Threshold Photoelectron Spectrum of Cyclobutadiene: Comparison with Time-Dependent Wavepacket Simulations.

The C4H4 isomer cyclobutadiene (CBD) is the prime model for antiaromaticity and thus a molecule of considerable interest in chemistry. Because it is highly reactive, it can only be studied under isolated conditions. Its electronic structure is characterized by a pseudo-Jahn-Teller effect in the neutral and a E ⊗ ß Jahn-Teller effect in the cation. As a result, recording photoelectron spectra as well as describing them theoretically has been challenging. Here we present the photoion mass-selected threshold photoelectron spectrum of cyclobutadiene together with a simulation based on time-dependent wavepacket dynamics that includes vibronic coupling in the ion, taking into account eight vibrational modes in the cation. Excellent agreement between theory and experiment is found, and the ionization energy is revised to 8.06 ± 0.02 eV.

Collision-driven state-changing efficiency of different buffer gases in cold traps: He(1S), Ar(1S) and p-H2(1Σ) on trapped CN-(1Σ).

We employ potential energy surfaces (PES) from ab initio quantum chemistry methods to describe the interaction of the CN-(1Σ) molecule, one of the small anions often studied at low temperatures, with other possible gases which can be employed as buffer in cold ion traps: the He and Ar atoms and the p-H2 molecule. These PESs are used to calculate from quantum multichannel dynamics the corresponding state-changing rate constants between the populated rotational states of the anion, the latter being in its electronic and vibrational ground states. The different cross sections for the collision-driven quenching and excitation processes at low temperatures are compared and further used to model CN- cooling (de-excitation) efficiency under different trap conditions. The interplay of potential coupling strength and mass-scaling effects is discussed to explain the differences of behaviour among the buffer gases. The advantages of being able to perform collisional cooling at higher trap temperatures when using Ar and p-H2 as buffer gases are also discussed.

Rotational state-changing collisions of C2H- and C2N- anions with He under interstellar and cold ion trap conditions: A computational comparison.

We present an extensive range of quantum calculations for the state-changing rotational dynamics involving two simple molecular anions that are expected to play some role in the evolutionary analysis of chemical networks in the interstellar environments, C2H- (X1Σ+) and C2N- (X3Σ-), but for which inelastic rates are only known for C2H-. The same systems are also of direct interest in modeling selective photo-detachment experiments in cold ion traps where the He atoms function as the chief buffer gas at the low trap temperatures. This study employs accurate, ab initio calculations of the interaction potential energy surfaces for these anions, treated as rigid rotors, and the He atom to obtain a wide range of state-changing quantum cross sections and rates at temperatures up to about 100 K. The results are analyzed and compared for the two systems to show differences and similarities between their rates of state-changing dynamics.