Phase-sensitive Detection Technique Research Articles

Magnetophonon Effect and Energy Band Parameters of InP

Magnetophonon effect has been studied in the transverse magnetoresistance of high purity epitaxial n-InP at temperatures between 77 and 300 K, by adopting the magnetic field modulation and phase sensitive second harmonic detection technique. Up to the 10th extrema are resolved at 88 K. The band edge effective mass is estimated to be 0.0813±0.001 m at 80 K and 0.0763±0.001 m at 282 K. The temperature dependence of the effective mass is discussed by the k · p perturbation theory and squared momentum matrix element is determined to be E p =16.6 [eV]. The energy bands of InP throughout the entire Brillouin zone has been calculated by diagonalizing a modified k · p Hamiltonian referred to 30 basis states at k =0 and a reasonable agreement with the present data is obtained.

Mean speed measurement of Ni and Pd molecular beams

The mean speed of Ni and Pd molecular beams effused from a nonequilibrium source has been measured by means of a phase-sensitive detection technique and is compared with the statistical mean speed. The mean speeds are determined to be 65,000 and 52,000 cm/s for Ni (1450 °C) and Pd (1320 °C), respectively. The ratio of the mean speed with respect to the statistical mean speed varies from 0.9 to 1.1, and from 0.92 to 1.18 for Ni and Pd, respectively, for the modulation frequency range 300–1200 Hz.

Magnetophonon Effect and Energy Band Parameters of InP

Magnetophonon effect has been studied in the transverse magnetoresistance of high purity epitaxial n-InP at temperatures between 77 and 300 K, by adopting the magnetic field modulation and phase sensitive second harmonic detection technique. Up to the 10th extrema are resolved at 88 K. The band edge effective mass is estimated to be 0.0813±0.001 m at 80 K and 0.0763±0.001 m at 282 K. The temperature dependence of the effective mass is discussed by the k · p perturbation theory and squared momentum matrix element is determined to be E p =16.6 [eV]. The energy bands of InP throughout the entire Brillouin zone has been calculated by diagonalizing a modified k · p Hamiltonian referred to 30 basis states at k =0 and a reasonable agreement with the present data is obtained.

Lower-hybrid wave resonance cone detection via CO2-laser scattering

Lower-hybrid waves are studied in the Princeton ACT-I steady-state toroidal plasma device using a radially scanning CO2 laser scattering system with both amplitude and phase sensitive detection techniques. Clearly defined resonance cones launched from external electrostatic antennas are seen to disappear as the plasma density is raised. Scaling of a lower-hybrid wave (LHW) laser signal with radio-frequency (rf) power in the presence of resonance cones shows nonlinearities associated with rf-induced changes in the effective laser scattering volume. Absolute fluctuation level estimates suggest that this occurs when eΦ/Te≥1. Wave-front curvature effects can cause a complete loss of resonance cone laser signals, even though probes indicate that cones are still present. Measurements of the wave k⊥ spectrum in the plasma show direct evidence for electron Landau filtering of the original wave k∥ spectrum launched from the antenna at the plasma edge, and strong dependence on antenna phasing. Finally, frequency shifts and loss of the resonance cone signal are associated with high levels of plasma density edge turbulence.

Effect of environment on the magnetic field modulation of exciplex luminescence

The effect of a magnetic field on the pyrene-N,N-dimethylaniline exciplex luminescence in methanol has been studied using an improved phase-sensitive detection technique. The wavelength dependence of B 1/2, the effect of isotopic substitution of the solvent, and the consequences of using mixed donors have been investigated. The results throw light on the environmental perturbation of the intersystem crossing process in solvent-separated radical pairs.

Direct electromagnetic generation of ultrasound in potassium

The direct electromagnetic generation of ultrasound has been studied in high-purity, single-crystal potassium. Electromagnetic acoustic transducers were used for both the generation and the detection of ultrasound, and a coherent phase-sensitive detection technique was used to measure the generated signals. The slow shear mode corresponding to the elastic constant ${C}^{\ensuremath{'}}$ was studied. The observed zero-field generation amplitude is three times greater than that predicted by the free-electron model which assumes specular scattering of electrons at the metal surface. A nonmonotonic variation of the generation amplitude for magnetic fields below the absorption edge has also been observed. The experimental results are compared with previous measurements and with a recent extension of the free-electron theory which assumes diffuse scattering of electrons at the metal surface.

