Ti:Sapphire Femtosecond Laser System
A Ti:Sapphire regenerative amplifier (Coherent Libra) operating at 1 kHz provides 800 nm fundamental pulses of 90 fs duration with 3.5 mJ of energy. Three optical parametric amplifiers (two OPerA Solos and a TOPAS + NDFG) are used to generate femtosecond pulses from UV to mid-IR wavelengths.
Time-Resolved IR Spectroscopy (TRIR)
TRIR spectroscopy is a powerful technique for studying excitations in a complex, multichromophoric systems. A vibrational spectrum with its comparatively narrow resonances is frequently easier to assign than overlapping electronic absorption spectra. Additionally, because vibrations are typically highly localized in space, TRIR can potentially reveal where an electronic excitation is localized in a larger molecule or macromolecule. In our laboratory, an intense UV or visible pump pulse is used for electronic excitation. The ensuing dynamics are probed with a weak mid-IR pulse in the double-bond stretching region (λ~6 μm). The extremely weak changes in mid-IR absorption induced by the pump pulse are measured with a liquid nitrogen-cooled, dual-row, 64-element HgCdTe array detector.
Femtosecond Transient Absorption (fs UV-Vis TA)
Similar to TRIR, the sample of interest is excited with an intense UV or visible pump pulse. The ensuing dynamics are probed with a second, much weaker UV or visible probe pulse. Signals are typically due to ground state bleaching, excited state absorption, or stimulated emission. A sensitive photomultiplier tube (PMT) coupled with a lock-in amplifier allows absorbance differences as small as 1×10-5 (10 μOD) to be measured.
Time-Correlated Single-Photon Counting (TCSPC) Emission Spectrometer
The TCSPC emission lifetime spectrometer with a microchannel plate PMT has the ability to record fluorescence lifetimes and spectra on time scales ranging from tens of picosecond to microsecond, depending on whether a Ti:Sapphire oscillator (Coherent Chameleon) or flashlamp is used for sample excitation. The fundamental of the Ti:Sapphire oscillator is widely tunable from 680 nm to 1080 nm, and is augmented by harmonics (SHG and THG) generated with an autotracking wavelength converter. The minimum time resolution of the emission spectrometer is on the order of 40 ps.
Multiple benchtop spectrometers are available for recording Circular Dichromism (CD), Fourier Transform Infrared (FTIR), and Ultraviolet-Visible (UV-vis) absorption spectra. A research-grade fluorometer is used to measure steady-state emission and excitation spectra and fluorescence quantum yields. Peltier-effect temperature cell holders provide programmable temperature control with many of these instruments, allowing for nucleic acid structure quantitation and melting curve (hypochromism) measurements.