Narrow homogeneous linewidths and slow cooling dynamics across infrared intra-band transitions in n-doped HgSe colloidal quantum dots.

Intra-band transitions in colloidal quantum dots (QDs) are promising for opto-electronic applications in the mid-IR spectral region. However, such intra-band transitions are typically very broad and spectrally overlapping, making the study of individual excited states and their ultrafast dynamics very challenging. Here, we present the first full spectrum two-dimensional continuum infrared (2D CIR) spectroscopy study of intrinsically n-doped HgSe QDs, which exhibit mid-infrared intra-band transitions in their ground state.

Wedge-Based Design for Phase Stable and Phase Accurate Heterodyne-Detected Sum-Frequency Generation Spectroscopy.

Phase sensitive and heterodyne-detected (HD) sum-frequency generation (SFG) spectroscopy offers the ability to separate the nonlinear susceptibility into its real and imaginary components. This provides information about the absolute orientation of molecules at interfaces while also producing an absorptive spectrum that is linear in spectral composition and can easily be decomposed into different spectral components. However, simultaneously obtaining phase accuracy and phase stability remains a challenge in SFG.

Full spectrum 2D IR spectroscopy reveals below-gap absorption and phonon dynamics in the mid-IR bandgap semiconductor InAs.

While the mid-infrared spectral region spans more than 3000 cm, ultrafast mid-IR spectroscopies are normally limited to the spectral bandwidth that can be generated in optical parametric amplifiers-typically a few hundred cm. As such, the spectral coverage in conventional two dimensional infrared (2D IR) spectroscopy captures only about 1% of the full potential 2D mid-IR spectrum. Here, we present 2D IR spectra using a continuum source as both the excitation and probe pulses, thus capturing close to the full 2D IR spectrum.

Interpreting Quasi-Thermal Effects in Ultrafast Spectroscopy of Hydrogen-Bonded Systems.

Vibrational excitation of molecules in the condensed phase relaxes through vibrational modes of decreasing energy to ultimately generate an equilibrium state in which the energy is distributed among low-frequency modes. In ultrafast vibrational spectroscopy, changes in the vibrational features of hydrogen-bonded NH and OH stretch modes are typically observed to persist long after these high-frequency vibrations have relaxed. Due to the resemblance to the spectral changes caused by heating the sample, these features are typically described as arising from a hot ground state.

Interferometric 2D Sum Frequency Generation Spectroscopy Reveals Structural Heterogeneity of Catalytic Monolayers on Transparent Materials.

Molecular monolayers exhibit structural and dynamical properties that are different from their bulk counterparts due to their interaction with the substrate. Extracting these distinct properties is crucial for a better understanding of processes such as heterogeneous catalysis and interfacial charge transfer. Ultrafast nonlinear spectroscopic techniques such as 2D infrared (2D IR) spectroscopy are powerful tools for understanding molecular dynamics in complex bulk systems.

Couplings Across the Vibrational Spectrum Caused by Strong Hydrogen Bonds: A Continuum 2D IR Study of the 7-Azaindole-Acetic Acid Heterodimer.

Strongly hydrogen-bonded motifs provide structural stability and can act as proton transfer relays to drive chemical processes in biological and chemical systems. However, structures with medium and strong hydrogen bonds are difficult to study due to their characteristically broad vibrational bands and large anharmonicity. This is further complicated by strong interactions between the high-frequency hydrogen-bonded vibrational modes, fingerprint modes, and low-frequency intradimer modes that modulate the hydrogen-bonding.

Strong intermolecular vibrational coupling through cyclic hydrogen-bonded structures revealed by ultrafast continuum mid-IR spectroscopy.

Cyclic hydrogen-bonded structures are common motifs in biological systems, providing structural stability and mediating proton transfer for redox reactions. The mechanism of proton transfer across hydrogen-bonded interfaces depends on the strength of the intermolecular coupling between bridging OH/NH vibrational modes. Here we present a novel ultrafast continuum mid-IR spectroscopy experiment to study the vibrational dynamics of the 7-azaindole-acetic acid (7AI-Ac) heterodimer as a model system for asymmetric cyclic hydrogen-bonded structures.

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.