Jonathan D. Spear

Collinear photothermal deflection spectroscopy with light-scattering samples

An analytical model that incorporates effects of light scattering was developed for dual-beam photothermal deflection spectroscopy. Thermal gradients induced by a modulated excitation beam deflect an optical probe beam which was treated as being of finite dimensions. Mechanisms by which thermal gradients produce refractive index gradients are discussed, with an explicit expression for dn/dT being derived. Experimental studies with suspensions of small latex particles in Nd(3+) solutions demonstrated that the model accurately predicts both the shape of the deflection signal and the attenuation of the signal due to light scattering. The absolute magnitude of the observed signal is approximately predicted by theory.

Low noise position sensitive detector for optical probe beam deflection measurements

We report the design of an optical position sensor that uses two discrete photodiodes electrically connected in parallel, with opposing polarities. A lens provides optical gain and restricts the acceptance angle of the detector. The response of the device to displacements of an optical spot is similar to that of a conventional bicell type position sensitive detector. However, the discrete photodiode design enables simpler electronic amplification with inherently less electrical noise than the bicell. Measurements by the sensor of the pointing noise of a focused helium–neon laser as a function of frequency demonstrate high sensitivity and suitability for optical probe beam deflection experiments.

Transverse photothermal beam deflection within a solid

The mirage effect within a transparent solid substrate was used for monitoring optical absorption of a thin film. Refractive index gradients, which accompany thermal gradients below the film‐coated surface, cause a probe laser beam to be deflected. The spectrum of copper, deposited onto a piece of clear acrylic, was recorded by this method of photothermal deflection. The influence of thermally induced mechanical stresses can alter the effective value of the thermo‐optic coefficient of the solid, dn/dT.

Collinear Photothermal Deflection Spectroscopy of Liquid Samples at Varying Temperature

The method of photothermal beam deflection was demonstrated for measuring the absorption spectrum of an aqueous Nd3+ solution at temperatures between 25°C and 90°C. Changes in the magnitude of the observed deflection can be correlated with the temperature dependence of both the absorptivity of the Nd3+ ions and the physical properties of the solvent, most notably the thermo-optic coefficient dn/dT.

Differential Photothermal Deflection Spectroscopy Using a Single Position Sensor

A differential dual-beam photothermal deflection spectrometer using a single position sensor is described. An excitation and a probe laser beam are directed simultaneously into a sample solution and a reference solution to provide real-time background correction. The probe beam that passes through the reference cell is folded back into the path of the sample-cell probe beam in such a way that only one position sensor is needed to monitor the deflection in both cells. Nd3+ solutions are used to demonstrate the effect of background correction and the sensitivity of this dual-beam photothermal deflection arrangement.

Shot noise in X-ray measurements with p-i-n diodes

The importance of shot noise is considered for situations in which p-i-n diodes monitor x-ray radiation. An expression for shot noise is derived in terms of the photon energy, the pair creation energy of the diode material, and the photocurrent. Statistical analysis shows that the Fano factor can be neglected for noise calculations. A lock-in amplifier measured the low frequency photocurrent noise from an unbiased silicon p-i-n photodiode that monitored radiation in the range of 6–16keV at a synchrotron beamline. With ordinary electronic amplification and shielding, shot noise dominated other noise sources for photocurrents exceeding about 5pA.

Photothermal deflection spectroscopy of an aqueous sample in a narrow bore quartz capillary

An apparatus to perform photothermal deflection spectroscopy on liquid samples within a cylindrical capillary is described. A tunable dye laser, modulated by an optical chopper, serves as an excitation source. The resolution of a spectral absorption peak near 575 nm, of a 1×10−3 M Nd3+ aqueous solution, demonstrates the effectiveness of the system. The sample is contained within a 75 μm internal diameter quartz capillary, typical of those used for capillary electrophoresis. Across the middle of the probed section of the capillary, the magnitude of the resolved peak is 5.2×10−5 absorbance units. A helium–neon laser, focused to a 1/e2 waist diameter of 40 μm, provides an optical probe beam across the center of the sample, overlapping the excitation beam at an angle of 3°. Maximum signal-to-noise ratio is achieved with the apparatus when the excitation beam is modulated at a frequency near 205 Hz. The deflection responsivity of the probe beam at this frequency is 650 nrad per μW of absorbed excitation radiat...

Lens correction for the implementation of cylindrical vessels in a spectrophotometer

A simple optical correction system for allowing cylindrical vials to be used as sample vessels in a spectrophotometer is described. Light within the spectrophotometer enters and exits the liquid samples through the curved glass wall of the vials. Absorption spectra can be obtained conveniently without the need for solution transfer into standard rectangular spectrophotometer cuvettes.