Collision-induced Spectroscopy Research Articles

Collision-induced spectroscopy with long-range intermolecular interactions: A diagrammatic representation and the invariant form of the induced properties

Collision-induced properties of two interacting molecules $a$ and $b$ are derived by means of a general diagrammatic method involving $M$ molecule-molecule and $N$ photon-molecule couplings. The method is an extension of previous graphical treatments of nonlinear optics because it exhaustively determines interaction-induced polarization mechanisms in a trustworthy and handy fashion. Here we focus on long-range intermolecular interactions. Retardation effects are neglected. A fully quantum-mechanical treatment of the molecules is made whereas second quantization for the electromagnetic field, in the nonrelativistic approximation, is implicitly applied. The collision-induced absorption, Raman, and hyper-Raman processes are viewed and studied, through guiding examples, as specific cases $N=1$, 2, and 3, respectively. In Raman $(N=2)$, the standard first-order $(M=1)$ dipole-induced dipole term of the incremental polarizability, $\mathrm{\ensuremath{\Delta}}\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\ensuremath{\alpha}}$, is the result of a coupling of the two photons with distinct molecules, $a$ and $b$, which perturb each other via a dipole-dipole mechanism. Rather, when the two photons interact with the same molecule, $a$ or $b$, the ($N=2$, $M=1$) graphs predict the occurrence of a nonlinear polarization mechanism. The latter is expected to contribute substantially to the collision-induced Raman bands by certain molecular gases.

Polarizabilities of atoms and molecules, new insights into an old subject

The importance of electric polarizabilities is discussed in detail. Emphasis is given not only to the dipole–dipole polarizability α ̂ , but also to the dipole–quadrupole, dipole–octopole and quadrupole–quadrupole polarizabilities Â, Ê, and Ĉ, respectively, as examples for the higher-order polarizabilities. In the case of the nonlinear polarizabilities we concentrate on the second hyperpolarizability γ ̂ and the quadrupole–dipole–dipole polarizability B̂. New experimental techniques such as the asymmetric white-light interferometry, atomic–beam interferometry, electric field deflection techniques, and collision-induced spectroscopy are discussed. Recent experimental and theoretical results are presented for the frequency, density, and temperature dependence of the polarizabilities. Special emphasis is given to new results concerning the frequency dependences of the mean dipole-polarizability of molecular iodine and of the polarizability anisotropy of carbon dioxide in the gas phase. An outlook is given for future work devoted to polarizabilities.