A new technique in infrared (IR) vibrational spectroscopy enhances the information gleaned from molecular spectra, giving a three-dimensional (3-D) look at vibrational energy relaxation (VER). The chemical phenomena in virtually all polyatomic liquids and solutions involve VER, an elementary process that is poorly understood. The fundamental mechanisms of VER may now be studied in unprecedented detail using a 3-D IR-Raman technique developed by chemists at the University of Illinois at Urbana-Champaign (UI; Urbana, IL).
The technique combines ultrashort pulses from a mid-IR laser with pulses of visible light to create two-color spectra that can be further resolved in time and frequency. Chemists Lawrence Iwaki and Dana Dlott used the pulses to excite particular vibrations in water, deuterated water (D20), and liquid methanol (CH3OH).
"Molecules have many specific vibrational transitions that can be used to identify a specific molecular motionfor example CH and OH stretching in methanol, and OH stretching and HOH bending in water," said Dlott, a UI professor of chemistry. "By adding an additional time and spectral dimension, we can monitor the femtosecond-time-scale flow of vibrational energy out of the vibration being pumped in the IR, through the molecule into other vibrations, and ultimately out into the rest of the liquid."
The first two dimensions in the 3-D technique are represented by a time series of incoherent anti-Stokes Raman spectra at a given mid-IR pump frequency. In the anti-Stokes spectrum, the intensity of each transition is directly proportional to the amount of excitation in the corresponding vibration. The third dimension involves changing which vibration is initially excited, by tuning the mid-IR pump pulse.
The experiment on methanol used a high-power (50-MW) optical parametric amplifier (OPA) pumped with a Ti:sapphire laser to generate a tunable picosecond mid-IR pulse and a fixed-frequency visible pulse (see Fig. 1). The OPA was seeded by quasi-CW narrow-band light from a single-longitudinal-mode diode-pumped Q-switched Nd:YAG laser, using crystals of potassium titanyl arsenate (KTA). A calcium fluoride (CaF2) Brewster prism separated the signal and the idler. The mid-IR pump pulse was focused on the sample, and the signal was then frequency doubled in a beta-barium borate (BBO) crystal to produce a 532-nm Raman probe pulse. Backscattered Raman light was then detected. The mid-IR pump pulse and 532-nm Raman probe pulse had spectral widths of 35 and 25 cm-1, respectively, and durations of 0.8 ps.
Of particular interest in the study of liquid methanol is how VER changes as the pump pulse is tuned through the methanol absorption, and how measuring the dependence of VER on pump frequency helps elucidate the fundamental mechanisms of VER (see Fig. 2). Initial results show that the dependence on pump frequency is not entirely destroyed by fast equilibration between molecular stretches. One surprising observation is instantaneous generation of some vibrational excitations when pumping a lower frequency. This may indicate that some vibrational predissociation does occur in methanol.
"We know what motions are expected between any two atoms in a molecule from Raman spectra," said Dlott. "What we don't know is how the energy kicks around to the other stretches." The answer has relevance to understanding molecular vibrations, including the quantum mechanics of nanomachines the size of a molecule.
