Ruth Signorell

General symmetry selection rules for the photoionization of polyatomic molecules

General rovibronic symmetry selection rules, which are applicable to any molecular symmetry, have been obtained for the photoionization of polyatomic molecules. The use of the molecular symmetry groups leads to a particularly transparent derivation. The photoelectron is characterized by a partial wave expansion in the orbital angular momentum quantum number l. For a given value of l, one-photon electric dipole transitions can only occur between neutral and ionic states that obey the rovibronic symmetry conditions Γrve (neutral) ⊗ Γrve (ion) ⊃ Γ* for l even and Γrve(neutral) ⊗ Γrve(ion) ⊃ Γ(s) for l odd, where Γ(s) and Γ* represent the totally symmetric and the antisymmetric representations, respectively. Combined with the wellknown angular momentum conservation selection rule Δ J = J + - J = l + , l + ½,…, l - ½, l - [EQUATION](where J + and J represent the total angular momentum quantum number of the ionic and the neutral state between which the photoelectronic transition occurs), these symmetry selectio...

HIGH RYDBERG STATES OF ARGON : STARK EFFECT AND FIELD-IONIZATION PROPERTIES

The Rydberg states with principal quantum number located below the ground state of the ion have been studied by pulsed field ionization following single-photon excitation out of the ground state of Ar. The linewidth of the tunable extreme ultraviolet (XUV) laser source used enabled high-resolution measurement of the Stark effect over a wide range of principal quantum numbers and electric field strengths. Particular attention was given to the ionization of high Rydberg states induced by DC and pulsed electric fields. The lowering (expressed in ) of the ionization threshold by DC electric fields is accurately described by when the electric field strength F is expressed in , a result that is in good agreement with predictions of the classical saddle-point model for field ionization. The field-ionization threshold is very sharp: its width decreases from 0.7 to when the DC field strength is reduced from 580 to . Apart from the Stark states located in a very narrow energy range around the saddle-point energy in the potential which are found to ionize very slowly, all Stark states located below the saddle-point energy have lifetimes exceeding several microseconds, whereas those located beyond the saddle-point energy ionize within less than 20 ns. The very slow field ionization that is observed in a narrow range of energies around the classical saddle point can be used to obtain high state selectivity in the pulsed field ionization. The pulsed field-ionization behaviour observed in argon suggests that the rule that is now commonly assumed in the analysis of pulsed-field-ionization (PFI) zero-kinetic-energy (ZEKE) spectra to describe the low-wavenumber onset of a line relative to the position of the corresponding field-free ionization threshold must be used with caution.

Structure of the ammonium radical from a rotationally resolved photoelectron spectrum

High-resolution zero-kinetic-energy photoelectron spectroscopy has been used to record the transition between the lowest bound state (3s 2A1) of the perdeuterated ammonium radical (ND4) and the X 1A1 ground vibronic state of the perdeuterated ammonium ion (ND4+). The spectra obtained are the first rotationally resolved photoelectron spectra ever measured for a tetrahedral molecule. The analysis of the rotational structure is accompanied by a description of the observed symmetry selection rules and propensity rules for core rotational angular momentum changes that characterize the photoionization process. Rotational constants (B0=2.8560±0.0037 cm−1 and B0+=2.9855±0.0037 cm−1) and centrifugal distortion constants (D0=(4.78±1.4)×10−5 cm−1 and D0+=(4.77±1.5)×10−5 cm−1) have been determined for the 3s 2A1 state of ND4 and the X 1A1 state of ND4+, respectively. The ionic rotational constant is in good agreement with the value B0+=2.9787±0.0029 cm−1 determined indirectly by Crofton and Oka (J. Chem. Phys. 86, ...

The first rotationally resolved spectrum of CH4

The pulsed-field-ionization (PFI) zero-kinetic-energy (ZEKE) photoelectron spectra of CH4 and CD4 have been recorded in the region 100880–104100 cm−1. From the analysis of the photoelectron spectra the first adiabatic ionization potential of CH4 and CD4 has been determined to be (101773±35) cm−1 and (102210±25) cm−1, respectively. A one-dimensional model for the pseudorotation between three equivalent C2v equilibrium structures shows evidence for a fluxional behavior of CH4+.

