Gerald J. Diebold

Chemical Generation of Acoustic Waves: A Giant Photoacoustic Effect

An anomalous photoacoustic effect is produced when a suspension of carbon particles in water is irradiated by a high-power, pulsed laser. The photoacoustic effect has an amplitude on the order of 2000 times that produced by a dye solution with an equivalent absorption coefficient and gives a distinctly audible sound above an uncovered cell. Transient grating experiments with carbon suspensions show a doubling of the acoustic frequency corresponding to the optical fringe spacing of the grating. The effect is thought to originate in high-temperature chemical reactions between the surface carbon and the surrounding water.

Photoacoustic "signatures" of particulate matter: optical production of acoustic monopole radiation.

Absorption of pulsed laser radiation by a single particle generates a photoacoustic wave whose time profile can be measured with a wideband pressure transducer. Solution of the wave equation for pressure in one, two, and three dimensions shows that the photoacoustic wave is determined by the geometry and dimensions of the particle, and by its sound speed and density relative to the fluid that surrounds it. Photoacoustic waves, referred to here as signatures, are reported in experiments in which fluid droplets, cylinders, and layers are irradiated with 10-nanosecond laser pulses.

Quantities, terminology, and symbols in photothermal and related spectroscopies (IUPAC Recommendations 2004)

This paper presents quantities, terminology, and symbols of terms related to photothermal phenomena and used in photothermal and related spectroscopies. The terms used in the literature to describe photothermal phenomena and methods are reviewed, and a glossary of terms is given. The origins of photothermal phenomena, as well as the relations among various photothermal effects, are summarized. The listed terms cover the terminology in transient grating, transient lens, photoacoustic spectroscopy, photothermal radiometry, calorimetry, interferometry, deflection, reflection, and other related spectroscopies, which use or are related to photothermal effects.

The photoacoustic effect generated by a spherical droplet in a fluid

In general, the absorption of amplitude‐modulated radiation by a body results in heating and a subsequent thermal expansion that gives rise to the emission of sound waves. Here, the properties of sound waves emitted by uniformly heated spherical droplets are found as a frequency domain solution to a boundary‐value problem for the acoustic pressure. The expression for the pressure is Fourier transformed to give the time domain pressure response to a delta function light pulse. Owing to the different sound speeds and densities in the sphere and its surrounding medium, the effects of reflection, dispersion, and phase cancellation can be seen in the acoustic signal.

Multiphoton ionization detection of NO scattered from solid surfaces

This paper describes experiments where resonantly enhanced multiphoton ionization was used to determine rotational state distributions of NO scattered from a copper surface in a high vacuum. A pulsed valve produced a high intensity beam of NO seeded in He with a well‐defined translational energy and a rotational temperature of 7 K. With a 0.28 eV incident beam and a surface temperature of 77 K the scattered beam was found to have a rotational temperature of 230 K. When the surface temperature was raised to 300 K, the scattered beam temperature approximated that of the surface. For the surface at either temperature, significant population of the X 2Π3/2 state was found. The variation of the scattered beam temperature with surface temperature is in qualitative agreement with calculations based on the hard cube model.

The photoacoustic effect generated by an isotropic solid sphere

In this paper the production of acoustic waves by irradiation of an optically thin, isotropic solid sphere is discussed. Solutions to the wave equation for the displacement potential are obtained which, together with the conservation equations for acoustic waves at an interface, give a frequency domain solution for the photoacoustic pressure. The time domain response of the sphere to a delta function heating pulse is obtained by Fourier transformation of the frequency domain expression. The photoacoustic waveform generated by a short radiation pulse is shown to depend on the radius of the sphere, the density and longitudinal sound speed of the sphere relative to those of the surrounding fluid, and the ratio of the transverse to longitudinal sound speed of the solid. Experiments are reported where a Q-switched laser was used to excite single spheres.

Photoacoustic waves generated by absorption of laser radiation in optically thin cylinders

The properties of photoacoustic waves radiated by an optically thin, infinitely long fluid cylinder can be found through solution of the wave equation for pressure. The amplitude and phase of the acoustic wave emitted by a cylinder irradiated by sinusoidally modulated light are first determined. The response of the cylinder to a pulse of radiation whose duration is short compared with the transit time of sound across the cylinder is then found by Fourier transformation of the frequency domain solution. The expressions derived here for the photoacoustic pressure are shown to give the radial eigenfrequencies of the cylinder, the frequency of oscillation of a cylindrical bubble, and a response of the cylinder to long light pulses proportional to the 1/2th time derivative of the optical pulse. The character of the photoacoustic waves is shown to be determined, in part, by repeated reflections of the outgoing cylindrical wave at the interface between the cylinder and its surrounding fluid. The results of sever...

Low density contrast agents for x-ray phase contrast imaging: the use of ambient air for x-ray angiography of excised murine liver tissue

We report a new preparative method for providing contrast through reduction in electron density that is uniquely suited for propagation-based differential x-ray phase contrast imaging. The method, which results in an air or fluid filled vasculature, makes possible visualization of the smallest microvessels, roughly down to 15 microm, in an excised murine liver, while preserving the tissue for subsequent histological workup. We show the utility of spatial frequency filtering for increasing the visibility of minute features characteristic of phase contrast imaging, and the capability of tomographic reconstruction to reveal microvessel structure and three-dimensional visualization of the sample. The effect of water evaporation from livers during x-ray imaging on the visibility of blood vessels is delineated. The deformed vascular tree in a cancerous murine liver is imaged.

The photoacoustic effect generated by an incompressible sphere

An incompressible sphere with a vanishing thermal expansivity suspended in a fluid can generate a photoacoustic effect when the heat deposited in the sphere by a light beam diffuses into the surrounding liquid causing it to expand and launch a sound wave. The properties of the photoacoustic effect for the sphere are found using a Greens function solution to the wave equation for pressure with Neumann boundary conditions. The results of the calculation show that the acoustic wave for fast heat liberation is an outgoing compressive pulse followed by a reflected pulse whose time profile is modified as a result of frequency dependent reflection from the sphere. For slow heat release by the sphere, the photoacoustic effect is shown to be proportional to the first time derivative of the heat flux at the particle-fluid interface.

Chemi-ionization reactions of Ca, Sr, Ba, and Yb atoms with the halogen and interhalogen molecules

Abstract Large variations are observed in the chemi-ionization cross sections for the thermal reactions of Ca, Sr, Ba, and Yb atoms with various halogen and interhalogen molecules, as well as chlorine dioxide. While the alkaline earth metal atom reactions generally produce ion products, this is only true for Yb + F 2 . The absence of ions for the other Yb reaction is explained on energetic grounds. For the reaction of a metal atom (M) with a halogen (X 2 ), the positive and negative ions are determined mass spectrometrically to be MX + and X − . For the interhalogens, both ion channels, MX + + Y − and MY + + X, are observed, the dominant channel being the one more exoergic. Absolute chemi-ionization cross sections are estimated to range from 0.001 to 1.6 A 2 , with an uncertainty of ± 100%. These cross sections decrease in the order BA > Sr Ca > Yb for all oxidizer gases other than F 2 . For a given alkaline earth metal, the cross sections decrease in the order F 2 > Cl 2 > I 2 , the only exception being Ba + F 2 . These large cross sections indicate that for all the reactions discussed here, the chemi-ionization channel is much more important than the chemiluminescence pathway.