HIGH-POWER MID-IR COMB GENERATION FOR CAVITY-ENHANCED 2DIR SPECTROSCOPY

Using frequency combs and optical cavities, we have previously demonstrated ultrafast transient absorption measurements with a detection limit of $\Delta$OD $= 1 \times 10^{-9} /\sqrt{\textrm{Hz}}$, enabling work in dilute molecular beams.\footnote{M. A. R. Reber, Y. Chen, and T. K. Allison, Optica \textbf{3}, 311 (2016).} Similar methods can be applied to multidimensional spectroscopy as well.\footnote{T. K. Allison, J. Phys. B: At. Mol. Opt. Phys. \textbf{50}, 044004 (2017).} Since molecules undergoing ultrafast dynamics have broad spectral features, cavity-enhanced ultrafast spectroscopy then demands broadband and widely tunable frequency combs. Here we present a frequency conversion setup for the generation of high power mid infrared frequency combs in the 3-10 $\mu$m region. The initial comb is generated using an Er:fiber oscillator with 100 MHz repetition rate. After nonlinear amplification, the comb is shifted in a highly nonlinear fiber (HNLF) to 1 $\mu$m and amplified to 10 W in a home built, multi-stage Yb:fiber amplifier. We have measured the output comb tooth linewidth to be less than 10 kHz and the pulse duration is 120 fs. This laser is then used as a pump for several nonlinear difference frequency generation stages seeded by additional HNLF-shifted combs. Cavity-enhanced mid-infrared combs in the 3-5 $\mu$m region will be applied to studying ultrafast dynamics of hydrogen-bonded clusters

Ultrafast internal conversion and photochromism in gas-phase salicylideneaniline

Salicylidenaniline (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 S$_1$ 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 parallel 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

Ultrafast internal conversion and photochromism in gas-phase salicylideneaniline

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

Widely tunable cavity-enhanced ultrafast spectroscopy

Generation of widely tunable frequency combs in the UV, visible, and infrared is discussed for use in cavity-enhanced transient absorption spectroscopy on gas-phase clusters. Progress towards cavity-enhanced two-dimensional spectroscopy is also presented

Widely tunable cavity-enhanced ultrafast spectroscopy

Generation of widely tunable frequency combs in the UV, visible, and infrared is discussed for use in cavity-enhanced transient absorption spectroscopy on gas-phase clusters. Progress towards cavity-enhanced two-dimensional spectroscopy is also presented