Ultraviolet irradiation of a manganese-tricarbonyl CO-releasing molecule (CORM) in water eventually leads to the liberation of some of the carbon monoxide ligands. By ultraviolet pump/mid-infrared probe femtosecond transient absorption spectroscopy in combination with quantum chemical calculations, we could disclose for the exemplary compound [Mn(CO)3(tpm)](+) (tpm = tris(2-pyrazolyl)methane) that only one of the three carbonyl ligands is photochemically dissociated on an ultrafast time scale and that some molecules may undergo geminate recombination.

We disclose the light-induced dynamics and ultrafast formation of several photoproducts from the manifold of reaction pathways in the photochemistry of 5-diazo Meldrum's acid (DMA), a photoactive compound used in lithography, by femtosecond mid-infrared transient absorption spectroscopy covering several nanoseconds. After excitation of DMA dissolved in methanol to the second excited state S(2), 70% of excited molecules relax back to the S(0) ground state. In competing processes, they can undergo an intramolecular Wolff rearrangement to form ketene, which reacts with a solvent molecule to an enol intermediate and further to carboxylate ester, or they first relax to the DMA S(1) state, from where they can isomerize to a diazirine and via an intersystem crossing to a triplet carbene. For a reliable identification of the involved compounds, density functional theory calculations on the normal modes and Fourier transform infrared spectroscopy of the reactant and the photoproducts in the chemical equilibrium accompany the analysis of the transient spectra. Additional experiments in ethanol and 2-propanol lead to slight spectral shifts as well as elongated time constants due to steric hindrance in transient spectra connected with the ester formation channel, further substantiating the assignment of the occurring reaction pathways and photoproducts.

A comparative study is carried out on two spectroscopic techniques employed to detect ultrafast absorption changes in the mid-infrared spectral range, namely direct multichannel detection via HgCdTe (MCT) photodiode arrays and the newly established technique of chirped-pulse upconversion (CPU). Whereas both methods are meanwhile individually used in a routine manner, we directly juxtapose their applicability in femtosecond pump-probe experiments based on 1 kHz shot-to-shot data acquisition. Additionally, we examine different phase-matching conditions in the CPU scheme for a given mid-infrared spectrum, thereby simultaneously detecting signals which are separated by more than 200 cm −1 . References and links 1. P. Stoutland, R. Dyer, and W. Woodruff, "Ultrafast infrared spectroscopy," Science 257, 1913-1917 (1992). 2. P. Hamm, S. Wiemann, M. Zurek, and W. Zinth, "Highly sensitive multichannel spectrometer for subpicosecond spectroscopy in the midinfrared," Opt. Lett. 19, 1642Lett. 19, -1644Lett. 19, (1994. 3. P. Hamm, R. A. Kaindl, and J. Stenger, "Noise suppression in femtosecond mid-infrared light sources," Opt. Lett. 25, 1798Lett. 25, -1800Lett. 25, (2000. 4. G. Cerullo and S. D. Silvestri, "Ultrafast optical parametric amplifiers," Rev. Sci. Instrum. 74, 1-18 (2003). 5. J. Moore, P. Hansen, and R. Hochstrasser, "A new method for picosecond time-resolved infrared spectroscopy: applications to CO photodissociation from iron porphyrins," Chem. Phys. Letters 138, 110-114 (1987). 6. T. P. Dougherty and E. J. Heilweil, "Dual-beam subpicosecond broadband infrared spectrometer," Opt. Lett. 19, 129-131 (1994). 7. M. Lim, T. Jackson, and P. Anfinrud, "Binding of CO to myoglobin from a heme pocket docking site to form nearly linear Fe-C-O," Science 269, 962-966 (1995). Current and future archive for earth and planetary atmosphere studies," J. Quant. Spectrosc. Radiat. Transf. 109, 1043-1059 (2008). 38. M. Kaucikas, J. Barber, and J. J. van Thor, "Polarization sensitive ultrafast mid-IR pump probe microspectrometer with diffraction limited spatial resolution," Opt.

Abstract. Ultraviolet irradiation of CO-releasing molecules (CORMs) in water eventually leads to the loss of several carbon monoxide ligands. We show for an exemplary manganese tricarbonyl CORM that only one ligand is photolyzed off on an ultrafast timescale and that some molecules may undergo geminate recombination.