Alfred S. Kwok

Vibrational anharmonicity and multilevel vibrational dephasing from vibrational echo beats

Vibrational echo experiments were performed on the IR active CO stretching modes (∼2000 cm−1) of rhodium dicarbonylacetylacetonate [Rh(CO)2acac] and tungsten hexacarbonyl [W(CO)6] in dibutylphthalate and a mutant of myoglobin-CO (H64V-CO) in glycerol–water using ps IR pulses from a free electron laser. The echo decays display pronounced beats and are nonexponential. The beats and nonexponential decays arise because the bandwidths of the laser pulses exceed the vibrational anharmonicities, leading to the excitation and dephasing of a multilevel coherence. From the beat frequencies, the anharmonicities are determined to be 14.7, 13.5, and 25.4 cm−1, for W(CO)6, Rh(CO)2acac, and H64V-CO, respectively. From the components of the nonexponential decays, the vibrational dephasing at very low temperature of both the v=0–1 and v=1–2 transitions are determined. At the lowest temperatures, T2≈2T1, so the v=2 lifetimes are obtained for the three molecules. These are found to be significantly shorter than the v=1 life...

Vibrational spectral diffusion and population dynamics in a glass-forming liquid: Variable bandwidth picosecond infrared spectroscopy

The temperature‐dependent vibrational population dynamics and spectral diffusion of the CO stretching mode of tungsten hexacarbonyl in 2‐methylpentane are observed from the room temperature liquid to the low temperature glass using picosecond infrared transient grating and pump–probe experiments. These experiments were performed between 10 and 300 K on the triply degenerate T1u asymmetric CO stretching mode at 1984 cm−1 using pulses with bandwidths narrower and wider than the absorption bandwidth of the transition. The rate of vibrational population relaxation (100≤T1<150 ps) is observed to decrease with increasing temperature. The orientational dynamics for this transition are observed on a faster time scale than the population relaxation. Although the liquid viscosity changes over 14 orders of magnitude, the orientational relaxation rate slows by less than one order of magnitude over the full temperature range. By comparing polarization‐dependent experiments performed with both narrow and broad bandwidt...

Phonon-induced scattering between vibrations and multiphoton vibrational up-pumping in liquid solution

Abstract Fast phonon-induced scattering between an infrared active and a Raman active vibration in solution is investigated using picosecond infrared pump/anti-Stokes Raman probe experiments. Population from the infrared active T 1u CO stretching mode of tungsten hexacarbonyl in carbon tetrachloride at 1980 cm −1 scatters to the Raman active E g mode at 2012 cm −1 but not to the A 1g mode at 2116 cm −1 . Equilibration occurs rapidly compared to the 700 ps population relaxation time. A power dependence of the line shape of the Raman mode indicates that significant population is pumped into high vibrational levels when high infrared pump powers are used.

Multilevel vibrational dephasing and vibrational anharmonicity from infrared photon echo beats

Abstract Vibrational photon echo experiments were performed on the asymmetric CO stretching mode of tungsten hexacarbonyl in glassy dibutylphthalate as a function of temperature using sub-picosecond infrared pulses (1976 cm −1 ) from a free electron laser. The echo decays display pronounced beats and are bi-exponential. The beats and bi-exponential decays arise because the bandwith of the pulses exceed the vibrational anharmonicity, leading to the excitation and dephasing of a multilevel coherence. From the beat frequency, the anharmonicity is determined to be 14.7 cm −1 . From the bi-exponential decay components, the temperature-dependent vibrational dephasing of both the v = 0 → 1 and v = 1 → 2 transitions are determined.

Vibrational photon echoes in a liquid and glass: Room temperature to 10 K

Picosecond infrared vibrational photon echo experiments were performed on the asymmetric CO stretching mode (1983 cm−1) of tungsten hexacarbonyl in 2‐methylpentane from room temperature to 10 K using a free electron laser. This is the first report of a room temperature infrared vibrational photon echo in a liquid.

Ultrafast infrared spectroscopy in biomolecules: Active site dynamics of heme proteins

Rapid advances in the generation of intense tunable ultrashort mid-infrared (IR) laser pulses allow the use of ultrafast IR pump-probe and vibrational echo experiments to investigate the dynamics of the fundamental vibrational transition of CO bound to the active site of heme proteins. The studies were performed using a free-electron laser (FEL) and an experimental set up at the Stanford University FEL Center. These novel techniques are discussed in some detail. Pump-probe experiments on myoglobin-CO (MbCO) measure CO vibrational relaxation (VR). The VR process involves loss of vibrational excitation from CO to the protein and solvent. Infrared vibrational echoes measure CO vibrational dephasing. The quantum mechanical treatment of the force-correlation function description of vibrational dynamics in condensed phases is described briefly. A quantum mechanical treatment is needed to explain the temperature dependence of VR in Mb-CO from 10 to 300 K. A molecular-level description including elements of heme protein structure in the treatment of vibrational dynamics is also discussed. Vibrational relaxation of CO in Mb occurs on the 10−11-s time scale. VR was studied in proteins with single-site mutations, proteins from different species, and model heme compounds. A roughly linear relationship between carbonyl stretching frequency and VR rate has been observed. The dominant VR pathway is shown to involve anharmonic coupling from CO through the π-bonded network of the porphyrin, to porphyrin vibrations with frequencies > 400 cm−1. The heme protein influences VR of bound ligands at the active site primarily via altering the through π-bond coupling between CO and heme. Preliminary vibrational echo studies of the effects of protein conformational relaxation dynamics on ligand dephasing are also reported.

