Transient Vibrational Absorption Spectroscopy is the time resolved study of the absorption spectrum of a sample as a function of time, following excitation from a femtosecond laser pulse. The excitation pulse can be tuned to any wavelength in the range ~235-1600 nm using an optical parametric amplifier (TOPAS), seeded with a 1 kHz, 40 fs, 800 nm pulse (Ti:Sapphire Spitfire ACE) to create pulse powers of around 10 uJ / pulse. The probe pulse is a tunable infra-red pulse also generated with a TOPAS to give wavelengths in the range of 1-15 microns with a bandwidth of hundreds of nm. The excitation pulse pases through the sample, refreshed for each pulse pair by use of either a flow cell or gravity jet apparatus, to prepare the sample in some electronically excited state. The IR probe then passes through the excited sample after some delay time, which can be varied by way of a gold retroreflector mounted on a delay stage which increases the path length of the probe pulse. The IR pulse is monitored with Mecury Cadmium Telluride detector, which also takes dark backgrounds and unpumped spectra to find the difference in absorption following excitation. By taking a number of these difference absorption spectra over time, a picture of the dynamics of the excited state can be revealed.
The stage that generates the delay between pump and probe pulses has a travel range of 500 mm with a resolution of 5 microns, reproducible to within 0.5 microns. This corresponds to a maximum temporal window of 3.3 nanoseconds at a resolution of 17 femtoseconds. The IR spectrometer will run for 8-12 hours for a single fill of liquid nitrogen. It consists of two 64 pixel linear arrays, one of which measures a reference beam, for shot-to-shot comparison with the probe beam measured on the other. It can sample the spectra at a rate of 1.9 kHz, thus fully sampling the 1 kHz laser. It possesses three separate gratings only a fully automated turret: 150 lines/mm blazed at 5 microns (6.2 nm/pixel resolution), 75 l/mm at 5 microns (12.4 nm/pixel resolution) and 50 l/mm at 6 microns (18.6 nm/pixel resolution).