Femtosecond isomerization dynamics in the ethylene cation measured in an EUV-pump NIR-probe configuration

Dynamics in the excited ethylene cation C₂H⁺ ₄ lead to isomerization to the ethylidene configuration (HC-CH ₃)⁺, which is predicted to be a transient configuration for electronic relaxation. With an intense femtosecond extreme ultraviolet pump pulse to populate the excited state, and a near infrared probe pulse to produce the fragments CH⁺ and CH⁺₃ (which provides a direct signature of ethylidene), we measure optimum fragment yields at a probe delay of 80 fs. Also, an H₂-stretch transient configuration, yielding H⁺₂ upon probing, is found to succeed the ethylidene configuration. We find that a simple single- or double-decay model does not match the data, and we present a modified model (introduction of an isomerization delay of 50 25 fs) that does provide agreement.