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

Area of Science:

• Physical Chemistry
• Molecular Dynamics
• Photochemistry

Background:

• Understanding molecular behavior under UV excitation is crucial for photochemistry.
• Alkyl halides exhibit complex photodynamics upon UV absorption.
• Femtosecond spectroscopy provides insights into ultrafast molecular processes.

Purpose of the Study:

• To investigate the ultrafast photodynamics of n-butyl bromide.
• To elucidate the dissociation and relaxation pathways following UV excitation.
• To determine the lifetimes of excited states and ion-pair intermediates.

Main Methods:

• Femtosecond time-resolved mass spectrometry.
• UV pump-probe spectroscopy at 400 nm.
• Analysis of fragment ion transients (C4H9+ and C4H8+).

Main Results:

• Direct C-Br bond dissociation from the A state occurs within 160 fs.
• Excitation to the 5p Rydberg state leads to multiple relaxation pathways.
• Ion-pair state lifetime determined as 10.8 ± 0.5 ps.
• Predissociation yielding H-Br (beta-elimination) and C4H8+ occurs within 3.0 ± 0.25 ps.
• Relaxation to the D state observed on a timescale of 30.2 ± 0.21 ps.

Conclusions:

• The study reveals distinct ultrafast dissociation and relaxation channels for n-butyl bromide.
• Femtosecond spectroscopy successfully tracks transient intermediates and provides state lifetimes.
• Results offer insights into the fundamental photophysical processes of alkyl halides.