Ion-Pair Formation in n‑Butyl Bromide through 5p Ryberg State Predissociation

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2022-12, Vol.126 (51), p.9651-9657
• Main Authors: Heald, Lauren F., Loftus, Colleen L., Gosman, Robert S., Sayres, Scott G.
• Format: Article
• Language: English
• Subjects: A: Structure, Spectroscopy, and Reactivity of Molecules and Clusters
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/acs.jpca.2c06777

The ultrafast photodynamics of n-butyl bromide are explored with femtosecond time-resolved mass spectrometry. Absorption of two UV (400 nm) pump photons induces the direct dissociation of the C–Br bond from the A state within 160 fs. Absorption of three UV pump photons excites the molecule into the 5p Rydberg state which undergoes several relaxation pathways including to the ion-pair state. Relaxation to the ion-pair state is tracked through the transient of the C4H9 + fragment and suggests an E state lifetime of 10.8 ± 0.5 ps, in close agreement with the tunneling time of smaller molecules. Predissociation from the 5p Rydberg states leads to the β-elimination of H–Br and formation of C4H8 + within 3.0 ± 0.25 ps. A portion of the excited parent molecule avoids the ion-pair formation and instead relaxes through the Rydberg excited state manifold into the D state within 30.2 ± 0.21 ps.