Ultrafast absorption spectroscopy of photodissociated CF2Br2: Details of the reaction mechanism and evidence for anomalously slow intramolecular vibrational redistribution within the CF2Br intermediate

Ultrasensitive ultraviolet-visible 20 fs absorption spectroscopy of low vapor pressure molecules in the gas phase

We describe an ultrasensitive pump-probe spectrometer for transient absorption measurements in the gas phase and in solution. The tunable UV pump and the visible (450-740 nm) probe pulses are generated by two independently tunable noncollinear optical parametric amplifiers, providing a temporal resolution of 20 fs. A homebuilt low gain photodetector is used to accommodate strong probe pulses with a shot noise significantly lower than the overall measurement noise. A matched digitizing scheme for single shot analysis of the light pulses at kilohertz repetition rates that minimizes the electronic noise contributions to the transient absorption signal is developed. The data processing scheme is optimized to yield best suppression of the laser excess noise and thereby transient absorbance changes down to 1.1 x 10(-6) can be resolved. A collinear focusing geometry optimized for a 50 mm interaction length combined with a heatable gas cell allows us to perform measurements on substances with low vapor pressures, e.g., on medium sized molecules which are crystalline at room temperature. As an application example highlighting the capability of this instrument, we present the direct time-domain observation of the ultrafast excited state intramolecular proton transfer of 2-(2(')-hydroxyphenyl)benzothiazole in the gas phase. We are able to compare the resulting dynamics in the gas phase and in solution with a temporal precision of better than 5 fs.

Ultraviolet ultrafast pump-probe laser based on a Ti:sapphire laser system

An ultrafast spectroscopic pump-probe apparatus for detection of ultraviolet time-resolved spectra is described. Frequency doubling of an 810-nm 0.3-TW Ti:sapphire laser system yields 15-mJ 165-fs pulses at 405 nm. We obtain pulse energies of 0.8 mJ by frequency tripling to 270 nm. An ultraviolet supercontinuum extending from 200 to 600 nm is also generated in water by use of the frequency-doubled 405-nm light. The system operates at a 5-Hz repetition rate and is useful for applications in deep-ultraviolet pump-probe subpicosecond spectroscopy.

Spectrally compensated sum-frequency mixing scheme for generation of broadband radiation at 193 nm

A dispersively compensated scheme for sum-frequency mixing of broadband ultrashort laser pulses is reported. An increase of the bandwidth of the sum-frequency mixing process by 12 times compared with the noncompensated bandwidth of the given crystal has been demonstrated. Mixing radiation at 266 and 707 nm in a 1-mm-thick beta-barium metaborate crystal by using the compensated scheme results in an output bandwidth of 0.6 nm at 193 nm, which corresponds to a minimum output pulse duration of 90 fs.