ECAMP 15

Understanding internal dynamics and decay processes is crucial in gas-phase molecular anion studies. Using our lab’s unique facility[1], we investigated the long-timescale energy dynamics of Ag$_2^-$ and Cu$_2^-$ dimer anions. Internally hot anions from a Cesium sputter source are accelerated to 4.2 keV and stored in an Electrostatic Ion Beam Trap (EIBT). A laser-assisted velocity map imaging (VMI) spectrometer records ejected electrons, while a separate detector captures neutral counterparts. Coincidence data are analyzed over trapping time, and internal energy dynamics are inferred from photoelectron spectra (PES) at different storage windows. Experiments were conducted using 635 nm and 380 nm lasers.

The PES reveal intriguing results, including previously unreported peaks and differential decay rates for various transitions. We simulated spectra by computing Franck-Condon factors for all allowed transitions from the Ag$_2^-$ ground electronic state to the Ag$_2$ ground electronic state. Calculations explain the spectral shape and indicate that faster-decaying transitions originate from higher rovibrational states.

Our results are compared with recent observations by Anderson et al. [2,3], who report spontaneous unimolecular decay of hot Ag$_2^-$ and Cu$_2^-$ dimer anions in the cryogenic DESIREE setup via fragmentation and electron emission. They attribute electron emission to vertical autodetachment (VAD) and fragmentation to tunneling dissociation, with the former showing a strong deviation from the BOA. However, their work does not provide any direct information on the internal energy distribution.

Our calculations rule out VAD and tunnelling dissociation as the cause for the faster decay of high rovibrational states. The origin of differential decay rates in the time dependent PES is still unclear. The experimental results and the possible causes for differential decay rates will be discussed at the conference.

We examine the possibility of non-BO transitions or excitation dependence of residual gas detachment. We pose the question whether the non-BO transitions in the case of electron-rich symmetric dimer anions are leading to radiative decay.