G.M. Gale

Ti:sapphire second-harmonic-pumped visible range femtosecond optical parametric oscillator

Abstract A β-barium borate (BBO) femtosecond optical parametric oscillator (OPO) pumped by the second-harmonic of a modelocked 82 MHz Ti:sapphire laser is described. With intracavity dispersion compensation, pulse durations down to 30 fs are obtained with a total average power up to 100 mW. The current wavelength range, with a 400 nm pump, is from 566 nm to 676 nm. Unusual tuning characteristics, including bichromatic emission, are presented in detail and explained theoretically.

Frequency-dependent vibrational population relaxation time of the OH stretching mode in liquid water

Abstract The vibrational population relaxation time has been measured across the inhomogeneously broadened OH stretching mode (ν3) of a 0.5% solution of HDO in D2O. Relaxation times vary from 0.5 ps at low frequency (3270 cm−1) to 1.0 ps at high frequency (3600 cm−1). This variation is attributed to a V–V transfer mechanism between ν3 and 2ν2.

Transient absorption of vibrationally excited water

We study the spectral response of the transition between the first and the second excited state of the O–H stretch vibration of HDO dissolved in liquid D2O with two-color femtosecond mid-infrared spectroscopy. The spectral response of this transition differs strongly from the fundamental absorption spectrum of the O–H stretch vibration. In addition, excitation of the O–H stretch vibration is observed to lead to a change of the hydrogen-bond dynamics of liquid water. We show that both these observations can be described with a refined quantum-mechanical version of the Lippincott–Schroeder model for hydrogen-bonded OH⋯O systems.

Coupling between molecular rotations and OH⋯O motions in liquid water: Theory and experiment

A new theory is proposed to describe spectral effects of the coupling between molecular rotations and OH⋯O motions in liquid water. The correlation function approach is employed together with a special type of development in which the coupling energy of these two motions is the expansion parameter. The isotropy of the liquid medium plays an essential role in this study. Based on this theory, a new infrared pump–probe experiment is described permitting a visualization of molecular rotations at subpicosecond time scales. Full curves relating the mean squared rotational angle and time, and not only the rotational relaxation time, are measured by this experiment. However, very short times where the incident pulses overlap must be avoided in this analysis. The lifetime of OH⋯O bonds in water is rotation–limited.

Vibrational relaxation of compressed nD2 fluid

Abstract Experimental observations of the vibrational population relaxation time of n D 2 fluid under pressures of up to 500 atm in the 25–85 K range are presented and described in terms of a semi-classical model for energy transfer in liquids. For comparison with the parameters of this model, a classical equivalent potential for quantum systems is derived from the “real” intermolecular potential.

Non-monotonic decay of transient infrared absorption in dilute HDO/D2O solutions

Abstract Transient infrared absorption of diluted HDO/D 2 O solutions is measured with 150 fs laser pulses. The signal intensity decays in a heavily non-exponential way, and its variation is often non-monotonic; no quantum beats are observed. These effects are ascribed to the simultaneous presence of solvent and population dynamics in low lying vibrational states, and to a partial cancellation of the bleach and absorption intensities.

Direct Picosecond Determination of the Character of Vibrational Line Broadening in Liquids

Vibrational transitions in liquids and solids may be broadened by many different physical processes. The diverse contributions to a given band cannot always be unambiguously separated in conventional spectroscopy, especially if the band is perturbed by neighbouring overlapping lines such as vibrational overtones and hot bands, and it is often particularly difficult to determine if the band contour is Gaussian, Lorentzian or of intermediate shape.

Biphonon dynamics in crystals: Nitrous oxide

Abstract Coherent picosecond excitation and probe techniques are employed to measure the dephasing times of the very weak 2v2 two-phonon resonance in solid N2O as a function of temperature. T2 times are also obtained for the v1 one-phonon transition which is in feeble Fermi interaction with 2v2.

Mid infra-red femtosecond pulse generation by wave-mixing: numerical simulation and experiment

Abstract Numerical simulations of the parametric difference-frequency-mixing process in the femtosecond regime are presented. The calculations indicate that sub- 100 fs pulses may be produced in the 3 to 5 μm range using available sources. Theoretical results are compared to experiments on asynchronous frequency-mixing of two femtosecond Ti:sapphire lasers.

Generation of highly coherent tunable mid-infrared femtosecond pulses by paramatric frequency-mixing at 82 MHz

Abstract Near-transform-limited femtosecond infrared pulses are generated in the 2.8 to 3.6 μm spectral region by parametric frequencymixing in KTP of the synchronised outputs of an 82 MHz femtosecond Ti: sapphire laser and a tunable visible-range femtosencond optical parametric oscillator pumped by the second-harmonic of the Ti: sapphire laser. Infrared pulse-widths down to 90 fs are measured and the dependence of pulse duration on frequency and mixing-crystal thickness is accurately described by a detailed calculation and a simple analytical expression