Fluorescence lifetime resolution with phase fluorometry

A phase fluorometer for the measurement of fluorescence lifetimes was constructed from commercially available components. The instrument was tested by using optical delays of 4 and 2 cm, showing an accuracy of 10 ps. Lifetimes, as short as 0.3 ns, obtained by the quenching of fluorescein by KI, were analyzed with a standard deviation of 3 ps. The lifetime resolving power was checked using mixtures of acridine and quinine–sulphate and least-squares fitting procedures. Accurate amplitude ratios were obtained with the technique of phase-sensitive detection [J. R. Lakowicz and H. Cherek, J. Biochem. Biophys. Methods 5, 19 (1981)].

Far-infrared photothermal ionization spectroscopy of semiconductors in the presence of intrinsic light

The equilibrium distribution of electrons and holes over shallow impurity states and energy bands of an ultrapure semiconductor is studied for the situation where the semiconductor is continuously illuminated with intrinsic light (i.e., radiation with energies of the order of the gap energy of the semiconductor). The response to additional injection of free minority or majority charge carriers into the energy bands—caused by photothermal ionization of minority or majority impurities, respectively—is separately investigated. The equilibrium and the response have theoretically been analyzed by means of a description with a set of rate of change equations. This analysis explains the usually observed behavior that photothermal ionization of minority impurities in ultrapure germanium under continuous illumination with intrinsic light gives rise to a decrease in electrical conductivity. The measured time evolution of the change in conductivity of an ultrapure germanium sample after the start of the photothermal process revealed a slow (∼5 ms) change, connected with minority impurities only, as well as a fast (<0.5 ms) change. The slow response time has been associated with the electron-hole recombination time, yielding a value 5×10−12 cm3 s−1 for the electron-hole recombination constant. It is demonstrated that in photothermal ionization spectroscopy, when using phase-sensitive detection techniques by means of a lock-in amplifier, such a simultaneous presence of a fast and a slow (i.e., of the order of magnitude of the chopping times applied) change in conductivity can cause artefacts in the spectra.

Photodissociation of ions within the source of a double-focusing mass spectrometer

Ions generated by electron ionization are photodissociated by shining laser light into the ion chamber of a double-focusing mass spectrometer. Photofragment ions are distinguished from other ions arriving at the detector by the technique of phase-sensitive detection. Fragment-ion spectra are obtained with the full resolution of the mass spectrometer and because low velocity ions having high number density are irradiated, large photofragment-ion currents are obtained. The dependence of relative photoabsorption cross-sections and competing dissociation reactions on photoexcitation energy have been studied. The influence of source temperature on competing photodissociation reactions has been investigated and photoionization of Ar + ions using 2.41 eV photons has been demonstrated.

Coherent Raman beats associated with superhyperfine structure in ruby

Coherent Raman beats associated with superhyperfine structure (shfs) due to Cr-Al interaction were observed in ruby. The rf modulated Stark field was used to create coherences between shfs sublevels under the irradiation with a cw ruby laser. From the beat frequencies high resolution spectra of shfs with width (HWHM) of about 10 kHz were obtained in the frequency range from about 4.5 to 6.5 MHz. The utility of phase sensitive detection technique combined with rf Stark modulation was demonstrated. The observed spectra were compared with the theoretical ones.

