Ultrafast continuum mid-infrared spectroscopy: probing the entire vibrational spectrum in a single laser shot with femtosecond time resolution.

Until now, ultrafast IR spectroscopy has been limited by the bandwidth of optical parametric amplifiers, typically 100-400  cm(-1). Here we present the first example of transient IR spectroscopy using a continuum laser source to probe the entire mid-IR region with ultrafast time resolution. The continuum source is based on focusing the fundamental, second harmonic, and third harmonic of 1 mJ, 25 fs, 800 nm pulses in air, generating ∼150  fs continuum mid-IR pulses that span the frequency range of <400 to >5000  cm(-1) or, conversely, <2 to >25  μm. We characterize the spectral and temporal properties of dicarbonylacetonato rhodium(I) in hexane. We further demonstrate the versatility of the method by measuring the very fast and broad (>1500  cm(-1)) spectral changes following IR excitation associated with the 7-azaindole-acetic acid heterodimer in carbon tetrachloride.