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We describe a nonlinear Er:doped fiber amplification scheme for producing fiber-delivered, 44 fs pulses with 100 kW peak power at arbitrary (0-60 MHz) repetition rate. Subsequent spectral broadening produces 80 THz of bandwidth. 

Salicylideneaniline (SA) is an archetypal system for excited-state intramolecular proton transfer (ESIPT) in non-planar systems. Multiple channels for relaxation involving both the keto and enol forms have been proposed after excitation to S1 with near-UV light. Here, we present transient absorption measurements of hot gas-phase SA, jet-cooled SA, and SA in Ar clusters using cavity-enhanced transient absorption spectroscopy (CE-TAS). Assignment of the spectra is aided by simulated TAS spectra, computed by applying time-dependent complete active space configuration interaction (TD-CASCI) to structures drawn from nonadiabatic molecular dynamics simulations. We find prompt ESIPT in all conditions followed by the rapid generation of the trans keto metastable photochrome state and fluorescent keto state in parallel. Increasing the internal energy increases the photochrome yield and decreases the fluorescent yield and fluorescent state lifetime observed in TAS. In Ar clusters, internal conversion of SA is severely hindered, but the photochrome yield is unchanged. Taken together, these results are consistent with the photochrome being produced via the vibrationally excited keto population after ESIPT. 

Broadband ultrafast optical spectroscopy methods, such as transient absorption spectroscopy and 2D spectroscopy, are widely used to study molecular dynamics. However, these techniques are typically restricted to optically thick samples, such as solids and liquid solutions. In this article we discuss a cavity-enhanced ultrafast transient absorption spectrometer covering almost the entire visible range with a detection limit of ΔOD < 1 × 10 −9 , extending broadband all-optical ultrafast spectroscopy techniques to dilute beams of gas-phase molecules and clusters. We describe the technical innovations behind the spectrometer and present transient absorption data on two archetypical molecular systems for excited-state intramolecular proton transfer, 1′-hydroxy-2′-acetonapthone and salicylideneaniline, under jet-cooled and Ar cluster conditions.