Photodissociation of ions within the source of a double-focusing mass spectrometer

Ions generated by electron ionization are photodissociated by shining laser light into the ionization chamber of the source of a conventional geometry, double-focusing mass spectrometer. Photofragment ions formed within the chamber fall through the full potential difference (3 kV) applied between the chamber and the earthed exit aperture. They are detected and recorded using the same procedure as that used to obtain a mass spectrum; the magnetic field is scanned with the electric sector voltage held constant at the value required for transmission of all 3 keV ions. Photofragment ions are distinguished from other fragment ions by using the technique of phase-sensitive detection. The laser beam is mechanically chopped and a lock-in amplifier connected to the detector records only those currents which are in phase with the chopping frequency. Fragment ion spectra are obtained with the full resolution of the double-focusing mass spectrometer and large photofragment ion currents are obtained. The dependences of relative photoabsorption cross-sections and competing dissociation reactions on photoexcitation energy have been studied. The influence of source temperature on competing dissociation reactions of photoexcited ions has been studied by working at one laser light wavelength and several source temperatures. Also, photoionization of Ar + ions using 2.41 eV photons has been demonstrated confirming that these singly charged ions can exist in highly excited, long-lived states.

A two-wavelength oscillation CO2 laser for differential absorption measurements of atmospheric trace molecules

A two-wavelength oscillation C02 laser was constructed for the differential absorption measurement of low concentrations of atmospheric trace molecules and pollution molecules. A diffraction grating was alternately angle-modulated for the two-branch oscillation using a single C02 laser. The output power of the two wavelengths was balanced to be equal. With these characteristics, the phase-sensitive detection technique can be applied to give highly sensitive detection of the received power difference, which is proportional to the absorption in the optical path of the transmitted beams. A differential absorption spectrometer was used to demonstrate the usefulness of this two-wavelength laser oscillation method incorporating the output power balancing technique. The concentration of ethylene molecules was detected in a calibration cell and was also measured in real time in the exhaust of a combustion engine. From these results it has been shown that this differential absorption scheme offers a minimum detectable absorption of approximately 0.8%.

Studies of phase characteristics of various photon echoes with a phase sensitive detection method

Various optical coherent transient signals in ruby have been examined by using an optical phase sensitive detection (PSD) technique. The PSD is performed by comparing the phase of the heterodyne beat signals obtained by the Stark switching technique with that of a local oscillator whose frequency is equal to the heterodyne beat frequency. The optical FID, two-pulse photon echoes, radiation locked echoes and notched echoes have been detected as υ-mode signals with small u-mode signals probably due to the asymmetry of the inhomogeneous broadening. The dependence of the phase of the radiation locked and notched echoes upon the driving pulses is also examined. The experimental results are in good agreement with theoretical predictions based on Bloch equations.

Design of a high-resolution mass spectrometer for studying the photodissociation of organic ions in the gas phase

The design and operating conditions of an apparatus to study photodissociation of ions is described. Ions in a large, double-focusing, mass spectrometer are irradiated with photons from an argon-ion laser. A path length of 706 mm for interaction between the ion and laser beams is obtained by passing the radiation through the ion source and then along the same path as that followed by the ions in the first field-free region of the mass spectrometer. By scanning the voltage of the electric sector, different fragment ions resulting from photodissociation are transmitted in turn to an electron multiplier. They are distinguished from ions resulting from unimolecular dissociation by mechanically chopping the laser beam and using the technique of phase-sensitive detection. The photodissociations of the molecular, positive ions of the three isomers of nitrotoluene have been investigated. It is found that, after excitation of the ions with 2.541 eV photons, the resulting fragmentation pathways are different from those for the corresponding unimolecular dissociations.

Photodissociation of mass-selected ions investigated using a modified ZAB—2F spectrometer

A commerical reversed-geometry mass spectrometer, the ZAB—2F instrument, has been modified for studies of photodissociation of ions. A quartz window fitted to a special magnet flight-tube allows radiation from an argon-ion laser to pass into the second field-free region of the spectrometer. Photodissociation of mass-selected ions is studied by recording ion kinetic energy spectra of the photofragment ions, the currents being measured using the technique of phase-sensitive detection. The initial studies using the modified spectrometer have been of the molecular positive ions of n-butylbenzene and 4-nitrobenzonitrile. The results demonstrate the control over fragmentation pathways that can be achieved by changing the radiation wavelength and hence the degree of photoexcitation of the reacting ions.