Enhancing stimulated Raman scattering of weaker gain Raman modes in microdroplets by seeding and efficient pumping

High-Q morphology-dependent resonances (MDRs) in microdroplets provide optical feedback for nonlinear processes such as stimulated Raman scattering (SRS). The SRS signal from weaker gain modes or from the minority species in a binary mixture droplet is small, because of depletion of the pump beam by the strongest-gain Raman mode. We present an effective- average Raman gain formula and discuss the factors that contribute to the enhancement of SRS intensity. Spatial overlap between pump wave and SRS wave is an important factor in determining the SRS intensity. We have observed that in a binary mixture microdroplet, the SRS of the majority species is more efficient (because of good spatial overlap) in pumping the minority species than the input laser itself. Laser-induced distortion can couple light efficiently into a droplet. We have used a variable number (3 - 6) of mode-locked 100-psec laser pulses focused on the droplet rim. The SRS emission from a minority species increases as the number of input pulses is increased, because cumulative laser-induced surface distortion couples more energy into the droplet. SRS signal at the Stokes shift of a weaker gain mode or of the mode of a minority species can be enhanced by seeding light at the SRS wavelengths. The seed signal is obtained from the fluorescence of a dye added to the microdroplet or from external coupling of light into the droplet. A lasing dye (Rhodamine 6G) is added to pure ethanol droplet. By using an excimer-pumped tunable dye laser as the excitation source, the weaker gain C-C-O mode (882 cm-1 Stokes shift) of ethanol is overlapped spectrally with the fluorescence and lasing of R6G. The SRS signal of the C-C-O mode is enhanced, because (1) the fluorescence of R6G at the SRS wavelength adds to the spontaneous SRS emission and (2) the gain from population inversion adds to the Raman gain. We have also observed enhanced SRS emission from a minority species (benzene) in dodecane by using internal dye seeding. We externally seeded ethanol SRS (2928 cm-1) in a binary mixture (12% ethanol and 88% water) microdroplet by using a frequency-doubled Nd-YAG pumped tunable dye laser. A large (factor of 45) enhancement in the ethanol SRS (integrated over 100 shots of a 10-ns laser) has been observed.

Vibrational Population Dynamics in Liquids and Glasses

The temperature dependent vibrational relaxation of the CO stretching mode of rhodium dicarbonyl acetylacetonate (Rh(CO)2(acac)) in dibutyl phthalate (DBP), and tungsten hexacarbonyl (W(CO)6) in 2-methylpentane (2-MP) were measured with IR pump and probe (PP) experiments. The experiments were performed with ∼1.5 ps pulses (spectral width Δv ≈ 12 cm-1) at λ ≈ 5 μm generated by the Stanford superconducting-accelerator-pumped free electron laser (FEL)1 and 10–15 ps pulses (Δv ≈ 2 cm-1) at the same wavelength generated by an LiIO3 optical parametric amplifier (OPA)2. Typical pulse energies used in the experiments were 10 nJ for the probe and 50–100 nJ for the pump. Both beams were focused to a ∼150 μm spot on a liquid cell with 400 μm optical path. The concentrations of the sample solutions were 6 × 10-3 M for Rh(CO)2(acac) and 4×10-3 M for W(C0)6. The differential absorption of the probe beam was measured as a function of the time delay between the pump and the probe pulses. The decay curves of the probe signal were fitted using a single or a double exponential function.

Picosecond vibrational photon echo experiments in liquids and glasses

An account is given of the first infrared vibrational photon echo experiments conducted in liquids and a glasses. The experiments were performed on the CO stretching mode of the solute tungsten hexacarbonyl (W(CO)6) at approximately 5.05 micrometers (approximately 1980 cm-1) in the solvents 2-methytetrahydrofuran, and 2-methylpentane (2-MP). In 2-MP, it was possible to observe the photon echo decay at room temperature and follow the temperature dependence from room temperature to 10 K. The photon echo experiments were conducted using the Stanford superconducting-linear-accelerator-pumped Free Electron Laser as the source of tunable ps infrared pulses.