Richard A. Forman

Frequency dependence of photoacoustic spectroscopy: Surface‐ and bulk‐absorption coefficients

Researchers seek improved ways to measure separately the surface‐ and bulk‐absorption coefficients of highly transparent materials. The case in which a laser beam modulated at angular frequency ω passes through the weakly absorbing windows of a gas cell which contains a nonabsorbing gas is investigated in this paper. In particular, the frequency dependences of the acoustic stresses in the gas which arise from the surface and bulk absorption are derived. An intermediate range of frequencies exists for which the acoustic stress due to surface absorption varies as ω−1 and has a 90 ° phase shift relative to the modulated laser beam and for which the acoustic stress due to bulk absorption varies as ω−3/2 and has a 45 ° phase shift. In addition, expressions for the acoustic stress and phase shift which are valid for all frequencies are given. These expressions enable one to develop numerical procedures by which the surface‐ and bulk‐absorption coefficients may be determined separately. Numerical examples for a ...

A novel method to detect nonexponential transients in deep level transient spectroscopy

In conventional Deep Level Transient Spectroscopy (DLTS) measurements, the analysis of the results is based upon the assumption of an exponential current or capacitance transient. We present experimental and computational results on a novel experimental method for determining when the assumption of exponentiality is not satisfied by the sample under study. The measurement may be performed without any changes in the conventional double‐boxcar DLTS system.

Silicon photodiode detector for fluorescence EXAFS

A large‐area silicon diode is used as a fluorescence detector for extended x‐ray absorption fine‐structure (EXAFS) measurements. A direct comparison of this diode detector relative to a gas ionization fluorescence detector is made. Advantages of the diode detector include: higher signal for a given photon flux (due to higher quantum efficiency), vacuum and cryogenic compatibility, freedom from microphonic noise, good linearity, extremely wide dynamic range, operation without high voltage or gas connections, very simple electronics, and low cost. A brief comparison with other detection methods for fluorescence EXAFS is given. Use of photodiodes for transmission EXAFS is discussed.

Second indirect band gap in silicon

Abstract We report the first observation of the г 25 → L 1 (second indirect) transition in Si based on optical absorption studies. The energy, (1650 ± 10) meV, measured for this critical point shows that there remains a large discrepancy between theoretical band structure calculations and experimental results for this material.

Photoacoustic methods for measuring surface and bulk absorption coefficients in highly transparent materials: theory of a gas cell

Researchers seek improved ways to measure the surface absorption and the bulk absorption coefficients of highly transparent materials. Procedures are presented here by which one may determine separately the surface absorption and the bulk absorption coefficients. For the case in which a laser beam modulated at angular frequency omega passes through the weakly absorbing windows of a gas cell containing a nonabsorbing gas, the temperature profiles in the cell windows and the temperature and acoustic pressure or stress profiles in the gas have been calculated. These calculations indicate that for sufficiently low frequencies and high ambient gas pressure, enough heat transfers from the cell windows to the gas to produce a detectable acoustic pressure signal at angular frequency omega in the gas. These calculations also enable us to state the necessary measurements for determining the surface and bulk absorption coefficients. Measuring the acoustic stress amplitude at the fundamental and higher harmonic frequencies and measuring the phase shifts of the frequency components of the acoustic stress with respect to the modulated laser beam give sufficient data by which one can determine the surface and bulk absorption coefficients. Numerical examples for a representative laser glass and air (nitrogen) are given.

Absorption coefficients of highly transparent solids: photoacoustic theory for cylindrical configurations.

The development of highly transparent solids for fiber optics, integrated optics, and high power lasers requires improved methods to measure very low absorption coefficients. For the case in which a laser beam, modulated at angular frequency omega, passes through a weakly absorbing solid which is surrounded by a confined, nonabsorbing gas, the temperature profiles in the solid and the temperature and pressure profiles in the gas have been calculated. The calculations suggest that for sufficiently low frequencies and high ambient gas pressures, enough heat transfers from the solid to the gas to produce a detectable acoustic-pressure signal at angular frequency omega in the gas. They also indicate that an absorbing layer at the solid-gas interface is not an essential mechanism for producing these detectable acoustic pressure signals. The model assumes that bulk absorption in the solid is the mechanism by which energy is transferred from the laser beam. Numerical examples for a typical laser glass are given.

Photoacoustic spectroscopy: a measurement technique for low absorption coefficients

The case in which a laser beam modulated at angular frequency omega passes through the weakly absorbing windows of a gas cell which contains a nonabsorbing gas is investigated in this paper. In particular, the frequency dependences of the acoustic stresses in the gas which arise from the surface and bulk absorption are derived. An intermediate range of frequencies exists for which the acoustic stress due to surface absorption varies approximately as omega(-1) and has an approximate 90 degrees phase shift relative to the modulated laser beam and for which the acoustic stress due to bulk absorption varies approximately as omega(-3/2) and has an approximate 45 degrees phase shift. Numerical examples for a representative laser glass and air (nitrogen) are given.

Nuclear Magnetic Resonance of Ti in Ti–H Systems

The nuclear magnetic resonance spectrum of 47,49Ti in powder samples of metallic titanium hydride TiHx has been studied over the range of hydrogen concentrations corresponding to 1.7 ≤ x ≤ 2.0. For TiH2 the Knight shift is found to be +0.26 ± 0.04%, and there is no change in the Knight shift as a function of hydrogen concentration over the range studied. The titanium resonance in all samples was quite broad, 20 Oe, so that the lines from the two isotopes were not resolved. As a function of hydrogen concentration, the line shape was found to remain constant, but a rapid decrease of line intensity was noted. In order to investigate the possibility of magnetic ordering suggested by earlier studies of the magnetic susceptibility of TiH2, studies of the magnetic‐field dependence and temperature dependence of the resonance were also performed. No dependence of Knight shift or line shape on magnetic field was observed over the range 9.0–15.0 kOe. No changes in Knight shift or line shape were observed in the temp...

Absorption coefficients in highly transparent solids: barothermal theory for cylindrical configurations

The development of highly transparent solids requires improved methods to measure very low absorption coefficients at laser wavelengths. For the case in which a laser beam passes through a weakly absorbing solid that is surrounded by a confined, nonabsorbing gas, the temperature profiles in the solid and the temperature and pressure profiles in the gas have been calculated. Our calculations suggest that sufficient heat transfers from the solid into the gas to produce an easily detected pressure rise in the gas.

Nuclear Magnetic Resonance of 7Li and 1H in Solid Lithium Imide, Lithium Amide, and Lithium Nitride

Nuclear magnetic resonance lines of 7Li nuclei have been observed in solid, polycrystalline samples of LiNH2, LiNH, and Li3N. Proton resonance lines have also been observed in the first two samples. In LiNH2 there is a splitting of the 7Li resonance line due to first‐order quadrupole effects which indicates a coupling constant of 119±4 kHz (estimated limit to maximum error) at room temperature. In Li2NH no quadrupole effects are seen which is in agreement with the fact that the lithium nuclei in this compound are situated in sites of cubic symmetry. In Li3N both first‐ and second‐order quadrupole effects have been observed simultaneously. From these data 7Li coupling constants for Li3N have been found to be 296±4 and 200±10 kHz at room temperature. An unexplained line was also observed at low magnetic fields for this compound. These effects were found to vary with temperature and with the purity of the sample.