Tracking Ultrafast Bond Dissociation Dynamics at 0.1 Å Resolution by Femtosecond Extreme Ultraviolet Absorption Spectroscopy

Visualizing the real-time dissociation of chemical bonds represents a challenge in the study of ultrafast molecular dynamics due to the simultaneous need for sub-angstrom spatial and femtosecond temporal resolution. Here, we follow the C-I dissociation dynamics of strong-field-ionized 2-iodopropane (2-C₃H₇I) with femtosecond extreme ultraviolet (XUV) absorption spectroscopy. By probing the iodine 4d core-level absorption, we resolve a continuous XUV spectral shift on the sub-100 fs time scale that accompanies the dissociation of the 2-C₃H₇I⁺ spin-orbit-excited ²E_1/2 state to yield atomic I in the ²P_3/2 state. In combination with ab initio calculations of the C-I distance-dependent XUV transition energy, we reconstruct the temporal evolution of the C-I distance from the Franck-Condon region to the asymptotic region with 10 fs and 0.1 Å resolution. The C-I bond elongation appears to couple to coherent vibrational motion along the HC(CH₃)₂ umbrella mode of the 2-C₃H₇ ⁺ fragment, whose effect on the I 4d XUV transition even at C-I distances of 3.5 Å points to the long-range nature of XUV absorption probing. Our results suggest that femtosecond XUV absorption spectroscopy, in combination with ab initio simulations of XUV transition energies, can be used to resolve the ultrafast structural dynamics of large polyatomic molecules.