R. S. Francis

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...

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.

Electronic dephasing in nonpolar room temperature liquids: UV photon echo pulse duration dependent measurements

The homogeneous dephasing times, T2, were obtained for the nonpolar chromophore diphenyl acetylene (DPA) dissolved in the room temperature nonpolar solvents 2-methyl pentane (2MP) and cyclohexane. The dephasing times were determined by comparing the pulse duration dependent resonant enhancement of the total two pulse four wave mixing signal to a numerically evaluated theory. Data was taken with 80 and 400 fs transform limited pulses at 300 nm, the electronic origin of DPA in the two solvents. In DPA/2MP, T2=120 fs and in DPA/cyclohexane, T2=500 fs. The analysis required concentration and power dependent measurements to quantitatively determine the resonant enhancement of the signal arising from the addition of DPA to the solvents above the nonresonant solvent only signal. Fundamental experimental difficulties inherent in performing UV femtosecond two pulse photon echoes, in particular the competition with nonresonant signal, are discussed. An efficient numerical algorithm for calculating photon echo decay...

Translational–rotational coupling in supercooled liquids: Heterodyne detected density induced molecular alignment

We present a new time domain technique for studying molecular orientational relaxation in viscous liquids. A molecular velocity gradient (acoustic disturbance) associated with a density change induced by weak absorption of a 1.06 μm excitation pulse, causes molecular alignment through translational–rotational coupling. Using an optical heterodyne detection method, molecular orientational relaxation is monitored. An eightfold experimental cycle, analogous to phase cycles in NMR, is used to separate the DIHARD signal (density induced heterodyne amplified rotational dynamics) from optical Kerr effect (OKE) contributions and thermal lensing effects. Calculations combining the Navier–Stokes equation with translational–rotational coupling are presented that describe the nature of the method. The method is analyzed theoretically and demonstrated with experiments on supercooled salol (phenyl salicylate). DIHARD experiments on salol combined with heterodyne detected OKE experiments are used to examine long time sc...

Crack propagation induced heating in crystalline energetic materials

A model is presented for time and spatial dependences of the heating of molecular vibrations and the possible initiation of chemical reaction from heat dissipated in the vicinity of a propagating crack in a molecular crystal. In the model, energy from a moving crack tip is released as phonons in proximity to the crack. Initially the phonons and the molecular vibrations are not in thermal equilibrium. Subsequently, there is a competition between excitation of molecular vibrations by multiphonon up-pumping and diffusion of phonons from the crack region. If the coupling between the locally hot phonon bath and the molecular vibrations is sufficiently large, a transitory high vibrational temperature will be achieved prior to eventual thermal equilibration with the bulk of the crystal. It is found that the peak vibrational temperature can be sufficiently high for a significant time period for chemical reactions to occur. The model calculates the local time-dependent vibrational temperature using reasonable values of the physical input parameters. For a crack tip moving near the speed of sound, the calculations show that vibrational temperatures can reach ∼800 K in 55 ps and exceed 550 K for ∼1 ns after the initial heating. This temperature change is sufficient to produce chemical reaction in a secondary explosive such as HMX, but given the duration and size of the heated region, a single crack should not result in self-sustaining chemical reaction. The role that cracks may play in shock sensitivity is discussed.

Density-induced heterodyne-amplified rotational dynamics: a new technique for studying orientational relaxation

Abstract A new time-domain method, density-induced heterodyne-amplified rotational dynamics (DIHARD), for studying the orientational relaxation of pure or mixed viscous liquids is described. Extremely weak absorption of an excitation pulse produces a density perturbation like that in a thermal lensing experiment. Through translational–rotational coupling, an orientational anisotropy is produced, which is heterodyne detected with a weak probe field. The DIHARD signal appears with the time for the velocity of sound to cross the excitation spot size and decays with the reorientational dynamics of the sample. DIHARD can provide data in situations where laser-excited optical Kerr experiments are difficult to perform.

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.