Electronic spectroscopy of transient molecules by mass-selected resonance enhanced multi-photon ionization

This work describes the experimental measurements and spectroscopic analyses of several transient molecules. These species are produced using pulsed laser ablation or electric discharge coupled with supersonic expansion and their electronic spectra are recorded using time-of-flight (TOF) based resonance enhanced multi-photon ionization (REMPI) spectroscopy. A combination of REMPI and TOF provides a powerful tool for unambiguous identification of the carrier of any spectral feature with a particular mass signature. Silver monosulfide (AgS) has been spectroscopically characterized in the gas phase for the first time and electronic transitions in the near-infrared region are assigned to the A2Σ+ − X2Πi electronic transition, based on a rotational analysis. The small difference in the rotational constant between the X2Π ground and A2Σ+ excited states indicates only a small change in Ag−S bond length on electron promotion. This is consistent with the dominant Frank-Condon factor for the 0−0 transition in the vibronic spectra. The electronic transition of titanium dioxide (TiO2) and zirconium dioxide (ZrO2) were observed in the visible and assigned to A1B2 − X1A1 system. There are many similarities in their spectra, such as a significant decrease in ω2 and ω3 and a slight decrease in ω1 upon excitation from the X 1A1 to A1B2 state. Also, long progressions are observed for both, suggesting a significant geometry change between the states. There is one striking difference; the observation of odd-ν3 quanta in the case of zirconium dioxide, which provides an evidence for vibronic coupling between A1B2 state and the next higher state, due to a breakdown of Born-Oppenheimer approximation. Silicon trimer (Si3) provides another interesting case for a breakdown of the Born-Oppenheimer approximation. The ground state electronic configuration of (Si3) in the high symmetry D3h geometry gives rise to 3A2´, 1A1´ and 1E´ lower states, amongst which 3A2´ is the least energetic state. However, due to Jahn-Teller and pseudo Jahn-Teller effects, the 1A1 component of the 1E´ term is stabilized and crosses the undistorted 3A2´ ground state to produce a global minimum of C2v geometry. The 3A2´ and 1A1 are nearly iso-energetic and both get populated, even under supersonic expansion conditions, giving rise to a complicated electronic spectrum in the visible region. Broad contours originating from the (2)3Σu− − X3Σg− transition of C6 and C8, and the 1Σu+ − X1Σg+ one for C7 and C9 have been identified in the ultraviolet using a 30 ps laser, indicating an excited state lifetime less than 30 ps (temporal width of the laser pulse). Spectroscopic measurements, other than providing information about the fundamental properties of molecules, can enrich our understanding of the universe. The gas phase spectra presented in this work provide a guide for detection of these molecules in stars, comets, dust clouds, etc. and to understand the molecular chemistry taking place in these environments.