coherence and quantum interference related phenomena, such as electromagnetically induced transparency (EIT) and Autler-Townes (AT) splitting, are presented in the latter part of this thesis. Fundamental concepts and relations are introduced and explained. The driven three-level cascade system is elucidated, including some of its experimental applications to alkali metal dimers, Na2 and Li2. A triple resonance spectroscopy experiment is described in terms of a three-laser, four-level inverted-Y excitation scheme, implemented in Na2. The accompanying density matrix formalism, providing the basis for theoretical simulations, is accounted for. From analysis of the results an absolute value of the electric dipole moment matrix element (transition moment) is extracted, using the AC Stark effect. Some recently reported unexpected experimental results and unforeseen features, occurring in Doppler broadened samples and related to the open character of molecular systems, are briefly commented