The modified Beer-Lambert law and the curve of growth for the atomic resonance transition Pb(7s( 3P o1) ← 6p 2( 3P 0))

We describe an experimental study of the modified Beer-Lambert law ▪ or ▪ applied to the resonance transition of atomic lead at λ = 283.3 nm (Pb(7s( 3P o 1) ← 6p 2( 3P 0))). γ is determined experimentally by two methods. The first involves the standard empirical procedure for time-resolved resonance line absorption following, in this case, the pulsed irradiation of lead tetraethyl in the presence of a buffer gas, yielding γ = 0.41 ± 0.04. The second employs a direct calibration of I 0/ I tr against the absolute concentration of atomic lead vapour in the temperature range T = 600–783 K, in equilibrium with solid lead, using the technique of phase sensitive detection. This yields γ = 0.54 ± 0.01. γ is also generated by calculation from the slope of the logarithmic form of the curve of growth using the three-layer model. Whilst such calculations can generate γ values considerably less than unity at high optical densities, they do not yield such values using realistic parameters at the optical densities at which the calibration using absolute atomic concentrations was carried out. It is therefore concluded that the disagreement between the measured and calculated values arises from limitations in the three-layer model. The time-resolved resonance absorption signals obtained in the empirical measurements of γ permit one to estimate a rate constant k for the reaction. ▪ of 2 × 10 −10 cm 3 molecule −1 s −1 (300 K)

A review of electronics design for auger electron spectroscopy

The technique of modulation phase-sensitive detection for Auger electron spectroscopy has changed little in principle since its inception in the late 1960's. However the 1970's have seen significant improvements with the introduction of highly-tuned preamplifiers, high performance oscillators, the use of integral rather than derivative spectra, and dynamical background substraction. These Improvements have largely been brought about by the availability of inexpensive integrated circuits (IC's) and by computer control. The authors have considered new modulation coupling approaches and the up-dating of high voltage ramp windowing circuits.

A Study of Magnetophonon Effect in n-InSb Using Field Modulation Technique

Precise measurements of the magnetophonon effect in n -InSb are made by making use of a magnetic field modulation and phase sensitive second harmonic detection technique. Obtained data show an excellent signal to noise ratio and up to the 14th extremum are resolved in the transverse configuration at 77 K. Second derivative of the magnetoresistance in the longitudinal configuration is extensively investigated in the temperature range of 89 to 171 K. A curve resolver analysis is applied to resolve new structure which appears between the ordinary magnetophonon resonance peaks and four peaks around the N =2 peak are resolved, which are assigned to the structure arising from the spin-conserving single and two LO phonon processes. Fourier transform technique is also adopted to obtain frequency spectrum of the magnetophonon structure. The higher harmonic terms and two LO phonon components are resolved for the first time by using a proper window function for Fourier analysis. The results are consistent with those...

Flux flow measurements in irreversible type II superconductors

We report new measurements of the flux flow characteristics of irreversible type II superconducting foils in transverse magnetic fields. Using a phase sensitive detection technique to measure differential resistivity it is found that the current-voltage curves exhibit remarkable fine structure the main features of which are shown to scale with the critical current.

Temperature Dependence of Galvanomagnetic Properties of Graphite between 4.2 K and 298 K

By means of a phase-sensitive detection technique, the basal-plane galvanomagneticp roperties of highly crystalline graphite have precisely been investigated as functions of temperature ( T ) down to 4.2 K and of magnetic field intensity ( H ) up to 6.1 kOe. The average mobility of majority carriers has been found to vary in proportion to T -1.6 in the range 40 K to 80 K and to T -1.2 above it, while a saturating trend due to the defect scattering comes about below it. The Hall coefficient is highly dependent on H at T <200 K, the field dependence being nearly in agreement with Soule's observation at 77 K and 4.2 K. The zero-field Hall coefficient behaves in a fashion consistent with Ono-Sugihara's theory over the most part of temperature range examined, but deviates remarkably at T <30 K. The magnetoconductivity tensors are determined as functions of T and H ; both diagonal and off-diagonal components depend considerably on the sample quality especially at T <100 K. Discussions are given with reference t...