Knowledge Bank

We have studied the mechanism of the molecular Stark shift for spectra in regions of high vibrational state density. In this high density regime (state densities $>10states/cm-1$) it is usually found that the vibrational spectra are highly perturbed by near-resonant states indicating the occurence of extensive intramolecular vibrational energy redistribution (IVR). We have measured the fully resolved (single eigenstate) spectrum of the acetylenic C-H stretch of propargyl alcohol in the presence of a tunable, homogenous, static electric field. These Stark effect measurements show that the Stark shift occurs by an interesting mechanism. The positions of the spectral transitions (the eigenvalues) are essentially independent of the electric field strength, however, the intensities (the eigenvector component of C-H stretch) are found to move across the individual eigenvalue positions. In this way the center-of-gravity of the transition moves with a Stark shift expected for a single, unperturbed rovibrational transition. The different behavior of the eigenvalues and eigenvectors as a function of the electric field strength explains why these vibrationally excited molecules are not deflected by inhomogeneous electric fields even though they are polar.