Verification of the vibrational exciton approach for CO2 and N2O nanoparticles

It is investigated how far the vibrational exciton approximation is suitable to describe the characteristic band shapes in the infrared spectra of CO2 and N2O nanoparticles. The particles typically contain between 50 and 104 molecules and have spatial dimensions between 1 and 10 nm. The accuracy of the exciton approach is estimated by comparison with experimental data and quantum chemical calculations for small clusters. The spectral changes due to different particle shapes and particle sizes are investigated with respect to the estimated accuracy. This includes the determination of a typical effective range for the dipole–dipole coupling.

Vibrational delocalization in ammonia aerosol particles

The present contribution investigates shape effects and surface effects in the infrared spectra of pure (NH(3)) and mixed (NH(3)-CO(2) and NH(3)-NH(2)D-NHD(2)-ND(3)) ammonia particles with sizes between about 1 and 50 nm. The particles investigated have been generated in a collisional cooling cell as aerosols at temperatures between 20 and 80 K. The contribution reveals that only the combination with a microscopic model leads to a comprehensive understanding of the various features observed in the experimental infrared spectra. As one of the major results, the corresponding exciton model explains why pronounced shape effects observed for pure particles only play a minor role in the case of mixed particles.

The permanentectric dipole moment of CH2D2: FIR el spectroscopy

The pure rotational spectrum in the far-infrared between 30 and 170 cm-1 and its absolute intensity has been measured for CH2D2 in the vibrational ground state by high-resolution interferometric Fourier transform techniques. The analysis of the integrated cross-sections in the essentially water-free spectrum results in an accurate value for the permanent, vibrationally induced ground state electric dipole moment of CH2D2|μ0| = (6·40±0·33) x 10-3D.The influence of centrifugal effects on intensities and on the determination of the permanent dipole moment was investigated. Although centrifugal effects are important for the explanation of single band profiles, they appear to be of little relevance for the resulting permanent dipole moment. A new, more general 9- dimensional dipole moment function for methane is derived from ab initio calculations and experimental band strength information of CHD3. Quantum Monte Carlo calculations using this function and a new, more general 9- dimensional analytical, anharmoni...

Isotope effects on vibrational excitons in carbon dioxide particles

Abstract The vibrational exciton approach is used to answer a long-standing question concerning the infrared spectra of CO2 particles: What causes the characteristic structure of the infrared absorption bands? The CO2 particles which are generated in a collisional cooling cell at 78 K consist of many thousands of molecules. Isotopically pure ( 12 CO 2 or 13 CO 2 ) particles as well as isotopically mixed ( 12 CO 2 / 13 CO 2 ) particles are investigated. Combining the experimental observations with exciton calculations, the band structure in the infrared spectra can be traced back to the shape of the particles.

The first adiabatic ionization potential of Ar2

The high resolution zero-kinetic-energy (ZEKE) photoelectron spectrum of Ar2 has been recorded between 116500 and 128500 cm−1. The spectrum consists of a progression of 52 vibrational bands in the A 2Σ1/2u+←X 1Σg+ (X 10g+ in Hund’s case (c) notation) photoelectronic transition. The absolute numbering of the vibrational progression in the A←X transition is achieved by measuring the isotope shifts of two vibrational bands of the 36Ar2 molecule. From the analysis of the vibrational progression the first adiabatic ionization potential of Ar2 has been determined to be 116593.5±6.0 cm−1 (14.4558±0.0007 eV) from which a dissociation energy D0 of 10601.2±6.0 cm−1 (1.3144±0.0007 eV) results for the A 2Σ1/2u+ ground state of Ar2+. The potential curve of the ground ionic state in the vicinity of the potential minimum is adequately represented by a Morse potential with ωe+=307.0±0.4 cm−1 and ωexe+=2.05±0.05 cm−1. The position of higher members of the vibrational progression with v+>25 cannot be fitted accurately with...

Large molecular aggregates: from atmospheric aerosols to drug nanoparticles

Large molecular aggregates with sizes ranging from subnanometers to microns are ubiquitous. As atmospheric aerosols they influence our climate, in interstellar space they are discussed as reactive sites, and in medicine small particles are considered as promising candidates to achieve a targeted drug delivery. The present contribution is focused on the characterization of the physical-chemical properties of these particles and on their targeted generation. One of the greatest challenges is to understand the properties of these aggregates on a molecular level. The latter point is discussed in detail focussing on the vibrational dynamics of these particles.