Breakdown of the Born-Oppenheimer approximation in ZEKE states of Ag2

Molecular Theory of ZEKE Spectroscopy

In this paper, we derive the double-resonance and single-resonance two photon absorption cross sections for zero electron kinetic energy (ZEKE) spectroscopy using the inverse Born–Oppenheimer approximation (IBOA) as a basis set. Quantum chemistry calculations can then be used to evaluate the ZEKE spectra. To show how to analyze the vibronic spectra of ZEKE spectroscopy, a couple of examples have been analyzed. Quantum chemistry calculations can be employed to obtain the potential surfaces of molecules and their ions, which can be used to calculate the Franck–Condon factors involved in the vibronic transitions. We also show that using the IBOA as a basis set, the autoionization of the ZEKE states can be calculated.

Mass-Analyzed Threshold Ionization Study of Vinyl Chloride Cation in the First Excited Electronic State Using Vacuum Ultraviolet Radiation in the 107−102.8 nm Range

Vibrational spectrum of vinyl chloride cation in the first excited electronic state, A2 A', was obtained by one-photon mass-analyzed threshold ionization (MATI) spectroscopy. Use of an improved vacuum ultraviolet radiation source based on four-wave sum frequency mixing in Hg resulted in excellent sensitivity for the MATI signals. From the MATI spectrum, the ionization energy to the A2 A' state of the cation was determined to be 11.6667 +/- 0.0006 eV. Nearly complete vibrational assignment for the MATI peaks was possible by utilizing the vibrational frequencies and Franck-Condon factors calculated at the DFT and TDDFT/B3LYP levels with the 6-311++G(3df,3pd) basis set. Geometry of the cation in the A2 A' state was determined by Franck-Condon fitting of the MATI spectrum.

One-Photon Mass-Analyzed Threshold Ionization Spectroscopy (MATI) of trans-Dichloroethylene (trans-C2H2Cl2): Cation Structure Determination via Franck−Condon Fit

One-photon mass-analyzed threshold ionization (MATI) spectrum of trans-C(2)H(2)Cl(2) was obtained by using vacuum ultraviolet radiation generated by four-wave mixing in Kr. The ionization energy determined from the position of the 0-0 band in the spectrum was 9.6306 +/- 0.0006 eV. Ten vibrational fundamentals for the cation were identified. The spectrum also displayed abundant overtones and combinations, most of which could be assigned adequately by comparing with the quantum chemical results. It was found that channel interaction was not important for this system. The equilibrium geometry of the cation was estimated through the Franck-Condon fit.

Vibrational assignment and Franck-Condon analysis of the mass-analyzed threshold ionization (MATI) spectrum of CH2ClI: the effect of strong spin-orbit interaction

Detailed analysis of the one-photon mass-analyzed threshold ionization (MATI) spectrum of CH(2)ClI is presented. This includes the determination of the ionization energy of CH(2)ClI, complete vibrational assignments, and quantum-chemical calculations at the spin-orbit density-functional-theory (SODFT) level with various basis sets. Relativistic effective core potentials with effective spin-orbit operators can be used in SODFT calculations to treat the spin-orbit term on an equal footing with other relativistic effects and electron correlations. The comparison of calculated and experimental vibrational frequencies indicate that the spin-orbit effects are essential for the reasonable description of the CH(2)ClI(+) cation. Geometrical parameters and thus the molecular shape of the cation are greatly influenced by the spin-orbit effects even for the ground state. Calculated geometrical parameters deviate substantially for different basis sets or effective core potentials. In an effort to derive the exact geometrical parameters for this cation, SODFT geometries were further improved utilizing Franck-Condon fit of the MATI spectral pattern. This empirical fitting produced the well-converged set of geometrical parameters that are quite insensitive to the choice of SODFT calculations. The C-I bond length and the Cl-C-I bond angle show large deviations among different SODFT calculations, but the empirical spectral fitting yields 2.191 +/- 0.003 Angstroms for the C-I bond length and 107.09 +/- 0.09 degrees for the Cl-C-I angle. Those fitted geometrical parameters along with the experimental vibrational frequencies could serve as a useful reference in calibrating relativistic quantum-chemical methods for radicals.

Zero kinetic energy photoelectron study of SO2+(X2A1) using coherent extreme ultraviolet radiation

Using our newly built extreme ultraviolet (XUV) photoelectron and photoion spectrometer, we have obtained the pulsed field ionization zero kinetic energy (ZEKE) photoelectron spectra of SO2+(X2A1)<--SO2(X1A1) by coherent XUV radiation in the energy range of 12.29-12.82 eV. The adiabatic ionization potential (IP) of SO2 is 12.3458+/-0.0002 (eV), which was determined by comparing the partially resolved rotational branch contour with the simulated one. Besides the bending vibrational mode (upsilon2) which was found to be exclusive in the photoelectron spectra (PE) reported previously, we also observed the other two modes: the symmetric stretching (upsilon1) and the antisymmetric stretching (upsilon3) vibrations. The fundamental of the symmetric stretching (upsilon(1)) is 1057 cm(-1) and the overtone of the antisymmetric stretching (2upsilon(3)) is 2494 cm(-1). The new vibrational progressions (upsilon(1)00)+, (1upsilon(2)0)+, (2upsilon(2)0)+, and (0upsilon(2)2)+ have also been observed, and these new observations suggested that the irregular structure of (0upsilon(2)0)+ assigned to the previous PE spectra should be reconsidered. The comparison of the intensities of these vibrational bands with the calculated Franck-Condon factors with harmonic approximation was also made.

MOLECULES IN HIGH RYDBERG STATES

The unusual properties of high molecular Rydberg states with principal quantum number n>>100 have enabled the development of new research tools for the study of molecules and ions in the gas phase. These tools range from spectroscopic techniques such as zero kinetic energy (ZEKE) photoelectron spectroscopy and mass-analyzed threshold ionization (MATI) spectroscopy to techniques that are suited to the investigation of photodissociation processes, bimolecular reactions, and state-selected ion-molecule reactions. This review summarizes recent progress in the understanding of the properties of high molecular Rydberg states and gives an overview of recent chemical applications of these states.