Ballistic or statistic - The few-femtosecond dissociative dynamics of a bifurcating Jahn-Teller system

Jahn-Teller (JT) systems are particularly attractive for ultrafast spectroscopy, as the JT-induced symmetry breaking leads to significant changes in the observables of the system, allowing for the results of the relaxation to be easily measured and interpreted. Highly symmetric systems can have multiple JT-active modes, which can result in bifurcating relaxation pathways. Few JT systems, however, have been measured on the few-femtosecond timescale that their relaxation occur on.

Photoionised tetrahedral silane (SiH₄) is an example of such a highly symmetric and unstable system, known to fragment into SiH₂⁺ and SiH₃⁺, . Thanks to the unprecedented time-resolution offered by Soft X-Ray (SXR) attosecond transient absorption spectroscopy (ATAS) we are able to capture the short-lived transient species involved in the JT fragmentation of SiH₄⁺ and for the first time untangle the competing dynamics that lead to the distinct fragments.

Our ATAS results show that bifurcation in the dynamics occurs immediately after ionisation. The SiH₃⁺ fragment is created as a result of the T₂×t₂ JT effect within 30 fs of ionisation through a strong activation of the ν₄ asymmetric bending mode. This dissociation is found to be ballistic; it proceeds without a barrier and creates vibrational coherence in the fragment which we experimentally observe. This wavepacket in the umbrella mode is found to dephase within 100 fs due to the large amplitude of the vibration.

The dynamics of the SiH₂⁺ dissociation are found to be the strikingly different. For this pathway, the initial motion of the system is driven by the T₂×e JT effect, bringing it into a local D_2d minimum along the ν₂ scissoring mode. We are able to experimentally identify the spectroscopic signature of this transient species through its vibrational quantum beats, and find that it is only able to dissociate further into the SiH₂⁺ fragment through the anharmonic transfer of vibrational energy from the e mode into the t₂ modes. This process occurs statistically, resulting in a loss of vibrational coherence and the negative exponential rise of the SiH₂⁺ fragment’s spectral signature with a 155 fs timescale.